Sustainable Composites for Lightweight Applications (Woodhead Publishing Series in Composites Science and Engineering) [1 ed.] 0128183160, 9780128183168

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Woodhead Publishing Series in Composites Science and Engineering

Sustainable Composites for Lightweight Applications Hom Nath Dhakal Sikiru Oluwarotimi Ismail

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2021 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-818316-8 For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisitions Editor: Gwen Jones Editorial Project Manager: Andrea Gallego Ortiz Production Project Manager: Anitha Sivaraj Cover Designer: Alan Studholme Typeset by TNQ Technologies

Preface

Environmental damage caused by using and disposing of plastics and composites is worrisome. If this trend continues, it could lead to unprecedented damage to our natural resources. For example, with new environmental legislations, the European Union (EU) environmental regulation aims to reduce CO2 emissions in automotive components by using new technology, improved design and overall weight reduction. Composite materials reinforced with carbon and glass fibres are extensively used as structural materials, owing to their excellent strengths and stiffness-to-weight ratios. These attributes make these materials very attractive options for critical industrial sectors, such as aerospace, marine and automotive. It is logical to think that being able to reduce the overall mass results in a significant volume reduction, which consequently leads to the use of less raw materials and overall CO2 reduction. However, composite reinforcements such as glass and carbon fibres, currently being used in the aforementioned transportation sectors, have low recyclability after the end-of-life and high overall energy consumption during their production. Due to these environmental and energy utilisation concerns, a new class of materials, including natural plant fibre-reinforced composites, are being introduced in these critical sectors. This book entitled Sustainable composites for lightweight applications is a reflection of several years of experience gained by the authors in the field of advanced and sustainable composite materials and manufacturing. In this instance, Professor Hom Nath Dhakal has been involved in the design and development of sustainable lightweight composite materials for over 20 years. He has over 30 years of teaching and research experience in the field of Mechanical Engineering, Materials and Manufacturing. Professor Dhakal has published in over 150 international peerreviewed journals, book chapters and conference proceedings. Professor Dhakal is a Chartered Engineer (CEng), a member of the American Society for Composites (MASC) and a Fellow of the Higher Education Academy (FHEA), the Institution of Engineering and Technology (FIET) and the Institute of Materials, Minerals, and Mining (IOM3) (FIMMM). Similarly, Dr Sikiru Oluwarotimi Ismail is currently a Senior Lecturer in Manufacturing Engineering and Materials. His research specialisation focuses on Advanced and Sustainable Materials and Manufacturing (Mechanical) Engineering: Design, development, testing, damage characterisation and optimisation of materials

x

Preface

(especially composites), innovative manufacturing processes/monitoring and optimisation of manufacturing. He has over 10 years of teaching and research experience in the aforementioned field. Dr Ismail has published in over 60 international peerreviewed journals, book chapters and conference proceedings. In addition, Dr Ismail is a Member of many national/local and international professional bodies: American Society of Mechanical Engineers (MASME), Institution of Mechanical Engineers (MIMechE), American Society for Composites (MASC), Chartered Engineer of Institution of Engineering and Technology (CEng MIET), a Fellow of Higher Education Academy (FHEA) and Royal Society for the encouragement of Arts, Manufactures and Commerce (FRSA).

Key features of this book The inception of the book was made when both authors were teaching undergraduate and postgraduate engineering modules at the University of Portsmouth, such as Sustainable Development and Environmental Management, Strategies for Resource and Environmental management, as well as Materials and Manufacture. The planning of this book was immersed and started in 2018, when the authors strongly felt that there was a knowledge gap in sustainable composites, especially applying them in lightweight applications. The main purpose of bringing up this book is to elucidate the importance of the development of environmentally friendly materials to the industrial sectors, with the required mechanical properties and functionality. This book introduces the exciting field of biobased composite materials. Some real-world examples have been provided to explain the relevant topics. For each individual chapter, an abstract, keywords and conclusions are provided.

Target audiences of this book The readers can explore important aspects of biobased composites, their properties, damage analysis and use in critical application areas. Future perspectives of sustainable composites have been systematically presented. The contents of this book will help to appreciate the sustainable materials of the future. Also, teachers, researchers, scientists and industries can appreciate the benefits and apply the knowledge for various applications.

Chapter highlights of this book This book has seven chapters. Chapter 1 introduces the general introduction of composite materials and their key features focusing on biobased composites. In Chapter 2, the structure and the morphological aspects of sustainable natural fibre reinforcements are highlighted. Interestingly, Chapter 3 discusses important aspects of lightweight composites, their properties and various applications. In Chapter 4, design, manufacturing processes and their effects on biocomposites properties are analysed

Preface

xi

and discussed. Moving forward, Chapter 5 focuses on the testing and damage characterisation of biobased composite materials. Chapter 6 considers the different improvement techniques and their influences on the properties enhancement. Finally, Chapter 7 summarises the various aspects and puts forward the future perspectives and the challenges of sustainable composite materials for various applications while moving forward in this thriving field. Hom Nath DHAKAL Sikiru Oluwarotimi ISMAIL

Introduction to composite materials 1.1

1

Background and context

A composite material is composed of at least two visually distinct materials, which combine to give properties superior to those of the individual constituents while retaining their respective chemical, physical and mechanical properties while contributing desirable properties to the whole (Hull and Clyne, 1996; Matthews and Rawlings, 1994). The primary reason why composite materials used for engineering applications are due to their high performance relating to improved specific strength and stiffness (strength to-weight-ratio). This attribute helps in reducing the overall weight of components. If one considers an automotive part, for example, the reduction of overall weight leads to the reduction of fuel consumption, increased performance and eventually leads to the reduction of CO2 emissions (Erbach, 2014). There are several factors that influence the overall performance of composite materials. Important properties such as tensile strength and modulus, impact resistance and fracture toughness behaviours (mode I, mode II and mixed-mode), vibration behaviour related to damping, thermal properties such as thermal decomposition, coefficient of thermal expansion (CTE), and thermal conductivity are directly related to the reinforcements types, their volume and geometry and how they were processed and prepared. Therefore, understanding these key parameters is very important in the process of design, manufacturing and service life of composites (Faruk et al., 2012; Monteiro et al., 2010; Paturel and Dhakal, 2020). Despite many research work directed in these composites, their structure-property relationships, reinforcement types and their influence on the various properties, their environmental impacts and end-of-life (EoL) aspects are still not fully understood. This chapter attempts to provide a basic introduction of composite materials and highlights the important aspects in terms of understanding their structure, properties, applications and EoL options while considering sustainable composites for lightweight applications. There are four main types of composites, namely, metal-matrix composites (MMCs), ceramic-matrix composites (CMCs), fibre-reinforced polymer matrix composites (FRPCs). Metal matrix composites consist of fibres reinforced with metal alloys. Such composites can withstand high temperatures, unlike other composites, but are heavy due to the presence of metal. MMCs are used in the automotive industry where metals and alloys are used as matrix material and reinforced with fibres or particulates. Under this, aluminium matrix composites are the most commonly used as matrix material. CMCs use ceramics as matrix material and reinforce with short fibres such as silicon carbide and boron nitride. CMCs have excellent resistance to high temperatures owing to the presence of ceramic. This is one of the major

Sustainable Composites for Lightweight Applications. https://doi.org/10.1016/B978-0-12-818316-8.00001-3 Copyright © 2021 Elsevier Ltd. All rights reserved.

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Sustainable Composites for Lightweight Applications

Table 1.1 Advantages and some drawbacks of FRPCs (Faruk et al., 2012; Dhakal et al., 2007; Bhardwaj, 2017). Advantages

Some drawbacks

• low density, high specific strength and stiffness (high strength-to-weight ratio) • less fatigue sensitive • economical (cost-effective) • corrosion resistance • good surface finish can be obtained • properties can be tailored in the fibre direction • low coefficient of thermal expansion (LTE) • part count reduction

• susceptible to chemical and solvent attacks • low reusability or recyclability in terms of carbon and glass fibre-reinforced composites • fluctuating cost • damage modes difficult to detect

advantages of the ceramic matrix. Therefore, these composites are used in hightemperature applications where compressive strength is more demanding than tensile and impact properties (Silvestre et al., 2015). Carbon and glass fibre-reinforced composites are very important materials in many engineering applications due to their lightweight, commercially available in continuous form and their corrosion resistance behaviour. Due to their attractive attributes, composites materials are extensively used in many critical applications such as aerospace, automotive, marine, construction and sports equipment. In the last 3 decades, polymeric composites have been one of the most attractive materials due to their versatile properties. One of the most commonly utilized composite types is Polymer Matrix Composites (PMCs), also known as fibre-reinforced plastics (FRPs). These composite materials are reinforced with many different types of fibres such as synthetic fibres (carbon, glass and aramids), natural biofibres (hemp, flax, jute, date palm kenaf among others) (Bhardwaj, 2017; Dhakal and Sain, 2019). Due to the several outstanding properties of composite materials, different key industry sectors are increasingly using composites instead of metallic materials in many structural and semi-structural applications. Some key advantages and drawbacks of synthetic fibre-reinforced polymer composites (FRPCs) against their metal counterparts are highlighted in Table 1.1. Growing environmental concern, high rate of depletion of petroleum-based materials; new stricter environmental regulations have resulted in research for alternative fibre-reinforced composites that are compatible with the environment. Natural fibrereinforced composites (NFRCs) have received much attention in recent years due to their many attractive properties such as high specific tensile strength and modulus compared to conventional glass fibres. Natural fibres represent an environmentfriendly alternative to conventional fibre reinforcements. Natural fibres are emerging as low cost, lightweight and biodegradable alternatives to glass fibres in composites (Dhakal et al., 2013; Dhakal et al., 2014). Moreover, the environmental impact of natural fibre versus glass and carbon fibres as reinforcements in composite fabrication

Introduction to composite materials

3

Table 1.2 Total energy required for production and cost of different for natural fibres versus glass and carbon fibres (Huda et al., 2008). Fibre types

Cost (US$/ton)

Energy (GJ/ton)

Natural fibres

200e1000

4

Glass fibres

1200e1800

30

Carbon fibres

12,500

130

NFRPCs fully / partially biodegradable

Applications

Composite development

NFRPCs

Amalagamation of natural fibre and polymer

Life cycle of NFRPCs

Scrap disposal

Fibre extractuion Landfills

CO2+ sun light Composting

Figure 1.1 Life cycle stages of natural fibre composites (Khan et al., 2018).

have been reported through the use of life cycle assessment (LCA). These various reports suggest that natural fibres exhibit significantly lower cost and lower energy consumption compared to glass and carbon fibres, which is shown in Table 1.2. The life cycle stages of natural fibre composites are illustrated in Fig. 1.1. However, due to their biochemical composition, these fibres are hydrophilic and need some kind of treatments to enhance the compatibility with hydrophobic thermoplastic and thermosets matrices. This shortcoming of natural fibre composites restricts the use of these composites in many non-structural and structural applications. The moisture absorption can lead to swelling of the fibres creating voids and microcracks at the fibre-matrix interface region resulting in a significant reduction of load transfer capability from the matrix to reinforcing fibres leading to reduction of mechanical properties. Many reported works on natural fibre composites have revealed that these shortcomings of natural fibre reinforcements have been minimised by modifying

4

Sustainable Composites for Lightweight Applications

fibre surfaces using various treatments achieving improvements in physical, mechanical and thermal properties by making compatible with different polymer matrices (Mohanty et al., 2003). The properties of fibre-reinforced composites depend on many factors, for example, types of matrices and reinforcement used, fibre volume fraction, fibre aspect ratio (length divided by the diameter of the fibre, L/d), fibre geometry and interfacial adhesion between reinforcement and the matrix. It is generally assumed that the higher the aspect ratio, the better the properties, up to a certain threshold. With longer thinner fibres the stress is able to travel along the fibres distributing the load more evenly than if the fibres are shorter; this, in turn, increases the mechanical properties. However, only the case up until a threshold aspect ratio is met, if the fibre aspect ratio is too high, i.e., the fibre size is increased, then fibre bunching can occur resulting in poor cohesion at the interface and a larger number of voids. At a low aspect ratio, the addition of reinforcement into the composite can create the phase of discontinuity, leading to heterogeneity structure and can result in poor mechanical performance. At a higher aspect ratio up to its threshold, the mechanical properties are expected to increase, because of good interfacial interaction between the matrix and the reinforcement (Hull and Clyne, 1996). In composite fabrication, the matrix wets the fibres, then the matrix and the fibres are bonded together and the resulting material, i.e. the composite, becomes far superior compared to the individual constituents. It is worth noting that these reinforcing fibres possess very high strength on their own, but when the load is applied, they break easily due to various defects. With this, outstanding mechanical properties can be achieved. When the polymeric matrix and fibres are combined, they provide significantly higher strength of resultant composite compared to individual fibre and matrix on their own. As can be seen in Fig. 1.2, the tensile properties of neat polypropylene (PP) and polylactic acid (PLA) have been significantly improved with the reinforcement of flax fibre onto PP and PLA. The tensile stress and modulus for both PP and flax/PP and flax PLA have been significantly improved. It is evident that the fibre reinforcement significantly contributes to the overall properties improvement of the resulting composites.

(a)

(b) 70

10

60

8

40

GPa

MPa

50 30

6 4

20 2

10 0

0 PP 0%

PLA 30%

40%

PP 0%

PLA 30%

40%

Figure 1.2 Comparison of tensile stress and modulus of (a) tensile stress of PP and PLA/flax composites in comparison to (b) modulus of PP and PLA/flax fibre-reinforced composites (Oksman et al., 2003).

Introduction to composite materials

1.2

5

Matrices and their types

The word “matrix” is a multipurpose word with wide applications and interpretations. In the case of material science, matrix refers to ceramic, metal or polymer matrices of a composite material. In the field of composite science, polymeric matrices are the most abundantly used materials. Poly comes from the Greek word for “many” and “mer” comes from the Greek word for “parts”. Therefore, a polymer is a long chain of molecules made up of from a series of repeating basic units called “mer”. Polymeric materials are most commonly used in every aspect of our lives. These materials are usually known as plastic and are the most widely used in composite manufacturing. These matrices are sub-divided into two types; thermoplastics and thermosets polymers. Fig. 1.3 shows a simple classification of different polymer matrices used in composites. The main differences between these two types of matrices are that thermoplastics can be re-heated and reformed to alter the shape or allow the shape of a component to be reformed, i.e., they are re-workable at temperature. Thermoplastic matrices are with high molecular weight, long chain molecules that can either be amorphous or partially crystalline (Stewart, 2011). Thermoplastic-reinforced composite materials are advantageous when strength, as well as improved toughness behaviour are important. The ability to reheat thermoplastic matrices offers re-workability and being able to formulate pellets, which allows greater freedom in manufacturing (Mallick, 2008; Stewart, 2011). Thermoplastics, as a matrix in material composites, have been growing gradually due to their recycling ability and rapid production cycle (Armentia et al., 2019). Thermosets matrices are resins, which cross-link during curing (hardening), resulting in glassy brittle solids such as epoxy, vinyl ester and polyesters. The cross-linked structures prevent the polymer from flowing and melting, which can provide the thermal stability of the polymer. The used or waste thermosets cannot be reused or recycled. Comparison between these two polymers in terms of their key advantages and disadvantages are highlighted in Table 1.3. Composites Matrix

Metal

Polymer

Thermosetting

Reinforcement

Ceramic

Biodegradable (natural/manmade)

Non-biodegradable

Thermoplastics

Biodegradable/non-biodegradable

NFRPCs (fully/partially biodegradable)

Figure 1.3 Classification of matrices and reinforcements for polymeric composites (Khan et al., 2018).

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Sustainable Composites for Lightweight Applications

Table 1.3 Key advantages and disadvantages of thermosets and thermoplastics polymers (Nordin et al., 2013). Advantages

Disadvantages

Thermosets

• low resin viscosity • good fibre wetting • excellent thermal stability once cured • chemically resistant • better creep resistant than thermoplastics

• prone to brittleness • non-recyclable via standard techniques • not post-formable

Thermoplastics

• recyclable • easy to repair by welding and solvent bonding • post-formable • good impact toughness

• poor melt flow • need to be heated above the melting point for processing purposes

1.2.1

Types and main functions and the properties of matrices

In order to get optimal composite properties, a proper cure of matrix (chemical reaction) is very important. The functions of a matrix are as follows: • • • • •

Binding of the fibres (reinforcements) and the transferring of the applied load to the fibres Isolates the fibres so that individual fibres can act separately (fibre-matrix interface) Provides good surface finish and machinability Provides protection to fibres from chemical attack and other mechanical wear Provides other secondary properties

In summary, the main functions of the matrix in reinforced composites are to support and transfer the stresses to the fibres (reinforcements), which carry most of the load and protect the fibres against physical damage and the environment. The commonly used thermosets and thermoplastics matrices and their physical and mechanical properties are illustrated in Table 1.4.

1.2.1.1

Epoxy resins

These resins are high-performance matrices in terms of their mechanical properties. Due to the cross-linked structure, the epoxy matrix has good dimensional stability. They have high strain to failure compared to other resins such as polyesters and vinyl esters. Epoxy resin has good chemical resistance, as well as less prone to moisture in comparison to polyesters and vinyl ester. However, epoxies are brittle polymers, and often the toughness of this resin is enhanced by using some additives. Generally, when the toughness is increased, the modulus and glass transition temperature (Tg) gets decreased.

1.2.1.2

Polyester resins

These are the most widely used resins, especially in the marine industry, in building yachts and boats. There are two types of polyester resins, namely saturated and

Introduction to composite materials

7

Table 1.4 Physical and mechanical properties of commonly used thermosets and thermoplastics matrices (Manaia et al., 2019). Density (g/cm3)

Young’s modulus (GPa)

Tensile strength (MPa)

Strain to failure (%)

Epoxy resins

1.1e1.4

3.0e6.0

35e100

1e6

Polyesters

1.2e1.5

2e4.5

40e90

4e7

Vinyl ester

1.2e1.4

3.1e3.8

40e90

2

HDPE

0.94e0.96

1.1e1.60

30e40

2e130

Polypropylene (PP)

0.89e0.92

1.0e1.4

0.02e0.04

20e400

PS

1.04e1.06

4e5

25e69

1e2.5

PLA

1.21e1.25

0.35e3.5

21e60

2.5e6

Matrix types

Thermosets

Thermoplastics

unsaturated. These resins are cost-effective resins, and the price is far lower than epoxy and vinyl ester. The mechanical properties, especially interlinear shear stress (ILSS) strain due to failure of these resins, are far lower than that of vinyl ester and epoxy. The unsaturated polyesters are the backbone of the composite industry, with 75% of the resin being used.

1.2.1.3

Vinyl ester resins

These resins have properties somewhere between epoxy and polyesters. They possess good toughness, as well as better repellence to water and other chemical attacks.

1.2.1.4

Phenolic resins

Phenolic resins are an attractive matrix, especially where fire resistance attributes are of importance. Phenolic resins show superior fire resistance behaviour compared to other thermosetting resins such as epoxy and vinyl ester.

1.2.1.5

Polyethylene

Polyethylene (PE) is a common yet extremely useful and cost-effective plastic polymer. PE is found nearly everywhere today, from plastic grocery bags, plastic wrap, drain pipes, milk cartons, to trash cans. PE is an easily processed thermoplastic, which can be made into a variety of shapes and forms, including tubing. An especially convenient quality of PE is its ability to be easily altered during processing to give a variety of forms that differ based on polymer chain length, density and crystallinity. These characteristics allow PE products to be tailored for a variety of uses. As an example, high-density PE (HDPE) has a comparatively more linear morphology and a higher degree of crystallinity than low-density PE (LDPE). HDPE is lightweight and

8

Sustainable Composites for Lightweight Applications

possesses good tensile strength, while LDPE exhibits good chemical resistance. PE can be further modified by resin manufacturers to increase its structural and functional properties. PE polymer chains can be extended to produce ultra-high molecular weight (UHMW) PE to give a very dense PE product. Linear LDPE (LLDPE) has a greater proportion of short branches resulting in greater flexibility.

1.2.1.6

Polypropylene

Polypropylene (PP) is a tough and rigid, semicrystalline thermoplastic produced from propene (or propylene) monomer. PP is among the most commonly used cheapest thermoplastics. PP has poor resistance to UV, impact and scratches with low service temperature. PP is widely used in various applications, including packaging (flexible and rigid), due to its good barrier properties, high strength and good surface finish. PP is also considered as one of the emerging polymers in the automotive industry. PP is used in medical applications due to high chemical and bacterial resistance, as well as good mechanical properties. The main applications in automotive include battery cases and trays, front and rear bumpers, fender liners, interior trim, instrumental panels and door trims. There are self-reinforced PP also available, which provides improved strength. The selfreinforced PP has attracted a great deal of attention in automotive applications recently due to their key advantages such as no skin irritation, easy to handle and good recyclability over glass fibre-reinforced composites. Additional advantages of self-reinforced PP are as follows: • Lightweight • Good toughness properties • Good strain to failure property.

1.2.1.7

Polystyrene

Polystyrene (PS) is one of the most commonly used thermoplastics. It is easy to process, and semi-finished products such as foams, sheets and films are produced from PS. Additional advantages of self-reinforced PS are as follows: • Lightweight • Optical clarity • Easy to process

Some disadvantages of PS are as follows: • Poor oxygen and UV resistance • Poor impact resistance.

1.2.1.8

Polylactic acid

One of the great attractions of using polylactic acid (PLA) is its green attributes. PLA is fully degradable, and PLA reinforced with natural fibres becomes 100% bio-based composites.

Introduction to composite materials

9

Additional advantages of PLA are as follows: • Lightweight, high glossy appearance • Good strength • Good transparency

Some disadvantages of PLA are as follows: • Poor toughness (highly brittle material) • Higher cost than other commonly used thermoplastic matrices.

1.3

Reinforcements and their types

In terms of reinforcements, commonly used in conventional composites, there are three main types: glass, carbon and ceramic fibres. These reinforcements provide required strength, durability and overall quality of the composite parts. Some of the factors governing fibre reinforcement contributions to the composites are; 1. 2. 3. 4. 5.

The mechanical properties of the fibre Adhesion (interaction) between the fibre and the matrix Volume or weight fraction of fibre in the composite Fibre orientation in the composites Manufacturing process used to make the fibre.

1.3.1 1.3.1.1

Conventional reinforcements and their types Glass fibres

Most of the glass fibre used in the composite is made from molten glass combined with silica. This gives glass fibre superior bulk properties such as strength, stiffness, flexibility and hardness. Due to these attributes, glass fibres are used in structural composites. The main types of glass fibre that are used in the production of reinforced composites are S-glass and E-glass. Glass fibres normally come in two types: • The low-cost general-purpose fibres (E-glass) and • The premium special-purpose fibres.

Among the key advantages for using glass fibres are the reduced costs, the inability to conduct heat and therefore increased potential for insulation, as well as its good mechanical properties and anti-corrosiveness. These fibres are extensively used in the automotive and marine sectors. However, nowadays, carbon fibres are used in these applications instead of glass fibres due to their high-strength-to weight ratio.

10

1.3.1.2

Sustainable Composites for Lightweight Applications

Carbon fibres

Due to their high strength-to-weight ratio, carbon fibres are one of the most attractive reinforcements in the manufacture of advanced structural composites compared to conventional load bearing materials such as steels. These fibres are light in weight (low density) and have excellent tensile strength and stiffness, high fatigue strength, low coefficient of linear thermal expansion and lower susceptibility to corrosion. These favourable attributes make them attractive an material in automotive, aerospace, sporting goods and many other applications. Carbon fibres are produced by using three different precursor feedstock, such as rayon, polyacrylonitrile and pitch. Due to their aforementioned attributes, these fibres are used in composites reinforcements where high strength and stiffness fatigue resistance, high-temperature applications, and chemical inertness are important. Despite their many attractive attributes, these fibres suffer from low impact resistance due to their brittleness behaviour. Carbon fibres have high electrical conductivity, which in many cases, can be disadvantageous. Additionally, the scale of their firmness and strength depends on the production processes. In many cases due to their high cost, carbon fibres are not used in may common commercial applications; however, they are extensively used in the aerospace industry where lightweight material contributes to the overall weight savings, which is a vital point compared to cost.

1.3.1.3

Ceramic fibres

Ceramic fibres as crystalline or amorphous synthetic minerals, which are characterized by their refractory nature, they are obtained mainly in the form of whiskers. They are normally composed of silica and alumina oxides. Most ceramic fibres are polycrystalline or polycrystalline oxides and are generally white in colour. These fibres are used where high temperature resistance is required. The synthetic fibres discussed above are created from unsustainable fossil-based materials through energy-intensive processes; hence, these fibres give a high carbon footprint as their production processes are high-energy intensive. Moreover, these synthetic fibres provide limited recyclability and non-biodegradability that have become a growing concern when disposing of waste end-of-life products. Glass fibre-reinforced composites are used in various applications. Glass fibrereinforced composites have proven to meet the structural and durability demands of automobile interior and exterior parts, various components used in the marine industry, among others. Good mechanical properties and well-developed manufacturing bases have aided in the insertion of fibreglass-reinforced plastics within the automotive and marine industry. However, glass fibre-reinforced composites exhibit few shortcomings, such as their relatively high fibre density (approximately 40% higher than natural fibre), difficult to machine, and poor recycling properties, not to mention the potential health hazards posed by glass fibre particulates (Tables 1.5 and 1.6). The energy consumption to produce a flax fibre mat obtained by using life cycle assessment (LCA) suggests (9.55 MJ/kg), including cultivation, harvesting and fibre separation, amounts to approximately 17% of the energy to produce a glass-fibre mat (54.7 MJ/kg) (Joshi et al., 2004).

Introduction to composite materials

11

Table 1.5 Advantages and drawbacks of natural fibres as composite reinforcements (Faruk et al., 2012; Dhakal et al., 2007; Prasad and Sain, 2003). Advantages

Some disadvantages

• low density and high specific strength and stiffness • fibres are obtained from renewable resources, for which production requires less energy, involves CO2 absorption, while returning oxygen to the environment • fibres can be produced at a lower cost than synthetic fibre • low hazardous manufacturing processes • low emission of toxic fumes when subjected to heat and during incineration at the end-of-life • less abrasive damage to processing equipment compared with that for synthetic composites • lower risk to human health (no skin irritation)

• lower durability compared to their synthetic fibre composites but can be improved considerably with treatment • high moisture absorption, which results in swelling • lower strength, particularly impact strength compared to synthetic fibres composites • greater variability of properties (depending on geographical location, local growing conditions and weather) • lower processing temperatures limiting matrix options (low decomposition temperature) • lower thermal resistance than synthetic fibre • heterogeneous size • lack of standard • prone to fire hazard

Table 1.6 Comparison of physical and mechanical properties of metallic, conventional reinforcements (carbon and glass) and natural hemp fibres and flax (bundles) with E-glass (Bledzki et al., 1996; Faruk et al., 2012; Gurunathan et al., 2015).

Young’s modulus (GPa)

Specific strength (MPa)

Specific modulus (GPa)   E r

Fibre types

Density (g/cm3)

Tensile strength (MPa)

Steel

7.8

1300

200

167

26

Aluminium

2.81

350

73

124

26

Carbon

1.51

2500

151

1656

100

E-glass

2.10

1100

75

524

28

Aramid

1.32

1400

45

1656

100

Hemp

1.4

690

30e70

453

21e50

Flax

1.5

345e1830

27e80

230e1220

18e53

12

Sustainable Composites for Lightweight Applications

1.3.2

Natural fibres and their types

These fibres are obtained from a natural source, such as plants, animals and minerals. There are certain benefits that have been attributed to the use of natural fibres as reinforcement. However, there are certain aspects of natural fibres, which make them disadvantageous to use. The following sections highlight the advantages and some drawbacks of natural fibres as reinforcements. The focus of the book being sustainable lightweight composites, natural fibre will be looked at in a more comprehensive way. • • • •

Natural fibre can be classified according to their origin in the following groups: Plant fibres (bast, leaf, fruit, seed, wood, grass) Animal fibres (wool, hair, silk) Mineral fibres (ceramics, metal).

1.3.2.1

Advantages and disadvantages of natural fibres

Natural fibres offer many advantages by virtue of being natural. They have a high strength to weight ratio due to their lower density. Moreover, they have many green attributes such as biodegradable, recyclable and renewable. Similarly, they have tremendous processing benefits in terms of tool wear and energy requirements. Despite their attractive attributes, natural fibres also have some drawbacks in comparison with their synthetic counterparts, such as glass and carbon fibres, especially in terms of their durability, their strength and the moisture absorption. Fibre property (physical and morphological) availability is another major factor in selecting a natural fibre to use as reinforcement, as this is often attributed to geographical location and local growing conditions (Dhakal et al., 2020). Table 1.5 summarises the main benefits and some drawbacks.

1.4

Main drivers of composite materials

Fibre-reinforced plastics are referred to as (FRP) composites, usually with carbon, glass, aramid or natural fibres embedded in a polymer matrix. Other matrix materials can be used, and composites may also contain fillers or Nano-materials such as graphene and layered silicates. The many component materials and different processes that can be used to make composites extremely versatile and efficient. They typically result in lighter, stronger, more durable solutions compared to traditional materials. However, a major driving force behind the development of composites has been the combination of the reinforcement and the matrix, which can be tailored to meet the required final properties of a component (Wambua et al., 2003). Composite materials are lightweight materials. For example, the density of carbon fibre is far lower than that of metallic materials. Industries such as aerospace, automotive, marine and construction are seeking strong and durable lightweight materials. Another key benefit of using composite materials is their directional properties, which can be tailored in order to achieve high strength and modulus.

Introduction to composite materials

13

The key drivers can be listed as follows: • High anisotropic properties • Sustainability of materials • High fatigue properties.

The key drivers of using composite materials in substations, compared to their metallic counterparts in structural and non-structural applications, lie in the specific strength and modulus of their reinforcements. The key mechanical properties (strength, stiffness and strain to failure) of composite materials are governed by the properties of reinforcements, their morphology, geometry, volume fraction and alignment. Table 1.6 illustrates the mechanical properties of metal and synthetic reinforcements. As can be seen, the specific properties of carbon and glass fibres are far superior to that of metals. Steel and aluminium are well-established materials, and they still top the list of materials used in the automotive sector due to their well-defined damage characterisation ease to manufacture and lower cost of manufacturing. A closer look at the comparative values of physical and mechanical properties of metal, synthetic reinforcements carbon and glass and hemp and flax fibres are presented in Table 1.6. The results demonstrate that carbon and glass fibres are superior to even metals in terms of their specific properties. While considering natural fibre composite reinforcements, hemp and flax fibres are the most commonly used materials. As expected, the ultimate tensile strength is particularly higher for carbon and glass fibre than that of hemp and flax fibres. However, if one considers the specific modulus of carbon, glass and aramid fibres (modulus/density), they are far superior to that of steel and aluminium. This signifies the greater potential for weight reduction. This is one of the key drivers for the substituting of metallic materials with lightweight and stronger composite materials (Gurunathan et al., 2015).

1.5

Application of sustainable composite materials

For the last decade, natural fibre-reinforced sustainable composites have been used in different industry sectors, including transport (automotive, marine, rail, aviation), sports, building/construction and medical applications. The detailed application areas have been extensively discussed in Chapter 3.

1.6

Summary

Due to the increase in environmental awareness amongst the industries, as well as consumer pressure and government legislations, the urgency to develop alternative lightweight materials to substitute heavy metallic materials, has already taken place. As there is a continual progress in replacing metallic materials with glass and carbon fibre-reinforced composites, a similar approach needs to be adapted to replace carbon and glass fibre composites with more sustainable natural fibre-reinforced composites with a continuous drive for property, durability and quality improvement with the

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Sustainable Composites for Lightweight Applications

enhanced end-of life-option. This will help the industry to reduce energy consumption arising from its materials and processes. If this is linked to the transport sectors, for example, this will then help in creating more fuel-efficient and lower emission vehicles. In order to realise the full potential of composite materials, it is important that from the design stage to raw materials extraction, production, use and endof-life stages are fully understood. This chapter has provided an overview of composite materials, their main characteristics and case for the need for sustainable lightweight composite materials for engineering applications.

References Armentia, S.L., Enciso, B., Mokry, G., Abenojar, J., Martinez, M.A., 2019. Novel application of a thermoplastic composite with improved matrix-fibre interface. J. Mater. Res. Technol. 8 (96), 5536e5547. Bhardwaj, S., 2017. Natural fibre composites:an opportunity for farmers. Int. J. Pure Appl. Biosci. 5, 509e514. Bledzki, A.K., Reihmane, S., Gassan, J., 1996. Properties and modification methods for vegetable fibers for natural fiber composites. J. Appl. Polym. Sci. 59, 1329e1336. Dhakal, H.N.P., Méner, E.L., Feldner, M., Jiang, C., Zhang, Z., 2020. Falling weight impact damage characterisation of flax and flax basalt vinyl ester hybrid composites. Polymers 12, 806. Dhakal, H.N., Sain, M., 2019. Enhancement of mechanical properties of flax-epoxy composite with carbon fibre hybridisation for lightweight applications. Materials 13 (1), 109. Dhakal, H.N., Skrifvars, M., Adekunle, A., Zhang, Z.Y., 2014. Falling weight impact response of jute/methacrylated soybean oil bio-composites under low velocity impact loading. Compos. Sci. Technol. 92, 134e141. Dhakal, H.N., Zhang, Z.Y., Guthrie, R., MacMullen, J., Bennett, N., 2013. Development of flax/ carbon fibre hybrid composites for enhanced properties. Carbohydr. Polym. 96 (1), 1e8. Dhakal, H.N., Zhang, Z.Y., Richardson, M.W., 2007. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos. Sci. Technol. 67 (7e8), 1674e1683. Erbach, G., 2014. Reducing CO2 Emissions from New Cars. European Parliamentary Research Service. Faruk, O., Bledzki, A.K., Fink, H.P., Sain, M., 2012. Biocomposites reinforced with natural fibres: 2000e2010. Prog. Polym. Sci. 37 (11), 1552e1596. Gurunathan, T., Mohanty, S., Nayak, S.K., 2015. A review of the recent developments in biocomposites based on natural fibres and their application perspectives. Compos. Appl. Sci. Manuf. 77, 1e25. Huda, M.S., Drzal, L.T., Ray, D., Mohanty, A.K., Mishra, M., 2008. Natural-fiber composites in the automotive sector. In: Properties and Performance of Natural-Fibre Composites. Woodhead Publishing, Oxford, UK, ISBN 9781845692674. Hull, D., Clyne, T.W., 1996. In an Introduction to Composite Materials, (Cambridge Solid State Science Series. Cambridge University Press, Cambridge, pp. IeVI. https://doi.org/10.1017/ CBO9781139170130. Joshi, S.V., Drzal, L.T., Mohanty, A.K., Arora, S., 2004. Are Natural Fiber Composites Environmentally Superior to Glass Fiber Reinforced Composites? Compos. Part A Appl. Sci. Manuf. 35, 371e376.

Introduction to composite materials

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Khan, M.Z.R., Srivastava, S.K., MK Gupta, M.K., 2018. Tensile and flexural properties of natural fiber reinforced polymer composites: a review. J. Reinforc. Plast. Compos. 37, 1435e1455. Mallick, P.K., 2008. Fiber-Reinforced Composites. United States of America: Taylor & Francis Group. Manaia, J.P., Manaia, A.T., Rodriges, L., 2019. Industrial hemp fibres: an overview. Fibres 7, 106. Matthews, F.L., Rawlings, R.D., 1994. Composite Materials: Engineering and Science. Chapman and Hall, London. Mohanty, A.K., Misra, M., Hinrichsen, G., 2003. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol. Mater. Eng. 276e277 (1), 1e24. Monteiro, S.N., Satyanarayana, K.G., Ferreira, A.S., 2010. Selection of high strength natural fibres. Matéria 15 (4), 488e505. Nordin, N.A., Yussof, F.M., Kasolang, S., Salleh, Z., Ahmed, A.M., 2013. Wear rate of natural fibre:long kenaf composite. Procedia Eng. 68, 145e151. Oksman, K., Skrifvars, M., Selin, J.F., 2003. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos. Sci. Technol. 63, 1317e1324. Paturel, A., Dhakal, H.N., 2020. Influence of water absorption on the low velocity falling weight impact damage behaviour of flax/glass reinforced vinyl ester hybrid composites. Molecules 25 (2), 1e16, 278. Prasad, B.M., Sain, M.M., 2003. Mechanical properties of thermally treated hemptreated hemp fibres in inert atmosphere for potential composite reinforcement. Materials Research and Innovation 7, 231e238. Silvestre, J., Silvestre, N., de Brito, J., 2015. Review article an overview on the improvement of mechanical properties of ceramics nanocomposites. Hindawi Publishing Corporation J. Nanomat. https://doi.org/10.1155/2015/106494. Article ID 106494. Stewart, R., 2011. Thermoplastic composites e recyclable and fast to process. Reinf. Plast. 55, 22e28. Wambua, P.W., Ivens, J., Verpoest, I., 2003. Natural fibres: can they replace glass in fibre reinforced plastics? Compos. Sci. Technol. 63, 1259e1264.

Further reading Agarwal, J., Sahoo, S., Mohanty, S., Nayak, S.K., 2019. Progress of novel techniques for lightweight automobile applications through innovative eco-friendly composite materials: a review. J. Thermoplast. Compos. Mater. 63, 1e36. Asdrubali, F., D’Alessandro, F., Schiavoni, S., 2015. A review of unconventional sustainable building insulation materials. Sustain. Mater. Technol. 4, 1e17. Bourmaud, A., Beaugrand, J., Shah, D.U., Placet, V., Baley, C., 2018. Towards the design of high-performance plant fibre composites. Prog. Mater. Sci. 97, 347e408. Dhakal, H.N., MacMullen, J., Zhang, Z.Y., 2016. Moisture measurement and effects on properties of marine composites. In: Marine Applications of Advanced Fibre-Reinforced Composites. Woodhead Publishing, Sawston, UK; Cambridge, UK, pp. 103e124. Dhakal, H.N., Zhang, Z.Y., Bennett, N., 2012. Influence of fibre treatment and glass fibre hybridisation on thermal degradation and surface energy characteristics of hemp/unsaturated polyester composites. Compos. B Eng. 43 (7), 2757e2761.

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Feraboli, P., Gasco, F., Wade, B., Maier, S., Kwan, R., Masini, A., Oto, L.D., Reggiani, M., 2011. Lamborghini “forged composites” technology for the suspension arms of the Sesto Elemento. In: American Society of Composites. 2011: Montreal. Hitchen, S.A., Kemp, R.M.J., 1996. Development of novel cost effective hybrid ply carbon-fibre composites. Compos. Sci. Technol. 56 (9), 1047e1054. Li, Y.X., Choy, X.J., Guo, C.L., Zhang, Z., 1999. Compressive and flexural behavior of ultrahigh-modulus polyethylene fibre and carbon fibre hybrid composites. Compos. Sci. Technol. 59 (1), 13e18. Madurwar, M.V., Ralegaonkar, R.V., Mandavgane, S.A., 2013. Application of agro-waste for sustainable construction materials: a review. Construct. Build. Mater. 38, 872e878. Mussig, J. (Ed.), 2010. Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications. John Wiley and Sons, Chichester. Mustafa, N.S., Omer, M.A.A., Garlnabi, M.E.M., Ismail, H.A., 2016. Reviewing of general polymer types, properties and application in medical field. Int. J. Sci. Res. 5 (8), 212e221. Pervaiz, M., Panthapulakkal, S., Birat, K.C., Sain, M., Tjong, J., 2016. Emerging trends in automotive lightweighting through novel composite materials. Mater. Sci. Appl. 7 (1), 26e38. Peças, P., Carvalho, H., Hafiz Salman, H., Leite, M., 2018. Natural fibre composites and their applications: a review. J. Compos. Sci. 2, 66. https://doi.org/10.3390/jcs2040066. Sarbu, A., Dima, S.O., Dobre, T., Udrea, I., Bradu, C., Avramescu, S., Mihalache, N., Radu, A.L., Nicolescu, T.V., Lungu, A., 2009. Polystyrene wastes recycling by lightweight concrete production. Rev. Chem. 60, 1350e1356. Yan, L., Chouw, N., Jayaraman, K., 2014. Flax fibre and its compositesda review. Compos. B Eng. 56, 296e317. Yan, L., Kasal, B., Huang, L., 2016. A review of recent research on the use of cellulosic fibres, their fibre fabric reinforced cementitious, geo-polymer and polymer composites in civil engineering. Compos. B Eng. 92, 94e132.

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2.1 Commonly used sustainable materials (plant-based natural fibres reinforcements in composites) Due to the environmental concerns, legislation for low emission materials, and consumer awareness towards the sustainable development aspirations, lightweight composite materials reinforced with plant fibres have been considered as an alternative to conventional fibre reinforcing materials (Bledzki and Gassan, 1999). The classification of natural fibres is illustrated in Fig. 2.1. For making composite materials sustainable and environmentally friendly, both matrices and reinforcements are expected to be originated from renewable resources. Natural fibres available for composite reinforcement can be classified into three main types (Gurunathan et al., 2015): 1. Lignocellulosic plant-based (hemp, flax, jute sisal, palm fibre, kenaf, date palm, etc.) 2. Animal-based (silk, wool and hair) 3. Mineral-derived (asbestos, wollastonite).

Lignocellulosic plant-based natural fibres can be further classified into: Bast fibres include flax, hemp, jute, kenaf and ramie. Leaf fibres comprising sisal, abaca, pineapple and henequen. Seed fibres, which are obtained from seeds, and a good example of such fibre is cotton and kapok. Just like plant-based fibres, natural fibres are further grouped as to their source. Stalk fibre include fibres from plant stalks such as rice, wheat, barley, typically extracted from plants. Grass and other fibre residue includes bamboo, bagasse, corn, widely available from tall grasses. Animal fibres include fibres that are obtained from hairy mammals such as sheep, which is the source of wool. Silk fibres, on the other hand, are obtained from insects such as spiders, which produce silk when making their cobwebs (Faruk et al., 2012). These reinforcements have outstanding properties such as abundant, biodegradable and recoverable after their end-of-life and in many cases, better specific properties (strength and stiffness) compared to their synthetic counterparts (Satyanarayana et al., 2009). Due to these several benefits, many European countries have already using biobased composites in automotive applications. Nonetheless, despite these, most of the polymers used to use in these composites are non-biodegradable. Especially when higher mechanical properties are required, matrices such as unsaturated polyester, epoxy, vinyl ester, polypropylene are used as they provide higher mechanical properties in comparison to biodegradable polymers, but these polymers are not fully biobased (Nayak et al., 2000).

Sustainable Composites for Lightweight Applications. https://doi.org/10.1016/B978-0-12-818316-8.00004-9 Copyright © 2021 Elsevier Ltd. All rights reserved.

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Sustainable Composites for Lightweight Applications

Natural fibers

Natural

Animal

Synthetic

Mineral Asbestos

Silk

Inorg. fiber

Org. fiber

Glass

Aramid/kavlar

Wool

Aromatic polyester

Hair

Polyethylene

Carbon Boron Silicacarbide

Cellulose/lignocellulose

Leaf

Seed

Fruit

Wood

Stalk

Flax

Sisal

Kapok

Coir

Bamboo

Hemp

Cotton

Oil palm

Soft wood

Rice

Banana

Wheat

Bagasse

Jute

Abaca

Loof ah

Barley

Corn

Ramire

PALF

Sabai

Henequen

Milk weed

Maize

Mesta

Oat

Rape

Rye

Esparto

Bast

Kenaf

Agave

Roselle

Raphia

Hard wood

Grass/reeds

Canary

Figure 2.1 Classification of natural fibres (Gurunathan et al., 2015).

The European Union’s directive requiring that all new vehicles needing to use 95% recyclable materials to achieve the end-of-life of vehicle by 2015 has led further motivation for the development of commercially viable lightweight composites. Under this directive, about 85% of these materials must be recoverable through re-use or mechanical recycling and about 10% through energy recovery or thermal recycling. Similar
GHGs regulation in North America, and directives are in place, for example, CAFE, in Europe, regulation 2020. For example, in Euro 6 regulation from 2020, a higher tax will be applied to vehicles exceeding 95 g/km of CO2 emission. In order to meet this target, original equipment manufacturers (OEMs) are undertaking various measures towards the utilisation of lightweight concepts as the weight reduction has direct benefits towards the reduction of CO2 emission (Pervaiz et al., 2016;Directive 2000/53/EC, n.d.). Despite these many attractive attributes, natural plant fibres are still not fully utilised to their full potential. The natural plant fibre reinforced composites have been used mainly for non-structural applications in automotive, construction and packaging industries due to their inherent moisture absorption behaviour resulting from their chemical compositions (cellulose, hemicelluloses, lignin and pectin) and natural variability (Dhakal et al., 2007). With this phenomenon, even with higher fibre contents, obtaining high or full potential strength and stiffness is difficult to achieve for natural

Sustainable natural fibre reinforcements and their morphological structures

19

fibre reinforced composites due to the fact that the moisture absorption behaviour of these fibres causes weak fibre matrix adhesion leading to ineffective stress transfer from matrix to fibres during various loading scenarios. To minimise this problem, many researchers have already undertaken various fibre surface modification processes, together with improved manufacturing processes (Nayak et al., 2000). It is worth noting that in addition to lack of compatibility between hydrophobic polymers and hydrophilic natural fibres leading to weak mechanical properties, fibre availability is a major factor in selecting natural fibres as reinforcement, often constrained geographically due to local growing conditions, as well as lack of established supply chain mechanisms. The following sub-sections will elaborate on the most commonly used and some emerging plant-based natural fibres and their main characteristics.

2.1.1 Hemp fibres Hemp (Cannabis Sativa L.) is one of the oldest, strongest and stiffest available natural fibre. The chemical constituents of mature hemp fibre are cellulose (74%), hemicellulose (18%), lignin (4%) and pectin (1%). Hemp is an annual plant, tall (2e5 m), robust, annual herbaceous plant, which is sown in spring and harvested in autumn. Hemp has been used as a basic raw material for the production of rope, traditional medicine, canvas and clothing for many years (Carus et al., 2013). Hemp is considered to be a multipurpose crop since it offers a source of reinforcing fibre in composites, oil and molecules (Andre et al., 2016). In many countries, the production of hemp fibre is limited due to the ban as this fibre contains narcotics. The flowering tops, as well as to some extent, the leaves of hemp produce resin secretions containing the narcotic 9-tetrahydrocannabinol (THC) for which marijuana and hashish are famous. However, the industrial hemp produces less than 0.3% THC, which cannot be used as a narcotic (Weiblen et al., 2015). For the last few decades, the industrial hemp has been one of the major sustainable reinforcements in sustainable lightweight composite materials. Just before 2000, hemp plants were common agriculture crops grown in the moderate climates for the production of ropes and shipping sails. Hemp and the flax are only commercial sources of long natural fibres grown in the UK. Originally native to Central Asia, it has since spread to every inhabited continent, region and country and widely cultivated in Europe (Richardson et al., 1998; Hepworth et al., 2000). For the last decade, hemp fibre has been very popular due to its versatile features: rage of food, strong fibre, and attractive agriculture features resistance to drought and pests, prevention of soil erosion, lower fertiliser, herbicides and pesticides and water requirement to grow in comparison to other plant fibres (Andre et al., 2016). The production and use of fertilisers and herbicides consume a significant amount of energy, and is hence, a very big burden to the environment. From the economic and environmental point of view, this natural fibre offers a significant opportunity to be used as a sustainable resource. Due to their high stiffness and strength, a primary requirement for the reinforcement of composite materials, these fibres are increasingly being used in composites reinforcements. The specific strength and stiffness that are comparable to those of glass fibres make these fibres attractive to the automotive and construction industry for the production of non-structural components.

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Sustainable Composites for Lightweight Applications

From both environmental and performance point of view, hemp offers excellent attractions as reinforcing materials in composites, especially for automotive applications (Karus and Kaup, 2002). The major disadvantage, however, is its low impact strength when compared to glass fibre reinforced plastic composites (Dhakal et al., 2012). Fig. 2.2 depicts a hemp plant, non-woven hemp mat and scanning electron microscopy (SEM) macrograph of hemp fibre.

2.1.2 Flax fibres Flax (Linum usitatissimum L.) is an annual plant, which grows from 0.5 to 1.5 m tall. Flax fibre is obtained from the stem of the flax plant. As most bast fibres, flax fibres are constituted of an outer wall of pectin, cellulose, hemicellulose and lignin. The cellulose content in flax fibres is approximately 71%, hemicellulose 18.6%e20.6%, lignin 2.2% and 1.2% wax. Flax is bast fibre extensively used in the textile industry, which possesses a smooth and straight texture (Dhakal et al., 2013). The mechanical properties such as strength and stiffness of flax fibres are comparatively higher than other natural plant fibres due to their high molecular weight and crystallinity of cellulose. The properties and the quality of flax fibres depend on the growing and harvesting conditions (retting and decorticating for specific uses), soil quality, use of fertiliser and the part of the stem where the fibres are extracted (position within the stem). Physical characteristics such as diameter, microfibrillar angle (as this fibril angle decreases, the mechanical properties increase), chemical compositions, as well as how the fibres were processed as it is well accepted that during the process, the fibres are damaged at varying degrees (Faruk et al., 2012; Zini and Scandola, 2011). Flax technical fibres (consisting of many elementary fibres bonded together) are seen as competitors to glass fibres for many applications because of their following key principal advantages: (1) (2) (3) (4)

Cheaper than glass fibres, Non-abrasive, biodegradable and less toxic than glass fibres, Higher specific strength and modulus than glass fibres, Less energy required to produce compared to glass fibres.

Due to their high strength and stiffness together with outstanding damping properties, this fibre has been used in many applications, including the automotive, marine and textile industries (Bos et al., 2002).

Figure 2.2 Hemp fibre (a) hemp plant, (b) non-woven hemp mat and (c) SEM of hemp fibre.

Sustainable natural fibre reinforcements and their morphological structures

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Flax fibres, as most of the plant fibres, are heterogeneous in their physical structure and chemical compositions, which poses a challenge when using these fibres for reinforcing composites for structural uses. Fig. 2.3 illustrates flax plant, non-woven flax mat and SEM images of flax fibre.

2.1.3 Jute fibres Jute (Genus Corchorus oliotorius), one of the most common bast fibres, acquired from the bark of jute plant is produced worldwide, which grows from 2 to 3.5 m tall, and fibres are extracted after harvesting. Bangladesh is one of the largest jute-producing nations, where jute is also known as the golden fibre of Bangladesh. Other countries like India, China, Myanmar and Nepal also produce high quantities of jute fibres. The main chemical constituents of jute fibre include cellulose (58%e65%), hemicellulose (20%e24%) and lignin (12%e15%). The high lignin contents make the fibre just brittle with lower extension to break in comparison to other bast fibres such as hemp. The low cost and non-abrasive nature of the jute fibres allow one of the most commonly used reinforcing alternatives, thereby resulting in significant cost savings in composite manufacture. The tabular (cellular) structure of the natural fibre (jute) provides good insulation against heat and noise. Jute, like many other natural plant fibres exhibit good mechanical properties (Faruk et al., 2014). As a result, they are attractive reinforcements in the manufacture of composites in the construction and automotive industries (Dhakal et al., 2014; Bourmaud et al., 2018; Sinha and Panigrahi, 2009). As expected from other natural plant fibres, jute fibre reinforced composites can provide low production energy and cost, good specific mechanical properties compared to conventional glass fibre reinforced composites. Fig. 2.4 illustrates the Jute plant and SEM image of jute fibre.

2.1.4 Kenaf fibres Kenaf belongs to the Hibiscus cannabinus L. family and is a herbaceous annual plant that can be grown under a wide range of weather conditions. This plant is easy to grow without much attention. Kenaf is a bast fibre and environmentally friendly biodegradable crop (Ramesh et al., 2018; Nishino et al., 2003). It is reported that this fibre grows

Figure 2.3 Flax fibre (a) mature flax plant, (b) non-woven flax mat and (c) SEM of flax fibre showing non-uniform diameter.

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Sustainable Composites for Lightweight Applications

Figure 2.4 Jute fibre, depicting (a) unidirectional jute fibre (b) jute yarn (c) SEM of untreated jute fibre (Aziz and Ansell, 2004).

to more than 3 m within 3 months, even in moderate ambient conditions. When natural fibres are used in structural/non-structural applications, the kenaf fibres reinforced composites have been very popular ones, especially in the automotive industry. However, due to their hydrophilic nature, they require prior treatment (Ramesh et al., 2018). These fibres also have been used for many decades as a rope, sacking and canvas. Fig. 2.5 depicts the kenaf elementary fibre and SEM images of treated and untreated kenaf fibre.

2.1.4.1 Advantages of kenaf fibres Kenaf fibre absorbs nitrogen and phosphorus included in the soil, and it accumulates carbon dioxide at a significantly high rate (Nishino et al., 2003). Like other natural fibres, kenaf fibre has many usual advantages that commonly used natural fibres have, which are: • • • •

They form a cellulosic source with ecological and economic benefits Low cost, low density, high specific properties (strength and stiffness) Kenaf fibres are fully biodegradable and are used in making sustainable biobased composites Extensively used as woven and non-woven mats in automotive industries

2.1.5 Date palm fibres Date palm fibres are derived from the date palm tree (Fig. 2.6). The date palm tree (Phoenix dactylifera L.) is a member of the palm tree family (Arecaceae), from this family derive more than 200 types of palm trees. Currently, it is estimated that there are more than 100 million date palm trees worldwide. This is one of the most important crops in North Africa and the Middle East. They are predominantly found in countries such as Saudi Arabia, Egypt, Iraq and Iran (Alawar et al., 2009). These fibres contribute significantly to the economy of these regions. The date palm fibres are emerging natural plant fibres. Although this fibre is cultivated worldwide for many years, these fibres are getting significant priorities in composite reinforcements in recent years due to their better water repellent properties and good thermal stability compared to other natural plant fibres. The diameter ranges from

Sustainable natural fibre reinforcements and their morphological structures

23

(a)

Nano scale microfibril Ø 5–10 nm Micro scale elementry fibre Ø 10–30 Pm

Cellulose

Micro scale fibre bundle Ø 50–100 Pm Kenaf pith Kenaf core Kenaf bast

Kenaf stem Ø 20–40 mm Kenaf tree

(b)

(c)

Figure 2.5 Kenaf fibre (a) microscale elementary fibre bundles (b) SEM image of un-treated kenaf fibre (c) SEM of treated kenaf fibre (Khalil et al., 2012; Yousif et al., 2012).

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Sustainable Composites for Lightweight Applications

Figure 2.6 Date palm fibre (a) date palm fibre location of the sheath, (b) sheath (c) processed raw date palm fibre (d) SEM micrographs of date palm single fibre.

100 to 1000 mm and a measured density of 0.917
0.127 g/cm3. The surface of date palm fibres is rough compared to other natural plant fibres. As can be noticed (Fig. 2.6), the outer surface shows some impurities and residues. The main features of the date palm fibres are: • Lower density (0.92 g/cm3) compared to other natural fibres. • Low moisture absorption (5%) compared to other common natural fibres: jute (12%), flax (10%) and sisal (11%).

Date palm fibres are extensively used for manufacturing rugs, huts and shades. Due to its high cellulose (46%) contents, these fibres possess good mechanical properties and low water absorption. Consequently, these fibres are weaker compared to flax fibres. After the harvesting of the date palm, large quantities of residues in the form of leaves and fronds are wasted each year. The by-products made from these wastes are mainly low-value products. Often they are burned in the agricultural land causing environmental and health hazards. The part of the date palm tree, which is often used as fibres is the leaf sheath. The sheath is the part of the tree, which surrounds the trunk of the plant attached to its lateral edges near the top of the trunk Fig. 2.6. The sheath is also known under the name of leaf and is often torn lose when pruning the leaves. Due to lower moisture absorption, these fibres have been successfully used as reinforcements in polymeric composites in recent years (Jawaid and Abdul Khalil, 2011).

Sustainable natural fibre reinforcements and their morphological structures

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2.1.6 Sisal fibres As common attributes of other commonly used natural fibres, sisal fibres (Agave sisalana), extracted from the leaves of the sisal plant, have the promising reinforcing potential for use in composite materials due to its low cost, low density, high specific strength and modulus, no health risk, easy availability in some countries and renewability. Sisal fibre has a high percentage of the lumen, which plays an important role in the overall properties of resultant composites. The main chemical constituents of sisal fibre include cellulose (70%), hemicellulose (12%), lignin (9%), Pectin (10%) and waxes (2%). The microstructure of sisal fibre is illustrated in Fig. 2.7 (Cheung et al., 2009; Nishino et al., 2003). Due to the large hollow lumen and porous microstructure, sisal fibres can be used for acoustic and thermal insulation applications. However, the large hollow section on sisal fibres can weaken the mechanical properties as the new cross-section area decreases, and the overall stress concentration increases.

Figure 2.7 (a) Sisal plant (b) sisal fibre (c) SEM image of sisal fibre showing hollow structure (Cheung et al., 2009; Nishino et al., 2003).

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Sustainable Composites for Lightweight Applications

2.1.7 Oil palm fibres The oil palm is one of the fibre materials belonging to the species Elaeis guineensis under the family Palmacea, and originated in the tropical forests of West Africa. Oil palm empty fruit bunch (OPEFB) and oil palm mesocarp fibres are two main fibrous materials obtained from palm oil mill (Jawaid et al., 2013). Empty fruit bunch fibre is one of the largest agricultural wastes biomass generated in countries like Nigeria, Malaysia and India. These fibres are the by-product of factories that process oil palm. The chemical constituents of OPEFB fibre are cellulose (65%), hemicellulose (24.2%), lignin (19%) and other extractives (8.9%) (Suksong et al., 2016; Sreekala and Thomas, 2003). These fibres have a lower density (0.7e1.55 g/cm3) than other natural fibres, which makes them attractive in lightweight composites as reinforcements. A large amount of lignocellulose materials such as oil palm fronds, trunks and empty fruit bunches generation leads to an enormous amount of empty fruit bunch (EFB) fibres. These fibres if not used properly, pose a serious environmental threat. The empty fruit bunches are mainly incinerated to produce bunch ash to be distributed back to the field as fertiliser. Approximately 15 million tons of this agriculture waste is generated by oil palm milling operation annually and part of it is burned in incinerators. The conventional method of burning these residues often creates environmental problems, in that it generates severe air pollution and is prohibited by the environment protection act. In abiding by these regulations, these residues are becoming expensive to dispose off (Izani et al., 2013; Rahman et al., 2007). Fig. 2.8 depicts oil palm empty fruit bunch and non-woven oil palm fibre mat for composite reinforcement. However, converting this waste into a useful product could save the environment from hazardous pollution. Currently, an extensive research provides an alternative way of optimising the usage of fibre obtained from oil palm into value added products. OPEFB is a form of fibrous lignocellulosic residue has significant potential as reinforcing materials in composites, Reinforcements in composite materials could be a way forward to utilise these waste agriculture biomasses. The EFB fibres possess some attractive properties such as good tensile strength and stiffness, good water repellence

Figure 2.8 Oil palm fibre from empty fruit bunch (a) fibre showing non-uniform diameter, (b) selected finer fibres and (c) non-woven oil palm fibre mat.

Sustainable natural fibre reinforcements and their morphological structures

27

behaviour after the chemical treatment of fibres. The work carried out by (Sreekala and Thomas, 2003) reported that the tensile strength and Young’s modulus of salinetreated OPEFB fibres were recorded at 273 MPa and 5.25 GPa, respectively. With these attractive mechanical properties, these fibrous wastes have been significantly utilised as reinforcements to produce biobased composites for various engineering applications in recent years (Sreekala and Thomas, 2003).

2.1.8 Banana fibres The banana fibres are extracted from the stem of the banana plant, which is the after product or often termed as a waste product of banana cultivation. These fibres can be normally obtained without incurring any additional costs of production for industrial uses (Joseph et al., 2002). The chemical constituents of banana fibre are cellulose (63%e64%), hemicellulose (19%), lignin (5%) and the moisture absorption is approximately 10%e11% (Seena et al., 2002). These fibres have already been used as reinforcements in composites and have shown comparable mechanical properties against other natural fibres. Fig. 2.9 shows raw banana fibres, SEM images of banana fibre reinforced composites and a cylindrical component made from banana-based epoxy composites.

