Structural health monitoring of biocomposites, fibre-reinforced composites and hybrid composites 9780081022917, 0081022913

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Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites

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Woodhead Publishing Series in Composites Science and Engineering

Structural Health Monitoring of Biocomposites, FibreReinforced Composites and Hybrid Composites Edited by

Mohammad Jawaid Mohamed Thariq Naheed Saba

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 © 2019 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: 978-0-08-102291-7 For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Gwen Jones Editorial Project Manager: Thomas Van Der Ploeg Production Project Manager: Poulouse Joseph Designer: Mark Rogers Typeset by TNQ Technologies

Dedicated to Parents of Dr. Mohamed Thariq Sufaidah Binti Mohd Musa-Mother Haji Hameed Sultan Bin Mohamed Sulaiman-Father

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Contents

List of contributors About the editors Preface 1

2

3

The effect of different fiber loading on flexural and thermal properties of banana/pineapple leaf (PALF)/glass hybrid composite Mohd Hanafee Zin, Khalina Abdan and Mohd Nurazzi Norizan 1.1 Introduction 1.2 Material and method 1.3 Morphology analysis 1.4 Thermal stability analysis 1.5 Results and discussion 1.6 Conclusion Acknowledgments References Biomass valorization for better aviation environmental impact through biocomposites and aviation biofuel Jia Tian Chen, Luqman Chuah Abdullah and Paridah Md. Tahir 2.1 Introduction 2.2 Summary References Further reading Structural health monitoring of aerospace composites Verma Rahul, Shukla Alokita, Kandasamy Jayakrishna, V.R. Kar, M. Rajesh, S. Thirumalini and M. Manikandan 3.1 Introduction 3.2 Failures and damages in composites 3.3 Micro-level failure mechanisms 3.4 Techniques used for aerospace composites 3.5 Conclusion References

xi xv xvii

1 1 3 4 5 6 16 16 16 19 19 29 30 31 33 33 34 35 44 48 49

viii

4

5

6

7

Contents

Recent advances and trends in structural health monitoring Shukla Alokita, Verma Rahul, Kandasamy Jayakrishna, V.R. Kar, M. Rajesh, S. Thirumalini and M. Manikandan 4.1 Introduction 4.2 State of the practice in bridge monitoring systems 4.3 Factors affecting measurement data 4.4 Benefits of structural health monitoring 4.5 Challenges for structural health monitoring 4.6 Advantages of structural health monitoring 4.7 Advance technology used for structural health monitoring 4.8 Conclusion References

53

Structural health monitoring of fiber polymer composites Kandasamy Jayakrishna, G. Rajiyalakshmi and A. Deepa 5.1 Introduction 5.2 Macroscopic behavior of fiber reinforced polymer 5.3 Microscopic results 5.4 Apparent moisture diffusivity 5.5 Conclusion References

75

Corrosion detection for natural/synthetic/textiles fiber polymer composites Nor Ishida Zainal Abidin, Mohd Faizul Mohd Sabri, Katayoon Kalantari, Amalina M. Afifi and Roslina Ahmad 6.1 Introduction 6.2 Destructive physical analysis methods 6.3 Nondestructive evaluation methods 6.4 Semi-analytical finite element method 6.5 Summary References Haptic-based virtual reality system to enhance actual aerospace composite panel drilling training Sivadas Chandra Sekaran, Hwa Jen Yap, Kan Ern Liew, Hafeez Kamaruzzaman, Chee Hau Tan and Razman Shah Rajab 7.1 Introduction 7.2 Results 7.3 Discussion 7.4 Conclusion Acknowledgments References

53 55 55 57 59 60 62 66 67

75 77 85 85 89 89 93 93 98 101 103 107 108 113 113 122 125 127 128 128

Contents

8

9

10

11

Maintenance and monitoring of composites Chhugani Tushar, Routray Ralish, M. Rajesh, M. Manikandan, R. Rajapandi, V.R. Kar and Kandasamy Jayakrishna 8.1 Introduction 8.2 Benefits of implementation of structural health monitoring 8.3 Challenges for structural health monitoring 8.4 Testing using nondestructive analysis 8.5 Comparison between NDT and SHM 8.6 Structural health monitoring 8.7 Emerging SHM technologies 8.8 Industrial applications of SHM 8.9 Conclusion References Synthetic/natural fiber properties of fire-designated zone of an aircraft engine: a structural health monitoring approach A.R. Abu Talib and I. Mohammed 9.1 Introduction 9.2 Methodology 9.3 Results and discussion 9.4 Conclusion Acknowledgments References Aerogel-based thermally sprayed coatings for aero-propulsion systems: a feasibility study based on structural health monitoring approach A.R. Abu Talib and M.I. Nadiir Bheekhun 10.1 Introduction 10.2 Methodology 10.3 Results and discussion 10.4 Conclusions Acknowledgments References Structural health monitoring of biocomposites, fibre-reinforced composites, and hybrid composite A. Hamdan, M.T.H. Sultan and F. Mustapha 11.1 Structural health monitoring application 11.2 Application of biocomposites, fiber-reinforced composites, and hybrid composite 11.3 Issues of SHM 11.4 Conclusion Acknowledgments References

ix

129 129 131 132 133 137 138 140 146 147 147 153 153 159 169 185 186 186

191 191 197 204 220 222 222 227 227 231 233 236 236 237

x

12

13

14

Contents

Fracture surface morphologies in understanding of composite structural behavior A. Deepa, Kandasamy Jayakrishna and G. Rajiyalakshmi 12.1 Introduction 12.2 Microhardness 12.3 Area of fracture and circularity ratio 12.4 Visual observation and scaling 12.5 Conclusion References An overview on the role of structural health monitoring in various sectors A. Sofi 13.1 Applications of composite materials 13.2 Literature review 13.3 Material properties 13.4 Conclusions References Fracture surface morphologies in understanding of composite structural behavior Hamid Essabir, Rachid Bouhfid and Abou el kacem Qaiss 14.1 Introduction 14.2 Processing of polymer composites based on short fiber 14.3 Problems related to the composites materials 14.4 Morphological properties in the composite materials 14.5 Composite properties (experimental results) 14.6 Conclusion Acknowledgments References

Index

243 243 246 248 252 255 255 257 257 262 268 273 274 277 277 279 281 282 284 290 291 291 295

List of contributors

Khalina Abdan Department of Biological and Agricultural Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Luqman Chuah Abdullah Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Malaysia A.R. Abu Talib Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia Amalina M. Afifi Roslina Ahmad

Department of Mechanical Engineering, University of Malaya Department of Mechanical Engineering, University of Malaya

Shukla Alokita School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India Rachid Bouhfid Moroccan Foundation for Advanced Science, Innovation and Research, Laboratory of Polymer Processing, Rabat, Morocco Sivadas Chandra Sekaran Researcher, Aerospace Malaysia Innovation Centre (AMIC), Kajang, Selangor, Malaysia Jia Tian Chen Centre of Excellence on Biomass Valorization for Aviation, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Malaysia; Aerospace Malaysia Innovation Centre (AMIC), MIGHT Partnership Hub, Jalan Impact, Cyberjaya, Malaysia A. Deepa School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India Hamid Essabir Moroccan Foundation for Advanced Science, Innovation and Research, Laboratory of Polymer Processing, Rabat, Morocco A. Hamdan Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; Aerospace Manufacturing Research Centre (AMRC), Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia Kandasamy Jayakrishna School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India Katayoon Kalantari

Department of Mechanical Engineering, University of Malaya

xii

List of contributors

Hafeez Kamaruzzaman Principal Consultant, STRAND Business Consulting Center, Petaling Jaya, Selangor, Malaysia V.R. Kar India

Department of Mechanical Engineering, NIT Jamshedpur, Jharkhand,

Kan Ern Liew Deputy Chief Technology Officer, Aerospace Malaysia Innovation Centre (AMIC), Kajang, Selangor, Malaysia M. Manikandan Department of Mechanical Engineering, Amrita College of Engineering and Technology, Nagercoil, Tamil Nadu, India I. Mohammed Department of Mechanical Engineering, Hassan Usman Katsina Polytechnic, Katsina, Nigeria Mohd Faizul Mohd Sabri Malaya

Department of Mechanical Engineering, University of

F. Mustapha Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia M.I. Nadiir Bheekhun Aerospace & Communication Technology Research Group, Faculty of Information Sciences and Engineering, Management and Science University Shah Alam, Malaysia Mohd Nurazzi Norizan Department of Biological and Agricultural Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Abou el kacem Qaiss Moroccan Foundation for Advanced Science, Innovation and Research, Laboratory of Polymer Processing, Rabat, Morocco Verma Rahul School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India Razman Shah Rajab Chief Executive Officer, Aerospace Malaysia Innovation Centre (AMIC), Kajang, Selangor, Malaysia R. Rajapandi National Institute of Technology Karnataka, Mangalore, India M. Rajesh School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India G. Rajiyalakshmi School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India Routray Ralish School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India A. Sofi

Associate Professor, VIT Vellore, Tamil Nadu, India

List of contributors

xiii

M.T.H. Sultan Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; Aerospace Manufacturing Research Centre (AMRC), Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), University Putra Malaysia, Serdang, Malaysia Paridah Md. Tahir Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Malaysia Chee Hau Tan Malaysia

Research Assistant, University of Malaya, Kuala Lumpur, Selangor,

S. Thirumalini School of Civil and Chemical Engineering, Vellore Institute of Technology, Vellore, India Chhugani Tushar School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India Hwa Jen Yap

Associate Professor, University of Malaya, Kuala Lumpur, Malaysia

Nor Ishida Zainal Abidin Malaya

Department of Mechanical Engineering, University of

Mohd Hanafee Zin Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Serdang, Selangor, Malaysia

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About the editors

Dr. Mohammad Jawaid is currently working as a fellow researcher (associate professor) at the Biocomposite Technology Laboratory, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia (UPM), Serdang, Selangor, Malaysia, and has also been a visiting professor at the Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia, since June 2013. He is also a visiting scientist at the TEMAG Laboratory, Faculty of Textile Technologies and Design at Istanbul Technical University, Turkey. He has more than 14 years of experience in teaching, research, and industries. His area of research interests includes hybrid reinforced/filled polymer composites, advance materials, (graphene/nanoclay/fire-retardant, lignocellulosic reinforced/filled polymer composites), modification and treatment of lignocellulosic fibers and solid wood, biopolymers and biopolymers for packaging applications, nanocomposites and nanocellulose fibers, and polymer blends. So far, he has published 20 books, 45 book chapters, more than 250 peer-reviewed international journal papers, and 5 review papers under the top 25 hot articles in Science Direct during 2013e18. Dr. Jawaid worked as a guest editor for special issues for Current Organic Synthesis and Current Analytical Chemistry, Bentham Publishers, UK; International Journal of Polymer Science, Hindawi Publishing; Inderscience Enterprises Ltd.; and IOP Conference Proceedings. He is an editorial board member of the Journal of Asian Science Technology and Innovation and Recent Innovations in Chemical Engineering Journal. In addition, he is also a reviewer of several high-impact international peer-reviewed journals for Elsevier, Springer, Wiley, Saga, etc. Presently, he is supervising 18 PhD students (6 PhD as main supervisor and 12 as a member of the supervisory committee) and 8 Master’s students (3 Master’s as main supervisor and 5 as a member of the supervisory committee) in the field of hybrid composites, green composites, nanocomposites, natural fiber-reinforced composites, nanocellulose, etc. Additionally, 13 PhD (3 PhD as main supervisor and 10 as a member of the supervisory committee) and 6 Master’s students (2 Master’s as main supervisor and 4 as a member of the supervisory committee) have graduated under his supervision from 2014 to 2018. He has several research grants at university, national, and international levels on polymer composites of around RM 3 million (USD 700,000). He has also delivered plenary and invited talks at international conferences related to composites in India, Turkey, Malaysia, Thailand, UK, France, Saudi Arabia, and China. Also, he is a member of technical committees of several national and international conferences on composites and material science. His H-index is 40 (Google Scholar) and 35 (Scopus).

xvi

About the editors

Assoc. Prof. Ir. Ts. Dr. Mohamed Thariq Bin Haji Hameed Sultan is a professional engineer (PEng) registered under the Board of Engineers Malaysia (BEM), a professional technologist (PTech) registered under the Malaysian Board of Technologists, a charted engineer (CEng) registered with the Institution of Mechanical Engineers, UK, and is currently attached to the Universiti Putra Malaysia as the Head of the Biocomposite Technology Laboratory, Institute of Tropical Forestry and Forest Products (INTROP), UPM Serdang, Selangor, Malaysia. Being Head of the Biocomposite Technology Laboratory, he is also appointed as an independent scientific advisor to the Aerospace Malaysia Innovation Centre (AMIC) based in Cyberjaya, Selangor, Malaysia. He received his PhD. from the University of Sheffield, UK. He has about 10 years of experience in teaching as well as in research. His area of research interests includes hybrid composites, advance materials, structural health monitoring, and impact studies. So far he has published more than 100 international journal papers and received many awards locally and internationally. In December 2017, he was awarded a Leaders in Innovation Fellowship (LIF) by the Royal Academy of Engineering (RAEng), UK. He is also the Honourable Secretary of the Malaysian Society of Structural Health Monitoring (MSSHM) based in UPM Serdang, Selangor, Malaysia. Currently, he is also attached to the Institution of Engineers Malaysia (IEM) as Deputy Chairman in the Engineering Education Technical Division (E2TD). Dr. Naheed Saba completed her PhD. in Biocomposites Technology from the Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Selangor, Malaysia, in 2017. She completed her Master’s in Chemistry and also her postgraduate diploma in Environment and Sustainable Development from India. She has published over 40 scientific and engineering articles in advanced composites. She edited one book from Elsevier and also published more than 15 book chapters for Springer, Elsevier, and Wiley publications. She has also attended a few international conferences and presented research papers. Her research interest areas are nanocellulosic materials, fire-retardant materials, natural fiber-reinforced polymer composites, biocomposites, hybrid composites, and nanocomposites. She is a recipient of the International Graduate Research Fellowship, UPM. She is a reviewer of several international journals such as Cellulose, Constructions and Building Materials, Journal of Materials Research and Technology, BioResources, Carbohydrate Polymers, etc. Her H Index is 14.

Preface

The book Structural Health Monitoring of Biocomposites, Fiber-Reinforced Composites and Hybrid Composites is part of a subseries, Testing, Modeling, and Analysis Under Composite Science and Technology. Structural Health Monitoring of Composite Structures offers a comprehensive review of established and promising technologies under development in the emerging area of structural health monitoring of aerospace, construction, and automotive composite structures. This book remarkably fills the gap in the published literature involving structural health monitoring of biocomposites, fiber-reinforced composites, and hybrid composites and provides a reference material for future research in natural and hybrid composite materials, which are currently in great demand due to their sustainable, recyclable, and eco-friendly nature as required in different applications. This book will describe the structural health monitoring methods and sensors related to specific composites. It assumes the value of discussion of the advantages and limitations of various sensors and methods, helping you to be updated with choices in your structure research and development. This book covers topics including: the effect of fiber loading on flexural strength, thermal and dynamic mechanical properties of banana-pineapple leaf fiber (PALF)woven glass hybrid biocomposites; biomass valorization for better aviation environmental impact through biocomposites and aviation bio-fuel; structural heath monitoring in aerospace composites; recent advances and trends in structural health monitoring; structural health monitoring of fiber polymer composites; corrosion detection for natural/synthetic/textile fiber polymer composites; performance of conventionally trained aircraft composite panel drillers versus immersive virtual reality-haptic trained drillers; maintenance and monitoring of composites; synthetic/natural fiber properties of fire-designated zone of an aircraft engine; a structural health monitoring approach, aerogel-based thermally sprayed coatings for aeropropulsion system: a feasibility study based on structural health monitoring approach; health monitoring of synthetic/natural woven fiber polymer composites; X-ray computed tomography analysis of damage evolution in carbon fiberereinforced laminated composites; fracture surface morphologies in understanding of composite structural behavior; an overview on the role of structural health monitoring in various sectors; and fracture surface morphologies in understanding of composite structural behavior. We are extremely thankful to all authors who are experts in the area of structural health monitoring who contributed book chapters in this edited book and supported

xviii

Preface

it by providing valuable ideas and knowledge. We are also grateful to Elsevier, UK supporting team, especially Jones Gwen, Thomas, Poulouse and Sandhya for helping us to finalize this book. Mohammad Jawaid Universiti Putra Malaysia, Serdang, Malaysia Mohamed Thariq Universiti Putra Malaysia, Serdang, Malaysia Naheed Saba Universiti Putra Malaysia, Serdang, Malaysia

The effect of different fiber loading on flexural and thermal properties of banana/pineapple leaf (PALF)/glass hybrid composite

1

Mohd Hanafee Zin 1 , Khalina Abdan 2 , Mohd Nurazzi Norizan 2 1 Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Serdang, Selangor, Malaysia; 2Department of Biological and Agricultural Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia

1.1

Introduction

The rising awareness of sustainability in aviation industry has led to various research studies being carried out in the natural resources domain. Natural fibers have been widely used as effective reinforcement in polymer matrices. Fillers in the form of fibers or particles are fabricated with polymers to obtain products with desired thermal, mechanical, and electrical properties. The properties of the fibrous composite materials are mainly dependent on their respective fiber properties. Other factors affecting the properties include the microstructural parameters such as fiber diameter, fiber length, fiber distribution, fiber orientation, volume fraction of the fibers, and packing arrangement of the fibers [1]. Natural fibers are biodegradable and a cheaper substitute for synthetic fibers, such as glass and carbon. They possess a lot of advantages, such as economical, low density, high toughness, ease of separation, and acceptable specific strength properties. Despite numerous research studies being done on natural fiberreinforced composites, natural fibers do have some drawbacks such as hydrophilic nature, which reduces the adhesive bonding with hydrophobic polymeric matric, as mentioned by [2]. Natural fibers are also prone to sensitivity to humidity and UV radiation, large variations in mechanical properties and low resistance on impact, as reported by [3]. To overcome this issue, most of the research has focused on improving interfacial properties between fiber reinforcement and polymer matrices through methods such as chemical treatment to enhance mechanical properties of the end products. In the structural application domain, fiber-reinforced composites have gained a lot of market potential due to the variety of usage. However, this market growth is

Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00001-0 Copyright © 2019 Elsevier Ltd. All rights reserved.