2.2 Influence of processing and chemical composition on the properties Natural plant fibres such as hemp, flax, jute and kenaf need to be separated from their barks. The fibres from these plants are typically extracted by retting, followed by mechanical processing such as scotching and hackling (Sultana, 1992). The processes used to manufacture conventional reinforcements such as carbon and glass are inherently different from those used in the production of natural fibre as composite reinforcements. As far as mechanical properties of commonly used natural fibres such as hemp, flax, kenaf and jute, for example, are concerned, their properties are relatively high but transforming these high-end mechanical properties to the resulting composites

Figure 2.9 (a) Raw banana fibres (b) SEM images of banana fibre reinforced epoxy composites fibres (c) banana fibre reinforced epoxy composite cylinder (Mohan and Kanny, 2019).

28

Sustainable Composites for Lightweight Applications

have been the challenge as fibre processing steps influences or affects the final properties of fibres. The fibre processing techniques introduce defects onto fibres at both micro and meso-levels. These defects created on the fibre surface, eventually degrade the mechanical properties of natural fibre reinforced composites (Gager et al., 2019). In the process of separation, different processes such as retting and scotching are utilised. These processes have significant effects on the quality of fibres, as well as in the chemical composition of these fibres (Merotte et al., 2016). Defects on the fibre surfaces as depicted in Fig. 2.10 can easily be introduced during these processes. These defects can significantly affect the overall mechanical and thermal properties of these fibres. The overall properties, therefore, depend on natural variability, as well as damage sustained during their processing.

2.2.1 Importance of fibre processing parameters The plants are taken out directly from the ground to harvest fibres such as hemp, flax and jute; this is done so as to retain the longest fibre length. The flower heads of the flax plant, for example, are then removed by rippling. After this, the plants are spread on the ground for retting. During this stage, the pectin layer, which binds the fibres to the bast tissue and stem, is removed. Retting is carried out by normally laying the fibres on the ground for around 3e7 weeks (dew retting). The retting process can weaken the middle lamella by the action of micro-organisms. The retting process is followed by breaking, scorching and hackling. In the breaking process, the fibre stems are passed between the rollers whereas the scorching separates the fibre bundles from the xylem. Hackling thins the fibre bundles by passing them through a series of combs (Zhang et al., 2005; Mohanty et al., 2001). The coarse fibre bundles are then combed into the hackling process. During this process, the ribbon-shaped fibre is refined towards a circular fibre structure, although recently, a few more fibre isolation methods have emerged. The dew retting process described is still the traditional and most common method used (Rowell et al., 2000).

Figure 2.10 SEM images of the surface of flax fibre showing complex fibre layers and defects (a) untreated (b) treated.

Sustainable natural fibre reinforcements and their morphological structures

29

Retting is controlled degradation of plant stems to allow the fibre to be separated from the woody core. The traditional method to separate the fibres from the plant is to cut and leave the stems on the field, where they are soaked during the night by the dew allowing natural bacterial degradation to take place (Kessler et al., 1998). Under these conditions, microorganisms grow and produce enzymes, which degrade the pectic substances, and the cortex fibres are progressively disassociated into fibre bundles and sub-bundles. This method is known as dew retting, is currently in practice, but the quality of fibres may vary due to variations in the climatic conditions. Alternatively, the plant stems are retted in water tanks to make the retting process more controlled. Other approaches include drying of stems artificially subjected to multiple passes in a system of rollers (Hobson et al., 2001). This can be an expensive process and can cause too much fibre damage (Zafeiropoulos et al., 2007; Zafeiropoulos and Baillie, 2007). The processing affects the final fibre aspect ratio, and thus, the mechanical properties of the product. Processing techniques such as internal mixing and injection moulding can cause high-fibre attrition. Fibres can be broken into smaller and shorter pieces due to various mechanisms: • Fibre-fibre interaction can also cause fibres to break; • Fibre-matrix interaction owing to the shear stresses acting in the viscous polymer; • Fibre wall interaction

Natural fibres have weak links as natural and artificial flaws, as well as kink bands. These weak links are the most probable rupture points along the fibre length when the fibre is mechanically stressed. When the natural plant fibres are obtained by using the above-mentioned process, the fibres go through some kind of disruption and damage. This phenomenon is described as micro-compression and kind bands. In fact, natural plant fibres experience extreme weather conditions such as wind, rain and these conditions exert some sort of forces. These conditions create some sort of damage. Moreover, the damages are worsened by various fibre-processing techniques used. Additionally, the manufacturing processes used to make the composites will further damage the fibres and their lengths, reducing the overall mechanical properties. The damage created during these processes, as well as inherent structural defects such as kink bands, lead to an overall reduction of properties (Oksman, 2001). A modern method such as steam-explosion and ultrasonic treatments to separate the main components of lignocellulosic biomass (cellulose, hemicelluloses, pectins and lignin), has been introduced as an alternative to the conventional process. However, there is an argument that these methods separate the material into its component fibre cells, and hence, destroying the structure of fibre bundles that the plant has provided. Also, these methods can be expensive. It seems that the only economical method currently available to separate fibres from stems is field retting despite the process having several drawbacks (Satyanarayana et al., 2007; Joffe et al., 2003). The fibre bundles extend continuously from bottom to top of the hemp plant; however, the single fibres are smaller units with lengths in the range of 5e55 mm (Vincent, 2000; Ranalli and Venturi, 2004). In taking composite materials as a whole, there are

30

Sustainable Composites for Lightweight Applications

many different material options to choose from in the areas of resins, fibres and cores, all with their own unique set of properties such as strength, stiffness, toughness, heat resistance, cost, production rate, etc. However, the end properties of a composite part produced from these different materials are not only a function of the individual properties of the resin matrix and fibre (and in sandwich structures, the core as well), but is also a function of the way in which the materials themselves are designed into the part and also the way in which they are processed. Natural plant fibres have high moisture absorption, and the low processing temperatures permissible. The processing temperature of the lignocellulose plant fibres is limited due to the potential fibre degradation at high temperatures. The polymer matrices that can be used are limited to low melting temperature plastics.

2.2.2 Chemical composition and their influences on the properties The major constituents (chemical compositions) of natural plant fibres include cellulose, hemicellulose, lignin, pectin, fat, waxes and water-soluble substances. Chemical composition plays an important role in the properties of natural fibres. The average chemical composition of commonly used natural fibres is illustrated in Table 2.1. When there is higher cellulose content, higher mechanical properties are achieved. Natural fibres themselves are considered as composites. As can be observed in Table 2.1 that coir fibres have the lowest cellulose contents and show the lowest tensile strength among all other natural fibres. Lower cellulose content has contributed to the lower strength and modulus. Natural fibres are mostly constituted of cellulose, a biopolymer of the plant sugar glucose. Other constituents present in natural fibres Table 2.1 Chemical composition and equilibrium moisture contents of selected natural plant fibres (Li et al., 2007; Rosa et al., 2010; Sukumaran et al., 2001; Sreekala and Thomas, 2003; Suksong et al., 2016). Fibre

Cellulose

Hemicellulose

Lignin

Wax (wt.%)

Moisture (wt.%)

Date palm fibre

46

e

20

e

e

Flax

71

18.6e20.6

2.2

1.5

7

Hemp

68

15

10

0.8

9

Jute

61e71

14e20

12e13

0.5

12

Bamboo

26e43

30

21e31

e

Kenaf

72

20.3

9

e

e

Cotton

82.7

5.7

e

e

e

Sisal

65

12

9.9

2

11

Oil palm fibre

65

24.2

19

e

e

8.9

Sustainable natural fibre reinforcements and their morphological structures

31

are hemicelluloses, pectin, lignin and waxes; flax fibres when considered for example, exhibit highest strength and modulus, which is attributed to the higher cellulose contents. Similarly, hemicellulose, pectin and lignin contents (percentages and interaction between them) play a significant role on the mechanical and thermal properties of natural plant fibres (Rouison et al., 2006). Chemical structures of cellulose, hemicellulose and lignin are depicted in Fig. 2.11. Similarly, due to their major chemical constituents, natural plant fibres such as flax, hemp, jute and bamboo absorb water. The moisture absorption is influenced by the humidity and duration they are exposed to the hygrothermal environments (temperature and humidity). The water absorption causes thickness swelling of fibres and causes a significant strength reduction compared to the dry specimens. Moreover, in order to attain higher mechanical strength of composites, reinforcing with higher fibre volume fraction is one of the ways that is generally employed. However, in the case of natural fibre reinforced composites, normally, higher the fibre volume fraction, greater the moisture absorption at wet environments, which induces significant weakness at the interface and reduces the mechanical properties significantly. Additionally, the chemical composition of natural plant fibres limits the temperatures at which they can be processed. Many authors have reported that this limits the use of these fibres as reinforcement in thermoplastic composites as these fibres start degrading at a rapid rate at approximately above 200e240 C with the degradation of hemicellulose (Bledzki et al., 2002).

(a)

(c)

O HO

OCH3

OH

OH

HO O

O

O

O H2C C OH

O OH

OH

CH2 n OCH3 O

(b)

H H

O H

H

H

O

O O

O H HO

H

OH H

CH3 C O

H

OH

H

H

O H

Figure 2.11 Chemical structure of (a) cellulose (b) hemicelluloses and (c) lignin.

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Sustainable Composites for Lightweight Applications

2.2.3 Cellulose structure Cellulose are made up of glucose polymeric structures which are repeated on the lattice. Usually, a single cellulose molecule may comprise of a few thousand glucose repetitions. The way glucose structures are connected together has a profound effect on the properties of the cellulose. Therefore, a wide range of properties can be expected from different cellulose-based fibres. One of the biggest limitations of cellulose-based fibres is that they carry a large density of voids due to the particular glucose structure. A large number of voids become water or moisture storing sites when the composite is immersed in liquid. Therefore, the moisture absorption is a great concern for cellulose-based composites. Various types of surface treatments may help in improving this feature.

2.2.3.1 Cellulose Cellulose is the basic structural component of all plant fibres. The molecular structure of cellulose is shown in Fig. 2.12. It is the most important organic compound produced by plants and the most abundant in the biosphere. The primary or outer wall of a cell consists of a thin network of cellulose microfibrils, irregularly and loosely arranged and incrusted with hemicellulose, lignin and other compounds. Cellulose is the most abundant natural polymer in the world and the most essential component of all plant fibres and found to be at a higher percentage than other constituents in natural plant fibres. It is an isotactic B-1, 4-polyacetal of cellulose. The basic unit, cellulose, is composed of two molecules of glucose. As a result, cellulose is often called a polyacetal of glucose (Eichhorn and Young, 2004). Cellulose is a semi-crystalline polysaccharide with a large amount of hydroxyl group present making natural fibres hydrophilic, and as a result, poor adhesion between fibre and matrices. This is one of the concerns of natural fibres when reinforced with hydrophobic polymers, making less compatible between them.

2.2.3.2 Hemicellulose Hemicelluloses are also found in all plant fibres. Hemicelluloses are polysaccharides bonded together in relatively short, branching chains. The hemicellulose fraction of the plant consists of a collection of polysaccharide polymers. Hemicelluloses usually consist of more than one type of sugar unit (Bledzki et al., 2002). Hemicellulosic polymers are branched, fully amorphous and have a lower molecular weight than cellulose. For example, in flax fibres, their open structure and availability of polar groups make hemicellulose and pectin susceptible to chemical degradation.

Figure 2.12 Molecular structure of cellulose.

Sustainable natural fibre reinforcements and their morphological structures

33

2.2.3.3 Lignin Lignin is a Latin word for wood. Lignin is the compound, which gives rigidity to the plant. Without lignin, plants could not attain great heights (as in trees) or rigidity found in some annual crops (straw). Lignin is a three-dimensional, highly complex polymer with an amorphous structure and high molecular weight. Of the three main constituents in fibres, it is expected that lignin would be the one with the least affinity for water (least water sorption). Another important feature of lignin is that it is thermoplastic, that is at temperatures around 90 C, it starts to soften, and at temperatures around 170 C, it starts to flow (Bledzki et al., 2002; Eichhorn and Young, 2004).

2.3 Mechanical, physical and morphological characteristics of plant fibres Due to their nature, plant fibres vary in their mechanical properties. This is because fibre morphologies vary due to different factors: plant fibres vary in their anatomy, morphology. The following sub-sections further discusses the morphological structures of different plant fibres. Moreover, the diameter also plays an important role; if the diameter decreases, the mechanical properties increase (Dittenber and Ganga Rao, 2012).

2.3.1 Morphological structure of natural fibres All plant species are built up of cells. When a cell is very long in relation to its width, it is called a fibre. For example, wood fibres are mostly 50 to100 times as long as they are wide. The length and width of some common natural fibres are illustrated in Table 2.2. Knowledge about fibre length and width is important for comparing different kinds of natural fibres. A high aspect ratio (length/width) in crucial in cellulose-based fibre composites as it gives an indication of possible strength properties. Hemp single fibre has an aspect ratio of 1000, which is good for mechanical properties. The fibre is like a microscopic tube (i.e., wall surrounding a central void referred to as the lumen). Moreover, when the cell wall is made up mainly (85% or more) of cellulose, hemicellulose and lignin, we talk about lignocellulose fibres, and this includes woody species, scrubs and most agricultural crops. Typical lignocellulose fibres from agriculture are found, for example, in straws, flax, hemp, jute and sisal. Non-lignocellulose fibres are fibres that do not contain lignin and are found in potatoes, beets and cotton, among other crops (Arbelaiz et al., 2005). Many natural fibres have a hollow space, so-called lumen, as shown in Fig. 2.12. In irregular distances, there are nodes dividing the fibre into individual cells. The surface of the natural fibres is rough and uneven which can give good adhesion to the matrix in a composite structure. Natural plant fibres are lignocellulose in nature, and they mainly contain cellulose, hemicellulose and lignin at varying concentrations. The reliability and long-term durability of natural fibres are influenced by the structure (microfibrillar angle, the fibre

34

Sustainable Composites for Lightweight Applications

Table 2.2 Mechanical and physical properties of selected natural plant fibres (Cheung et al., 2009; Pickering et al., 2016). Tensile strength (MPa)

Young’s modulus (GPa)

Elongation (%)

Density (g/cm3)

Diameter (micro meter)

Length (mm)

Date palm fibre

58e230

0.3e7.5

5e50

0.9e1.2

100e1000

20e250

Flax

345e1035

27.6

2.7e3.2

1.5

10e25

10e65

Hemp

690

70

1.6

1.4

25e35

5e55

Jute

393e773

26.5

1.5e1.8

1.3

25e200

0.8e6

Bamboo

140e230

11e17

e

0.6e1.1

14

2.7

Kenaf

930

53

1.6

e

1.14e11

12e36

Cotton

287e800

5.5e12.6

7e8

1.25

10e34

2.7

Sisal

511e635

9.4e22

2e2.5

1.15

7e47

0.8e8

Oil palm (empty fruit)

130e248

3.58

9.7e14

0.7e1.55

191e250

0.8e0.9

S-glass

4570

86

2.8

2.5

e

e

Fibre

diameter, fibre surface characteristics) and chemical compositions (cellulose, hemicellulose and lignin) content. The strength and stiffness of plant fibres mainly depend on the percentage of cellulose content and microfibrillar angle. Similarly, the performance of natural fibres as reinforcements also largely depends on operating environments (temperature and humidity) and the presence of surface defects and the hydrophilic nature of fibres itself. Additionally, the performance also largely depends on the source of origin, length and diameter, and the retting process used. These are the key concerns for these reinforcements to be used fully in structural composites as reinforcements. The cell walls of natural fibres are predominantly made up of a number of layers including a primary wall (the first layer deposited during cell development) and the secondary wall (S), which comprises of three sub-layers (S1, S2 and S3) as depicted in Fig. 2.13. A hole located in the centre of the elementary fibre is called a lumen. Such a hierarchical organisation produces multi-interphase regions with different morphological characterisations. The interphase transition regions are usually small, which induces challenges to achieve an accurate evaluation of the nanoscopic interfacial properties. Therefore, it is of necessity to develop a reliable method to characterise the interfacial properties of plant fibres and their reinforcing composites at the nanoscopic level (Fig. 2.14).

(b) Secondary wall S3

Lumen

rTs1

nS1=4 – 6 Lamina

X lumen

Helically arranged crystalline microfibrils of cellulose

Secondary wall S2

Ts2

nS2=32 – 150 Lamina

Spiral angle

Z nS3=0 – 6 Lamina

Secondary wall S1

rTs3 Ts2

Primary wall Amorphous region mainly consisting of lignin and hemicellulose

Y Cell

Disorderly arranged crystalline cellulose microfibrils networks

rTs1

Primary wall

Figure 2.13 The structure of natural fibre (a) typical cell wall representation (Bourmaud et al., 2018; Bledzki and Gassan, 1999).

Sustainable natural fibre reinforcements and their morphological structures

(a)

35

36

Sustainable Composites for Lightweight Applications

(a)

(b)

(c)

(d) S1

Middle lamella

S2 S3

Elementary fibres

P Stem

Bundle

(e)

Secondary cell wall

Primary cell wall

Elementary fibre

S2 layer

Fibre bundle

Lumen Middle lamella Tricellular junctions

Stem transverse section Fibre bundle section

Wood or shives

Figure 2.14 Multi-scale structure of flax bast fibre (a) stem of flax plant (b) bundle of flax fibre (c) representation of elementary fibre (d) S2 layer of elementary fibres (Bourmaud et al., 2015; Goudenhooft et al., 2019).

Sisal leaf fibre and jute bast fibre, for example, have multi-wall structures. A schematic diagram of the microstructures of sisal leaf fibre and jute bast fibres are also illustrated in Fig. 2.15(a) and (b), respectively. As can be seen from the diagram that the microstructure is very complex, comprising of many elements. These parameters, such as average diameter of the primary wall, spiral angle, significantly influence the overall mechanical properties of sisal fibre (Li et al., 2017).

2.3.1.1 Primary and secondary cell walls Generally, there are two walls inside the natural fibres, and they are primary and secondary walls. The natural fibres are made up of cells, and the wall that encompasses the cell is called the primary cell wall. Then, the secondary cell wall is placed between the plasma membrane and the primary cell wall, as shown in Figs 2.14 and 2.15. The cells are protected by the secondary cell walls. Also, those walls are built of layered sheaths of cellulose microfibrils, and fibres positioned in parallel inside each layer. The lignin makes the secondary cellular wall much less leaky and less flexible to water than the primary cell wall (Eichorn et al., 2001). In most cases, the secondary cell wall involves cellulose with lignin, other polysaccharides, and glycoprotein. Also, it comprises layers (S1, S2 and S3), as shown in Fig. 2.14.

Secondary wall Pectin Primary wall

(b)

Lumen Elementary fibre

Stick

Fibre bundle

After retting

Microfibril

P

S2 Lumen S1 S3

Jute reed section (with meshy structure)

Technical fibre Lumen S3

Amorphous polymers (Amorphous region) MFA, θ

Secondary S2 wall

Cellulose microfibrils (Crystalline region)

Orientation angle 7–9°

Micro fibrils

Secondary wall Primary wall

Middle lamella

Cell wall

Sustainable natural fibre reinforcements and their morphological structures

(a)

S1

Primary P wall

Amorphous cellulose

Fibre Crystaline cellulose

Single elemetary fibre 37

Figure 2.15 Microstructures of (a) sisal fibre and (b) jute fibre (Li et al., 2017).

38

Sustainable Composites for Lightweight Applications

2.3.1.2 Lumen The inside structure of the tubular shape of natural fibre is called the lumen. The structure of lumen is a factor in determining the properties of natural plant fibres as shown in Figs 2.14 and 2.15. The structure and shape of the lumen play an important role in determining the properties of the natural fibre. In addition, an increased lumen space results in a lower density of the fibre. Besides, the shape of the lumen also dictates the load-bearing properties and stiffness of the fibre. A hollow tubular structure generally offers better specific strength and stiffness. Therefore, the lumen in the natural fibres provides a natural way of reducing the density of the fibre, and therefore, the total mass of the composite. Such a hierarchical organisation produces multi-interphase regions with different morphological characterisations. Without loss of generality, the interphase transition regions are usually small, which induces challenges to achieve an accurate evaluation of the nanoscopic interfacial properties. Therefore, it is of necessity to develop a reliable method to characterise the interfacial properties of plant fibres and their reinforcing composites at the nanoscopic level (Li et al., 2017; Eichorn et al., 2001).

2.3.2 Effects of variable morphological structure and mechanical properties From Table 2.2, it can be observed that the variation in fibre dimensions is significantly high in both length and diameter for many natural plant fibres. This variation can influence the properties as well as modelling parameters. When numerical results are desired, the input parameters are important, and when there is a large dimensional variation, the accuracy of the modelling prediction could be compromised. In such a situation, experimental work is required to complement the numerical results. Nonetheless, the tensile strength and modulus of plant-based fibres are reasonably high; given the other attributes such as low density, lower cost and higher specific strength and modulus, the aforementioned shortcomings can be compensated, and these fibres have significant potential to be used as sustainable reinforcing materials in composites. Fibre length is a critical factor in obtaining maximum strength potential. It is well accepted that as the fibre volume fraction is increases up to its threshold value, the mechanical properties also are increased, but the fibre length must be greater than the critical value. Although the great variation in dimensions is not desired, at the same time, this gives opportunities for selecting fibres of different dimensions for different purposes. Another important factor that significantly affects the properties of natural plant fibre is fibre defects, including kink bands and crack running along the fibre bundles. Kink bands are folds or bends in the fibre walls. These are areas of low strength, and when the fibre is loaded, kink starts to extend, leading to the failure of the fibre. Under the tensile strength, for example, the kink bands and crack running along the fibre act as stress concentration factors, and as a result, these points can be sites for the initiation of delamination and fibre matrix deboning. SEM of hemp fibre showing kink band and fibre split along its longitudinal axis are shown in Fig. 2.16.

Sustainable natural fibre reinforcements and their morphological structures

39

Figure 2.16 SEM micrographs of an example of (a) “kink bands,” and (b) fibre split/crack observed in hemp fibres.

The structural performance and reliability of biocomposites depend on many factors, including fibre architecture, aspect ratio, orientation of fibres (uni-directional or transverse), fibre volume fraction and processing technique used, among others. These factors eventually influence the fibre-matrix interface. It is obvious that the reinforcement must be stronger if it is to give good mechanical properties. The bond between the matrix and the reinforcement (fibres) is critical since good interfacial adhesion between the matrix and the fibres transfers the stress from the matrix to the fibres improving the mechanical properties of the composites (Bisanda and Ansell, 1991). Fig. 2.16(a) illustrates the variation in thickness along the span of a single fibre strand of hemp fibre. Materials heterogeneity and diameter variation is one of the issues of natural plant fibres. The thickness variation affects the mechanical strength of the composite material since the strength at the thinner section could be very much lower compared to the thicker section. When the diameter is larger, the loadbearing ability could be higher than the small diameter section. When fracture behaviour is required to investigate, fibre properties are important parameters. Therefore, it is an important aspect to investigate to understand the characteristics of the fibre. It is evident that there is a direct correlation between the mechanical properties of natural plant fibres and surface defects and kink bands. Kink bands are present on the surface of flax, hemp and jute fibres for example. The kink bands are not desired as these act as stress concentration points, whereupon, loading can initiate debonding, leading to matrix microcracking.

2.4 Effects of variable morphology on properties The quality and reliability of natural fibres are influenced by their structure (microfibrillar angle, fibre surface characteristics, the fibre diameter) and chemical composition (vis. cellulose, hemicellulose and lignin content). The properties of natural fibres depend on their lignocellulosic contents. Higher non-cellulose contents such as

40

Sustainable Composites for Lightweight Applications

hemicellulose, lignin, pectin and wax contents influence the properties and minimises the interfacial interaction between fibres and the matrix. The key for the property enhancement of the fibres lies in removing non-cellulosic components using various surface treatments without damaging the cellulose. Additionally, non-cellulosic contents such as hemicellulose and lignin contribute to hydrophilic nature and promote early thermal degradation during various processing. Moreover, the performance of natural fibres as reinforcements are significantly influenced by operating environments (temperature and humidity) and the presence of surface defects and the hydrophilic nature of fibres itself (Faruk et al., 2012). Notwithstanding, other parameters such as harvesting techniques, agronomic practices, genotype significantly influence the overall fibre quality (fineness, aspect ratios) of natural fibres (Placet et al., 2017; M€ussig and Amaducci, 2018). The structural shape and the morphology of reinforcing fibres influence the overall properties of resultant composites. Hemp and flax fibres, for example, exist in various morphological structures: technical fibres consisting of a number of elementary fibres bonded together by pectineus gums. Lignin, located in the middle lamella, provides the rigidity to the cell wall (Peji
c et al., 2020). For example, the hollow structure of fibre morphology can provide improved vibration and energy absorption properties. Morphologies of some fibres are unidirectional, polygonal and non-uniform. This significantly influences the overall mechanical properties. Moreover, the overall performance and various properties of composites depend on properties of constituents, their size, shape, structure and overall morphological characteristics. Amongst them, just the orientation of fibre and how the load is applied (either parallel to the fibre orientation or perpendicular to the fibre orientation) plays a significant role in the overall mechanical performance (Fig. 2.17). Tensile properties of flax and jute fibre reinforced PP composites presented by Tanguy et al. (2018) clearly shows how the direction of loading influences the properties. Their results exhibited a significant difference in tensile properties between longitudinal and transverse directions (Fig. 2.18).

0°

45°

90°

E-modulus [MPa]

Composite

5000 4000 3000 2000 1000

45

0

22.5°

0°

22.5°

45°

67.5°

90°

PP

67.5°

Figure 2.17 Influence of fibre orientation on the mechanical properties of PP/lyocell composites (Cordin et al., 2018).

Sustainable natural fibre reinforcements and their morphological structures

(a) 250

(b) PP-Flax 0° PP-Jute 0°

A B

100 50 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Strain (%)

Stress (MPa)

Stress (MPa)

200 150

41

10 9 8 7 6 5 4 3 2 1

PP-Flax 90° PP-Jute 90°

A

B

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Strain (%)

Figure 2.18 Influence of fibre orientations (longitudinal and transverse) on the tensile properties of PP/flax (a) and PP/jute composites (b) (Tanguy et al., 2018).

They reported that the unidirectional properties of reinforced composites, for example, is influenced by different parameters including mechanical properties of reinforcements itself, the property of matrix, the fibre aspect ratio, fibre volume fraction, as well as their morphological structures. It is well accepted that the overall mechanical properties of composites depend on the adhesion between fibre and the matrix (Armentia et al., 2019; Dhakal et al., 2007). This topic is covered and extensively discussed in the next chapter, Chapter 3.

2.5 Physical and mechanical investigation of single fibres and fibre bundles 2.5.1 Importance of single fibre and fibre bundle properties It is well established that natural plant fibres bear rough surface topography due to some chemical materials present in their surfaces such as lignin, pectin and waxy substances. It is a well-accepted fact that natural fibres have high variability both physically and chemically. The properties of composites start with the properties of fibres. If fibres have good strength and stiffness, then it will be able to transfer the load from the matrix to fibres. It is, therefore, important to have a good understanding of individual fibre (elementary fibre) and fibre bundle (technical fibre) properties so that the overall properties of composites can be predicted well. Fig. 2.16 illustrates the non-uniform diameter and irregular surface properties of flax fibre. This property variation can be a challenge when calculating or modelling the properties of heterogeneous natural fibres. The understanding of fibre-matrix adhesion, key mechanisms and their influence on the overall properties of composite materials play a significant role. In conventional composite reinforcements such as glass and carbon fibre, in order to characterise the interlaminar shear strength (ISS), the stress analysis testing is normally used. However, due to property and morphology variation, the techniques used for conventional fibres

42

Sustainable Composites for Lightweight Applications

may not be fully applicable to natural fibre-reinforced composites. Therefore, single fibre testing has been one of the most popular techniques employed to measure the ISS of natural fibres instead of fracture mechanics analysis. Commonly used testing methods for fibre-matrix adhesion are subsequently elucidated, according to Zhou et al. (2016). Single fibre fragmentation test (SFFT): In single fibre fragmentation testing, a single fibre is entirely covered in a polymer matrix, which is then loaded in tension mode. The fibre covered inside the resin breaks into smaller debris at locations where the fibre’s axial stress reaches its tensile strength. From this test, a critical fibre length (lc) is determined. The average interfacial shear stress can be calculated using the formula: s ¼ sf d=2lc

(2.1)

where, s is average interfacial shear strength, s is fibre strength, d is fibre diameter and lc is critical fibre length. Single fibre pull-out test (SFPT): In SFPT testing, the fibre is implanted in a block of the matrix, where the free end of the fibre is gripped, and a load is applied continuously. The load-displacement is measured as the fibre is being pulled out. When the load required to pull the fibre from the block is determined, the resultant interfacial shear strength is then calculated using the following formula. F ¼ spdl

(2.2)

where, F is maximum load, s is fibre-matrix shear strength, pd is fibre circumstance and l is embedded fibre length. The main difference between SFFT and SFPT, according to Zafeiropoulos et al. (2007), Zafeiropoulos and Baillie (2007) is that SFPT has no stochastic data reduction system. It has been highlighted that measuring critical fibre length and critical fibre strength pose challenges for natural fibres. Micro-bond test (MT): In this, a small amount of resin is first applied on the surface of the fibre in the form of a droplet creating a shape of an ellipsoid. Then after that, a shear force is applied by restraining the bead by the opposing knife edges. The applied load and blade displacement is recorded. Then the average shear stress is calculated by using the formula: s¼

F pdl

(2.3)

where, F is maximum load, s is fibre-matrix shear strength, pd is fibre circumstance and l is embedded fibre length. Table 2.3 illustrates the mechanical and physical properties of important natural plant fibres both single and fibre bundles (Pickering et al., 2016; Bisanda and Ansell, 1991; David and Hota, 2012; de Farias et al., 2009; Faruk et al., 2012; Paiva et al., 2007).

Sustainable natural fibre reinforcements and their morphological structures

43

Table 2.3 Mechanical and physical properties of plant-based natural fibres (single fibres and their bundles).

Property Density (g/ cm3)

Flax Single/Bundle

Hemp Single/ Bundle

Jute Single/ Bundle

1.45

1.48

1.46

Bamboo Single/ Bundle 1.4

E-glass Single/Bundle 2.55

Tensile strength (MPa)

1500

800

900

550

800

400

950

750

2400

2000

Tensile modulus (GPa)

75

55

65

40

30

10

50

30

74

70

Specific strength (MPa/g/ cm3)

1030

550

600

370

550

275

680

535

940

780

Specific modulus (GPa/g/ cm3)

52

38

44

27

21

7

36

21

29

27

Strain (%)

2

1.5

1.6

1.8

1.9

3

It can be observed that the specific mechanical properties of natural plant fibres are similar or even higher than that of glass fibres. Moreover, it is reported by many literatures that the energy consumption for the manufacture of natural fibre non-woven mats, including cultivation, harvesting and fibre separation, only amounts to a third, respectively, a fifth of the energy necessary for the manufacture of glass-fibre mats. This reality provides a tremendous opportunity to use sustainable lightweight natural plant fibres in composite reinforcements. It is well appreciated that the strength and stiffness of the unidirectional composites, for example, is directly related to the strength of its single fibre. The overall properties of resultant composites, therefore, depend on the strength and stiffness of single fibres. It is evident from Table 2.3 that the single fibre provides higher mechanical properties than the fibre bundles for all the natural plant fibres. Similarly, the specific strength and modulus of flax fibre is comparable to the properties of glass fibre, even slightly higher. This is one of the key properties of natural plant fibres that needs to be exploited. It is, at the same time, important to consider the differences in properties between single and fibre bundles. As explained in the previous section, defects are created in natural plant fibres due to the growing conditions, harvesting, retting and the extraction of the fibres.

44

Sustainable Composites for Lightweight Applications

It is well established that the interface between fibres and matrix is an important phenomenon when considering the load transferability of matrices to fibres during mechanical loading. It is a well-accepted phenomenon that the final mechanical properties of composites depend on the effective fibre/matrix adhesion or load transfer capability of the matrix to the fibres. Moreover, it is also believed that the weak viscoelastic interphase between single fibres in the reinforcing fibre bundle is responsible for the stiffness reduction of natural fibre reinforced composites. The measurement of strength and modulus of single fibre and bundles become complicated by the inherent variability and due to errors encountered during these testing. Single fibre may contain cell-wall defects linking to local misalignments of cellulose microfibrils originating during growth and during the processing. These defects are also known as kink-bands, nodes and slip planes. In order to have a reliable set of data, a large number of samples need to be tested. Flax fibres, for example, are present in the outer part of flax stems in the form of single fibres assembled into bundles (10e40 single fibres). The microfibrillar angle, crystalline and amorphous phases and size of the lumen parameters are significantly influenced by the plant health, growing conditions, and hence, influence the mechanical properties of single and fibre bundles. It is established that the fibre bundle has less mechanical properties than the single fibre. Nonetheless, the fibre bundles are the main constituents of composite materials as they are composed of single fibres that are held together by pectin-rich interphase, also known as middle lamella. Jute fibre has microfibrillar angle of 8 degrees and high cellulose content. One of the unique features of jute fibres in comparison to other natural fibres such as flax is that their length is significantly shorter, 0.8e6 mm in comparison to that of flax, 10e65 mm. Due to their short fibre length, the mechanical properties of jute fibres are obtained not from a single fibre but from the fibre bundles. It is worth noting that fibre length influences the mechanical properties of the resultant composites. The aspect ratio (length divided fibre diameter) provides surface areas, which are important parameters for high mechanical strength and stiffness. The tensile strength and modulus of fibre bundle are reported for flax fibre in the range of 300e600 MPa and 30e37 GPa, respectively, as it can be seen that there is a large property gap between single fibre and fibre bundles. The differences have been attributed to viscoelastic shearing within the weak-rich interphase between elementary fibres. The reported work by suggested a large variation on the tensile strength of flax single fibre ranging from 600 to 2000 MPa. It is obvious that the variation is contributed to the different parameters such as growing conditions, extraction methods used, etc. The values reported for modulus and strain to failure lies in the range of 60e80 GPa and 1.5%e2.0%, respectively (Fig. 2.19). Fig. 2.20 depicts the tensile stress-strain curves for flax and hemp single fibres. As can be seen, flax fibre displays higher stress compared to hemp fibre (Marrotte et al., 2018; Marrot et al., 2013).

Sustainable natural fibre reinforcements and their morphological structures

45

1200 Flax fibre Flax bundle Jute bundle

Stress (MPa)

1000

800

600

400

200

0 0.0

1.0

0.5

2.0

1.5

2.5

3.0

3.5

Strain (%)

Figure 2.19 Tensile stress versus strain curves for flax and jute fibres (single and bundles) (Baley and Bourmaud, 2014).

1000

Stress (MPa)

800

600

400

200

0 0.0

Flax Hemp 0.5

1.0

1.5

2.0

2.5

3.0

Strain (%)

Figure 2.20 Tensile stress versus strain curves for flax and jute fibres (single and bundles) (Marrotte et al., 2018).

The difference in stiffness between elementary fibres and technical fibres has recently been attributed to a viscoelastic shearing within the weak pectin-rich interphase between elementary fibres. Fig. 2.21 illustrates the SEM images of jute and flax bundle cross-section and longitudinal views.

46

Sustainable Composites for Lightweight Applications

Figure 2.21 SEM images of fibres: (a) jute bundle cross-section, (b) flax bundle cross-section, (c) jute longitudinal bundle view, and (d) flax longitudinal bundle view (Tanguy et al., 2018).

2.6 Summary The use of plant-based natural fibres as reinforcement in composite materials compared to their synthetic counterparts such as glass and carbon fibres provide multiple benefits, including but not limited to low density, non-abrasive processing, abundant, recyclability, biodegradability, excellent specific strength and stiffness. From the environmental sustainability point of view, replacing synthetic fibres with sustainable natural plant fibres would attract significant attention and acceptance in lightweight composite applications. Although the use of natural plant fibres as a reinforcement in composites provides multiple benefits, some inherent drawbacks of these fibres place some key challenges that can limit the full application of these materials. Understanding of their interfacial shear strength using single fibre testing approaches is important to use and appreciate in determining the critical length and interlaminar shear strength. Therefore, there has to be continued concerted efforts by the industry and research communities to minimise and overcome those challenges so that these outstanding environmentally friendly materials are utilised to their full potential in semi-structural and structural lightweight applications.

Sustainable natural fibre reinforcements and their morphological structures

47

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Zafeiropoulos, N.E., Dijon, G.G., Baillie, C., 2007. A study of the effect of surface treatments on the tensile strength of flax fibres. Part I. Application of Gaussian statistics. Compos. Appl. Sci. Manuf. 38 (2), 621e628. Zhang, J., Henriksson, H., Szabo, I.J., Henriksson, G., Johansson, G., 2005. The active component in the flax-retting system of the zygomycete rhizopus oryzae sb is a family 28 polygalacturonase. J. Ind. Microbiol. Biotechnol. 32, 431-156. Zhou, Y., Fan, M., Chen, L., 2016. Interface and bonding mechanisms of plant fibre composites: an overview. Compos. B Eng. 101, 31e45. Zini, E., Scandola, M., 2011. Green composites: an overview. Polym. Compos. 32 (12), 1905e1915.

Further reading Ishikawa, T., 2014. Overview of CFRP (carbon fiber reinforced plastics) application to future automobiles. J. Soc. Automot. Eng. Jpn. 68. M€ ussig, J. (Ed.), 2010. Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications. Wiley, Chichester, UK. Omar, F., Andrzej, K.B., Hans-Peter, F., Mohini, S., 2012. Biocomposites reinforced with natural fibers: 2000e2010. Prog. Polym. Sci. 37, 1552e1596. Reddy, N., Yang, Y., 2004. Structure of novel cellulosic fibres from cornhusks. Polymer preprints, American chemical society, division of polymer chemistry. In: American Chemical Society, Division of Environmental Chemistry; Papers Presented at the Philadelphia, PA Meeting, Division of Polymer Chemistry, vol. 45, p. 411 (2). Wambua, P., Ivens, J., Verpoest, I., 2003. Natural fibres: can they replace glass in fibre reinforced plastics? Compos. Sci. Technol. 63, 1259e1264.

Lightweight composites, important properties and applications 3.1 3.1.1

3

Lightweight composite materials: requirements and their key features Lightweight concept

Composites are defined as a macroscopic mixture of two or more distinct materials having a finite interface between them. The ability to tailor composites for a specific application is one of the biggest advantages of using these materials. Thus, the composite materials are used in a variety of fields ranging from, aerospace, sports, motorsport and the civil industry, among other applications. The lightweight concept, as far as composite materials are concerned, involves using improved design, which entails important performance requirements and puts a high priority on weight reduction. Equally, the lightweight concept emphasis on the end-of-life of a product at the design stage, the use of multifunctional and recyclable materials and efficient manufacturing processes. This involves, for example, improving the performance of composites by aligning fibres correctly, and reducing defects by using correct manufacturing techniques. Application of lightweighting principles is further linked to economics of parts and components. One of the examples of lightweight materials is carbon fibre-reinforced polymer composites (CFRP). Epoxy-based carbon fibre-reinforced composite contains a reinforcement structure of carbon fibres and a matrix phase of epoxy, which holds or binds all together. Carbon fibre composites have a high strength to weight ratio, high tensile modulus to weight ratio (depending on the anisotropy), high rigidity and a high fatigue strength. Also, due to it its lower density (1.5e2.0 g/cm3) compared to glass fibre-S2 (2.46 g/cm3), the specific strength and modulus of carbon fibre-reinforced composite are significantly higher (131 GPa cm3/g) than glass fibre-reinforced composites (35.3 GPa cm3/g). Carbon fibre is also corrosion resistant and has great heat resistant properties. Carbon fibre through the years is becoming more and more affordable, but due to the complexity of the manufacturing process, it can be hard to manufacture parts correctly and this results in high costs. One of the drawbacks of carbon fibre composites is an end-of-life option in which it is not recyclable and consumes a high amount of energy for production. In addition, despite being an excellent lightweight material, carbon fibre-reinforced thermosets composite possess low ductility. This means that when a load is applied, it breaks without giving any warning, manifesting a low toughness behaviour. The behaviour is not desirable in many critical applications (Faruk et al., 2014).

Sustainable Composites for Lightweight Applications. https://doi.org/10.1016/B978-0-12-818316-8.00006-2 Copyright © 2021 Elsevier Ltd. All rights reserved.

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Sustainable Composites for Lightweight Applications

Transport was the largest source of CO2 emissions (27% in 2017 and 2018), the majority arising from road transport, according to the report produced by the UK Automotive Sustainability Report (2019). With new environmental legislations, for example, EU regulation is aiming to reduce CO2 emissions from 132.2 g/km by 2020 in automotive components (from EU report on reducing CO2 emissions from passenger cars). The automotive industry is currently moving towards a low carbon economy, lower fuel consumption and lower running costs. The weight of the vehicles, for example, contributes towards fuel consumption, which eventually leads to greenhouse gases (GHGs). It is clear that the lightweighting approach can contribute towards the reduction of GHGs in the transport sector. It is a general assumption that a 10% weight reduction is known to improve fuel efficiency by 5%e7% (Taub et al., 2007; Mohanty et al., 2018). Automobile original equipment manufacturers (OEMs) and related parts manufacturers in the supply chain are seeking to achieve lightweight by developing lightweight materials. In the last decade or so, OEMs are producing newer models with significant weight reduction. Recently, there has been a growing interest in the use of natural fibres for composites design and manufacturing with the aim to reducing the overall weight of the vehicles.

3.1.2

Lightweight drives

The main drivers for lightweight composite materials to replace metal parts are the challenges face by our society with regard to unprecedented environmental degradation due to the use of fossil-based raw materials. In order to reduce this trend, a viable alternative is required, and lightweight biobased materials can contribute significantly towards this (Mohanty et al., 2018). The key drives for lightweight materials include: • • • • • • •


and ELVs) Government legislation towards low carbon emissions (CAFE Consumers behaviour towards greener materials Companies striving to becoming good steward of the resources Improved environmental performance Cost saving from per Kg weight reduction Enhancements in fuel efficiency and range Avoiding the issues of corrosion especially related to metallic materials

EU proposals for CO2 reduction from new cars and vans after 2021 were agreed in 2018. According to these proposals, a 15% CO2 reduction will be required by 2025. Further, an ambitious plan was brought forward, and according to which, the further reduction target is 37.5% for cars and 31% for vans. Also, end- of-life vehicles legislation (ELVs), for example, expects about 80% of components by weight need to be recovered after their end-of-life. In order to achieve the ELVs goal, one of the approaches, which has been successfully employed in the UK, is the remanufacturing approach. In this approach, a high value used product is returned to its original performance with warranty that the product is equivalent to its original or even better than a newly manufactured product. It is claimed that remanufacturing usually uses 85% less energy than manufacturing and reduces raw materials consumption significantly.

Lightweight composites, important properties and applications

3.1.3

55

Achieving lightweighting potentials

Lightweighting can be achieved by using an optimised design, by choosing correct materials and manufacturing techniques. The fundamental expectation of this approach is using less and lighter material to support the applied load in structures or components. This requires delivering functional requirements of the products with the use of less or lighter materials possible. For example, using concepts such as design for manufacture and assemble, the components are decided at the design stage on how they are going to be assembled/disassembled and reused at the end of their life. Thermoplastic-based composite materials have better end-of-life options than thermoset-based composites as far as recycling is concerned. However, thermoset-based composites provide higher mechanical properties and low-cost manufacturing when compared to thermoplastic-based composites. Thermoplastic composites offer the following additional benefits compared to thermosets matrices. • Increased impact resistance • Fast processing of pre-impregnated materials • The ability to reshape the products

However, thermoplastic matrices come in solid-state, and due to this, it is difficult to impregnate the fibres while using certain manufacturing techniques. This can lead to a high cost of components. Another negative aspect of using thermoplastic-based composites is propensity for creep behaviour and a high internal tension caused by thermal expansion differences between the thermoplastic and reinforced fibres after cooling down typically from relatively high temperatures. As far as using thermoset-based composites for lightweight applications are concerned, there are several benefits attached including: • New optimised manufacturing techniques such as resin transfer moulding (RTM) and out-of-autoclave techniques • Good mechanical and corrosion properties • Low investment cost • Superior surface finish

The key requirements for the development of lightweight materials include: • • • •

Reduced weight Reduced the cost Long term durability under the harsh environments Improved design methods for integrated structures so that fewer parts are required to make.

3.1.4

Lightweighting benefits

Several potential benefits can be gained from the utilisation of lightweight thermosets and thermoplastics composite materials. These include design flexibility, the high strength-toweight ratio in comparison to their metal counterparts. The lightweight structures can provide increased fuel efficiency, which can reduce the overall carbon footprint.

56

Sustainable Composites for Lightweight Applications

In fact, lightweight aspires to improve product performance, quality improvement and cost reduction. In the context of environmental concerns, the lightweighting approach motivates towards using fewer materials, less energy for materials extraction, which in turn can help to solve some of the environmental and sustainability challenges. The concept of lightweighting through materials design and development is very relevant, relating to sustainability. Key benefits of lightweighting: • • • •

Reduction of the overall weight of the component Less use of raw materials (helping to realise sustainability aspirations) Reduced fuel consumption (Leading to improved environmental performance) Reduce costs

Fig. 3.1 illustrates the benefits of using natural fibre-reinforced composites in comparison with fossil-based composites. The key parameters used to compare are important factors towards the overall CO2 reduction in comparison to glass fibre composites. Cost-saving through the reduction of overall weight is a primary driver towards the adaption of less heavy parts, lightweight. The main underlining fact to this approach as far as transport sector is concerned is that the lighter the weight of the parts, the lesser the fuel consumption, leading to lesser environmental damage and higher cost-effect. In recent years, the electric car has been put forward as a way to minimise the carbon footprint. However, there is a challenge of reducing the overall weight of the batteries. If lightweight materials can be used, then it can compensate for the heavy battery weight in some way. Table 3.1 depicts the cost benefits, as suggested by Taub et al. (2019).

GHG emissions in %: fossil- and hemp-based composites compared

100%

Hemp-based composites; accounted for carbon storage Hemp-based composites; not accounted for carbon storage

80%

Fossil-based composites

60%

40%

20%

4

5

6

7

8

9

Hemp/PP vs GF/PP battery tray

Hemp fibre/PTP vs GF/PES bus exterior panel

Hemp fibre/epoxy vs ABS automotive door panel

Hemp fibre/PP vs PP composites

Hemp fibre/PP vs GF composites

Hemp fibre/PP vs GF/PP mat

0%

Figure 3.1 Greenhouse gas emission potential comparison between natural hemp fibre composites versus glass fibre-reinforced composites (Akampumuza et al., 2017).

Lightweight composites, important properties and applications

57

Table 3.1 Lightweighting benefits towards the cost. Vehicle types

Value of lightweighting ($/kg)

Light vehicle

$4.50/kg

Heavy vehicle

$5e11/kg dry van dedicated routes

Heavy vehicle

$13e24/kg bulk carriers

Despite several advantages of carbon and glass fibre composites, the ever-worsening environmental situations, new environmental legislations, research and development into more sustainable, environmentally friendly and cost-effective materials have been the focus in recent years. With this new scenario, there is a paradigm shift into the use of lightweight materials such as natural fibre-reinforced composites and biocomposites as alternative materials to carbon and glass fibres due to their positive ecological attributes and attractive specific strength and modulus. Moreover, environmental sustainability is one of the key aspects of the lightweighting approach. Towards this, natural fibres such as flax, hemp, jute and kenaf have been attractive alternative reinforcing materials due to their lower density (1.2e1.6 g/cm3) compared to glass fibre-reinforced composites (2.46 g/cm3), ensuring the production of lightweight composites together with sustainable and renewable attributes. Over the past decades, the application of natural fibre-reinforced polymer composites (NFRPCs) materials in some industry sectors has increased significantly. The applications of NFRPCs have been mainly non-structural or semi-structural components where the use in primary structural systems has been limited because of several limiting factors, including lower mechanical properties, lack of enough test data, damage mechanisms not fully understood and processes parameters are limited for composites design and manufacturing. This becomes more apparent for natural fibre-reinforced composites and biobased composites due to the fact that these materials are not as tested and established as conventional reinforcements, to some industry sectors for use on primary structural systems, an in-depth understanding of design, failure modes and their structure-property relationships is paramount. Further barriers include weak fibre matrix interface, environmental degradation and susceptibility to thermal and oxidative degradation (Dhakal et al., 2007a,b; Dhakal et al., 2012; Faruk et al., 2012; Bourmaud et al., 2018).

3.2

Important properties

For any new device or components to be used in semi-structural or structural applications, key properties required to investigate are mechanical (strength, stiffness and toughness), thermal (long-term durability and degradability), and environmental performances (response of materials under harsh operating conditions). Similarly, the parts should be able to be manufactured using a cost-effective process, which has influences on the various properties. While developing lightweight composites

58

Sustainable Composites for Lightweight Applications

using natural fibres as reinforcements, balancing these parameters becomes even more important than conventional composites as these are a relatively new class of materials, many parameters are still not fully understood, and they are under development. Many of the application areas require balanced properties. For the automotive frame, for example, it requires high strength, stiff materials with long-term durability attributes. When the components are required of high stiffness, they generally have lower impact toughness. In the case of composite materials, two constituents (reinforcements and matrices) play an important role in the overall properties of composites. The following sections discuss the important properties and various contributing factors for such properties for lightweight composites.

3.2.1

Mechanical properties of biobased composites

The overall performance of natural fibre-reinforced composites depend on the key mechanical properties. The mechanical properties of natural fibre-reinforced composites depend on a number of parameters such as volume fraction of the fibres, fibre aspect ratio (L/d), fibre matrix adhesion, stress transfer ability at the interface, the orientation of fibres, microstructure and morphology of fibres among others. Most of the studies on conventional fibre composites involve the study of mechanical properties as a function of fibre content, the effect of various treatments on the mechanical properties and prediction of modulus and strength using well-established models and comparison with experimental data. When new lightweight materials such as natural fibre composites and biocomposites are investigated, mechanical properties, mainly strength and stiffness, first need to be predicted and influencing factors are understood. It is well established that the mechanical properties of plant fibre-reinforced composites mainly depend on the strength and the stiffness of the reinforcements along with other parameters. For example, fibre morphology and geometrical aspects significantly influence the overall mechanical properties of reinforced composites. Surface modifications and chemical treatments are employed in order to improve the compatibility between fibre and matrices, especially in the case of natural fibre composites (Zafeiropoulos et al., 2002).

3.2.1.1

Tensile properties

A tensile test is conducted to determine the tensile strength, tensile modulus, elastic limit, proportional limit, elongation, and reduction in cross-sectional area. The tensile properties of composites are enhanced with the reinforcement of fibres as fibres possess higher mechanical properties (strength and stiffness) than that of matrices. The test results conducted for many natural fibre-reinforced composites indicate that the tensile strength decreases after reaching the threshold volume fraction of fibres, which is attributed due to the matrices not being able to wet the fibres and as results, the fibre matrix interface becomes weak (Dhakal et al., 2007a,b). The weak fibre matrix interface would not be able to transfer the applied load from the matrix to fibres. In such a scenario, fibre pull-out takes place, and load transfer between fibre and matrix becomes weak because of reduced fibre matrix interfacial adhesion. Another reason for lower tensile properties is related to incompatibility between hydrophilic natural fibres and hydrophobic

Lightweight composites, important properties and applications

59

polymer matrices. In this situation, adhesion between reinforcement and the matrix becomes not so strong, and upon the application of load, the delamination at the interface and interphase takes place instead of fibre breakages. When there is a strong interface between fibre and matrix, upon the application of load, the fibres are normally broken and there is less delamination (Dhakal and Sain, 2019). Moreover, most of the thermoset matrices are brittle in nature. When natural fibres are reinforced, the overall composites become more ductile as most of the natural fibres are ductile in nature. The work presented by (Dhakal et al., 2007a) on hemp fibre-reinforced unsaturated polyester composite suggests that tensile strength and modulus of composites increased up to the critical fibre volume fraction, also called threshold fibre volume fraction, above that, the strength and modulus were decreased. On the other hand, it has also been reported that voids contents in the composite significantly influences the tensile properties. Higher void contents reduce the tensile properties of the non-woven hemp/UP composites. Tensile properties are often considered one of the key properties where most research and development work has been focussed on the investigation of the mechanical properties of natural fibre-reinforced composites. Typical tensile properties of plant fibre-reinforced composites in longitudinal and transverse directions are presented in Table 3.2. The reported data presented in Table 3.2 reveals that the stiffness and strength for aligned untreated flax/epoxy composites are approximately 5 GPa and 68 MPa, respectively. The reported data reveals that the stiffness and tensile strength for randomly oriented untreated hemp in fibre composites is 2.7 GPa and 33 MPa, Table 3.2 Typical reported tensile properties of important plant fibre composites in longitudinal and transverse directions. Fibre volume fraction (Vf)

Flax/ epoxy

Aligned

(Wf ¼ 0.50)

4.67

68.12

Dhakal and Sain (2019)

Flax/PP

Random

0.14

3.4

36

0.14a

3.4

39

Hornsby et al., (1997)

Aligned

0.51b

28.7

288

Madsen (2004)

Random

0.20

5.4

77

Oksmann (2000)

Aligned

0.60

45.0

1020

Gamstedt et al. (1999)

(MA) was used as a compatibilising agent Silane treated For comparison.

b c

Ultimate stress (MPa)

Fibre orientation

Glassc/ PP

a

Stiffness (GPa)

Fibre/ matrix

References

60

Sustainable Composites for Lightweight Applications

respectively. Additionally, the stiffness and strength for the composites where hemp fibres are aligned in the direction of tension forces are 27.6 GPa and 277 MPa, respectively. It is evident that the greatest stiffness and strength in the fibre composites are obtained when the fibres are aligned in the direction of the tension force (load applied parallel to the fibre direction). In other words, to achieve the optimal tensile properties, fibre orientation plays a significant role. Similarly, the stiffness and strength for flax/PP composite in the transverse direction (load direction not parallel to the fibre) is 3.4 GPa and 39 MPa, respectively. Moreover, the stiffness and strength for flax fibre-reinforced in the longitudinal direction are 28.7 GPa and 288 MPa, respectively. As expected, the addition of the natural fibre results in a stiffness and strength value in the longitudinal or aligned direction, which is significantly higher than that in the random and transverse direction. Also shown in Table 3.2 are the tensile properties of glass fibre-reinforced composites. This demonstrates that glass fibre composites are superior to plant fibre composites irrespective of fibre orientation, and the ultimate tensile strength, in particular, is higher for glass fibre composites. These results certainly provide encouraging steps in replacing glass fibre-reinforced composites by natural fibre-reinforced lightweight composites for some structural and non-structural applications. The most important benefit of using natural fibre-reinforced composites instead of glass fibre composites is the environmental aspect. Natural fibre composites are renewable, biodegradable, has less processing energy and provides recycling possibilities. One important aspect to note in this is that natural fibre composites are susceptible to moisture absorption, and as a result, the mechanical properties such as tensile strength and modulus are significantly reduced due to the weak fibre matrix interface created as a result of moisture ingress. This aspect has been elaborated in the section. In order to withstand the applied load, the properties of both reinforcements and the matrices are important. As much as the properties of fibres, the properties of the matrices are crucial, as it is the role of the matrix to protect the fibres and maintain the fibre as straight columns to prevent them from buckling and transfer the load to fibres. The properties of reinforced composites depend on the type, shape, and orientation of the reinforcing agents, the length of the fibres, and the volume fraction (percentage) of the reinforcing material. Short fibres are less effective than long fibres, and their properties are strongly influenced by time and temperature. Long fibres transmit the load through the matrix better and thus are commonly used in critical applications. Along with the fibre volume fractions, fibre wetting in the matrix phase and high fibre aspect ratio is important in order to get improved tensile properties. Moreover, the manufacturing process (optimised manufacturing process) and void contents are equally important factors that contribute towards the optimised tensile properties of natural fibre composites. For achieving optimal strength and stiffness of natural fibre-reinforced biobased composites, various parameters as mentioned above, need to be optimised. Fig. 3.2 depicts the advantage of biobased composites in comparison to conventional composites and metallic materials. It can be seen the advantages of biobased composites. Their specific properties are only lower than 15%e20% in comparison to carbon fibrereinforced composites. Natural fibre-reinforced biobased composites, therefore, present unique mechanical properties.

Longitudinal stiffness [GPa]

225 200 175 150

Flax / PP

125

Glass / PP

100

Carbon / PP

75

Steel

50

Aluminium

25 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Specific stiffness [GPa.cm3/kg]

(b)

Longitudinal stiffness E 250

Longitudinal specific stiffness E/ρ 140 120 100 Flax / PP

80

Glass / PP

60

Carbon / PP Steel

40

Aluminium

20 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90%100%

Fibre volume fraction

Fibre volume fraction

Longitudinal specific stiffness in bending E1/3/ρ

(c) Specific bending stiffness [GPa1/3 .cm3/kg]

4,0 3,5

Lightweight composites, important properties and applications

(a)

3,0 2,5

Flax / PP

2,0

Glass / PP Carbon / PP Steel Aluminium

1,5 1,0 0,5 0,0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Fibre volume fraction

Figure 3.2 The absolute and longitudinal tensile properties comparison of composites against established steel and alloys (Pil et al., 2016). 61

62

Sustainable Composites for Lightweight Applications

To summarise the tensile properties of natural fibre-reinforced lightweight composites, it is important to remember that the structures, physical and chemical compositions of reinforcements significantly influence the final properties. Notwithstanding, the effects of processing parameters and variables, the critical length of the fibres, the orientation of the fibre, wettability of the fibres, the fibre/matrix adhesion and most importantly, the intrinsic qualities of the fibres.

3.2.1.2

Flexural properties

Flexural properties of natural fibre composites generally determined by using a threepoint bending test. The test is conducted by placing a specimen onto two rounded supports and is subjected to a load that is central to the supports. The test provides the flexural properties, including flexural strength, modulus and strain of the composite materials at failure. Flexural stress/strength refers to the amount of force being applied to the test specimen; flexural strain is the percentage of strain that has been exerted onto the outer surface of the specimen. The flexural modulus is the tendency of the composite to bend and is worked out by the ratio of stress to strain in flexural deformation. Flexural modulus is generally dependent on the fibre/matrix interface bonding (Aziz and Ansell, 2004). As applied to tensile properties, the higher volume fraction of fibre up to the threshold point increases the flexural properties. However, flexural strength may also decrease as the fibre loading is increased beyond its critical fibre volume fraction. As above this, the resin may not be able to wet, and stress transfer from matrix to fibres weakens as the result of a weak fibre matrix interface. Stress concentrations are experienced at weak points due to low adhesion forces that exist between the matrix and the fibres. Flexural stress is calculated using the following formula. Flexural Stress ¼ sf ¼

3FL 2bh2

(3.1)

Where, f ¼ Flexural stress, in megapascals (MPa) F ¼ Load in Newtons (N). L ¼ Span, in millimetres (mm). h ¼ Thickness of the specimen, in millimetres (mm). b ¼ Width of the specimen, in millimetres (mm). Whereas flexural modulus is calculated using the following formula:
 L3 DF Flexural Modulus ¼ Ef 4bh3 Ds Where, Ef is the flexural modulus of elasticity, expressed in megapascals (MPa). s is the difference in deflection between s00 and sʹ F is the difference in load F00 and load Fʹ at s00 and sʹ respectively.