2

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

somehow limited due to lack of toughness of FRP. The mechanical properties of natural fiber-reinforced composites are significantly improved with the incorporation of synthetic fiber such as glass or carbon fiber. Hybrid composites refer to a combination of two or more different materials combined in a common matrix. Natural fibers have great potential to be utilized as the substitution material especially for interior aircraft cabin parts. However, since natural fibers do not have adequate strength to substitute for conventional synthetic fiber, therefore hybrid composites are preferred. Hybridization of two or more different materials is found to be an effective approach to design materials with different requirements and applications. Problems of high-cost synthetic fibers and low mechanical properties of natural fibers can be overcome by blending them in a common matrix. By having a hybrid composite, what lacks in one material can be complemented by the strength of the other material. Previous study by Thwe and Liao [4] on bamboo/glass fiber-reinforced polymer matrix hybrid composite depicted the improvement of tensile strength and modulus for the hybrid composites by more than half as compared to unhybridized composites. In order to produce composite sample with better mechanical properties, it is important to reduce hydrophobicity of the natural fibers through chemical modifications, as reported by Alavudeen et al. [5]. Various chemical modifications have been conducted to improve fiber-matrix adhesion, including alkaline treatment, benzoylation, acetylation, and many more. Alkaline treatment is one of the most common and economical methods to improve hydrophobicity of natural fibers. Asim et al. [6] investigated the effect of alkaline and silane treatment on mechanical properties of kenaf and pineapple leaf fibers. They found out that silane and NaOH treatment resulted in reduction of fiber diameter, due to removal of noncellulosic material, as well as enhancing the tensile properties, as compared to untreated fibers. Another study by Mahuya and Chakraborty [7], on alkaline treatment for bamboo, depicted that alkaline treatment resulted in lattice transformation from cellulose-I to cellulose-II. However, above 20% NaOH concentration, mechanical properties started to deteriorate. It is vital to determine the suitable composition of each element in hybrid composite in order to obtain optimum desired mechanical properties. Moreover, by selecting the appropriate composition between fibers and matrix, impact strength can be improved through fiber matrix adhesion and internal stress transfer. Fiber loading is one of the important factors affecting the strength of composite sample. Fiber loading determines the load transfer within the composite sample, thanks to the fiber-matrix bonding. Stronger interfacial adhesion between fiber and matrix will lead to better mechanical properties, as reported by Mohsen et al. [8]. This chapter focused on investigating the effect of different fiber loading on flexural properties of banana/pineapple leaf fiber (PALF)/glass hybrid composite. Alkaline treatment was conducted to improve adhesion between natural fibers and epoxy matrix. Samples were fabricated using hot compression method, and underwent flexural test per ASTM: D790.

The effect of different fiber loading on flexural and thermal properties

1.2

3

Material and method

Loose banana pseudo stem fiber and PALF were obtained from a local company in Johor, Malaysia. The fibers were cut into 40 cm length. The synthetic fiber selected was dry-woven E-Glass fabric (WR600) roving of 600 gsm. Woven glass fiber was cut into 30 cm
30 cm size. Resin system used for the experiment was Epoxy Resin DM15, with ratio between base and hardener (MEKP) of 5 to 1. The material properties for banana and PALF are shown in Table 1.1. In this study, banana fiber was chemically treated with 6% sodium hydroxide (NaOH) for 2 h immersion time, while PALF was treated with 6% NaOH for 3 h soaking period. The fibers were dried for 24 h, followed by a postcure in the oven at 60 C for 8 h in order to ensure removal of moisture from fiber. Dry reinforcement was weighted per the calculated ratio. The composites reinforced with three different volume percentages of fibers (30, 40, and 50 wt%) were prepared for the study using the following formula: Vf ¼

Wf =rf ½ðWf =rf Þ þ ðWm =rm Þ

where, Vf is fiber volume fraction (%), Wf and Wm are the weight (g) of fiber and matrix, respectively, and rf and rm are the density (g/cm3) of fiber and matrix, respectively. The hybrid composite layup was set up following the combination shown in Table 1.2. Fig. 1.1 shows the setup of composite layup using the Wabash Hot Press Machine 50MT. The layup sequence is shown in Fig. 1.2. The layup sequence with glass woven as the core in the middle is designed in such a way that most of the stress transferred due to flexural loading is absorbed by the natural fiber layer at top and bottom layers in the composite sample. Flexural test was conducted as per ASTM: D790 on 10 samples for each combination, and average values were obtained for analysis (Fig. 1.3).

Table 1.1 Material properties for banana, PALF fiber, and glass woven composites Properties

Banana fiber

Pineapple leaf fiber

Glass fiber

Density (kg/m )

1350

1520

2550

Tensile Strength (MPa)

212

413

3400

Young’s Modulus (GPa)

8

34

71

Elongation at break (%)

2

1.6

3.4

3

4

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 1.2 Composite layup combination Combination

Abbreviation

Glass fiber

Banana

PALF

Matrix

Glass Banana PALF

(GBP)

1

1 (0.25)

1 (0.25)

1

30% Glass Banana

(GB30)

1

1 (0.30)

0

1

40% Glass Banana

(GB40)

1

1 (0.40)

0

1

50% Glass Banana

(GB50)

1

1 (0.50)

0

1

30% Glass PALF

(GP30)

1

0

1 (0.30)

1

40% Glass PALF

(GP40)

1

0

1 (0.40)

1

50% Glass PALF

(GP50)

1

0

1 (0.50)

1

Figure 1.1 Composite specimen fabrication using hot compression method.

Banana/PaLF Glass Banana/PaLF

Figure 1.2 Layup sequence of hybrid composite.

1.3

Morphology analysis

The surface microstructure of untreated and treated fibers were observed using Hitachi S-3400N scanning electron microscopy (SEM) with the setting of 5.0 kV and a magnification between 300 and 800 times. The morphological analysis was carried out to examine the existence of interfacial adhesion between fibers and epoxy matrix. The

The effect of different fiber loading on flexural and thermal properties

5

Figure 1.3 Flexural testing as per ASTM: D790.

samples were coated with a thin layer of gold prior to scanning observation in order to increase the sample conductivity as well as prevent electrostatic charging during sample examination. The images were analyzed to investigate the distribution of natural fibers in the polymer matrix and their nature of interaction with each other.

1.4

Thermal stability analysis

The thermogravimetric analysis (TGA) was performed to evaluate the thermal properties of hybrid biocomposites of banana, PALF fiber, and woven glass using the TA Instrument TGA Q500 with a temperature setting between 30 and 600 C and a rate of 10 C/min. The sample weights are between 4.9 and 8.9 g. Thermogravimetric analysis measures the amount and the changing rate of material weight as a function of time or temperature in a controlled atmosphere [9]. TGA can also be used to characterize the effect of decomposition, oxidation, and dehydration on material’s weight loss or gain. Nitrogen gas with flow rate of 20 mL/min was used, and each sample was run for 3 h. The results are presented in graphical formdTG and derivative thermogravimetric (DTG) curves. For TG curve, at the point of major weight loss, the 5% degradation onset was conducted, while for DTG curve, the maximum rate of weight loss sample was identified. Dynamic mechanical analysis (DMA) tests were conducted to analyze the viscoelastic properties of the samples at different temperatures. The test involves the application of an oscillatory strain at different temperatures and low frequencies, to a specimen. Elastic storage modulus (E0 ) is the ratio of the elastic stress to strain, which indicates the ability of a material to store energy elastically. The ratio of viscous stress to strain is called viscous or loss modulus (E00 ) and is the measure of a material’s ability to dissipate energy. The ratio of the viscous modulus to the elastic one is the tangent of the phase angle shift between stress and strain.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

1.5 1.5.1

Results and discussion Flexural test

Fig. 1.4 shows the flexural strength for different fiber loading for banana/PALF/glass composite combination. The graph depicted increasing trend with higher fiber volume fraction. The flexural strength for GB and GP composite increased from 30% to 40%, and showed reduction at 50% fiber volume. At 40% fiber volume, GP40 recorded higher flexural strength of 105.10 MPa, which is equivalent to 16% difference as compared to GB40, which recorded 90.95 MPa. The GBP sample illustrated the lowest flexural strength of 60.68 MPa. Based on the results, it can be seen that 40% fiber volume yielded the highest flexural strength for both GB and GP composite samples (Fig. 1.5). During the flexural test, it was observed that the composite specimen did not fail completely. For this hybrid composite, natural fiber and epoxy resin absorbed most of the force applied during bending, since the fibers are placed on both top and bottom layers, as shown in Fig. 1.6. The shear created between the matrix and fibers resisted the external force, and when the stretching side failed, the force is transferred to the glass fiber core. Higher fiber volume means greater shear created throughout the fibers and matrix, hence resulting in greater force needed to break the sample. This result is reflected through higher flexural stress recorded for higher fiber volume sample. According to previous study, fiber volume fraction greater than 50% resulted in reduction of the mechanical properties. This is due to the fact that the resin content is insufficient to be absorbed by natural fibers in the composite sample, thus affecting overall mechanical properties. Insufficient resin reduced the fiber-matrix bonding, increased sample brittleness, hence making the sample fail prematurely in flexural test.

Flexural strength (MPa)

120.0 100.0

105.087 90.951 88.067

84.919

80.0

81.773

72.845

60.680

60.0 40.0 20.0 0.0 GB30

GB40

GB50 GP30 GP40 Fibre loading combination

GP50

GBP

Figure 1.4 Flexural strength for different fiber loading for banana/PALF/glass composite combination.

The effect of different fiber loading on flexural and thermal properties

7

Specimen 1 to 10

Flexure stress (MPa)

140 120

Specimen # 1 2 3 4 5 6 7 8 9 10

100 80 60 40 20 0 0

1

2

3 Flexure strain (%)

4

5

6

Figure 1.5 Typical stressestrain curve for banana glass hybrid composite sample under flexural loading. Squeezing / compression

Glass fibre core

Natural fibre Layers

Stretching / tension

Figure 1.6 The stress transfer diagram for hybrid biocomposite subjected to axial loading.

Flexural moduli of the composite samples are shown in Fig. 1.7. The flexural modulus illustrated almost similar pattern to flexural strength. The flexural modulus indicated highest value at 40% fiber volume for both GB and GP composite samples. However, GP40 recorded superior flexural modulus of 7.61 GPa, as compared to GB40, which recorded 4.24 GPa, indicating 79.59% difference. The lowest flexural modulus was obtained from GBP samples of 2.07 GPa. Flexural modulus indicates the stiffness of the material. The higher the flexural modulus, the stiffer the material, and vice versa. Flexural modulus is related to tensile strength. Therefore, the skin materials with higher tensile strength are able to absorb more force, hence resulting in higher flexural modulus.

1.5.2

Image analyzer

Fig. 1.8(aed) showed the composite samples observed under the digital image analyzer to measure the fiber diameter as well as fiber packing for different fiber loading. For each composition, 10 samples were observed, and for each sample, 10 different measurements were taken and averaged. Based on the observations, it can be seen that the fiber packing increased with increment of fiber weight percentage. This is most noticeable for composite sample with 50 wt% for both banana and PALF glass hybrid. The average measurement of fiber diameter for banana is 0.20 mm, while PALF fiber diameter recorded average of 0.10 mm.

8

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Flexure modulus (GPa)

9.0 7.613

8.0 7.0 6.0 5.0 4.0

3.384

4.239

4.019

4.311

3.903

3.0

2.069

2.0 1.0 0.0

GB30

GB40

GB50

GP30 GP40 Fibre loading combination

GP50

GBP

Figure 1.7 Flexural modulus for different fiber loading banana/PALF/glass composite combination.

(a)

(d)

(b)

30wt%

30wt%

(e)

(c)

40wt%

40wt%

(f)

50wt%

50wt%

Figure 1.8 (aec): Image analyzer of banana-glass hybrid biocomposites. (def): Image analyzer of PALF-glass hybrid biocomposites.

At 50 wt%, fiber bundles were obviously noticeable throughout the sample, which could affect stress transfer from matrix to fiber reinforcement subjected to flexural loading.

1.5.3

Scanning Electron Microscopy

Fig. 1.9(aeg) shows the surface morphology of the composite sample at the failure point due to flexural loading. Fig. 1.9(a) depicted fractured surface for 30% weight fiber volume that contains cavities, fiber pullout, and fracture with aggregation of fibers. Lower reinforcement density led to low mechanical properties, as can be seen on Fig. 1.4. This is due to

The effect of different fiber loading on flexural and thermal properties

(a)

9

(b)

WD 8/16/2017 HV Mag Spot Det 12:28:08 PM 5.00 KV 5 000 x 7.2 mm 3.0 ETD

20 µm ITMA

HV WD Spot Det 8/8/2017 Mag 12:46:06 PM 5.00 KV 5 000 x 7.5 mm 3.0 ETD

20 µm ITMA

Glass banana (40%)

Glass banana (30%)

(c)

(d)

8/16/2017 HV Mag WD Spot Det 12:10:17 PM 5.00 KV 5 000 x 5.3 mm 3.0 ETD

20 µm ITMA

WD 8/16/2017 HV Mag Spot Det 11:22:20 AM 5.00 KV 5 000 x 5.1 mm 3.0 ETD

Glass banana (50%)

20 µm ITMA

Glass PALF (30%)

(e)

(f)

8/8/2017 HV Mag WD Spot Det 3:28:06 PM 5.00 KV 5 000 x 5.7 mm 3.0 ETD

Glass PALF (40%)

20 µm ITMA

WD Spot Det 8/8/2017 HV Mag 2:58:17 PM 5.00 KV 5 000 x 6.2 mm 3.0 ETD

20 µm ITMA

Glass PALF (50%)

Figure 1.9 (aec): Surface morphology of the banana-glass hybrid biocomposite sample at the failure point due to flexural loading. (def): Surface morphology of the PALF-glass hybrid biocomposite sample at the failure point due to flexural loading. (g): Glass Banana PALF.

10

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

inadequate strength to withhold the load transfer from epoxy matrix when subjected to flexural loading. At 50% weight fiber volume, the fiber packing showed higher density. However, due to hydrophilic nature of banana fiber, more resin is needed to coat the entire fibers in the composite sample. Inadequate resin resulted in low mechanical properties, as depicted in Figs. 1.4 and 1.5. This is due to insufficient physical adhesion between fibers and epoxy matrix. Fig. 1.9(def) illustrated surface morphology for glass-PALF composite with different fiber weight volume composition. The presence of epoxy resin deposited along the PALF fiber showed existence of interfacial adhesion between matrix and fiber, hence contributed to good mechanical properties of composite sample. At 40% weight fiber volume, the sample showed fiber breakage subjected to flexural loading. This indicated that the stress has been transferred from epoxy matrix to PALF fiber prior to sample failure. At 50% weight fiber volume, insufficient epoxy matrix introduced numerous voids leading to a lower flexural modulus of the composite, which behave as plastic material resisting the deformation. Fig. 1.6(g) showed surface morphology of glass-banana-PALF composite sample. The lower flexural strength of GBP composite may be attributed to the restriction of macromolecular mobility and deformability imposed by the presence of glass woven fiber in epoxy matrix.

1.5.4

Thermogravimetric analysis

TGA analysis curves of composite samples are shown in Fig. 1.10(a and b), and DTG curves are shown in Fig. 1.10(c and d), and the data are simplified in Table 1.2. For banana hybrid composite samples, the major degradation is as shown in Fig. 1.10(a), which occurred in the temperature range of 273e427 C at a peak at 349 C, while for the PALF hybrid composite samples, degradation occurred in the range of 253e426 C at a peak at 341 C. From the graph, it can be noticed that there are four phases of thermal degradation. The first mass loss, recorded between 30 and 100 C, could be due to moisture evaporation. The next phase is between 130 and 380 C for hybrid biocomposites, due to the decomposition of the components hemicellulose, cellulose, and lignin. According to a previous researcher, the hemicellulose decomposed at 220 C and completed at 315 C. The cellulose decomposition will only take place after hemicellulose has completely decomposed. Cellulose has better thermal stability, due to the crystalline structure of their cellulose chain. Lignin possessed the highest thermal stability as it decomposes slowly, starting from around 160 C and extending up to 900 C. This is due to the fact that lignin is the toughest component that gives rigidity to the plant materials. As lignin is completely decomposed, the leftover residue is inorganic material such as sodium hydroxide (NaOH), in the fiber that can only be decomposed at very high temperature of 1700 C and above. As mentioned in a current study, after the final thermal degradation, black carbonaceous residues were present [10]. Fig. 1.10(a and b) indicated an increase decomposition temperature for banana and PALF, due to higher fiber content (40%). For banana at 40% fiber weight volume, the

The effect of different fiber loading on flexural and thermal properties 100

(a)

336.96°C 349.12°C

B30 B40 B50 GBP

100

(b)

60

40

20

319.46°C

341.14°C

313.78°C 60

40

20

0 0

100

200 300 400 Temperature (°C)

500

12

B30 B40 B50 GBP

(c)

0

600

0 10

100

200 300 400 Temperature (°C)

500

8 6 4 2

600 B30 B40 B50 GBP

(d)

8 Deriv. weight (%/min)

10 Deriv. weight (%/min)

P30 P40 P50 GBP

336.96°C

80 Weight (%)

Weight (%)

80 327.29°C 328.03°C

11

6 4 2 0

0 –2

–2 0

100

200

300 400 Temperature (°C)

500

600

0

100

200

300 400 Temperature (°C)

500

600

Figure 1.10 (a): Effect of fiber loading on thermal stability of the banana glass hybrid biocomposites. (b): Effect of fiber loading on thermal stability of the PALF glass hybrid biocomposites. (c): Effect of fiber loading on derivative thermogravimetric analysis of the banana glass hybrid biocomposites. (d): Effect of fiber loading on derivative thermogravimetric analysis of the PALF glass hybrid biocomposites.

onset degradation is 349.12 C, while for PALF with similar setting is 341.14 C. The ash content for banana and PALF at 40% fiber weight volume are 10.35% and 9.381% respectively. Fig. 1.10(c and d) illustrated DTG curves for banana and PALF hybrid biocomposites. Based on DTG curve, there was a minor peak in temperature range between 310 and 350 C. At this temperature, fiber was heated and the moisture content evaporated due to heating process. Peak temperature from DTG occurred at 411.48 C with 10.07 C/min rate indicating increase in the maximum rate of weight loss at 0.4Vf for banana hybrid composite samples. There was a similar pattern for PALF whereby the maximum peak temperature was recorded at 403.24 C with the rate of 9.36 C/min. (Table 1.3)

1.5.5

Dynamic mechanical analysis

The DMA has been an instrumental tool to study the behavior of composite structure. DMA is an effective method to investigate the relaxations in polymers, as well as behavior of the materials under multiple conditions of stress, temperature, and phase

12

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

composition of fiber composites, and its function in determining the mechanical properties. Storage modulus cure of both banana-glass and PALF-glass hybrid biocomposite are shown in Fig. 1.11(a and b). Fig. 1.11(a and b) showed the effect of temperature on the dynamic modulus (E0 ) of banana, PALF, and glass hybrid biocomposite with varying fiber weight fractions (0.3e0.5Vf) at a frequency of 20 Hz. At low temperature, there is slight variation on storage modulus for matrix and composites, indicating lesser contribution of the fibers toward the stiffness of the material at low temperature. For both banana and PALF hybrid composites, storage modulus decreases with temperature. Higher fiber weight fraction imparts stiffness to the composite sample. However, the stiffness decreases with increasing temperature. At low fiber weight fraction (0.3Vf) for PALF, lowest storage modulus was recorded at 70 C, as compared to 0.4 and 0.5Vf. This is due to the rise in molecular mobility of the polymer chains above the Tg. The drop in modulus passing through the glass transition temperature is reduced for higher fiber loading composites, indicating the reinforcement effect of banana and PALF that improved sample strength and rigidity. Based on the graph, for glass PALF composite there is a shift in glass transition temperature for sample from 0.3Vf to 0.4Vf, which is from 77.92 to 100.46 C, respectively. This indicated that higher fiber loading increases rigidity of the sample, which leads to higher temperature needed to move the molecular structure, before the sample changes from glassy to rubbery stage. The composite containing 50 wt% PALF glass fiber displayed a decrease in damping peak (Tan d) amplitude with regard to 40 wt% sample by 24.9% to 75.09 C. Increase in fiber packing led to excessive absorption of resin matrix, which in turn increased brittleness of the composite sample that resulted in lower glass transition temperature. However, for PALF, the trend is slightly different whereby the sample with lowest fiber loading (30 wt%) exhibited the highest glass transition temperature of 112.70 C, whereas the highest fiber loading (50 wt%) shifted the Tg to 91.35 C.