(3.2)

Lightweight composites, important properties and applications

63

Flexural properties of natural fibre-reinforced polymer composites have been widely reported. Banana fibre-reinforced unsaturated polyester composites were prepared using a compression moulding technique, and various properties, including the flexural strength and modulus were investigated (Kenned et al., 2020). The study reports that flexural strength at break and flexural modulus showed an increasing trend with the introduction of banana fibre as reinforcement into the unsaturated polyester matrix. It was reported that the matrix on its own showed a brittle failure behaviour. As fibre reinforcement was increased from 10 to 40 wt.%, the flexural strength was increased. However, beyond the threshold value of 40 wt.% of reinforcements, the flexural strength was decreased. The reasons for the decrease at higher fibre volume fraction was attributed to less strong fibre matrix interface as it is difficult to for matrix to wet the fibres at higher concentrations (over the threshold value). As for the tensile, the flexural properties of natural fibre composites depend on several factors. The most important are critical fibre length, optimal fibre volume fractions, fibre orientation and fibre matrix compatibility. Fibre morphology, geometrical aspects also play important roles in the overall properties of fibre quality, and hence, the properties of resultant composite materials. The flexural properties of natural fibre-reinforced composites alter with varying fibre volume fractions. Hemp fibrereinforced unsaturated polyester composites, for example, increased in flexural strength and modulus with an increase of layered of hemp reinforcement, optimal properties achieved at four-layered hemp fibres (21% FVF) according to (Dhakal et al., 2007a). The void content was another factor mentioned in the many reports published in the field of natural fibre composites that significantly influences the mechanical properties.

3.2.1.3

Impact properties

Impact testing is used to measure the impact toughness of the material used. This is described as the toughness and the ability of the material to absorb energy due to sudden loading. Toughness takes into account the ductility and strength of the material being tested. Composite materials are very likely to undergo impact damage during their service life, which in turn may reduce its structural strength and load-carrying capacity, often leading to catastrophic failure. The impact strength of any composite is dependent on the number of fibres contained in a composite and also the type of impact testing being used. The impact strength of a composite increases with an increase in fibre content; however, this is limited once the critical volume fraction is attained. The reason for this is because the addition of fibres creates points of stress. There are four different types of impact testing classified (Andrew et al., 2019): • Low-velocity impacts are considered when impacts occur at a velocity of 0-11 m/sec, • High-velocity impacts are considered when impact velocity is above 11 m/sec, Ballistic impacts are considered when impact velocity is above 500 m/sec. • Hypervelocity impacts are considered when impact velocity is above 2000 m/sec

Impacts can be at different velocities and energy levels, an example of some of these impacts are tool dropping, hail strike (weather), ballistic (military), bird strike,

Sustainable Composites for Lightweight Applications

Compression strength after impact

Backside fibre failure from ice impact

BVID•

64

Not visible

Visible on the back surface

Visible on both faces Impact energy

* BVID = barely visible impact damage

Barely visible impact damage (BVID) No penetration

Type 1 Delamination

Type 2 Backside fibre failure with minor delamination

Visible impact damage (VID) Penetration Type 3 Through-thickness cracks in recurring diamond shape pattern

Type 4 Extensive throughthickness cracks

Type 5 Clean hole

Increasing velocity/energy

Figure 3.3 Effects of various impacts levels on the damage of composite structures (Kim et al., 2003).

lightning strike, runway debris and towing damage. These impacts range from low velocity to high velocity, respectively. Each of these impact velocity/types can result in different damage characteristics on composite components and can take place under different environmental conditions. Fig. 3.3 shows a rough guide to the response of composite materials for a range of impact velocity or incident impact energies. Higher energy levels yield severe damage (Fig. 3.3). Natural fibre composites are considered susceptible to impact loadings. Lowvelocity impact damage is one of the serious threats to composite parts, especially to natural fibre-reinforced biobased composites. How they respond to various impact loadings and the capacity of the composites to withstand various impact conditions during their service life is important to understand. Published scientific reports suggest that many natural fibre-reinforced composite materials are very sensitive to impact loading. There are several published papers highlight that assessing and predicting impact resistance of a composite material is always challenging since the damage consists of different forms and mechanisms such as delamination at the interface, fibre breakage, matrix cracking and fibre pull out (Richardson and Wisheart, 1996). Normally, under the low-velocity impact test, the composite is not damaged fully and still capable of performing its primary function, whereas, in a high-velocity impact test, the composite is completely ruptured or penetrated by the impact striker (Thanomslip and Hogg, 2003).

Lightweight composites, important properties and applications

65

The impact loading on a particular composite involves relatively high contact forces acting over a small area for a period of short duration. Local strains generated at the point of contact between the two solids (projectile and composite specimen) result in the absorption of energy. When a projectile strikes a laminated composite, fracture damage such as delamination, matrix cracking and fibre breakage frequently occur. When the low-velocity impact damage process is involved, the internal damage created in the structures is not visible to the naked eyes. Dhakal et al., (2007b) analysed and compared the effects of low-velocity impact damage behaviour of hemp fibre-reinforced unsaturated polyester composites in comparison with chopped strand matt E-glass fibre-reinforced unsaturated polyester composites. They reported that the time elapsed for damage initiation to penetration to be shorter for unreinforced polyester than for hemp fibre-reinforced samples. The results showed that there was a significant improvement in the impact properties of neat polyester when non-woven hemp matt was used as reinforcing fibre. Neat polyester matrix showed brittle failure behaviour (Fig. 3.4(a)). As can be expected in this study, the unreinforced polyester under impact loading succumbed to brittle fracture and failure. Upon reinforcing the polyester with four and then five layers of hemp fibre, the impact resistance improved significantly. The hemp fibre-reinforced samples, in this case, showed a more ductile fracture and failure, suggesting that with the fibre volume increased, the samples are able to absorb

(a)

Front face UPE

Back face UPE

(b)

Front face 5 layered hemp

Back face 5 layered hemp

Figure 3.4 Low-velocity impact damage patterns (a) neat unsaturated polyester sample showing brittle failure front and rear sides of impacted surfaces (b) front and rear faces of hemp fibrereinforced unsaturated polyester samples showing ductile failure pattern (Dhakal et al., 2007b).

66

Sustainable Composites for Lightweight Applications

more energy (Fig. 3.4(b)). Their report highlighted that natural fibres, generally are ductile and are key factors to improve the impact resistance of composites. Although the study found that strength and stiffness were found to be lower in hemp fibre-reinforced polyester than chopped strand E-glass fibre-reinforced composites, results show that total energy absorption by the hemp fibre-reinforced composites was comparable to that of chopped strand E-glass fibre-reinforced composite. It is worth noting that the hollow structure of hemp fibre positively influenced the impact energy as it increases the damping property, which helps in dissipating the energy better than glass fibre-reinforced composites. Dhakal et al. (2012) investigated the influence of impactor geometry and impact velocity on hemp fibre-reinforced unsaturated polyester composites. Three different types of impactor geometries used were: hemispherical, 30, and 90 at four varying impact velocity levels: 2.52, 2.71, 2.89 and 2.97 m/s, using a drop-weight impact test machine. The results indicated that the impacted specimen absorbed higher impact energy and withstood higher loads when a hemispherical shaped impactor was used compared to 30 and 90 impactor. As part of understanding the influence of incident energy levels and the repeated impact on the damage behaviour of flax/unsaturated polyester composites fabricated by using vacuum bagging technique, Dhakal et al., (2019) conducted low-velocity falling weight impact testing using three different energy levels: 25, 27 and 29 J at room temperature with repeated impact scenario. It is reported that the peak load increases almost linearly with the impact energy until perforation of the plates is observed, and afterwards the peak load reduces significantly. Their findings suggested that the absorption of impact energy significantly decreases with repeated impact damage. The energy absorption values for the post-impacted specimens were the lowest compared to all other specimens. This was mainly due to matrix cracking and fibre breakage resulting in low impact resistance. The influence of thickness on the impact damage behaviour of plain woven flax fibre-reinforced epoxy composites fabricated using hand lay-up technique were investigated by Wang et al. (2016). Three different types of specimens used in their work were of two-layered, four-layered, and six-layered thickness under the drop weight impact testing using different incident energies up to penetration using a hemispherical impactor diameter of 16.5 mm. The two-layered flax fibre specimen showed microcracks on the impact side at an energy level of 10 J. In addition, a dented area of diameter 40 mm was also observed on the specimen with an approximate depth of 5 mm. However, with an increase in impact energy level to 15 J, the cracks increased, and perforation of the specimen occurred. In the case of four-layered flax fibre specimen, visible damage such as microcracks were seen on the specimen at an energy level of 25 J. The perforation of the four-layered specimen occurred at an energy level of 30 J. For six-layered specimen, the appearance of cracks started to occur at an energy level of 50 J. This specimen had larger damage compared to the other two types of specimens, and therefore, higher absorption of energy. The perforation of a six-layered flax fibre specimen occurred at an energy level of 70 J. Moreover, dents were observed only for two-layered specimens due to plastic deformation caused by bending under impact loading. Their report concluded that the thickness of the specimen has a

Lightweight composites, important properties and applications

67

16000 14000

Force, N

12000 10000 8000 6000 4000

20 J 40 J 80 J

2000 0 0

1

2

3

4

5

6

7

8

9

10 11 12

Time, ms

Figure 3.5 Force vs time traces for impacted GFRP specimen (Khomenko et al., 2017).

significant influence on the damage behaviour of flax fibre-reinforced epoxy composite. Higher thickness of flax composites requiring greater penetration energy, an indication of higher impact resistance of composites. A similar thickness effect was reported by Dhakal et al. (2014a) on jute fibre-reinforced methacrylated soybean oil (MSO) biocomposites, using different fibre architectures. However, the incident impact energy used on those studied samples were far smaller than the reported work by Wang et al. (2016). Moving forward, Khomenko et al. (2017) reported on GFRP specimens impacted at energy levels of 20, 40 and 80 J that the peak load for all impacted specimens increased at a higher incident impact energy level. The recorded force at the maximum peak was approximately 7, 12 and 15 kN for specimens impacted at an energy level of 20, 40 and 80 J, respectively. Moreover, the impact force versus time response was similar (sinusoidal) for all impacted specimens. The first drop in the curve was seen due to the initial strike of the impactor causing delamination and matrix cracking. However, with further increase in the force caused a second drop near the peak of the curve. This was mainly due to the more extensive damage area on the impacted specimen, such as fibre breakage, delamination, and appearance of cracks. These observed behaviours are depicted in Fig. 3.5. Investigation onto the effect of temperatures on the low-velocity impact damage response of jute fibre-reinforced unsaturated polyester was carried out by Dhakal et al. (2014b). The composites were subjected under impact testing at a lowvelocity, indicating that the impact was a non-penetrating impact. The results indicated that temperature is a major influencing factor on the energy absorption and damage characteristics. The impact condition had affected the property of residual flexural strength. When the temperature was between 30 and 50 C, the composite specimens showed the greatest fraction of its preliminary strength in contrast to the same specimens, which were tested at higher temperatures around 75 C. Their work suggested

68

Sustainable Composites for Lightweight Applications

that as the temperature increases, there was a reduction in the maximum impact force, which is depicted in Fig. 3.6. The pre- and post-impact test outcomes predominantly depicted that the mode of failure of jute fibre-reinforced unsaturated polyester composites were mainly caused due to delamination despite the changes in the impact temperature. Furthermore, Paturel and Dhakal (2020) studied the low-velocity response of flax/ vinyl ester composites at different temperatures. The report proposed that glass/flax/ vinyl ester hybrid systems were able to withstand higher load at elevated temperatures. An interesting observation made in this study was that as far as the impact damage resistance is concerned, the water immersed flax specimens displayed improved energy absorption than dry specimens. This was attributed to a weak fibre matrix interface allowing higher impact energy to dissipate. Their report concluded that the hybrid system was a viable way to prevent moisture ingress effects on flax/ vinyl ester composites and a way forward strategy to achieve improved impact resistance structures.

Parameters influencing the impact damage characteristics of composites There are several important parameters that influence the impact damage behaviour of composites materials, including: • • • • • • •

Fibre geometry and volume fraction Resin types and their toughness Thickness variation Impactor tup geometry Fabric types and stacking sequence Hygrothermal environments (service conditions (temperature, humidity) Incident energy applied

3.2.1.4

Fatigue properties

Fatigue failure takes place in composite materials as a result of cumulative cyclic loading and random loading in their service lifespan. Fatigue failure resembles, for example, property deterioration during the service life of composite materials. Compared to a metallic material, the fatigue life measurement of composites is a complex process because it fails under various damage modes and involves various fibre matrix properties. Under the cyclic loadings, polymers fail well below their failure loading under monotonic loading. Therefore, fatigue failure prediction and evaluation plays an important role for measuring the long-term performance of composite materials. However, measuring the fatigue behaviour of composites is always challenging and complex (Szebényi et al., 2020). Lightweight composite materials often expose to the harsh environment and cyclic loading and inherently susceptible to fatigue failure. Fatigue failure under the repetitive loading accumulated over a period of time causes structural components to fail catastrophically, even below their ultimate strength (Qi et al., 2020). This scenario becomes a serious issue for natural fibre composites because inherently,

(b) 1500

1500

Ambient 50ºC 75ºC

Ambient 50ºC 75ºC

Contact force (N)

Contact force (N)

Lightweight composites, important properties and applications

(a)

1000

500

0 0

0.5

1

1.5

2

2.5

3

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Stress

natural fibre-reinforced composites have structural variation (heterogeneity) and when these materials can fail in a catastrophic manner at levels much lower than ultimate strengths due to internal damage accumulation over a period of time (Mahboob and Bougherara, 2018). Therefore, predicting fatigue failure in fatigue critical components is important while designing a part or component. There is a considerable amount of reported work covering fatigue behaviour of natural fibre composites; however, not enough data to have confidence while analysing the failure mechanisms of natural fibre composites under fatigue loading (Shah et al., 2013). Despite many attractive attributes, natural plant fibre composites are inherently susceptible to long-term durability due to moisture absorption and prolonged repetitive loadings such as fatigue and vibration (Barbiere et al., 2020). In addition, the failure mechanism under accumulative loading is complex and not fully understood for natural fibre composites compared to conventional glass and carbon fibre composites. Due to lack of enough experience on how natural fibre composites fail, their failure mechanisms under such loading, these composites are not yet fully utilised in structural applications (Mahboob and Bougherara, 2018). In their comprehensive fatigue test evaluation of hemp fibre-reinforced epoxy composite (Barbiere et al., 2020). They reported the influence of moisture absorption (ambient, wet and dry conditions) on the fatigue life by applying three levels of fatigue stresses: 60%, 75% and 90% of the tensile strength. They concluded that moisture immersed samples exhibited lower tensile strength and lower fatigue sensitivity. Another interesting result of the fatigue behaviour of flax fibre-reinforced epoxy composites is by Jeannin et al. (2019). It is highlighted in their findings that the quasi-static strength and rigidity were significantly affected by the ageing conditions, but the fatigue strength was improved. There are various models applied to predict the cumulative failure of composites under fatigue loading. According to an extensive review carried out by Mahboob and Bougherara (2018), most of the fatigue testing has been undertaken using constant stress amplitude tests. Under the fatigue cyclic loading conditions, the material is subjected to stress over the time between smax and smin as a sinusoidal, as shown in Fig. 3.7, where the sm is the mean stress, and the range is defined as Ds, meaning smax  smin :

V max

'V

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Figure 3.7 Constant stress amplitude fatigue failure.

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To measure the fatigue life, a commonly used method is a typical stress-life (SeN) diagram, using power-law, and regression equation: Smax ¼ S0 N b (Fig. 3.7). Where, Smax is the maximum stress applied, N is the number of cycles to failure, S0 is the single cycle (static) ultimate strength of the material and b is the material fatigue strength coefficient. The work undertaken by Shah et al. (2013) highlights that carbon fibre-reinforced epoxy composites outperforms the fatigue behaviour of natural fibre composites and glass fibre-reinforced composites. However, the fatigue behaviour of flax fibre-reinforced composites is comparable to glass fibre-reinforced composites. Moreover, natural fibre composites have displayed an improved damage accumulation rate (slower) in comparison to glass fibre composites. Although the causes of this behaviour are not so clear, the following could have attributed to such behaviour (Shah et al., 2013; Liang et al., 2012; Baley, 2002): 1. Due to strain hardening at cumulative cyclic loading exhibited by NFCs. 2. NFCs show slower stiffness degradation compared to glass fibre-reinforced composites during their fatigue life. 3. The progressive reorientation of microfibrils towards the loading direction

The above-highlighted observation is certainly encouraging when employing fatigue-loading situations on NFCs. However, glass fibre composites possess significantly higher static properties than NFCs, and therefore, overall fatigue performance of GFRCs is far higher than that of NFCs (de Vasconcellos et al., 2014). Owing to their natural variability and inherent moisture absorption characteristics, the fatigue damage rate of natural fibre composites is significantly higher than that of synthetic fibresreinforced composites. Key elements influencing the fatigue life of natural fibre composites (Awaja et al., 2016): 1. 2. 3. 4.

Damage accumulation at the fibre/matrix interface Material (fibre and matrix) variability Fibre types, treated or untreated and fillers or additives types Fatigue parameters (stress amplitude, intensity and frequency, temperature,

3.2.1.5

Creep behaviour

Creep is defined as the time-dependent deformation (deformation over time) or timedependent viscoelastic properties of a material subjected to continuous stress. Creep failure, also called permanent deformation, involves a combination of elastic deformation and viscoelastic behaviour of polymer composites. To measure this timedependent failure, knowledge on how materials respond the long-term loading is essential. The increased demand for sustainable lightweight materials such as natural fibre-reinforced composites, withstanding short- and long-term loading, depends on short and long-term creep behaviour at different stress and temperature levels. Creep failure generally occurs at high temperatures. For natural fibre composites to be used in structural and semi-structural applications, they need to withstand harsh environmental conditions, including high temperatures during their service life. In

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Strain

Creep rupture

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Figure 3.8 A typical strain time curve under creep testing.

such environments, polymeric composites lose their mechanical strength. It has been reported that the creep behaviour of natural fibre composites showed poor performance in certain operating conditions (Dhakal et al., 2009). Thus, it is important to have a comprehensive understanding of their long-term dimensional stability and durability relating to their viscoelastic behaviour at harsh service conditions. Creep behaviour is generally expressed as strain versus time (ε as a function of time, t) as depicted in Fig. 3.8. The creep behaviour of polymers normally consists of four stages. There are three important regions, stages under the creep testing. Instantaneous deformation, ε0 : In this stage, when an external load is employed, the material responds with an instantons strain ðε0 Þ due to the elastic/plastic deformation of the polymer. Stage I (primary creep): In this stage, strain increases at a rapid rate and slows with time. The rapid increase in strain is related to the non-uniformity of fibres in the composites, which contributes to the continuance of the fracture of fibres. If the applied stress is high, the strain time curve becomes non-linear because of damage initiation in materials. Stage II (secondary creep): In this stage, the strain rate is stable, has a slow uniform rate, which is an indication that materials are capable of sustaining the applied stress. Stage III (tertiary creep): In this stage, the strain rate accelerates and terminates when martial breaks or rupture (point of fracture). For example, in composites, this scenario is attributed to damage at fibre intersections, deboning at the fibre matrix interface and finally the material fails. Rupture of the composite at this stage can be influenced by several factors such as fibre matrix interface bonding, fibre defects, voids and stress level and temperature applied. This rupture stage is not desirable in the composite. Creep performance is measured using either tension, flexural and compressive testing modes. In such modes, the permissible or ultimate strength of the materials are first determined. Several factor contribute towards the creep failure of polymer and composites. The creep rupture behaviour, also called lifetime prediction, is therefore, important to understand when designing composites and structures.

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Dhakal et al. (2009) report on hemp fibre-reinforced unsaturated polyester composites containing varying fibre volume percentages subjected to short-term flexural creep tests. They also evaluated the influence of stress levels and temperatures. Their findings suggested that the creep behaviour is significantly influenced by both stress levels and temperature. Creep performance deteriorated as temperature increased above the glass transition temperature. Fig. 3.9 shows the strain/time curves for hemp/UP specimens, as reported by Dhakal et al. (2009), which are found to have typical strain-time curves showing an instantaneous elastic strain on loading followed by a period of slow linear deformation. At the start point, the sudden increase in strain reflects the existence of constant elasticity. Similarly, the report highlights that the recovery can be divided into instantaneous contraction and creep recovery. A constant deformation indicates that the instantaneous deformation consisting of both plastic and elastic deformation. Their work also investigates the creep behaviour at higher temperature (50 C). They

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Figure 3.9 Creep responses of composite with different fibre concentrations at a constant stress of (a) five and (b) 15 MPa at 25 C (Dhakal et al., 2009).

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suggested that at higher temperature, the matrix, UP, becomes softer, and the creep strain becomes higher. A similar influence (increase in creep deformation) at a higher temperature was reported for hemp-reinforced UP composites. In general, when predicting the creep rupture of composite materials at different stress levels and elevated temperatures, the following factors significantly influence the overall creep behaviour (Dhakal et al., 2009; Nunez et al., 2004; Morreale et al., 2017). • • • • • •

Viscoelastic behaviour of matrix materials Fibre alignment and flaws in the fibres Fibre volume fraction Void contents Fibre matrix interfacial strength Creep parameters (stress levels, temperature and time).

The stress and temperature play an important role in the creep failure of composite materials. The strain time relationship behaviour (creep rupture) of composites is influenced by the magnitude of stress levels. Therefore, creep performance (strength and modulus) decreases with increased stress and temperature. The creep compliance increases with the increase in temperature because of the increased free volume of the amorphous fraction of the polymer matrix. This is because, particularly at the high temperatures, the material approaches melting and experiences viscous flow (Park and Balatinez, 1998; Varela-Rizo et al., 2010). At high temperatures, the matrix loses its stiffness, and as a result, creep strain increases. Moreover, Greco et al. (2007) evaluated the flexural creep behaviour of woven composite. Their work explained that the first stage of rapid increase in strain was attributed to the non-uniformity of reinforcing fibres in the composites, which contributed to the continuance of the fracture of fibres. At the high-stress levels, the strain time curve becomes non-linear because of damage initiation. Their work suggests that the creep failure under the flexural mode can take place due to fibre alignment in tension and microbuckling in the compression side. The final damage can take place due to the deboning of fibre and matrix at their interface region.

3.3

Thermal stability of biobased composites

In recent years, the use of fibre-reinforced polymer (FRP) composites, including carbon fibre-reinforced polymer (CFRP), glass fibre-reinforced polymer (GFRP) and natural fibre-reinforced polymer composites (NFRPCs) have become an important part of many applications. It is well-accepted that various environmental conditions affect the physical, mechanical and thermal properties. When these composites are exposed to outdoor applications, it causes degradation, including solar Ultra Violet (UV) rays, hot/cool cycling, humidity and other environmental pollutants. It is important that the environmental effects of the weathering process on composites are fully understood to prevent the damage of the parts and components, as well as health and safety issues. With a focus on the understanding of the changes to the thermal properties of the composites due to different environmental conditions, the life span of the composites can be optimised.

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Composites, both synthetic and natural fibre-reinforced, are prone to weakening and degradation of their mechanical and thermal properties, particularly when exposed to high temperatures. In addition, natural fibres are more vulnerable to the effects of oxidative degradation (Wielage et al., 1999; Azwa et al., 2013). The chemical composition of natural fibres as reinforcements in composites influences the overall properties of composites. Natural fibres such as flax, hemp, jute and kenaf absorb moisture at different conditions. The chemical compositions not only influence the moisture absorption behaviours, but they also limit the temperature such that they can processed. In addition, Dorez et al. (2013) reported on the thermal and fire properties of flax, hemp, and bamboo-reinforced Polybutylene succinate (PBS)-based biocomposites. They used ammonium polyphosphate (APP) as a fire-retardant agent. The influence of fibre type and fire retardant on the thermal stability and fire properties were investigated. It was reported that the degradation process of lignocellulosic fibres occurred in four key steps, including: 1. Release of water vapour by the fibres covering first mass loss between 50 and 150 C. 2. The second step corresponding to the depolymerisation of the hemicellulose and cleavage of glycosidic linkages of cellulose between 250 and 370 C. 3. The degradation of cellulose at the main peak between 340 and 370 C.

Often, degradation behaviour obtained from TGA is compared with the cone calorimeter test results. The thermal decomposition and thermal stability of the composites are assessed by thermogravimetric analysis (TGA). TGA is extensively used to measure the rate and quantity of weight change (physical and chemical) in composite samples as a function of temperature. The key parameters measured in TGA include weight loss at onset temperature (Tonset), degradation temperature at different percentages of mass loss, final decomposition and remaining of residue (char yield), also known as char. In TGA testing, the materials are exposed to increasing temperature and controlled atmospheres (oxygen and nitrogen). More also, Dhakal et al. (2012) investigated the degradation of hemp fibre using TGA and suggested that the onset of decomposition temperature started around 240 C with the degradation of hemicellulose. This temperature indicates that up this temperature, the material is thermally stable. This critical temperature needs to be taken care of when natural fibres are used in composites manufacturing processes such as injection and extrusion processes. When natural fibres, especially reinforced in thermoplastics matrices, processing parameters such as temperature and time become very critical. TGA test results can indicate the thermal stability of different materials. As far as thermal degradation of natural fibre-reinforced composites are concerned, their physical and chemical compositions play a significant role. Thermal degradation behaviour of natural fibres (lignocellulose fibres) occurs at different stages (Azwa et al., 2013; Dhakal et al., 2012; Methacanon et al., 2010). The key stages are as follows: Stage 1: the mass loss region between 50 and 120 C is attributed to the release of moisture absorbed by the fibres (evaporation, desorption of residual moisture trapped). Stage 2: The mass loss between 250 and 370 C corresponds to the depolymerisation or decomposition of hemicellulose.

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Stage 3: The degradation of lignin. The degradation temperature range is larger for lignin (from 200 to 500 C). Lignin helps in char formation. Low lignin content, for example, for flax, can contribute to higher decomposition temperature.

3.3.1

Thermal degradation and stability of biobased composites

Thermal stability and decomposition behaviour of composites are measured using various techniques. One of the popular techniques is thermogravimetric analysis (TGA). Natural fibre as reinforcements in composites has several limitations. One of the main drawbacks of natural fibre composites being the degradation of natural fibre at low temperatures compared to their synthetic counterparts. The main degradation of natural fibres takes place at around 200e240 C. In such a condition, these reinforcements are mainly used for commodity plastics such as PP, PE and PVC, which have a melting temperature of about 185 C. Besides, the work of Bhattacharyya and Kim (2017) on different natural fibre composites and compared with glass fibre composites highlighted the thermal stability of polymer and natural fibre composites. Fig. 3.10 illustrates the TGA traces of natural fibre composites with reference flax-glass epoxy composites covering both thermoplastic and thermosets matrices. Acceptable processing temperature of natural fibre composites depends on other constituents such as chemical composition, fibril angle, fibre morphologies and others. It is worth noting that degradation temperature directly influences the processing temperatures. For example, flax fibre degradation takes place between 270 and 400 C due to the thermal decomposition of cellulosic structures of the fibre, whereas glass fibre does not show any sign of degradation below 800 C. Thus, with a similar matrix, the flax fibre shows a poor resistance to high temperature, with a degradation of 80% at 400 C. This makes flax fibre-reinforced composites venerable at high temperature processing and applications in comparison to glass fibre. Thermal stability of polymer and composites can be enhanced by various methods, including grafting, surface treatments and use of various surface coatings. Fig. 3.11 depicts TG and DTG curves of uncoated and coated flax fibres.

3.3.2

Flammability behaviour

NFRPCs are gaining gradual acceptance for use in many critical industry sectors due to their lightweight, low cost and better specific properties than glass fibre composites. However, their susceptibility to environmental degradation and poor flame retardancy characteristics, sensitivity to their processing temperature and flammability (fire behaviour) issue pose challenges for use in these critical industry sectors in various fireproof related demanding applications despite many additives and treatment processes have been developed to improve long-term durability and flammability issues (Norouzi et al., 2015). Polymer normally does not possess high flammability resistance characteristics. Similarly, when natural plant fibres are reinforced in different polymers, the overall flammability properties of resultant composites are influenced. The lignocellulosic fibres such as flax, jute, hemp and kenaf fibres are easily ignited when they are put into

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contact with fire. These fibres burn quite fast, leaving a small amount of char, depending on lignin contents. For improvement to fire resistance behaviour various fire different flame-retardants (FRs) additives and chemical treatment methods are employed. In this way, the char yield properties are improved. There are several techniques available to measure the fire properties of composites. Cone calorimeter is a widely employed technology to investigate fire-retardant behaviour of polymer and composites in a small scale. Flammability characteristics of fire properties of polymer and composites can be determined by measuring different parameters such as heat release rate, time taken for ignition, smoke generation, etc. The main testing techniques to measure the fire properties of composites include (Norouzi et al., 2015): 1. Cone calorimeter testing: This technology measures and monitors important fire properties in accordance with the oxygen consumption principle by deriving oxygen consumption rate into heat release rate (HRR) during the combustion of specimens. This is one of the most used and reliable testing method, which enables the analysis of various fire response parameters as listed below (Schartel et al., 2003; Norouzi et al., 2015): • Time to ignition (TTI, s), • Time to peak • Total heat release rate (THR, kW/m2), • Total heat release (THR(t)) • Peak of the total heat release rate (PHRR, kW/m2) • Mass loss rate (MLR) and • Effective heat of combustion (EHC) • Total smoke produced (TPS) 2. Limiting oxygen index (LOI): This is a parameter that characterises in which the lowest percentage of oxygen in the mixture with nitrogen at which the test specimen ignites and burns on its own. LOI is a useful, small-scale test, which correlates to the ignitability in polymers. According to Gou and Tang (2011), any material with an LOI less than 21% can easily burn in air, and they are flammable; a material with an LOI greater than 21% can reduce the flame after removal of the igniting source. It is a well-established phenomenon that materials with an LOI greater than 28% are generally flame retardants, and materials between the thresholds of 21% < LOI < 28% can be referred to as slow-burning materials (Norouzi et al., 2015). It is clear that LOI provides rough guidance to indicate the relative flammability of polymer and composites. Limiting oxygen index (LOI) is used to express the relative flammability of polymers and composites. This provides rough guidance, such as composites with higher LOI will show lower flammability compared to low LOI. The formation of char is a good indicator to suggest the flammability property of polymer and composites.

UL-94: This technique can help to classify the flammability behaviour of polymers and composites according to their burning behaviour as V-0, V-1 and V-2. As reported, in the V-2 category, the burning of sample stops within 30 s on a vertical configuration and flaming drips are permitted in this category. In V-1 category, burning discontinues within 30 s on a vertical sample and flaming drips are permissible as long as they are not burned. Whereas, in the V-0 category, burning stops within 10 s on a vertical sample and flaming drips are allowable as long as they are not ignited (He et al., 2007).

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Figure 3.12 Cone calorimeter technology, showing (a) schema of a cone calorimeter, (b) stages and fire, fire properties and (c) cone calorimeter set-up detail (Fateh et al., 2016; Schartel and Hull, 2007).

In order to evaluate the fire properties of polymer and composites, the development of the HRR over time, in particular the value of its peak/maximum (pHRR or HRRmax), is usually measured. The total heat release (TRR, kJ/m2) is obtained by integrating the HRR vs time curve. Furthermore, the cone calorimeter test also enables the analysis of fire response parameters such as the time of ignition (TOI), time of combustion or extinction (TOF), mass loss during combustion, quantities of CO and CO2 and total smoke released (TSR) (Schartel and Hull, 2007). A schema of cone calorimeter is illustrated in Fig. 3.12. Furthermore, Kim et al. (2015) investigated the effects of wool, fire-retardant additive and polypropylene viscosity (MFI) on fire retardant and mechanical performance of the PP-short wool fibre composites. Cone calorimeter technology was used to measure the fire properties. Their findings demonstrated significant enhancement of fire-retardant properties of PP/wool 30 wt.% composite with APP 20 wt.%. A decrease in PHRR (w82%) and an increase in time to PHRR (w170%), compared to those of neat PP, were reported as evidenced of improved fire properties, as depicted in Fig. 3.13. Similarly, Bhattacharyya and Kim (2017) reported that HRR traces of composite laminates showed more fluctuations than that of short fibres as depicted in Fig. 3.14(a). Compared to flax/epoxy composites, glass/epoxy composites exhibited significantly slower heat and smoke and ignited slower than natural flax/epoxy composites during the combustion process. This is attributed to the inert nature of glass fibres.

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3.3.2.1

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Parameters influencing cone calorimeter performance

There are several parameters that influence the fire properties investigated by the cone calorimeter. Some of them are listed below, as per the work conducted by Schartel and Hull (2007). Physical observation: This is a crucial parameter needing to take extra care while running calorimeter testing in order to understand the burning pattern of specimens (surface rise, char formation and cracking, bubbling and sparking). Visual observation becomes an important part of the test and should be reported and recorded. Sample thickness: The important fire properties also largely depends on the sample size and especially the thickness. While presenting and discussing the results, this parameter should be correlated. It is reported that thermally thin samples exhibit decreased ignition time compared to thick samples. Type of fibres: When considering the fibres for reinforcements, their structural morphologies, chemical composition, as well as their fibre weaving and orientation, also play an important role in fibre properties. Distance between sample and cone heater: this parameter also influences the fire properties.

3.3.2.2

Ways for improvement of fire properties of natural fibre reinforcements and composites

There are several methods employed to improve the fire performance of natural fibrereinforced sustainable composites. For the last decade, nanoclay has been extensively used as an additive to improve the thermal stability and fire-retardant properties of composites. Nanoclay being cross-linked thermally and physically, helps to improve the fire-retardant behaviour. The addition of graphene weakens the reaction of flame retardant PP to a small flame. Lower loading of graphene is observed to improve the swelling of char, resulting in better insulation of the char and decrease in heat and smoke release. It was reported that flammability (fire behaviour) of wool/PP composites was significantly improved by adding wool and ammonium polyphosphate (APP). The cone calorimeter and vertical burn tests exhibited a significant decrease in HRR and a direct flame self-extinguishment of composites, respectively (Kim et al., 2015). Expanded graphite (EG) has also been used to improve the flammability of polymers. This flame retardant expands to its original volume up to 300 times upon heating at the temperature of 900 C. The expanded graphite acts as an insulating layer, and hence, reduces dripping (Schartel et al., 2003). Several reported work suggests that both EG and APP contribute to improving fire properties (for example, PHRR and THR). However, it is mentioned that APP can produce smoke and carbon monoxide production per unit mass loss. Their work highlighted that EG was a more attractive fire retardant as this reduced both fire risk and fire hazard. However, APP improved fire risk but increased fire hazard.

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Another way to improve fire properties is by blending of two polymers, which involves blending the polymer with lower fire properties into the polymer with higher fire properties. For example, if phenolic, which has higher fire resistance is mixed with unsaturated polyester resin (UPE), then the good fire properties of phenolic resin can help in improving the fire properties of UPE resin (Kandola and Nazare, 2007; Chiu et al., 2000). The use of nanoparticles such as nanoclays (layered silicate, montmorillonite (MMT) organically modified clays) have been extensively used to improve the fire properties of polymer and composites. Although clays are not fire retardant themselves, however, their incorporation into the polymer matrices can help to reduce PHRR. Nanoparticles help in the melt flow of polymers. Moreover, clay contributes to the formation of a carbonaceous-silicate char that helps in increasing thermal stability by acting as a heat barrier. Also, Kozlowski et al. (2007) reported an improvement in the fire properties of poly (lactic acid) with the incorporation of organically modified montmorillonite to fabricate a platelet structure of nanocomposites. Nanoparticles not only improve the thermal stability but also equally enhances the mechanical properties of composite materials. With a small amount, two to five wt.% nanoclay mixed into polymers, especially exfoliated structure, contributing to a significant of mechanical properties are achieved (Yang and Nelson, 2011; Norouzi et al., 2015). A novel work on using extracellular polymeric substances (EPS), such as EPSflocs and EPSgranules, were extracted from activated and aerobic granular sludge, respectively, and tested as bio-based flame retardant materials have been researched (Kim et al., 2020). From these substances, flax fibre was coated, and flammability behaviour was investigated by employing a vertical burning test. Significant differences were observed during the burning test, where coated flax exhibited far superior flammability properties than uncoated, reference flax samples. The detail of the images is depicted in Fig. 3.15 (Kim et al., 2020). Similarly, fire resistance of natural fibre composites can also be improved by thermal coatings such as ceramic or mineral fibres. These coatings can be bonded to the composite, which reflects heat away from the composites, and as a result, this increases the ignition resistance. Treated fibres have display significantly reduced pHRR significantly (Fig. 3.16). The application of the flame-retardant matrix helped to reduce pHRR values greatly. Also, the modifications helped overall flame retardance behaviour. Moving forward, Maqsood et al. (2020) comprehensively investigated the influence of different flame retardants on the thermal stability and flame retardancy behaviour of polylactic acid (PLA) yarn. The acidic and carbonic sources, as well as a modified polyester-based plasticiser, were used as flame retardants. Their report achieved a significant decrease (59%) in the heat release rate with modified flame retardant PLA in comparison to neat PLA without any additives (Fig. 3.17). The flame retardant used this study were a phosphorous-nitrogen-based non-halogenated with commercial name (EXP PP/37), the kraft lignin (KL) powder, and a modified polyester-based plasticising agent (PES).

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Figure 3.15 The uncoated flax fabric, EPSflocs coated and EPSgranules coated flax fabrics before, during and after burning. (a) and (b) are EPSflocs and EPSgranules coated flax fabrics before burning, respectively. (c), (d) and (e) are uncoated flax fabric, EPSflocs and EPSgranules coated flax fabrics after burning for 5 s, respectively. (f), (g) and (h) are uncoated flax fabric, EPSflocs and EPSgranules coated flax fabrics after burning for 12 s, respectively, (i), (j) and (k) are uncoated flax fabric, EPSflocs and EPSgranules coated flax fabrics after the burning was completely finished (Kim et al., 2020).

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350 300

PLA/EXP10/PES10

250

PLA/EXP15/PES10

200

PLA/EXP15/PES10/KL5

150 PLA/EXP15/PES10/KL7

100 50 0

Total heat release/MJm–2

(b)

500

70 60

PLA

50

PLA/EXP10/PES10

40

PLA/EXP15/PES10

30 PLA/EXP15/PES10/KL5

20

PLA/EXP15/PES10/KL7

10 0

0

50 100 150 200 250 300 350 400

Time/s

0

50 100 150 200 250 300 350 400

Time/s

Figure 3.17 (a) Heat release rate of net PLA, PLA/EXP/PES and PLA/EXP/PES/KL fabrics (b) total heat release curves of neat PLA/EXP/PES and PLA/EXP/PES/KL (Maqsood et al., 2020).

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Heat release rate/kW m–2

(a)

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87

Figure 3.18 Thermal conductivity mechanisms comparison between (a) crystalline materials (metal) and (b) polymers (Burger et al., 2016).

3.3.3

Thermal conductivity measurements

The thermal conductivity of composites is related to the material’s capacity to transport or conduct heat. Therefore, specific heat capacity is always linked to thermal conductivity. The thermal conductivity of materials depends on how their crystal structures are arranged. Crystalline materials are different from amorphous materials in terms of their capacity to transfer heat. The crystalline and semi crystalline polymers for example will give greater thermal conductivity compared to amorphous polymers. Overall, polymers are far less conductive than metals. Fig. 3.18 shows the thermal conductivity mechanisms crystalline and amorphous polymers by illustrating as a Newton pendulum, as described by Burger et al. (2016). It is clear that crystallinity is an important factor when measuring the thermal conductivity of materials. It is important to know the thermal conductivity properties of natural fibre composites, especially when they are required for use in heat-related applications. The information on thermal conductivity plays an important role while selecting parts for various applications. Thermal conductivity of natural fibre-reinforced composites have different behaviour than that of glass and carbon fibre composites due to their hollow morphological structures. The thermal conductivity of natural fibrereinforced composites also greatly influenced by the conductivity and the type of matrices. Matrices are insulating materials, so they have limited thermal conductivity. As mentioned above, generally, the thermal conductivity of polymer depends on their types, amorphous or crystalline. For example, the conductivity of amorphous polymers increases with temperature up to its glass transition temperature (Tg). Depending on the applications, they can be made more electrically and thermally conductive by adding conductive fillers such as graphite, metallic particles, carbon black and carbon nanotubes (Han and Fina, 2011). Moreover, thermal conductivity also depends on the filler contents level. The specific heat capacity of the composites decreases with increasing particle contents.

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Average thermal conductivity (W/m.K)

0.30

0.29

0.28

0.27

0.26

0.25

0.24 Neat (AR0) PCL

Aspect ratio 19

Aspect ratio 26

Aspect ratio 30

Aspect ratio 38

Figure 3.19 Thermal conductivity showing the influence of aspect ratio of hemp/PCL biocomposites (Dhakal et al., 2020).

There are two types of thermal conductivities measured for fibre-reinforced composites: 1. The in-plane thermal conductivity (to thermal conductivity in the direction parallel to fibre axis) and 2. Through-thickness thermal conductivity (refers to thermal conductivity in the direction perpendicular to fibre axis).

Due to the structure and morphology of natural fibres, natural fibre-reinforced composites have low through-thickness conductivities. It also becomes a challenge to measure the thermal conductivity of natural fibres and resultant composites. Fig. 3.19 shows the influence of fibre aspect ratio on the in-plane thermal conductivity of hemp fibre-reinforced poly(e-caprolactone) biocomposites. Their report highlighted that the thermal conductivity depended on the aspect ratio where it increased up to the fibre threshold values, as well as the interaction between fibre matrix interface. The thermal conductivity on composites reported that the through-thickness conductivity is far lower than in-plane conductivity. Thermal conductivity, for example, depends on various parameters such as morphology, density and homogeneity of materials. Measuring conductivity in the through-thickness direction is always more challenging than in the in-plane (Ming et al., 2015). The thermal conductivity of composites decreases as fibre volume fraction increase; however, with increase in fibre angle, thermal conductivity increases. Mounika et al. (2012) reported on the thermal conductivity of bamboo fibre-reinforced polyester composites in comparison to glass/polyester composites. Fibre volume fraction plays an important role in thermal conductivity. The thermal conductivity of polyester was highest compared to different bamboo composites (Fig. 3.20).

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Thermal conductivity (Wm–1k–1)

0.3 0.25 0.2

Vf = 0.304

0.15 0.1 0.05 0 Glass FRPC

Bamboo FRPC

Bamboo short FRPC

Polyester resin

Figure 3.20 Thermal conductivity of bamboo/polyester composites (Mounika et al., 2012).

a - LDPE b - SRP (20% sisal) c - GSRP (50/50 SRP/GRP) d - GRP (20% glass)

Thermal conductivity (W/mK)

0.58

d

0.50

c

0.42 0.34

b a

0.26 0.18 0.10 120

160

200

240

280

320

360

Temperature (K)

Figure 3.21 Thermal conductivity of bamboo/polyester composites (Kalaprasad et al., 2000).

Similarly, a study carried out by Kalaprasad et al. (2000), highlighted that thermal conductivity and thermal diffusivity for sisal-reinforced polyethylene (SRP), glassreinforced polyethylene (GRP) and sisal/glass hybrid fibre-reinforced polyethylene (GSRP), reported that GSRP showed increased thermal conductivity than nonhybridised sisal-reinforced composite (SRP), due to the addition of the glass fibre, as depicted in Fig. 3.21.

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Thermal conductivity (W/mK)

0.8

0.6

0.4

0.2

0.0 0 wt% GnPs

0.1 wt% GnPs

0.3 wt% GnPs

0.5 wt% GnPs

Figure 3.22 Through-plane thermal conductivity of the GNPs/carbon fibre-reinforced epoxy composites (Wang and Cai, 2019).

3.3.3.1

Ways improving the thermal conductivity of polymer matrix composites

Several reported works have highlighted ways to improve the thermal conductivity of polymers and composites. Incorporation of thermally conductive fillers into less conductive polymers are common practice. For example, the work reported by Wang et al. (2015) recommends that thermal composites of epoxy composites were significantly enhanced with the incorporation of expanded graphite (EG) particles. The key to achieving such improvement was attributed to the proper dispersion of EG in the epoxy matrix. Similarly, the reported work by Wang and Cai (2019) achieved a significant improvement in through-plane thermal conductivity of carbon fibre laminated composites by incorporation of 0.3 wt% of graphene nanoplatelets (GNPs) as illustrated in Fig. 3.22. The thermal diffusivity of the composite plates in their work was measured using the laser flash method. The conductivity was calculated using Eq. (3.3). K ¼ a r Cp

(3.3)

Where, K ¼ thermal conductivity (W/m K), a ¼ thermal diffusivity (mm2/s), r ¼ densities of the samples (g/cm3), Cp ¼ specific heat capacity (J/g K). Idumah and Hassan (2016) added exfoliated graphene nanoplatelets (GNP) to kenaf fibre-reinforced PP composites. Their study reported enhancement in thermal conductivity by 88% with 3 phr GNP loading. According to various published works in the thermal conductivity of polymeric composites, the key parameters that influence the thermal conductivity can be listed as follows: 1. Defects found in crystalline structures according to the comprehensive work presented by Burger et al. (2016). They reiterated that thermal conductive will always be decreased

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91

Point defect

Dislocation

Grain boundary

Figure 3.23 Phonon scattering in crystalline materials as a result of various defects (Burger et al., 2016). 2. 3. 4. 5. 6. 7. 8. 9.

Mechanisms involved and fillers alignment Filler chemical nature Filler content types and dispersion Aspect ratios and sizes Polymer physical structure Polymer particle interactions and interface Temperature Processing techniques used in the fabrication of composite laminates.

when defects are presented in the structure, as illustrated in Fig. 3.23. They suggested that any phonon scattering caused by defects leads scattered transfer of waves through the crystal.

3.4

Environmental effects (water absorption) and their influence in different properties

Moisture ingress in composites creates negative influences. Long-term exposure in harsh environments (high temperatures and humidity) during their service life, for example, can lead to a significant deterioration on the properties, especially for natural fibre-reinforced composites and biocomposites. Natural fibre-reinforced composites, due to inherent affinity to moisture, degrade and lose their structural integrity when they are exposed to humidity and extreme weather conditions. The extreme hygrothermal environments (conditions such as temperature, humidity), corrosion and UV radiation weaken the fibre matrix interface and severely influence the mechanical, physical and thermal properties. Moreover, at the elevated temperatures, the reduction in mechanical properties accelerates. In the process, moisture first attacks in the matrix and then reaches the fibres, which then leads to a weak fibre matrix interface and degrades the mechanical properties (Dhakal et al., 2007a; Akil et al., 2009). Enough points have been already made in the earlier sections that there has been a significant uptake in the application of natural fibre-reinforced composites for nonstructural applications due to their many attractive attributes. Their semi-structural

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and structural supplication largely depends on how their mechanical properties and damage behaviour are accurately evaluated, understood and applied in the design process. Water absorption creates heterogeneous structures in composite materials, which not only influences the mechanical properties but effects glass transition temperature, thermal conductivity and storage modulus as a result of damaged fibre matrix interfaces. It is, therefore, the study of moisture absorption and its effects on various properties of natural fibre composites that have attracted significant attention from academia and industry, especially in the last decade. The following sections highlight the hygrothermal behaviour and its influences on the properties of natural fibre composites.

3.4.1

Moisture diffusion mechanisms in composites

Concerning the natural fibre-reinforced composites, the presence of hydroxyl group and complex chemical compositions and morphological structures, a high percentage of moisture absorption takes place when exposed to moisture and humid conditions. This phenomenon also restricts the compatibility or adhesion with non-polar polymer matrices. The absorption of moisture by a natural fibre composites leads to fibre swelling. The moisture ingress further leads to the degradation of fibre matrix interface. When the fibre matrix interface is weakened, the load transfer capability from the matrix to the reinforcing fibres is reduced, and the overall mechanical properties are significantly compromised. According to (Dhakal et al., 2007a) water is absorbed along the fibree matrix interface, which can lead to a swelling of the fibre or a hydrolytic breakdown of any chemical bonding between the fibre and the matrix. Moisture absorption tests are conducted following different standards. Normally, moisture absorption tests are conducted at room temperature and elevated temperatures with different humidity. It is generally accepted that accelerated moisture absorption tests can reduce the time required for moisture absorption test to reach the saturation moisture uptake. The moisture absorption is measured, using Eq. (3.4), which allows calculating the percentage of water absorption in the polymer composites by measuring the weight difference between the samples immersed in water and the sample in dry condition (Dhakal et al., 2015). Mt ¼

Wt  W0  100 W0

(3.4)

where, Mt is moisture uptake, and W0 and W are the mass of the specimen before and during ageing, respectively. The moisture content versus square root of time then is normally plotted. From the moisture uptake curve (moisture gain versus the square root of time), the rate at which water moves from the surface to the interior of the specimens is described by the water diffusion coefficient. Diffusion coefficient Dx, an important parameter in Fick’s law, is used to determine the rate of water absorption and calculated by using Eq. (3.5), from the initial stage of the diffusion process curve; pffi 2 Mt = t  h Dx ¼ p 4  Mt


(3.5)

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(a)

Fibre swells after moisture absorption

93

(b)

Capillary mechanismwater molecules flow along fibre-matrix interface

Matrix microcrack around swollen fibres Water diffusion through bulk matrix

(c)

(d)

Water soluble substances leach from fibres

Ultimate fibre-matrix debonding

Figure 3.24 Effects of water in the fibre matrix interface (Azwa et al., 2013).

Where, Mt the maximum water uptake of the sample (%), h the thickness of the pffi samples (m) and t the time square root (s). The main reason for natural fibre composites degrading after prolonged moisture absorption is due to their chemical compositions. The structural integrity of cellulose, hemicellulose and pectin, for example, is significantly altered as the result of moisture absorption, especially at the elevated temperatures. Generally, when natural fibre-reinforced composites are exposed to hygrothermal environments water molecules can penetrate the composites by three different mechanisms (Dhakal et al., 2007a; Akil et al., 2014; Espert et al., 2002; Robert et al., 2010): 1. Water molecules ingress through microgaps between polymer chains. 2. Capillary transport process taking place via gaps and flaws at the fibre-matrix interface due to the manufacturing defects or poor wettability resins into fibres. 3. Swelling of fibres (especially in the case of plant fibres such as flax, hemp, jute and kenaf) causing the expansion of the microcracks (stress cracking) formed in the matrix.

The three mechanisms highlighted above are illustrated in Fig. 3.24 where water acts on cellulosic fibre and polymer matrix interfacial bond according to the four following stages, as suggested by Azwa et al. (2013) and (Dhakal et al., 2007a). (a) Micro cracking of the brittle thermosetting resin occurs. (b) As the composite cracks and gets damaged, capillarity and transport via micro cracks become active. (c) The capillarity mechanism involves the flow of water molecules along the fibreematrix interfaces and a process of diffusion through the bulk matrix.

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(d) The water molecules actively attack the interface, resulting in the debonding of the fibre and the matrix leading to premature failure.

It is clear from the above three paths that both the matrix and fibre/matrix interface, and moreover, manufacturing flaws/defects, as well as the morphology of reinforcements, can contribute to moisture ingress in the composite materials. In the case of natural fibre composites, fibres are more susceptible to moisture absorption. With the polar nature of fibres, reinforced in hydrophobic matrices, the fibre-matrix interface and interphase regions are always a weak point where hydrolysis reaction takes place and causes interfacial damage leading overall deterioration of various properties (Azwa et al., 2013; Dhakal et al., 2007a). Moisture absorption-related behaviours, considering the above-described mechanisms, are normally assigned into three main categories: 1. Linear Fickian behaviour: in this moisture absorption, after an initial increase due to weight gain resulting from moisture absorption, gradual equilibrium is reached. 2. Non-Fickian behaviour: after an initial increase due to weight gain resulting from moisture absorption, no equilibrium is achieved. 3. Pseudo-Fickain behaviour: If the moisture absorption behaviour lies in between Fickian and non-Fickian, then often it is termed as Pseudo-Fickain behaviour.

The above mentioned moisture absorption behaviours were described by (Dhakal et al., 2007a) where hemp fibre-reinforced unsaturated polyester composites. The samples were immersed at room temperature (25 C) and elevated temperature (100 C). The implication was that the composite moisture was directly proportional to the fibre volume fraction. The water absorption behaviours of the composites were illustrating Fickian behaviours at room temperature and displaying non-Fickian behaviours at elevated temperatures.

3.4.2

Effects of moisture diffusion the mechanical properties

The effects of moisture absorption on the mechanical properties of natural fibre composites have been extensively studied. There are many studies reporting the effects of moisture absorption on the various mechanical properties of natural fibre composites. The reported work by Dhakal et al., (2007a), for example, on the effects of hemp fibre reinforcement on the mechanical properties (tensile and flexural) of unsaturated polyester composites extensively presented the moisture absorption and its influence on the mechanical properties. Moisture absorption at room and elevated temperatures had a significant effect on the tensile and flexural properties. Their findings highlighted that the moisture absorption was directly proportional to fibre volume fraction. At a higher fibre volume fraction of hemp fibre, the percentage of moisture absorption was increased. It was correlated that a high percentage of moisture absorption led to a significant reduction in tensile and flexural properties, which was attributed to a weak fibre matrix interface due to moisture absorption. Their report further highlighted that the rate of moisture ingress (diffusion coefficient) increased gradually with higher hemp fibre contents, which was attributed to the cellulose contents of the hemp fibre (Bismarck et al., 2002; Dhakal et al., 2007a). In their work, it was reported that strain to failure (tensile and flexural) was increased with imposture absorption. Furthermore,

Lightweight composites, important properties and applications

95

15,0

12,5

90 80

Weight gain (%)

10,0 70 7,5

Weight gain measurements Tensile modulus measurements

60 50

5,0

40

Decrease of tensile modulus (%)

100

2,5 30 0,0 0,0

2,5

5,0

7,5

10,0

12,5

15,0

17,5

20 20,0

√t (h)

Figure 3.25 Effects of water absorption on the tensile modulus (Habibi et al., 2019).

their work highlighted that the temperature has a substantial influence on the loadbearing capability. At high temperature, the tensile and flexural stiffness and strength were reduced compared to room temperature. Similarly, the work by Habibi et al. (2019), as presented in Fig. 3.25, shows a significant decrease on the mechanical properties at three different temperatures (room, 50 and 75 ). Their report suggests that the tensile strength of room temperature immersed specimen was decreased by 15% in comparison to dry specimens. Similar to the tensile strength, the tensile modulus decreased significantly. The interesting observation is that the tensile strain to failure was significantly increased with moisture absorption. The decrease in strength and modulus and an increase in strain (approximately by 205%) is attributed to a weak fibre matrix interface due to moisture ingress and induced plasticisation. Compared to dry specimens, the moisture immersed specimens at 50 C deceased the tensile modulus and strength by 35% and 45%, respectively. An increase in temperature to 75 C, leads to a reduction of modulus and strength by 57% and 53%, respectively. Additionally, Akil et al. (2009) studied the moisture absorption behaviour of pultruded jute fibre-reinforced unsaturated polyester composites at three different environments: distilled water, seawater and acidic solution. It was observed that in all three environments, flexural properties were decreased with increased moisture absorption (Fig. 3.26). The reason attributed for such behaviour was moisture absorption contributing to weak fibre matrix interface. Additionally, when plant fibres such as jute are exposed to water, it swells, and as a result, microcracks are formed in brittle matrix such as UPE, which creates a path for transport of water through the fibre matrix interface. Their work further suggested that strain to failure increased with an increase in moisture content. This behaviour was attributed to the cellulose contents of the jute

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(a) 400

Flexural strength, MPa

350 300 250

Standard

200

1st day

150

1st week 2nd week

100

3rd week

50 0 Distilled water

Sea water

Acidic solution

Environmental condition 100 90 80

Maximum flexural strain, ×10

–3

mm/mm

(b)

70

Standard

60 50

1st day

40

1st week

30

2nd week

20

3rd week

10 0 Distilled water

Sea water

Acidic solution

Environmental condition

(c)

4

Flexural modulus, MPa

3 3 Standard

2

1st day 2

1st week 2nd week

1

3rd week

1 0

Distilled water

Sea water

Acidic solution

Environmental condition

Figure 3.26 Effects of moisture absorption on (a) flexural strength, (b) maximum flexural strain and (c) flexural modulus for pultruded jute fibre-reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) seawater and (c) acidic solution (Akil et al., 2009).

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Flexural strength (MPa)

(a) 140

121.1

120

80

98.1

89.9

100 61.4

64.8 51.0

60 40 20 0

(b) 120 Flexural modulus (GPa)

Unconditioned Conditioned

97

100

Type ,

Type ,,, 107.1

Unconditioned Conditioned

87.3 76.3

80 60

50.6 37.9

40

Type ,,

31.1

20 0

Type ,

Type ,,

Type ,,,

Figure 3.27 Effects of hygrothermal exposure on the flexural properties of flax/PP and flax/ carbon/PP hybrid composites. Type I represents flax/pp composites. Type II represents flax/ carbon hybrid composites with flax ply at the surface. Type III represents flax/carbon hybrid composites with carbon ply at the surface (Cheng et al., 2020).

fibre. With moisture, plasticisation gives rise to plastic deformation, as well as cellulose degradation, and makes jute fibre more flexible, which contributes to an increase in failure strain as a result of moisture absorption. The flexural properties of flax and flax/carbon hybrid composites reported by Cheng et al. (2020) suggested that as for the tensile properties, the moisture ingress influenced the flexural properties of natural flax fibre composites. Fig. 3.27 illustrates the comparison of flax and flax PP composites and their hybrids. It is evident from the results that flexural strength and modulus decreased significantly due to moisture ingress leading to plasticisation of flax fibre, as well as PP matrix. A similar observation was reported by Paturel and Dhakal (2020). In their work, the flax and flax/glass hybrid composites were investigated in terms of moisture absorption (Fig. 3.28) and its influence on the low-velocity impact damage characteristics. The flax/vinyl ester composites and their hybrids were immersed at two different temperatures (room and 70 C). It was observed that flax hybrid composites absorbed far less moisture than flax/VE composites (Fig. 3.28). The sample at elevated

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G6-RT G6-HT F6-RT F6-HT GF4G-RT GF4G-HT

4.5

Weight gain (%)

4 3.5 3 2.5 2 1.5 1 0.5 0

0

5

10

15

20

25

30

Time1/2 (h)

Figure 3.28 Moisture absorption versus square root of time at room temperature and high temperature (Paturel and Dhakal, 2020).

(a)

(b)

(c)

(d)

5 days

10 days

30 days

Figure 3.29 Surfaces of single flax fibres: (a) unaged flax fibre, reference; (bed) aged flax fibres (Cheng et al., 2020).

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99

temperatures reached to saturation in a short period of time compared to room temperature moisture immersed samples. Natural fibre absorbs more moisture from the surrounding environment than glass fibres. In the short-term moisture absorption, some composites can recover their mechanical properties upon the drying of the composites. However, if the samples are immersed for a prolonged period, the properties are irreversible (Cheng et al., 2020). Fig. 3.29 depicts the SEM images of flax fibre at dry and water immersed conditions for various periods. The surfaces of flax fibres are damaged as the immersion period is increased. This further promotes moisture absorption and leads to deterioration in the mechanical properties of composites.

3.5

3.5.1

Numerical modelling of mechanical properties and damage behaviour of natural fibre-reinforced biobased composites Background

Composite structures and their related properties and damage mechanisms are traditionally analysed by using the macroscopic/microscopic approaches. The failure modes of composite materials can be measured by various techniques, the main being experimental testing, finite element analysis (FEA) and analytical methods. Numerical modelling is extensively used to analyse composite structural damages. Finite Element Method (FEM) is a popular technique used to predict the damage behaviour of composite materials. Moreover, in recent years, FEM has been used in modelling of natural fibres, as well as natural fibre-reinforced composites (Xiong et al., 2018). There are many reported works on the prediction of bulk properties of composite materials. Among the analytical models, Hashin and Shtrikman (1963), Voigt-Reuss type bounds Voigt (1889), Mori and Tanaka (1973) and Rule of mixtures (ROM) are reliable models used to predict the bulk properties (strength and modulus) of composite materials as an analytical method. Although these analytical methods are attractive, in recent years, due to the development of fast computers, the analytical methods are replaced with numerical modelling. The modelling of composite materials, especially natural fibre composites, is a difficult task as it involves heterogeneous components. Upon loading, composite materials undergo many possible damage mechanisms before the ultimate failure of the material.

3.5.2

Predicting mechanical and damage behaviour of natural fibres and composites

Over the last decade, sustainable composite materials such as natural fibre-reinforced biobased composites are becoming an increasingly attractive alternative for lightweight applications covering important industrial sectors such as automotive, marine and construction. The full exploitation of these composites largely depends on how accurately one can predict their bulk properties and damage behaviours.

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Different theoretical models to predict the mechanical behaviour of natural fibre composites have gained significant attention in recent years. The benefits of using such models are included but not limited to: • Cost effective as experimental studies take times and require materials and equipment • It is also easy to use and can predict long term behaviour of materials

Numerical modelling can be carried out using various methods such as finite difference approach (FDA), boundary element approach (BEA) or finite element analysis (FEA). In general, FEA is widely used to study the mechanical behaviour of composite materials compared to FDA and BEA. In FDA, the body is separated in equal element sizes. The body is discretised into equal and square or rectangular elements grid, and hence, this approach is not applicable in curve surfaces and model with complex geometry. It is also not applicable in cases where stress concentrations are varied. The BEA consists of separating the only boundary into different numbers of elements. Line elements in 2D or surface elements in 3D modelling are used. The discretisation of boundary is carried out into a number of finite elements, where loading is applied, and shape functions are used to calculate the unknown for each element. Finite element analysis (FEA) is a numerical technique, which helps to find approximate solutions to mathematical models that involves partial differential equations. This is one of the most effective analytical tools in the context of dynamic loading. In FEA, the body is discretised into a finite number of elements. Different shapes of elements can be utilised based on the problem. Different shape functions are utilised to solve the problems, and results are re-assembled to achieve the solution for the whole problem (Becker and Karamanlidis, 2004; Liu et al., 2015). There are various commercial FEA software available. The most commonly used include, but are not limited to, ANSYS, ABAQUS and LS-DYNA and Multiscale Designer. These software provide FE codes with an interface to ensure that the user has an understanding with minimum efforts.

3.5.2.1

Finite element method

In this approach, the problem under consideration is divided into a number of small segments of a simple basic shape, known as “spatial discretisation”, with each of the simpler shapes being known as an “element”, and the whole collection of elements being known as a “mesh”. For each element, the relevant properties of the element are then predicted in a simple way. In the case of structural analysis, the relationship between forces, displacements, strains and finally, stresses are evaluated, following the theory of elasticity. In FEA, the body is discretised into a finite number of elements. Different shapes of elements can be utilised based on the problem. Different shape functions are utilised to solve the problems, and results are re-assembled to achieve the solution for the whole problem.

3.5.2.2

Boundary element method

In this approach, only the surface or boundary of the problem under consideration is considered, as the name implies. The boundary is divided into a number of small segments over which the transformed governing differential equations, in the form

Lightweight composites, important properties and applications

(a)

(b)

25

30

Experiment, long fiber

Experiment, long fiber

FEA, 1000Pm fiber

25

20 15 FEA, 167 Pm fiber

10

Experiment, short fiber

5 0 0.00

Stress (MPa)

30

Stress (MPa)

101

FEA, 1000Pm fiber

20 15 10

FEA, 167 Pm fiber Experiment, short fiber

5

0.01

0.02

0.03

Strain

0 0.00

0.01

0.02

0.03

Strain

Figure 3.30 Stress versus strain traces of non-aligned fibre composites (a) without and (b) with 10% microcells (Kern et al., 2016).

of integral identities, are numerically integrated. As in the finite element analysis, provided that the boundary conditions are satisfied, a system of linear algebraic equations emerges, for which a unique solution can be obtained.

3.5.2.3

Finite difference method

In this approach, the body is separated into equal element sizes. The body is discretised into equal and square or rectangular elements grid, and hence, this approach is not applicable in curve surfaces and models with complex geometry. It is also not applicable in cases where stress concentrations are varied. For composite materials to be used in semi-structural and structural applications, predicting their mechanical performance using numerical modelling becomes important. There are several models established for glass and carbon fibre composites. However, not many developed for natural fibre-reinforced biobased composites. The development of an accurate prediction model is difficult due to the following factors: • • • • •

Their natural variability (non-uniform fibre diameter) Their variable microfibril angles Their non-straight shape and non-straight alignment Their complex architectures (a large portion of the lumen) Some natural fibres such as bast fibres having a hollow structure (lumen) needing more complex mesh to analyse

While considering the above factors, the reported work by Kern et al., have presented aligned and non-aligned fibre models. Their work highlights that the net stress versus strain traces were analysed from the FEA models and then were compared with the experimental results. It has shown a good agreement between FEA and experimental results which is illustrated in Fig. 3.30. They also mentioned that FEA model underpredicted the effect of fibre length (Kern et al., 2016).

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Moreover, natural fibre-reinforced composites and biocomposites are a relatively new class of materials. When new materials are developed, it is important that these material properties can be analysed using certain design criteria and materials parameters. Due to these variations, properties and damage prediction of natural fibre composites are largely carried out by using experimental studies rather than numerical modelling. The designers consider some of the drawbacks of natural fibre composites such as quality variations, susceptibility to moisture absorption, quality variations and microbial growth as weaknesses and disadvantages. These drawbacks are not within the boundaries of the most commonly available software, which were mainly designed for the testing and modelling of classical conventional composite materials. Some researchers have developed a numerical model based on the finite elements method to predict the micro and macro mechanical behaviours of composite materials. The experimental and numerical comparison made for banana fibre-reinforced PP composites by Monzon et al. (2019) suggests that FEA model can be used for natural fibre composites but requires software with the capability of full meshing covering fine filaments. They suggested Multiscale Designer software capable of single unit cell simulation with matrix so that micro-mechanic level analysis can be carried out. In their work, experimental results were compared with the numerical modelling with accepted error in comparison to experimental results. The use of numerical methods provides several advantages compared to experimental studies. The main advantages being time and cost. For experimental studies, very often, expensive equipment is required, which adds cost. In numerical modelling, mechanical behaviour can be predicted by using various software using material characteristics such as strength, modules and fibre diameters. However, to model and predict the accurate mechanical behaviour of composites, numerical results are important to correlate with experimental studies. In other words, the numerical models are important to validate with the experimental results. Due to the heterogeneous behaviour of composites, the modelling of failure behaviour and mechanisms is always challenging. Therefore, using some simple models, a realistic behaviour of composites can be predicted. These numerical behaviours and values are important while designing composite materials, which, in turn, will help to reduce fully relying on experimental results. Finite element method has been widely applied in modelling the mechanical behaviour of natural fibres, and natural fibre-reinforced composites. One of the popular techniques used for predicting the mechanical behaviour unidirectional composites is to produce a micromechanical model by considering representative volume (RVE), a constitutive description of (Sun and Vaidya, 1996): • the reinforcing fibres and their packing sequence and the polymer matrices, • fibre matrix interface characterisation, • and appropriate boundary conditions on the RVE.

As the performance of biobased composites is significantly influenced by these parameters, so they do for the mechanical (static and dynamic) behaviours prediction of biobased composites using a certain model.

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103

25 Flax/pp composites

Moisture (%)

20

15 Flax/carbon hybrid composites 10

64 hrs

Type I - experiments Type I - fick’s model Type II - experiments Type II - fick’s model Type III - experiments Type III - fick’s model

5

0 0

2

4

6

8

10

12

14

16

18

Square root (time/h)

Figure 3.31 Moisture absorption behaviour of flax/pp composites and flax carbon hybrid composites (Cheng et al., 2020).

3.5.3

The prediction of static mechanical properties of composites using FEA

The prediction of static mechanical properties of composites using different theoretical models was carried out by Dayo et al. (2017). Fig. 3.31 shows experimental and Series or inverse rule of mixtures (IROM), Halpin-Tsai and Nielsen models used to predict the Young’s modulus of treated hemp fibres-reinforced polybenzoxazine composites. Their work reported that some models were closer to experimental than others. They suggested from their work that the best estimation of Young’s modulus was Nielsen and Halpin-Tsai. More so, inputting the right material parameters is key to get an accurate FEA result. If there are wrong parameters introduced, the prediction can completely go wrong, and the system safety can be compromised. Therefore, it is normal practice to validate FE simulation via analytical and experimental results. Therefore, in order to validate the accuracy and efficiency of FEA, experimental tests are normally conducted. One of the approaches, which has been implemented in recent years to reduce the risk of moisture absorption on the degradation of mechanical properties is the hybridisation technique. Both experimental data and theoretical calculations have been used to predict the moisture absorption curves. Inputting the right material parameters is key to get an accurate FEA result. If there are wrong parameters introduced, the prediction can completely go wrong, and the system safety can be compromised. Therefore, it is normal practice to validate FE simulation via analytical and experimental results. Therefore, in order to validate the accuracy and efficiency of FEA, experimental tests are normally conducted. In addition, Bauroni and Dhakal (2019) investigated the mechanical behaviours of flax and flax fibre-hybridised glass fibre composites by using a model. Their results showed that mechanical properties, especially low-velocity impact properties largely

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depend on fibre volume fraction, fibre matrix interface, properties of fibres and matrix. Their work predicted the experimental and numerical results of the low-velocity impact on flax and its hybrid composites (Fig. 3.32). The experimental results were shown in good agreement with the numerical modelling for impacted composites.

3.6

Applications of lightweight natural fibre composites

The functional requirements of a component dictate the properties. The applications of biobased composites, for example, depends on several factors. For some applications, wear and moisture resistance behaviours are important. For other applications, thermal stability is more important. For example, under-hood automotive parts, impact resistance and heat deflection temperature with reduced flammability properties are important. Over the past decades, biobased composites have been extensively used in various industry sectors such as construction, transport, marine and sports due to their excellent specific properties, corrosion resistance and low processing energy requirements, among others. Among the various sectors, transport, specifically these composites are predominantly used in automotive industries mainly for reinforcement of door panels, passenger rear decks, pillars and boot linings. In addition to the automotive industries, the aerospace and construction and low-cost building industries are investigating the possibility of using natural fibres as a reinforcing material as alternatives to conventional glass fibres.

3.6.1

Automotive application (road vehicles and land transport)

Body panels, engine components- rocker covers, whole car bodies in some cases. Performance road cars utilise carbon fibre composite for both chassis, body and engine components such as rocker covers and air intakes. Due to their lightweight,

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Figure 3.33 Production of door frame using hemp fibre as reinforcement (Peças et al., 2018).

renewability and positive environmental benefits, biocomposites have gain popularity in the automotive industry. According to a report published by ECCC (2016a; WHO), the transport sector contributes the highest greenhouse gases (GHGs) in the World. In the current scenario, there is a significant demand and expectation that the transport sector considers environmental aspects as a high priority along with other aspects such as cost and passenger comfort. The GHGs are related to the burning of fossil fuel or energy consumption in all stages of the vehicle parts: raw materials extraction, production, use and disposal/re-use). Despite several advantages outlined, carbon fibres are expensive materials. It takes a large amount of energy to produce carbon fibre composites. Moreover, the high cost of producing carbon fibre (in terms of cost and energy consumption) makes it difficult to be used in automotive applications when the OEMs in this sector are seeking to bring the overall cost down due to high competition. There are efforts directed in bringing the cost down and using the recycled carbon fibres as composite reinforcements. Nevertheless, these reinforcements take a massive amount of energy for production and manufacturing (Agarwal et al., 2019). If the overall vehicle weight can be reduced, then the fuel consumption can also be reduced. In this context, lightweight, sustainable biobased composites can contribute significantly. Natural fibre composites have been extensively used in interior parts of automotive applications, as depicted in Fig. 3.33. This component was produced using 50% PP and 50% hemp non-woven mat. This indeed provides technological, ecological and economic benefits. Moreover, non-woven hemp/PP-based composites can also have an important impact on damaged properties. The automotive industry in Europe extensively uses short natural fibre-reinforced (hemp, flax, kenaf) composites with thermoplastic and thermoset matrices. These composites are mainly used in non-structural interior applications using mainly non-woven mats, as these composites do not possess high mechanical strength required for semistructural or structural applications (Fig. 3.33). Moreover, many automotive OEMs use

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natural fibre-reinforced composites in trim parts such as door panels, seat cushions, cabin linings and parcel shelves. Reported works suggest that GHGs generated by automotive vehicles counts for more than a quarter of GHGs generated (Pervaiz et al., 2016). It makes every sense that the automotive sector is now at the forefront of using natural fibre-reinforced lightweight composites. Plant fibre-reinforced composites have been widely used by automotive industries in Europe and North America due to their economical, technological and green credentials. The adaption of lightweight materials in transport sector has been a key drive towards weight saving initiatives. The extensively used fibres and semi-products are raw fibres and non-woven mats, and the composites. These composite parts possess moderate mechanical properties, which makes them well qualified for interior parts (Fig. 3.33). "The most environmentally friendly thing you can do for a car that burns gasoline is to make lighter bodies" (Henry Ford).

When Henry Ford said this, he was trying to move using steel parts to biocomposites parts, but the aim was not achieved due to World War II. The weight reduction in the automotive industry is still an on-going process as with most areas of transport through all the parts of a vehicle design. Other areas of performance can be benefited from a lighter vehicle, and from the manufacturer’s side it can also reduce the production cost, which can then help to decrease the fuel consumption and lead to overall GHGs emissions when the lighter vehicle could be made. Also, after the end-of-life of a vehicle, the parts are expected to be re-used or recovered. For this, biobased composites possess very attractive attributes, which has attracted the attention of OEMs in automotive sector. These materials can be recycled as new reinforcements, which still can hold many important properties (Mohanty et al.,

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Figure 3.35 Natural plant fibre application in the current E-Class Mercedes-Benz (Akampumuza et al., 2017).

2018). Fig. 3.34 shows the emphasis given by the North American automotive OEMs of lighting drive. For example, hemp fibre-reinforced polyester composites were used in the “Lotus Eco Elise” concept car, where hand lay-up and vacuum bagging techniques were used to form the parts. It was reported that the use of hemp fibre replacing glass fibres helped to reduce the Eco Elise’s weight by 32 kg resulting in a higher fuel economy and a better performance in comparison to the standard Elise S. Majority of the car manufacturers in Germany use natural fibre composites in various parts. Fig. 3.35 depicts the use of plant fibre parts used in the current E-Class Mercedes-Benz. Daimler/Chrysler has also manufactured door trim panels using a biocomposite plastic comprising 25% hemp, 25% kenaf and polypropylene. Due to the high sensitivity of plant fibres towards thermal stability, low flammability characteristics and the susceptibility to moisture absorption, the natural fibrereinforced composites are mainly used in interior parts. Besides their use in trim parts, plant fibres are also used for thermo-acoustic insulation. Another well-established field of application is the use of coconut fibres bonded with natural latex for seat cushions. For this application, the ability of plant fibres to absorb a large amount of humidity leads to an increased comfort that can not be reached with synthetic materials. An important step towards higher performance applications was achieved with the door panels of the Mercedes-Benz E-Class. The wood fibre materials previously used for the door panels were replaced by a plant fibre-reinforced material consisting of a flax/sisal fibre mat embedded in an epoxy resin matrix. A remarkable weight reduction

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Figure 3.36 Applications of natural fibre composites by various original equipment manufacturers (Akampumuza et al., 2017).

of about 20% was attained, and the mechanical properties, important for passenger protection in the event of an accident, were improved. Recently, plant fibres have also been used in exterior components such as the engine and the transmission covers (Adekomaya, 2020). As highlighted in the earlier sections, due to the high specific strength and stiffness and the excellent damping properties, have promoted the use of flax fibre composites in semi-structural, as well as structural components, by replacing synthetic E-glass fibres in many in automotive parts. Motorsports is one of the popular application areas of lightweight composites. Carbon fibre reinforced composites (CFRP) and glass-fibre reinforced composites (GFRP) as outstanding lightweight materials which are used in many components including wings, aerofoils, side panels, nose cones, air boxes, and chassis components. Ranging from formula students projects to Formula One (F1) standard components, race teams use composites for the bodywork and aero packages of the racing cars. Motorsport teams are now using composites for the structural elements of the cars, such as carbon fibre monocoques, tubular structures and suspension components for potential reduction of weight and CO2 emissions Pervaiz et al., (2016). Both thermoplastic- and thermoset-based composites are used in automotive applications. These are due to advanced manufacturing techniques being proven in the aerospace industry. Fig. 3.36 depicts the advantages of natural fibre composites for

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Figure 3.37 Vibration damping related properties of different natural non-woven fibre-reinforced PP composites (Hadiji et al., 2020).

automotive lightweight applications, which is significantly lighter than conventional chassis with improved stiffness. Fig. 3.37 shows the vibration damping properties of non-woven natural fibrereinforced PP composites in comparison with glass/PP composites. The key parameters used to compare are important factors towards the overall damping analysis of natural fibre composites. It is evident from Fig. 3.37 that non-woven natural fibre composites exhibited the highest damping ratio compared to glass fibre-reinforced PP composites. The key properties compared were: bending modulus, fibre volume fraction, composite density, loss factor and porosity content. Due to these unique damping parameters of important natural fibre-reinforced composites, these materials can serve as alternative lightweight composites in the automotive industry (Hadiji et al., 2020).

3.6.2

Aerospace and related application

Carbon fibre and glass fibre-reinforced composites were developed with the aim of its usage in the aerospace sector. Carbon fibre-reinforced composites with the combination of properties such as lower density, high strength to weight ration attracts for both structural and non-structural applications in civil and military aircraft (Soutis, 2005). The Dreamliner aircraft (Boeing 787), for example, used 50% by weight continuous carbon fibre composites in order to reduce the overall weight of the aircraft for enhanced fuel economy. These applications demand excellent performance in terms

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of high strength and stiffness, high operating temperatures, high thermal conductivity, among others. For natural fibre-reinforced composites to be used in aerospace applications, it is a long way away. However, if the fire-resistance behaviours of these composites are brought to the acceptable level, then there is a possibility of using these composites in non-load bearing applications in interior parts. With the classic fibrereinforced polymer composites (FRP and GRP), however, there are often considerable problems with respect to re-use or recycling after the end-of-tlife. Moreover, when it comes to the important mechanical properties such as vibration damping and toughness, carbon fibre composites do not necessarily exhibit the highest performance. With excellent damping properties of flax fibres as reinforcements, using these fibres as hybrid materials together with carbon using high-performance manufacturing technology, natural fibre-reinforced hybrid composites have a significant potential to be used in the non-structural applications in the aerospace sector.

3.6.3

Marine applications

Carbon and glass fibre-reinforced thermosets (epoxy, vinyl ester and polyester) composites (GFRP) are extensively used in marine applications due to their costeffectiveness and higher mechanical performance, as well as ease of fabrication (formability), and room temperature curing ability. Due to the lightweight nature of composites, boats can use structures and skins made from composite materials, and they not only benefit from the lightweight properties of composites but the composites can be easily repaired if they become damaged. Due to the cost factors, glass fibrereinforced composites are extensively used in marine industry as the cost of carbon fibres is almost five times than that of glass fibres. Glass fibre-reinforced composite applications include yachts, sailboats, dinghies and lifeboats, due partly to its corrosion resistance, lightweight and resistance to degradation by water. Any materials that require to be applied in marine applications will need to go through harsh environments during their service life. Moisture absorption elated issues such as weak fibre matrix interface, blistering, degradation due to the marine environment are important criteria to be understood. Therefore, lightweight, long-term durability and high strength and stiffness are prerequisite design criteria for composites to be used in marine applications. In recent years, concerns over microplastics generated by marine components and the harm that has caused to the marine lives have been highlighted by many scientific research papers as a serious concern. Therefore, there is a great potential for the use of sustainable, environmentally friendly biodegradable composites. However, due to their vulnerability to harsh environments, inconsistent long-term durability has hindered the use of these composites in marine outdoor applications. However, significant efforts have been made to make these composites withstand harsh environments (glass/carbon/flax hybrid composites for example) as these composite have significant potential to be used as lightweight, sustainable materials in marine applications. In recent years, composite sandwich structures have been used in the marine industry with the main aim being the forming of lightweight structures with high strength and stiffness. In sandwich structures, two skins are placed topsides usually, glass,

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carbon and aramid fibres and thick foam cores (usually polyvinyl chloride (PVC), polystyrene and honeycomb) are placed in the middle. The key benefits of using sandwich structures in marine components are cost and weight saving. After many years of using non-renewable conventional reinforced composites, the marine sector is in the search for more sustainable composites with environment and end-of-life as a focussed agenda. Going forwards for using a lightweighting approach in the marine sector requires a new shift, using natural fibre-reinforced biobased materials. For this, the matrices need to go more towards biodegradable/renewable from conventional ones. Similarly, with regard to reinforcements, the need it to move from glass fibres to more sustainable such as flax, hemp, kenaf and date palm fibres, for example. With this new approach, sustainability aspirations can be realised; however, the shortcomings of sustainable renewables mentioned need to be overcome.

3.6.4

The building construction application

Carbon fibre and glass fibre-reinforced composites have also been used extensively in construction industries. The applications include composite cables as an anchor for an earthquake-resistant building, bridges, precast concrete. There are numerous other applications where carbon and glass fibres can be used. They include turbine blades, gearbox, flywheel energy storage, conductive reinforcements for fuel cell and solar panel supports. Building construction materials consume about 40% of the World’s global energy, 25% of the global water, 40% of the global resources this is due to the majority of materials used come from conventional materials such as steel, cement and bricks where these conventional materials consume a significant amount of electrical and thermal energy (Asdrubali et al., 2015). In recent years, there have been efforts to replace conventional building materials with lightweight composite materials. Moreover, efforts are being made even to replace non-renewable carbon and glass fibres by natural fibres such as hemp, jute, kenaf, coir, flax and date palm fibres (Mark and Fam, 2019; Yorseng et al., 2020). These materials offer tremendous benefits in building materials, including they being renewable, lower cost, biodegradable and good mechanical and acoustic properties. Additionally, there are several reported works that highlighted the use of solid wastes generated from agricultural and industrial production and are used as reinforcements in construction and building materials. These initiatives not only provide the required mechanical properties but also provide economic and environmental benefits. Building and construction industries are aiming to use low carbon materials. This concept aims to correlate the amount of energy used (embodied energy), raw materials and energy intensity (Yan et al., 2016). While considering low carbon materials in building and construction sector, many literatures indicate that amongst the more common construction materials considered, the lowest energy option or low carbon material is timber, while the highest or high carbon material is steel, with concrete in between. Furthermore, if sustainable low

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carbon materials are used in buildings, it can significantly reduce the overall energy consumption. There is a strong view that the replacement of high-energy intensive processes and materials can be replaced by the use of low carbon materials, with better design and recyclable materials (Yan et al., 2016). As detailed by Faruk et al. (2012), with businesses recently having an increasing environmental awareness and with a view to the ability to recycle end-of-life products, the need for the use of natural fibres over synthetic fibres is becoming more prevalent. The reduced cost and increased performance of NFRPCs go hand in hand with the drive to increase a company’s green credentials in inciting more research into the uses and capabilities of natural fibre composites. A significant encouragement that can be taken from the above sections is that the specific properties of hemp and flax fibres are comparable to glass fibre especially. This indicates that the specific mechanical properties of hemp fibres are approaching the properties of glass fibres. Because of the density of hemp and flax fibres is lower than that of glass fibre, the reinforcement of hemp and flax to the polymeric matrix reduces the density of the composite as a whole. These properties make hemp fibres attractive environmentally friendly reinforcing lightweighting reinforcements in composites (Gurunathan et al., 2015). It is well established that the key factor in the development of today’s modern buildings has been the use of structural steels due to their unique properties. However, the development of structural composites has demonstrated that advanced composites can be viable alternatives to steel due to advanced manufacturing processes and understanding of their damage mechanisms. This is due in part to improved material performance, but more particularly, the development of new linear processing techniques. Continuous pultrusion processing methods produce low cost sections, which have high specific stiffness, constant linear properties and good environmental stability, making them suitable for primary construction components. However, a principal limitation in the use of composites in mainstream construction has been the high environmental cost in the manufacture of energy incentive synthetic fibres, and the problems associated with their subsequent disposal at the end of their life (Ahmad et al., 2014). Natural fibre-reinforced (rice husk/phenolic) composites have been successfully tested and used to make temporary shelters. The rice husk particleboards are manufactured in various densities, thickness, types and grades to suit a wide range of applications (Arjmandi et al., 2017). The important market for the flax and hemp short fibres is their use in ecological thermal insulation materials. In many countries, this market is growing faster than the total market for insulation materials (Hussain et al., 2019; Saheb and Jog, 1999).

3.6.5

Other applications

Carbon fibre-reinforced composites have been used in sporting goods due to their lightweight and strength-to-weight ratio. Fishing rods, tennis racquets, race bicycles and prosthetics. Due to their excellent strength and damping properties, flax fibres have been used to make bicycle frames by hybridisation with carbon fibres. As indicated in the earlier

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sections, despite having high strength and stiffness, carbon fibres are expensive materials, and they lack impact toughness (energy absorbing capabilities when a sudden load is applied) required for a bicycle frame (Amiri et al., 2018). Their work reported that by hybridising flax fibres into carbon, their frame exhibited similar strength and stiffness as commercially available carbon, titanium and aluminium frames while exhibiting superior damping properties. These outstanding properties were achieved by maintaining 40 wt.% bio-content. Fig. 3.38 shows the vibration damping of different materials where bidirectional flax outperforms the other important materials. With these unique properties, natural fibre-reinforced flax composites can also be sued in making fishing rods and other sporting equipment.

3.7

Conclusions

This chapter overviewed the importance of lightweight composites reinforced with sustainable and renewable plant-based fibres, which meet the expectation of lightweighting aspiration and drives, as well as providing several benefits, including performance, environmental and cost efficiency. While considering important mechanical properties, it was evident that the mechanical properties of natural fibre composites were comparable to the properties of conventional glass fibre-reinforced composites, mainly, the specific properties. A careful consideration is required when using lightweight composites in harsh environments, such as at humid, high temperature applications as these composites are susceptible to these harsh service conditions. The numerical modelling of natural fibres composites requires special attention as the composites have inherent property variations. With various surface modifications and optimised manufacturing processes already developed, the drawbacks of the composites can be eradicated. Therefore, the key application sectors, such as automotive, construction, marine and aerospace, can use the lightweight composites for making various environmentally friendly components.

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Jeannin, T., Gabrion, X., Ramasso, E., Placet, V., 2019. About the fatigue endurance of unidirectional flax-epoxy composite laminates. Compos. B Eng. 165, 690e701. Kalaprasad, G., Pradeep, P., Mathew, G., Pavithran, C., Thomas, S., 2000. Thermal conductivity and thermal diffusivity analyses of low-density polyethylene composites reinforced with sisal, glass and intimately mixed sisal/glass fibres. Compos. Sci. Technol. 60 (16), 2967e2977. Kandola, B.K., Nazare, S., 2007. An overview of flammability and fire retardancy of fibre reinforced unsaturated polyester composites. In: Proceedings of Fibre Reinforced Composites Conference, Port Elizabeth, South Africae. Kenned, J.J., Sankaranarayanasamy, K., Binoj, J.S., Chelliah, S.K., 2020. Thermo-mechanical and morphological characterization of needle punched non-woven banana fiber reinforced polymer composites. Compos. Sci. Technol. 185, 1e13. Kern, W.T., Kim, W., Argento, A., Lee, E.C., Mielewski, D.F., 2016. Finite element analysis and microscopy of natural fiber composites containing microcellular voids. Mater. Des. 106, 285e294. Khomenko, A., Karpenko, O., Koricho, E., Haq, M., Cloud, G., Udpa, L., 2017. Quantitative comparison of optical transmission scanning with conventional techniques for NDE of impact damage in GFRP composites. Compos. B Eng. 123, 92e104. Kim, H., Welch, D.A., Kedward, K.T., 2003. Experimental investigation of high velocity ice impacts on woven carbon/epoxy composite panels. Compos. Appl. Sci. Manuf. 34 (1), 25e41. Kim, N.K., Mao, N., Lin, R., Bhattacharyya, D., van Loosdrecht, M.C.M., Lin, Y., 2020. Flame retardant property of flax fabrics coated by extracellular polymeric substances recovered from both activated sludge and aerobic granular sludge. Water Res. 170, 1e8. Kim, N.K., Lin, R.J.T., Bhattacharyya, D., 2015. Effects of wool fibres, ammonium polyphosphate and polymer viscosity on the flammability and mechanical performance of PP/ wool composites. Polym. Degrad. Stabil. 119, 167e177. Kozlowski, M., Iwanczuk, A., Frackowiak, S., Szczurek, T., 2007. Biocomposites of functional properties. In: International Sustainable Materials, Polymers & Composites Conference, Warwick. Liang, S., Gning, P.B., Guillaumat, L., 2012. A comparative study of fatigue behaviour of flax/ epoxy and glass/epoxy composites. Compos. Sci. Technol. 72 (5), 535e543. Liu, J., Zhang, Z.Q., Zhang, G.Y., 2015. A smoothed finite element method (S-FEM) for largedeformation elastoplastic analysis. Int. J. Comput. Methods 12 (4), 1e26. Madsen, B., 2004. Properties of Plant Fibre Yarn Polymer Composites. PhD Thesis. Department of Civil Engineering, Technical University of Denmark, Denmark. Series R: No 082. Mahboob, Z., Bougherara, H., 2018. Fatigue of flax-epoxy and other plant fibre composites: critical review and analysis. Compos. Appl. Sci. Manuf. 109, 440e462. Maqsood, M., Langensiepen, F., Seide, G., 2020. Investigation of melt spinnability of plasticized polylactic acid biocomposites-containing intumescent flame retardant. J. Therm. Anal. Calorim. 139, 305e318. Mark, K., Fam, A., 2019. Performance of flax-FRP sandwich panels exposed to different ambient temperatures. Construct. Build. Mater. 219, 121e130. Methacanon, P., Weerawatsophon, U., Sumransin, N., Prahsarn, C., Bergado, D.T., 2010. Properties and potential application of the selected natural fibers as limited life geotextiles. Carbohydr. Polym. 82, 1090e1096. Ming, Y., Chi, H., Blaser, R., Xu, C., Yang, J., Veenstra, M., et al., 2015. Anisotropic thermal transport in MOF-5 com-posites. Int. J. Heat Transf. 82, 250e258.

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Design, manufacturing processes and their effects on biocomposite properties

4

4.1 Introduction and context In an attempt to improve the inherent properties of the biocomposite materials, different designs and manufacturing processes have been adopted. These design and manufacturing processes involve the selection of natural fibres or filler (reinforcements) and matrices used, consideration of sizes/part design, processing: fibre surface treatments, the stacking sequences, fibre orientation, compaction methods and curing techniques, among others. Importantly, all these processes determine the properties (mechanical, physical, electrical and thermal) of a particular biocomposite. Therefore, the intended enhancement of biocomposite materials begins from the design stage to the manufacturing phase, as all these stages are carefully monitored to avoid defects and alteration in the expected behaviours. Also, the functionality of biocomposite materials in engineering applications depends on the products of design and manufacturing processes, whereby the biocomposites have undertaken. The design and manufacturing processes of biocomposites have some challenges today. These include, but are not limited to, increasing applications of biocomposites, unpredicted materials responses (mainly non-linear properties), and availability of limited design data, because there are numerous varieties or species of natural fibres and matrices. In addition, the interactions between hydrophilic natural fibres and hydrophobic matrices, as well as presence of variable lumens (the centred holes) in many natural fibres are critical challenges that are confronting design and manufacturing processes of several biocomposites (Rudin and Choi, 2013). These natural limitations have a tendency of creating voids within the biocomposites. The presence of void, probably during the manufacturing process, has undesirable implications on the total quality of the fabricated products. Hence, various design techniques and manufacturing processes will be extensively considered, as well as their influences on the several properties of some commonly used biocomposite materials within this chapter to capture this concept comprehensively.

4.2 Eco-design and sustainability (design for environment and design for manufacturing) 4.2.1 Eco-design There are numerous advantages from the products manufactured from composite materials. One of these benefits includes environmental friendliness due to their

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sustainable sources, biodegradability, renewability and corrosion resistance, as characterised by the natural or biofibres. Also, composite materials have lower weight when compared with some metals, alloys and synthetic fibre. Therefore, the overall weight of the cars is reduced when natural fibre-reinforced polymer (FRP) composites are used in the car or aeroplane components. These components can be car/aeroplane engine cover, body, seat, dashboards and among other interior parts of the cars/aeroplanes. As the overall weight of the car and aeroplanes are reduced, the fuel consumption will be reduced. Hence, it offers fuel savings throughout the lifetime of cars and aeroplanes. This results in a reduction in the quantity of carbon dioxide (CO2) release to the atmosphere, among other toxic gases from fossil fuels. Consequently, a better and cleaner environment is maintained. Also, natural fibres provide good acoustic insulating properties due to their hollow structure. Therefore, noise pollution can be reduced. In recent years, the use of natural FRP composite has gained much interest as a low cost, environmentally friendly alternative for the more commonly used reinforcing materials, such as glass and carbon fibres. Cost is one of the essential factors that influence the sustainability of a material. Several natural fibres, especially jute, coir and bamboo, are much cheaper (unit price) when compared with some commonly used synthetic fibres, including carbon and E-glass (Fig. 4.1). This consequently reduces the total cost of producing natural FRP composites. There has been increasing pressure from both nationally and internationally users, designers and manufacturers to lessen the environmental damage caused by the use of non-renewable materials by using more sustainable and environmentally friendly materials as reinforcements in the composite manufacturing. This has resulted in many manufacturers searching and adopting preventive measures to seek and minimise the environmental impact of composites after their service life (Dhahak et al., 2013, 2014; Thakur et al., 2014; Rodi et al., 2016).

$12.00

Cost range

Cost per weight ($/Kg)

$10.00

$8.00

$6.00

$4.00

$2.00

$0.00 Carbon E-glass

Abaca Bamboo

Coir

Cotton

Flax

Hemp

Jute

Kenaf

Ramie

Sisal

Figure 4.1 Difference between natural and synthetic fibres in terms of cost per weight (Lotfi et al., 2019).

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Natural fibres, as a main constituent/reinforcement of biocomposites, are abundantly available, sustainable and biodegradable. Natural fibres include, but are not limited to, hemp, jute, sisal, bamboo, kenaf, rice husk and straw, wheat straw, oil palm, henequen, curaua, olive pit/husk, coir, pineapple leaf, ramie, choir, abaca, flax, date palm, banana, pineapple and bagasse (fibrous residue of sugarcane stalk). They have some outstanding mechanical properties. For example, hemp fibres have better tensile strength at break of 550e1110 MPa and tensile modulus of 58e70 GPa when compared with other naturally available plant fibres: date palm, jute, flax, to mention but a few (Bledzki and Gassan, 1998; Dhakal et al., 2007; Mohanty et al., 2002; Bourmaud et al., 2017). Some of the mechanical properties of natural fibres are quite comparable or better than that of synthetic fibres (such as E-glass), as shown in Fig. 4.2 and comprehensively reported by Pickering et al. (2016). These significant properties of natural FRP biocomposites have greatly increased their areas of engineering application, as designs for both environment and manufacturing are carefully considered starting from the initiation journey of design concept. More also, although it is well understood that the processing of natural FRP biocomposites is safer than that of synthetic FRP composites with respect to health and the environment, there are a lot of environmental and sustainability issues still associated with the design and manufacturing processes of biocomposite materials today. These issues are chronologically discussed hereafter.

4.2.2 Sustainability Sustainability is one of the key developmental concepts of the next generation of products (materials) and processes (manufacturing). This gave birth to the advent of sustainable, eco-efficient, biodegradable and environmentally friendly biocomposite materials as a suitable substitute to the synthetic, non-renewable fibre-reinforced polymer composites. Sustainability encompasses land use, energy efficiency, resource availability, soil conversation, biodiversity, environmental impact and impact on social community (Quarshie and Carruthers, 2014). Therefore, the design for biocomposite materials should improve biodiversity, land use and energy efficiency, as

100

E-modulus (GPa)

Specific modulus

80 60 40 20 0 E-glass

Hemp

Flax

Figure 4.2 Comparison of some natural fibres with synthetic (E-glass) fibre (Mohanty et al., 2002).

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well as conserve soil and resources available, reduce impact on both environment and social community, as consumer demands and satisfaction are met through design and manufacture of biocomposite materials. Also, biomass is available annually in a large quantity, nearly 70% of these crops, for energy use (Wool and Sun, 2005). Some are used as energy crops, crop residues and biogas, while the remaining proportion ends up as wastes. Soil quality is maintained with some residues that returned to the land. Some animal manures or beddings are produced from some of these crop residues. Biocomposite materials, with bio-based constituent(s), have been considered as a solution to the fast reducing petroleum supply and growing environmental threat (Mohanty et al., 2002). These natural constituents are readily available all over the world. They are available at a very large quantity and probably, throughout a year in thousands of tones (Lotfi et al., 2019). They are renewable, recyclable and biodegradable. Biocomposites with both bio-fibre(s) and bio-based matrix, commonly called ‘green composites’ are biodegradable; prone to environmental and microbial degradation after disposal, without having an adverse environmental effect. Hence, they are environmentally acceptable and commercially viable for those who are engaging in planting/farming of these natural fibres, as shown in Fig. 4.3(a). Also, some of these bio-based materials (fibres and matrices) are originated from plants. The photosynthesis reaction enables plants to maintain carbon dioxide neutrality, as they respire (take in) carbon dioxide. Therefore, the quantity of carbon dioxide in the atmosphere is reduced. This process is further explained with the aid of Fig. 4.3(b). The solar energy required by the plants is a sustainable energy source.

4.2.3 Design for environment Design for the environment is an approach towards achieving design goals without jeopardising or affecting the environment and human health throughout the stages of the product or process life cycle. It aims at improving product quality and cost, as well as minimising or eliminating all environmental impacts of a product over its life cycle. It involves five different product (biocomposite) life cycle stages: materials, production, distribution, use and recovery, as depicted in Fig. 4.4. All the processes involved in each of these stages are properly designed and monitored to maintain or protect a healthy environment. An environment includes the earth, and both atmosphere and hydrosphere, where plants and animals exist. Also, the selection of non-renewable resources and the release of toxic or inorganic substances to the environment during the five stages of design for the environment must be avoided, especially after use and recovery stages, as illustrated in Fig. 4.4.

4.2.3.1 Materials Basically, composites are desirable products of the combination of reinforcements (fibres/fillers) and binders (matrices), designed and produced for a specific engineering application. Biocomposites are composite materials of either biofibre or bio-based matrix, or both. The main reinforcing elements or materials of biocomposites are from natural sources, as bio-fibres/fillers. Some fibres are originated from plants/vegetables, while others are from either animals or minerals. Examples of fibres under each of the

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125

(a) Renewable

Recyclable

Triggered biodegradable

Environmental acceptability & commercial viability

Sustainable

(b) Moulding

Plastic converters

Biopolymeric materials

Trays, spoons, etc

Bio Biowaste collection

Innovation: polymer production Composting Renewable resources (cellulose, corn, etc.)

2

CO Photosynthesis

Figure 4.3 (a) Sustainability of the bio-based materials for biocomposites and (b) carbon dioxide sequestration (Mohanty et al., 2002).

aforementioned sources have been discussed in the previous chapters. Additional examples of natural sources of biofibres have been reported (Sanjay et al., 2015, 2018; Akil et al., 2011). Furthermore, the matrix can be classified into petrochemicalbased and bio-based. Petrochemical-based matrices are thermosetting and thermoplastic, while examples of bio-based matrices include wheat, gluten, soybean, starch, gelatin, polyhydroxybutyrate (PHB) and polylactic acid (PLA), among others (Lotfi et al., 2019; Sanjay et al., 2015). It is very important to ensure that the depletion of natural resources and deforestation are avoided in searching for biomaterials for biocomposite manufacturing. Biofibres and bio-based matrices are renewable and abundant resources (Table 4.1), most of them are very compactable to produce an improved fully biocomposite material. Also, some are non-hazardous and environmentally friendly with respect to health issues. Some of their wastes are water-based and biodegradable. These factors are very germane when designing biocomposite materials for good environments. This aspect has been extensively discussed in the previous Chapter two.

126

Sustainable Composites for Lightweight Applications Non-renewable resources

Resources

Post-industrial recycling

Materials

Renewable resources

Post-consumer recycling Natural decay

Natural “biological” life cycle Toxics

Recovery

Organics

Deposit

Production Remanufacturing

Product “industrial” life cycle

Distribution

Reuse

Inorganics

Use

Figure 4.4 Biocomposite double life cycles (Ulrich and Eppinger, 2012).

In addition, toxic materials (fibre and matrix) must be eliminated or carefully handled. Care must be taken to avoid toxic raw materials; in process, use and after use (Fig. 4.4). There are some materials that are not originally toxic, but they become harmful after they have reacted with fluids (liquids and gases), heat and ultraviolet rays from sunlight. These potentially harmful materials must be carefully considered during the design of biocomposites. Also, the use of raw materials should be reduced. The design of biocomposite materials to be used as a compartment/part of a system should encourage not too many raw materials and quantity. During materials selection, the use of recovered and recycled materials should be welcomed. This is required to reduce discards, minimise waste, time and energy during the production stage.

4.2.3.2 Production In this stage, there must be a reduction in the use of process materials. Process materials that can be fully recovered and recycled must be specified and adopted during the production phase of the design for the environment. Similar to the material stage, the toxic process materials must be eliminated. Also, processes with high energy efficiency should be selected. The use of clean and renewable (solar, water and wind) sources of energy must be encouraged or preferred during the production stage, rather than fossil fuels. The purpose of these preferences is to keep the working and general environments safe and clean from toxic fumes, gases and hazardous noises. Air and water pollution from industrial emissions and discharges, as well as waste generated during the production of biocomposites must be either well controlled or avoided. For example, processing of natural (hemp) fibres have a very good environmental impact, especially when compared with a synthetic or conventional fibres, such as glass. A comparison of hemp fibre with glass fibre is presented in Table 4.2. During

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127

Table 4.1 Yearly production of various important natural fibres, their origins, species and largest producer countries (Lotfi et al., 2019). Fibre type

Origin

Species

Largest producer countries

World production (103 tons)

Coir

Fruit

Cocos nucífera

India, Vietnam, Sri Lanka

100

Kenaf

Stem

Hibtscus connabtnus

India, Bangladesh, United States

970

Flax

Stem

Linum usitatissimum

Canada, France, Belgium

830

Bamboo

Stem

(>1250 species)

China, India, Indonesia

30,000

Abaca

Leaf

Muso textilis

Philippines, Ecuador, Costa Rica

70

Jute

Stem

Corchorus capsularis

India, Bangladesh

2500

Sisal

Leaf

Agave sisolana

Tanzania, Brazil, Kenya

378

Ramie

Stem

Boehmeria nivea

China, Brazil, Philippines

100

Cotton

Seed

Gossypium sp.

China, India, United States

25,000

Banana

Leaf

Musa indica

Brazil, India

200

Silk

Animal

Silkworms, honeybee

China, India, Europe

202

Wool

Animal

Sheep, alpaca, camel

Australia, New Zealand, China

2000

Hemp

Stem

Cannabis sativa

China, France, Philippines

215

Pineapple

Leaf

Ananas comosus

Philippines, Thailand, Indonesia

74

Agave

Leaf

Agave fourcroydes

Columbia, Cuba, Mexico

56

Kapok

Fruit

Ceiba pentandra

China, India

316

Bagasse

Stem

d

Brazil, India, China

75,000

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Sustainable Composites for Lightweight Applications

Table 4.2 Comparison of environmental impacts during the production of 1 kg of glass and hemp fibres (Shahzad, 2011). Fibres Parameters

Hemp

Glass

Power consumption (MJ)

3.4

48.3

CO2 emission (kg)

0.64

20.4

SOx emission (g)

1.2

8.8

NOx emission (g)

0.95

2.9

BOD (mg)

0.265

1.75

their processing stages, lesser energy is required when compared to the synthetic (carbon and glass) FRP composites. Additionally, the production of controllable scraps and wastes should be reduced. More also, the production technique must be applied to minimise the total volume of materials used. The production process must be well planned in order to avoid rejects and reduce material waste, using clean, highly efficient production processes and few manufacturing steps as much as possible.

4.2.3.3 Distribution After the first two stages (materials and production) of design for the environment have been successfully accomplished, there is need for the biocomposite materials to be transported to the product manufacturing companies, such as automobile, aerospace, telecommunication, power/energy, sports/games or recreation, marine/naval, to mention but a few, where they will be used or processed to products. In another way, biocomposite products are also required to be moved to the market places. Therefore, it is a part of the design for the environment to ensure that the most effective and energy-efficient shipping plan is embraced and emission from transport is reduced. Both air pollution from transportation emissions and waste generation from packaging for distribution must be well controlled or avoided. In addition, harmful and hazardous packaging materials must be eliminated. If it is possible, packing should be eliminated; if not, reuse packaging materials should be encouraged. This should be considered during biocomposite design. The concept of lightweight biocomposite is also a good factor to be considered, with ideas of folding, disassembly or nesting to distribute biocomposite materials for use.

4.2.3.4 Use As a part of the design for the environment, it is of great importance to ensure that the useful biocomposite life is extended. During the use of biocomposite products, discharges of harmful substances should be eliminated and energy consumption should be reduced. For instance, the use of lightweight biocomposite materials in transportation industries to manufacture vehicles and aircraft has significantly reduced the

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amount of fuel consumption. Therefore, it has saved our environment from being attacked with excessive carbon dioxide, sulphur dioxide and other toxic gases that are released from the combustion of fossil fuel and their effects on ozone layers and ecosystem at large. Biocomposite products must be used under intended conditions, under clean and efficient servicing operations. Also, minimal maintenance is required when biocomposite materials are in use. They do not require painting or coating to prevent rusting, because they possess good corrosion and wear resistance properties, unlike metals and some alloys. Biocomposite materials can be reused and recycled, and therefore, biocomposite materials with excellent recovery should be encouraged. This will lead to the final stage of the design for the environment, known as recovery, as subsequently discussed.

4.2.3.5 Recovery The design of biocomposite materials involves natural fibres or bio-based matrix or both that can be recycled continually, without compromise quality and effective performance. These natural materials or constituents of biocomposites can completely return to the earth’s natural cycles to create new natural materials for sustainability. This concept has been fully and earlier explained, using Fig. 4.4. During recovery, incompatible materials must be easily separated. Easy disassembly of biocomposite parts should be considered during the design stage, either by using hands or simple tools. The biocomposite components must be designed to support easy recovery and manufacturing. It is expected that the waste volume for landfill deposits, as well as incineration, is reduced. Fig. 4.5 further illustrates a complete life cycle of biocomposite materials, with emphasis on recovery.

4.2.4 Design for manufacture The possibility of using biocomposite materials in many areas of engineering application has emphasised the need for design for manufacture. Biocomposite materials are manufactured using different processes: hand lay-up, compression moulding and extrusion, to mention but a few. These processes and their effects on the properties of various biocomposite materials are extensively discussed in the next section. The expected properties and intended areas of application of a biocomposite are important determinant factors to be considered during design for its manufacture. Close to these factors is the geometry of the biocomposite products, among others. Importantly, Mohanty et al. (2002) proposed a tri-corner approach in designing for the manufacture of biocomposites with excellent or desirable strength: (i) effective, (ii) low cost and efficient biofibre treatment, effective blending and functional matrix modification, and (iii) choice of efficient biocomposite processing conditions, as illustrated schematically in Fig. 4.6. A deeper discussion on each of these approaches can be found in the subsequent sub-sections and other chapters. Moreover, it is expected that biocomposite design for manufacture supports a reduction in the costs of manufacturing, components, assembly, supporting production and considers the impact of design for manufacturing decisions on other unavoidable

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Recycling

Oil production

Incineratrion Refinery

Eco-Cycle

Biorefinery

Biodegradation

Green chemistry

Biodeterioration Biofragmentation Assimilation Biomass Emissions of VOCs nanoparticle release degradation during service life

Renewable raw material

Figure 4.5 Detailed life cycle of sustainable biocomposites (Vilaplana et al., 2010).

Bio-composite processings

Efficient biofibre surface treatments

Synergism

Matrix polymer modification

High performance bio-composite formulation

Figure 4.6 Proposed tri-corner approach to manufacture high efficient biocomposites (Mohanty et al., 2002).

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factors. Recently, design for manufacture has been combined with a design for assembly, as a simultaneous effort. They are jointly referred to as a design for manufacture and assembly (DfMA). DfMA has numerous benefits over the traditional design approach, where many manufacturing processes and post-manufacturing operations are required. A single part of biocomposite materials can be manufactured, just like forged or cast materials (Fig. 4.7(b)), instead of Fig. 4.7(a). A typical example is shown in Fig. 4.8, where an interior carpet of a car’s door is made by hemp fibre-reinforced polyethylene biocomposites. Furthermore, there are numerous advantages of having effective DfMA (single biocomposite part), as shown in both Figs. 4.7(b) and 4.8 over the traditional design approach. These benefits include, but are not limited to, the following: ⁃ ⁃ ⁃ ⁃ ⁃ ⁃ ⁃ ⁃

Reduced costs (of materials, design, manufacturing and assembly). Ease of design, manufacturing and installation/assembly. Not susceptible to failure, hence longer durability is guaranteed. Lower time/effective time management from design to installation/repairs or recycle stages. Use of less equipment, labour and facilities. Effective waste management. Ease of maintenance and repairs. Better aesthetics and reduced weight.

4.3 Manufacturing processes and their influences on properties of bio-composites Manufacturing processes are the series or stages of techniques undergo by the biocomposites before they are manufactured into semi or finished products for several engineering applications. These processes vary from one biocomposite to another. They determine the properties of the biocomposites. Therefore, they are selected based on the inherent properties of the fibres/fillers bio-fibres/natural or synthetic and matrices/binders (thermoplastics or thermosets), as reinforcing and binding elements, respectively. Also, the following factors determine the type of suitable process for manufacturing of a particular biocomposite: fibre loading, quantity, process parameters (production speed, temperature and pressure), surface treatments, costs, properties (of raw materials and final products), size, the complexity of shape and applications

Figure 4.7 (a) Traditional design approach (11 parts) and (b) DfMA approach (single part).

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Figure 4.8 Single components containing natural fibres (flax, hemp and sisal) reinforced composite parts used in Mercedez-Benz. (From Holbery, J., Houston, D. Natural-fiberreinforced polymer composites in automotive applications. JOM 58, 80e86 (2006). https://doi. org/10.1007/s11837-006-0234-2).

of the biocomposites, among others. The size of the biocomposite is a leading factor among all these factors (Ho et al., 2012). Both compression and injection moulding processes are suitable for a small to medium-sized components, because of their fast processing cycles and simplicity. Nonetheless, open moulding and autoclave processes are often used to manufacture large structures (Ho et al., 2012). Therefore, suitable manufacturing techniques must be utilised to enhance the interfacial bonding, properties and produce an optimal biocomposite material, without manufacturing-induced defects. There are several methods for manufacturing different biocomposite materials both in laboratory/small and commercial/large scales, but the most common techniques have been extensively and hereafter discussed. These processes include, but are limited to, hand and spray lay-ups, injection moulding, compression moulding, extrusion, resin transfer moulding, filament winding, automated fibre placement, autoclave and out-of-autoclave, as well as additive manufacturing.

4.3.1 Hand and spray lay-ups 4.3.1.1 Hand lay-up Hand lay-up is the most basic, simplest and oldest process of manufacturing biocomposites. This method has been widely used for many years due to its simple principles to operate and teach. However, resin mixing, laminate resin contents and laminate qualities are very dependent on the skills of laminators. This method involves the placement of plies or fabric layers and succeeding application of matrix in the

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mould manually, to produce a biocomposite laminate (Lotfi et al., 2019). In other words, in the hand lay-up method, resins are impregnated into fibres by hand, which are in the form of woven, knitted, stitched or bonded fabrics. This is usually accomplished by rollers or brushes, with the increased use of nip-roller type impregnators for forcing resin into the fabrics by means of rotating rollers and a bath of resin. Laminates are left to cure under standard atmospheric conditions. According to Knoeller (2018), hand lay-up is a process whereby resin material is rolled into the already placed reinforcing fibres in the mould, after a release film and gel coating. This manufacturing process can be divided into four sequential stages, as thus listed. i. ii. iii. iv.

Mould preparation Gel coating Lay-up Curing

A suitable mould must be prepared and used for the hand lay-up process, except the biocomposite will be joined directly to another structure. A mould can be very simple or with some shapes, such as curves and edges. Mould with several shapes must be assembled and dismantled in parts or sections before lay-up and after curing to remove the biocomposites. A certain amount of fibres is cut and placed in the mould in a specific direction(s) before a catalysed resin is added to the well-laid fibres. The mixing and contents of the resin, as well as the quality of the biocomposite laminate, greatly depend on the skills of the operator or designer, and hence, it is a labour-intensive manufacturing process. Thereafter, impregnation of the fibres with resin is done using a roller or brush, as depicted in Fig. 4.9(a), or in a slightly modified configuration (Fig. 4.9(b)). Furthermore, the advantages of this method include simple operation and low cost. It is suitable for creating a corrosion-resistant and structural portion. Hand lay-up process does not involve too much of fibre loading, unlike other processes. It is suitable for fibre-reinforced thermosetting polymer biocomposites and many material (fibre and matrix) types. It accommodates higher fibre contents and longer fibres, when compared with the spray lay-up. An increased natural fibre content significantly increases the stiffness property and improves the impact strength of biocomposite (Arbelaiz et al., 2005b; George et al., 2010). However, both water uptake and odour of biocomposite increase with an increase in the fibre content. It requires less capital and infrastructure when compared with other methods. It supports versatility, the use of longer fibres and low tooling cost. Also, it can be used for several years (durability). It is very easy to operate and has simple principles to teach. These advantages have made hand lay-up attractive application in the transportation sector, to manufacture marine, military and aircraft structures. For instance, it is used for the fabrication of bulletproof composite panels from ramie fibre-reinforced epoxy composites and commonly used to fabricate polymer matrix composite parts for the American aerospace industries (Lei et al., 2006; Knoeller, 2018), big tanks/containers, boat/ship hulls, deck, swimming pools, aircraft and automotive components in transportation industries. Other typical applications of this technique include the manufacturing of standard wind-turbine blades and boats. Several different hybrid fibre-reinforced

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(a)

Resin

Hand roller Fibreglass reinforcements

Gel coat Mold Release film

(b)

Vacuum regulator

Vacuum pump Breather Vacuum port Non-porous teflon

Pipe

Caul plate

Vacuum bagging

Composite panel

Alumium base table Non-porous teflon for dam structure

Sealant for dam structure

Sealant for vacuum bagging

Figure 4.9 Hand lay-up process, showing (a) a simple illustration and (b) complete set-up with vacuum bagging process (Cucinotta et al., 2017; Hakim et al., 2017).

polymeric biocomposites have been successfully manufactured using hand lay-up technique, with various common matrices: polyester resin (Singh et al., 1995; Ahmed and Vijayarangan, 2008; Athjayamani et al., 2010; Khanam et al., 2010; Ramesh et al., 2013; Isa et al., 2013; Alavudeen et al., 2015), epoxy resin (Venkateshwaran and ElayaPerumal, 2010; Jawaid et al., 2010, 2013; Zhang et al., 2012; Ramnath et al., 2013, 2014; Boopalan et al., 2013; Santulli et al., 2013; Dhakal et al., 2013; Srinivasan et al., 2014; Guermazi et al., 2014; Boroujeni et al., 2014; Yahaya et al., 2014a,b,c, 2015; Dong and Davies, 2015; Gupta and Srivastava, 2016), among others. Nonetheless, the mixing of resin, resin content and quality of laminates are significantly depended on the skills of the laminators. Production rate and the possibility of achieving a high fibre volume fraction with hand lay-up process is less and difficult, respectively, especially in comparison with other automated processes. In addition, this method creates high void contents. It generally supports a low volume production of biocomposites; hence it is not economical. High viscosity resin is difficult to use in

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this technique. The lower viscosity of the resins implies that they have an increased propensity to penetrate clothing, among others. Resins need to be low in viscosity to be workable by hand. This generally compromises their mechanical/thermal properties due to the need for high diluent/styrene levels. To remove the entrapped air or reduce occurrence of voids and pores, and obtain an even distribution of the resin within the fibres, a better fibre-matrix interaction and a desired thickness, rolling, brushing or squeezing of the wet biocomposites must be carried out, using hand rollers.

4.3.1.2 Spray lay-up Spray lay-up method is very similar to the hand lay-up technique. It is an advancement in hand lay-up. The main difference is that a spray gun is used to spread pressurised resin and chopped fibres, as depicted in Fig. 4.10. The matrix material and fibre can be sprayed separately or simultaneously. Other stages of manufacturing are very similar to, if not exactly, the same as the hand lay-up. The spray lay-up process supports high fibre volume fraction in biocomposites and the production of various sizes of components. It is a continuous process. It accommodates several materials, as moulds. Re-spraying can be used to correct errors. Therefore, the spray lay-up method is good for fabricating lower load-carrying components, such as small boats, fairing of trucks, bathtubs, to mention but a few. However, it is sometimes slow, inconsistent and environmentally unfriendly, and it has no control of fibre orientation. Moving forward, natural reinforcements (plant fibres such as sisal hemp, coir, jute, banana, flax, cotton and nettle) in the form of chopped short fibres, particles, flakes and fillers, as well as matrices (polyester, unsaturated polyester, epoxy, phenolic resin, polyvinyl ester and polyurethane resin), among others, are suitable materials for spray lay-up manufacturing process. Fiber Resin catalyst pot Air pressurized resin

Chopper gun

Optional gel coat

Figure 4.10 A simple illustration of a spray lay-up process (Balasubramanian, Sultan, and Rajeswari, 2018).

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4.3.2 Vacuum bagging moulding A vacuum is used in the vacuum bagging process, as the name implies. In this process, the vacuum is introduced to eliminate unwanted enclosed air, excess resin or gases, as earlier illustrated in Fig. 4.9(b). A complete woven mat or fabric form and matrix layup are covered or sealed up with a non-adhering film of polyvinyl alcohol or nonporous teflon or nylon to create a vacuum bag within the mould (Fig. 4.11). Often, the air under the vacuum bag is sucked by an atmospheric pressure to produce a desirable and sustainable biocomposite laminate after the curing stage at a particular or room temperature has been completed. Also, the advantages of vacuum bagging manufacturing process include, but are not limited to, the following: ⁃ Removal of any air or voids within the resin and reinforcement lay-up, which leads to greater surface finish and mechanical properties. ⁃ Any volatile organic compounds produced in the addition of a catalyst or during the curing process are removed and confined safely. This also helps the aforementioned benefit, because if these volatile organic compounds are removed, they have no effect on the finished biocomposite products. ⁃ Possibility of delamination defect in biocomposite parts is also reduced, as the resin is encouraged to move between the layers and into an absorbent part within the bagging system.

However, the setbacks of this process are as follows: ⁃ Stringent measures are to be taken to ensure that there are no leaks within the vacuum system. This can be tested before introducing resin by turning the vacuum on and clamping the system to seal the vacuum, and the pressure should be noted on the generator and the system left to 10e15 min. Therefore, any reduction in pressure indicates a leak within the system. ⁃ Ensure that a thermally stable sealant tape is used. The tape must remain stable and hold the seal around the edge under curing and exothermic conditions. ⁃ Bridging may occur when there is an intricate or complex part that does not allow the bag to completely press against the part. Pleats may be used to prevent this unwanted condition or the use of pressure pads made from a flexible material, such as rubber, is also encouraged.

Toughened glass mould

Alumium foil

Sealing tape

Composite laminates

Release agent

Peel ply

Perforated relase film

Breather

To vaccum Vacuum hose

Vacuum bag

Figure 4.11 A simple arrangement of the vacuum bagging system (Hang et al., 2015).