Table 1.3 TGA analysis of banana, PALF, and glass hybrid composites

Sample

% Mass loss change from 278C to first inflection

% Mass loss from first to second inflection points

Total % of mass loss

Onset of degradation (8C)

B30

10.44

72.89

83.33

327.29

B40

7.479

81.73

89.21

349.12

B50

10.60

69.38

79.98

328.03

P30

5.559

81.41

86.97

319.46

P40

7.656

82.20

89.86

341.14

P50

7.479

80.34

87.82

313.78

GBP

7.108

81.75

88.86

336.96

The effect of different fiber loading on flexural and thermal properties

2000

(a)

13

B30.001 B40.001 B50.001 GBP.001

Storage modulus(MPa)

1500

1000

500

0 20

40

60

80

100

Temperature(ºc)

Storage modulus(MPa)

2000

(b)

120 140 160 Universal V4.5A TA instruments

GBP.001 P30.001 P40.001 P50.001

1500

1000

500

0 20

40

60

80

100

Temperature(ºc)

120 140 160 Universal V4.5A TA instruments

Figure 1.11 (a): Effect of fiber loading with temperature on the storage modulus values of the banana glass hybrid biocomposites. (b): Effect of fiber loading with temperature on the storage modulus values of the PALF glass hybrid biocomposites. (c): Effect of fiber loading with temperature on the tan delta values of the banana glass hybrid biocomposites. (d): Effect of fiber loading with temperature on the tan delta values of the PALF glass hybrid biocomposites.

14

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

0.6

(c)

B30.001 B40.001 B50.001 GBP.001

67.33ºC 0.5379 75.09ºC 0.5076 77.92ºC 0.4424

100.46ºC 0.4362

Tan delta

0.4

0.2

0.0 20

40

60

80 100 Temperature(ºc)

120

140

0.6

(d)

160

P30.001 P40.001 P50.001 GBP.001

67.18ºC 80.16ºC 0.5369 0.5384 91.35ºC 0.4635

112.70ºC 0.3806

Tan delta

0.4

0.2

0.0 20

40

Figure 1.11 cont'd.

60

80 100 Temperature(ºc)

120

140

160

The effect of different fiber loading on flexural and thermal properties

15

Generally, PALF hybrid exhibits higher Tg than banana hybrid samples in all fiberloading composition. This result indicated that PALF has higher thermal stability as compared to banana fiber. The dynamic Tg refers to the temperature where the middle point of E0 versus temperature curve or the region where E0 increases with increasing frequency at constant temperature. Another way to define Tg is by looking at the maximum of where the Tan d occurs or maximum of where the E00 occurs, as mentioned by Saba et al. [11]. Glass transition temperature is the temperature range where a thermosetting polymer changes from a ‘‘glassy,” rigid state to a more pliable or ‘‘rubbery” state. There is a difference between melting temperature (Tm) and Tg, as at melting point the material begin to melts while at (Tg) the material gets softer. Melting temperature of crystalline form is usually higher than the Tg, provided the polymer has both crystalline and amorphous form. Above the Tm temperature, the polymer is in the rubbery stage, while below it, the polymer is in the glassy or brittle stage. The glass transition is a transition that happens to amorphous polymers at Tg. The higher the Tg, the greater the cross-linked density, which then leads to higher polymer modulus value of the system. The effects of cross-linking on the various regions of the DMA curve are visible in rubbery and glass transition region. However, in the glassy region, both the loss and storage moduli are independent of the degree of cross-linking. Thus, highly cross-linked thermoset polymer has much larger loss and storage moduli, indicating the tighter network structure and higher stiffness, whereas the polymer of lightly cross-linked shows considerable smaller storage and loss modulus, as also reported by previous researchers [12] (Table 1.4). Previous study reported that the reinforced fiber could improve E’ due to the stiffening effect of fiber with matrix and eventually decreased the damping curve of polymer matrix [13]. They further acknowledged that damping property of fiber-reinforced composite materials depends on various parameters such as frictional resistance, fiber/matrix interface, interphase zone, matrix cracking, and fiber breakage.

Table 1.4 DMA analysis of banana, PALF, and glass hybrid biocomposites Combination

Storage modulus

Loss modulus

Tan delta

B30

60.11

70.73

77.92

B40

77.94

91.73

100.23

B50

53.16

66.91

75.13

P30

96.64

107.34

112.63

P40

61.16

70.81

80.34

P50

65.18

81.79

91.39

GBP

31.13

31.13

67.38

16

1.6

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Conclusion

Increasing fiber volume improves the flexural strength of the composite samples. The optimum fiber volume composition is 40 wt% for both banana and PALF hybrid composites. At 40 wt%, banana glass hybrid composite yielded flexural strength of 90.951 MPa, while PALF glass hybrid recorded 105.087 MPa. The flexural modulus also indicated similar trend with 4.219 and 7.613 GPa for banana-glass and PALFglass hybrid composite at 0.4Vf. TGA test suggested that 40 wt% offered optimum onset degradation temperature for both banana-glass and PALF-glass hybrid composite. DMA analysis showed a shift in the Tg for banana-glass hybrid composite, from 30 to 40 wt%, indicating optimum condition that contributed to molecular structure stability of the composite sample.

Acknowledgments This study was conducted in a collaborative laboratory known as the “Sustainable Aviation,” a joint lab with Aerospace Malaysia Innovation Centre, Universiti Putra Malaysia, and University of Nottingham Malaysia Campus, under Project no. BFeP06.

References [1] Annie Paul S, Boudenne A, Ibos L, Candau Y, Joseph K, Thomas S. Effect of fiber loading and chemical treatments on thermophysical properties of banana fiber/polypropylene commingled composite materials. Compos Part A Appl Sci Manuf 2008;39(9):1582e8. [2] Lee SH, Wang S. Biodegradable polymers/bamboo fiber biocomposite with bio-based coupling agent. Compos Part A Appl Sci Manuf 2006;37(1):80e91. [3] Benítez AN, Monzon MD, Angulo I, Ortega Z, Hernandez PM, Marrero MD. Treatment of banana fiber for use in the reinforcement of polymeric matrices. Meas J Int Meas Confed 2013;46(3):1065e73. [4] Thwe MM, Liao K. Effects of environmental aging on the mechanical properties of bamboo-glass fiber reinforced polymer matrix hybrid composites. Compos Part A Appl Sci Manuf 2002;33(1):43e52. [5] Alavudeen A, Rajini N, Karthikeyan S, Thiruchitrambalam M, Venkateshwaren N. Mechanical properties of banana/kenaf fiber-reinforced hybrid polyester composites: effect of woven fabric and random orientation. Mater Des 2015;66:246e57. [Internet] Available from: http://www.sciencedirect.com/science/article/pii/S0261306914008553. [6] Asim M, Jawaid M, Abdan K, Ishak MR. Effect of alkali and silane treatments on mechanical and fibre-matrix bond strength of kenaf and pineapple leaf fibres. J Bionic Eng 2016;13(3): 426e35. [Internet] Available from: https://doi.org/10.1016/S1672-6529(16)60315-3. [7] Das M, Chakraborty D. Influence of alkali treatment on the fine structure and morphology of bamboo fibers. J Appl Polym Sci 2006;102(5):5050e6. [8] Nasihatgozar M, Daghigh V, Lacy TE, Daghigh H, Nikbin K, Simoneau A. Mechanical characterization of novel latania natural fiber reinforced PP/EPDM composites. Polym Test 2016;56:321e8. [Internet] Available from: https://doi.org/10.1016/j.polymertesting.2016. 10.016.

The effect of different fiber loading on flexural and thermal properties

17

[9] Boopalan M, Niranjanaa M, Umapathy MJ. Study on the mechanical properties and thermal properties of jute and banana fiber reinforced epoxy hybrid composites. Compos Part B Eng 2013;51:54e7. [Internet] Available from: http://linkinghub.elsevier.com/ retrieve/pii/S1359836813000887. [10] Mariatti M, Jannah M, Abu Bakar A, Abdul Khalil HPS. Properties of banana and Pandanus woven fabric reinforced unsaturated polyester composites. J Compos Mater 2008;42(9):931e41. [Internet] Available from: http://jcm.sagepub.com/cgi/doi/10.1177/ 0021998308090452. [11] Saba N, Jawaid M, Alothman OY, Paridah MT. A review on dynamic mechanical properties of natural fibre reinforced polymer composites. Constr Build Mater 2016;106: 149e59. [Internet] Available from: http://www.sciencedirect.com/science/article/pii/ S0950061815307479. [12] Thomas S. Dynamic mechanical behavior of short coir fiber reinforced natural rubber composites. Compos Part A Appl Sci Manuf 2005;36:1499e506. [13] Kumar SMS, Duraibabu D, Subramanian K. Studies on mechanical, thermal and dynamic mechanical properties of untreated ( raw ) and treated coconut sheath fiber reinforced epoxy composites. J Mater 2014;59:63e9. [Internet] Available from: https://doi.org/10. 1016/j.matdes.2014.02.013.

Biomass aviation through aviation

valorization for better environmental impact biocomposites and biofuel

2

Jia Tian Chen 1,2 , Luqman Chuah Abdullah 3 , Paridah Md. Tahir 4 1 Centre of Excellence on Biomass Valorization for Aviation, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Malaysia; 2Aerospace Malaysia Innovation Centre (AMIC), MIGHT Partnership Hub, Jalan Impact, Cyberjaya, Malaysia; 3Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Malaysia; 4Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Malaysia

2.1 2.1.1

Introduction Aviation environmental impact

The expansion of international civil aviation over the past two decades has been significant [1]. Air traffic is accounted by revenue passenger kilometer (RPK), and this has historically doubled every 15 years [2]. The forecast for the next 20 years is an increase between 4.3% and 4.8% in air traffic per year, with the main driver being Asia Pacific in terms of growth percentage [1]. Current global civil aviation accounts for approximately 2% of man-made carbon dioxide (CO2) emissions [3]. In terms of energy consumption in the transportation sector, globally jet fuel contributes 11%, and is projected to increase to 14% within the next 20 years [4]. Other than CO2, an aircraft also emits other greenhouse gases and particles that impact the climate, such as water vapor, nitrogen oxides, sulfur oxides, and soot [5]. Fig. 2.1 shows the RPK, as a representation of air traffic increase, from 1970 to 2010 (Airbus, 2010). According to the International Air Transport Association (IATA), CO2 contribution by the aviation industry may grow by up to 5% by 2050 [6]. However, the environmental impact of air travel is best measured by radiative forcing (RF) effect of its emissions. RF is the measurement of the effect of energy the atmosphere faces due to greenhouse gas emissions, or forcing agents, which triggers a cooling or heating effect expressed in watts per square meter (W/m2) [7]. RF can be uniquely found on flight paths due to the jet engine emissions of nitrogen oxides, sulfur oxides, water vapors, and particulates (like soot). The emission of CO2 is known to be a long-lived greenhouse gas that warms the earth, however, this is not unique only to the aviation industry. An interesting indicator would be water vapor emissions from the jet engines, these emitted vapors form contrails that have atmospheric effects similar to clouds [8,9]. Due Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00002-2 Copyright © 2019 Elsevier Ltd. All rights reserved.

20

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites RPK (trillion) ICAO total traffic

Airbus GMF 2010

10.0

8.0 Air traffic will double in the next 15 years

Air traffic has doubled every 15 years

6.0

4.0 20-year world annual traffic growth 4.8%

2.0

0.0 1970

1980

1990

2000

2010

2020

2030

Figure 2.1 Increase in RPK, as air traffic change, over time and prediction [2].

to the cloud-like effect, contrails absorb and emit thermal infrared radiation leading to positive RF (warming). Contrails can also form into larger forms, known as contrail cirrus, which has a higher RF effect. Fig. 2.2 shows the RF effect of new contrails and the warming effect after formation of larger contrail cirrus clouds [10]. New contrails are clouds that are less than 5 h from jet engine emission, and these account for a positive (warming) RF of the atmosphere by 4.3 mW/m2. However, older contrails that have formed into larger contrail cirrus, will affect the atmosphere eight times more (37.5 mW/m2). Lee et al. have estimated that aviation RF contributes a total increase of the climate by 0.078 W/m2, and this may increase by a factor of four over 2005 levels [9]. 90 45 0 –45

4.3 mW/m2

–90 –180

–90

0

90

180

90 45 0 –45

37.5 mW/m2

–90 –180

–300

–90

–30 –10 –3

0 Longitude –1

1.

90

3. 10. 30.

180

300.

Figure 2.2 Radiative forcing (RF) effects of new contrails (above) and after effects (below) [10].

Biomass valorization for better aviation environmental impact through biocomposites

21

In order for the aviation industry’s impact to the environment to be minimal, a global movement spearheaded by Air Transport Action Group (ATAG) in 2008 was put in place. To reduce the environmental footprint, major aviation leaders committed to three targets at the 2008 Aviation and Environment Summit [11]: • • •

Annual fuel efficiency utilization of 1.5% per annum Net carbon emissions cap by 2020 (carbon neutral growth) 50% reduction in net aviation CO2 emissions by 2050 from 2005 levels.

This has led to the International Civil Aviation Organization (ICAO), under the United Nations, to have the aviation industry pledge reduction in its emissions through four pillars of innovation [11]: • • • •

Product Technology Operations and Infrastructure Economic Measures Sustainable Fuel.

Although the aviation industry has had a strong record in decreasing its emissions, the four pillars of innovation are foreseen to enable the industry to reach its 2050 target of 50% net reduction in aviation CO2 emissions, and related greenhouse gasses. The vision of reduction in CO2 emissions by the global aviation industry can be summed up in Fig. 2.3. In Fig. 2.3, it can be seen that technology and biofuels are the two largest contributors to reach 2050’s 50% carbon dioxide reduction goal. The red line represents predicted emissions pending zero action on any of the four pillars, and the green line represents the predicted trajectory toward its 2050 goal. Technology is split into various sectors: (1) aircraft technology, (2) operations, and (3) infrastructure. Improvements in aircraft technology are accounted for by introduction of better aerodynamic

Emissions assuming no action

No action

Aircraft technology (known), operations and infrastructure measures

Carbon-neutral growth 2020

Biofuels and radically new technologies

Gross emissions trajectory

Economic measures

Technology

CO2 Emissions

Operations Infrastructure Biofuels and radical tech CNG 2020

–50% by 2050 Not to scale

2005

2010

2020

2030

2040

2050

Figure 2.3 International Civil Aviation Organization (ICAO) action plan until 2050 (four pillar strategy) [11].

22

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

properties into the aircraft, such as wingtips; and weight reduction of the plane, such as increased usage of composite materials, lightweight paints, and lightweight seats. Improvements in operations are targeted at optimized flight paths, fuel savings for takeoff and landing procedures, and weather-based flight paths. Finally, infrastructure is aimed at improvements and modernization of air traffic management systems at airports. The green portion of the action plan, biofuel and radically new technologies, is seen as the major contributor to reach the 50% reduction in CO2 emissions of the industry. While new radical technologies are in development, such as hybrid-electrical propulsion, fuel celledriven environmental control system, and assisted take-off and landing, these technologies are still immature and their effect on the reduction of environmental impact is still in consideration. Biofuel, in this instance,biofuel made for aviation jet fuel grade, is the best contender. Numerous test flights have already been performed globally and its affect in reduction of CO2 emission and improved fuel performance is well recorded [3]. The utilization of jet biofuel and biocomposite contributes to the aviation industry’s reduction in CO2 and improving the sustainability of the industry. Jet biofuel improves fuel performance, lowers soot and CO2 emissions. Biocomposites improve the sustainability factor of a plane, rather than composite fibers derived from nonrenewable sources. Biocomposites have a long development roadmap, and the usage of renewable, sustainable sources is key for implementation onto an aircraft.