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4.3.3 Injection moulding Injection moulding is used to force the measured quantity of the mixture of molten polymer and fibre into a predetermined mould cavity (Fig. 4.12). Initially, the thermoplastic polymer used for an injection moulding process is modified to plastic pellets. Now, the already chopped fibres with pellets are fed separately through a funnelshaped feed hopper into a heated compression barrel to fabricate fibre-reinforced polymer biocomposites. The heated barrel accommodates a rotating screw or screws (in case of the twin-screw extruder). The solid pellets are converted to a viscous liquid through heat produced from interfacial friction between the barrel, pellets and screw (Nystr€ om, 2007). The viscous liquid is required for an easy movement through the sprue nozzle of the injection machine. Therefore, the screw helps to generate heat, develop shear force for mixing the fibres and polymer and functions as a piston to push the biocomposite (fibre-reinforced molten polymer) through sprue nozzle to the close mould cavities. The extruded product is compressed into the closed mould cavities to produce a desirable geometry, after solidification and freezing (cooling) of the fibre orientation and distribution. Finally, the anticipated biocomposite product is ejected from the mould cavity. The compounding process considerably affects the fibre shortening, thermal deterioration and fibrillation at initial stages and the final properties of the biocomposite properties. The following various compounding methods are recommended for injection moulding of natural fibre-reinforced thermoplastics: mixing (cascade mixing), pull-drill process (bast fibres), extruder, pultrusion (bast fibres), pelletising (with matrix) and hybrid fibre non-woven pre-consolidation and cut (Faruk et al., 2012). The modulus distribution of the biocomposites produced by injection moulding is critically affected by the fibre orientation and residual stresses. Residual stresses could cause an earlier fracture of pure thermoplastic polymers and their biocomposites, which significantly affect the mechanical properties and quality (when it causes

Carbon fibre roving

Hopper

Real - time monitoring system

Quantitative feeding system Screw

Mould

Heater

Vent hole

Injection moulding machine

Figure 4.12 Descriptive set-up of the direct fibre feeding injection moulding system (Yan and Cao, 2018).

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shrinkage, stress cracking, undue deformation and warpage) of the final biocomposite products. Residual stress distribution in the injection moulded components also affect their dimensional accuracy and other structural properties (Kim et al., 2002). An occurrence of fibre attrition is another limitation of the injection moulding process because it decreases the fibre length during processing (Pickering et al., 2016). However, optimal properties of the injection moulded biocomposites are achievable by carefully selecting, monitoring and improving the following three principal parameters: ⁃ Process: Melt and mould temperatures, speeds and pressures of both screw and injection process. The thermal and rheological properties of the matrix depend on the selected temperature and pressure for the moulding process. Too high temperatures can cause fibre degradation. For example, natural fibres need process at lower temperatures, around 240
C (Monteiro et al., 2012), or otherwise, they will degrade. This limits the thermoplastic matrices used, which must possess melting points lower than the degradation temperature of the concerned fibres (Pickering et al., 2016). ⁃ Materials: Molten polymer rheology and reinforcement/fibre type and volume content. The viscosity of the matrix is very germane during injection moulding, and therefore, this process is generally limited to fibre content of less than 40 m% in a biocomposite (Pickering et al., 2016). For example, semi-crystalline polylactic acid exhibited a greater shear viscosity than that of amorphous polylactic acid (Fang and Hanna, 1999). ⁃ Geometric: Size and shape of the mould cavity, location of the injection gates and the vents.

Several studies have been carried out, moving forward, on the manufacturing of natural fibre-reinforced with renewable polymeric biocomposites using injection moulding technique (Huda et al., 2005a,b, 2006a,b; Serizawa et al., 2006; Nystr€om, 2007). Studies have further shown the suitability of injection moulding method to fabricate natural fibre-reinforced thermoplastic biocomposites (Arbelaiz et al., 2005a; Khan et al., 2009; AlMaadeed et al., 2012; Kumar et al., 2013; Asaithambi et al., 2014; PérezFonseca et al., 2014; Bledzki et al., 2015; Gupta and Srivastava, 2016).

4.3.4 Compression moulding As the name implies, the moulding of the biocomposites is carried out under the action of pre-determined heat, high compression force or pressure. The compression moulding process combines a hot-press method and an autoclave process. A hotpress method is performed with or without a close mould, but an autoclave process involves a closed operation, whereby thermoplastic prepregs are sequentially laid up on a mould, followed by the vacuum bagging of the whole laminate, heating process at a pre-determined heat-pressure operational cycle and curing, before the final biocomposite is manufactured (Mallick, 2008; Hu and Lim, 2007). Furthermore, it is the widest choice for high volume biocomposite components, used with thermoset and thermoplastic matrices and short or long fibres. For instance, close to 70 wt% fibre can be used, and between 1 and 10 mm thickness can be manufactured. Compression is very similar to the hand lay-up process. The main difference between both processes is the presence of a closed set of matching dies during curing

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after pressure and heat are applied during compression moulding. Compression and hand lay-up mouldings are suitable for the manufacturing of small and large composites parts (Lotfi et al., 2019). More also, this manufacturing method for biocomposites involves two halves of a mould (female and male), and it utilises pressure and heat to produce parts. The pre-cut and measured quantity of mat, chopped or switched fibres are stacked together. The pre-heated closed mould cavity accommodates the fibres, then pressurised before applying temperature in order to melt the compounds and the molten compounds conform to the shape of the mould cavity (Fig. 4.13). Next, the component is ejected after the mould is opened. For reducing the damage of the fibre, the fibre is gently placed inside the mould with no vigorous motion and shear stress. Noticeably, this manufacturing process of biocomposite with long fibres produces a higher fibre volume fracture. Short and long fibres, as well as fibre mats can be used, when they are pre-mixed with the compounds. They reinforce the biocomposites and significantly decrease shrinkage of the final biocomposite part. Also, an increased filler content acts as a heat sink within the material and reduces the total quantity of heat released (Mallick, 2008), and improves the anisotropy of the final compression moulded products (Dumont et al., 2003). The compression moulding process generally involves two traditional initial charges: bulk moulding compound (BMC) and sheet moulding compound (SMC), according to Ho et al. (2012). The surface of the female mould cavity is usually covered by the charges (Mallick, 2008). Plastic materials are mingled with short fillers and fillers before they are placed in the mould cavity in the case of BMC. Conversely, SMC involves cutting of long fibre sheets according to the mould cavity, before putting them in the surface of the mould. The desired thickness of the compounds is sequentially built up by adding a sheet layer upon layer with resin on the fibre sheet (van Voorn et al., 2001). The physicomechanical properties of a flax fibre-reinforced SMC product have been enhanced (van Voorn et al., 2001). Pressure

Movable mould Charge

Fixed mould Ejector pin

Moulded part

Figure 4.13 A simple illustration of a compression moulding (Rajak et al., 2019).

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For example, the mechanical properties and morphology of flax fibre-reinforced melamine-formaldehyde composites manufactured by compression moulding method were studied by Hagstrand and Oksman (2001). The mechanical properties were compared with that of glass fibre. The result showed that flax fibre-reinforced biocomposites exhibited lower mechanical properties against glass fibre. However, considering the cost and the density, the properties of flax fibre biocomposites were more competitive than that of glass fibre composites. The study also highlighted that compression moulding method helped to increase fibre-matrix interfacial adhesion. Moreover, the flammability of flax fibre-reinforced polypropylene biocomposites has been fabricated by the compression moulding process and investigated (Helwig and Paukszta, 2006). The heat release rate and mass loss rate, along with the mechanisms of thermal decomposition, were discussed. The results obtained depicted that the heat release rate depended on the fibre volume in the biocomposites. The characteristic of the biocomposite resembled that of lignocellulosic materials when its fibre volume exceeded 20%. Admittedly, the benefits of using the compression moulding process include, but are not limited to, the following. ⁃ Good dimensional tolerances under applications of pressure and temperature. ⁃ Process is very repeatable, as being used in the automotive industry for the production of small to moderate-sized parts at high volumes. ⁃ Low void content is achieved due to the application of appropriate pressure. ⁃ Minimal scrap. ⁃ High surface quality and impact strength of products. ⁃ Low labour requirements. ⁃ Can be used alongside with the prepreg and preform composites. ⁃ Compression moulding process attracts few limitations, as subsequently stated. ⁃ Equipment cost is high. ⁃ Depends on the size of the component. ⁃ Inappropriate for structural components.

In addition, there are many reported studies on the manufacturing of natural fibrereinforced renewable polymeric biocomposites using a compression moulding technique (Huda et al., 2005a,b, 2006b; Serizawa et al., 2006). Studies have further shown the suitability of compression moulding technique to fabricate both fibre-reinforced thermoset and thermoplastic biocomposites (Xian-bao et al., 2006; Athijayamani et al., 2009; Graupner et al., 2009; J unior et al., 2013; Dhakal et al., 2013; Shanmugam and Thiruchitrambalam, 2013; Zhang et al., 2013; Arthanarieswaran et al., 2014; Gupta and Srivastava, 2016). Moreover, it has been reported that the tensile strength of fibres reduced by 10% in only 10 min between temperatures of 150 and 200
C (Herrmann et al., 1998). An optimum compression temperature of nearly 80
C has been used to manufacture jute yarn-reinforced bacterial co-polyester biopol biocomposites with an improved range of mechanical properties (Mohanty et al., 2000). Van de Velde and Kiekens (2003) recorded the highest strength for a unidirectional and multidirectional non-woven flax-reinforced polypropylene biocomposite at 200
C. Therefore, processing parameters such as curing temperature, pressure and cycle time must be correctly selected and controlled. They vary with various materials

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and thicknesses of the plies or sheets used. For instance, high compaction pressure in the compression moulding process can elongate the flax or hemp fibre used. Thus, the fibre can be crushed and eventually lower the overall mechanical properties of the FRP biocomposites.

4.3.5 Vacuum resin infusion The vacuum can also be applied to the mould cavity to assist resin in-flow, being drawn into the fabrics (Fig. 4.14). This is known as vacuum-assisted resin injection (VARI). Once all the fabric is wet out, the resin inlets are closed, and the laminate is allowed to cure. Both injection and curing processes can take place at either ambient or elevated temperature. Generally, curing time depends on the type of polymer used for the biocomposite manufacturing process. Benefits of resin infusion include, but are not limited to: ⁃ Good tolerance control, as tooling controls and dimensions, give high repeatability. ⁃ Prototype tooling costs are low. ⁃ Volatile emissions, such as styrene from unsaturated polyester, are controlled as a close mould tooling process is used. ⁃ Very little waste is produced. ⁃ Process can be automated, and hence, productivity and efficiency are both enhanced. ⁃ Integration of heating up of the mould for the matrix.

4.3.6 Pre-impregnated resin The pre-impregnated resin can be used. This technique may involve the use of prepreg or simply be a manual lay-up method. The location of where the prepreg layout is carried out should be an environmentally controlled room. This is necessary to control the temperature and minimise dust particles. Workers are often required to wear protective clothing in order to further reduce contamination. Problems may occur with prepreg lay-up if silicone comes in contact with the prepreg. An occurrence of this compromises Resin channel

Bag sealing tape

Bag Vacuum channel

Resin valve Fibreglass and reinforcements

Mixed resin

Figure 4.14 Schema of vacuum resin infusion process (Cucinotta et al., 2017).

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the surface of the prepreg and its ability to bond with another material. Prepreg materials also need to be kept under refrigeration in order to preserve their shelf life, as curing can occur when the material is exposed to ambient temperature. Prepreg can be applied by manual hand lay-up or automation process for mass production. Similarly, the advantages of using prepreg material include the following: ⁃ As the resin is already impregnated, there is no need to add liquid resins, which can often be messy in the hand lay-up method. ⁃ Reduction in the chances of getting air bubbles within lay-up and the possible presence of voids is also reduced. ⁃ Shelf life of prepreg is comparatively high if kept under refrigeration and free from contaminants.

4.3.7 Extrusion Manufacturing of a fibre-reinforced biocomposite materials can be done through an extrusion process, by steadily forcing materials through a pre-designed and uniform cross-sectioned die, as depicted in Fig. 4.15. This is possible by mixing softened bead-like pellets or crude polymer (matrix material) with pre-treated or crude lignocellulosic fibres (fibre bundles/reinforcements) before they are continually fed into a single or twin-screw extruder. Fig. 4.15 shows an extrusion compounding process of spent coffee grounds-reinforced polypropylene (SCG/PP) composites (Sohn et al., 2019). Both reinforcement and polymer matrix can be concomitantly fed into the screw extruder (Gallos et al., 2017). Malkapuram et al. (2009) stated that twinscrew extruders exhibited better fibre dispersion, and hence, mechanical performance than the single screw system. The extrusion process is suitable for both natural fibres, renewable and cellulosebased polymers. Uniform fibre dispersion is one of the important factors to consider during this process because it determines the properties of the biocomposite. Hence, a twin-screw extruder produces a high shear process towards good fibre dispersion (Oksman et al., 2009; Arrakhiz et al., 2012).

+ SCG/PP composites Spent coffee grounds Air-cooled fans

Nozzle

Polypropylene

Hopper

Heater 1 Heater 2 Heater 3 Heater 4

Screw

Cutting machine

Conveyor belt

Extrusion machine

Figure 4.15 An extrusion process of biocomposite material (Sohn et al., 2019).

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More also, the extrusion process is a fast, dimensional repeatable and economic process. It produces high fibre volume fraction and high structural properties of biocomposite products. However, the extrusion process of biocomposite parts is limited to component size because of the quantity of fibre and the extruding force required. It is also limited to constant or near-constant cross-section components (Lotfi et al., 2019). Tungjitpornkull and Sombatsompop (2009) reported a better tensile modulus of compression moulded glass fibre-reinforced wood/polyvinyl chloride (WPVC) composites than that of twin-screw extrusion counterparts, as shown in Fig. 4.16. This improved property was attributed to the less fibre breakage and thermal degradation of the PVC molecules. Consequently, a longer fibre length, higher specific density due to lesser void contents (manufacturing defect), and stronger composite was produced with compression moulding in comparison with extrusion counterpart.

4.3.8 Resin transfer moulding Resin transfer moulding (RTM) has become very popular to fabricate fibre-reinforced polymer biocomposites, due to its capability to produce high volume laminates with cost-effectiveness and low void content. It bridges the gap between capital intensive compression moulding and labour intensive hand lay-up method (Sreekumar et al., 2007). This process is a closed moulding process, whereby resin is transferred (injected) under pressure over the already placed fibre preform (woven mat, fabrics or chopped strand mat), as schematically shown in Fig. 4.17. Generally, these mats or fabrics are made from natural plant fibres, such as banana, hemp, sisal, flax, nettle fibres to fabricated biocomposite laminates. Epoxy, polyester, vinyl ester, methyl methacrylate and phenolic resins are commonly used in RTM. The fabrics used are sometimes pre-pressed to the mould shape and held together by a binder. These ‘preforms’ are then more easily laid into the mould tool. A second mould tool is then clamped over the first, and resin is injected into the mould cavity until it is filled up, 16

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14 12 10 8 6 4 2 Compression moulding

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0 0

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Figure 4.16 Effect of compression moulding and twin-screw extrusion techniques on tensile modulus of composites (Ku et al., 2011).

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Resin

Vent

Mould

Figure 4.17 Illustration of a resin transfer moulding process (Rajak, 2019).

using single or multiple inlet ports (multiple injection gates). The component is then removed from the mould after cooling (Sreekumar et al., 2007). A full chemical reaction between the resin and its catalyst (curing) is required through the post-curing process. Both fire retardancy and surface finish of the biocomposite can be enhanced using appropriate mineral fillers. The mould temperature, mould configuration, resin viscosity, vent control, resin injection pressure, fibre mat permeability, gate location and configuration, preform placement methods, preform permeability and architecture, are important process parameters in RTM technique (Ho et al., 2012). They require careful selection, monitoring or control and optimisation to avoid manufacturing defects. For instance, mould deformation and fibre preform wash-out defects can occur by excessive injection pressure. Also, pre-mature resin gelation and its resultant short shot flaws are induced by an excessive high mould temperature (Ho et al., 2012). RTM is a very useful technique to produce high volume and low-cost biocomposite parts. Therefore, RTM is characterised with the following benefits: ⁃ Increased productivity through automation, as well as good temperature and pressure control. ⁃ Improved quality, as a result of the process being consistent. ⁃ Good dimensional tolerances and surface finish due to the use of suitable pressure in the process. ⁃ Required lower temperature, and therefore, thermomechanical degradation is avoided. ⁃ Manual, semi-automated and highly automated processes are possible. ⁃ Possibility of a combination of a wide range of reinforcing materials (fibres and fillers) to achieve numerous desired orientations. ⁃ Production of near net shape components is possible; hence it decreases the material waste. ⁃ Nearly zero air enclosure or entrapment (voids). ⁃ Production of uniform component thickness, large and complex components with high strength to weight ratio. ⁃ Due to pressure being utilised in this process, higher fibre volumes can be obtained, but high injection pressure is not required.

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The RTM is widely used in the automobile industry to produce car body components, in addition to other products, such as bathtubs and big containers. However, the following setbacks characterised the RTM process: ⁃ ⁃ ⁃ ⁃

The size of the biocomposite laminate is limited by the mould cavity. It requires complex mould design. It does not accommodate all reinforcing materials content. Tooling cost is relatively high.

In addition, there are many studies on RTM of sustainable natural fibre-reinforced polymeric biocomposites for lightweight applications in various engineering sectors, especially in automotive and aerospace (Ferland et al., 1996; Ikegawa et al., 1996; Willians and Wool, 2000; Kim and Daniel, 2003; Warrior et al., 2003; Goutianos et al., 2006; Sreekumar et al., 2007; Rassmann et al., 2010; Salim et al., 2011; Francucci et al., 2012). Importantly, Sreekumar et al. (2007) reported that the mechanical (tensile and flexural strengths, Young’s and flexural moduli) properties of sisal fibrereinforced biocomposites manufactured by RTM were greater than that of compression moulded counterparts. These improved mechanical behaviours of RTM biocomposite samples were attributed to their lower water absorption and void content, as a resultant advantage of better fibre-matrix interfacial adhesion. Also, Sebe et al. (2000) used the RTM technique to manufacture a series of hemp fibre-reinforced polyester biocomposites. The results obtained depicted a proportional increase in mechanical (impact and flexural properties) with an increased quantity of fibres during formulation. Sreekumar et al. (2007) reported a maximum water absorption and void content with compression moulded sisal-leaf fibre-reinforced polyester biocomposites when compared with RTM counterparts.

4.3.9 Automated fibre placement Automated fibre placement, as depicted in Fig. 4.18, is an innovative technique of manufacturing large and complex composite structures. Some of the setbacks of the hand lay-up method, especially the low productivity, can be improved with the automation of a programmed robotic system, as described in Fig. 4.19. An uninterrupted layering and building of biocomposite structure is performed as a robot places the Incoming tape

Tape feed

Tape cut Consolidation force

HGT

Process heat

Direction of travel

Previous ply Tool

Figure 4.18 The Automated fibre placement process (Lotfi et al., 2019).

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Fibre feeding system

Robotic arm Spool with fibre tape

Head

Figure 4.19 Sub-components of an automatic fibre placement system (Kozaczuk, 2016).

continuous fibre-reinforced composite tape. The incoming tape is heated either by laser or hot nitrogen, before pressing to the mould for good compaction. Each ply can be laid using diverse orientations and angles (Lotfi et al., 2019). The main benefits of using automated fibre placement process when compared with other manufacturing techniques include, but are not limited to, reduced costs of labour, manufacturing time and material scraps or wastes, it supports producibility and repeatability. Some large unique structures can be produced by this process. Consequently, this process is now popular and affordable for several aviation industries today, such as Spirit, Boeing and Airbus. Nevertheless, the limitations from mould shape, head geometry and roller diameter, as well as ply edges caused by cut tapes, have reduced the application of this advanced method (Kozaczuk, 2016; Lotfi et al., 2019). Also, it is relatively expensive. Sometimes, it results in a distortion in the process of thermoplastic biocomposites.

4.3.10 Filament winding There are other processing methods widely used in the fabrication of natural fibrereinforced polymer composite materials. One of them is filament winding. This process involves the continuous movement of unwinding fibre strands through a resin container, where complete impregnation occurs before the resin-impregnated strands are passed to a rotating mandrel in a controlled pattern to give a specific fibre orientation (NPTEL, 2019b). The simple schematic illustration of the process is depicted in Fig. 4.20. From Fig. 4.20, it is evident that fibre creel, resin-impregnated system, carriage and rotating mandrel are the main components of this process. Filament winding is primarily suitable for hollow, circular or oval sectioned parts, such as pipes and tanks, to mention but a few. Filament winding has been used by Li (2015) to manufacture fibre-reinforced polymer, as shown in Fig. 4.21. Similarly, the fabrication of natural fibre-reinforced polymer biocomposites has been done with this process. For instance, filament winding was used successfully to fabricate hemp-reinforced thermoplastic polypropylene

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Fiber spools

Resin bath Nip rollers

Rotating mandrel

Figure 4.20 The schematic illustration of the filament winding process (Wang et al., 2020).

composites (Madsen, 2004). In this experimental study, the hemp fibre yarn was aligned by filament winding with a custom-built winding machine. The film stacking method for fibre/matrix mixing in a compression moulding was also applied in this work. The study had concluded that the commingled filament-winding method was found to be much more optimal approach for fibre/matrix mixing, when compared with the film-stacking method with a combination of compression moulding. More also, filament winding was recently used to manufacture a basalt fibre-reinforced epoxy composite pipes of diameter, thickness and filament winding angle of 100 mm, 6 mm and 55 degrees, respectively (Prabhakar et al., 2019). The results obtained from the drop weight low-impact tests conducted showed a higher breaking strength at a maximum impact energy of 60.42 J at an impactor’s height of 1.25 m, when compared with other conventional material pipes.

Figure 4.21 Manufacturing of fibre-reinforced polymer with filament winding (Li, 2015).

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Furthermore, the fibre tension produces the required compaction. This is the reason why fibre tension is a vital factor to be carefully considered in the filament winding process. The fibre tension depends on fibre type, geometry and winding pattern produced by the rotating mandrel. An optimum fibre tension must be determined and used to avoid fibre surface fracture, which eventually leads to final breakage. Heat, commonly from an oven, is used to cure the biocomposite before the biocomposite part is finally removed from the metallic, collapsible rubber or soluble plaster mandrel using appropriate technique. Benefits of filament winding include: ⁃ Possibility of achieving a high strength to weight ratio and high fibre volume ratio. ⁃ An automated process, therefore it involves minimal labour, high production volume (very fast process), efficiency and cost saving, because fibres are not necessarily converted to fabric prior to use. ⁃ Control or metering of the resin is possible. ⁃ A specific direction of fibre orientation is easily achieved, with good structural properties of the laminates. ⁃ Possibility of design flexibility in biocomposite parts due to change in materials, winding patterns and curing option. ⁃ Excellent fibre distribution, placement and orientation are obtainable with a high measure of uniformity or consistency. ⁃ Use to manufacture biocomposite parts that require accurate tolerances ⁃ Requires a less and low cost materials to fabricate high strength biocomposite part, when compared with other biocomposite manufacturing processes. ⁃ Supports the production of variable sizes of components (NPTEL, 2019b). ⁃ Suitable for commercial production because of its low cost, high flexibility and repeatability (Sofi et al., 2018).

Nevertheless, there are few shortcomings that are associated with the application of the filament winding process, as subsequently highlighted. ⁃ ⁃ ⁃ ⁃ ⁃ ⁃

Expensive mandrel for some applications and large components. Surfaces of some biocomposite parts may be occasionally unacceptable. Relatively high capital investment. Within a single layer winding, it is impossible to change the fibre direction. Production of a reverse curvature (female feature) is impossible. The mechanism requires accurate and skilful control to achieve uniform fibre distribution and orientation (NPTEL, 2019b).

Filament winding process can be used to manufacture numerous composite products for military and defence sector (missile and rocket motor cases), aerospace industry (aircraft fuselages), sports/game (golf shafts), oil and gas (storage tank, pipelines and gas cylinders), marine/naval (vessels and sail boat mast), building/construction (ducting and cement mixture), among others (fishing rods). As technology advances with the advent and application of sophisticated machine centre and engineering software, filament winding can be used to manufacture complex engineered non-spherical and noncylindrical composite products. Also, a conventional 2-axis lathe type filament winding machine has been improved to higher degrees of freedom (Minsch et al., 2017). Independent monitoring of all movements of the entire process is possible today, using a computer controlled machine. Also, a robotic filament winding technique has been

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adopted for industrial application, as recently reviewed by Quanjin et al. (2018a). Further, recent studies on this process have been reported in an attempt to advance the innovative equipment, re-design and manufacture different optimised axial filament winding machines and processes (Mateen et al., 2018; Quanjin et al., 2018b, 2019).

4.3.11 Autoclave moulding The autoclave moulding method is very similar to the vacuum bagging, with few changes. Heat and pressure that are required by the biocomposites during the curing stage are supplied by the autoclave machine (Fig. 4.22(a)). This process involves firmly stacking of prepregs in a mould following a specific sequence. A release gel is applied to the surface of the mould to avoid the sticking between the polymer and the mould surface. In addition, this process allows the use of cores and inserts. Then, vacuum bagging follows to remove all possible entrapped air between the layers, as earlier explained (Fig. 4.22(b)). Afterward, the whole assembly is moved to the

(a)

(b) Bleeder pack Membrane

Prepreg pack Cork dam Seal

Fan Mould

Heaters Pressure

Vacuum

Figure 4.22 (a) An autoclave machine, (b) its internal components and process schematic (Halley, 2012; Dixit et al., 2016).

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autoclave machine, where both heat and pressure are applied to aid uniform and effective distribution of the matrix, as well as good fibre-matrix interfacial adhesion or bonding for a definite time interval. This stage is called curing. Later, the composite component is removed from the mould after cooling of the assembly and removal of the vacuum bag, sequentially. Autoclave moulding process embraces the following advantages: ⁃ ⁃ ⁃ ⁃ ⁃ ⁃ ⁃

Applied to both fibre-reinforced thermosetting and thermoplastic polymer composites. Better inter-layer adhesion. Good control of both fibre and resin. Proper and sufficient fibre wetting. Degree of uniformity in component solidification is high. Supports high fibre volume fraction in the composite component. Absence of void content in the final component due to the benefit of the vacuum bagging mechanism. ⁃ As a part of the benefits of the vacuum bagging process, a better interfacial bond with inserts and cores is often achieved. ⁃ Used to manufacture high strength to weight ratio parts.

For profiting from the aforementioned advantages of the autoclave moulding process of manufacturing fibre-reinforced polymer composites, this process has been widely used to manufacture numerous engineering parts mainly by aerospace, marine and military companies. These products include, but are not limited to, aircraft components, military, marine and space crafts, as well as missiles. Despite the aforementioned benefits of the autoclave moulding process, it has the following few drawbacks: ⁃ Low production rate. ⁃ Restriction on composite component size, which depends on the size of the autoclave machine. ⁃ Involvement of skilled labour. ⁃ Expensive technique for processing composite.

4.3.12 Out-of-autoclave moulding In a bid to reduce rigid manufacturing environment and significant acquisition, tooling and operation costs that are associated with traditional autoclave moulding techniques, an out-of-autoclave (OoA) moulding was invented to produce autoclave-quality components, using vacuum bag-only (VBO). These costs are much with large components. In addition, the OoA process has gained attention today, especially due to its lower cure pressure through VBO, which consequently eliminates autoclave-induced manufacturing defects. However, a lower cure pressure must be used with care; it may result in a relatively low mechanical performance (mostly toughness), out-time of only a week and high porosity or uneven resin bleed, especially within high fibre volume fraction (Centea et al., 2015). OoA composite manufacturing technique is performed in a closed mould, where vacuum, pressure and heat are applied by means other than an autoclave. Examples

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of methods under this technique include, but are not limited to, resin transfer moulding and vacuum-assisted resin transfer moulding. Comparatively, the energy consumption in the autoclave method is relatively higher than that of the OoA method, due to higher cure pressure and temperature required. These consequently increase the operational costs and reduce the sustainability of the conventional autoclave technology. Also, the size of the autoclaved composite material or part is limited by the capacity of the autoclave machine. Hence, it hardly supports flexibility in sizes, from small to big composite parts.

4.3.12.1 Autoclave and out-of-autoclave curing processes Along with different manufacturing processes employed in the composite fabrication process, the curing methods and parameters equally play an important role in the final properties of the composites. Because the curing process determines resin-rich areas, porosity formation, adhesion between reinforcement and the matrices, the two key curing processes employed in composite fabrication are subsequently elucidated. ⁃ Autoclave curing: This is the most commonly used curing process used in the fabrication of primary and secondary composite structures. It uses combined pressure and vacuum, which helps to achieve composites with a very small amount of void contents and a highly reliable performance. However, this process uses a significant amount of energy and requires high operating costs, as previously discussed. ⁃ Oven or out-of-autoclave curing: In this process, high-quality composites are obtained. However, the OoA process is not much used to fabricate primary structural components, unlike the autoclave process. In recent years, due to its low operating costs and environmental benefits, the OoA process had been employed in composite manufacturing.

4.3.13 Additive manufacturing Advancement in technology has produced additive manufacturing (AM) technique, commonly referred to as 3D printing. AM or 3D printing is a process whereby desired components are built from a three-dimensional (3D) computer-aided models in a successive layer-by-layer pattern, through the ejection of already prepared materials via a nozzle (Berman, 2012; Parandoush and Lin, 2017). Manufacturing of parts began many decades ago with traditional subtractive manufacturing (SM) technology. SM technology involves the removal of unwanted parts from a whole material, mainly using machining processes. Due to the drawbacks of SM and several benefits of AM technology, attention and/or attraction of many manufacturing industries to the use of AM technology have been widely increasing. Disadvantages of SM technology include, but are not limited to, longer production time, lesser accuracy, higher cost of manufacturing, a greater possibility of failure and accident, higher energy consumption, higher scrap/wastes and inability to produce complex geometry or product with many intricacies. With the effective application of AM technology, the listed challenges can be easily eliminated.

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The introduction of AM processes to the manufacturing of FRP composite manufacturing can be traced back to a few years ago. Today, AM technology has been developed in many sectors, such as building and construction, digital art, tissue/biomedical engineering, architectural design and importantly, composite manufacturing, to mention but a few. Recently, the use of robots in manufacturing is promoting various innovative FRP composite manufacturing techniques. Advances in AM or 3D printing techniques of FRP composites include fused deposition modelling (FDM), extrusion, selective laser sintering (SLS), stereolithography (SLT), laminated object manufacturing (LOM) and recently, the four-dimensional (4D) printing of active FRP composites (Parandoush and Lin, 2017), as simply and briefly explained in Fig. 4.23(aef), respectively. The 4D printing is a process that accommodates the use of smart or active materials, which can respond to external stimuli (such as heat, chemical, cold, among others) and change to pre-programmed shapes or self-transformed structures. The fourth dimension denotes time (Mitchell et al., 2018). The choice of process to use depends on the nature of reinforcement and matrix of the anticipated composite product, cost, quality, properties, quantity, volume/size, time, among other factors. AM of FRP composites has improved the efficiency of AM with the capability to fabricate highly customised components with better mechanical properties when compared with unreinforced polymeric products. Nevertheless, with the increasing benefits of AM technology in the field of FRP composite manufacturing, there are still some concerns that require attention to further optimise it. They include fibre orientation, blockage of nozzle and printer heads due to filler entanglement, the formation of a void, poor fibre-matrix interfacial adhesion, increased curing time, agglomerate and non-homogeneous FRP composite formation, as well as predictive modelling and simulation. Although, a few studies have been carried out on 4D printing of active FRP composites (Ge et al., 2013; Li et al., 2017; Miao et al., 2017; Rajkumar and Shanmugam, 2018; Zhang et al., 2019; Piedade, 2019; Ahmed et al., 2020), finite element methods (Weinan et al., 2007; Pineda et al., 2013; Zhang and Xu, 2013), analytical and modelling techniques (Li et al., 2002; Modniks and Andersons, 2013; Melenka et al., 2015) of additively manufactured FRP composite materials, but there are still opportunities for cutting edge research in a bid to continuously improving the AM or 3D and 4D printing of FRP composite materials/products.

4.3.14 Brief comparison among manufacturing processes The choice of use of the above-mentioned manufacturing process depends on cost contribution (from materials, labour and tooling) and required energy intensity. Therefore, the attention of many biocomposite manufacturers has been shifted from the extrusion process to either resin transfer moulding or compression moulding technique because of the high energy intensity required for the extrusion process. This is the reason why the foremost manufacturing techniques for natural fibre-reinforced polymer biocomposites are compression, resin transfer and injection moulding. For instance, Lotfi et al., (2019) conducted a comparative study on energy consumptions of extrusion, hand lay-up, resin transfer moulding, compression moulding and

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(a) Liquifier head Extrusion nozzles Part Support Print bed Support material spool (if necessary)

Build platform

Build material spool

(b) Fused filament fabrication

Other approaches Prepreg composite filament

In–situ coating with liquid resin To extruder

Impregnated fibre

Dry fibre

In–situ fusion with molten thermoplastic Dry fibre

Molten thermoplastics

Liquid deposition modelling Randomly oreinted short fibres in paste or liquid

Cross-section view Fibre

Matrix

Aligned fibres

Inter–layer boundaries

Voids

Figure 4.23 Additive manufacturing technologies, showing (a) fused deposition modelling, (b) extrusion, (c) selective laser sintering, (d) stereolithography, (e) laminated object manufacturing and (f) 4D printing of active FRP composites (Ning et al., 2015; Goh et al., 2019; Shahzad et al., 2014; Pan and Patil, 2017; Ahn et al., 2012; Miao et al., 2017), respectively.

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Laser Mirror scanner

(c)

XY deflection Roller / scraper

f-Tlens Protective atmosphere

Y

X Z

Feed container Build cylinder Overflow container

(d)

DMD based projection unit

Z-stage Electrostatic deposition unit Platform Particle collection plate

Z

Built physical resin model

X Particles dropped on the resin surface

Liquid resin

Mirror

(e)

Laser beam X-Y moving optic head

Laser Heated roller

Current laser Part layer contour Previous layer

Material sheet Material supply roll

Figure 4.23 cont’d.

Fabricated part and support material Platform

Waste take-up roll

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(f) Top down Heat

Cool

Side view

Swelling

t=0

t = 5 min

Swelling

t=0

t = 25 min

Figure 4.23 cont'd.

automated fibre placement and recorded approximately 19, 15, 13, 12 and 3 MJ/kg, respectively. It was observed that the automated fibre placement exhibited minimum energy intensity among the manufacturing methods that were considered. Also, the gross costs (including sum of tooling, materials and total labour cost/unit) of approximately $953, 619, 534, 463 and 446 were spent on resin transfer moulding, hand layup, automated fibre placement, compression moulding and extrusion, respectively. This implies that the extrusion method, with the lowest cost of $446, was the most economical process, among others (Fig. 4.24).

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(a) 1000 Tooling cost/unit

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CM: compression moulding

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Figure 4.24 Comparison of (a) cost per unit and (b) energy intensity of selected composite manufacturing methods (Lotfi et al., 2019).

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Similarly, the ultimate tensile strengths (UTS) and elastic moduli (EM) of hand layup, vacuum infusion and vacuum bagging manufactured composite samples have been comparatively studied (Abdurohman et al., 2018). The results obtained show that samples fabricated through the vacuum infusion method recorded the highest UTS and EM, as presented in Fig. 4.25. In moving forward, both quality and mechanical behaviour performance of OoA prepreg and autoclaved composite materials have been studied and compared (Sutter et al., 2019). From the result obtained, it was evident

(a) 400 346.15 350 300

271.298 260.982

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(b) 12000 10000

10673.4 9221.9 8660.52

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Vacuum bagging

Figure 4.25 Comparison among (a) ultimate tensile strengths and (b) elastic moduli (MPa) of differently manufactured FRP composite laminates (Abdurohman et al., 2018).

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from the early results that OoA manufactured composite material recorded a decrease in both quality and mechanical property performance after a prolonged out-time. Notwithstanding, it exhibited a similar performance to the autoclaved materials when subjected to an ambient shop floor handling, within a few days of processing duration. As technology advances to improve the manufacturing process of sustainable and lightweight biocomposite materials and parts, a combination of different processes has been designed and developed. These include, but are not limited to, liquid compound moulding processes (such as RTM light and compression RTM), extrusion-injection and extrusion-compression moulding with different screw, mould and die designs (Faruk et al., 2012), in addition to the application of automated systems (robots) and efficient software packages.

4.4 Key drivers for cleaner production or green manufacturing The concept of cleaner production includes the development and application of new, reliable, efficient, simple and new technologies, as well as intense, innovative activities that are better than the existing conventional or traditional ones in terms of environmental protection and waste minimisation. Biocomposites are environmentally friendly products, unlike metallic products and their alloys. They possess the following properties to support cleaner production: biodegradability, renewability, sustainability and recyclability, especially when both reinforcements and matrices are bio-products (green composites). Both the life cycle of biocomposite materials and their manufacturing processes support the goals of cleaner production. Cleaner production involves the use of cleaner and renewable sources of energy, such as sun, wind and water, and the possibility of building the recycling process into the manufacturing process of biocomposite materials, which makes the entire process cleaner. Biocomposite waste can be recycled. Therefore, our environments (air, water and land) are protected from harmful substances, called pollutants. Also, there is an assurance of satisfactory end-of product disposal. In addition, composite materials are recently used to produce filters for removing particles from gas streams. Hence, air pollution is reduced. For instance, powerful and efficient filters are manufactured by combining woven and non-woven fabrics. These filters are used to clean the air of commercial plants from several particles. Also, filtration fabrics are produced for paper industries, whereby a high volume of water is removed for each tonne of paper produced. This is achieved through the following strategies: 1. Decrease the water pollution by chemicals, using a higher density of yarns with greater retention capability in newly developed woven fabrics. 2. Reduce the energy consumption per production unit by decreasing the water content of paper with newly developed ultra-thin fabrics. 3. Increase or promote the use of recycled fibres by developing new yarns and fabrics that possess both sticky and contaminant resistance properties.

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Figure 4.26 A CAD illustration of three dimensional woven based composites. (From Gereke, T., Cherif, C., 2019. A review of numerical models for 3D woven composite reinforcements. Composite Structures 209, 60e66).

Also, the application of three-dimensional woven-based composites (Fig. 4.26) in aviation industry helps to reduce the quantity of fuel consumption and resultant carbon emissions, noise patters, as a considerable weight of the aircraft and noise level are reduced. For example, there is a 20% decrease in fuel consumption of a Boeing 787 aeroplane compared with other similar aircraft. There is a decrease in the energy required to manufacture the aircraft, quantity of scrapped materials and use of spare parts due to better composite properties that yield a greater wear resistance and durability or life span (Mourad, 2012).

4.5 Manufacturing defects Biocomposite materials have attracted wide applications due to their better inherent properties: corrosion and wear resistance, high strength and stiffness, low cost and density, sustainability and renewability, as well as recyclability and biodegradability, when compared with some metals and their alloys. Nevertheless, these outstanding properties can be compromised and destroyed due to the wrong choice of manufacturing processes and poor management or control of process parameters, which consequently cause various types of manufacturing defects. These defects could be caused during materials processing by environmental, mechanical and/or human factors. Fig. 4.27 shows various manufacturing defect formations and their associated or dependent relative process design parameters. Defects alter the materials, as well as mechanical, thermal, optical, acoustic and electrical properties of the manufactured biocomposites. The functionality of both fibres and matrices, which are the main constituents of a reinforced biocomposite, as well as coupling agents and fillers, can be adversely affected with an incorrect selection of both processes. It has been reported that nearly 44% of failures in fibre-reinforced polymer composites is caused by the manufacturing process (Knoeller, 2018). The possible common defects associated with the manufacturing of biocomposites and their causes include microcracks and cracks, temperature effects, moisture absorption, inclusions or contamination, porosity (void or pores), among others, as simply depicted in Fig. 4.28. They are subsequently and extensively discussed.

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Winding/draping/forming design parameters: • • • • •

Pre-load tension Holding force profile Pre-shear Direction Starting point

Curing design parameters: • Pulling speed • Temperature profile • Tool geometry

Misalignment Shear deformation

Curing stage

Wrinkles Thermal properties variation

Winding/draping/forming stage

Residual stresses Incomplete cure

Fibre volume fraction/thickness variation Permeability variation

Distortion Cure induced voids and cracking

Resin rich area Voids

Filling/consolidation stage

Dry spots

Filling design parameters: • • • • •

Injection rate/pressure Mould filling temperature profile Gates/vents locations Gates/vents numbers Pressure profile

Figure 4.27 Various manufacturing defect formations and their dependent process design parameters (Struzziero et al., 2019). Delamination

Broken fibre

Debonding

Wrinkle

Porosity

Void

Resin rich

Matrix crack

Foreign object

Blister

Figure 4.28 Common manufacturing-induced defects of fibre-reinforced composites (Bowkett and Thanapalan, 2017).

4.5.1 Microcracks and cracks Both matrix and reinforcements expand and contract at different temperatures because of their dissimilar coefficients of thermal expansion (CTE). A volumetric contraction of the matrix occurs, the CTE of matrix is often higher than that of reinforcements (Wisnom et al., 2006). This phenomenon occurs after thermoplastic polymeric

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1  in 100

Matrix micro-cracks within plies

Delamination (separation) at ply boundary

Figure 4.29 Formation of matrix microcracks within plies, showing their consequent delamination defect.

biocomposites are cooled to their service temperature subsequent to moulding process. Thermal stresses are developed in the fibres and around matrix during cooling and heating operations, due to the different contraction rates of fibres and matrix (Parlevliet et al., 2006). An interface de-bonding probably occurs when the magnitude of residual stresses is more than the yield strength of the biocomposite, later results in a transverse or microcracks (Timmerman et al., 2003; Parlevliet et al., 2006; Parlevliet et al., 2007). In addition, microcracks can occur from the stresses formed within thermoset polymer biocomposites during curing. These stresses increase whenever the temperature increases. This causes a rise in the size or shape and density of the cracks and possibly a delamination defect (Wisnom et al., 2006). Also, the stress concentration increases at ply interfaces. Therefore, the formation of microcracks within these interfaces often results in delamination (Fig. 4.29). Microcracks are tolerated at low densities in materials engineering practice. However, experimental results have confirmed that delamination occurs when a critical microcrack density is reached in the presence of multiple microcracks (Parlevliet et al., 2007; Knoeller, 2018). Therefore, it may be a regrettable decision to accept microcracking within biocomposite materials during the manufacturing process, as it damages the materials by reducing their mechanical (CTE, cyclic or fatigue and longitudinal stiffness) properties (Timmerman et al., 2003). The crack occurs when there is an actually visible separation in a material. It initiates as a microcrack and propagates when the total local energy exceeds that which the material can absorb (Strong, 2008). Cracking is a processing flaw frequently associated with the use of a gel coat; a protective layer applied to the mould before reinforcement is placed, of a biocomposite material. This gel coat must be capable of withstanding large stresses during moulding and de-moulding stages. There is an additional stress added to the biocomposite parts whenever there is sticking between the mould and the material, except when an alternative improved technique is used. A well prepared and treated gel coats with release agents could either prevents or minimises the sticking effect. Crack leads to delamination that reduces both strength and stiffness of the biocomposite, and eventually, causes a catastrophic fracture.

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4.5.2 Temperature effects Manufacturing of biocomposite involves thermal process, whereby temperature changes or heat transfers between the fibres and the matrix. This results in thermal stresses and strains in the biocomposites. Heat energy causes a free expansion of the laminate layers in a biocomposite laminate type. This causes internal stresses and failure if this expansion is constrained by the adjacent laminate layers of a biocomposite with different oriented reinforcements or fibres. The thermoplastic matrices may not be able to continuously transfer loads efficiently, prior to untimely failure, if the temperature higher than the glass transition temperature is allowed (Knoeller, 2018). Additionally, the use of improper time and/or temperature during the curing process is one of the principal and common sources of temperature effects.

4.5.3 Moisture absorption Biocomposites have a tendency of absorbing moisture or water through fibre, matrix, fibre-matrix interface, areas that have been already affected by porosity, microcracking, cracking and delamination. The fibre absorbs less moisture or other liquid such as water than the matrix materials through capillary action before the moisture is absorbed by the matrix. Consequently, the chemical composition of the liquid resin is altered. There is also a decrease in the glass transition temperature and mechanical properties, such as strength and elasticity (Knoeller, 2018). When moisture is restricted within reinforcements, due to the barriers caused by the reinforcements against moisture absorption, there is always a swelling effect. If this effect is prolonged and later freeze, it causes fibre-matrix de-bonding or an inter-laminar delamination. Therefore, materials that are susceptible to moisture absorption must be kept in a low humidity environment and moisture must be removed from the thermoset polymer matrix biocomposites before they are subjected to a high-temperature curing process to preclude expansion and resultant delamination (Knoeller, 2018).

4.5.4 Inclusions or contamination The physical and mechanical properties of biocomposite materials are often affected by an enclosure of foreign bodies, especially solid materials. Obviously, an encapsulated strange material possesses different properties, when compared with the main constituents of the biocomposites. Energy fields and structural stresses can be transmitted through inclusions. For instance, foreign particles such as irrelevant or unrelated fibres and pieces of a plastic release film/peel ply that are not removed from the surface of prepreg can cause contamination defects (Knoeller, 2018).

4.5.5 Porosity (void or pores) Porosity can be described as the volume fraction of small microvoids (a space that is neither occupied by fibre nor matrix) in a biocomposite material (Fernlund et al., 2016). A space occupied by entrapped volatile gases or air (non-solid foreign material)

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is known as void. Trapped gas when mixing resin or bridging ply and wrinkling, insufficient use of adhesives/matrices, inappropriate pressure used during curing cycle, as well as dissolved and absorbed gases and water can result in voids or blisters. Conversely, pores result from insufficient fibre wetting by the matrix or insufficient infiltration of the fibre tows (Matzkanin and Yolken, 2007). Both voids and pores are sometimes used interchangeably. Precisely, voids and pores are formed under (or beneath) and on the surface of fibre-reinforced biocomposites, respectively. These two defects can be detected by using ultrasound, acoustic, acousto-ultrasonics, X-ray, to mention but a few techniques. Porosity has a very significant effect on the mechanical behaviours of biocomposites, among other important properties. It is not easy to avoid in biocomposites. Therefore, much concentration has been given to minimise it in a biocomposite material. It is caused due to the inclusion of air during the manufacturing process, especially when there is poor wettability of the natural fibres, presence of lumens in most plant fibres (such as hemp and flax), low compatibility of fibres or hollow feature within natural fibres and/or fibre bundles. Although these hollow features and lumens could become closer during the manufacturing process when subjected to high pressure (Madsen et al., 2009; Pickering et al., 2016). In moving forward, there are different types of voids and porosity. Depending on the mode and location of formation, voids could be inter-laminar (between plies), fibre tow within partially impregnated fibre tows or resin (fully enclosed by resin), as depicted in Fig. 4.30. Both inter-laminar and fibre tow voids are commonly named bulk voids because of their connection to the breathing network and they can be removed by de-bulking. Similarly, types of porosity depend on their various shapes and sizes. There are spherical, cylindrical and microporosity, as shown in Fig. 4.31. Also, there is surface porosity, which occurs on the biocomposite laminate surface, as depicted Fig. 4.32. The image was captured by light reflecting off the surface. It has no significant impact on the mechanical behaviours, but it affects the aesthetic appearance and aerodynamic properties of the biocomposites.

Epoxy (matrix)

Perpendicular carbon fibre tows Fiber tow void

Parallel carbon fibre tows

Interlaminar void

Resin void

Figure 4.30 Typical types of voids and their locations of formation (Fernlund et al., 2016; Farhang, 2014).

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Spherical porosity

Micro porosity

Cylindrical porosity

Resin rich region

Fibre rich region

Figure 4.31 The micrographs of different types of porosity and their features: shapes, sizes and locations (Hakim et al., 2017).

Figure 4.32 Formation of surface porosity (black) on a small and flat MTM45-1/CF2426A prepreg laminate (Wells, 2015).

Admittedly, porosity has been identified as one of the crucial manufacturing defects of biocomposite materials. It is quite uneasy to obtain 0% porosity in a biocomposite; the presence of it above a certain limit can be greatly disadvantageous to the properties, especially mechanical (moduli, delamination resistance, inter-laminar shear, flexural/ bending, compressive, static and fatigue strengths), water/moisture absorption behaviours and structural reliability of the final biocomposite products. However, it can be managed or reduced through the void sinks mechanism. This mechanism involves an application of vacuum evacuation, bubble mobility and increased matrix pressure. It depends on the materials (fibre and matrix) and the manufacturing process used. Therefore, in order to monitor and reduce the formation of porosity during composite manufacturing, Fernlund et al. (2016) proposed a model by assuming that gas is an ideal gas and flows according to Darcy’s law. Hence, the governing differential Eq. (4.1) is applied to formulate Eq. (4.2).

Design, manufacturing processes and their effects on bio-composite properties

vp K v vp   $ p$ vt m vx vx

165

! ¼0

(4.1)

where ; p and t represents porosity, pressure (Pa) and time (s), respectively and K, m and x denote gas permeability (m2), gas dynamic viscosity (Pa.s) and distance (m), respectively. t ¼

   1 m  1 F pv 0:6 2 In  L  p0 K 0:9  p0

(4.2)

where t  ; p0 F , pv and L are minimum required de-bulk time, initial pressure, a specific or final porosity level, uniform pressure and length, respectively. In addition, porosity in natural fibre-reinforced polymer biocomposites increases with the fibre content. It increases at a high rate if the geometrical compaction limit is exceeded, which depends on the types of fibres used and the fibre orientation. Its levels could be critical because of the high effect on the mechanical behaviours of natural FRP composites. For instance, Madsen and Lilholt (2003) reported that porosity fractions increased from 0.04 to 0.08 when the fibre weight fractions increased from 0.56 to 0.72 in a non-treated flax fibre-reinforced polypropylene biocomposite. It is important to quantify for this defect in a model in order to have a better prediction of the properties (axial stiffness and tensile strength) of the concerned biocomposite. Porosity defects can occur in a biocomposites if the matrix flow path is increasingly complex during the manufacturing process stage (Baghaei et al., 2014). Further explanation was given on the effects of alkali treatment, hemp yarn and non-woven types on porosity of the hemp-reinforced polylactic acid (PLA) biocomposite samples. The maximum improvement in terms of mechanical properties was achieved with alkalitreated hemp/PLA yarn when compared with the untreated counterparts. This was attributed to the better compact and close packing of the PLA/hemp yarns within the biocomposite system, different from the PLA/hemp non-woven biocomposite sample. Moreover, an occurrence of porosity depends on the type of manufacturing process and degree of process parameters considered. During the extrusion process, relative high screw speed could lead to high porosity due to non-uniform dispersion of the fibres and shorter residence time, and subsequently, reduce the mechanical properties (such as tensile strength) of the biocomposites (Ku et al., 2011). In addition, a low curing pressure (Fernlund et al., 2016), high humidity (Grunenfelder and Nutt, 2010; Kay and Fernlund, 2012), insufficient de-bulk or gas removal (Kay and Fernlund, 2012) and deficient vacuum (Kay and Fernlund, 2012; Centea and Hubert, 2014) during OoA processing of prepregs could result to porous biocomposite materials. However, a low porosity in OoA prepreg processing is possible by keeping volatiles in solution, through vacuum evacuation of trapped air and resin infiltration, as a three-step approach recommended by Fernlund et al. (2016). Hakim et al. (2017) reported that the presence of porosity reduced the mechanical (both mode I cyclic strain energy release fatigue life and mode I static inter-laminar fracture toughness) of carbon

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fibre-reinforced polymer composites. Matzkanin and Yolken (2007) stated that approximately 7% of inter-laminar strength can be reduced for every 1% of voids present in a fibre-reinforced polymer composites, up to a void content of 4%. In addition, a nearly 5% and 50% of strength and fatigue life of a fibre-reinforced polymer composite product are reduced for each 1% increase in voids. Therefore, these composites are more liable to degradation in harsh environments. Between 2% and 2.5% void limit is accepted in practice (Knoeller, 2018). Both voids can be excluded from the final fibre-reinforced polymer biocomposite product by drying prepreg in a room with a controlled humidity before lamination and pores should be eliminated during the OoA process sincerely pores are almost impossible to eliminate from the prepreg (Wood and Bader, 1994).

4.5.6 Other manufacturing defects Other manufacturing defects in biocomposite materials include fibre and/or resin starved and rich areas (Fig. 4.31), usually caused due to non-uniform or uncontrolled distribution of fibres and flow of resin during moulding, respectively. Also, there are fibre kinking or waviness and misalignment of fibres. During prepreg preparation, pultrusion and filament winding processes, and improper tension of fibres causes a very complex phenomenon, called fibre kinking. Fig. 4.33 depicts an optical microscopic cross-section of a well-examined squared panel specimen after curing, with both low and high random waviness or kinking defect, and their failure progressions (Fig. 4.34).

(a)

(b)

High waviness specimen

304 mm

Area with high waviness

(c)

Low waviness specimen

304 mm

Figure 4.33 A well examined (a) composite panel, showing both (b) high and (c) low fibre waviness manufacturing defects (Elhajjar et al., 2016).

Design, manufacturing processes and their effects on bio-composite properties

High waviness specimens

(b)

Low waviness specimens

Fibre kinking

Damage progression

(a)

167

Delamination onset

Fibre fracture

Progressive delamination

Figure 4.34 Failure evolution of (a) high and (b) low fibre waviness or kinking defect of a composite panel (Elhajjar et al., 2016).

Fibre misalignment is resulted from washing out of fibres by excessive flow of resin, non-conformity of the pre-selected lay-up and/or fibre filament winding arrangement and misoriented fibres (Fig. 4.35). Scratching or cutting of fibres often result to a broken filaments or fibre fracture, as another common biocomposite material manufacturing-induced defect.

1000 Pm

Figure 4.35 Fibre misalignment defect during the manufacturing process (Krishnamurthy, 2006).

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Last, there is a need for all the aforementioned manufacturing defects to be properly detected and characterised. Therefore, the subsequent Chapter five discusses different techniques used to characterise these defects. Importantly, Chapter six provides some improvements in designs and processes (manufacturing techniques) that are required and relevant to minimise the defects and enhance the properties of FRP biocomposites.

4.6 Conclusions The natural sources, better production processes and inherent properties of several fibres and matrices used for the manufacturing of reinforced polymeric biocomposites support the concepts of eco-design and sustainability. The era has gone when the most dependable and used engineering materials were only metals and alloys. The renewability, recyclability and biodegradability of many biocomposite products have been enhancing the design for environment. Actually, product design for the environment involves material, production, distribution, use and recovery stages. Each of these stages is very important and cannot be compromised in order to protect environments from harmful processes and products. Evidently, biocomposite technology is a key driver for cleaner production in terms of a low amount of energy consumption and waste emission, as well as the use of renewable energy, during processing. Biocomposite materials are easy to manipulate into final useful or desirable products, through well-designed manufacturing procedures. These include correct choice of fibres and matrices, suitable matrix polymer modification, efficient bio-fibre surface treatment and optimal manufacturing process. With the present advancement in processing and production technologies for fibrereinforced polymeric biocomposite, compression moulding, resin transfer moulding, extrusion and injection moulding techniques are the major manufacturing processes. But, an innovative design and development of a single method and combination of different processes are increasing. For example, extrusion-injection and extrusioncompression moulding with different screw, mould and die designs, the advent of AM or 3D and 4D printing, in addition to the application of automated or robotic systems with the help of efficient engineering software packages. However, fibre-reinforced polymeric biocomposites are susceptible to some manufacturing defects, as discussed. These defects could be caused during materials processing by environmental, mechanical and/or human factors. Defects reduce the quality of materials, as well as mechanical, thermal, optical, acoustic and electrical properties of the manufactured biocomposites. Hence, manufacturing defects must be avoided at all costs to maximally benefit from the outstanding properties of sustainable and lightweight biocomposites during various applications.

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Testing and damage characterisation of biocomposite materials 5.1

5

Introduction and context

The detrimental effects of synthetic materials to the environment in terms of pollution and the need for material sustainability have informed a recent and increased research on biocomposites worldwide, thus leading to noteworthy achievements in eco-friendly green technology (Faruk et al., 2012). Also, it has been reported that the mechanical behaviours of biocomposite materials are preferred when compared with their synthetic counterparts, having a comparative advantage in terms of cost and energy consumed in their production (Sanjay et al., 2018). These biocomposites are abundantly available by nature’s endowment and have found applications in such fields as electronics, aircraft, automobiles, sports (Shekar and Ramachandra, 2018), as well as construction, thus leading to the elimination of wastes in structural works (Sanjay et al., 2018). However, while biocomposites have gained commercial successes in these fields, care must be taken to ensure that the material matches the application of interest (Dicker et al., 2014). A number of variables affect the properties of biocomposites and these include fibre source environment, methods employed in their processing, type of fibre and fibre modification (Faruk et al., 2012).

5.2

Testing methods for damage characterisation and their importance

During their service life, composites materials undergo various loading conditions. Moreover, depending on loading methods employed, the damage modes and mechanisms involve complex scenarios where composites can lose its structural integrity fully or partially. In order to prevent such damages and failures, reliable damage detection and monitoring techniques are paramount important. Among these techniques, non-destructive evaluation (NDE) or also called non-destructive testing (NDT), is one of the most commonly used methods. This testing method requires that the damage on the surface of materials and their interiors are duly identified and characterised without cutting or modify the material in a sense that causes damage, i.e., materials are inspected, evaluated and characterised for defect assessment following established standards for testing and materials standards (ASTM E2533, 2017), without changing the fundamental features of the material nor causing any harm or damage to the material undergoing the test. Worthy of mention is the cost-effectiveness that these NDT

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Sustainable Composites for Lightweight Applications

techniques provide to either a sample or whole material investigation (Gholizadeh, 2016). NDT methods in biocomposites have found applications in quite a lot of fields. These include, but are not limited to, nuclear industry, manufacturing, storage tanks, aerospace, nuclear industry, tube and pipe production industries, security, military and defence, as well as characterisation of manufacturing defects in biocomposites. There are numerous techniques that have been designed and utilised in the NDT of various biocomposites. These include the following methods: n n n n n n n n n n n

Visual inspection (VI)/testing (VT) Ultrasonic examination Thermographic testing Radiographic testing Electromagnetic testing Acoustic emission inspection Acousto-ultrasonic testing Shearography testing Computed tomography scanning X-ray micro-computed tomography examination Scanning electron microscopy.

Summarily, various categories of NDT and their evaluation techniques are illustrated in Fig. 5.1. The explanations, applications, benefits and drawbacks of the aforementioned NDT techniques are subsequently explained. Other commonly used, available and improved testing and evaluation techniques are listed as thus: optical testing, magnetic particle examination, liquid penetrant inspection, digital image correlation, vibration method, digital X-ray radiography and infrared thermography inspection, although, a few of these methods have been briefly discussed under some main NDT techniques.

Non-destructive testing & evaluation

Visual inspection

Acoustic wave-based

Optical techniques

Imaging techniques

Electromagnetic fields

Visual and optical testing Liquid penetrant Dye penetrant

Acoustic emission Nonlinear acoustics Ultrasonic testing Acoustoultrasonic

Infrared thermography Terahertz testing Shearography Digital image correlation

X-ray radiography

Eddy-current Remote field testing testing Magnetic Magnetic particle flux leakage testing inspection

Neutron radiography γ -ray radiography

Figure 5.1 Broad classification of NDT and their related evaluation techniques (Wang et al., 2020).

Testing and damage characterisation of biocomposite materials

5.2.1

181

Visual inspection or testing

Usually, visual inspection (VI) or testing (VT), as the utmost used and simple form of NDT, has the advantage of saving time, as well as money. This is because other tests may not be necessary or a reduction in the number of other types of testing. Most importantly, visual inspection (VI) has the merit of fast processing. The process is also relatively affordable because it does not require any equipment, although not without its intrinsic demerits. The main drawback of VI includes the ability to examine external defects and damage to bio/composite materials. It cannot detect internal and/ or microscopic damage, such as inter-laminar delamination, voids, particle inclusion, cracks, fibre fracture, wavering and kinking, to mentioning but a few. This limitation is ascribed to human sight capacity. For instance, Dhakal et al. (2014a) visually examined the effects of temperatures and low-impact velocities on jute FRP biocomposites, as shown in Fig. 5.2(a). The same visual method was used to examine the influence of low-velocity impact load on jute FRP/methacrylated soybean oil biocomposite samples (Dhakal et al., 2014b). Similarly, De Rose et al. (2012) conducted a visual inspection of hemp/unsaturated polyester biocomposite samples after post-impact static and cyclic flexural tests.