2.1.2

Sustainable biomass for aviation

Biomass is seen as the next generation source for the aviation industry, as biomass absorbs carbon dioxide, it limits and reduces the aviation industry’s impact. However, caution has to be noted that the biomass has to be sustainable and renewable. Sustainable biomass means no competition with food, no land use change in accordance to sustainability criteria, minimal impact to the environmental/ecosystem, and is renewable (able to be grown), compared to fossil fuel based whose resources are finite. The need for sustainable and renewable biomass sources is best illustrated by Table 2.1. Currently, bioenergy (from biomass) is the world’s largest source of renewable energy, accounting for 14% out of 18% of the renewable energy sector. This places bioenergy at an estimate of 10% of global energy supply. This biomass covers both sustainable and nonsustainable biomass [12]. Of the bioenergy, only 4% of the total Table 2.1 Comparison of renewable, nonrenewable, sustainable, and nonsustainable Sustainable

Non-sustainable

Renewable

A reliable source that meets annual demand

A renewable source that does not meet demand

Nonrenewable

Producing sufficient sustainable biomass for 10 years only

Supply only lasting for 40 years

Biomass valorization for better aviation environmental impact through biocomposites

23

can be considered as biomass usage for biofuel, which includes biodiesel, bioethanol, and biogas. The world’s largest biofuel producers from biomass are South America (largely Brazil), North America, followed by Europe [13]. These energy crops, or biomass, can be sectioned into three generations of biomass feedstock. It is sectioned as such to fundamentally understand the differences between its renewable and sustainable differences. First-generation feedstock are crops such as corn, sugarcane, soybean, vegetable oil, wheat, rapeseed, peanuts, and a number of other food crops. First-generation biofuels are produced directly from these food crops. However, in 2008, it was estimated that if biofuels from first-generation crops were to satisfy 20% of the growing biofuel demand, there will be no crop balances for food. In 2007, the United Nations Food and Agriculture Organization saw world food prices increase by 40% within 12 months, due to the influence of biofuel being derived from feedstock such as sugarcane, corn, rapeseed oil, palm oil, and soybeans [14]. This meant that first-generation biomass was renewable, but not sustainable, for usage of biofuel or for creation of a biocomposite industry. The first-generation debate brought about the need for second-generation feedstock, and later third generation, which are not in direct competition with food. Secondgeneration biofuels are derived from nonfood feedstock, for example, switchgrass, Jatropha, waste vegetable oil, municipal solid waste, lignocellulosic sources, etc. These second-generation sources tends to be grown on marginal lands, land that cannot be used for “edible” crops. Compared to first-generation, second-generation feedstock are processed differently and will require additional processing steps to convert the feed into fuel. Recently, third generation entered into the spotlight as biofuel derived from algae. Third-generation feedstock provides numerous advantages over second and third generation, from a larger array of carbon sources, significantly higher yields, better land efficiency, diversity of algae species for added-value products, good potentials of products, and different methods in cultivation. However, the biggest drawback for algae is the large amount of water and nutrients (nitrogen and phosphorus) to grow them, and the investment costs for large-scale implementation [15]. Combined, biomass feedstock in 2007 was approximated at 47.2 EJ (exajoule) per year; in 2016, this was estimated to be 56 EJ per year. The main driver behind the increase in feedstock availability is the biodiesel and bioethanol industry [16]. This is due to the increase in countries adopting biofuels to improve their greenhouse gas impact, as part of the United Nations Framework Convention on Climate Change.

2.1.3

Biocomposites

A material can be considered a biocomposite when one part of its composition, whether it is its matrix or reinforcements, is derived from natural sources [17]. Biocomposites are made in part, with natural fiber, which can be derived from a plant’s seed, leaf, bast, fruit, and/or stalk. This is limited to such due to the need for cellulose, hemicellulose, and lignin, which is able to form a matrix and is held together in a framework to yield a structured composite shape. The usage of lightweight composite materials, increased utilization of composites on aircraft, and the pending introduction of biocomposites are all contributing factors

24

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

toward lowering the weight of an aircraft, and for biocompositesd“greening” the aircraft. With decrease in aircraft weight, fuel savings and reduction of CO2 can be accomplished. The outlook for utilizing biocomposite for aircraft parts is attractive; however, to develop biofibers for aviation requires stringent testing and validation, and the research is not yet mature enough. The limiting factor for now, is the need to treat and process the natural fiber to rival current composite fibers’ properties and component strength. This is accounted by natural fiber’s lack in homogeneity, compared to glass and carbon; it has a higher tendency to absorb moisture; and the compatibility with current resin used in aerospace manufacturing [18]. A method to fast track the implementation of natural fiber into aerospace manufacturing is through hybridization, a mix of conventional glass and/ or carbon fiber with natural fiber; this is currently undergoing prototyping and validation, but is seen as a faster means to market than 100% utilization of natural fiber for aerospace parts [19]. However, natural fibers will inherently be limited to secondgeneration biomass feedstock due to the need for lignocellulose material. The usage of natural fibers like coconut coir, banana stem, pineapple leaves, sugar cane bagasse, kenaf fiber, oil palm empty fruit bunch (EFB), and bamboo fiber has been documented for aerospace part prototyping [18e21].

2.1.4

Jet biofuel

Global Kerosene Jet A-1 production and demand is approximately 80 billion gallons litre per year, or 302.8 billion liters per year [22]. To understand the magnitude of jet biofuel required globally, taking into account the mandates by various countries, a 2%e3% global replacement with jet biofuel will equate to roughly 6.06e15.14 billion liters per year. According to ICAO, the United States, Canada, China, Japan, the European Union, and Indonesia have implemented a jet biofuel mandate between 2% and 3% of their annual consumption. This is the result of the aviation industry’s pledge back in 2008 with ATAG, and in 2017 marks the introduction of a carbontrading scheme by ICAO known as Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) [23]. The CORSIA scheme will go into its pilot phase in 2020, and will see the implementation (a global market-based measure) of trading “emissions” units. Aircraft operators will be required to pay for any CO2 emission growth at their 2020 levels (reported). The amount of CO2 to be offset by 2025 is in the range of 142e174 million tons of CO2 emissions. The price of carbon is also estimated from a low 6e10 USD/ton of CO2-eq to a high estimate of 20e33 USD/ ton CO2-eq. To produce jet biofuel sustainably, the feedstock has to be sustainable (second or third generation) and the process to convert has to be certified. Under the ASTM: D7566 “Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbon” [24], only five processes have been certified under international standards that govern Kerosene Jet A-1 usage on board an aircraft. Synthesized hydrocarbon from biomass is in essence jet biofuel. Conventionally, Kerosene Jet A-1 is certified under ASTM: D1655 “Standard Specification for Aviation Turbine Fuel,” without any synthetic hydrocarbons [25]. In order to produce synthetic hydrocarbons, or jet

Biomass valorization for better aviation environmental impact through biocomposites

25

biofuel, the five processes are: (1) Fischer-Tropsch Synthetic Kerosene with Aromatics (SKA), (2) Fischer-Tropsch Synthetic Paraffinic Kerosene, (3) Hydroprocessed Esters and Fatty Acids, (4) Alcohol-to-Jet, and (5) Direct Sugar to Hydrocarbon/Synthetic Isoparaffin. These were recognized and certified in 2009 (FT-SPK), 2011 (HEFA), 2016 (ATJ), and 2014 (DSHC/SIP). All FT fuels are able to be blended up to 50%, HEFA fuel is also up to 50%, DSHC/SIP can be blended up to 10%, and finally ATJ up to 30% blend. A simplified process flow can be seen in Fig. 2.4 of the various jet biofuel processes [26]. Conventional Kerosene Jet A-1 (via MEROX process) composition is approximately 21% aromatics, 25% paraffin, 11% isoparaffin, and 43% cycloparaffin. Compared to fuel derived from HEFA; 10% paraffin, 90% isoparaffin; FT derived, 3% paraffin, 88% isoparaffin, 9% cycloparaffin; and lastly, DSHC derived is >95% isoparaffins. The difference in composition is noticeable and, therefore, is unable to fully (100%) substitute conventional Kerosene Jet A-1, and requires blending instead. Asides from these five processes, there are numerous other, not yet certified, pathways that utilize second-generation and third-generation feedstock to yield jet biofuel. Most notably are hydrotreated depolymerized cellulosic jet, catalytic hydrothermolysis, hydrothermal liquefaction, synthesized aromatic kerosene, and synthesized kerosene. Utilization of jet biofuel is a reality, and efforts are underway worldwide to push for adoption. This is needed to understand the performance and benefits of jet biofuel.

Vegetable oil, used cooking oil, fat

Hydroprocessing

50%

Hydrothermal liquefaction Algae

Pyrolysis

Bio-oil

Gasification

Fischer-tropsh synthesis

Syngas

50%

Lignocellulose Gas fermentation

Alcohol synthesis

Hydroprocessing

Alternative jet fuel

Hydrolysis Waste gas CO, CO2, H2

Olefins Glucose

Hydrocarbons

Alcoholic fermentation

Dehydration & oligomerization

Alcohols 30%

Sugar & starch

Fermentation

Farnesane

10%

Biological process

Chemical process

Thermal process

Feedstock

Intermediate product

= ASTM certified (maximum blending level indicated)

Figure 2.4 Simplified process flow of certified processes to produce jet biofuel [26].

26

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites KLM / skyNRG Alaska /hawaї bioenergy

Airbus / air canada / biofuelnet

Core jetfuel

Advanced biofuel flighpath SAFUG NARA

Lufthansa / GEVO Airbus / rostec

aireg

BA/solena

NEWbio

NISA

Bioport holland

Airbus / ENN

lufthansa / solena ASCENT

United / altair

Farm to fly CAFFI

Itaka

INAF

Bioqueroseno

GE / D’arcinoff Qatar univ. biofuel project

Cathay / fulcrum

MSBRC SEASAFI

Airbus / malaysia Qatar airways / byogy Green aviation initiative

BBP GOL / boeing

Virgin / lanzatech

QSAFI

GOL / amyris-total Arg. national initiative

Boeing / SSA / SKYNRG

AISAF

Virgin / skyNRG

Avianca / byogy Quantas / solena Stakeholders action group

Projects

Airlines /fuel producers

Figure 2.5 Global activity map of jet biofuel, test flights by airlines and aerospace industry [11].

Fig. 2.5) is a world map of various activity on jet biofuel test flights, production, and projects [11]. Some of the efforts have been summarized in the table below to link the relationship between the airline, jet biofuel producer, its feedstock and process utilized, and contractual ties (Table 2.2). To replace 2%e3% with jet biofuel of current global aviation jet fuel demand, a total of roughly 6e15 billion liters per year is required. This demand is currently not met by any jet biofuel producers around the world, and only a handful of countries around the world have implemented mandates on jet biofuel. However, through rigorous testing and analysis, it is a known fact that jet biofuel combustion process is cleaner and more efficient than conventional Kerosene Jet A-1. Jet biofuel offers customization beyond the ability of conventional Kerosene Jet A-1, as the production of Kerosene Jet A-1 is in harmony with other products derived from crude oil, i.e., gasoline/petrol, diesel, and petroleum products. Jet biofuel processes enable a certain level freedom of conversion and equilibrium reaction. For example, most synthetic kerosene-grade fuel offers more straight chain chemical groups than conventional jet fuel, which limits the soot formation during combustion in the jet engines. Cleaner burn, limited soot formation will mean increased engine performance, easier maintenance cycles, and better fuel efficiency. Stability of the fuel also increases through the introduction of more cyclic components within the fuel, without a decrease in fuel performancedjet biofuel is able to be tailored. The lack of aromatics in synthetic kerosene is an advantage, as aromatics do not provide clean combustion and encourage soot formation, unlike Kerosene Jet A-1, where aromatics are still in abundance. Aromatics are heavy molecules, with limited hydrogen, and therefore limited energy content. Aromatics increase the fuel density without offering

Biomass valorization for better aviation environmental impact through biocomposites

Table 2.2 Overview of airlines that have agreements with jet biofuel producers Airline

Producer

Feedstock-pathway

Description

United Airlines

AltAir

HEFA

AltAir production capacity from 2016, 15 million gallons at a 30:70 blend, purchase agreement.

Cathay Pacific

Fulcrum Bioenergy

MSW-FT

Long-term supply agreement for fuel from municipal solid waste, located in Nevada. Supply starting from 375 million gallons (US) over 10 years (2% of Cathay’s annual fuel consumption). Airline strategy to achieve carbon neutral growth by 2020.

Southwest Airlines

Red Rock Biofuels

Forest residues e FT

Supply agreement for 3 million gallons per year.

GOL (Brazil)

UOP/AmyrisTotal

HEFA e inedible corn oil and used cooking oil/. SIPsugarcane

Target blending of 1% of biofuel in 2016, achieved 200 flights with 4% mixture for World Cup (92,000 L of HEFA UOP fuel). SIP achieved first flight in September 2014.

Oslo Airport

Neste Oil

NESTE e NEXBTL

First airport in the world to introduce jet biofuel.

Air France

Amyris-Total

SIP-Sugarcane

1 year series of weekly flights between Toulouse and Paris, using 10% blend, completed in September 2015.

KLM

SkyNRG

HEFA-used cooking oil

18 weekly intercontinental flight from Amsterdam to Aruba, using 20% blend of biofuels made from used cooking oil.

Lufthansa

Neste Oil

HEFA-used cooking oil

Part of Project burnFAIR.

SAS (Norwegian)

SkyNRG Nordic

HEFA-used cooking oil

Airport-based biofuel supply facilities in Norway and Sweden.

Virgin

Lanzatech/ Swedish biofuels

Waste gases e alcohol to Jet

In pipeline to be used.

27

28

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

(a)

(b)

(c)

(d)

Figure 2.6 Experimentation with Kerosene Jet A-1 and various synthetic aviation fuel with a Rolls-Royce combustion rig [30]. Description of a to d can be seen in the paragraph below.

the added boost of energy density [27e29]. Experiments on soot/coke formation were performed, and the results can be seen in Fig. 2.6. Experimentation performed by Pucher et al. [30], in the figure above. (a) is with conventional Kerosene Jet A-1, (b) is with 2% fatty acid methyl esters (FAME) mixture with conventional Kerosene Jet A-1, (c) is with 50% synthetic Kerosene Jet A-1 blended with conventional fuel, and (d) is with 100% synthetic Kerosene Jet A1. These fuels were combusted in a Rolls Royce T-56-A-15 combustion rig to mimic a typical jet engine. As seen, the vast difference is between (a and d), with 100% synthetic Kerosene Jet A-1 there is a vast difference in soot/coke formation at the fuel nozzle and igniter plug, an almost absence of coke formation. However, blended fuel (b and c) still saw significant coke formation, with (c) flaring betterdinjector holes are relatively clear compared to (b and a) [30]. From a CO2 standpoint, jet biofuel production has been compared with conventional Kerosene Jet A-1. In this study [31], vegetable oil using HEFA process was used to compare with Kerosene Jet A-1 (MEROX process). The results can be seen in the graph in Fig. 2.7. Vegetable oil conversion with HEFA can yield between 20% and 73% better CO2eq/MJ compared to Kerosene Jet A-1. However, current HEFA fuel is more expensive to produce compared to Kerosene Jet A-1. A study by Massachusetts Institute of Technology (MIT) used a discounted cash flow rate of return approach (DCFROR)

Biomass valorization for better aviation environmental impact through biocomposites

29

Jet A1 Pure vegetable oil from rape seed Hydrotreated vegetable oil from palm oil* Hydrotreated vegetable oil from palm oil Hydrotreated vegetable oil from sunflower Hydrotreated vegetable oil from rape seed 0

10

20

30

40

50

60

70

80

90

(gCO2eq/MJ)

Figure 2.7 Conventional Jet A-1 production compared with HEFA-based jet biofuel production on CO2 emissions per MJ [31].

[29], HEFA fuel best scenario cost was 2.2 times more expensive than Kerosene Jet A1 (MEROX), with its worst case scenario being up to 2.6 times more expensive, where Kerosene Jet A-1 fuel price is approximately USD 1.82/gal (USD 0.48/L).

2.2

Summary

Biomass valorization is needed for a better aviation environmental footprint and this can be accomplished by biocomposites and jet biofuel. The key to enabling the aviation industry to adopt biomass-derived products is for the biomass to be sustainable and renewable. Sustainability of the feedstock and its abundance is needed to support a growing aviation industry sustainably, while ensuring the safety standards that the industry is known for. First-generation biomass is not known for its sustainability, due to its competition with food. Biocomposites therefore need to utilize secondgeneration sustainable feedstock, however, development of biocomposites is currently limited to its ability to treat and process natural fibers to rival its glass and carbon fiber (fossil-derived) counterparts. The development of biocomposites is much watched by the aviation industry, which sees it as an integral part to “green” the industry’s carbon footprint, and the lightweight properties of composite are needed to meet the industry’s goal of reduction in CO2 emissions. However, biocomposites are still not mature enough to be adopted by the industry, however, hybridization (mixing biocomposites with conventional composites) is seen as a means to fast track the ability to introduce biocomposites into the market for aviation. Lastly, for jet biofuel, the maturity is gaining ground with up to five certified pathways/methods to produce jet biofuel. However, like with biocomposites, the source needs to be sustainable and be sufficiently abundant to supply a very large global demand (at least 6 billion liters annually for global consumption) for jet biofuel. The costs to produce synthetic fuel are still high, as seen with recent studies, but development of the jet biofuel is not at a dead end. Reduction of production price is still hopeful, coupled with the inherent increase in fossil fuels, the price will eventually be competitive. Still, with movements of CORSIA and the aviation industry, the cost of jet biofuel remains high, even in the face of a growing industry, the considerable operational benefits jet biofuel can offer, and the corresponding environmental benefits.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

References [1] Alberici S, Sp€ottle M. Roadmap for a Meta-standard for sustainable jet fuels. Ecofys; 2016 [Internet] Available from: http://www.ecofys.com/. [2] Leahy J. Airbus global market forecast 2010e2029. Airbus; 2010 [Internet] Available from: www.airbus.com. [3] Air Transport Action Group. Aviation benefits beyond borders. ATAG; 2016 [Internet] Available from: http://aviationbenefits.org/media/. [4] Energy Information Administration. International energy outlook 2016. EIA; 2016 [Internet] Available from: http://www.eia.gov/forecasts/. [5] Intergovernmental Panel on Climate Change. Aviation and the global atmosphere 1999, Summary for policymakers. A special report of IPCC Working Groups I and III. IPCC; 1999 [Internet] Available from: https://www.ipcc.ch/pdf/special-reports/spm/av-en.pdf. [6] IATA. The IATA technology roadmap report. 3rd ed. International Air Transport Association; June 2009 [Internet] Available from: https://www.iata.com/reports. [7] IPCC. IPCC climate change 2007: Synthesis report. IPCC; 2017 [Internet] Available from: https://ipcc.ch/pdf/assessment-report/. [8] Lee DS, Fahey DW, Forster PM, Newton PJ, Wit RCN, Lim LL, Owen B, Sausen R. Aviation and global climate change in the 21st century. Atmos Environ 2009;43:3520e7. [9] Lee DS, Pitari G, Grewe V, Gierens K, Penner JE, Petzold A, Prather MJ, Schumann U, Bais A, Berntsen T, Iachetti D, Lim LL, Sausen R. Transport impacts on atmosphere and climate: Aviation. Atmos Environ 2010;44:4678e734. [10] Burkhardt U. Contrail cirrus and their climate impact. In: Proceedings of the Wakenet Workshop, DLR e Institute for atmospheric Physics; 2010 June 10. Germany: DLR; 2010. [11] IATA. IATA technology roadmap [IATA]. 4th ed. IATA; 2013. Available from: https:// www.iata.org/whatwedo/environment/document. [12] World Energy. World bioenergy Association 2016. WBA; 2016 [Internet] Available from: https://www.worldenergy.org/publications/. [13] Araujo K, Mahajan D, Kerr R, Silva M. Global biofuels at the crossroads: an Overview of technical, policy, and investment complexities in the sustainability of biofuel development. MDPI Agric 2017;7:32. [14] Tenenbaum DJ. Food vs fuel: diversion of crops could cause more hunger. Environ Health Prospect 2008;116(6):A254e7. [15] Katiyar R, Kumar A, Gurjar BR. Microalgae based biofuel: challenges and opportunities, biofuels, green energy and technology. Springer; 2017. [16] World Energy Counsel. World energy resources 2016. World Energy; 2016 [Internet] Available from: https://www.worldenergy.org/publications/. [17] Joshi SV, Drzal LT, Mohanty AK, Arora S. Are natural fiber composite environmentally superior to glass fiber reinforced composites. Composites Part A 2004;35:371e6. [18] Gopi S, Balakrishnan P, Sreekala MS, Pius A, Thomas S. Green materials for aerospace industries. Biocom High Perf App 2017:307e18. [19] Zin MH, Razzi MF, Othman S, Liew KE, Abdan K, Mazlan N. A review on fabrication method of bio-sourced hybrid composites for aerospace and automotive application. IOP Conference. Mat Sci Eng;152. [20] Deepak JJR, Arumuga PV, Amuthakkannan P, Arun Prasath KA. Review on biocomposites and bioresin based composites for potential industrial applications. Rev Adv Mater Sci 2017;48:112e21.

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[21] Haris MY, Laila D, Zainudin ES, Mustapha F, Zahari R, Halim Z. Preliminary review of biocomposites materials for aircraft radome application. Key Eng Mat 2011;471e472: 563e7. [22] Davidson C, Newes E, Schwab A, Vimmerstedt L. An Overview of aviation fuel markets for biofuels stakeholder. Technical Report NREL/TP-6A20e60254. National Renewable Energy Laboratory; July 2014 [Internet] Available from: http://www.nrel.gov/docs/. [23] Carbon Market Watch. The CORSIA: ICAO’s market based measure and implications for Europe. Carbon Market Watch; October 2016. Available from: https:// carbonmarketwatch.org. [24] ASTM. ASTM D 7566, standard specification for aviation turbine fuels csynthesized hydrocarbon. 2016. version revision a. [25] version revision a ASTM D 1655 “standard specification for aviation turbine fuels”. 2016. Available from:, https://www.astm.org/. [26] El Takriti S, Pavlenko N, Searle S. Mitigating international aviation emissions, risks and opportunities for alternative jet fuels. The International Council on Clean Transportation; March 2017. [27] Edwards T, Maurice LQ. Surrogate mixtures to represent complex aviation and rocket fuels. J Prop Power 2001;17:461e6. [28] Bisio A. Aircraft fuels e energy, technology and the environment1. John Wiley and Sons Inc.; 1995. p. 257e9. [29] Coordinating Research Council Inc. Handbook of aviation fuel properties. CRC report No. 635. 3rd ed. 2004. [30] Pucher G, Allan W, Poitras P. Emissions from a gas turbine sector rig operated with synthetic aviation and biodiesel fuel. J Eng Gas Turbine Power 2010;133(11). [31] Malina R. HEFA and FT jet fuel cost analyses. MIT; November 2012.

Further reading [1] ATAG. Air Transport Action Group. Fact sheet: delivering fuel efficiency. ATAG; 2016 [Internet] Available from: http://www.atag.org/component/.

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Structural health monitoring of aerospace composites

3

Verma Rahul 1 , Shukla Alokita 1 , Kandasamy Jayakrishna 1 , V.R. Kar 2 , M. Rajesh 1 , S. Thirumalini 3 , M. Manikandan 4 1 School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India; 2Department of Mechanical Engineering, NIT Jamshedpur, Jharkhand, India; 3 School of Civil and Chemical Engineering, Vellore Institute of Technology, Vellore, India; 4 Department of Mechanical Engineering, Amrita College of Engineering and Technology, Nagercoil, Tamil Nadu, India

3.1

Introduction

The effect that the aerospace industry has had on our lives and over the world economy in the last few decades is hard to ignore. Beside the undeniable points of advantages like being quick, affordable and providing the ability to travel to far-off places, there are numerous more ways in which aerospace industry has proven to be a boon in our lives [1e3]. The impact of aeronautic trade on world financial markets is even more articulated [4,5]. The current worldwide market is inconceivable without the presence of aerospace industry. Additionally, innovative work for aviation applications is at the front line on designing accomplishments and numerous new innovations have occurred in different fields. Moreover, recent technical, social, and economic requests have brought about new confrontations for aircraft designers and operators in manufacturing, maintaining, and monitoring of aerospace equipment [6,7]. To bring about the innovations in the aerospace industry, research has shifted toward advanced materials that have potential usage in the aerospace industry with the approach of materials engineering [8,9]. The innovative approach to alter materials provided a whole new range of bringing about alterations in the conventional designs of material as well. This further resulted in better performance of the aircraft and therefore a better future for aviation. Initially aluminum dominated the aerospace industries because of its distinct properties like being lightweight, inexpensive, and easily available [10e12]. Due to these properties, up to 70% of the aircraft was initially made of aluminum. Other materials that were being used were titanium, graphite, fiberglass, etc. but in small amounts of 3%e7%. The aluminum alloys were widely used due to their higher stiffness and strength as compared to pure aluminum metal [10,13e16]. However, the aluminum alloys proved unsatisfactory in the long run as drawbacks like corrosion and material failures such as cracking and separations were discovered [17].

Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00003-4 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

With time, due to technical amelioration the materials used in aerospace industries changed. Presently, most of the components are made of comparatively lighter materials than aluminum and its alloys, such as carbon fiber-reinforced polymers [18e21]. The high-performance fiber-reinforced composites satisfy all the needs that existed since the early decades of aerospace industries like higher operating temperature, modulus of rigidity, high design flexibility and lighter weight in addition, to some recent needs like better radio frequency compatibility and less detectable airframes for defense purposes [22e24]. Thus in recent years fiber-reinforced composites have dominated all major aviation applications [15,21,25]. Composites are polymer matrices that are reinforced with natural or man-made fibers or other engineered reinforcing material. The polymer matrix shields the inner fibers from all external and environmental damages while simultaneously distributing the load equally among the fibers. At the same time, fibers provide stiffness and strength to the reinforcedpolymer matrices for better applicability of composites. The type of matrixefiber interaction decides the properties of the composites. However, the basic fiber strength almost directly affects the tensile strength of the composite [26]. The tensile strength of the composite structure is higher than that of conventional materials like the aluminum alloys. Therefore less maintenance is required for the composite structures than other conventional materials that were used in past [27]. The maintenance necessary for the composite structures is due to the less frequent and extremely complex material failures in them. The failure of conventional metallic structure is a commonly observed phenomenon. The damages in the metallic structure are generally caused due to exhaustion of structures by being subjected to continuous loading and unloading. The fatigue damages in composites are essentially quite complex than in the conventional materials like metals in which damage was mainly in the form of cracks due to tension or compression [25]. The reason for these distinct failure characteristics of the composites is their fibrous nature. The fatigue mechanisms of the composites depend on layer orientation and fiber sequences in the matrix. The failure in these types of matrix materials is visible at the matrixefiber interface, matrix layers interfaces, and eventually in fibers.

3.2

Failures and damages in composites

The state of the structure in which they fail to give adequate output is termed as failure of the composites. It is the failure of the system to function satisfactorily. The failure measurement depends on a number of qualitative and quantitative factors like strength of the structures, structural stiffness of the composite, yield capacity of the composite, bendability available, resistance to corrosion, resistance to impact, resistance toward thunderstorms and lightning, and fatigue due to loading and unloading cycles [28,29]. The failures in composites do not take place in one step; rather, they are multilevel events as depicted in Fig. 3.1. Complete failures are caused due to multiple damages occurring in sequential order. These multiple damages are generally micro-level damages and are often termed “local damages.” This implies that the final rupture of the

Structural health monitoring of aerospace composites

35

Failure in composite

Macro-level failure mechanism

Micro-level failure mechanism

Matrix-level failure mechanism

Fiber-level failure mechanism

Fiber fracture

Fiber buckling

Fiber splitting and radial cracking

Manufact uring defect

Loading generating transverse stresses

Coupled fibermatrix level failure mechanism

Fiber bending Fiber pullout

Matrix cracking

Coupled micromacro failure mechanism

Fiber breakage and interfacial debonding

Transverse matrix cracking

Fiber failure due to matrix cracking

Fiber interfacial cracking

Figure 3.1 Different levels of failure in composites.

matrix is the one that occurs after the gradual development of local ruptures. The phenomenon of this stepwise rupture is termed “damage growth.” The categories for the types of damage mechanisms are: • • •

Micro-level failure mechanisms Macro-level failure mechanisms Coupled micro-macro-level failure mechanisms

3.3

Micro-level failure mechanisms

The micro-level damage mechanisms further have three divisions: fiber-level damage mechanism, matrix-level damage mechanism, and coupled fiber-matrix-level mechanism. The following subsections discuss these mechanisms in detail [30,31].

3.3.1

Fiber-level failure mechanism

The most destructive way for the failure in laminate is the fiber failure mode. This happens because the fiber is considered to be the material that carries load. There are different stress components on the basis of which the failure of fibers can occur. Some major damage mechanisms are fiber fracture, fiber buckling, fiber bending, fiber splitting, and radial cracking.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

3.3.1.1

Fiber fracture

When the value of stress or strain acting symmetrically to the axis in the fiber goes beyond the axial strength of the fiber, it becomes inevitable for the fiber to break down into two or more segments corresponding to its length, which can be visualized in Fig. 3.2. Failure of fiber has more possibilities to take place in fibers that are brittle in nature. Fiber fractures are the most destructive ruptures that can occur in a fiber composite as compared to the other modes of fiber damages. Also, this type of fiber failure is caused due to fracture that tends to occur during shearing. The rupture of fiber composite comes into play when the value of strain exceeds the value of maximum permissible stress [33,34].

3.3.1.2

Fiber buckling

The phenomenon in which a layer of fiber tends to buckle due to intensive load applied on fiber is known as fiber buckling [36]. Generally, when a compressive load is applied in the axial direction of the fiber, buckling of the fiber takes place, as seen in Fig. 3.3. The buckling of fiber is caused mainly due to compressive stress if applied in the direction of its axis. It may also be termed “kinking” of fiber. The value of critical stress for which the buckling is supposed to occur depends on parameters like properties of fiber material, matrix properties, and how the fibers are distributed in the matrix [37]. The fiber buckling or kinking mostly takes place in the area where there are nonsymmetric alignments of fiber or in the area with some local defects. On the basis of observations and experiments, it can be concluded that buckling or kinking of fiber takes

Force

Force

Fracture

Figure 3.2 Fiber fracture [32].

Figure 3.3 Fiber buckling [35].

Structural health monitoring of aerospace composites

37

place in a well-defined area. The buckling region is also known as the “kink band.” The kink band will always remain inclined at a particular angle to the direction of fiber. This mechanism acts as a device that activates other failure mechanisms and can lead to more complicated and catastrophic failures in fibers.

3.3.1.3

Fiber bending

Bending describes the distinctive nature of small elements in a fiber matrix composite under an external load in such a way that the force applied is in a direction perpendicular to the axis, usually passing through the center of gravity of matrix [38]. In bending, the fibers present on the external surface are subjected to a tensile force while the fibers in the inner layer experience compressive force, see Fig. 3.4. As the radius of bending decreases, the strain difference between outer and inner fibers increases. Beyond a certain minimum radius of bending, the tensile strain on outer fiber reaches high value, due to which the outer layer of fiber starts cracking. The radius at which the outer surface of the bent sheet shows cracks is called minimum bend radius. The parameters on which bending of the fiber depends are properties of the fiber and the alignment of fiber through the whole matrix.

3.3.1.4

Fiber splitting and radial cracking

Fiber splitting is the phenomenon that takes place when the value of hoop stress exceeds the maximum value of permissible stress. Similarly, radial cracking of fiber is also caused due to exceeding hoop stresses. Fiber splitting and radial cracking takes place due to hoop stress. A normal stress in the direction parallel to the axis of cylindrical symmetry (Fig. 3.5) is known as “hoop stress” or “circumferential stress.” The radial cracks in fibers are verified to have the fracture stress and are strongly correlated to the type of imperfection triggering crack nucleation.

Bending

Figure 3.4 Fiber bending.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Fiber radial cracking

Matrix

Interface

Fiber radial cracking

Figure 3.5 Fiber radial cracking.

3.3.2

Matrix-level failure mechanisms

Matrix-level damage mechanisms are the damages that tend to take place on the surface as well as inside the surface of the matrix. There are two main damage mechanisms in the matrix: matrix cracking and fiber interfacial cracking. These are explained in the following subsections.

3.3.2.1

Matrix cracking

As the value of stress increases the maximum allowable stress in the matrix, cracking of matrix takes place [40]. In a unidirectional lamina, two types of matrix cracks will develop, in which the crack can be either perpendicular or parallel to the direction of the fibers. For the cracks to be perpendicular, the axial stress developed should be tensile in nature (Fig. 3.6) whereas for the cracks to be parallel to the direction of fiber, the in-plane transverse stress in the lamina should be tensile in nature [41]. The crack developing in the direction perpendicular to the direction of the fiber leads to lowering of the modulus of rigidity to lesser extent than those cracks that develop in the direction parallel to the fibers. The formation of transverse stresses in the plane of the lamina often remain undiscovered, which in turn leads to massive failures in the entire component [42].

3.3.2.2

Fiber interfacial cracking

The feeble interface between matrix and fiber is ruptured when the in-plane transverse stress is tensile in nature. Thus, the failure is initiated in this matrix region. The cracks then sprout along the direction of the length of the fiber [43,44]. The interface between the matrix and the fiber subsequently leads to debonding. This phenomenon is termed “transverse fiber debonding.”

3.3.3

Coupled fiber-matrix-level failure mechanism

Fiber-level damage mechanism [30,31] and matrix-level damage mechanism [45,46] seldom take place individually. Fiber-level damage leads to the matrix-level damage,

Structural health monitoring of aerospace composites

39

Load

Matrix

Nanofibers

Crack opening

Slipping regions

Microfibers

Figure 3.6 Matrix cracking [39].

which are collectively known as coupled fiber-matrix-level damage mechanism. The various fiber-matrix-level damage mechanisms are fiber pullout, fiber breakage and interfacial debonding, transverse matrix cracking, and fiber failure due to matrix cracking [47] and are described in following subsections.

3.3.3.1

Fiber pullout

In a flexible fiber-reinforced polymer composite there is no clear failure throughout its cross-section; instead, patches are created in the polymer, which contains various cracks that clearly represent the failure [48]. In this type, fiber-matrix system permits deflection at the fiber-matrix boundary so as to ease the process of fiber bridging, which is an extremely important source to prevent crack formation leading to major losses in the matrix [49]. When the material system eventually fails, it reflects excessive fiber sliding and pullout taking place inside the patches, as shown in Fig. 3.7. The amount of energy that has evolved during fiber pullout process depends upon the length of that particular part of fiber that has been pulled out and on its internal capacity to carry load [50,51].

3.3.3.2

Fiber breakage and interfacial debonding

When the area joining the two faces of a fiber breaks due to defect near the tip of the broken fiber, it acts as an area where the stress concentration is at its peak. This consequently will lead to failure and breaking of bonds at the interface of the fiber; see Fig. 3.8.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Fiber bridging of crack

Fiber pullout

σ

Fiber hole left

σ

Figure 3.7 Schematic diagram of fiber pullout. Fiber breakage

Interfacial debonding

Figure 3.8 Fiber breakage and debonding.

3.3.3.3

Transverse matrix cracking

The failure between the two faces of the fiber that is responsible for breaking of fiber from the matrix is more likely to act as an area for stress concentration of transverse tensile stress that coexists along the same plane. As the value of stress surpasses the maximum point of limiting stress in the matrix, formation of a thick crack in the matrix takes place.

3.3.3.4

Fiber failure due to matrix cracking

There is a huge possibility that the crack formed in a matrix can easily break off at the interface even at low level of strain; with higher strain values, there can be fiber failure due to exceeding value of stress at the crack tip as compared to the fracture stress of the fibers.

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Delamination

Figure 3.9 Delamination.

3.3.4

Macro-level failure mechanisms

The two adjacent layers in the composites are held together with the help of resin that is present as a fine layer between the composite layers [52]. This thin layer of resin also helps in distributing displacements and stress along the composite layers apart from joining them together. Due to various internal and external factors, when this resin layer weakens, transferring of stress and also distribution of loading are hindered. This hindrance effect further damages the layer of resin, which results in separation of proximate composite layers; see Fig. 3.9. This type of failure is called “delamination.” Macro-level damages in composites are mainly in the form of delamination [53]. Delamination reduces the life of the structure. It is the major failure before which final failure becomes inevitable. The major types of macro-level failures that occur are failures due to manufacturing defects and failures due to loading-generated transverse stresses, which are discussed in detail in the following subsections [54].