(a)

(b)

Front

Rear

Figure 5.2 Visual examination results of the impact damage responses on front and rear/back faces of (a) jute and (b) hemp FRP/unsaturated polyester biocomposite samples (Dhakal et al., 2014a; De Rose et al., 2012).

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The results obtained are shown in Fig. 5.2(b). In addition, VI has been used to assess elastic deformation, crack initiation and propagation when investigating the influence of water absorption on hybrid flax/basalt FRP composites (Almansour et al., 2017).

5.2.2

Ultrasonic testing

In contrast to visual inspection, ultrasonic testing (UT) involves the use of some electronic components, such as a transmitter, receiver circuit, transducer and display device. With the UT testing method, the location of the crack, size of the flaw, orientation and other relevant features could be determined based on the signal information. Some of the high points of ultrasonic testing are its scanning speed, good flaw detection, good resolution and field deployment. However, it has disadvantages with respect to set up and skill requirements, i.e., ultrasonic testing is difficult to set up and requires a highly-skilled labour to ensure the accuracy of scanning. Assembly lines with the repetitive part design is an excellent application of ultrasonic testing. Generally, ultrasonic NDT has two approaches that are used in various fields. These are pulse-echo and through transmission approaches. In these approaches, the frequency of the sound waves used for detecting internal flaws in a material is quite high and lies between 1 and 50 MHz. Three modes which characterised ultrasonic testing are transmission, reflection and backscattering. All these modes depend on transducer range, coupling agents and frequencies. Defects present in homogeneous materials can be easily located using the pulseecho ultrasonic method. Here, waves scattering on flaws, wave energy loss transit time and energy loss as a result of attenuation are of utmost importance to the operator. This helps in locating material inconsistency, whether homogeneous or otherwise. Measurements of ultrasonic pulse velocity have been reported to be quite suitable for detecting, locating, imaging and quality control of large defects (Gholizadeh, 2016). The second approach, i.e., through transmission ultrasonic method ensures the transducer and receiver are placed at a fixed position away from the composite specimen and not on the sample surface. It is quite dissimilar from the common traditional ultrasonic techniques. Also, the transmission ultrasonic method has advantage of application to complex geometries where traditional transducer and receiver cannot make contact with part surface. Moreover, Fig. 5.3 depicts an experimental set-up. It can be observed that the ultrasonic wave passed through the defect-free Carbon fibre reinforced polymer (FRP) panel, but it rebounded by the internal defect present in the similar carbon FRP structure, as shown in Fig. 5.3(a) and (b), respectively. The UT results are compared with that of thermography testing subsequently.

5.2.3

Thermography testing

Thermography testing (TT) method is also called thermal imaging. Defects have the potential of changing the thermal properties of a material, such as thermal conductivity and amplitude, among others. When these defects move deeper into a part surface, they

Testing and damage characterisation of biocomposite materials

(a)

Water

Receiver

Emitter

183

(b)

CFRP panel

Tank

Figure 5.3 Experimental design of ultrasonic testing, showing the reaction of its wave to both (a) defect-free and (b) internal defect in the carbon FRP composite panels, respectively (Duan et al., 2019).

have a tendency of producing reduced or lower heat fluctuation when compared with material with a nearer defect on a part surface. Hence thermography examination is applicable for thin sample parts. A general rule is that thermography testing is not capable of picking defects with sample part depth greater than its diameter. When a part experiences impact damage or delamination, it causes a flaw which changes the thermal radiation of the impacted region of the material. This method has the advantage of inspecting large part surfaces. Also, it does not require a coupling, thus allowing for part inspection with only one side accessibility. Another point is that it does not have to couple distinguished thermography testing from many other types of inspections. However, the need for expensive and sensitive instruments, highly skilled inspection workers and lack of defect clarity in the part surface depth forms the demerits of this type of testing. A type of thermography testing, such as the Infrared thermography testing (IRT), records thermal radiation that is discharged from the surface of a specimen with the aid of an infrared camera, as illustrated in Fig. 5.4. The results of pulsed-TT are very similar to UT. For instance, Duan et al. (2019) compared the impact damage responses of 35 carbon FRP composite samples, with thickness and impact energy ranged from 2.3 to 4 mm and 10e36 J, respectively. The results obtained are shown in Fig. 5.5. As for the drawbacks of each of these non-destructive examinations, pulsed-TT and UT could not detect 45 and 0
oriented defects, respectively.

5.2.4

Radiography testing

Radiographic testing (RT) has been reported as the most widely applied inspection method. Delamination as the highest critical and commonly reported damage that FRP composite materials experience, often results in an air pocket damage. The delamination is clearly detected during RT only, provided the X-ray beam and its orientation is not perpendicular to each other. Many types of radiography exist with their various unique applications. When parts are not too thin or thick, then traditional RT is most appropriate. When parts are thin, in the range of 1e5 mm, the appropriate type of radiography is low voltage radiography. Gamma rays (g-rays) radiography applies to thick

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Infrared sensor

Heating lamps

Infrared waves Infrared wave absorption Defect Periodic heating

Heat diffusion

N images Amplitude and phase images FFT

i -th pixel

Figure 5.4 Illustration of a lock-in thermography technique, depicting thermal amplitude and phase micrograph generation (de Oliveira et al., 2020).

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 5.5 Comparative examination results between (a-d) UT and (e-h) pulsed-TT images carbon FRP composite samples (Duan et al., 2019).

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b a

c

a

d p

a

P

d

Washer edge d = 6 mm [90
/45
/45
/0]s

Figure 5.6 (i) Damaged bolted joint sample, showing (ii) various X-ray radiographic images of drilling-induced damage at ultimate load: (a) transverse matrix cracks, (c) delamination, (b) 0
and (d) 45
axial splits, respectively (Atas and Soutis, 2013).

parts as they penetrate the composites and have shorter wavelengths. Another type of radiography used specifically for detecting delamination and minor FRP sample matrix cracks is the penetrant-enhanced radiography. These radiography types help to detect non-uniform fibre distribution, large voids, translaminar cracks, inclusions, as well as fibre disorientation. The defects/damage are associated with weld lines and fibre wrinkles. Additionally, there are several types of radiographic testing techniques, with particular applications of each. These methods include computed radiography, digital radiography, film radiography and computed tomography. The X-ray computed tomography (XCT) can be described as a non-destructive technique for examining the interior structures of solid materials/FRP composite samples and also to digitally obtain information about their three-dimensional (3D) features and properties. XCT has a comparative benefit over projection radiology, due to the 3D captured micrograph of the FRP composite samples that it provides, i.e., the projection radiology generates a 2-D image only. Hence the data from XCT is simple and quickly readable. Its results are also reliable because XCT alters the observation scale from macroscopic to microscopic scale. More discussion on XCT can be found in subsequent sub-chapters. For instance, local damage mechanisms and joint strengths of bolted joints in different oriented carbon FRP composite laminates were identified (Atas and Soutis, 2013), using penetrant enhanced X-ray radiography testing (Fig. 5.6). This was done in an attempt to reduce the drilling-induced delamination damage on drilled holes of the samples in question. Another comprehensive study has been reported, whereby three different X-ray imaging inspections (micro-computed tomography, computed and digital radiography) were concomitantly applied to examine the presence of voids in a glass FRP composite sample (Rique et al., 2015).

5.2.5

Electromagnetic testing

As the name suggests, the electromagnetic testing method employs electricity and magnetism in the detection and evaluation of faults, fractures, corrosion or other material conditions. The approach here is that both magnetic fields and electric currents (or either of them) are induced within a test FRP composite sample, while the electromagnetic signal is detected. Some identified electromagnetic (EM) techniques are Eddy current (EC) inspection, Microwave open-ended waveguide imaging,

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(a)

Vertical linear axles

Manually adjustable linear axles Horizontal axle

Rotational axle Emitting coil

(b) Eddy current

Driver coil

Eddy current

Driver coil

In-plane waviness

Pickup coil

Pickup coil

Figure 5.7 Eddy current inspection: (a) measurement rig and (b) schematic induced EC in UD carbon FRP material without fibre waviness (left) and with in-plane waviness (right). (Berger, D., Lanza, G., 2017. Development and application of eddy current sensor arrays for process integrated inspection of carbon fibre preforms. Sensors 14 (4), 1e12.; Mizukami, K., Mizutani, Y., Todoroki, A., Suzuki, Y., 2016. Detection of in-plane and out-of-plane fibre waviness in unidirectional carbon fibre reinforced composites using eddy current testing. Composites Part B: Engineering 86, 84e94).

Couple spiral inductors, alternating current field measurement (ACFM), magnetic flux leakage (MFL) and remote field testing (RFT). They all have different physics as governed by their unique partial differential equations (PDEs). On moving forward, EC inspection depends on Faraday’s law, whereby the discontinuity in the conductivity distribution causes a change in the coil impedance, as illustrated in Fig. 5.7. The currents flow from one fibre to another through their contact points, along the fibres. This method is often used to detect damage in carbon FRP

Testing and damage characterisation of biocomposite materials

187

composites, provided defects are reflected in the change of the impedance recorded by its analyser. The EC inspection is commonly used for crack, impact damage, fibre breakage and corrosion detections, among other defects. For instance, Mook et al. (2001) employed special static, as well as rotary EC probes to identify some defects and characterised the shape of the damaged area after impact test. The local defects included impact damage, resin-rich zones, fibre breakage and delamination, as well as fibre orientation. Similarly, an EC theta probe was used to detect purposely induced low impact defects, produced at the energy of 0.25 J. Also, the correlation between the impact energy level and the signal phase was obtained (Koyama et al., 2011, 2013).

5.2.6

Acoustic emission inspection

Acoustic emission (AE) has proven to be an effective method for analysing imperfections. Material defects, such as fibre-matrix de-bonding, matrix microcracking, localised delamination or fibre pull-out and breakage, generate mechanical vibrations and produce stress waves at the origin, which concentrically disperse. Then, a group of piezoelectric detects the stress waves with high sensitivity. Moreover, two aspects uniquely distinguish the AE method from other widely used non-destructive testing methods. Firstly, the signal origin is considered, whereby the sound from the energy that is released in the FRP composite sample is captured in this method, as against the supply of energy to the object. The second distinguishing feature of the AE technique is its ability to address dynamism in a material, i.e., AE discerns between stagnant and developing defects significantly. Other merits of the acoustic emission method are its high sensitivity, assurance of process control with permanent sensor mounting without having to dismantle for specimen clean up and allowance of the use of multiple sensors which account for global and fast inspection. It is also worthy of note to mention that AE is useful in the detection of different types of damage due to fatigue loading, i.e., AE can detect fatigue damage types such as fibre fractures, fatigue cracks, fibre-matrix de-bonding, matrix microcracks and delamination. The disadvantage, however, of acoustic emission testing is that it requires a highly skilled inspector to map its data with the exact type of damage mechanism. Furthermore, an extensive literature review has been conducted by De Rosa et al. (2009) to report some applications of AE testing. It was used to monitor mechanical properties of natural FRP composites, such as interface investigations in single FRP composite tests, damage progression and detection of failure mechanisms, as well as crack propagation. For instance, AE has been used to understand the influence of fibre weaving on behaviours of three glass FRP composite materials (Mi et al., 2020). These weaved samples were a uniaxial, biaxial and triaxial direction, with fibre orientations/ ply sequences of [0
]8, [þ45
, 45
]4 and [0
, þ45
, 45
]4s, designated as UD, 2AX and 3AX, respectively. The signal amplitudes of the samples depicted the magnitude of their tensile strengths, especially after fast Fourier transformation (FFT), as shown in Fig. 5.8. AE also showed the damage patterns (matrix cracking, fibre pull-out, as well as fibre-matrix de-bonding), stress-strain curves and strain rates of the three samples. It was concluded that AE could effectively monitor the structural health of fibre-resin composite materials with complex fibre weavings.

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(b)

(a)

2000

Amplitude (dB)

Amplitude (dB)

1 0.5 0

−0.5 −1 3AX

300 2AX UD

0

100

200

Time (s)

1500 1000 500 0 3AX

1000 800 600 400 200 Frequency (kHz)

2AX UD 0

Figure 5.8 The AE waveforms of the three glass FRP composite laminate samples and their FFT outcomes (Mi et al., 2020). 100

Matrix cracking Interface failure Fiber breakage

Partial power 1 (%)

80

60

40

20 0 0

100

200 300 400 500 600 Weighted peak frequency (kHz)

700

Figure 5.9 AE results of carbon fabric laminate depicting various failures under flexural testing (Ali et al., 2019).

Moving forward, Ali et al. (2019) employed AE to analyse microscopic matrix cracking, interface failure and fibre breakage in woven carbon FRP composite laminates, as shown in Fig. 5.9. AE root-mean-square was applied to analyse the drilling evolution and drilling-induced damage mechanism of 16-layers glass FRP composite laminates (Ravishankar and Murthy, 2000). Andrew et al. (2016) also characterised damage and residual strength of repaired glass FRP composite laminates with the AE method, among others reported studies on the application of the AE testing technique.

5.2.7

Acousto-ultrasonic testing

As its name implies, the acousto-ultrasonic testing (AUT) type of testing combines AE and UT types. It is precisely applied to evaluate the level of impact of internal failures, irregularities and heterogeneity in FRP composite structures. This testing method is quite useful in spotting and assessing non-critical flaws in composites. It also helps to indicate accumulated damage in a structure caused by impact or fatigue, as shown in Fig. 5.10. From Fig. 5.10, the current and baseline signals depicted a fatigue crack

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189

1.5

Baseline Current

Normalized amplitude

1.0 0.5 0.0 −0.5 −1.0 −1.5

0

30

60

90

120

150

Time (us)

Figure 5.10 Acousto-ultrasonic results, showing the presence of fatigue crack (current) and notched prior to the fatigue treatment (baseline) (Su et al., 2014).

and notched prior to the fatigue treatment, respectively. However, a low-point of this method is the compulsory pre-calculations and setup prior to testing. Another disadvantage of this method is that individual large flaws, such as voids or delamination, cannot be detected.

5.2.8

Shearography testing

This type of examination depends on the laser optical technique. Composite failure often occurs by stress concentrations. The strain concentration level that surrounds a specific defect determines the criticality of the defect. Shearography is often used on foam and honeycomb structures. It also has an advantage of less susceptibility to noise when compared with many other types of NDT techniques. This allows for the use of less-skilled workers without extensive training to inspect and determine whether a part is useable or not. However, one major drawback of shearography is the difficulty in characterising other defect types apart from delamination and dis-bonding. This makes pairing with other NDT techniques inevitable in order to identify certain defects in composites (Fig. 5.11). Therefore, there are laser digital shearography in addition to laser and digital types.

5.2.9

Computed tomography scanning

The computed tomography (CT) scanner is typically a large, box-like machine with a short tunnel in the middle. The narrow examination table slides in and out with the electronic X-ray and X-ray tube detectors in the gantry. A computer workstation is used for imaging information processes. A CT system contains an X-ray source, an X-ray detector, a rotary table and a data process unit for visualisation, computation

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(a)

(b)

Shearography system

Image of disbonds

(c) Laser source

Loading frame

Display and processing

CCD camera

Polariser

Shearing device

Loading frame

Figure 5.11 (a) Testing for bonded insulation defect on the outer part of a rocket body, (b) dis-bond defect between the outer foam insulation and skin of the rocket, through (c) shearography system (Bossi and Giurgiutiu, 2014; Wang et al., 2020).

CT scanner

Detector

X-ray source

Rotary table

Figure 5.12 A CT scanning machine.

and data analysis of the results measured (Fig. 5.12). A CT system generates crosssection images through the projection of emitted photons beam and an object plane using angle positions that is angled while performing one revolution (Cantatore and Muller, 2011). When the emitted photons go through an object, some of them are scattered, absorbed with the rest transmitted. The attenuation represents the absorbed or

Testing and damage characterisation of biocomposite materials

(a)

(b) mm 3 2.5 2 1.5 1 0.5 0 −0.5 −1

191

μm 400 300 200 100 0 −100 −200 −300 −400 −500 −600 −700

Figure 5.13 CT scans of conventional and ultrasonically assisted drilled hemp FRP composite laminate samples (Wang et al., 2019).

scattered X-rays often result from interactions with an object (Hsieh, 2009). The attenuation prevents some X-ray from reaching the detector. The transmitted photons go through the object at an individual angle and are collected on the detector, which is then visualised by the computer. The visualisation creates a complete reconstruction of the object being scanned. Bartscher et al. (2007) stated that the 3D grey value data structure that is derived in this manner depicts the distribution density of the electron within the object measured. Unlike other visual scanning technologies that capture a point cloud, the CT scanning develops numerous X-ray images. They are combined to form a voxel data set. A voxel represents volume element or volumetric pixel, and it deploys X-ray attenuation effectively in distinguishing parts. For example, surface CT scans of conventional and ultrasonically assisted drilled hemp FRP composite laminate samples exhibit fibre push-out and burrs, as shown in Fig. 5.13 (Wang et al., 2019). These damages are not effectively observed, using a visual inspection and optical microscopy testing.

5.2.10 X-ray micro-computed tomography examination An example of model X-ray micro-computed tomography (mCT) scan XT H 225. XTH 225 (Fig. 5.14). It is ideal for a wide range of sample sizes and materials. It possesses three interchangeable sources: optional 225 kV rotating target, 180 kV transmission target and 225 kV reflection target. This model provides a microfocus X-ray source, high image resolution, a large inspection volume and readily available for ultrafast CT reconstruction. Also, its application area is wide, and it includes small castings, complex mechanisms and plastic parts together with natural specimens and several engineering materials. This instrument is advantageous because of its straightforward inspection automation, low cost-of-ownership, easy system operation, proprietary 225 kV microfocus X-ray source with a focal spot size of 3 mm and a high level of safety. It captures data very fast, together with images of high quality. From Fig. 5.14, part 1 shows the X-ray source (microfocus X-ray tube), part 2 indicates the sample in the rotating stage and part 3 depicts the phosphor detector.

192

(a)

(b) SDD

1

3

Fibre-optic taper

SOD

Micro-focus X-ray tube

1

2

2

Rotating stage

Controller

Phosphor CCD detector camera

Acquisition and display computer Trends in biotechnology

Figure 5.14 (a) XT H 225 X-ray mCT scanner, showing its set-up and (b) scanning/working mechanism.

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The microfocus X-ray source illuminates the object and a planar X-ray detector collects magnified projection images. Based on hundreds of angular views acquired while the object rotates, a computer synthesises a stack of virtual cross-section slices through the object. Then, scroll through the cross sections can be done, interpolating sections along different planes to inspect the internal structure and selecting simple or complex volumes of interest. Measurement of 3D morphometric parameters and the creation of realistic visual models for virtual travel within the object is achieved. This technology is of the order of >two to three micrometres. Moreover, an X-ray mCT scanner can be used to assess the barely visible impact damage failure, delamination of the layer of composite, matrix cracking, fibres breakage and fibres pull out the matrix, among other damage. It can assess FRP composite manufacturing defects. The X-ray mCT machine has asimilar working principle to CT scanner, but with better magnification. It is possible to observe an impacted FRP composite laminate sample with a scale/lamina of one failure to show the different mechanisms of the impact failure (Dhakal et al., 2018a,b). More also, Ismail et al. (2016) used X-ray CT micrographs to show peel-up and push-out inter-laminar drilling-induced delamination damage and fibre uncut on drilled holes of carbon and hemp FRP composite samples, as depicted in Fig. 5.15(a) and (b), respectively. Moreover, in recent years, this technique has been used to model the crack and damage progression in composite materials. In which, a high-resolution synchrotron X-ray CT is utilised to map the crack distribution, porosity formation and defects detection, among others, using 3D reconstruction for FE modelling of natural fibre and biodegradable matrix. For instance, Jiang et al. (2020) employed or established 3D finite element model to investigate the water diffusion response of jute/PLA composite based on the X-ray CT technique, as depicted in Fig. 5.16. Peel-up delamination

Feed direction Push-out delamination

(a)

Back

Front Uncut fibres

Uncut fibres

Small burrs

(b)

HFRP

Figure 5.15 X-ray CT micrographs, depicting (a) delamination damage and (b) fibre uncut and burrs on drilled holes of carbon and hemp FRP composite samples (Ismail et al., 2016).

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Mould flow

direction

(a)

3D FE model

(b)

(e)

Z Threshold segmentation X

y Element model

Assembley, material parameters and boundary conditions assign

(d) (i)

(ii)

(ii)

Import Abaqus software PLA matrix

Jute fibres

PLA matrix

Jute fibres

Figure 5.16 3D model established for jute fibre and PLA matrix, depicting (a) 3D view, (b) 3D volume representation by introducing a threshold segmentation, (c) 3D rebuilding element model of jute fibre and PLA matrix after surface and mesh generation, (d) element model of jute fibre and PLA matrix using ABAQUS software and (e) Already built 3D finite element model with assembled jute fibre and PLA matrix, assigned material parameters and boundary conditions (Jiang et al., 2020).

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(c) (i)

Surface and mesh generation

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195

5.2.11 Scanning electron microscopy A scanning electron microscope (SEM) focuses electron beam over a prepared FRP composite sample surface to create an image or micrograph. A comprehensive working principle of SEM is depicted in Fig. 5.17. The electrons in the beam interact with the sample to produce various signals that can be used to obtain information about the surface topography and composition. The trimmed and sputter-coated sample is attached to a mount with putty, as shown in Fig. 5.18(a). Then, it is loaded onto a platform and closed in the sample chamber (Fig. 5.18(b)) before the vacuum is turned on. When the right pressure is reached, the beam starts; first, at a low intensity to avoid bombardment damage to the sample and then switched to a higher one ready for imaging. Images of the sample can be taken at a low magnification to show an overview of the damage, followed by increasing levels of magnification at key points in order to better identify and view any specific failure mechanisms. Furthermore, SEM provides an excellent technique for examination of surface morphology of natural and treated biofibres. For example, the micrograph of untreated biofibres, as shown in Fig. 5.19(a), indicates the presence of impurities on the surface, mostly waxes and oil. Waxes and oils provide a protective layer to the surface of the

FE-gun

Condensor

EsB detector Filter grid

Beam booster

Inlens SE detector Gemini objective

Magnetic lens

Scan coils Electrostatic lens Sample

Figure 5.17 Detailed illustration of ZEISS EVO LS 10 scanning electron microscopy.

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Figure 5.18 (a) Coated and mounted SEM sample and (b) SEM vacuum chamber loading platform.

(a)

0406 5.0KV

X500 10Pm WD35

(b)

0421 5.0KV

X500 10Pm WD35

Figure 5.19 SEM micrographs of hemp fibre (a) before and (b) after NaOH treatment.

fibres. However, a surface free of impurities is obtained after alkali treatment, as presented in Fig. 5.19(b). In addition, numerous studies have been conducted to show various applications of SEM. For instance, Dhakal et al. (2018c) were able to detect entanglement of some garnet abrasive particles within both peel-up and push-out types of drilling-induced inter-laminar delamination damage on drilled hole surfaces of hybrid carbon/flax FRP composite samples, as shown in Fig. 5.20. Similarly, various damage caused by drilling (Ismail et al., 2016; Niamat et al., 2019), wear, scratch (Parikh and Gohil, 2017; Akpan et al., 2018; Rajini et al., 2019; Chanda et al., 2019; Chegdani and Mansori, 2019; Belotti et al., 2019; Cheng et al., 2020), impact, flexural and tensile (Dhakal et al., 2019; Thiagamani et al., 2019; Wu et al., 2019; Nagaprasad et al., 2020) tests on different FRP composite samples have been observed and well analysed using SEM examination. These damages include, but are not limited to, fibre fracture and breakage, matrix melting and cracking, voids, crack, de-bonding, resin-rich region, filler accumulation, delamination, fibre pull-out and uncut.

Testing and damage characterisation of biocomposite materials

Mag =

102 X

EHT = 15.00 kV WD = 17.0 mm

Mag = 102 X

Signal A = SE1 I Probe =

50 pA

EHT = 15.00 kV Signal A = SE1 WD = 18.5 mm I Probe = 50 pA

Date : 28 Jul 2017

200 μm

Mag = 102 X

EHT =15.00 kV WD = 19.5 mm

Signal A =SE1 I Probe = 50 pA

Date :2 Aug 2017 Time :9:33:59

20 μm

Mag = 500 X

EHT = 15.00 kV WD = 19.0 mm

Signal A = SE1 I Probe = 50 pA

Date :26 Jul 2017 Time :9:58:53

Time :9:55:44

Date :26 Jul 2017 Time :10:03:16

197

Figure 5.20 SEM micrographs, showing entanglement of garnet abrasive particles within interlaminar delamination after drilling operation (Dhakal et al., 2018c).

5.3

Damage mechanisms and types (key factors for improving damage resistance)

The inherent properties of FRP composite materials make their failures or damage quite different from that of homogeneous materials, such as metals (Fig. 5.21). It has been reported that more than 50% of composite failure occurs within the first 20% of its life. This implies that composite structure can sustain cracks in its environment. Generally, damage in the form of crack initiation occurs in a metal after more than 75% of its fatigue life (Jollivet et al., 2013). Mechanisms of damage in metals are well understood more that of FRP composites. Defects and damage in composites are associated with their manufacturing process and in-service life stages, respectively. Damages of various composite structures are inevitable, either after a short- or longterm service. Thick-wall, large-scale composite, composite laminates, sandwich and smart structures are subjected to various kinds of loads/forces or stresses as structural, semi-structural and/or non-structural components. Composite constituents, types of loads and loading conditions determine the modes and mechanisms of damage. For instance, impact or flexural load, P induces combined (a) compressive, (b) shear and (c) tensile stresses, which they eventually orchestrate the final failure of the impacted composite sandwich structure (Fig. 5.22), after the decrease in mechanical properties

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Damage

Delemination

Composite

Metal

Cracks & delamination Fiber failure

Damage in early cycles

Propagation & cracks

Nb cycle to failure

Nb cycles nb cycles for crack initiation

Figure 5.21 Comparison between fatigue damage progression in FRP composite and metallic materials (Jollivet et al., 2013). Impact location

Impact load, P

Surface deformation

Delamination

Skin (a) Compression (b) Shear

Core Skin

(c) Tension Support

Support

Matrix cracks due to bending

Fibre breakage

Matrix cracks due to shear

Figure 5.22 (a) Analysis of sandwich composite panel and (b) composite laminate (right) under impact loading (Greenhalgh, E.S., 2009. Defects and damage and their role in the failure of polymer composites. In: Greenhalgh, E.S. (Ed.). Failure analysis and fractography of polymer composites, Chapter 7, pp. 365e440. Woodhead Publishing: Cambridge, UK).

due to matrix cracking, fibre-matrix de-bonding and fibre breakage. These damages could similarly be caused by fatigue, lightning strikes and such alike. There are numerous damages associated with FRP composite materials. These include, but are not limited to, crack, de-bonding, delamination and waviness, through a particular evolution. Fig. 5.23 summarily depicts evolutions of some flaws/defects and damage associated with composite manufacturing and in-service/applications, respectively. The first aspect of composite manufacturing defects has been already and extensively discussed in the previous sub-chapters of this chapter. Therefore, the second part that deals with various damage or failures of FRP composite structures, their types, modes and mechanisms are hereby subsequently elucidated.

5.3.1

Damage types and mechanisms

Damage types and mechanisms depend on the types and locations of the failure. Some of the common types and mechanisms include interface damage or fibre-matrix de-bonding, as shown in Fig. 5.24(a). There is a matrix cracking or matrix damage.

Testing and damage characterisation of biocomposite materials

199

Scale dimension Nano-scale

Micro-scale

Meso-scale

Macro-scale

Manufacturing-induced flaws/defects

Matrix cracking

Nanoscale particles & imperfections

Fibre waviness

Voids, porosity, inclusions Fibre wrinkling

Fibre kinks/breakage

Misalignment of plies Misalignment of fibre orientation Laminate warping and buckling In-service damage evolution Yielding and failure of molecular, crystalline, or cross-linked networks

Matrix cracking

Delamination

Interface debonding Micro-buckling and waviness Fibre fracture and pull-out

Figure 5.23 Evolution of FRP composite manufacturing defects and in-service damage (Wang et al., 2020).

Stress

Stress

a a

b

C a

10 Pm

C

10 Pm

Figure 5.24 (a) fibre-matrix interfacial de-bonding and (b) matrix micro-cracks, caused by (c) locally strained area around the fibre (Arif et al., 2014).

In matrix cracking, stress, such as residual, could concentrate in an area to develop defects or flaws, which later promote crack propagation, as depicted in Fig. 5.24(b). Transversal de-bonding is followed by matrix failure, pull-out and longitudinal fibre failure. Others are compressive and shear damage. They can lead to delamination of the composite materials. Fibre fracture or breakage/fragmentation occurs when fibre ruptures through the through-thickness direction. Higher impact energy can cause fibre damage. Other types are described in Fig. 5.25. Detailed mechanisms of these and other damage initiations and propagations are elucidated later.

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Type A

Type B Type C

Type F horizontal

Type D

Type E

Inspection surface

Opposite skin of inspection area

Figure 5.25 Various damage, showing Types A: Delamination, B and E: dis-bonding, C: crack, D: crush and F: fluid ingress in a composite sandwich structure (Olympus, 2020).

5.3.2

Failure or damage modes

Inter-laminar damage or delamination is one of the main failure modes in the impact damage. Also, trans-laminar damage occurs when the final damage stage tallies with the fibre failure. Matrix damage can occur in mode I as an intra- and inter-laminar damage (Fig. 5.26). Delamination means the separation of individual layers and often ensues from the weak interface between the fibre and matrix. It is an inter-laminar crack between resin-rich area and fibre plies. Delamination at the interface layers can cause final failure. Delamination damage is commonly reported when FRP composites are subjected to compression load. Another factor that could contribute to the susceptibility of composites to delamination is the brittle nature of the matrix resin, which binds the laminates. Delamination can occur due to buckling and compressive loadings, among other stresses. It occurs often due to weak fibre-matrix interfacial adhesion. Delamination modes include modes I, II and III, which are caused by opening or peeling, sliding shear and tearing shear, respectively (Fig. 5.27). In composite drilling, it could be either peel-up (entry) or push-out (exit) delamination (Fig. 5.28), when the drilling thrust force is higher than the threshold value.

(a)

(b)

20 μm

(c)

100 μm

10 μm

Figure 5.26 SEM micrographs of (a) inter-laminar, (b) intra-laminar and (c) trans-laminar damage in FRP composites (Jollivet et al., 2013).

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201

Figure 5.27 The basic fracture modes in delamination, depicting (a) mode I (Opening or peeling), (b) mode II (Sliding shear) and (c) mode III (tearing shear).

Mode I

Mode III

HSS drill bit Feed direction

a

Drill bit rotation

b

Peel-up Composite laminate

delamination Push-out delamination

Mode I

Mode II

Figure 5.28 Schematic illustration of (a) peel-up and (b) push-out types of delamination and their associated damage modes during drilling of FRP composite laminate (Ismail et al., 2016; Ojo et al., 2017).

5.3.3

Failure or damage mechanisms associated with FRP composites

The damage begins at a nano- or micro-scale level when it can be detected only by using some non-destructive techniques, as previously discussed. At this first stage, there is a change in either molecular, crystalline or cross-linked structures as a result of initial yielding and failure. Shortly after this phase, damage such as matrix cracking, interfacial de-bonding and buckling and waviness progressively occur as the composite material failures in-service from micro (mm) to meso (mm)-scale level, while carrying loads. Fibre fracture or cracking and pull-out are associated at the mesoscale stage. Without control, these failures continue and later develop into macro (m) scale leveled damage, such as delamination. It is necessary to note that the failure of the matrix precedes the fibre failure/crack. The damage progression is clearly illustrated in Fig. 5.29. Finally, the mechanical properties (strengths and stiffness), structural integrity, load-carrying capacity and service time of the composite component are decreased prior to the final rupture (Wang et al., 2020).

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Matrix cracking Matrix Fi

be

r

Delamination Layer n–1 Matrix crack growth: Layer n Over thickness of ply Along fibre-matrix interface Layer n+1 matrix er

Failure

Damage index

fib

Stage II

Percent of life

Fi

Stage III

Stage I

Fiber cracking Matrix be

r

100 %

Figure 5.29 Damage mechanisms and evolution in unidirectional FRP composite structures (Romanowicz and Muc, 2018).

In other words, low-velocity impact modes of damage follow four distinctive stages: (a) matrix failure - where matrix crack occurs parallel to the fibres under combination effects of compression, shear and tensile (as previously discussed). (b) delamination e occurs by inter-laminar stresses. (c) fibre failure e orchestrates by the combination of tensile and compressive fibre buckling and (d) penetration e This is peculiar to high, ballistic and hypervelocity impact response types, where impactor or penetrator finally penetrates and completely perforates the FRP composite structures (Razali et al., 2014). Various damage modes (Fig. 5.30) can lead to perforation.

Due to the anisotropic and heterogeneous nature of FRP composite materials, they primarily and commonly exhibit brittle factures. Other uncommon fractures include ductile or in a combined form: ductile-brittle. For instance, Skalskyi et al. (2020) employed AE signals to estimate and rank various types and mechanisms of fracture in four different samples of Twaron 1000 and 1014 aramid fibre reinforced epoxy composites: untreated, silicone oil treated, plasma and 10% polystyrene-co-glycidyl methacrylate (PS-GMA) and 100% PS-GMA particle treated FRP composites, designated as samples 1, 2, 3 and 4, respectively. The optimum or best composite was sample 4, followed by samples 3 and 2, while sample 1 recorded the least performance. Furthermore, matrix cracking and shifting mechanisms in matrix recorded brittle and ductile types of fractures, while the interfacial de-bonding and fibre breakage of fracture mechanisms exhibited ductile-brittle, brittle and ductile, as well as ductile-brittle brittle types of fractures, as depicted in Fig. 5.31.

Testing and damage characterisation of biocomposite materials

Delamination

203

Impact location Surface buckling

Matrix cracks due to bending

Matrix cracks due to shear

Fiber breakage

Figure 5.30 Schematic illustration of various common impact damage modes of FRP composite laminate (Shyr, T-W., Pan, Y-H., 2003. Impact resistance and damage characteristics of composite laminates. Composite Structures 62, 193e203).

Type of fracture The number of events, %

80

Ductile (plastic deformation) Ductile-brittle (microcracking) Brittle (crack growth)

60

40

20

0

1

3

2

4 Type of sample

A, mV ΠT 0.16

1 0

0

0.08

60

−1

20

40

t,

μs

30 t, μs

A, mV

ΠT 0.08

0.6 0

0 400 0 800 f , kHz

0.04

30 20

−0.6 0

20

40

t, μs

t, μs

0

10 400 0 800 f , kHz

Figure 5.31 Various fracture types of aramid FRP composite samples and their recorded corresponding recorded number of AE events, as well as waveforms and continuous wavelet transform from AE signals of the tensile fractured samples 1 and 3. Adapted from Skalskyi, V., Stankevych, O., Zosel, T., Vynnytska, S., Thomas, H., Pich, A., 2020. Ranking of fibre composites by estimation of types and mechanisms of their fracture. Eng. Fract. Mech. 235, 1e13.

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10:59:11

10:59:12

28.7°C

(b)

49.1°C

28 45 26

24

22 21.2°C

40 35 30 26.5°C

Figure 5.32 Crack pattern and temperature distribution of (a) plain and (b) notched samples at failure (Belmonte et al., 2017).

It was concluded that fracture resistance of FRP composites increased with their interfacial bond. Also, there was a decrease and an increase in the number of AE signals associated with brittle and ductile-brittle damage, showing macro-crack growth and microcracking fractures, respectively. The decrease was attributed to the fibre surface treatment. Moving forward, Belmonte et al. (2017) studied the damage mechanisms on a plain and notched short glass FRP composite samples subjected to fatigue loading at room temperature and humidity. Fractographic features, such as ductile-brittle matrix damage response, fibre failure or pull-out and degree of the glass-polyamide interfacial bond, were observed with the aid of infrared thermography, optical and electron microscopy. The crack pattern and temperature distribution of the two samples at failure are shown in Fig. 5.32. Further results showed that the plain sample failed due to unstable fatigue crack propagation (FCP), as illustrated in Fig. 5.33, while, crack initiation, stable and last unstable crack propagation, respectively, characterised the fracture steps or failure modes of the notched samples (Fig. 5.34). These failure modes determined the damage mechanisms of the samples, as matrix fracture responses were microductile and brittle on the fractured surfaces produced by stable and unstable FCP, respectively, as depicted in both Figs 5.33 and 5.34. Other damage mechanisms in FRP composite materials can be summarised in Fig. 5.35.

5.3.4

Damage detection in FRP composite structures

Several experimental methods of non-destructive examinations of damage in FRP composite structures have been previously explained in this chapter. Detection of the presence of damage and prediction of their locations are possible with many structural health monitoring techniques, which have been developed in the last few decades (Su et al., 2006; Diamanti and Soutis, 2010; Raghavan and Cesnik, 2007). The most famously used techniques are the guided wave base method. They are very popular because of their ability to detect small size damage, large detection zone, as well as their low attenuation. Lamb waves, the most famous among them, are applied to a localise damage, using piezoelectric as an actuator to transfer Lamb wave-imaging to the FRP composite structure, before the examination.

c

20 μm

(c)

b

Hochsp = 5.00 kV Arbeitsabstand = 10mm

Signal A = SE2 Vergroserung = 1.00 K X

Hochsp = 5.00 kV Arbeitsabstand = 11mm

Testing and damage characterisation of biocomposite materials

20 μm

(b)

(a)

Signal A = SE2 Vergroserung = 1.00 K X

Figure 5.33 (a) Tensile fractured plain sample and (b) its SEM micrographs, showing the morphology of the fractured surface at crack initiation and (c) caused by unstable crack propagation (Belmonte et al., 2017).

205

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(a) d

60 μm

(b) c

b

EHT = 5.00 kV WD = 11.4 mm

u

u

S

S

60 μm

(c)

EHT = 5.00 KV WD = 11.3 mm

Signal A = SE2 Mag = 1.00 K X

60 μ m

(d)

Signal A = SE2 Mag = 1.00 K X

EHT = 5.00 KV WD = 11.0 mm

Signal A = SE2 Mag = 1.00 K X

Figure 5.34 (a) Tensile fractured notched sample, (b) its SEM micrographs, showing the morphology of the fractured surface at crack initiation, (c) the end of stable FCP and (d) caused by unstable FCP (Belmonte et al., 2017). Note: s and u represent stable and unstable FCP on the fractured surface. Damage mechanisms in composite materials

Fiber microdefects

Matrix microdefects

Fiber splitting (exfoilation) including organic fiber. Fiber surface crack especially in glass fibers. Fiber rupture (tension breakage). Fiber buckling in compression.

Volumetric pores especially in powdered-metal matrix. Transverse, axial and shear matrix microcracks. Idiabatic shear band (dynamic loading).

Matrix macrodefects Matrix transverse cracks in transverse layers. pseudo-matrix macrocracks.

Fiber-matrix microdefects Axial microcrack at ends ruptured fibers debond. Transverse micro crackings at ends of ruptured fibers, Shear microcracks directed at 45° from the applied loading’s direction.

Interlayer microdefects Interlaminar micro cracks delaminations. Intralaminar micro cracks which may also lead to loss of ply’s stability.

Figure 5.35 Summary of damage mechanisms in FRP composite structures (Lurie and Minhat, 2015).

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207

Moving forward, another structural health monitoring technique that is mostly suitable for complex FRP composite structures and has been extensively used is called wave-based imaging (Sohn et al., 2011; Rogge and Leckey, 2013; Radzienski et al., 2013; Kudela et al., 2015). Details of this method are available in their reported studies. Zhao et al. (2007) introduced a RAPID method. This method enables probabilistic inspection of damage in FRP composite materials through the reconstruction of the algorithm. A wind panel was tested and the presence and location of damage were obtained, implying the capability of the RAPID technique. Other structural health monitoring methods include, but are not limited to, delay-and-sum, cross-correlation and windowed energy arrival, as effectively used by Michaels and Michaels (2007), Veidt et al. (2008) and Sharif-Khodaei and Aliabadi (2014), respectively. Moreover, composites structures are often subjected to heat from fire, engine, electric wires/cables, exhaust wash and sun, due to their various areas of applications during service life. The effects of heat energy cause resin degradation and decrease in room-temperature mechanical behaviours. The affected FRP composites may appear undamaged to the visual examination, traditional, ultra-modern NDT methods and evaluation systems, below a particular exposure threshold, and without knowing that up to 60% of their initial strengths have been lost (Pereira et al., 2013; Razali et al., 2014). This phenomenon is referred to as incipient thermal damage (ITD). In a bid to solve the challenge of ITD in FRP composite components, many techniques have been adopted. These methods include, but are not limited to, the use of fluorescent thermal damage probes. They are embedded in the composite matrix during the manufacturing stage (Howie et al., 2012). The aviation industry also used the Fourier transform infrared spectroscopy technique to identify composite components that require repairs (Fu et al., 2014). Laser-induced fluorescence technique is another similar technique introduced for ITD detection (Fisher et al., 1997). Due to the necessary understanding of the failure throughout the whole thickness of FRP composite structure, the effect of composite thickness in terms of the decision on a part replacement and/or cost-effectiveness during repairs, Lindgren et al. (2006) employed sonicinfrared technique (also referred to as a sonic IR) to detect ITD through FRP composite structure thickness. The effectiveness and accuracy of the sonic IR technique depend on the standoff distance of the ultrasonic horn, which is used for the heating of the composite locally. An advanced NDT tool, known as thermography technique, has been introduced to evaluate ITD. This method includes the use of non-destructive heating equipment for sample excitation and infrared camera for collection of the full-time history of the surface heating. The thermography technique produces a similar sensitivity of ITD determination via the thickness of the composite material, excluding the requirement of a surface profile-following fixture (Razali et al., 2017).

5.3.5

Key factors for improving damage resistance

The service life of FRP composite materials depends on their damage resistance. Damages are undesirable responses in any material, but unfortunatelys they are inevitable. Several properties of composite reduce with time, under some certain conditions. However, these properties can be improved right from an effective design stage of

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composite structure, because good damage resistance is a function of an improved property. This initial stage involves suitable selection of compactible reinforcements (fibres, particulates and fillers) and matrix, efficient process parameters, among others. These activities are very germane prior to the second stage of manufacturing. In addition, many damages in composites can be traced to poor manufacturing processes that produce defects, such as voids, porosity, the inclusion of foreign objects, to mention but a few. It also involves the use of suitable curing parameters (temperature and pressure) and techniques. In a simple word, the improvement in damage resistance of a composite material should involve a defect-free manufacturing process. For avoiding repetition, details on manufacturing defects associated with FRP composite materials can be obtained from the previous Chapter 4, under Section 4.5 of this book. Close to this factor is the use of efficient manufacturing methods: injection, compression, resin transfer moulding, automated fibre placement, filament winding, extrusion, additive manufacturing/3D printing, vacuum resin infusion, autoclave and out-of-autoclave moulding and effective techniques for enhancing properties of FRP composite structures. For instance, hybridisation technique improves some mechanical (tensile, impact and flexural) behaviours, z-pinning increases delamination resistance, fibre surface treatments (either chemical, physical, biological, among other methods) enhance the resistance to interfacial de-bonding and delamination by improving the fibrematrix interfacial adhesion, weaving, braiding, knitting and such alike can improve resistance to impact damage, to mention but a few. Section 4.3 of Chapter 4 extensively discussed all these methods/techniques with relevant and self-explanatory figures. Last, there is no multi-purpose FRP composite material; composite is designed and manufactured for a particular application. Wrong use of composite materials could reduce and destroy their properties, and hence, result in catastrophic failure and short service life. Therefore, all the composite structures must be used correctly and according to their stated conditions of optimum service in order to maintain their outstanding properties and performances.

5.4

5.4.1

Characterisation of damage modes using destructive and non-destructive damage analysis techniques (SEM, X-ray micro CT, AE, AFM, etc.) Categorisation of NDT methods for FRP composite materials

Composites literally suggest a material composition with more than one base material, such that the structure and properties of the individual base materials remain unchanged, i.e., they do not form an alloy. Sometimes, it may be challenging to choose an appropriate NDT method for a particular inspection of the composite structure for aerospace applications. But, this difficulty has been solved using standards (ASTM E2533, 2017) as a practical manual or guide. Testing situations and material applications are the determinants of the NDT method categorisation. The following are some of the categories.

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209

NDT

Through transm.ultrasonic

Visual

Shearography

Holography

Infrared

Thermography

Radiography

Liquid penetrant

methods

Penetrant

Non-Contact

Electromagnetic

Contact methods

Magnetic

Eddy current

Traditioal ultrasonic

techniques

Figure 5.36 Contact and non-contact NDT techniques.

5.4.2

Contact versus non-contact techniques

The fundamental NDT techniques are broadly divided into contact and non-contact techniques. Each of these methods has its definite applications in FRP composite material examination and evaluation. To obtain data or results from the contact method, reliability, good contact between the surface of the composite material being tested and the sensor is required. NDT testing methods that require good contact include UT, EC, penetrant, magnetic, as well as electromagnetic testing. The non-contact approach is performed without physical interaction or contact between the measuring sensor and the FRP composite material being inspected. Non-contact technique also helps to accelerate the process of collecting data. This category includes transmission ultrasonic, thermography, shearography, RT and VI methods. Optical methods, such as shearography and thermography, are mostly used methods within the non-contact category. Fig. 5.36 summarises various examples under the two categories or basic types of NDT techniques, as adapted (Gholizadeh, 2016).

5.4.3

Inspection type versus NDT methods

Many inspection types have been reported in the literature for the evaluation of composite materials. For instance, ultrasonic testing has been widely applied to identify the damage, assess defects and monitor health in various aerospace composite structures. Different researchers have also proposed other methods, which include thermographic inspection, vibration techniques, infrared thermography, X-ray computed tomography and shearography. Ultrasonic testing is the most applied method for monitoring the composite structure of aircraft wing-box. This method also has wide applications: identification of

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Table 5.1 Inspection types and NDT methods (Gholizadeh, 2016). Inspection types

NDT methods

n

Identification of damage in composite parts of aircraft Assessment of composite components of aircraft Health monitoring of composite parts of aircraft.

Shearography Vibration techniques Ultrasonic inspection Infrared thermography Thermographic inspection XCT

n

Health monitoring of composite structure of aircraft wing-box

Ultrasonic examination

n

Structural health monitoring

Ultrasonic testing

n

Damage in GFRP

Thermographic testing Radiography

n

Auto-detection of impact damage in carbon FRP composite components Characterisation of damage in carbon FRP composite structures

Thermographic testing Radiography

n

Assessment of manufacturing defects and impact damage in glass FRP/epoxy composite components

Infrared thermography

n

Damage evaluation in composite sandwich components parameters that influence damping properties of composite structures The structure behaviour Dynamic behaviours for detection of damage in composite parts Statistical recognition/identification and restoration assessment of skin damage in composite components.

Vibration methods

n n

n

n n n n

Multiple cracks detection

Neutron radiography

damage, assessment of defects, as well as structural health monitoring (SHM) of composite components of aircraft. Table 5.1 shows different types of inspections and the methods used in each case.

5.4.4

Physical behaviours and structural integrity

There are several methods of testing composite structures. However, attention must be given to efficiency, safety and cost of the operation when determining best method to adopt. Composite failures are majorly caused as a result of material defects. These failures can manifest as fibre pull-out, fibre de-bonding, matrix cracking and fibre fracture. Structural integrity thus utilises advanced NDT approach to determine, detect and

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211

Table 5.2 Class of NDT techniques according to the detecting factors (Gholizadeh, 2016). Class

Use

Evaluation of both mechanical and physical behaviours, as well as detection of the material defects in various composite structures.

Measurement of: n n n n n n n n n n

To evaluate the structural integrity of the composite components, after manufacturing.

amount of fibre portion dynamic mechanical analysis mechanical behaviours: Stiffness and strength elastic constants delamination occurrence material content initiation of damage and succeeding damage progression construction connected with laminate resin cure condition fibre-matrix interfacial adhesion condition.

Detection of: n n n n

fibre breakage mechanical rubbing fibre pull-out micro-cracks and de-bonding

localise size of damage. Table 5.2 presents the NDT categories based on the factors that they evaluate. On moving forward, material mechanical properties are quite important as they determine whether or not the material can be manufactured. They also determine the performance and service life of the material. Therefore, the knowledge of the mechanical behaviours is of the essence for proper physical and mechanical characterisation of composite parts. Hence, this aspect of composite engineering has attracted a lot of studies.

5.5

Experimental and numerical modelling of damage modes and mechanisms

Detailed knowledge and understanding of FRP composite damage modes and mechanisms are very important in order to guide an effective composite design, development/fabrication and applications. Therefore, numerical formulation or modelling and simulation are carried out to mimic and evaluate the real damage response and evolution in composite structures, based on previously obtained relevant experimental results/data (Razali et al., 2017). Numerical analysis is relevant

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to reduce the cost of materials and fabrication, production time and other challenges in the manufacturing of FRP composite materials. The use of the finite element method is very rampant in modelling and simulating material property degradation, various damage modes and mechanisms in FRP composites, using several simulation software, such as ANSYS, ABAQUS and LS-DYNA and Multiscale Designer, among others. Examples of the results obtained are given towards the end of this chapter. Some structural health monitoring or damage detecting and analysing methods that have been previously discussed, such as Lamb wave, wave filled imaging, RAPID, delay-and-sum, cross-correlation and windowed energy arrival, require a comparison between baseline and real-time data obtained from sensors. Experimental tests still remain the most dependable technique of obtaining the required baseline data. This is either impossible or can only be achieved with an enormous job because it requires testing a large FRP composite component in every single condition during its in-service life. Therefore, the finite element method is employed to obtain the required baseline data. However, drawbacks of the finite element method include expensive computation (Chakherlou and Yaghoobi, 2010) and its invalid equations in discontinuities, such as crack tips. Application of meshless methods, for example, peridynamics, is one of the ways out of these limitations. Therefore, Yaghoobi and Chorzepa (2015, 2017) used peridynamics and micropolar peridynamics (Chorzepa and Yaghoobi, 2016) to model fibre reinforcement in cementitious composites and solve the complex and unguided fracture response of FRP composite beams. In addition, the use of the spectral finite element method was another effective and alternative method of solving the limitations of the finite element method towards detecting damage in FRP composite structures. This method was first made prominent by Doyle (1997). Fourier-based spectral finite element method was employed and the results showed that it was very computationally efficient when compared with the finite element method. However, the main limitation of the spectral finite element method includes modelling of realistic composites components and complex features. Moreover, the wavelet spectral finite element was introduced with still some challenges. In an attempt to further improve on the wavelet spectral finite element method, wavelet spectral finite element-based user-defined element was formulated for one-dimensional composite beams to model complex structures (Khalili et al., 2014; Khalili et al., 2017). Wavelet spectral finite element-based user-defined element method was improved to effectively simulate delamination damage in composite beams (Khalili et al., 2015a). After much efforts, wavelet spectral finite element-based user-defined element method was further developed, and it was able to simulate two-dimensional composite laminate structures (Khalili et al., 2015b; Khalili et al., 2016), with computational efficiency and ability to model realistic structures and provide baseline data for the structural health monitoring benefits.

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213

Furthermore, FRP composite damage detection, among other heterogeneous and anisotropic complex materials, is very essential and hence a continuous possibility today, especially with machine learning methods coupled with the frequency response functions (Abbasi et al., 2015). Therefore, an Euler-Bernoulli model has been formulated to mimic the response of damaged FRP composite parts with possibility of inspecting different modes of delamination by embedding artificial immune-based method (Mohebbi et al., 2012; Mohebbi et al., 2013; Babaei et al., 2013, 2014).

5.5.1

Impact damage

Impact property of an FRP composite material is the ability of the material to resist the sudden release of load. It can be classified into four different categories, based on the velocity ranges: n

n n n

Low-velocity impact (LVI): Less than 20 or 40 m/s, such as dropping of hand tools on a material (usually less than 31 m/s), ship collision, vehicle impact and crash-worthiness, among other impact events. This often causes barely visible impact damage (BVID) response or effect on the FRP composite materials. High-velocity impact (HV): Between 40 and 240 m/s, such as bird striking and/or colliding with composite structures, such as aircraft. Ballistic velocity impact (BVI): Greater than 240 m/s, such as a projectile fired from a gun, free-falling bombs and missiles, among other military activities. Hypervelocity impact (HVI): Up to 15,000 m/s, such as orbit debris roving in outer-space, space vessels and meteoroid impact (Razali et al., 2017).

For instance, Petrucci et al. (2015) reported an experimental investigation on impact and post-impact damage behaviours of various hybrid FRP composite laminates. The impact responses showed that the glass-hemp-basalt (GHB) hybrid FRP composite laminate sample recorded a minimum performance, and the flax-hemp-basalt (FHB) sample slightly recorded better impact resistance than the glass-flax-basalt (GFB) combination. Importantly, the SEM micrographs obtained show their variable modes of fractured surfaces morphologically (Fig. 5.37). The possibility of delamination damage was observed with the FHB samples when AE was used to monitor the post-impact flexural tests. Conclusively, the FHB hybrids recorded better impact damage resistance that other hybrid samples. Similarly, Sarasini et al. (2016) experimental investigated into the low velocity falling weight impact and 4-point flexural damage tolerance of carbon/flax FRP hybrid composites, at different energy level ranged from 5 to 30 J. From the result obtained, it was evident that there was interfacial damage and cracks in the flaxbased laminates under impact and flexural loads, as depicted in Fig. 5.38(a) and (b), respectively. But, the presence of outer flax laminates (acting like skins) exhibited a greater impact damage resistance, preventing crack propagation within the laminates. Monitoring through digital image correlation supported preliminary

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Figure 5.37 SEM micrographs of the FRP hybrid composites, showing brittle fractured hemp fibre surfaces of (a) GHB, (b) FHB and (c) glass-flax fibre layers of GFB laminates (Petrucci et al., 2015).

identification of associated failure modes and mechanisms of the hybrid FRP composite laminate structures. Summarily, the failure modes of impact loading include matrix cracking and breakage, fibre cracking and breakage, fibre pull-out and delamination, among others (Razali et al., 2017). The aforementioned modes of damage associated with FRP composite structures depend on materials (fibre/reinforcement and matrix) properties, target geometry, impact velocity, impactor/projectile nose shape and relative mass, support conditions, as well as the target geometry, among other factors.

5.5.2

Fatigue life model

Various approaches have been employed to model fatigue damage in FRP composite materials. Residual strength and stiffness degradation or commonly called damage

Testing and damage characterisation of biocomposite materials

215

Figure 5.38 (a) Interfacial damage and (b) crack paths in flax-based FRP composite laminates (Sarasini et al., 2016).

mechanism theories, are the most used fatigue life models (Zhang et al., 2015). These models accepted that the fatigue damage in composites is caused as a result of changes in their material properties (Romanowicz and Muc, 2018). Various loading parameters are considered for fatigue residual strength or stiffness models recently available and reported (D’Amore et al., 2016; D’Amore and Grassia, 2017; Mejlej et al., 2017; Stojkovic et al., 2017). One of the popular and effective model is Epaarachchi and Clausen model (Epaarachchi and Clausen, 2003), as stated in Eqs. (5.1)e(5.4) (Romanowicz and Muc, 2018).  
  s su 0:64 jsinðqÞj 1 u a Nfb  1 ¼ 1 fb 0:64 jsinðqÞj smax smax ð1  4Þ

(5.1)

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Where,

4¼

8 > < R

9 for  N < R < 1  tension  tension and tension  compression > = 1=R for 1 < R < þN  compression  compression > ;

> :

(5.2) And R¼

smin smax

(5.3)

where Nf and su represent the numbers of cycles to failure and material ultimate stress in the loading direction, respectively. And, q, f and R are the smallest angles between the loading direction and fibre direction, loading frequency and stress ratio. While a and b denote the material constants obtained from experimental fatigue tests. smin and smax stand for the minimal and maximal applied fatigue stress in the loading direction, respectively. From Eq. (5.1) the number of cycles to failure is expressed as Eq. (5.4). "

1 Nf ¼ 1 þ f

5.5.3



su 1 smax



su smax

0:64 jsinðqÞj

1

ð1  4Þ

f 0:64 jsinðqÞj

#1 b

b

(5.4)

Thermal effects

Yang et al. (2015) used the finite element method to study the effects of temperature on low-velocity impact damage in woven carbon/epoxy composite sandwich structures with closed-cell polymeric foam core. The composite panels were subjected to energy levels of 10 and 50 J, under cold, room and high or elevated temperature dry of 45.6
C (50
F), 21
C (70
F) and 82.2
C (180
F), simply designated as CTD, RTD and ETD, respectively. The models depicted the face sheets, inter-laminar/delamination damage progression and temperature effect, with the aid of ultrasonic inspection and high-speed cameras. From the results obtained, it was observed that the larger damage areas for both impact energy levels were caused by the larger temperature exposure (Fig. 5.39a), resulting in visible damage with high penetration rate and indent or impact depth (Fig. 5.39(b)). However, the low energy level of 10 J produced a barely visible impact damage with no fibre fracture. Also, the simulation results for both energy levels are shown in Fig. 5.40(a) and (b).

Testing and damage characterisation of biocomposite materials

(a)

217

6 35 5

Damage area, (in2)

4

25

20

3

15 2

Damage area, (cm2)

30

10 1 5

0 ETD-50J

(b)

ETD-10J

RTD-50J

RTD-10J

CTD-50J

CTD-10J

0

1 24 22 0.8

20

16

0.6

14 12 0.4

10

Indent depth, (mm)

Indent depth, (in)

18

8 6 0.2 4 2 0

0 ETD-50J

ETD-10J

RTD-50J

RTD-10J

CTD-50J

CTD-10J

Figure 5.39 Comparison of effects of temperatures and energy levels on (a) impact damage zone and (b) indent depth, showing visible and barely visible damage responses at 50 and 10 J, respectively (Yang et al., 2015).

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(a)

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Shear damage view

Core damage Damage

Core crushing in limited zone under impactor

Cohesive layer damage

Delamination in the top facesheet

Figure 5.40 Low-velocity impact damage simulation results, exhibiting (a) barely visible and (b) visible impact failure, at 10 and 50 J, respectively (Yang et al., 2015).

In like manner, Buenrostro and Whisler (2018) employed both experimental and finite element analyse to investigate the impact, flexural and compressive behaviours of polyurethane foam (PUF) and glass fibre reinforced polyurethane form (GFRPUF). From the results obtained (Fig. 5.41), it was observed that GFRPUF composite specimens were more brittle than the PUF samples under flexural bending.

Testing and damage characterisation of biocomposite materials

(b)

219

Shear damage view

Continuum

Cohesive layer damage

damage in facesheets Fiber breakage at the top and bottom facesheets

Core damage

Delamination

Damage

Foam is completely crushed from red to green zone

Figure 5.40 cont'd.

Also, GFRPUF samples exhibited a better impact resistance property than PUF. This can be traced to the reinforcing effect of glass fibres on PUF material and better effective distribution of impact energy or force than the un-reinforced PUF samples. However, GFRPUF samples recorded a lower quasi-static compressive damage resistance when compared with the PUF counterparts, due to the manufacturing process and observed large voids in the GFRPUF composite samples. Both experimental and numerical simulation (finite element) results are depicted Fig. 5.41(a) and (b), respectively, and in a combined form to show their dynamic compressive damage responses or results (Fig. 5.41(c)).