3.3.4.1

Manufacturing defects

Delamination is primarily the failure of the thin bonding resin layer between the composite layers [55]. Manufacturing defects are thus the major factors that cause delamination [56]. Irregular and unsystematic arrangement of lamina or mishandling during curing, such as curing at lower temperature or pressure or curing for improper durations, cause the bonding to weaken. Formation of air pockets or inclusions due to other impurities serve as the key factors for the failures.

3.3.4.2

Loading-generated transverse stresses

For macro-level damage the failure of the thin layer of resin is mandatory [57,58]. Transverse stresses are mainly responsible for the failure of this layer [59]. During loading this interface can fail under compressive stresses or tensile stresses [60]. While compressive stresses can cause failure in the direction of the plane through buckling, the failure is most commonly due to tensile load acting perpendicular to the axis of the plane [61].

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

3.3.5

Coupled micro-macro failure mechanism

One of the most significant failure mechanisms among other failure mechanisms is the transverse matrix cracking of the lamina [62,63]. It is the combined result of microlevel and macro-level damage mechanisms. The coupled micro-macro failure mechanism takes place when the transverse crack propagates to magnify damage, or a crack is formed between the adjacent layers, or when partial delamination takes place [64]. The final coupled micro-macro damage is formed in any of three situations. The first situation takes place when the transverse crack, which is a micro failure, passes through the full thickness of the surface and may travel to the adjacent lamina, resulting in breakage or delamination of the composite. The second situation that can arise is one in which the crack initiates at the adjacent interface. Due to the formation of this crack, the stress is saturated in the area, somewhere between the neighboring layers, which further results into the weakening of the interface. This is the major reason for the starting of delamination crack in the interface. Moreover, the development of the delamination is the mechanism that causes the final failure of the laminate. The third possible situation is the one that is marked by the partial delamination caused due to the interfacial cracks produced in the surface through the whole thickness. The delamination of fiber can give a start to this transverse crack in the adjacent layers. The transverse crack then begins to break the interfacial bonds of the further layers and thus results in the complete failure of the laminate. Hence, the delamination and transverse cracking of the lamina are strongly coupled.

3.3.6

Structural health monitoring

Health monitoring is the scientific process of nondestructively identifying the damages and failures in structures. It helps to deduce different types of operational and environmental loads that act on the structure, such as mechanical damage caused by loading, growth of damage as the component operates, and the future performance of the components as damage accumulates. Structural health monitoring is carried out in four main steps: • • • •

Operational evaluation Data accession, fusion and cleansing Feature extraction Statistical modeling for feature discrimination

3.3.7

Operational evaluation

In order to apply the structural health monitoring strategy on a system, operational evaluation is the elementary step. It elaborates the problems and its consequences, which lead to monitor the conditions of various infrastructures. Operation evaluation comprises providing solutions to various questions such as, what type of damage is defined for the structure under monitoring, what environmental and operational

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43

conditions are present under which the system to be monitored is functioning, what are the restrictions of receiving data about the structural health of the system due to its operational conditions, and what are the advantages of the execution of structural health monitoring for the safety of life and property?

3.3.8

Data accession, fusion and cleansing

The data accession or acquisition part of the health monitoring comprises the methodology of the structural health monitoring process [65]. Based on the type of structural health monitoring technique being done, the type of sensors required are decided [66]. The other things taken into account at this stage are the number of sensors used, the positions where sensors are placed, and the data storage hardware employed. Since the conditions under which data is measured are variable, normalization of the collected data plays an important role in describing the failure in the system. Normalization of the data is done when it is difficult to directly interpret the inputs. This process also includes accumulation of the results obtained by various sensors to arrive at a final result that can describe the failure in the system with the least error percentage. Data cleansing is the process that is required further to give more-refined results of the damage detection. While conducting dynamic tests, filtration and decimation are the two most important data cleansing procedures [67].

3.3.9

Feature extraction and information condensation

The most critical part of structural health monitoring is extracting the inferences and developing a conclusion. This process involves discovering the properties that are prone to damage and also investigation of the damage-affected areas. In recent studies, the linear modal properties are the features mainly being used for detecting the damage. The process of fusion of collected data from the various processes and sources, followed by combining the data to prepare a summarized result, is known as “condensation.”

3.3.10 Statistical modal development The statistical model development is the final step of structural health monitoring that has not been explored in the recent studies. This process includes two categories. When the collected data is from damaged and undamaged parts, the statistical modeling comes under general classification also known as supervised learning, like group classification and regression analysis techniques; whereas if the data is received only from damaged structures, then it is known as unsupervised learning.

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3.4

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Techniques used for aerospace composites

A number of research studies have been done to evaluate the results of defects on the strength and life of composite structures apart from evaluating the extent of damage [68,69]. The development of nondestructive testing techniques is always done simultaneously with the evolution of damage detecting techniques. Countless research and studies have been conducted to find the most trustworthy nondestructive examination techniques to detect the areas affected by damage or failure and the characteristic features of the failure in the aerospace composite structures. The most widely used damage detecting methods in the aerospace composites are eddy currents (electromagnetic testing), ultrasonic inspection, optical methods, acoustic emissions, vibration analysis, radiography, thermography, and Lamb waves. All these techniques are concisely explained in the following subsections, with their advantages, disadvantages, and uses to identify the failures in the composite structures.

3.4.1

Visual inspection

Visual inspection can be done with “naked eyes.” For this type of inspection to take place, the surface of material that has to be inspected should be spic-and-span [70]. The surface should be properly scintillated for proper detections. To decrease errors in the inspection results and enhance resolutions, some techniques such as impactsensitive coatings, liquid penetrants, and magnetic particles were developed [71]. This type of detection technique is used only for the area existing near the surface of a material.

3.4.2

Shearography method

Another important method for the monitoring of composites is shearography. This method is speckle-shearing interferometry for the calculation of displacement gradients at the outer most region of the structure [72]. In this type of technique, a coherent point source of light is used to brighten up the object and an image-shearing camera is used for monitoring purposes [73]. In order to evaluate the strains produced in the object, images in stress and unstressed conditions are captured by the camera. These images are then compared with each other. The differences in the images are further examined for determining the effects of stresses and strains during loading and unloading. The strain concentration developed in the region helps in determining any failure present in that region of the structure. The most unique method for measurement of loading arrangements in composite materials is vacuum stressing. As the depth or diameter of the defect increases, the deviations evaluated in the displacement pattern decrease.

3.4.3

Transient thermographic technique

Transient thermographic technique is generally used to obtain the graphical representation of the differential heating in the objects or the thermal images of the object

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45

Flash lamp

Heat conduction

Defect To PC

IR camera Light

IR radiations

Figure 3.10 Schematic diagram of transient thermographic technique [74].

whose monitoring has to be done. This method works on the principle of infrared imaging science; see Fig. 3.10 [75]. The thermographic cameras are sensitive to longinfrared radiation of the electromagnetic spectrum (ranging from 9000 to 14,000 nm). The cameras detect these radiations emitted by various structures to produce images called thermograms, which enables us to see the environment even in the absence of proper visible illumination. Through this camera, warmer and cooler objects are differentiated very easily against the surrounding irrespective of presence or absence of visible light. This technique is best used for homogenous materials through which heat passes uniformly [76]. The defect present in a homogenous material causes high thermal opposition to the passage of heat. As the depth of the defect increases, the probability of finding the exact result will be decreased. As the concentration of the amount of energy incident on the surface is increased, the chances of getting a precise result will also increase. This method of monitoring the structures is very useful for the composite materials having a honeycomb structure. An extensive number of methods exist to monitor the aerospace composite structures by this technique depending on the way that the structures are heated and the way results are interpreted. In order to detect delaminations, corrosion, cracks present in the surface and voids, thermal wave propagation including lock-in thermography and pulse thermography is used [77]. Other techniques such as vibrothermography also came into existence, in which thermal wave propagation is used in combination with elastic wave propagation to further enhance the testing results. The process is considered to be extremely capable of detecting damage in hazardous or inaccessible parts of aerospace structures.

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3.4.4

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Eddy current inspection

Eddy current testing is considered to be one of the most trustworthy techniques for monitoring aerospace composites [78]. This method of inspection works on the principle of the interrelationship between the material under testing and the type of origin of the magnetic field [79]. This interrelationship gives rise to eddy currents on the surface that have to be tested. The alteration in the values of eddy current flow helps to detect and to calculate even minute cracks present in the structure. One of the unique advantages of this technique is the absence of the need to prepare the surface. Most of the other methods are dependent on sensors, which are affected by dirt and oils on the surface and therefore require thorough surface preparation. The eddy current sensors are not sensitive to any such external agents like oil, dirt, dust, or humidity and are therefore considered to be more trustworthy [80]. This method of testing allows the spotting of cracks or failures in an extensive variety of conductive materials both nonferromagnetic as well as ferromagnetic, unlike other nondestructive techniques like magnetic particle method, which is constrained to ferromagnetic metals. Another advantage of the eddy current method of inspection over other methods is that the inspection can be enforced without any firsthand physical contact between the test piece and the eddy current sensor. This method of testing is highly developed in the inspection of materials, specifically in the aerospace industries. The in-depth studies and progress in the instruments, as well as highly precise eddy current sensors developed over the last few decades, have made eddy current testing the most used inspection technique among all other techniques.

3.4.5

Ultrasonic inspection technique

Ultrasonic inspection technique is the inspection method that uses high-energy sound waves in the form of a concentrated pulse produced by a device called a transducer and a pulse receiver, as seen in Fig. 3.11 [81]. The pulses produced should always lie in the range of 1e50 MHz in order to evaluate defects and characteristic parameters of the defects, like dimensions, position, and the structural properties of the defects. One of the most popular C-scan methods can produce very precise measurements of the flaw position and flaw dimensions [82]. An approximate idea of the fault characteristics can be achieved by quantification of the time of flight between the reflections terminating from the front surface and going toward the back surface, passing through the flaws. Ultrasonic inspection consists of another long-used technique for monitoring of structures, that is, the immersion technique in water. The major disadvantage of this method is the requirement of removal of the part of the structure that is supposed to be monitored by immersion in the water.

3.4.6

Vibration-based damage identification technique

The aerospace industry initially started its damage identification techniques based on vibrational properties during the period when manufacturing of the space shuttle was at its peak [83]. Vibration-based detection inspection (VBDI) proved to be extremely

Structural health monitoring of aerospace composites

Transducer (wave exciter)

47

Pulse reciever (wave detector)

V1

V2

Damaged region Lamb waves

Figure 3.11 Ultrasonic inspection method.

helpful in detecting and elucidating the effect of vibrational deviations on a structure to detect the presence of local changes caused by damages. In proper working conditions, all types of machines generate their own typical vibrations. These vibrations are often linked to periodic motions occurring during the operation of machines, like the rotating electric fields, meshing-gear teeth, or shaft rotations. These vibrations are then converted into electric signals with the help of various transducers for analyzing and concluding the failure characteristics. VDBI gives immediate results and therefore can be used for intermittent and permanent monitoring for proper application of Structural Health Monitoring (SHM) principles. The detection identification technique can also be coupled with automatic shutdown mechanisms. The arrangement used in VDBI will prohibit further operations of the machine on detection of any damage to prevent the spreading of damage in the embryonic stage itself, thus saving the components from catastrophic results [84].

3.4.7

Optical inspection method

Optical methods consist of further subdivisions such as holography, photoelasticity, Moire techniques, and various other methods that have the ability to detect damage very effectively and precisely. The damage is characterized based on the change in the intensities of transmission, diffraction properties, and fringe patterns obtained due to interference. In optical method detection of damage, the most commonly used sensors are optical-fiber sensors (Fig. 3.12) and fiber Bragg grating (FBG) sensors. These sensors work on the principle

Optical fiber

Capillary tube

Epoxy bead

Gage length, l

Figure 3.12 Schematic diagram of optical-fiber sensor.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

of evaluation strain and temperature readings. The major disadvantage of FBG sensors is its high operating cost, improper alignment of optical fibers to the structure, and also the connector that decreases the sensitivity of the detection technique. During the last two decades, many studies regarding the correlations between rigidity and fatigue damage changes in composite structures have concluded that the change in the rigidity of the composite structures helps provide critical information about their structural status. Optical-fiber systems are the most used inspection technique in the recent years for monitoring the stiffness of composite structures. Optical-fiber methods include the Fabry-Perot (FP) sensor, optical fiber time domain (OFTD) sensor, and the Bragg Grating (BG) sensor for the inspection of composites. The FP sensors can produce extremely precise results for temperature variations, magnetic fields, mechanical vibrations, and acoustic waves. The FP sensor is a type of interferometric sensor that can compute strain accurately due to its high-resolution quality. However, the data accession is a very complicated process as it comprises a high data sampling frequency and fringe counting. The BG sensor is one of the best sensors for the inspection of composite structure due to the absence of mechanical coupling or splicing and is useful for simultaneous transmission of multiple signals. Thus it is commonly used as an embedded sensor. But, the instruments required for the inspection and the data processing for evaluating the shift in the wavelength is expensive. OFTD sensors are good for distributed sensing technique. However, the instruments required for inspection and a data accession system including high data sampling rate are expensive. The intensity-based optical fiber (IBOF) sensor, however, serves as a contrast to the other optical sensors as it is very cost-effective. Apart from the process and further maintenance, the IBOF has a less-complicated installation process. The cost-effectiveness is achieved as the instruments used to construct this optical sensor are like a photodiode and its white light source is very basic and inexpensive. Since a high data-sampling rate is not required even under high-loading frequency, real-time monitoring can be carried out easily. But this type of intensity-based optical system causes problem in results as it suffers deflections due to bending or twisting of fibers or the connections or by source fluctuations. These systems do not provide a precise result on the real field applications in comparison to the interferometric sensors. However, IBOF sensors are quite durable under fatigue loading.

3.5

Conclusion

Health monitoring of composite structures used in the aerospace have been reviewed and emphasized that strength, stiffness, yield capacity, bendability, and resistance against corrosion, impact, lightning, and fatigue due to cyclic loading influence on durability of composite structure. In this chapter, four different stages involved in structural health monitoring and different techniques have been explained to detect and prevent the failures due to different loading and environmental conditions in the aero structure. Detailed review on fiber buckling, fiber splitting, fiber cracking, fiber

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fracture, and fiber bending, and cracks in the matrix, etc. have been carried out to prevent catastrophic results. Further applicability of different techniques have been discussed, including visual inspection, shearography method, transient thermographic technique, eddy current technique, ultrasonic inspection technique, vibration-based damage identification technique, and optical inspection method.

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[65] Sohn H, Farrar CR, Hemez FM, Czarnecki JJ. A review of structural health review of structural health monitoring literature 1996-2001. Los Alamos National Laboratory; 2002. [66] Lynch JP, Loh KJ. A summary review of wireless sensors and sensor networks for structural health monitoring. Shock Vib Digest 2006;38(2):91e130. [67] Rice JA, Mechitov K, Sim SH, Nagayama T, Jang S, Kim R, Spencer Jr BF, Agha G, Fujino Y. Flexible smart sensor framework for autonomous structural health monitoring. [68] Asp LE, Sj€ogren A, Greenhalgh ES. Delamination growth and thresholds in a carbon/ epoxy composite under fatigue loading. J Compos Technol Res 2001;23(2):55e68. [69] Baker AA. Repair of cracked or defective metallic aircraft components with advanced fibre compositesdan overview of Australian work. Compos Struct 1984;2(2):153e81. [70] Staszewski W, Boller C, Tomlinson GR, editors. Health monitoring of aerospace structures: smart sensor technologies and signal processing. John Wiley & Sons; 2004. [71] McCrea A, Chamberlain D, Navon R. Automated inspection and restoration of steel bridgesda critical review of methods and enabling technologies. Autom Construct 2002; 11(4):351e73. [72] Hung YY. Shearography: a new optical method for strain measurement and nondestructive testing. Opt Eng 1982;21(3). 213391. [73] Dutta S, Pal SK, Mukhopadhyay S, Sen R. Application of digital image processing in tool condition monitoring: a review. CIRP J Manuf Sci Technol 2013;6(3):212e32. [74] Sfarra S, Ibarra-Castanedo C, Lambiase F, Paoletti D, Di Ilio A, Maldague X. From the experimental simulation to integrated non-destructive analysis by means of optical and infrared techniques: results compared. Meas Sci Technol October 1, 2012;23(11):115601. [75] McCullough RW. Transient thermographic technique for NDI of aerospace composite structures. In: Thermosense XXVI, vol. 5405. International Society for Optics and Photonics; 2004. p. 390e403. [76] Luong MP. Fatigue limit evaluation of metals using an infrared thermographic technique. Mech Mater 1998;28(1):155e63. [77] Sakagami T, Kubo S. Applications of pulse heating thermography and lock-in thermography to quantitative nondestructive evaluations. Infrared Phys Technol 2002;43(3): 211e8. [78] Song SJ, Lee HB, Kim YH, Shin YK. Eddy current testing. Korea Institute of Nuclear Safety; 2004. [79] García-Martín J, G
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Shukla Alokita 1 , Verma Rahul 1 , Kandasamy Jayakrishna 1 , V.R. Kar 3 , M. Rajesh 1 , S. Thirumalini 2 , M. Manikandan 4 1 School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India; 2School of Civil and Chemical Engineering, Vellore Institute of Technology, Vellore, India; 3Department of Mechanical Engineering, NIT Jamshedpur, Jharkhand, India; 4 Department of Mechanical Engineering, Amrita College of Engineering and Technology, Nagercoil, Tamil Nadu, India

4.1

Introduction

In recent years, researchers from academic institutes, government agencies, and industries have been concentrating more on structural health monitoring (SHM) in different fields such as civil, marine, mechanical, military, aerospace, power generation, and offshore oil and gas. In engineering, the age, material type, service condition, and layout of the structures influences their performance. Other than performance, safety, reliability, and serviceability of any engineering structure are crucial aspects [1]. Hence, it is important to use technology to monitor the engineering structure by evaluation and assessment [2]. SHM technology is employed for various applications worldwide. For example, long-term monitoring systems have been implemented to monitor large structures in various countries such as Europe [3e5], the United States [6,7], Canada [8,9], Japan [10,11], Korea [12,13], China [14e16], and other countries [17e19]. Development of SHM helps in detecting damage and analyzing strategies, which further helps to increase the service life of engineering structures or components by avoiding their failure [20]. In general, engineering structures fail due to damage in the material and due to certain geometric properties including some boundary conditions of that system, which negatively affect their performance. The main aim of SHM is to alert the system in initial stages of initiation of damage and avoid further propagation of failure with the help of continuous monitoring by structurally integrated sensors. In general, SHM is used to monitor the structure by measuring strain, load, displacement, impact, pH level, moisture, crack width, vibration signatures, and presence of cracks. In SHM process, dynamic responses, extraction of damage-sensitive features, and statistical analysis are used to monitor the structure [21]. Wang et al. [22] analyzed merits and limitations of different gear damage monitoring techniques using vibration measurement. David et al. [23] present a critical review for diagnostics of rotating machinery using acoustic emission technology. Loutas et al. [24] used vibration and acoustic emission recordings method to monitor the condition of gears. They used Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00004-6 Copyright © 2019 Elsevier Ltd. All rights reserved.

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an oil debris monitoring (ODM) system along with vibration and acoustic emission recordings method to increase the diagnostic capacity. Tandon and Choudhury [25] analyzed the vibration and acoustic measurement method to find the damage in the rolling element of bearings. Fadden and Smith [26] also reviewed vibration-based monitoring of rolling element of bearings by high-frequency response method. Staszewski et al. [27] used active and passive approaches to monitor the aerospace composite structure. They estimated the severity in composite plates using 3-D laser vibrometry to locate delamination. Strain waves transmitted from an impact that are applied to the aircraft composite structure were detected using piezoceramic sensors. Damage detection is important to ensure the service life and performance of the structure. In recent years advances in SHM have been taking place at a faster pace. Recent developments in the material field influence the performance of structures. Especially, low-weight materials such as composite materials and alloys are used to replace conventional materials for different engineering applications. Main advantages associated with light-weight composite materials are high strength-to-weight ratio and easy manufacturing. These properties increase the usage of composite materials. Hence, it is important to monitor the damage occurring in the composite material. In the composite structures, strength is based on fiber-matrix interaction, fiber pullout, debonding of fiber-matrix, and crack formation. Similarly, geometry monitoring is also important. Henceforth, it is important to monitor the performance of composite structures continuously for safety. Complying with the current trends, researchers used various nondestructive approaches in this area such as ultrasonic testing, X-rays, vibration/modal analysis, and numerous optical methods such as shearography or holography [28]. Todd et al. [29] analyzed the geometry changes using eight-degree-of-freedom spring-massdamper vibration-based damage detection in a system. Montalvao et al. [30] reviewed vibration-based SHM for composite material for damage detection, localization, and assessment for certain kinds of structures. Kang et al. [31] used carbon nanotube material to form a piezoresistive strain sensor for SHM. Higher van der Waals attraction forces associated with carbon nanotube produces strain measurement. The reason for using strain measurement is that, higher van der Waals attraction force allows axial slipping of the smooth surfaces of the nanotubes. FabryePerot interferometer (EFPI) and fiber Bragg grating (FBG) sensors were employed by Leng and Asundi [32] to monitor the curing process of carbon fiber composite laminates. Results revealed that both embedded EFPI and FBG sensors detect the damage occurring in the composite material due to curing process. Murukeshan et al. [33] used FBG sensors in the curing process of composite material. It was also used to analyze the mechanical changes for 3- and 4-point bending. Kalamkarov et al. [34] measured the strain produced in pultruded carbon fiberereinforced composite rods using optical sensors. Cracks and corrosion occurring in aircrafts have been monitored using piezoelectric wafer active sensors. Lau et al. [35] used optical fiber sensors for SHM of civil infrastructure elements. They carried out experimental investigations to measure the strain of composite-strengthened concrete structures by fixing single- and multiple-point strain measuring techniques. Ciang et al. [36] improved the safety of wind turbines by monitoring the wind turbine system to avoid downtime and to lower the frequency

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of sudden breakdowns. The failure of structures is common due to damage occurring in the materials and geometry. It is very important to analyze the structural response of engineering structures with respect to various factors such as loading conditions, environment, materials, boundary conditions, etc. Hence, it is important to monitor the structure from time to time to avoid failure. In this chapter, recent developments in SHM are discussed for various engineering applications.

4.2

State of the practice in bridge monitoring systems

Long-term structural health monitoring systems are successfully employed in the monitoring of bridges [37]. Each monitoring technique is useful in various fields based on their advantages such as in the monitoring of underground tunnels, aerospace composites, submarines, and bridges. Among all the advantages of SHM systems, one major advantage is the monitoring of megastructures like bridges [37]. Ni YQ et al. [38] reviewed that about 40 bridges with spans of 100 m or longer in the world are being evaluated with SHM systems. Famous examples of such bridges are the Great Belt Bridge in Denmark [3], the Tsing Ma Bridge in Hong Kong [39], the Akashi Kaikyo Bridge in Japan [40], the Commodore Barry Bridge in United States [41], the Seohae Bridge in Korea [42], and the Confederation Bridge in Canada [43]. Long-term monitoring techniques have been implemented at the time of construction on some recently constructed bridges, such as the Shenzhen Western Corridor, the 4th Qianjiang Bridge, the Sutong Bridge, and the Stonecutters Bridge [44]. In the research of KY Wong et al. [21], there was major improvement in the monitoring system of the Stonecutters Bridge when compared to the Tsing Ma Bridge. The improvement includes the addition of advanced sensors such as corrosion sensors, hygrometers, barometers, and pluviometers [37]. There are some specific types of sensors like corrosion sensors, fiber-optic sensors, and strain gauge sensors that can be employed only at the time of manufacturing of the bridge [37]. In recent years, the main focus of monitoring systems was on the monitoring of bridges’ durability, integrity, and reliability [37]. This was clearly visible in the Sutong Bridge. The majority of embedded sensors in the Sutong Bridge are a foundation stability and safety monitoring system, with the intent of bringing longevity to the life span of the bridge and making the bridge more durable [37].

4.3

Factors affecting measurement data

SHM is an effective way to enhance the life span of a structure. In recent years, there have been huge advancements in the technology of SHM systems such as plantation of embedded sensors at the time of manufacturing and the statistical analysis of the structure to determine the present health of the structure [45]. There are numerous benefits of implementation of SHM, such as improving public safety, improving life span of the structures, and reducing effective costs of construction [2]. However, to meet the

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recent needs for proper monitoring of the structures, a number of technical factors and environmental factors need to be considered while conducting normalization and fusion of data [46]. The measurement data given by most of the monitoring techniques is very precise, but there are some factors such as environmental factors, on-site construction defects, and mixing of monitoring techniques that affect the measured data. These factors are discussed in the following subsections.

4.3.1

Environmental factors

The major difference between structural health monitoring and conventional measurement systems is the damage diagnostic and prognostic methods [46]. In the past decades, numerous studies by Ko JM et al. [46] on the methods of structural damage identification have been conducted and stated in detailed reviews [19,47]. The most extensively studied methods are vibration-based damage detection methods [46]. Vibration-based damage detection methods work on the principle of using measured changes in dynamic features to analyze the alterations in physical properties that can lead to structural degradation [46]. There are various environmental conditions such as humidity, solar radiation, wind, and temperature that play a major role in changing the modal parameters, which can further cause irreparable damage to the structure. The assessment results of numerous vibration-based damage detection methods practiced on bridges reflected that the environmental effects were the main reasons for reducing the on-field applicability of modal-based methods [6,48]. For better performance of damage detection methods, it is mandatory to differentiate the anomaly changes in dynamic features caused by structural damage from simple changes due to the variations in environment and operational fluctuations [46]. Numerous investigations concluded that temperature is the major source causing modal variability; the temperature may reach 5%e10% for highway bridges due to the variations in modal frequencies [46]. Ko JM et al. [49], with a one year of data measurement on Ting Kau Bridge, give the effectiveness of different statistical learning algorithms [50] for modeling the effect of temperature on modal frequency. Due to variation, the modal frequency can exceed the tolerance frequency value of a structure, which leads to the failure of structure. With in-depth knowledge of effects of environment on modal properties and including well-defined corrections in the suitable detection method, it is feasible to be able to find the smallest structural damage in the future [51e54].

4.3.2

On-site construction defects

The manufacturing of aerospace and automotive structures is done in a very precise way, whereas civil infrastructures are not prepared with such accuracy. Therefore, there is increased possibility of error in design in civil structures. Exact dimensions are not achieved at the actual construction site as described in the blueprints of projects. These complications create problems for applying monitoring methods on the structures for the analysis of data. Initially, complete analysis of the structure is done with respect to the blueprint of the structure and construction is done accordingly,

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but after the completion of a structure the actual analysis performed by monitoring techniques may incur errors due to the presence of defects like dimensional errors, different composition of the material with respect to the blueprint, and improper installation of embedded sensors. These errors are the causes that produce different measurement data on the monitoring of structures [55,56].

4.3.3

Misinterpretations due to mixing of data by different monitoring techniques

The construction of megastructures is not feasible by a single company. In order to manufacture a megastructure, numerous companies are appointed to handle various departments like the construction management, management for the production of raw material, financial management to control the overall expenditures, and also for the monitoring of structural health. When it comes to the monitoring of huge structures, various specialists are appointed to obtain the most precise results. Different specialists employ different types of monitoring techniques on the same structure. The complexity of structures does not favor each monitoring technique and causes errors in the analyzed data. When different monitoring techniques are applied on a single structure, such as optical-fiber sensors, eddy current sensors, thermography, and others, the final collected data is difficult to interpret and thus leads to improper conclusions. This misinterpretation alters the resultant analysis to a large extent. The final results can seldom help to reach a common conclusion, which can cause overlooking of actual damage, thus making catastrophic damage inevitable [57].

4.4

Benefits of structural health monitoring

Modern human society is marked by a number of structures like bridges, roads, railways, skyscrapers, etc. Any country or state will develop and prosper only if they carefully maintain and monitor these key structures. Maintaining and monitoring the structures will not only prevent economic losses but will also ensure public health and safety. It is evident that if the important structures are not properly monitored and the prescribed guidelines for their maintenance are not met, this leads to catastrophic results. These accidents are a setback to the national as well as world economies and claim huge numbers of lives. In recent times, improved technological support is used to effectively analyze this structural health. With the increasing realization of the importance of structural health monitoring, various automated tools are being developed for the benefit of society. The government is also implementing stringent rules regarding procedures and the duration of the maintenance cycles of massive structures. This decreases capital investments and also reduces the risk due to accidents. These benefits of structural health monitoring are described in the following subsections.

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4.4.1

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Enhanced public safety

One of the major benefits of SHM is enhancement of public safety. The advanced SHM methods use sensors to collect data and then carry out an appropriate analysis. The government ensures public safety by mandating municipalities to employ the monitoring of all crucial structures. When the structures start aging, they start developing many cracks and weak points. The SHM process can help in detecting the cracks and faults in old structures. When the faults are known, the management can either rectify them or isolate the structure for public safety. In the case of new structures, faulty manufacturing techniques can render a number of weak points in the structures. Very often during the design phase of a structure, the operational environmental conditions are not well considered. SHM ensures that the structures are adapted to the operational environment, therefore reducing the risk associated with its usage [58,59].

4.4.2

Early risk detection

The application of SHM helps the engineers to recognize poor structural health and risk associated with the structure in the early stages. This early detection helps in prevention of events like floods, which are caused by damaged dams, pipelines, and dykes. The detection of faults is done by the sensors, which are installed in the system itself. The sensors monitor the change in water level and therefore identify the minor leaks. The engineers can rectify the minor leaks and thus further prevent the collapse of the entire structure. SHM is an effective method to collect geotechnical information about various structures like roads, bridges, buildings, and other such civil structures. If the movement of the ground is detected in time, the failures of structures due to landslides and earthquakes can be prevented to a great extent [2,60].

4.4.3

Improved life spans

Regular structural health monitoring helps in rectification of cracks and failures in their initial stage. This not only improves their efficiency but also enhances their life span. The traditional methods of SHM like the visual inspection technique do not provide such accuracy. This makes optical inspection method undependable for enhancement of the longevity of a structure. The smallest failure can easily be monitored by advanced techniques being used in SHM like the optical method, transient thermographic method, and eddy current method. The smallest cracks that are detected can be easily rectified. The propagation of cracks that will further lead to structural failure is thus postponed [61].

4.4.4

Cost effectiveness

Apart from having various benefits like increased life span of the structures and ensured public safety, implementation of proper SHM also reduces the short-term and long-term expenditures associated with structures. The business industry is

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particularly in favor of proper SHM to increase their overall profits. Detection of small failures and immediate rectification can help in lengthening the maintenance schedules of the entire structure. The fixing of failures in the initial stages helps to avoid major damage to structures and thus saves on expenditures involved in demolition and rebuilding of the entire structure [6,62].

4.5

Challenges for structural health monitoring

SHM is an efficient way to safeguard national property. Although SHM has various advantages, like maintaining only when required, which reduces capital expenditures and increases the life span and improves public safety, there are certain limitations of structural health monitoring. Limitations of SHM are that when neglected this can cause great damage to the structures as well as to public around it. Aktan et al. [63] state that the failure of civil infrastructure systems to perform at their expected level might decrease the national gross domestic product by almost 1%. However, improved structural health monitoring of civil infrastructures can help in improving the performance ratio. Differences in the shapes and sizes of the structures and also the age of the structures influence the SHM technique involved. The differences make it difficult for establishing a standard method for all the structures, which could further save time and efforts. The type of SHM system applied on any structure is based on several factors like shape and size of the structure. The structural health monitoring of a bridge like Tsing Ma Bridge in Hong Kong and a building like Shanghai Tower in the same city will be different. The Tsing Ma Bridge is the longest suspension bridge in the city with the main span of 1377 m, and the Shanghai Tower is 632 m tall. The structural differences in these two buildings thus demand different types of SHM techniques to be used [39]. The Tsing Ma is equipped with a wind and structural health monitoring system. The entire SHM system is comprised of 6 anemometers, 110 strain gauges, 115 temperature sensors, 3 data acquisition outstations, 2 displacement transducers, 19 accelerometers, 10 level sensing stations, 7 weigh-in-motion stations, and 14 GPS rover stations. The Shanghai Tower is monitored by a system comprised of 400 sensors of 11 types like the strain sensors, together with 11 substations [64]. Similarly, an old structure like Steccata church in Parma, Italy, and a new building constructed in the same city with similar environmental conditions will have different types of health monitoring systems involved. The church is monitored with a laser Doppler vibrometer technique, a noncontact detection technique providing data with great reliability and accuracy; in contrast, a recently constructed building will be installed with embedded sensors to measure the data [65,66]. The variations in the structures demand unique monitoring techniques to be employed with each structure. It becomes difficult when there is a lot of construction. Analyzing each structure individually is an enormous task and is prone to defects. Thus standard policies for employment of monitoring methods can save time and capital expenditures as well as reduce the errors involved in collection of measurement data.

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4.6

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Advantages of structural health monitoring

Earlier, nondestructive evaluation (NDE) was used to monitor structures. Disadvantages associated with NDEs are that it is not suitable for bigger structures and requires specialized equipment and skilled labor. In order to improve the monitoring process, researchers developed the SHM technique by integrating sensors and actuators inside of structures. Also, the implementation of SHM reduces the monitoring downtime when compared to NDE. SHM provides quality input and eliminates failures, which helps in unpredictable environments and which accrues a stable level of reliability over its service life. Main advantages associated with implementation of SHM in structures are to be able to avoid the failure and ensure safety of the structure and avoid the uncountable human loses due to accidents. Holmes et al. [67] describe an alternative approach for NDE. The full matrix of time domain signals from every transmitterereceiver pair is captured and postprocessed. Performance of algorithm was matched by measuring its ability to image a point-like reflector. Drinkwater and Wilcox [68] reviewed ultrasonic arrays for nondestructive evaluation. Ultrasonic array increases the inspection quality and reduces inspection time. Meola et al. [69] carried out experimental investigations for NDE of aerospace material with lock-in thermography to determine damage such as delamination, impact damage, and fatigue failure used in aerospace materials such as composites, hybrid composites, sandwiches, and metals. Kinra [70] developed ultrasonic NDE to resolve the problems associated with conventional ultrasonic measurement techniques for thin specimens. They converted time domain signals collected from thin specimens into frequency domain using the fast Fourier transform algorithm. Clark et al. [71] analyzed the damage that occurred in concrete and masonry bridges located in the United States using infrared thermography to the NDT. They experienced problems to deduct the debonding and concrete spalling utilizing solar heating when the temperature is extremely low. Common repair schemes in military and civilian aircrafts made of metallic or composite patches are resolved by adhesively bonding the patches instead of mechanically fastening them. However, it is difficult to identify an adhesively bonded repair because of bonding of batch in the structure, according to Genest et al. [72]. Similarly, during crime investigation detection, identification of body fluids and DNA analysis is a crucial step. The main disadvantage associated with most of the current methods is design to detect a specific body fluid. So, investigators should have emphasized more different types of tests and finalizing the tests for different blood products. The case can be failed in the court because of variation of small deviation in the biological evidence, according to Virkler and Lednev [73]. Rausche [74] summarized most common NDTs used for deep foundations to analyze the intimate contact between pile material and soil, causing dissipation. Pulse echo method, transient response method, vibration method, two accelerometer method, bending wave, cross hole sonic logging, case method, and single hole sonic logging method are commonly used methods to detect this type of failure. Main disadvantages associated with the above-mentioned tests are the limitation of length, requirement of experience for interpretation, and requirement of an exposed section

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of pile. A further drawback for the tests is that the results may be difficult to interpret if sufficient free distance between accelerometers is not available. However, as compared to conventional measuring technique, NDE offers advantages that are important to resolve the problem and enhance the structures stability and durability. Tremendous developments in the electronic, sensor, and nonconventional methods help in the development of SHM. Peairs et al. [75] carried out research on impedance-based structural health monitoring. Surface-bonded piezoelectric transducers used to acquire the excitation signal measure the impedance of structure. This indicates the damage that occurred in the system. FBG sensors were used to ensure the safety and integrity of the civil structure by Majumder et al. [76]. Advantages associated with FBG sensors are light weight, immunity to electromagnetic interference and harsh environment, and ability to be multiplied for distributive measurement. FBG is used to measure the strain measurement under static and dynamic environment. Lynch and Loh [77] analyzed the advantages of wireless sensor and sensor network for SHM. The main advantages associated with wireless sensor technology in SHM are that it is inexpensive to install and extensive wiring is not required between sensors, and there is no further need of a data acquisition system. Raghavan and Cesnik [78] reviewed the guided-wave SHM. They discussed different transducer technologies, including both conventional and nonconventional piezoelectric transducers. Lu and Michaels [79] analyzed the advantages of diffused ultrasonic waves for SHM. Diffused ultrasonic waves offer advantages of simplicity of signal generation and reception, sensitivity to damage, and large area coverage. Experimental results revealed that for a small aluminium plate specimen, a high probability of damage detection can be achieved (over 95%) with a probability of false alarm of approximately 5%. This high probability can be achieved even with temperature variations of more than 30 C. Generally the smallest detectable flaws have dimensions in the range of 1e2 mm. Park et al. [80] summarized software and hardware issues of impedance-based SHM and carried out experimental and theoretical studies of various structures using high-frequency structural excitations. Nagayama and Spencer [81] carried out experimental investigations on damage analysis of civil infrastructure using smart sensors. Results revealed that implementation of smart sensor technique for the monitoring of structures can detect damage effectively. Wang et al. [82] conducted experimental and theoretical investigations on the applicability of time-reversal concept to guided waves in plate-like structures. They achieved temporal and spatial focusing with the help of time reversal concept. Lynch [83] reviewed wireless SHM for civil structures and invented the key design future of wireless sensing. Wireless sensing unit is associated with various factors such as collocation of computational power and precision of integration through computing sensors with self-interrogation of measurement data. The Alamosa Canyon Bridge in New Mexico was employed with wireless sensing units to analyze its performance. Forced vibration test revealed the accuracy and reliability of wireless sensor system in the monitoring of structure. Crack formation and its random propagation reduce the life span of the structure, thus decrementing its performance. Researchers made a number of additional attempts to model the cracks in the beam structure. Local stiffness reduction, discrete spring

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models, and complex models in two or three dimensions are the three major categories to design the cracks in the structure. Friswell and Penny [84] addressed effects of the excitation for breathing cracks and considered the nonlinear effect. Introduction of sensor and wireless technology in SHM improves the efficiency of the damage deduction.