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PUF, front

GFRPUF, front

PUF, back

GFRPUF, back

(a)

(b)

S,Mises SNEG, (fraction = -1.0) (Avg: 75%) +8.0e+07 +7.3e+07 +6.7e+07 +6.0e+07 +5.3e+07 +4.7e+07 +4.0e+07 +3.3e+07 +2.7e+07 +2.0e+07 +1.3e+07 +6.7e+06 +0.0e+00

(a)

S,Mises SNEG, (fraction = -1.0) (Avg: 75%) +8.0e+07 +8.0e+07 +7.3e+07 +6.7e+07 +6.0e+07 +5.3e+07 +4.7e+07 +4.0e+07 +3.3e+07 +2.7e+07 +2.0e+07 +1.3e+07 +6.7e+06 +0.0e+00

PUF

GFRPUF

back

back

(b)

Figure 5.41 Impact damage responses of PUF and GFRPUF composite sandwich samples, showing their (a) experimental and (b) finite element results at a velocity of 220 m/s (1.2 kJ), and (c) both results of dynamic compression of GFRPUF and PUF cores. Adapted from Buenrostro, E., Whisler, D., 2018. Impact response of a low-cost randomly oriented fibre foam core sandwich panel. J. Compos. Mater. 52 (25), 3429e3444.

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221

4.5 PUF actual PUF FEA 3% fiber actual 3% fiber FEA

4 3.5

Force (kN)

3 2.5 2 1.5 1 0.5 0 0

0.005

0.01

0.015 Time (s)

0.02

0.025

(c)

Figure 5.41 cont'd.

5.6

Conclusions

Testing and damage characterisation of FRP bio/composite materials are more crucial and required in many manufacturing industries today. This is because FRP bio/composite structures are widely utilised in critical-safety applications, such as automobile, aircraft and marine primary constructions, where efficiency, cost and safety are important determinant factors. Therefore, several testing and damage characterisation techniques that are relevant to FRP bio/composite materials have been comprehensively elucidated in this chapter. Various destructive and non-destructive testing techniques, either contact or non-contact methods, can be used to detect and analyse manufacturing defects and test damage in FRP bio/composite structures internally (microscopically) and/or externally. Each of these methods has benefits and drawbacks. It is evident that two or more methods can be used to reduce their limitations, inspect and analyse a single damage, such as inter-laminar delamination, crack initiation and propagation, de-bonding, fibre fracture or kinking, to mention but a few. Therefore, the choice of their application depends on the nature of defect and damage, their penetrating or scanning capability, properties of the FRP composite materials and extent of analysis required, among others. More also, there is a possibility of an increase in the future concerning the number of examination techniques, efficiency, accuracy, fast analysis/processing of data and decision making with rapid advancement in science, technology and engineering (metrology): use of highly intelligent and automated/robotic inspection systems or diagnostic systems, machine and deep learning, artificial intelligence and robotics,

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as well as artificial neural network coding/algorithms. The future is already here! The trend has started as a combination of two or more traditional or conventional techniques is fast increasing. These include, but are not limited to, acousto-ultrasonic testing (a combination of acoustic emission and ultrasonic inspections), digital X-ray radiography and infrared thermography inspection, to mention but a few.

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Sustainable composites and techniques for property enhancement 6.1

6

The context of sustainability in composites (comparison of sustainability of biocomposites versus conventional composites through a details life cycle analysis)

The concept of sustainability has become an object of serious focus of advanced technology, especially in the 21st century. This is necessary for continuous processes and products in the field of composite technology in order to meet the ever-increasing needs and satisfaction of the users and manufacturers. Sustainability cuts across many aspects of life. It involves social, ecological and economic relationships, as well as a balance among earth, life and environment to achieve equitable, bearable and viable inter-dependent benefits among its elements (Fig. 6.1). The natural sources of several bio-fillers and fibres, as well as matrices, have tremendously contributed to the sustainability of biocomposite materials for engineering applications. For instance, biofibres such as jute, pineapple, kenaf, choir, hemp, sisal, flax, cotton, rice husk, date palm, to mention but a few, are processed for the reinforcement of numerous biocomposite components. These natural fibres combined with either petroleum-based matrices to produce bio-based composites or natural matrices, such as polycaprolactone (PCL), polylactic acid (PLA), among others, to fabricate complete biocomposite parts. These components are used in transportation (aerospace, marine and automobile), telecommunication and other engineering sectors. Moreover, natural (plant/vegetable, animal and mineral) fibres and matrices are renewable, biodegradable and sustainable, due to their reliable sources when compared with the conventional composites. Complete synthetic composites are mainly products of the combination of synthetic fibres (carbon, glass, Kevlar, among others) and petroleum-based matrices (epoxy, vinyl ester, among other resins). Sustainability of biocomposites is enhanced through good productive farming or agricultural practices. Most of the natural plant fibres can be grown or cultivated in many countries/continents and harvested more than once in a year; after short periods to produce biocomposites. For example, rice, flax, maize/corn and hemp can be planted and harvested up to 2e4 times in a year, depends on a climatic/geographical condition and land suitability. Other natural perennial plant fibres, such as coconut, date palm, wood, oil palm and bamboo, can produce a huge quantity of fibres for continuous production of fibres and/or fillers for reinforcement of the biocomposite materials. These plants can exist for many years, similar to the stable availability of many natural or biobased matrices/resins.

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Environment

Economic

Equitable

Social

Sustainable Viable

Bearable

Life

Earth Ecological

Figure 6.1 The key elements of sustainability (Yemm, 2013).

Conversely, the increasing rate of the depletion of the petroleum-based matrices and synthetic fibres, in addition to their tedious, harmful, high capital cost of processing and hazardous products made conventional composites to be relatively less sustainable, when compared with biocomposites. With the current trend in fossil fuel consumption rate by the geometrically increasing world population, there is a possibility that many wells of fossil fuel will be exhausted, if there are no new petroleum oil wells detected. Therefore, the manufacturing of many conventional composites will be directly and severely affected, if not totally stopped. With the recent government legislation on recommendation, interest and promotion of the use of renewable energies to prevent environments from pollution, it is obvious that the attention of many composite manufacturing industries will shift to the production of biocomposites. This is because the manufacturing of biocomposites consumes lesser energy and releases lesser CO2, among other toxic gases to the atmosphere, when compared with processing of conventional composites from synthetic carbon and glass fibres, as well as petroleum-based matrices. The process of biocomposite is almost harmless, considering the health of the workers. Environmental pollution is majorly caused by releasing toxic fumes and gases, such as CO2, SO2 and NOx, among other pollutants. These gaseous pollutants are very rampant with the manufacturing processes of synthetic composites (Pervaiz and Sain, 2003;Toffe et al., 2019). In addition, considering the life cycle analyses of both conventional composites and biocomposites. Although many conventional composite parts can be easily recyclable, but with higher release of aforementioned pollutants/gases. Significantly, complete product (composite) environmental life cycle analysis involves five main stages: raw material extraction, production, distribution, use and disposal/recovery. The other

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first four stages have been directly and indirectly elucidated. Now, considering the last stage on product disposal/recovery after use, both partial and full biodegradability of biocomposite materials support a good disposal mechanism. Biocomposites are discomposed and easily returned to the soils, due to the nature of their compositions. Soil regains nutrients from the disposed biocomposite parts and supports the growth of the plants/vegetables, animals and production of minerals for the fabrication of more natural fibres and matrices. Well-grown plants respire by absorbing more CO2, and therefore, preventing global warming effect and depletion of ozone layers. However, complete synthetic composites are non-biodegradable. Hence, they are liable to cause environmental pollution, whereas, biocomposites are commonly referred to as environmentally friendly products because they thereby support environmental sustainability.

6.2

Inherent properties of natural fibres of biocomposite materials

The inherent properties of several natural fibres significantly depend on their structures and chemical compositions. These two factors are functions of growing/climatic condition, harvest time (Pickering et al., 2007), extraction technique (Bos et al., 2002), fibre treatment and storage procedures of the plants/fibres (Pickering et al., 2016). Based on the natural source and structure of plant fibres, flax fibres, for example, are produced in the stems of flax bast plants. Flax is a cellulose polymer with a more crystalline structure, which makes it stronger and stiffer to handle (Yan et al., 2014). The length of the flax plant ranges up to 900 mm, which possesses strong fibres all along the stem. These fibres have an average diameter of nearly 14 mm. At a macroscopic level, a stem consists of bark, phloem, xylem and a central void when described from outside to the inner region. Looking at a cross section of a fibre bundle at the mesoscopic level, it consists 10e40 fibres linked together mainly by pectin. The fibre has a complex hierarchical microstructure with different length scales and different materials in varying proportions. The outer cell wall, known as the primary cell wall, is 0.2 mm and coats the thicker outer secondary wall (S3). The S3 is responsible for the strength of the material and encloses the lumen, and S2 holds the bulk of the fibre and is the thickest cell wall, as shown in Fig. 6.2. Additionally, the S2 comprises molecules of crystalline cellulose microfibrils and amorphous hemicellulose, orientated at 10 degrees off the fibre axis. It gives flax high tensile strength. The S1 consists of concentric cylinders with a channel in the middle called the lumen. Lumen contributes to the low density, high water uptake (hydrophilic nature), and wettability of several natural/plant fibres. Each layer contains cellulose microfibrils, which arranges parallel to each other. The fibrils of the fibre form an angle with the fibre direction. This angle could affect the strength of the fibres, and usually, the fibre is more ductile, if the microfibrils have a spiral orientation. The microfibrils contribute approximately 70% of the weight of an individual flax fibre (Yan et al., 2014). The secondary cell wall (S1eS3) has a very crystalline structure, making it strong and stiff along the fibre length. The orientation

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S1 =10°

S2

Secondary cell wall

S3

P

Primary cell wall

Figure 6.2 Schematic microstructure of a flax fibre cell (Yan et al., 2014).

of the structure and amorphous regions between it implies that these properties will differ greatly in the lateral direction (Fig. 6.3). Therefore, it significantly affects the properties of the biocomposites made with them. Furthermore, flax fibres are one of the strongest natural fibres. The tensile strength (mechanical property) of elementary fibres has been observed to be around 1500 MPa, whilst the technical fibres exhibited approximately 800 MPa at a 3 mm clamped length. The modulus of flax fibre varied, due to the diameter of the elementary fibres. It ranged from 39 to 78 GPa for 35e5 mm, respectively (Bos et al., 2006). Also, there are variations in the geometries or dimensions of the same fibre, due to the differences in their diameter, morphology and length. The dissimilarities have been attributed to the different climatic/environmental conditions, ages, soil types and geographical regions, where such agricultural produce (plant fibres) are grown before harvest and process stages. These variations contributed to the inhomogeneous quality and dimensional instability of natural fibre-based biocomposites. An aspect ratio, the ratio of fibre length to its diameter, is an important factor that determines the mechanical properties and damage responses of biocomposite materials. Due to the inconsistent diameters of many natural fibres, it is thus difficult to obtain a consistent fibre aspect ratio without approximation within a single biocomposite material. The strengths of biocomposites increase with fibre aspect ratios. For instance, the influence of drilling process parameters and fibre aspect ratios on surface roughness and delamination drilling-induced damage responses of hemp fibre-reinforced polycaprolactone (HFRP) complete biocomposite samples has been investigated (Ismail et al., 2016).

Elementary fibre

Cellulose microfibril

Lumen

1 – 1.5 m

Secondary walls

Cellulose macrofibril

S3 Crystallised regin

S2 Primary wall

Hemicellulose& lignin matrix

S1 Amorphous regin

Woody body (xylem)

Bast fibre bundle

2 – 4 mm

S2 walls with 30 - 150 lamina

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Technical fibre

Cross-section of the plant’s stem

Hydrogen bonds between microfibrils

Pectin between technical fibres

100 – 200 Pm

10 – 40 Pm

ca. 50 nm

233

Figure 6.3 A detailed flax fibre structure, showing its macro to the nanoscale level. Adapted from Woigk, W., Fuentes, C.A., Rion, J., Hegemann, D., van Vuure, A.W., Dransfeld, C., Masania, K., 2019. Interface properties and their effect on the mechanical performance of flax fibre thermoplastic composites. Compos. Appl. Sci. Manuf. 122, 8e17.

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The optimal HFRP biocomposite sample was recorded with the smallest aspect ratio of 19, implying that the machinability of biocomposites (materials) reduces with an increase in their fibre aspect ratios (strengths). In moving forward, natural fibres and fillers contain hydroxyl groups, which means they attract moisture. This causes weak or poor fibre/filler-matrix interfacial adhesion when embedded with hydrophobic polymer matrices. The weak fibre-matrix interfacial bonding prevents sufficient load transfer from matrices to fibres (reinforcements). Due to this phenomenon, biocomposites possess poor mechanical properties compared to their conventional counterparts. However, this limitation has been reduced by introducing several suitable fibre treatments (chemically - using compatibilising agents, physically, additively and biologically) and overall property enhancement techniques (hybridisation, stitching, pinning, weaving, knitting, among others). These treatments and techniques are later elucidated comprehensively. In addition, natural fibres and bio-resins possess biodegradation properties. They could not resist microbial attack. Therefore, they can completely disintegrate into natural ecosystems, including lakes, oceans, active sludge, marines and natural soils. Still on the advantage from the inherent properties of natural reinforcements (fibres/fillers) and binders (matrices/resins) of biocomposites, they can exhibit chemical transformation, under actions of microorganism and biological enzymes. The major structural component of all plant fibres is cellulose. Bast pineapple, ramie, curaua, kenaf, jute/ flax and hemp fibres with higher cellulose contents of approximately 81, 76, 74, 72, 71 and 68 wt%, respectively (Faruk et al., 2012) and microfibrils exhibit higher performance. Cellulosic fibre and/or bio-polymer materials present in biocomposites are easily attacked by microorganisms, thus enable their easy decomposition, unlike synthetic/conventional fibre and/or petroleum-based materials. Biodegradability, caused by activities of microorganisms, involves hydrolytic depolymerisation, whereby giant cellulose materials are broken down to lower molecular weight compounds to produce monomeric glucose elements. Microorganisms attack starch fast (Reddy et al., 2016). This property supports biocomposite materials to be eco-friendly or environmentally friendly products. Several studies on the biodegradability property of biocomposites have been conducted and reported within this book. Briefly, it has been studied that flax fibres are more biodegradable than polylactic acid (PLA) within the same biocomposite material. Various microorganisms and soil water attacked flax fibre faster than PLA, under the same burial time during soil burial test (Alimuzzaman et al., 2014).

6.3 6.3.1

Improvement of reinforcements and matrices through various treatments and fillers Fibre treatments

Due to some drawbacks of natural fibre-reinforced polymer (FRP) biocomposite materials, it is necessary to treat natural fibres to reduce challenges of weak interfacial bonding formed with polymers, hydrophilicity and poor thermal stability. The processes of mechanical interlocking, chemical, electrostatic and inter-diffusion bonding produce fibre-matrix interfacial bonding (Matthews and Rawlings, 1999; Pickering

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et al., 2016). The mechanical interlocking depends on the surface roughness of the natural fibres, and electrostatic bonding is influenced by the metallic interfaces and chemical bonding forms in the presence of chemical groups in the matrix and on the surface of the fibre used. The interfacial strength obtained from the chemical bonding depends on the density and type of the bonds, with the application of coupling agents. The coupling agent acts as a bridge, and thus, reduces the incompatibility and provides a chemical bond between the fibre and matrix. An interfacial interaction between the molecules and atoms of the fibres and matrix produces an inter-diffusion bonding. Importantly, it is possible to have multiple types of bonding at the same interface and time (Pickering et al., 2016). Many studies have been conducted to improve on the properties of the natural fibres, and consequently, improve the compatibility between fibres and the matrices (Roy et al., 2020). One of the key motivations of various surface treatments of natural fibre is that it enhances the wettability of the fibres so that matrices wet fibre sufficiently, which leads to enhanced adhesion between the fibres and the matrices. These biocomposite properties include, but are not limited to, mechanical (toughness, tensile strength, modulus, elongation at break, impact, flexural, interfacial strength, among others), physical (wettability) and thermal stability. Mainly, several techniques under chemical, physical, additive and biological treatments have been used (Pommet et al., 2008; Quarshie and Carruthers, 2014). The detailed treatments are subsequently discussed.

6.3.2

Chemical treatments

It is well established that there is an incompatibility between the surface of hydrophilic cellulose fibres and hydrophobic polymeric matrices. Importantly, the chemical modification method contributes towards the compatibilisation of fibres and matrices in which the efficiency of wetting is significantly improved. Therefore, the surfaces of natural fibres are treated with some chemicals to improve the fibre-polymer interfacial bonding or adhesion, by reducing their incompatibility. There are numerous coupling mechanisms in biocomposite materials. These include, but are not limited to: n n n n n n

Acid-base effect Restrained layers Chemical bonding Weak boundary Wettability Deformable layers (Faruk et al., 2012).

Also, there are better improvements and numerous reported studies on chemical treatments than physical treatments. Chemical modifications of natural fibres are performed using alkali, zirconate, acryl, peroxide, titanate, isocyanate, permanganate, acrylonitrile, benzyl and acetyl treatments, as well as maleated anhydride grafted coupling agent (Singh et al., 1996; Faruk et al., 2014; Pickering et al., 2016). The commonest one is the alkaline treatment, using sodium hydroxide (NaOH), commonly known as a caustic soda (Quarshie and Carruthers, 2014), followed by silane, acetyl and maleated anhydride grafted coupling agents. This surface modification technique is considered one of the effective methods to improve interfacial

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compatibility. In order to make natural fibres as reinforcements in composites, the requirement for improving the properties of natural fibres has been recognised as important steps established by a number of investigations. In alkaline treatment, the components such as hemicellulose and pectin present on the fibre surface are aimed to remove. Non-celluloses, including fats, wax and pectin were removed in the work carried out by Sawpan et al. (2011). The SEM images shown in Fig. 6.4 depict the difference between untreated and surface-treated hemp fibres. For instance, Beckermann and Pickering (2008) treated hemp fibres with a solution of 5 wt% NaOH/2 wt% Na2SO3 to improve thermal stability, crystallinity index, Young’s modulus, lignin reduction, tensile strength and fibre separation of hemp fibres/maleic anhydride grafted polypropylene biocomposites. Similarly, Bera et al. (2010) treated jute fibres with 4%, 8%, 10%, 14% and 28% NaOH with maleic anhydride grafted polypropylene (MA-PP) and vinyl trimethoxy silane (VTMO) coupling agents to improve or increase the tensile, interfacial strength and flexural properties of the resultant biocomposites. The optimum tensile properties of the jute fibres were obtained with 4% NaOH treatment. Furthermore, it was observed that the MA-PP treated fibres exhibited a higher tensile strength of 145% increase when compared with a 42% increase with VTMO counterpart, at a fibre volume fraction of 40%. Also, MA-PP treated Biocomposites recorded a better interfacial strength against fibre pull out. Bachtiar et al. (2008) improved the tensile modulus properties of alkali-treated sugar palm fibre-reinforced epoxy composite when compared with the untreated counterpart. However, this improvement depended on the concentration of the alkali and the soaking time, as depicted in Fig. 6.5. The fibre surface modification using alkaline treatment not only removes the hemicellulose and lignin but also removes the waxes, fatty acids and other organic residues. Similarly, Liu et al. (2004) used alkali treatment to improve the mechanical (tensile and flexural) and impact strength properties of Indian grass fibre-reinforced soy protein-based biocomposites. These enhancements were attributed to the reduction in the fibre inter-fibrillary region by eliminating hemicellulose and lignin, increased

Figure 6.4 Surface morphology of (a) untreated and (b) treated hemp fibre obtained from SEM characterisation (Sawpan et al., 2011).

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(a) 4000

Tensile modulus (MPa)

3800

3600

3400 1 h. soaking time 3200

4 h. soaking time 8 h. soaking time

3000 0.25 M

Untreated

0.5 M

Concentration of alkali

Tensile modulus (MPa)

(b) 4000 3800 3600 3400 0.25 M NaOH 3200 0.5 M NaOH

h 8

h 4

h 1

Un

tre

at

ed

3000

Soaking time

Figure 6.5 The effects of (a) alkali/NaOH treatment and its concentrations and (b) soaking time on average tensile modulus of sugar palm-reinforced epoxy composite (Bachtiar et al., 2008).

homogeneous distribution of the bio-filler within the matrix and fibre aspect ratio in the biocomposite and consequently, improved efficiency of fibre reinforcement and fibrematrix interfacial bonding. Natural fibre surface impurities are removed while these various treatments are employed. It is important to bear in mind that optimal wt% of treatment must not be compromised. If there is an excessive amount of treatment employed, this will damage the fibre instead of improving the properties. Therefore, the percentage concentration of the alkali and the soaking time are important, as shown in Fig. 6.6(a) and (b).

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(a) 16

3 Tensile strength

Modulus of elasticity

2 1.5

8

1 4

Modulus (GPa)

Tensile strength (MPa)

2.5 12

0.5 0

0 A

C

B

D

E 3

30 Bend strength

Modulus

2

20 15

1

10

Modulus (GPa)

Bend strength (MPa)

25

5 0

0 A

(b)

B

D

C

E

70

Impact strength (J/m)

60 50 40 30 20 10 0 A

B

C

D

E

Figure 6.6 The (a) tensile, flexural and (b) impact strength properties of grass fibre-reinforced soy-based biocomposites of (A) soya plastic, (B) raw fibre-reinforced plastic composites, (C) 5% alkali at 2 h, (D) 10% alkali at 2 h and (E) 10% alkali at 4 h FRP composites (Liu et al., 2004).

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There are other reported studies on improvements of several mechanical, thermal stability and moisture resistance properties of natural fibres using alkali treatments (Goda et al., 2006; Gomes et al., 2007; Ibrahim and Hadithon, 2010; Islam et al., 2010; Kabir et al., 2011). The work carried out by Dhakal et al. (2012) used NaOH treatment on the hemp fibre and fabricated hemp fibre reinforced unsaturated composites. Their findings suggest that the surface treatment significantly improved thermal stability and surface characteristics because of 10% NaOH treatment. In the same study, they also highlighted that the wetting behaviour of treated hemp fibre was improved, as shown by the contact angle measurement. Furthermore, acetylation is another type of chemical treatment for the improvement of interfacial adhesion, mechanical (tensile, stiffness and flexural) properties, biological/fungal resistance, the thermal and dimensional stability of natural fibre-reinforced polymer composites/biocomposites (Khalil et al., 2001; Tserki et al., 2005; Bledzki et al., 2008). Sometimes, acetylation can be done after alkaline pretreatment for optimal performance. The natural fibres are soaked in acetic acid, treated with acetic anhydride before the final stage of washing (Quarshie and Carruthers, 2014; Wallenberger and Weston, 2004). Esterification occurs during this process due to the reaction between acetyl groups (CH3CH-) and hydroxyl groups (-OH), as illustrated in Fig. 6.7 (Hill et al., 1998; Pickering et al., 2016). The fibre surfaces are rendered more hydrophobic, thus increases the fibre-matrix interfacial compatibility, reduces the moisture uptake and biological or fungal attack by replacing the hydroxyl groups of a natural fibre cell wall with acetic anhydrides (Quarshie and Carruthers, 2014). This treatment is very common to woods for dimensional stability environmental protection, without reducing other important mechanical (strength and modulus) properties of the concerned natural fibres. Care must be taken to avoid overtreatment of fibres during the acetylation process because it causes fibre cellulose degradation and cracking with the catalyst used. Consequently, the mechanical properties (especially impact strength) of the overtreated fibres and resultant biocomposites are reduced (Bledzki et al., 2008; Pickering et al., 2016). Also, Wallace (2005) reported the use of a coupling agent, known as polymethylene-polyphenyl-isocyanate, to enhance the interfacial bonding (strength and stiffness) between wood fibre and polypropylene (PP) of the wood FRP composites. The synthetic isocyanate has adverse health effects. Therefore, Lee and Wang (2006) manufactured a bio-based lysine diisocyanate to improve the adhesive bond and hence, enhance the mechanical properties and water resistance of bamboo fibrepolylactide biocomposites.

O OH

+

H 3C H 3C

C

O O

CH3 + H3C

O C

C O

C OH

O

Figure 6.7 Esterification reaction between an acetic anhydride and a hydroxyl groups of a natural fibre cell wall (Pickering et al., 2016).

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In addition, there is a silane treatment. Silanes are very effective molecules used as a surface modifier or a grafting mediator used in composite materials. Due to their silicon-oxygen structure and larger bond angles than hydrocarbon equivalents, they are more popular as an interfacial modifier (Tingju et al., 2013). Primarily, silane treatment involves hydrolysis of alkoxy groups on with water to produce silanol (Si-OH) group and later reacts with eOH groups presents on the surface of natural fibres, as shown in Fig. 6.8. This reaction could lead to the formation of either hydrogen of covalent bonding. Silanes increases the strength of natural fibre-reinforced polymer biocomposites by increasing the hydrophobicity of their natural fibres. A large increase in strength is associated with covalent bonding between silane and matrix used (Rachini et al., 2012; Pickering et al., 2003). The most rampantly used silanes are methacryl, alkyl, amino and glycidoxy silanes. An improved interfacial bonding of both conventional and natural FRP composite materials is possible with the use of an organo-silane coupling agents. For instance, Valadez-Gonzalez et al. (1999) achieved a stronger interaction between short henequen fibres with an organo-silane coupling agent. Consequently, the mechanical properties, such as tensile strength of the biocomposite, were improved, as depicted in Fig. 6.9. The reaction illustrated in Fig. 6.8 can be simply represented as Eqs (6.1) and (6.2), according to Rachini et al. (2012). R
SiðOR1 Þ3 þ 3H2 O/R
SiðOHÞ3 þ 3R1 OH

(6.1)

R
SiðOHÞ3 þ Fibre
OH/Fibre
OðXÞ2 Si
R þ H2 O

(6.2)

Where, R and R1 represent functional carbon and carbon chains, respectively and X denotes eOH or eOSi. Xu et al. (2009) modified the surface of kenaf fibre with a silane coupling agent and improved the interfacial adhesion between the natural fibres and the polystyrene matrix. This was performed after an occurrence of a condensation reaction between the alkoxysilane and hydroxyl groups of kenaf cellulosic fibres. Consequently, the resultant kenaf fibre-reinforced polystyrene biocomposites exhibited a lower tan d and greater storage modulus, when compared with the untreated counterparts. This implies a higher interfacial adhesion between the treated fibres and the matrix resin. Pothan and Thomas (2003) chemically treated the surface of banana fibres using two different silanes: A174 (gamma- methacryloxypropyltrimethoxysilane) and OH

OH OH

+

2 OH

Si

O

R’

OH H

OH

Si

R’

OH

OH O

OH

Si

R’ + H2O

OH

Figure 6.8 Silane reacts with natural fibre surface, where hydrogen bonding (Pickering et al., 2016).

R0

and . denote organic group and

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30

Tensile strength (MPa)

25 20 15 10 5 0 FIB

FIBNA

FIBSIL

FIBNASIL

Fibre treatment

Figure 6.9 The effects of different fibre surface treatments on the tensile strength of a 20%v/v high-density polyethylene-henequen fibre composites. FIB, untreated henequen; FIBNA, henequen fibre treated with an aqueous alkaline solution; FIBSIL, henequen fibre treated with a silane coupling agent; FIBNASIL, henequen fibre treated first with alkaline solution and then with a silane coupling agent. (Valadez-Gonzalez et al. 1999).

A151 (vinyl triethoxy silane), as well as 0.5% NaOH. Fig. 6.10 depicts the structures of the types of silanes used. Silane A151 treated fibres recorded the highest overall polarity parameter when compared with other treatments. Also, the highest storage modulus was recorded from the same silane A151 treated banana fibre-reinforced polyester biocomposites. While, the lowest hydrogen-bond donating acidity and maximum shift in the loss modulus curve were recorded from the silane A174 treated fibres and its biocomposites, respectively after pre-treatment with 0.5% NaOH. However, banana fibres treated with silane A151 recorded the highest value of hydrogenbond donating acidity. In conclusion, it was evident that silane A174 was an ideal coupling agent for enhancing banana fibre-polyester matrix interfacial adhesion. Hence, an improvement in the storage modulus of banana fibre-reinforced polyester biocomposites has been achieved through chemical modification. Cantero et al. (2003) reported on the influence of different chemical treatments on flax fibre-polypropylene compatibility, considering the surface energy and mechanical properties of the resultant biocomposites. Both natural flax and flax pulp kinds of flax fibres were modified. Maleic anhydride-polypropylene copolymer (MA-PP), maleic anhydride (MA) and vinyl trimethoxy silane (VTMO) were the three applied CH3

OC2H5 CH2

CH

Si

OC2H5

CH2

C

OC2H5 A151

C O O

(CH2)3

OCH3 Si OCH3 OCH3

A174

Figure 6.10 The structures of A151 and A174 silanes used for modification (Pothan and Thomas, 2003).

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treatments. Their results showed that the polar component of the flax fibre surface energy was reduced using the three treatments. Evidently, the 10% MA-PP treated fibrereinforced biocomposites exhibited the greatest mechanical (tensile and flexural strengths) and other properties (total surface energy), whereas both MA and VTMO treated fibre biocomposites exhibited similar values when compared with the untreated counterparts. Similarly, Ismail and Abdul Khalil (2000) experimentally investigated into the influence of silane coupling agent treatment and partial replacement of oil palm empty fruit bunch fibres (ground into oil palm wood flour) by silica on mechanical properties of the resultant reinforced biocomposites. Based on their report, the addition of a silane coupling agent increased the cure time, scorch time and the mechanical properties (hardness, tear strength, tensile strength, fatigue life and tensile modulus). Also, the addition of silane coupling agent and partial replacement of oil palm wood flour by silica improved the interfacial interaction between the rubber and the filler. In a similar manner, Abdul Khalil and Ismail (2000) treated oil palm empty fruit bunch (EFB) and coconut (Coir) mat fibres with titanate (neopentyl (diallyl) oxy tri (dioctyl) pyro-phosphate titanate) or silane (g-methacryloxypropyltrimethoxy silane), before fabricated into a resultant non-woven fibre mat-reinforced polyester biocomposites. The results obtained from the soil burial (biological) tests conducted were dependent on the silane and acetylation treatments. Both untreated and titanate treated fibres biocomposites recorded a high decrease in both tensile and impact strengths. The acetylated fibre biocomposite exhibited the highest bioresistance to decay or degradation after 12 months of exposure, followed by silane treated FRP biocomposites, cast resin and 4-ply glass FRP composite that were included in the experiment for the purpose of clear comparison. The high soil degradation exhibited by the fibre composites were related to high reductions in their mechanical properties (impact and tensile strengths). Moreover, maleated coupling is extensively utilised todays to improve the properties of the natural fibre-reinforced polymer biocomposites. Comparatively, maleated coupling has a fundamental difference when compared with other chemical treatments. Maleic anhydride is used to modify the fibre surfaces and the polymeric matrices to obtain better mechanical properties and fibre-matrix interfacial adhesion (Faruk et al., 2012). The maleic anhydride grafted polymers are coupling agents, extensively used to enhance the performance of biocomposite materials. Commonly, maleic anhydride is used with compatible polymer matrices. For instance, there is a maleic anhydride grafted polypropylene matrix (MA-PP). MA-PP type is widely used in many reported works, as subsequently discussed. It reacts with eOH groups on the natural fibre surfaces to produce either hydrogen or covalent bonding. There are many reported studies on the application of coupling agents, such as commonly used maleic anhydride grafted polylactic acid or polylactide and polypropylene (MA-PLA and MA-PP) towards enhancement of the properties of natural fibrereinforced polymer biocomposites. A 0.5% MA-PP (G-3015) concentration in toluene was used to modify the jute fibre surfaces (Mohanty et al., 2004). They reported a better fibre-matrix interfacial adhesion that resulted in an approximately 72.3% increase in the flexural strength and significantly reduced water absorption of the treated jute fibre-reinforced polypropylene biocomposite. Average fibre lengths, fibre loading

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and impregnation time of 6 mm, 30% and 5 min were used, respectively. Similarly, the influence of maleic anhydride (MA) on the mechanical properties (impact strength, hardness, Young’s and flexural moduli) and swelling behaviour of banana, hemp and sisal fibre-reinforced novolac biocomposites have been studied (Mishra et al., 2000). They observed improvements in these properties with all the maleic anhydride treated fibre biocomposites than the untreated counterparts. Evidently, there was an increase in all the mechanical properties and a decrease in both steam and water absorptions, as the maleic anhydride acted as a compatibiliser. Yang et al. (2007) reported an increase in the mechanical (tensile strength and modulus) properties of rice-husk flourreinforced polypropylene biocomposites, using MA-PP coupling agents. This was attributed to the enhancing effect of compatibilising agents on the filler-matrix interfacial adhesion. However, the tensile strengths of the biocomposites decreased with an increased reinforcing filler (rice-husk flour) loading. Also, the addition of the compatibilising agents almost had no significant influence on both notched and unnotched Izod impact strengths of the biocomposites. Moving forward, Liu et al. (2009) used both maleic anhydride grafted polyethylene (PE-g-MA) and maleic anhydride grafted styrene/ethyleneebutylene/styrene triblock polymer (SEBS-g-MA) to improve the impact property and interfacial adhesion between banana fibre and high-density polyethylene (HDPE)/Nylon-6 blends. The HDPE/Nylon-6 based banana fibre-reinforced biocomposites exhibited better strengths and moduli with the addition of SEBS-g-MA when compared with counterpart HDPE based biocomposites. Additionally, both biocomposite systems exhibited slightly different thermal stability, apart from an additional decomposition peak caused as a result of minor Nylon-6 in the biocomposite with HDPE/Nylon-6 blends. Gassan and Bledzki (2000) treated the jute fibre surfaces with a maleic anhydride polypropylene copolymer (MAH-g-PP), as a coupling agent and consequently, improved the jute fibre-polypropylene interfacial bonding of the biocomposites. This better fibre-matrix interfacial adhesion decreased by nearly 30% loss-energy by nonpenetration recorded after the impact test on the concerned biocomposite samples. But, with greater loss energies at different temperatures: cold and warm of
30 and 70 C test conditions, respectively, when compared with a room temperature of 23 C. Furthermore, various improvements on properties and interfacial adhesion of different natural fibres and their fibre-reinforced polypropylene (FRPP) biocomposites with a widely used maleic anhydride grafted polypropylene (MAH-g-PP or PP-gMAH or simply, MA-PP), as a coupling agent have been reported. These include, but are not limited to, trans-crystallinity of jute FRPP biocomposites (Bledzki et al., 2001), surface properties and water uptake behaviour of different types of flax, hemp and cellulose fibres (Bismarck et al., 2002), thermal and crystallisation properties of sisal FRPP biocomposites (Joseph et al., 2003), dynamic-mechanical properties of flax and hemp FRPP biocomposites (Wielage et al., 2003), wetting behaviour of flax FRP biocomposites (Aranberri-Askargorta et al., 2003), characterisation of corn FRPP biocomposites: effects of micro-sized cellulose based corn fibers (Fuqua and Ulven, 2008) and tensile properties of Hildegardia FRPP biocomposites (Li et al., 2010), as reported by Faruk et al. (2012).

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In addition, as earlier and broadly discussed, Beckermann and Pickering (2008), Bledzki et al. (2010), Bera et al. (2010), as well as Franco-Marqu
es et al. (2011) treated hemp, abaca, jute and newsprint/paper (natural) fibres, respectively, with different concentrations of MA-PP before they were fabricated into their respective biocomposites. Summarily, several mechanical (impact, tensile, flexural strengths and stiffness), thermal, physical and importantly interfacial adhesion or bonding of the natural fibres/ polypropylene biocomposites were improved. Evidently, among all these surface treatments, the use of the MA-PP coupling agent could be regarded to be the most successful method of enhancing the interfacial bonding. It provided almost twice the strength of the concerned biocomposite result obtained with silane treatment (Bera et al., 2010). This significant improvement with MA-PP, especially in mechanical performance, has been attributed to its wettability on natural fibres and improved dispersion (Kazayawoko and Balatinecz, 1997). In a like manner, maleic anhydride grafted polylactic acid or polylactide (MA-PLA) significantly increased the thermal stability, mechanical (tensile, flexural and impact strengths), interfacial adhesion and other important properties of PLA matrix natural (kenaf, bamboo and ramie) FRP composites (Avella et al., 2008; Kang et al., 2013; Yu et al., 2014), respectively. Also, enzyme treatment has been used successfully to improve the properties of biocomposite materials. The use of enzyme treatment on natural fibres is gaining increasing attention today because of its environmental benefits and eco-friendliness (Faruk et al., 2014); for example, Bledzki et al. (2010) modified abaca fibres by fungamix and natural enzymes, before being prepared into abaca fibre-reinforced polypropylene biocomposites. The fungamix modified abaca fibre biocomposites exhibited an increase of 85% in elongation at break, 45% in tensile strength, 35% in flexural strengths, 25% in impact strength and the lowest moisture absorption of 45%, and when compared with the unmodified fibre composites, and among others: maleic anhydride grafted polypropylene (MA-PP) and natural digestion system (NDS) modified abaca fibre biocomposites, as shown in Fig. 6.11(aed), respectively. Also, the modification supported a better resistance in both acid and base medium, as well as an improved decomposition temperature. A comprehensive and critical review on effects of different alkaline treatments (including a percentage of concentration) and coupling agents (such as MA-PP) on mechanical properties tensile strength, elongation at break, tensile modulus, Young’s various biocomposites (old newsprint-filled reinforced polypropylene, high-density polyethylene and linear low-density polyethylene/flax fibre-reinforced polypropylene, corn chaff-reinforced polypropylene, jute fibre-reinforced/poly (butylene succinate), coir fibre-reinforced polypropylene, hemp fibre-reinforced plylactic acid, kenaf fibre-reinforced polypropylene, jute fibre-reinforced polypropylene, bleached hemp fibre-reinforced 1-pentene/polypropylene copolymer, polypropylene/hemp fibres, micro winceyette fibre-reinforced thermoplastic corn starch, flax fibre-reinforced/highdensity polyethylene, high-density polyethylene-reinforced with hemp fibres, hardwood fibres and rice hulls fibres) have been reported by Ku et al. (2011). From this review, it was evident that both coupling agents and alkaline treatments improved the mechanical and other properties of biocomposites to a certain threshold value.

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The chemical surface treatment has been reported to have a long-term mechanical performance of natural FRP composites in both wet and humid conditions, with regards to enhancements of instantaneous mechanical behaviour of biocomposite materials (Singh et al., 1996).

6.3.3

Physical treatments

The main aim of using physical treatment methods is to improve the fibre-matrix adhesion through the increase of surface characteristics. With physical treatments, the functional surface and structural properties of natural fibres can be enhanced using plasma, corona discharge, ultraviolet (UV), fibre beating, heat treatment and electron radiation. The chemical compositions of the natural fibres are not extensively changed with physical treatments. Physical treatments improve the mechanical properties by enhancing the fibre-matrix bonding, therefore interfacial adhesion of the biocomposites. Another physical technique is called plasma treatment. The use of plasma treatment helps to roughen the fibre surfaces. This treatment is conducted in oxygen and using a vacuum chamber. It accommodates a variety of fibre surface modifications, depending on the nature and type of the gases used. Plasma treatments can be carried out in a lowtemperature condition to enhance the surface roughness of the natural fibres. It results in hydrophobicity at fibre surfaces, and thus, an increase in the surface roughness of the fibre. Therefore, the interfacial adhesion is increased (Sinha and Panigrahi, 2009; Pickering et al., 2016). For instance, Seki et al. (2009) modified the jute fibre surfaces by subjecting the fibres to a low-temperature oxygen plasma treatment. They investigated the effects of this treatment on the interlaminar shear strength (ILSS) and flexural strength (mechanical) properties of the jute fibres/high density polypropylene composites. They obtained the best interfacial bonding with the biocomposite, using an optimum plasma power of 60 W for 15 min, followed by 30 W with nearly 47% and 32% increase, respectively, compared with the untreated biocomposites. The mechanical properties, such as storage moduli of the plasma treated FRP composites depend on different strain frequencies, hybridisation of the reinforcements, plasma treatment speed and/or scan rate, as depicted in Fig. 6.12. More also, Marais et al. (2005) investigated the effect of cold helium plasma-treated flax fibre-reinforced unsaturated polyester biocomposites on their mechanical (stiffness) and water permeation properties. Their results showed that the plasma treatment enhanced the fibre-matrix adhesion and produced better stiffness. Also, Martin et al. (2000) improved the mechanical properties of a plasma-treated sisal-high density polyethylene biocomposites. Sinha and Panigrahi (2009) studied the effect of argon cold plasma treatment on the structural and wettability of jute fibres, as well as flexural strength of its reinforced unsaturated polyester biocomposite. Their result showed that the application of plasma treatment caused an occurrence of rough surface morphology and hydrophobicity in the jute fibres. This was attributed to a decrease in lignin, phenolic, secondary alcoholic groups or oxidation and hemicelluloses after plasma treatment, as shown in the Fourier transform infrared-spectroscopy results obtained. Consequently, these phenomena improved the interfacial jute fibre-thermosets adhesion, as depicted from micrographs of the plasma-treated fibres biocomposites obtained from the scanning electron microscope used. There was a nearly 14% increase in

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Figure 6.12 Influences of different strain frequencies, hybridisation of the reinforcements, plasma treatment speeds and/or scan rates on plasma treated FRP composites. Cho, B-G., Hwang, S-H., Park, M., Park, J.K., Park, Y-B., Chae, H.G., 2019. The effects of plasma surface treatment on the mechanical properties of polycarbonate/carbon nanotube/carbon fibre composites. Composites Part B: Engineering 160, 436e445. 247

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the mechanical (flexural) strength of the 10 min plasma-treated fibre biocomposites, among all the treated fibres, when compared with the raw/untreated fibre biocomposites. In addition, plasma treatment causes a crystallisation, etching, formation of free radicals, chemical implantation and polymerisation (Mohanty et al., 2005), among other effects. Many manufacturers prefer this treatment to others because it is cost effective, environmentally friendly and sustainable. However, it has few problems of scaling up for a commercial purpose and depends on atmospheric conditions, which vary from place to place. Seki et al. (2010) treated jute fibres using oxygen and different powered plasma treatments in radio frequency (RF) and low frequency (LF) plasma reactors. It was reported that mechanical (tensile, flexural and short beam shear) properties of the jute fibre-reinforced polyester biocomposites were enhanced. The LF and RF oxygen plasma jute treated fibre-reinforced polyester biocomposites exhibited a significant increase in interlaminar shear strength (ILSS) of 19.8 and 26.3 MPa, respectively, when compared with 11.5 MPa for the untreated jute fibre-reinforced polyester biocomposite. There were improvements in both tensile and flexural strengths of jute fibre-reinforced polyester biocomposites with oxygen plasma treatment for both LF and RF plasma systems. Evidently, a better improvement on the mechanical performance of the jute fibre-reinforced polyester was achieved with an RF oxygen plasma system in comparison with LF counterpart. Similarly, an increased level of corona treatment has been used to tremendously improve the wettability of some natural fibres, such as woods (Wallace, 2005) and surface energy of the cellulosic fibres. It causes both physical and slight chemical changes to the fibres, such as an increase in the surface polarity and fibre roughness. It increases the compatibility between hydrophilic fibres and hydrophobic matrix. For example, Ragoubi et al. (2010) reported on a significant increase in or improvements on mechanical (tensile strength, compressive stress-strain and 30% of Young’s modulus) properties of corona treated hemp fibre-reinforced polypropylene biocomposites. Likewise, Pizzi et al. (2009) enhanced the mechanical (surface contact angles, hardness, bending/flexural, tensile strength and modulus of elasticity) of corona treated nonwoven flax and hemp fibre mat-reinforced 50/50 natural resin matrices (mimosa flavonoid tannin with hexamine and mimosa tannin with both hexamine and glyoxalated organosolv lignin) lower density thicker and higher density thin biocomposites. However, the application of corona treatment to the three-dimensional (3-D) surfaces such as natural fibres is never found easy (Gassan and Gutowski, 2000; Ragoubi et al., 2010). Furthermore, Gassan and Gutowski (2000) improve the mechanical (flexural and tensile strengths, storage modulus, polarity and tenacity) of tossa jute fibres/epoxy biocomposite, using both corona discharge and ultraviolet treatments on the natural fibres. These treatments increased the fibre polarity. However, the corona treatment decreased the flexural strength of the jute fibres, and consequently, the flexural strength of the biocomposite, while UV treatment enhanced the flexural strength of the biocomposite to nearly 30% under optimum treatment conditions. Also, the jute yarn tenacity (tensile strength) decreased due to the oxidation reaction at the fibre surface. The fibre modulus of elasticity (stiffness) was not affected with various corona energy levels. But, there was nearly 15% improvement in storage modulus at optimal treatment condition of 60 C.

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Also, fibre beating causes fibre defibrillation, and thus, increases the fibre surface area to promote better mechanical interlocking. For example, Beg and Pickering (2008) enhanced the tensile strength of kraft fibre-reinforced PP biocomposites with modest levels of fibre beating due to the improved interfacial bonding. The unbeaten fibre biocomposite tensile strength of 41 MPa was increased to a threshold value of 45 MPa of beaten fibre composites at an optimum beating time of 5.5 min. Noticeably, it was observed that the tensile strength of the beaten fibre composites reduced upon further beating, shortly after the optimum beating time. Close to fibre beating is the heat treatment. As its name implies, this is a process of heating the fibres near their degradation temperature. This process affects the fibre chemical (chain scission, the formation of carboxyl, carbonyl and peroxide, as well as free radical production groups), mechanical and physical properties (Pickering, 2008). Other changes include the degree of polymerisation, water content, strength, cellulose crystallinity and chemistry. The heat treatment is very similar to both plasma and corona treatment because it depends on the composition, time and temperature of the gases used during the treatment. For instance, the crystallinity index of kenaf fibres has been increased with heat treatment (Cao et al., 2007). Consequently, this caused an improvement of its mechanical (tensile strength and elongation at break) properties. A maximum or an optimal tensile strength of the treated fibre was obtained at 140 C, which later decreased upon further heating. Rong et al. (2001) improved the mechanical properties of unidirectional (UD) sisal-reinforced epoxy biocomposite after heating the sisal fibres at 150 C for 240 min in an air-conditioning oven. Additionally, electron radiation produces free radicals that assist crosslinking between the hydrophilic bio-fibres and the hydrophobic matrix. The interfacial bonding between natural fibres (single hemp, ramie, flax and cotton)/fibre bundles and polymer (polypropylene) has been enhanced with electron radiation of 10 MeV at intensities of 5, 15 and 33 kGy on the fibres (Huber et al., 2010). The interfacial shear strength of the biocomposites increased up to 50%. This was attributed to the formation of radicals with the cellulose chains of the natural fibres and the molecules of the polypropylene, causing a crosslinking, and consequently, and improved adhesion between the fibres and the matrix. Moreover, both hydrophilicity (rate of moisture uptake) and thermal degradation of natural fibres can be decreased by using hydrothermal treatments. For instance, Wallenberger and Weston (2004), as well as Stamboulis et al. (2000) used this method to reduce the thermal degradation and improve the moisture and thermal resistance of treated Duralin flax, as dew retting process was eliminated, when compared with the untreated counterpart.

6.3.4

Additive treatments

This method aims to enhance the properties of matrix materials, as well as fibres so that the overall properties of composites are improved. The natural fibre-polymer matrix interfacial bonding can be enhanced by impregnating the natural fibres with a suitable

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matrix. This method of fibre treatment is not common, unlike the two aforementioned and extensively discussed methods. Additive treatments involve using either a solvent exchange method or by infusion method, whereby monomers are infused into the fibres followed by polymerisation with the aid of a catalyst, radiation or heat, as reported by Mohanty et al. (2005).

6.3.5

Biological treatments

Similar to chemical treatments, the biological process could be used to modify and improve the surface of natural fibres to produce enhanced biocomposites. For example, Pommet et al. (2008) deposited cellulose nanofibrils on the surfaces of both natural hemp and sisal fibres, using bacteria (Acetobacter xylinum), as shown in Figs 6.13 and 6.14, respectively. The nanofibrils acted as substrates during the fermentation stage of bacterial cellulose. After a single fibre pull-out/tensile test was carried out, a significant improvement in the biologically treated fibre biocomposites was observed. This was attributed to an improved interfacial bond between the 5%e6% bacterial cellulose treated fibres and the polymeric matrices (cellulose acetate butyrate and polylactic acid) used. Recently, Roy et al. (2020) investigated the effects of various chemical treatments on the performance of jute fibres filled natural rubber (NR) composites. They used three different types of surface treatments, namely; alkali treatment, combined alkali/stearic acid treatment and combined alkali/silane treatment. The influence of surface treatments on the jute fibres was characterised, using advanced techniques such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and field emission scanning electron microscopy (FESEM). The morphology from their report suggested that surface roughness of treated fibres were found rougher than the nontreated, as shown in Fig. 6.15. The rough surface acted as a mechanical interlocking, which contributed to improved properties. Their results exhibited that there was a significant improvement in the properties of jute fibre-reinforced NR biocomposites. The improvement in the properties was attributed to strong fibre-matrix interaction due to various surface treatments used.

Figure 6.13 SEM micrographs of (a) untreated natural and (b) bacterial cellulose modified hemp fibre surfaces (Pommet et al., 2008).

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Figure 6.14 SEM micrographs of (a) untreated natural, (b) sisal fibre with the attachment of bacterial cellulose and (c) acetone treated and bacterial cellulose modified sisal fibre surfaces (Pommet et al., 2008).

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Figure 6.15 XRD patterns for treated and non-treated jute fibres (Roy et al., 2020).

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6.4

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Approaches towards overall property enhancement via hybridisation, pinning, stitching, among others

Several efficient techniques have been developed towards overall property enhancement of fibre-reinforced polymer biocomposite laminates. These approaches improve or increase damage tolerance and properties of biocomposites, such as the ultimate strength, delamination resistance and impact damage tolerance, among others. Although, as the application of biocomposite increases, there may be a need for further improvement of these approaches using new technology. The most widely used approaches are stitching, hybridisation, pinning, knitting, weaving, braiding and tufting, as subsequently expatiated.

6.4.1

Stitching

Stitching is a method adopted to enhance the interlaminar fracture toughness of biocomposite, among many textile techniques of inserting of a through-thickness yarn into traditional two-dimensional (2-D) fibre reinforcements or preforms. Other methods include, but are not limited to, knitting and weaving. Stitching is probably the most commonly used method due to its versatility and ease of handling (Beier et al., 2007; Velmurugan and Solaimurugan, 2007; Tan et al., 2010). It is considered to be one of the main methods for the automated and cost-reduced production of complex textile preforms, later used for liquid composite moulding of high performance fibre-reinforced polymeric biocomposites (Beier et al., 2007; Weimer and Mitschang, 2001). It enhances the impact damage resistance, interlaminar fracture toughness, lap joint strength, delamination crack growth resistance and tolerance of the final biocomposites with suitable and efficient thread and pattern (Mouritz et al., 1997). For instance, Yudhanto et al. (2015) enhanced the tensile strength of composites stitched a with thicker thread because of the effective hindrance of edge delamination. Also, Velmurugan and Solaimurugan (2007) reported that stitching increased the Mode I delamination toughness of fibre polymeric-reinforced composite up to 20 times greater than the unstitched counterpart sample. But, Jain et al. (1998) recorded an enhanced Mode I delamination resistance up to 4 times; lower than that of Velmurugan and Solaimurugan. They study crack propagation of the fibre-reinforced polymer composites, depending on stitch density, thread diameter and type. These results were similar to the results observed by Jain and Mai (1994), as well as Dransfield et al. (1998). The stitching pattern could be lock, modified lock, chain or zigzag type (Fig. 6.16). In detail, Fig. 6.17 depicts a clear description of a stitched pattern through a quasi-isotropic composite laminate. Due to the aforementioned benefits associated with the use of stitching techniques, its potential applications in automobile parts include floor panels, bumper bars and door frame members. And stitching is used as a part of techniques for manufacturing some aircraft structures, such as wing panels, fuselages and blade stiffened parts (Mouritz et al., 1999; Mouritz and Cox, 2000).

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Figure 6.16 Illustration of the seam (a) lock, (b) modified lock, (c) chain, (d) plain and (e) zigzag patterns of stitching (Beier et al., 2007; Velmurugan and Solaimurugan, 2007; Mouritz and Cox, 2000). Needle thread

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Sustainable composites and techniques for property enhancement

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However, damaging effects of stitching on fibre preforms for biocomposites include fibre waviness or distortion, resin crack, crimping, induced undulations of the fibres, broken in-plane fibres, increased resin-rich zone and porosity, among others. Consequently, they lead to the degradation of the mechanical (flexural, tensile and compressive) properties of the composites (Khan and Mouritz, 1996; Velmurugan et al., 2004). In moving forward, some of the stitch-induced damages are illustrated in Fig. 6.18. In addition, the final extent of fibre damage depends on tension on stitch thread, the stitch type, alteration in effective fibre volume fraction, stitch span, consideration process during curing of the biocomposites, type of stitch yarn material, stitch density, the diameters of the stitching yarn and the needle (Mouritz et al., 1997; Yudhanto et al., 2015). The typical range of stitch diameter is 0.1e1.0 mm, above this range often results in detrimental distortion in the fibre architecture (Mouritz and Cox, 2010). Furthermore, the influences of through-thickness stitching of natural fibre on the mechanical (Mode I interlaminar fracture toughness and tensile) properties of flax fibre-reinforced epoxy biocomposite laminates have been reported (Ravandi et al., 2016). Preforms of woven fabric and unidirectional (UD) flax fibre laminates were stitched in various stitch densities, using flax yarn and cotton thread. A decrease in tensile properties (strength and stiffness) of the composites was similarly observed with both stitch materials (flax yarn and cotton thread), as shown in Fig. 6.19(a) and (b). This was attributed to the imperfection that resulted by the stitching. The interlaminar fracture toughness, GIC of the composite was not significantly enhanced with cotton thread stitch, but flax yarn stitch improved this property by at least 10% at the lowest stitch fibre area fraction. Evidently, the flax yarn stitch exhibited a higher fracture toughness of the biocomposite laminates than the cotton thread stitch, as depicted in Fig. 6.19(c).

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In the following year, the influences of through-thickness stitching of natural fibre on the low-velocity impact response of continuous flax fibre-reinforced epoxy biocomposite laminates have been additionally studied (Ravandi et al., 2017). Similar impact behaviour of both flax and cotton stitched biocomposites was observed, regardless of differences in their stitch fibre type and thickness. Besides, the intra-laminar fracture toughness of the stitched woven flax fibre-reinforced biocomposite laminate reduced by nearly 16% with flax yarn stitch, while the intra-laminar fracture toughness of the cotton yarn stitched laminate was decreased by only 5%. Hence, it was evident that the flax yarn stitch reduced the impact resistance of the composite laminate under low-velocity impact, and the use of flax yarn stitch is unfavourable to the higher structural performance applications of the composite. More also, the effect of water absorption on the Mode I interlaminar fracture toughness of stitched and unstitched hybrid flax/basalt fibre-reinforced vinyl ester biocomposite laminates have been experimentally investigated (Almansour et al., 2017). Using three types of composite laminates: flax fibre-reinforced vinyl ester (FVE), unstitched hybrid (FBVEu) and stitched hybrid (FBVEs). The results obtained showed that the water immersed FVE laminate exhibited decreased Mode I interlaminar fracture toughness initiation and propagation by an average of 27% and 10%, respectively, when compared with the dry counterparts, whereas that of water immersed FBVEu and FBVEs biocomposite laminates were enhanced by nearly 15% and 17%, compared with the dry counterparts. These further established the fact that basalt fibre hybridisation improves both durability and water/moisture absorption resistance of natural fibre-reinforced polymeric biocomposites. It was observed that FVE laminate sample recorded the highest water absorption or weight gain of 5.38% before a plateau was reached around 5 weeks (900 h). Importantly, both FBVEs and FBVEu types of hybrid biocomposite laminates exhibited similar water absorption responses of 3.70% and 3.57%, respectively, because of the same balanced lay-up stacking sequence used. Comparatively, FBVEs recorded higher weight gain or water uptake than the FBVEu counterpart, as shown in Fig. 6.20. This can be attributed to the absorption property of the stitching thread, materials and more space created by needle and thread for water to lodge (water ingress). Hence, stitched hybrid natural fibrereinforced biocomposite attracts an increasingly small percentage of water uptake, and consequently, limits the stitching performance and some mechanical properties. These results were further evaluated and confirmed by using efficient microscopic examination techniques: scanning electron microscopy (SEM) and X-ray computed micro-tomography (mCT). In addition, in the following year, the same authors advanced the experimental investigation to cover Mode II interlaminar fracture toughness of the same hybrid stitched and unstitched flax/basalt-reinforced vinyl ester polymeric biocomposite laminates. They reported that the crack length of FBVEu dry biocomposites recorded high delamination resistance and an increased crack propagation resistance. The delamination phenomenon was effectively arrested by the stitch yarn used in FBVEs laminates and resultantly caused an increase in the initial interlaminar fracture energy, GIIC init. by 62%. However, the propagation of interlaminar fracture toughness, GIIC prop. was reduced by 36% (Almansour et al., 2018).

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Figure 6.20 Comparative water absorption responses of stitched and unstitched hybrid natural flax/basalt fibre-reinforced biocomposite laminates (Almansour et al., 2017).

6.4.2

Hybridisation

The technique of combining more than one type of fibre-reinforced in the same matrix of a biocomposite system is called hybridisation. With the use of the hybrid system, the mechanical properties of natural fibre-reinforced polymeric biocomposites can be improved. In recent years, the hybrid technique has been used to overcome some of the drawbacks of natural fibre-reinforced bio-based composites, which tailors the important properties of two reinforcements in the same matrix. It is one of the widely used techniques in the contemporary world of composite technology. The hybridising material (sisal fibre) can be arranged in different stacking sequences (Fig. 6.21); either as skins (Fig. 6.21(a)) or core (Fig. 6.21(b)). The combined inherent properties of the two or more constituent fibres provided greater advantages over the use of conventional single fibre-reinforced polymeric biocomposites. Other attractive factors of employing hybridisation technique include, but are not limited to, the following benefits:

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(a) - SHHS (b) - HSSH (c) - HHSH (d) - HSHS

Figure 6.21 Stacking sequences of hybridising sisal fibre mats into hemp fibre mat-reinforced polymeric biocomposites (Kumar et al., 2019).

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n

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Weight and cost reduction. Both weight of a hybrid biocomposite of a certain thickness and its cost of production are lower than that of synthetic fibre composite counterparts. Improvement to environmental effects since more natural fibres are used to obtain biocomposite laminates of higher thickness, density and strengths. Enhancement of mechanical, thermal and other properties of the natural fibre-reinforced polymeric biocomposites, compared with the synthetic fibres (glass, carbon and such like) reinforced polymeric composites. Application where high performance is required (Almansour et al., 2017).

However, hybrid natural fibre-reinforced polymeric biocomposites still have some reduced limitations initially associated with the single natural fibre-reinforced polymeric biocomposites, despite their aforementioned outstanding properties. Some of the major drawbacks of single natural fibre-reinforced polymeric biocomposites have been extensively discussed in the earlier chapters. In addition, natural fibrereinforced polymeric biocomposites have not been widely used as structural components, due to their lower strength and stiffness, as well as higher moisture absorption properties (Faruk et al., 2012). Therefore, some of the challenges of hybrid biocomposite systems are thus briefly highlighted. n

n n n n

Moisture absorption and swelling effect over a long duration, due to the combination of more hydrophilic natural plant fibres, probably the hybridising fibre has a higher hydrophilic nature. Higher cost of manufacturing occasionally. Requirement of special manufacturing process(s) or technique(s) in a few cases. Possibility of more or complex cracking, delamination effects, among other manufacturinginduced defects and post-manufacturing/test damage. Problem of incompatibility (weak interfacial adhesion) between two or more different natural fibres and a single matrix.

There are greater benefits derived from the properties and application of hybrid natural fibre-reinforced polymeric biocomposites over single fibre-reinforced polymeric biocomposites and much more over the conventional fibre-reinforced polymeric composite. Therefore, many studies have been conducted and reported on hybrid biocomposite systems. For instance, due to the excellent chemical, acoustic, mechanical and thermal properties of basalt fibres, they have been used as hybridising materials into several natural fibre-reinforced polymeric biocomposites (Dhakal et al., 2015; Fiore et al., 2016, 2017). The post-impact, quasi-static and cyclic flexural properties of hybrid hemp/basalt fibre-reinforced unsaturated polyester biocomposite were enhanced (Dhakal et al., 2015). The hybrid hemp/basalt (HB) reinforced biocomposite exhibit greater flexural stress-strain (modulus) of non-impacted biocomposite samples and normalised flexural strength of impacted samples under different impact energies of 3, 6 and 9 J, when compared with single natural hemp (H) fibre-reinforced unsaturated polyester biocomposite, as depicted in Fig. 6.22. Similar results on higher values of flexural strength, modulus and Mode II interlaminar fracture toughness of hybrid flax/basalt fibre-reinforced vinyl ester biocomposite laminates were obtained by Almansour et al. (2015), when compared with the single or non-hybrid flax fibrereinforced laminate counterparts.

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(a) 180

H HB

160

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140 120 100 80 60 40 20 0 0.0

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1.0 Hemp Hemp/basalt 0.9

0.8

0.7

0.6

0.5 0

2

4

6

8

10

Impact energy (J)

Figure 6.22 (a) The stress-strain curves of hemp and hemp/basalt biocomposites before impact test and (b) varied residual normalised flexural strength with impact energy (Dhakal et al., 2015).

Similarly, the effects of hybridisation technique with basalt fibre as double external layers on the mechanical (impact and flexural strength), water absorption and modulus properties of flax fibre-reinforced epoxy biocomposites under 0, 15, 30, 45 and 60 ageing days of salt-fog environment conditions have been investigated (Fiore et al., 2016). The basalt and flax fibres were arranged to form two basalt fibres (skin)-six flax fibres (core)-two basalt fibre (skin) stacking sequence of flax/basalt biocomposite laminates, using a balanced lay-up method. The hybridised flax/basalt biocomposite

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(a)

261

120 V [MPa]

Flax

100

Flax–basalt

80 60 40 20 0 0.0

(b) 8000

H [%]

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E [MPa]

6.0

8.0

Flax

6000

Flax–basalt

4000 2000 0 –2000 –4000 –6000 H [%]

–8000 0.0

2.0

4.0

6.0

8.0

Figure 6.23 The effects of hybridisation technique on mechanical properties of flax fibre biocomposites, showing (a) stress-strain and (b) modulus estrain curves of flax and flax/basalt fibre-reinforced unaged biocomposites (Fiore et al., 2016).

laminate exhibited greater impact, flexural modulus and strength by 28%, 49% and 71% without ageing day, respectively, when compared with non-hybridised flax fibre biocomposite counterpart (Fig. 6.23). In addition, the hybrid flax/basalt laminates possessed a greater water uptake resistance property than that of the non-hybrid flax fibre laminate, due to the presence of hybridising material (basalt fibre) skins. The hybrid flax/basalt and non-hybrid fibre laminates recorded maximum water uptake values of 9% and 23%, respectively, before a plateau was reached by both laminates after an approximately aging time of 700 h (Fig. 6.24). This result was very similar to that of Almansour et al. (2017) with the same hybrid flax/basalt fibre-reinforced biocomposite, but different polymeric matrix (vinyl ester) was used. However, the impact strengths (durability) of the hybrid biocomposite laminates were lower after 15, 30, 45 and 60 ageing days, when compared with the non-hybrid flax biocomposite counterparts, as depicted in Fig. 6.25. The letters A, B, C, D and E represented 0, 15, 30, 45 and 60 ageing days, respectively.

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Flax Water uptake [%p/p]

30

Flax–basalt 25 20 15 10 5 0 200

0

600

400

800

1000

1200

1400

1600

Aging time [h]

Figure 6.24 Influence of basalt fibre, as a hybridising material on water uptake of flax fibrereinforced biocomposite laminate (Fiore et al., 2016).

Fiore et al. (2017) hybridised jute fibre-reinforced epoxy biocomposite laminates with basalt fibre, using intercalated and sandwich-like basalt-jute stacking sequences, as well as ply-substitution approach. The durability of the jute-basalt interply hybrid laminates exposed to salt-fog was improved. Evidently, the optimum durability results of mechanical properties (quasi-static flexural modulus and impact peak load) were obtained with sandwich-like configuration after 90 days of ageing, when compared with jute fibre-reinforced and intercalated configurations. Admittedly, the improvement in properties of natural fibre-reinforced polymeric biocomposite can be attributed to the source of hybridising basalt fibre. Basalt fibres are obtained from the natural mineral 55

Flax

50

Flax–basalt

Impact strength [KJ/m2]

45 40 35 30 25 20 15 10 5 0

A

B

C

D

E

Figure 6.25 Comparative effects of 0, 15, 30, 45 and 60 ageing days of salt-fog environment conditions on impact strengths of both hybrid flax/basalt fibre-reinforced epoxy polymeric biocomposite and non-hybrid flax fibre counterparts (Fiore et al., 2016).

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produced when rocks are melted. It is a non-harmful resource, hence considered as an environmentally friendly fibre when compared with the synthetic glass and carbon fibres (Sarasini et al., 2013; Fiore et al., 2016). Consequently, basalt hybridisation into hydrophilic natural fibres enhances the mechanical properties and water absorption resistance of biocomposites by improving the interfacial adhesion between fibres and matrix. With these benefits, basalt fibres have potential of being used as hybridising materials of natural fibre-based biocomposites for structural applications (Sim et al., 2005). Moreover, Aji et al. (2012) studied the effects of hybridisation on mechanical (tensile, impact and flexural strengths) properties kenaf/pineapple leaf fibre-reinforced high-density polyethylene biocomposites. Both tensile and flexural of the biocomposites were increased due to the addition of the pineapple leaf fibre. The kenaf fibre enhanced both impact strength and water absorption resistance. In like manner, the influence of hybridisation on the mechanical properties of another natural banana and sisal fibres has been evaluated (Idicula et al., 2010). The addition of banana fibres improved the tensile properties of the hybrid short banana/sisal fibre-reinforced polyester biocomposites. The biocomposites with the ratio of banana to sisal (3:1) at 67 vol% fibre content recorded a maximum tensile strength of 58 MPa. This was attributed to the smaller diameter of banana fibres when compared with the sisal fibres, better stress transfer and improved banana-polyester interfacial bond present in the biocomposite system.

6.4.3

Pinning

Pinning is usually carried out on prepregs using a hand-held ultrasonic device (Tong et al., 2002; Mouritz, 2007), with a higher possibility of distortion compared with weaving. Also, the influence of the pinning technique on FRP composite damage and mechanical responses depends on the impact of energy, damage mechanisms (distribution and modes) and laminate layup (Francesconi and Aymerich, 2018). Zpinning (Fig. 6.26) is quite different from other techniques because it is the only method applicable to the reinforcement of the prepreg laminates, usually in the direction of through-thickness (Mouritz, 2007).

z-pin

Figure 6.26 A typical pinned composite (Mouritz and Cox, 2010).

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An ultrasonically assisted method has been used to improve the insertion of z-pins into FRP composite samples (Hoffmann et al., 2019). From the results obtained, there were microstructural changes, absence of fibre waviness, as well as resin-rich regions that surrounded each steel pin. Additionally, lack of friction and poor pin-laminate interfacial bonding were observed. The volume content of z-pins in FRP composite materials improved their mode I interlaminar fracture properties. It enhanced the delamination, fatigue and compression resistance of composite materials, especially after impact damage, as investigated and reported by several studies (Cartié et al., 2006; Zhang et al., 2008; Cartié et al., 2009; Pingkarawat and Mouritz, 2014, 2016; Mouritz and Koh, 2014; Pegorin et al., 2015; Ladani et al., 2016). To reduce variance associated with the pinning technique and obtain better results necessitated the use of a robotic insertion device or automatic multi-pin insertion system (Qinghua et al., 2014). The automatic system was capable of inserting multiples of pins during the preparation of Z-pin FRP composite laminates has been developed. It has 5 times more efficiency than the extant machines. The innovative system enhances accuracy, productivity and supports commercialisation of zpin technology. Pinning resulted in high fibre breakage and waviness, crimping, resin pockets and inplane fibre distortion around a pin inserted into biocomposites (Fig. 6.27), if not properly handled by an experienced manufacturer. Resin-rich or pocket zones occur around the pin, where fibres have been displaced by the inserted pins in the biocomposite system.

(a) Load axis T

Pin

Resin pockets

(b)

Figure 6.27 Micrograph of a pinned biocomposite, showing the defects of (a) resin-rich, fibre distortion or waviness and (b) crimping defect (Chan et al., 2006; Mouritz, 2007; Mouritz and Cox, 2010).

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Furthermore, cohesive modelling has been used to predict the behaviour of zpinned FRP composites. The simulation observations obtained were very close to the experimental results. It was evident that the fracture resistance curve must be discretised along the z-pin field in order to accurately capture the delamination dynamics in FRP composites (Ranatunga and Clay, 2012). In moving forward, a new finite element model has been developed to predict mode I interlaminar fracture toughness of z-pinned FRP composite laminates. Three different z-pinned materials were inserted into the composites used. These included high strength z-pins (stainless steel and titanium) and soft material (copper). It was observed that the model accurately predicted the crack growth resistance curves, as well as the interlaminar fracture behaviours of the FRP composites with various z-pins (Fig. 6.28). The lines and points denote the finite element prediction and experimental values obtained, respectively. The titanium and steel z-pinned composites failed by pull-out, while the copper z-pinned sample failed by both pull-out and rupture. Several methods have been used to evaluate and determine the through-thickness resistivity of FRP composites. For instance, the use of metal z-pins decreased the resistivity, and therefore, it depended on the electrical conductivity of the z-pin material. An increase in z-pin conductivity produced a decrease in the resistivity. The copper z-pin has a higher conductivity, compared with other materials, as reported (Pegorin et al., 2018).

6.4.4

Knitting

Interlaminar fracture toughness (kJ/mm2)

Fabric reinforcements are required for the biocomposite manufacturing processes, such as resin transfer moulding, vacuum infusion and hand or spray lay-up (as earlier and extensively discussed in Chapter 4). Natural fibres (such as flax, hemp, sisal,

12

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8

Cu (pullout)

Cu (rupture)

4

Unpinned 0

0

20

40

60

Crack growth (mm) Unpinned

Copper

Titanium

Steel

Figure 6.28 Experimental and finite element results of the interlaminar fracture toughness of a z-pinned FRP composite material, showing the effects of different z-pin materials (Blacklock et al., 2016).

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among others) can be warp knitted into either uniaxial or biaxial fabrics. Then, it can be woven into biaxial plain weaves. For instance, flax fibres were warp knitted by Goutianos et al. (2006). Warp knitting can be carried out using a warp knitting machine with weft insertion, whereby each warp thread is knitted to form interlaced loops, and fibre yarn straight inlays are introduced in the weft direction. Therefore, the fabric products are crimp-free in both directions. The fire yarns can be oriented in a uniaxial direction. Second, yarn can also be inserted in the warp direction to produce a 2-layer biaxial woven non-crimp fabric system. Numerous studies have been conducted on the properties of several knitted composite materials. Mechanical behaviours of 3D stitched woven-knitted basalt fabric biocomposites, to start with, were investigated (Hu et al., 2010). Four categories of knitted configurations were considered (Fig. 6.29). From the results obtained, it was observed that the mechanical properties of the 3D stitched woven-knitted composites were significantly influenced by the type of knitted structure. The interlock structure recorded the maximum energy absorption and tensile strength, while the plain knit structure exhibited the maximum Young’s modulus, when compared with other types. Figs 6.30e6.32 show tensile, bending and impact responses obtained by the four

(a)

(b)

Warp

(c)

Weft

(d)

Figure 6.29 Various knitted arrangements adopted: (a) varied plain knit, (b) 1 x 1 rib, (c) interlock and (d) Milano (Hu et al., 2010).

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(a) 700 600

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150 Rib 100

50 Varied plain knit 0 0.00

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0.06

0.08

0.10

Strain

Figure 6.30 Tensile properties of the four samples, showing their responses at both (a) warp and (b) weft directions (Hu et al., 2010).

samples, respectively, at both warp and weft directions. Similar results on mechanical performances of an interlock2 knitted composite samples have been reported (Demircan et al., 2015), with maximum tensile, 3-point bending, as well as 3-point bending impact responses, when tested at weft direction and compared with the tuck-s-miss, plain and tuck counterparts. Also, interlock knitted composite exhibited highest interlaminar fracture toughness when compared with rib, full Milano and uniweave types

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25

(b) 300 Interlock

Bending stress (MPa)

250 Milano 200 Varied plain knit 150

100

50

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0 0

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10

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Deflection (mm)

Figure 6.31 Bending or flexural properties of the four samples, showing their responses at both (a) warp and (b) weft directions (Hu et al., 2010).

(Kim et al., 2005). The knitted composite samples recorded nearly 10e20 times interlaminar fracture toughness greater than the uniweave counterpart (Fig. 6.33). The mechanical performances (impact, tensile and flexural strength and modulus) of knitted composite materials can be traced to their laminate fibre lay-up systems, especially in sandwich laminates, as recently investigated on by Skrifvars et al. (2019), using uniaxial warp-knitted/non-woven viscose fabrics/unsaturated polyester sandwich biocomposites.

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(a)

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2

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0 0

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8 10 12 Displacement (mm)

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8000

Interlock Varied plain knit Milano

7000 6000

Rib

5000 4000 3000 2000 1000 0

0

3

6

9

12

15

18

Displacement (mm)

Figure 6.32 Impact properties of the four samples, showing their (a) load-displacement and (b) specific energy absorbed-displacement plots, taken at the highest speed of 4.43 m/s (Hu et al., 2010).

6.4.5

Weaving

Weaving is a process of producing fabric by interlacing the fibre elements. Weaving could take various patters, as illustrated in Fig. 6.34. An innovative multi-axis 3D weaving technique for composites has been studied, where multi-axis 3D woven fabrics possessed several layers without delamination defect, as a result of the presence of both z-fibres and in-plane behaviours and Hand bias yarn layers (Bilisik, 2012). A typical illustrative 3D woven pattern is depicted in Fig. 6.35.

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8 Interlock Rib Full milano Uniweave

GIC[kJ/m2]

6

4

2

0 0

25

50 75 Crack Length,a [mm]

100

Figure 6.33 Interlaminar fracture toughness responses of the four types of knitted composites (Kim et al., 2005).

There are various weaving machines available today to produce different plain weave fabrics of various yarns and their densities in the warp and weft directions. A 3D weaving (Fig. 6.36) is generally carried out on sewing machines and computercontrolled looms to achieve consistent properties to an extent. Weaving produces the least damage when compared with stitching and pinning, though it depends on

(a)

(b)

(c)

(d)

(e)

(f)

Figure 6.34 Various fabric weaving, showing (a), (b), (e) plain, (d) unidirectional, (c) and (f) twill 2/2 (Karahan and Karahan, 2014).

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271

Z yarns

Warp yarns

Figure 6.35 A typical illustrative 3D woven pattern (Yao et al., 2012).

the processing conditions and fibre arrangement (Mouritz and Cox, 2010). However, one of the frequent setbacks of weaving is the crimping defect, as illustrated in Fig. 6.37. Furthermore, the ineplain fibres constrained to tows oriented at 0 and 90 in 3D woven biocomposites are commonly referred to as warp and weft directions, respectively, as illustrated in Fig. 6.38(a). Additionally, yarn can be stitched and combined with another knitted yarn to form a biaxial weft knitted fabric for biocomposite materials of highly improved properties, as shown in Fig. 6.38.

(a)

Woven z-binder

(b)

Woven z-binder

Figure 6.36 A typical 3D (a) orthogonal and (b) angle interlocked woven composites (Mouritz and Cox, 2010).

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Filler

Stuffers

Warp weaver

Figure 6.37 Crimping defect in interlock woven fabric (Mouritz and Cox, 2010).

(a)

(b)

(c)

Warp (wale) (0º) Weft (course) (90º) Stitch yarn

Weft yarn

Biaxial weft knitted fabric

Warp yarns

(d)

Stitch yarn

Weft yarn

Warp yarn

5 mm Biaxial weft knitted fabric

Figure 6.38 Representative illustration of the structure of a biaxial weft knitted fabric, showing its (a) warp and weft yarns, (b) stitch yarn with knitted fabric, (c) schema and (d) micrograph biaxial weft knitted fabrics (Demircan et al., 2015).

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More also, the influence of fibre weight contents of kenaf/Kevlar fibre on the mechanical behaviours of woven hybrid laminated biocomposites has been reported (Yahaya et al., 2014). The mechanical performances of the hybrid composites were differently influenced by the fibre weight contents, with the optimum ratio of 78/22, when compared with other hybrid counterparts and tested under impact loading. This indicated the combined benefits of weaving and hybridisation techniques on biocomposite materials. In moving forward, the influence of hemp/PLA yarn laminate, 8harness satin hemp/PLA and basket hemp/PLA woven fabric structures on mechanical (tensile, flexural and impact strengths) and water absorption performance of hemp unidirectional woven fabric-reinforced polylactic acid biocomposites has been investigated (Baghaei et al., 2015). The optimal mechanical behaviours were obtained from the satin fabric pattern, with expressively minimum porosities and fibre misalignment. Both weave patters exhibited higher storage modulus and water absorption than the yarn laminate biocomposite and neat PLA matrix, respectively.

6.4.6

Braiding

Braiding is a smart manufacturing technique for tubular structures, including hollow shafts, tube and struts (Fig. 6.39). It is similar to hybrid filament winding and weaving methods of manufacturing biocomposite components. Braided fabric or reinforced composite components are usually characterised with high quality, good dimensional stability, low cost, better fatigue life, efficient fibre orientation, delamination and impact resistance. Process parameters such as braid angle is an important factor that determines the quality of braided biocomposite materials. For instance, it was reported that an increase in braid angle of a natural hemp fibre/bio-matrix (ecopoxy) tubular-braided biocomposites caused an increase in its void content (Bruni-Bossio et al., 2019). Therefore, smaller braid angles of 40 (with a void fraction of 1.5%) and 50 were preferred to 60 . However, the total void content was decreased with the application of vacuum-bagging. The presence of voids reduces the mechanical properties (elastic moduli and ultimate strengths) of braided cellulose-reinforced composites (Qamhia et al., 2015), as well as tensile strength and ductility of finite element modelled 3D braided composites (Dong and Gong, 2018). These results have been similarly reported (Dong and Huo, 2018). Conversely, highest bending stiffness and impact resistance has been observed with the highest braid angle of 45 circular braided composites when compared with 15 and 30 samples (Zhou et al., 2015). In addition, it has been observed that the braided thermoplastic matrix composites exhibited a greater impact resistance when compared with the braided thermosetting matrix based composite (Lei et al., 2017). This established the need for the right choice of matrix in a braided composite material, in addition to a smaller braided angle.

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Figure 6.39 Braiding technology (simulation) in composite manufacturing. Sun, X., Kawashita, L.F., Wollmann, T., Spitzer, S., Langkamp, A., Gude, M., 2018. Experimental and numerical studies on the braiding of carbon fibres over structured end-fittings for the design and manufacture of high performance hybrid shafts. Production Engineering 12, 215e228.

6.4.7

Tufting

The process of tufting is schematically presented in Fig. 6.40. It is a one-side sewing method, whereby a thread binder is inserted through a ply stack, using a needle. This property enhanced technique can be totally automated for ease of change in parameters, using robots (Dell’Anno et al., 2016). It can probably be referred to as an advancement of frequently used stitching. Few studies on the application of tufting techniques are available. A reduced delamination and enhanced energy absorption have been achieved under impact loading, although they depended on the tufting pitch (Deconinck et al., 2014).

Needle Ply stack Tufting

Loop Figure 6.40 Schematic description of a tufting method (Deconinck et al., 2014).

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6.5

275

Summary and further evaluation

Sustainability is an indispensable concept that must be considered at every stage of manufacturing processes through any technique. Both production processes and biocomposite products are expected to be sustainable in order to keep meeting the insatiable human needs. There is a high possibility that biocomposite sustainability will definitely increase with advancement in material engineering, especially in composite technology. The inherent properties of natural fibres and matrices determine their mechanical performances and other properties. Therefore, the need for fibre treatment is very germane. Natural or bio-fibres treatments are possible chemically, physically, additively and biologically. Among these treatment methods, both chemical and physical techniques are widely employed. In addition, several approaches have been adopted to enhance the overall property of several biocomposites, including hybrid types. These techniques include, but are not limited to, hybridisation, pinning, stitching, weaving, knitting, braiding and tufting. The main benefit of these approaches include improvement of the mechanical behaviours (tensile, flexural, impact, interlaminar fracture toughness), among other properties of biocomposite materials. The application of these technologies are prone to change and improvement as new biocomposite materials are designed and developed. Therefore, the quest to improve on these manufacturing techniques remains a continuous task in the field of composite engineering.

6.6

Conclusion

The sustainable composites and techniques for their properties’ enhancement have been extensively elucidated in this chapter. Comparatively, it was evident that biocomposites support the concept of sustainability more than conventional composites, considering the five stages of product environmental life-cycle analysis, especially material, production and disposal/recovery stages. Due to the inherent properties of natural fibres and matrices, there are needs for fibre treatments towards a better reinforcing performance within a particular biocomposite system. Among all the methods of fibre treatments, chemical (acetylation) treatment has been generally accepted for the treatment of natural fibres, because it supports a continuous process and reduces the moisture uptake greatly, not at the detriment of other important properties of the fibres (especially stiffness and strength). Other aforementioned property enhancement techniques could support the functionality and durability of biocomposite parts. However, there is no specific technique that could improve all the properties of all types of biocomposites; instead, a combination of methods may serve better. Summarily, it is evident that enhancement of performance behaviours of biocomposite materials depends on the nature of the reinforcements (fibres/fillers) and matrices (binders), manufacturing techniques/approaches, fibre treatments, process parameters used and areas of application.

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Future outlooks and challenges of sustainable lightweight composites 7.1

7

Journey of composite materials towards sustainability

Reducing the weight of the components is one of the major drives of sustainable lightweight composites under which materials used are expected to provide both weight saving and required functional properties. This is an innovative new class of promising approach, as this provides efficient materials utilisation, the high strength-to-weight ratio in comparison to their metal counterparts leading to increased fuel consumption, benefits to the environment as this can reduce the carbon footprint significantly (Pervaiz et al., 2016; Bourmaud et al., 2018; Dhakal et al., 2013). For the last few decades, the use of advanced lightweight composite materials derived from renewable sources, especially in high-technology industry sectors: automotive, aerospace, marine and construction, has increased significantly in the quest for reducing reliance on heavy metal parts and non-renewable conventional composites moving towards more sustainable composite materials with less carbon footprint attributes. However, there are still some questions whether the lightweight composite materials and lightweighting approach can provide structural properties such as strength, modulus and long-term durability required, and still meet the environmental expectations (Holbery and Houston, 2006; Faruk et al., 2012; Mohanty et al., 2018). The environmental impact such as global warming potential (GWP), ecotoxicity, natural resource depletion and acidification are far less for sustainable lightweight composites compared to glass and carbon fibre-reinforced composites (Wu et al., 2018). If the inherent drawbacks of these composites can be addressed, their acceptance in the key industrial sectors is expected to grow significantly in the future. This can help the composite sector as a whole towards achieving sustainable development aspiration; an economically, environmentally and socially sustainable future. The following sections highlight and discusses the outlook and key challenges for lightweight materials and initiatives.

7.2

Market outlook and supply chain scenario

Public expectations for greener products, government legislation towards less dependence on fossil fuels, due to their attractive attributes such as high strength to weight

Sustainable Composites for Lightweight Applications. https://doi.org/10.1016/B978-0-12-818316-8.00003-7 Copyright © 2021 Elsevier Ltd. All rights reserved.

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ratio, high resistance to chemical and corrosion together with compliance with the ‘lightweighting’ materials, composites have tremendous scope of application. The emergence of these lightweight composites has motivated high technology sectors, including automotive, marine, aerospace, sports leisure, construction and wind energy moving towards taking benefits from these new class of lightweight, sustainable materials. The composite materials hold a significant market share in today’s economy (Qu et al., 2019). One of the key sectors using lightweight composites is the automotive sector. Major original equipment manufacturers (OEMs), including Toyota, Mercedes, BMW, Audi and Ford, amongst others, use carbon fibre for reinforcements to reduce the overall mass of the vehicles where CO2 emission is expected to be reduced in the overall life span of the products. Similarly, biobased thermoplastic composites have been utilised in this sector (Shah, 2013; Almansour et al., 2017; Ishikawa et al., 2018). According to the report published by IDTechEX (Collins, 2017), the forecast for all synthetic fibres used in composite parts will exceed $9 bn by 2027. It can also be envisaged emerging reinforcements such as flax, hemp and kenaf are gaining demand and popularity, due to their green credentials. However, due to the lack of an established sustainable supply chain network, it is accepted that there is not enough supply to meet emerging demands.

7.3 7.3.1

Challenges of achieving properties for lightweight applications Materials and manufacturing process

Despite their attractive attributes, resolving their weaknesses, such as poor recyclability after their end-of-life, especially for carbon and glass fibres, low mechanical performance, especially for natural fibre-reinforced lightweight composites and improving their long-term durability under various harsh environments remain as challenges to make these materials and lightweight approach full commercially viable. Moreover, some of the materials, such as natural fibres used in lightweight applications have been considered difficult to maintain their supply chain (Dhakal et al., 2007; Rohan et al., 2015). In order to achieve high quality, with required mechanical properties at a low cost is a big challenge in achieving lightweight aspirations. Processing automation using latest technology is important. However, the sustainable lightweight reinforcements such as natural fibres require a different set of processing parameters than the glass and carbon fibres. Reducing the cycle time without reducing the impregnation ability and mechanical properties are challenging tasks. It is important that instead of creating waste materials at the end of life, recycling and re-using materials instead of using virgin materials. When new materials are developed, it is important that material properties can be analysed using certain design criteria and materials parameters. The designers consider

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some of the drawbacks of natural fibre composites such as quality variations, susceptibility to moisture absorption, non-uniform diameter and microbial growth as weaknesses and disadvantages for modelling. Additionally, safety issues relating to matrices, processing techniques used and the costs are concerned challenges (Takahashi et al., 2012).

7.3.2

Recyclability and end-of- life option

When selecting appropriate matrices and reinforcements for lightweight composites, the recyclability aspect is of prime concern. Lightweighting materials such as glass and carbon fibres are attractive materials due to their many good attributes. However, the poor recyclability and recovery aspect poses a significant challenge. This is one of the biggest issues that OEMs in the transport sector face. This issue becomes even greater when using thermoset matrices. Several studies suggest that the demand for lightweight materials are expected to rise due to a rise in the demand for environmentally friendly, low emission vehicles. There is a strong correlation between vehicle weight and CO2 emission per km. Many companies in the transport sector have put their priorities to reduce CO2 emissions. There is a clear drive to reduce the vehicle weight, which can be achieved by using lighter materials, increase of the functionalities of the parts and improvement in the car system and powertrain design. Therefore, optimising performance with maintaining the environmentally sustainability is of a significant drive. More importantly, after the end-of-life consideration is crucial, as these materials cannot be landfilled as these materials leach toxic substances. Hence, end-of-life (EoL) management of lightweight composites, especially glass and carbon fibre-reinforced composites, is of a huge challenge. Additionally, an in-depth understanding of lightweight materials and their environmental impact arising from important stages of their life cycle such as raw materials extraction, processing, production, use and disposal is very important. It is vital that there is a credible life cycle analysis (LCA) technique used in order to determine the environmental impact of various products and processes.

7.3.3

Long-term durability

Understanding long-term durability in different harsh environments is still a challenge for natural fibre-reinforced lightweight composites and biobased composites (Dhakal et al., 2014). Overcoming some key obstacles such as inadequate impact toughness, reduced long-term stability in certain environmental conditions (temperature, humidity and UV radiation) is critical (Wang et al., 2006). Moreover, applications of these materials in automotive, marine, aerospace, wind turbine require a thorough understanding of how these materials behave under different loading conditions (Pickering et al., 2016). The relationship between microstructure and various mechanical properties by using suitable characterisation techniques is crucial. Achieving acceptable mechanical properties, low density, environmentally sustainable materials coupled with long-term structural durability, remain key challenges (Das et al., 2017; Dhakal et al., 2018).

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7.4

Sustainable Composites for Lightweight Applications

Future outlook

The prospective of use and continuous development of lightweight and ultralightweight composite materials present an exciting potential of meeting sustainable materials and low carbon economy (LCE). To fully realise the benefits of these technical challenges, such as understanding design, selection of materials and costeffective manufacturing process, acceptable strength and modulus, failure mechanisms together with establishing material structure-property is very important. For achieving these properties, some advanced materials, manufacturing and characterisation technologies are emerging, which include: • Graphene incorporated natural fibre composites, the inclusion of nanoparticles into composites for enhanced thermal and functional properties, automated fibre placement technology, additive manufacturing (AM), non-destructive characterisation techniques such as MicroComputed Tomography (m-CT) capable with either in-situ experiments or numerical simulations to assess the physical properties of materials, as well as the measurement of volumetric variability (fibre distribution and matrix volume contents). • New materials and various manufacturing processes have emerged in the last few years, which offer significant opportunities in reducing the overall weight of the components. Hybrid composites such as fibre/metal laminates (FMLs), hybridising natural fibres with carbon and glass fibres could displace steel and alloys in many applications. However, the cost factor and their interlayer compatibility can be a challenging issue (Dhakal et al., 2015; Sarasini et al., 2013). • Utilisation of cellulose nanofiber-reinforced composites and nanotechnology concepts in natural fibre composites and biobased lightweight composites can play a significant role in improving water repellence, coating and barrier properties.

Admittedly, the current development of the manufacturing process, natural fibrebased lightweight new sustainable materials and their technological innovation in key technology sectors (automotive, marine, aerospace and construction) will provide answers to the shortcomings of new lightweight materials and processes. In this context, a paradigm shift from fossil fuel-based economy to circular economy and involvement of multidisciplinary research initiatives must receive high priority with the involvement of key stakeholders, including legislative bodies, OEMs, academic institutions and the consumers.

References Almansour, F.A., Dhakal, H.N., Zhang, Z.Y., 2017. Effect of water absorption on Mode I interlaminar fracture toughness of flax/basalt reinforced vinyl ester hybrid composites. Compos. Struct. 168, 813e825. https://doi.org/10.1016/j.compstruct.2017.02.081. Bourmaud, A., Beaugrand, J., Shah, D., Placet, V., Baley, C., 2018. Towards the design of highperformance plant fibre composites. Prog. Mater. Sci. 97, 347e408. Collins, C., 2017. IDTechEX Report. Composite Materials for the Automotive Sector - Who Needs Who More? IDTechEx Research Article.

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Das, O., Kim, N.K., Sarmah, A.K., Bhattacharyya, D., 2017. Development of waste based biochar/wool hybrid biocomposites: flammability characteristics and mechanical properties. J. Clean. Prod. 144, 79e89. Dhakal, H.N., Zhang, Z.Y., Richardson, M.O.W., 2007. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos. Sci. Technol. 67, 1674e1683. Dhakal, H.N., Zhang, Z.Y., Guthrie, R., MacMullen, J., Bennett, N., 2013. Development of flax/ carbon fibre hybrid composites for enhanced properties. Carbohydr. Polym. 96, 1e8. https://doi.org/10.1016/j.carbpol.2013.03.074. Dhakal, H.N., Zhang, Z.Y., Bennett, N., Lopez-Arraiza, A., Vallejo, F.J., 2014. Effects of water immersion ageing on the mechanical properties of flas and jute fobre biocomposites evaluated by nanoindentation and flexural testing. J. Compos. Mater. 48, 1399e1406. Dhakal, H.N., Sarasini, F., Santulli, C., Tirillo, J., Zhang, Z., Arumugam, V., 2015. Effect of basalt fibre hybridisation on post-impact mechanical behaviour of hemp fibre reinforced composites. Compos. Part A Appl. Sci. Manuf. 75, 54e67. Dhakal, H., Bourmaud, A., Berzin, F., Almansour, F., Zhang, Z., Shah, D.U., Beaugrand, J., 2018. Mechanical properties of leaf sheath date palm fibre waste biomass reinforced polycaprolactone (PCL) biocomposites. Ind. Crop. Prod. 126, 394e402. Faruk, O., Bledzki, A., Fink, H.-P., Sain, M., 2012. Biocomposites reinforced with natural fibers: 2000-2010. Prog. Polym. Sci. 37, 1552e1587. Holbery, J., Houston, D., 2006. Natural-fiber-reinforced polymer composites in automotive applications. JOM J. Min. Met. Mater. Soc. 58, 80e86. Ishikawa, T., Amaoka, Masubuchi, Y., Yamamoto, T., Yamanaka, A., Arai, M., Takahashi, J., 2018. Overview of automotive structural composites technology developments in Japan. Compos. Sci. Technol. 155, 221e246. Mohanty, A.K., Vivekanandhan, S., Pin, J.-M., Misra, M., 2018. Composites from renewable and sustainable resources: challenges and innovations. Science 362, 536e542. https:// doi.org/10.1126/science.aat9072. Pervaiz, M., Panthapulakkal, S., KC, B., Sain, M., Tjong, J., 2016. Emerging trends in automotive light- weighting through novel composite materials. Mater. Sci. Appl. 7, 26e38. Pickering, K.L., Efendy, M.G.A., Le, T.M., 2016. A review of recent developments in natural fibre composites and their mechanical performance. Compos. Part A Appl. Sci. Manuf. 83, 98e112. https://doi.org/10.1016/j.compositesa.2015.08.038, 2016. Qu, Z., Pan, X., Hu, X., Guo, Y., Shen, Y., 2019. Evaluation of nano-mechanical behaviour on flax fiber metal laminates using atomic force microscope. Materials 12, 3363. Rohan, K., McDonough, T.J., Ugrestic, V., Potyla, E., Henning, F., 2015. Mechanical study of direct long fiber thermoplastic carbon/polyamid 6 and its relation to processing parameters. In: Proceedings 14th Annual SPE Automotive Composites Conference & Exhibition, 1 June 2015. Sarasini, F., Tirillo, J., Valente, M., Valente, T., Ciof, S., Iannace, S., Sorrentino, L., 2013. Effect of basalt fibre hybridization on the impact behavior under low impact velocity of glass/ basalt woven fabric/epoxy resin composites. Compos. Part A Appl. Sci. Manuf. 47, 109e123. Shah, D.U., 2013. Developing plant fibre composites for structural applications by optimising composite parameters: a critical review. J. Mater. Sci. 48, 6083e6107. https://doi.org/ 10.1007/s10853-013-7458-7. Takahashi, J., Uzawa, K., Matsuo, T., Yamane, M., 2012. Technological Challenges for Realizing Ultra Lightweight Mass Production Automobile by Using CFRTP. ITHEC 2012, Bremen, Germany.

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Wang, W., Sain, M., Cooper, P.A., 2006. Study of moisture absorption in natural fiber plastic composites. Compos. Sci. Technol. 66, 379e386. https://doi.org/10.1016/j.compscitech. 2005.07.027. Wu, Y., Xia, C., Cai, L., Garcia, A.C., Shi, S.Q., 2018. Development of natural fibre reinforced composite with comparable mechanical properties and reduced energy consumption and environmental impacts for replacing automotive glass-fibre sheet moulding compound. J. Clean. Prod. 184, 92e100.

Further reading Hughes, M., 2012. Defects in natural fibres: their origin, characteristics and implications for natural fibre-reinforced composites. J. Mater. Sci. 47, 599e609. https://doi.org/10.1007/ s10853-011-6025-3.

Index Note: ‘Page numbers followed by “t” indicate tables, “f ” indicates figures and “b” indicate boxes.’ A Acoustic emission (AE), 187e188, 188f Acousto-ultrasonic testing (AUT), 188e189, 189f Additive manufacturing (AM) technique, 151e152, 153f Agave sisalana, 25, 25f Alkaline treatment, 235e237, 236fe237f Animal fibres, 17 Autoclave curing, 151 Autoclave moulding, 149e150, 149f Automated fibre placement, 145e146, 145fe146f B Banana fibres, 27, 27f Bast fibres, 17 Biobased composites cone calorimeter performance, 83 cone calorimeter testing, 79, 80f environmental conditions, 74 fibre-reinforced polymer (FRP) composites, 74 flammability behaviour carbonaceous-silicate char, 84 cone calorimeter performance, 83 cone calorimeter testing, 79, 80f expanded graphite (EG), 83 extracellular polymeric substances (EPS), 84 flammability properties, 76e79 heat release rate (HRR), 80, 81f limiting oxygen index (LOI), 79 nanoclay, 83 natural fibre composites, 84, 85f peak of the total heat release rate (PHRR), 80 polylactic acid (PLA) yarn, 84, 86f smoke production rate curves, 80, 81fe82f

UL-94, 79 uncoated flax fabric, 84, 85f unsaturated polyester resin (UPE) resin, fire properties, 84 flammability properties, 76e79 heat release rate (HRR), 80, 81f lignocellulosic fibre degradation, 75 limiting oxygen index (LOI), 79 natural fibre-reinforced biobased composites boundary element approach (BEA), 100e101 failure modes, 99 finite difference approach (FDA), 100e102, 101f finite element analysis (FEA), 100 Rule of mixtures (ROM), 99 static mechanical properties prediction, 103e104, 103fe104f peak of the total heat release rate (PHRR), 80 smoke production rate curves, 80, 81fe82f thermal and fire properties, 75 thermal conductivity measurements aspect ratio, hemp/PCL biocomposites, 88, 88f bamboo fibre-reinforced polyester composites, 88, 89f crystalline and semi crystalline polymers, 87 fibre-reinforced composites, 88 natural fibre composites, 87 polymer matrix composites, 89e91, 90fe91f thermal degradation behaviour, natural fibres, 75 thermal stability and decomposition behaviour, 76, 77fe78f thermogravimetric analysis (TGA), 75

292

Biocomposites applications, 121, 179 bio-based constituent(s), 124 biomass, 124 cleaner production/green manufacturing, 158e159, 159f damage mechanisms and types composite manufacturing and in-service/ applications, 198, 199f delamination damage, 200, 201f fatigue life model, 214e216 fibre-matrix de-bonding, 198e199, 199f finite element method, 211e212 fibre reinforced polymer composite and metallic materials. See Fibre reinforced polymer composites (FRPCs) impact damage, 213e214, 214fe215f initiations and propagations, 198e199, 200f inter-laminar damage or delamination, 200, 200f intra-laminar damage, 200, 200f matrix micro-cracks, 198e199, 199f numerical analysis, 211e212 peridynamics and micropolar peridynamics, 212 thermal effects, 216e219, 217fe218f, 220f wavelet spectral finite element, 212 damage resistance, 207e208 design and manufacturing processes, 121 additive manufacturing (AM) technique, 151e152, 153f advantages, 131 autoclave moulding, 149e150, 149f automated fibre placement, 145e146, 145fe146f compression moulding, 129, 131e132, 138e141, 139f cost contribution, 152e155 cost reduction, 129e131 design for manufacture and assembly (DfMA), 129e131, 131f economical process, 152e155, 156f elastic moduli (EM), 157e158, 157f extrusion, 129, 142e143, 142fe143f, 152e155 filament winding, 146e149, 147f

Index

hand lay-up, 129, 132e135, 134f injection moulding processes, 131e132, 137e138, 137f natural fibre-reinforced polymer biocomposites, 152e155 open moulding and autoclave processes, 131e132 out-of-autoclave (OoA) moulding, 150e151 pre-impregnated resin, 141e142 resin transfer moulding (RTM), 143e145, 144f spray lay-up, 135, 135f traditional design approach, 129e131, 131f tri-corner approach, 129, 130f ultimate tensile strengths (UTS), 157e158, 157f vacuum bagging moulding, 136, 136f vacuum resin infusion, 141, 141f distribution, 128 eco-design, 121e123, 122fe123f functionality, 121 life cycle stages, 124, 126f loading methods, 179e180 manufacturing defects failure evolution, 166, 167f fibre kinking, 166, 166f fibre misalignment defect, 167, 167f fibre-reinforced composites, 159, 160f formations and process design parameters, 159, 160f inclusions/contamination, 162 microcracks and cracks, 160e161, 161f moisture absorption, 162 porosity (void or pores), 162e166, 163fe164f temperature effects, 162 materials bio-based matrices, 125, 127t biofibres, 125, 127t design, 126 petrochemical-based matrices, 124e125 toxic materials, 126 mechanical behaviours, 179 natural fibres, 123 natural limitations, 121 non-destructive testing (NDT)

Index

acoustic emission (AE), 187e188, 188f acousto-ultrasonic testing (AUT), 188e189, 189f computed tomography (CT) scanner, 189e191, 190fe191f contact vs. non-contact techniques, 209, 209f electromagnetic testing, 185e187, 186f inspection type vs. NDT methods, 209e210, 210t physical behaviours and structural integrity, 210e211, 211t radiographic testing (RT), 183e185, 185f scanning electron microscope (SEM), 195e196, 195fe197f shearography testing, 189, 190f thermography testing (TT) method, 182e183, 184f ultrasonic testing (UT), 182, 183f visual inspection (VI) or testing (VT), 181e182, 181f x-ray micro-computed tomography (mCT) scan, 191e193, 192fe194f production, 126e128, 128t recovery, 129, 130f reliable damage detection and monitoring techniques, 179e180 soil quality, 124 sustainability, 123e124, 125f Biomass, 124 Braiding, 273, 274f C Cannabis Sativa L., 19e20, 20f Carbon and glass fibre-reinforced composites, 2 Ceramic-matrix composites (CMCs), 1e2 Cleaner production/green manufacturing, 158e159, 159f Compression moulding, 131e132 advantages, 140 autoclave process, 138 bulk moulding compound (BMC), 139 fibre-matrix interfacial adhesion, 140 vs. hand lay-up process, 138e139 hot-press method, 138 illustration, 139, 139f processing parameters, 140e141 sheet moulding compound (SMC), 139

293

Computed tomography (CT) scanner, 189e191, 190fe191f Cone calorimeter testing, 79 D Date palm fibres, 22e24, 24f Design for manufacture and assembly (DfMA), 129e131 E Eddy current (EC) inspection, 186e187, 186f Elaeis guineensis, 26e27, 26f Epoxy resins, 6 Euro 6 regulation, 18 Extrusion compounding process, 142e143, 142fe143f F Fibre reinforced polymer composites (FRPCs) advantages and drawbacks, 2t failure/damage mechanisms crack pattern and temperature distribution, 204, 204f ductile-brittle, 202 fatigue crack propagation (FCP), 204, 205f fractographic features, 204 fracture resistance, 204 guided wave base method, 204 incipient thermal damage (ITD), 207 low-velocity impact modes, 202, 203f macro scale leveled damage, 201 matrix cracking and shifting mechanisms, 202, 203f nano- or micro-scale level, 201 progression, 197, 198f, 201, 202f RAPID method, 207 structural health monitoring techniques, 204, 207 tensile fractured notched sample, 204, 206f Twaron 1000 and 1014 aramid fibre reinforced epoxy composites, 202 wave filled imaging, 207 Fibre swelling, 92

294

Filament winding advantages, 148 applications, 148 basalt fibre-reinforced epoxy composite pipes, 146e147 commingled filament-winding method, 146e147 conventional 2-axis lathe type filament winding, 148e149 description, 146 fibre-reinforced polymer manufacturing, 146e147, 147f film stacking method, 146e147 hemp fibre yarn, 146e147 military and defence sector, 148e149 robotic filament winding technique, 148e149 schematic illustration, 146, 147f Flax fibres, 20e21, 21f scanning electron microscope (SEM) images, 45, 46f tensile stress-strain curves, 44, 45f 4D printing, 152 Fused deposition modelling (FDM), 152 G Genus Corchorus oliotorius, 21, 22f Glass fibre-reinforced composites, 10, 11t Glass fibres, 9 Grass fibre, 17 Guided wave base method, 204 H Hand lay-up advantages, 133e134 applications, 133e134 with common matrices, 133e134 curing, 133 description, 132e133 gel coating, 133 illustration diagram, 133, 134f lay-up, 133e134 low volume production, 134e135 mould preparation, 133 nip-roller type impregnators, 132e133 production rate, 134e135 set-up, 133, 134f Hemp fibres, 19e20, 20f Hibiscus cannabinus L., 21e22, 23f

Index

I Incipient thermal damage (ITD), 207 Injection moulding processes, 131e132, 137e138, 137f J Jute fibres, 21, 22f scanning electron microscope (SEM) images, 45, 46f tensile stress-strain curves, 44, 45f K Kenaf fibres, 21e22, 23f Knitting configurations, 266e268, 266f interlaminar fracture toughness responses, 266e268, 270f interlock structure, 266e268 tensile, bending and impact responses, 266e268, 267fe269f 3D stitched woven-knitted basalt fabric biocomposites, 266e268 warp knitting, 265e266 L Laminated object manufacturing (LOM), 152 Leaf fibres, 17 Lightweight composites advantages, 55e57, 56f, 57t aerospace application, 109e110 automotive application chassis, body and engine components, 104e105 E-Class Mercedes-Benz, 107, 107f Formula One (F1) standard components, 108 fuel consumption, 105 hemp fibre door frame, 105, 105f motorsports, 108 natural fibre-reinforced composites, 105e106, 108e109, 108f North American automotive OEMs, 106e107, 106f plant fibre-reinforced composites, 106 vibration damping, 109, 109f weight reduction, 106e107 biobased composites. See Biobased composites

Index

building construction application, 111e112 carbon fibre-reinforced polymer composites (CFRP), 53 cost benefits, 56, 57t creep behaviour creep compliance, 74 definition, 71 hemp fibre-reinforced unsaturated polyester composites, 73 instantaneous deformation, 72 lifetime prediction, 72 natural fibre composites, 71e72 stage I (primary creep), 72 stage II (secondary creep), 72 stage III (tertiary creep), 72 strain vs. time, 72e74, 72fe73f stress and temperature, 74 drivers, 54 environmental properties, 57e58 environmental sustainability, 57 epoxy-based carbon fibre-reinforced composite, 53 fatigue properties carbon fibre-reinforced epoxy composites, 71 constant stress amplitude tests, 70, 70f failure prediction and evaluation, 68 flax fibre-reinforced composites, 71 hemp fibre-reinforced epoxy composite, 70 heterogeneity, 68e70 natural fibre composites, 70e71 property deterioration, 68 stress-life (SeN) diagram, 71 flexural properties, 62e63 greenhouse gases (GHGs) reduction, 54 impact properties composite structure damage, 63e64, 64f energy absorption values, 66 flax fibre-reinforced epoxy composite, 66e67 flax/unsaturated polyester composites, 66 flax/vinyl ester composites, 68 four-layered flax fibre specimen, 66e67 glass fibre-reinforced polymer (GFRP) specimens, 67, 67f hemp fibre-reinforced samples, 65e66 high-velocity impacts, 63 hypervelocity impacts, 63

295

impact velocity levels, 66 jute fibre-reinforced methacrylated soybean oil (MSO) biocomposites, 66e67 jute fibre-reinforced unsaturated polyester, 67e68, 69f low-velocity impacts, 63e65 neat polyester matrix, 65, 65f parameters, 68 plain woven flax fibre-reinforced epoxy composites, 66e67 six-layered specimen, 66e67 toughness, 63 two-layered flax fibre specimen, 66e67 unreinforced polyester, 65e66 long-term durability, 287 marine application, 110e111 market outlook, 285e286 materials and manufacturing process, 286e287 recyclability and end-of-life option, 287 requirements, 55 sporting goods, 112 supply chain scenario, 285e286 surface modifications and chemical treatments, 58 sustainability, 285 tensile properties adhesion, 58e59 glass fibre-reinforced polymer (GFRP) composites, 60 lower tensile properties, 58e59 natural fibre-reinforced biobased composites, 60, 61f, 62, 64 plant fibre composites, 59e60, 59t reinforced composites, 60 stiffness and strength, 59e60 threshold fibre volume fraction, 59 thermal properties, 57e58 thermoplastic-based composite materials, 55 water absorption diffusion coefficient, 92e95 fibre matrix interface, 93e94, 93f fibre swelling, 92 flax/PP and flax/ carbon/PP hybrid composites, 97, 97f flax/vinyl ester composites, 97e99 hemp fibre reinforcement, 94e95

296

Lightweight composites (Continued) heterogeneous structures, 91e92 hygrothermal environments, 91 linear Fickian behaviour, 94 low-velocity impact damage characteristics, 97, 98f moisture absorption tests, 92e93 moisture uptake curve, 92e93 natural fibre composites, 93 non-Fickian behaviour, 94 pseudo-Fickain behaviour, 94 pultruded jute fibre-reinforced unsaturated polyester composites, 95, 96f single flax fibres, 98f, 99 tensile modulus, 95, 95f Lignocellulosic plant-based natural fibres, 17 Limiting oxygen index (LOI), 79 Linear Fickian behaviour, 94 Linum usitatissimum L., 20e21, 21f M Matrices advantages and disadvantages, 5, 5f classification, 5, 5f epoxy resins, 6 functions, 6 phenolic resins, 7 physical and mechanical properties, 6, 7t polyester resins, 6e7 polyethylene (PE), 7e8 polylactic acid (PLA), 8e9 polymeric matrices, 5 polypropylene (PP), 8 polystyrene (PS), 8 thermoplastics, 5, 6t thermosets polymers, 5, 6t vinyl ester resins, 7 Metal-matrix composites (MMCs), 1e2 N Nano-materials, 12 Natural fibre-reinforced composites (NFRCs), 2e3 environmental impact, 2e3 vs. glass and carbon fibres, 2e3, 3t life cycle stages, 2e3, 3f mechanical properties, 4 tensile properties, 4, 4f

Index

Natural fibre-reinforced polymer composites (NFRPCs), 57 Natural fibres vs. glass and carbon fibres, 2e3, 3t NDT. See Non-destructive testing (NDT) NFRCs. See Natural fibre-reinforced composites (NFRCs) Non-destructive testing (NDT) acoustic emission (AE), 187e188, 188f acousto-ultrasonic testing (AUT), 188e189, 189f computed tomography (CT) scanner, 189e191, 190fe191f contact vs. non-contact techniques, 209, 209f electromagnetic testing, 185e187, 186f inspection type vs. NDT methods, 209e210, 210t physical behaviours and structural integrity, 210e211, 211t radiographic testing (RT), 183e185, 185f scanning electron microscope (SEM), 195e196, 195fe197f shearography testing, 189, 190f thermography testing (TT) method, 182e183, 184f ultrasonic testing (UT), 182, 183f visual inspection (VI) or testing (VT), 181e182, 181f x-ray micro-computed tomography (mCT) scan, 191e193, 192fe194f Non-Fickian behaviour, 94 O Oil palm empty fruit bunch (OPEFB), 26 Oil palm fibres, 26e27, 26f Out-of-autoclave (OoA) curing, 151 moulding, 150e151 P Petrochemical-based matrices, 124e125 Petroleum-based materials, 2e3 Phenolic resins, 7 Phoenix dactylifera L., 22e24, 24f Plant-based natural fibres reinforcements banana fibres, 27, 27f cellulose structure hemicellulose, 32

Index

lignin, 33 moisture absorption, 32 molecular structure, 32, 32f chemical compositions, 30e31, 30t, 31f classifications, 17, 18f complex fibre layers and defects, 28, 28f composite reinforcement, 17 date palm fibres, 22e24, 24f European Union’s directive, 18 fibre bundles, 44 fibre processing techniques, 27e28 dew retting, 29 fibre isolation methods, 28 hackling process, 28 injection moulding, 29 internal mixing, 29 micro-compression, 29 retting process, 28e29 steam-explosion, 29 ultrasonic treatments, 29 flax fibres, 20e21, 21f scanning electron microscope (SEM) images, 45, 46f tensile stress-strain curves, 44, 45f hemp fibres, 19e20, 20f jute fibres, 21, 22f scanning electron microscope (SEM) images, 45, 46f tensile stress-strain curves, 44, 45f kenaf fibres, 21e22, 23f lignocellulosic plant-based natural fibres, 17 mechanical processing, 27 middle lamella, 44 moisture absorption behaviour, 18e19 morphological structure flax bast fibre, 34, 36f high aspect ratio, 33 length and width, 33, 34t lumen, 33, 38 and mechanical properties, 33, 34t, 38e39, 39f natural fibre, 33e34, 35f and physical properties, 33, 34t primary and secondary cell walls, 36 reliability and long-term durability, 33e34 sisal leaf fibre and jute bast fibre, 36, 37f strength and stiffness, 33e34 temperature and humidity, 33e34

297

non-cellulose contents, 39e40 non-structural applications, 18e19 oil palm fibres, 26e27, 26f physical and mechanical investigation, 42, 43t composite reinforcements, 41e42 fibre-matrix adhesion, 41e42 interlaminar shear strength (ISS), 41e42 micro-bond test (MT), 42 single fibre fragmentation test (SFFT), 42 single fibre pull-out test (SFPT), 42 PP/lyocell composites, mechanical properties, 40, 40f sisal fibres, 25, 25f strength and modulus, 44 strength and stiffness, 43 tensile properties, 40, 41f tensile strength and modulus, 44 Polyester resins, 6e7 Polyethylene (PE), 7e8 Polylactic acid (PLA), 8e9 Polymeric matrices, 5 Polymer matrix composites (PMCs), 2 Polypropylene (PP), 8 Polystyrene (PS), 8 Porosity (void or pores) defects, 165 description, 162e163 extrusion process, 165e166 fibre-reinforced biocomposites surface, 162e163 hemp-reinforced polylactic acid (PLA) biocomposite samples, 165 inter-laminar and fibre tow voids, 163 natural fibre-reinforced polymer biocomposites, 165 OoA prepreg, 165e166 small and flat MTM45-1/CF2426A prepreg laminate, 163, 164f types, 163, 163fe164f void sinks mechanism, 164 Pre-impregnated resin, 141e142 Pseudo-Fickain behaviour, 94 R Reinforcements carbon fibres, 10 ceramic fibres, 10 glass fibres, 9 natural fibres, 12

298

Resin transfer moulding (RTM) advantages, 144 automobile industry, 145 description, 143e144 hemp fibre-reinforced polyester biocomposites, 145 process parameters, 144 schematic diagram, 143e144, 144f sisal fibre-reinforced biocomposites, 145 sustainable natural fibre-reinforced polymeric biocomposites, 145 S Scanning electron microscope (SEM), 195e196, 195fe197f Seed fibres, 17 Selective laser sintering (SLS), 152 Shearography testing, 189, 190f Silk fibres, 17 Single fibre fragmentation test (SFFT), 42 Single fibre pull-out test (SFPT), 42 Sisal fibres, 25, 25f Spray lay-up, 135, 135f Stalk fibre, 17 Stereolithography (SLT), 152 Stitching advantages, 253 areal stitch fractions, 255, 256f basalt fibre hybridisation, 257 composite laminates, 257 damaging effects, 255 description, 253 FBVEu dry biocomposites, 257 fibre distortions, 255, 255f flax yarn stitch, 255 microscopic examination techniques, 257 mode I delamination toughness, 253 mode I interlaminar fracture toughness, 257 mode II interlaminar fracture toughness, 257 natural fibre, 255, 257 patterns, 253, 254f quasi-isotropic composite laminate, 253, 254f two-dimensional (2-D) fibre reinforcements, 253 water absorption responses, 257, 258f Sustainable composites, 13 additive treatments, 250e251

Index

biological treatments Acetobacter xylinum, 251, 251f field emission scanning electron microscopy (FESEM), 251 Fourier transform infrared (FTIR) spectroscopy, 251 jute fibres filled natural rubber (NR) composites, 251 x-ray diffraction (XRD), 251, 252f braiding, 273, 274f cellulosic fibre and bio-polymer materials, 234 chemical treatments acetylation, 239 alkaline treatment, 235e237, 236fe237f banana fibres, 240e241, 241f bio-based lysine diisocyanate, 239 biocomposite, tensile strength, 240, 241f coupling agents, 242e243 coupling mechanisms, 235 enzyme treatment, 244, 245f esterification, 239, 239f flax fibre-polypropylene compatibility, 241e242 HDPE/Nylon-6 based banana fibre-reinforced biocomposites, 243 Hildegardia FRPP biocomposites, 243 kenaf fibre-reinforced polystyrene biocomposites, 240 maleated coupling, 242 maleic anhydride (MA), 241e242 maleic anhydride grafted polyethylene (PE-g-MA), 243 maleic anhydride grafted styrene/ ethyleneebutylene/styrene triblock polymer (SEBS-g-MA), 243 maleic anhydride-polypropylene copolymer (MA-PP), 241e244 NaOH treatment, 236, 237f, 239 natural fibre surface impurities, 236e237 polymethylene-polyphenyl-isocyanate, 239 silane A151 treated fibres, 240e241 silane treatment, 240e242, 240f soil burial (biological) tests, 241e242 tensile, flexural and impact strength properties, 236e237, 238f vinyl trimethoxy silane (VTMO), 241e242

Index

damage tolerance and properties, 253 environmental life cycle analysis, 230e231 environmental pollution, 230 fibre treatments, 234e235 flax fibres bio-resins, 234 crystalline structure, 231 drilling-induced damage responses, 232e234 macro to nanoscale level, 231e232, 233f mesoscopic level, 231 primary cell wall, 231 schematic microstructure, 231, 232f secondary wall (S3), 231e232 tensile strength, 232 weak fibre-matrix interfacial bonding, 234 government legislation, 230 hybridisation advantages, 258e259 flax/basalt biocomposite, 260e261, 261f flax fibre-reinforced biocomposite laminate, 260e261, 262f hybrid hemp/basalt (HB) reinforced biocomposite, 259, 260f jute fibre-reinforced epoxy biocomposite, 262e263 kenaf/pineapple leaf fibre-reinforced high-density polyethylene biocomposites, 263 natural fibre-reinforced polymeric biocomposites, 259 salt-fog environment conditions, 260e261, 262f stacking sequences, 258e259, 258f inter-dependent benefits, 229 key elements, 229, 230f knitting configurations, 266e268, 266f interlaminar fracture toughness responses, 266e268, 270f interlock structure, 266e268 tensile, bending and impact responses, 266e268, 267fe269f 3D stitched woven-knitted basalt fabric biocomposites, 266e268 warp knitting, 265e266 natural sources, 229 physical treatments

299

cold helium plasma-treated flax fibre-reinforced unsaturated polyester biocomposites, 246e249 corona treatment, 249 electron radiation, 250 fibre beating, 250 Fourier transform infrared-spectroscopy, 246e249 functional surface and structural properties, 246 heat treatment, 250 hydrothermal treatments, 250 plasma treatment, 246, 247f, 249 ultraviolet treatments, 249 pinning, 263e265, 263fe265f productive farming/agricultural practices, 229 stitching advantages, 253 areal stitch fractions, 255, 256f basalt fibre hybridisation, 257 composite laminates, 257 damaging effects, 255 description, 253 FBVEu dry biocomposites, 257 fibre distortions, 255, 255f flax yarn stitch, 255 microscopic examination techniques, 257 mode I delamination toughness, 253 mode I interlaminar fracture toughness, 257 mode II interlaminar fracture toughness, 257 natural fibre, 255, 257 patterns, 253, 254f quasi-isotropic composite laminate, 253, 254f two-dimensional (2-D) fibre reinforcements, 253 water absorption responses, 257, 258f tufting, 274, 274f weaving advantages, 273 biaxial weft knitted fabric, 271, 272f crimping defect, 270e271, 272f definition, 269 patterns, 269, 270f 3D woven pattern, 269, 271f

300

T Thermography testing (TT) method, 182e183, 184f Thermoplastics, 5, 6t Thermosets polymers, 5, 6t 3D printing, 151 Tufting, 274, 274f U Ultrasonic testing (UT), 182, 183f

Index

V Vacuum bagging moulding, 136, 136f Vacuum resin infusion, 141, 141f Vinyl ester resins, 7 Visual inspection (VI), 181e182, 181f X X-ray micro-computed tomography (mCT) scan, 191e193, 192fe194f