4.7

Advance technology used for structural health monitoring

Construction of massive structures such as bridges, buildings, dams, pipelines, aircraft, ships, etc. is unavoidable. Introducing developments in new fields such as materials, design, manufacturing, and technology must ensure the safety of these structures. The inevitable natural disasters like climate changes, unexpected earthquakes of high magnitude, and severe environmental conditions are major setbacks to the safety of massive structures. Industrial disasters and natural disasters do not take place under similar circumstances. While natural disasters are unfortunate and inevitable events, industrial disasters are results of pure technological and human failure. This was clearly seen in 1984, in the city of Bhopal, India. In the middle of the night when the entire city was asleep, highly poisonous gases were released from the Union Carbide pesticide plant [85]. The accident killed 3000 people initially; updated figures indicate 8000 casualties at the time and a total of 12,000 since [86]. The final catastrophic event, also famous for being the “worst industrial disaster,” could have easily been avoided if the faults in the system were detected by proper structural health monitoring. The event was caused when an uncontrolled chemical reaction led to the release of methyl isocyanate (MIC) gas into the air. The accident was a direct result of a cheap engineering solution to a known maintenance problem. Not only were the prescribed standards overlooked and low-quality materials were used but also the monitoring schedules were lengthened. The lengthening of the monitoring schedule made it difficult to identify and rectify the cracks and dislocations in the plant. The management had also cut short the safety systems and the sensor alarms to save money, which further caused the accident [87,88]. The failure of a system is similar to that of the metal. It begins in the form of small cracks, which spread and grow unless rectified within a certain time. When any weak point is encountered by these cracks, the further propagation is magnified. In the Bhopal plant, the microscopic cracks were developing continuously since there was no proper structural health monitoring employed. Following this, when the reaction in the MIC tank became uncontrollable, this rendered a number of weak points, which magnified the existing propagation [89]. This resulted in the final accident, famously known as the “Bhopal gas tragedy.” One of the many accidents that also lie in the roster of accidents due to poor structural health monitoring is the crash of American Airlines flight 587. On November 12, 2001, the airbus A300 crashed shortly after takeoff. All 260 people died onboard the plane, including 250 passengers and 10 crew members [90]. Before the takeoff of an aircraft, a proper monitoring of the structural health of the aircraft has to be done. But

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due to the negligence of the ground crew members, the importance of proper monitoring was overlooked for quite a long time for this airbus A300. The result of this negligence was seen as the reason of the crash, attributing the problem to the debonding and delamination of carbon fiber present in the composite. This was the final inevitable result of the micro-level damage, like fiber cracking, matrix cracking, fiber buckling, etc., which must have been taking place already for a long time. It was later found on further investigation that before the actual fiber delamination and debonding, small cracks had formed on the surface and it was corroded. The aircraft was made of composites and alloys of aluminum and titanium [91]. The component of the aircraft that undergoes high stress concentration was made of composite and was not supposed to be made of composite. As the flight took off, it directly entered into a high turbulence zone, which caused stress concentration beyond its design limit in the wings as well as in the vertical stabilizer. Investigations by the National Transportation Safety Board suggested that the vertical stabilizer was separated as soon as the airbus A300 took off, causing the aircraft to lose control and crash [92]. Avoidance of such an accident was easily possible with the help of monitoring operations using various sensors and could have helped prevent the world’s second most destructive aircraft crash. Researchers proposed numerous methods to ensure the safety of massive structures under dynamic environments [93]. They introduced various smart sensors such as optical-fiber Bragg grating (OFBG) and polyvinylidene fluoride (PVDF) sensors, equipped with self-sensing mortar and carbon fiberereinforced polymer (CFRP). It also includes a wireless sensor network called stochastic dynamic damage locating vector (SDDLV), which is one of the most promising algorithms to ensure the safety of structures by effective damage detection (Jang et al. [94]). Wireless sensor networking for structural health monitoring was studied in this article. In this work they used stochastic dynamic damage location method to validate the wireless sensor network for the structural health monitoring system. In an experimental setup they used three-dimensional truss structures and lmote2 sensors to acquire the signals for the structures. Additionally, the authors proposed the decentralized damage identification methods to reduce the data transition traffic. Marin et al. [95] used SDDLV approach to monitor the damage using vibration-based damage localization and compared the results with finite element model of a structure for both reference and damaged states. Sim and Spencer [96] proposed a multiscale approach to measure the acceleration and strain measurements of massive bridges and compared the experimental results with numerical methods. Hill [97] introduced OFBG sensors to monitor damage for civil infrastructures. Fig. 4.1 shows the encapsulated OFBG strain sensors developed by Ou and Zhou [98] and sense minimum strain 1e2 mm. Ou et al. [99] developed the FibreReinforced Polymer FRP-OFBG sensor as shown in Fig. 4.2 to measure the strain of concrete.

Figure 4.1 Encapsulated OFBG strain sensors.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Figure 4.2 CFRP-OFBG bars.

Chan and Zhou [100] used FRPeOFeOFBG sensing unit to measure the interface strain. They found that OF-FBG sensing principle along a single fiber is effective to measure the temperature and strain. Zhou et al. [101] analyzed the sensing performance of OFBG for rough civil infrastructure. Results stated that the sensing performance under harsh environment and durability is significantly enhanced due to the FRP materials. They introduced OFBG to monitor the system of Aizhai Bridge located in China. Most of the civil structures fail due to formation of cracks and their random propagation. In order to monitor the formation of cracks and their propagation in civil structures, the sensing speed of sensors used in the civil structure must high. High sensitivity and toughness, compatibility with matrix, area sensing, adaptive to complex surface, high sensitivity coefficient, and fast response associated with PVDF have become popular in SHM to monitor the civil structures’ cracks and strains. Duan et al. [102] used fiber Bragg optic sensors for measurement of strains and PVDF sensors for monitoring the structural local response by measuring cracks and fatigue life gauges for accumulative fatigue damage. Rathod et al. [103] evaluated the quasi-static and high-frequency dynamic strain using large-area PVDF thin films with the help of linear strainevoltage relationship. Similarly, dynamic strain was sensed by comparing the measured signals from PVDF films with the continuous surface electrodes using piezoelectric wafer sensors as a reference. Ren and Lissenden [104] used PVDF multielement Lamb wave sensors to analyze the mode content. They bonded PVDF sensors directly to the wave surface and confirmed that the curved surfaces have low mass, low profile, low cost, and minimal influence on passing Lamb waves. Ke et al. [105] used PVDF nanocomposites to improve the conductivity and piezoresistive sensitivity. Loyola et al. [106] analyzed damage that occurred in fiberreinforced glass composite. They achieved damage detection by monitoring multiwalled thin film carbon nanotube, which altered the electrical conductivity. Hurlebaus and Gaul [107] analyzed the damage diagnostics using piezoelectric films. They used a thin self-sensing actuating layer of PDVF copolymer to identify the location and size of cracks and delamination in composite; see Fig. 4.5. Rosa and Sarasini [108]

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Figure 4.3 Fiber optic sensor.

used piezopolymeric-based in-situ damage detection technique, which is based on acoustic emission with PVDF sensors for different fields, such as automotive, aerospace, and complex civil structures. Wireless ultrasonic SHM system has been developed by Zhao et al. [109] to inspect the aircraft wing. Small and light-weight piezoelectric discs bonded to different parts of the aircraft wing were used to monitor the crack formations and corrosion. Bremer et al. [110] developed a fiber-optic crack sensor and two different fiberoptic moisture sensors to detect the moisture ingress in concrete-based building structures. They developed the fiber-optic crack sensor based on a textile net structure that transfers elongation due to the presence of cracks in the concrete structures. Gianti et al. [111] recorded vibrations in pendulum using vibration sensors coupled with optical fibers. Total reflection of fiber in fiber-optic sensor provides the intensity of vibration due to loss caused by the bending of the fibers. Microbending of fiber is shown in Fig. 4.4. Fiber-optic sensor (Fig. 4.3) can be used to determine the damage in tunnels due to excessive loading caused by corrosion, displacement, mechanical pressure, flooding, and landslides. Thomas and Khatibi [112] analyzed the reduction in the flexural stiffness of carbon/epoxy laminates using piezoelectric wafer active sensors (PWAS) under impact loading, and performance of PWAS was monitored using impedance analysis. Soh et al. [113] used piezoceramic transducer patches to monitor the damage that occurred when the destructive load is applied on a prototype reinforced concrete bridge. Raghavan and Cesnik [114] conducted guided wave testing for SHM by surface-bonded/embedded piezoelectric wafer transducers.

Signal “leaking out” from the fiber Macrobending

Figure 4.4 Microbending scheme.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites Exploded view of delaminated region

Clamped region

50 mm

25 mm

229 mm

Figure 4.5 Schematic of the delamination modeling procedure.

Reliable damage detection is critical for the use of composite resources in different applications. Zhou and Sim [115] reviewed development of in situ fibre-optic damage detection and assessment systems implanted in fiber-reinforced composite structures for detecting damage, its location and also assessing the nature of that damage. Kessler et al. [116] carried out experimental and analytical survey for in situ damage detection in composite materials. They used finite element approach for modal analysis and Lamb wave techniques on the quasi-isotropic graphite/epoxy test specimens containing specific damage to their composite structure. Further, experimental analysis has been carried out to verify the analytical results obtained from finite element model. They concluded that for small amounts of global damage in composite structures, passive modal analysis method was reliable, however, active Lamb method was sensitive for all types of local damage present between the sensor and actuator. Similarly, they used frequency response method to detect the damage occurring in composite material [117]. They compared the changes in the natural frequency and modes of the material using a laser vibrometer with 2-D finite element model. On comparing results, it was concluded that at low frequency models accurately predicted the response but generally fail at a higher frequency. Acousto-ultrasonic waves method has great potential to detect the damage that occurs in metallic and composite structures. Initially, the wave was introduced through the structures, and was used to propagate long distances. The main drawback associated with this method is that it requires a greater number of transducers for monitoring large structures. Mallet et al. [118] used a new scanning technique based on laser vibrometry to detect the damage occurring in aluminium plates and validated them with numerical simulations. Leong et al. [119] used a commercial laser vibrometer designed for vibration/modal analysis, which is used to detect the crack propagation in composite plate.

4.8

Conclusion

Monitoring of structures using recent advances and trends in structural health monitoring has been reviewed and emphasized. In this chapter, benefits of implementation of SHM, such as enhancement of public safety, early risk detection, improvement in the life span of the structure, and decrease in the capital expenditures involved, are discussed. The state of the practice in bridge monitoring systems for some famous bridges

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like the Tsing Ma Bridge, Commodore Barry Bridge, and Great Belt Bridge have been listed. Factors affecting measurement of data influence the overall analysis and thus the final monitoring results are not obtained accurately. Factors like environmental effects, on-site construction defects, and mixing of data by different monitoring techniques significantly alter the measurement data obtained. Further challenges in SHM due to unstandardized policies are also reviewed. Lastly, advantages of SHM have been explained with the help of some catastrophic disasters due to improper implementation of required SHM techniques.

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[111] Gianti MS, Prasetyo E, Wijaya AD, Berliandika S, Marzuki A. Vibration measurement of mathematical pendulum based on macrobending-fiber optic sensor as a model of bridge structural health monitoring. Proc Eng December 31, 2017;170:430e4. [112] Thomas GR, Khatibi AA. Durability of structural health monitoring systems under impact loading. Proc Eng December 31, 2017;188:340e7. [113] Soh CK, Tseng KK, Bhalla S, Gupta A. Performance of smart piezoceramic patches in health monitoring of a RC bridge. Smart Mater Struct August 2000;9(4):533. [114] Raghavan A, Cesnik CE. Finite-dimensional piezoelectric transducer modeling for guided wave based structural health monitoring. Smart Mater Struct November 9, 2005; 14(6):1448. [115] Zhou G, Sim LM. Damage detection and assessment in fibre-reinforced composite structures with embedded fibre optic sensors-review. Smart Mater Struct October 7, 2002; 11(6):925. [116] Kessler SS, Spearing SM, Soutis C. Damage detection in composite materials using Lamb wave methods. Smart Mater Struct April 5, 2002;11(2):269. [117] Kessler SS, Spearing SM, Atalla MJ, Cesnik CE, Soutis C. Damage detection in composite materials using frequency response methods. Compos B Eng January 31, 2002; 33(1):87e95. [118] Mallet L, Lee BC, Staszewski WJ, Scarpa F. Structural health monitoring using scanning laser vibrometry: II. Lamb waves for damage detection. Smart Mater Struct February 4, 2004;13(2):261. [119] Leong WH, Staszewski WJ, Lee BC, Scarpa F. Structural health monitoring using scanning laser vibrometry: III. Lamb waves for fatigue crack detection. Smart Mater Struct October 31, 2005;14(6):1387.

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Structural health monitoring of fiber polymer composites

5

Kandasamy Jayakrishna, G. Rajiyalakshmi, A. Deepa School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India

5.1

Introduction

An examination assessed the harm advancement conduct by considering the impact of the material structure and water retention [1]. Harm perception was led by the coordination of nonruinous and coordinate perception techniques. Applicant material fortifications were T300-3k plain woven texture (PW) and T700S-12k multihub sewn texture (MA) [2]. The impacts of water assimilation on the exhibitions of pressure after effect (CAI) and post impact fatigue (PIF) were little in PW carbon-fiber strengthened epoxy framework composite (CFRP) covers. Alternately, PIF properties of waterassimilated MA CFRP overlays definitely diminished than those of dry ones [3]. CAI quality was not influenced by water retention. PIF execution of dry MA CFRP was genuinely higher than that of the others. From the exact perception, a few confirmations of interfacial disintegration caused by water assimilation were affirmed in both PW and MA CFRP covers. Another study analyzed the impact of fullerene scattering on the mechanical properties of CFRPs [4]. Mechanical properties, for example, strain, pressure, open-gap pressure, CAI, authoritative, short shaft shear, and interlaminar break durability were assessed. Strain and pressure qualities expanded 2%e12% by scattering 0.5% of fullerene into the grid gum. Moreover, interlaminar crack durability of the composite was enhanced by around 60% [5]. It was found that a small measure of fullerene (0.1e1 wt%) expanded the disappointment strain of epoxy pitch itself, in this manner enhancing the CFRP quality. Elgabbas et al., directed experimentaleexpository examination on the basic conduct of precast prestressed empty center reinforced concrete (RC) pieces fortified in flexure by CFRP covers [5a,6]. Remotely reinforced and close surface mounted (NSM) covers were utilized. The CFRP territory and utilizing transverse safe haven were additionally explored. Results showed that NSM system brought about ideal reinforcing productivity [7]. The expanded bond quality likewise brought about full actuation of the NSM covers at disappointment. In any case, the NSM flexural reinforcing level ought to be precisely intended to stay away from horrible shear-pressure disappointment mode [8,9]. Direct productivity was related with the remotely fortified method due to the untimely debonding. In any case, this proficiency was streamlined by utilizing transverse CFRP covers as port, which recoordinated deholding further from the covers’ finishes and postponed failure, however, at

Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00005-8 Copyright © 2019 Elsevier Ltd. All rights reserved.

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much lower deformations than those of the control piece [10]. A similar expository investigation was led by Reaza et al. utilizing the harmony/similarity strategy [11], which is a semi-logical postpreparation technique for calculation of up-to-this-point inaccessible through-thickness variety of interlaminar (transverse) shear worries in the region of the bilayer interface circumferential re-contestant corner line of an inside part-through round tube-shaped opening debilitating an edge-stacked covered composite plate [12]. A co-sort triangular composite plate component, in view of the presumptions of transverse inextensibility and layer-wise steady shear-edge hypothesis (LCST), is used to first process the in-plane anxieties and furthermore, layer-wise through-thickness normal interlaminar shear stresses, which fill in as the beginning stage for calculation of through-thickness dissemination of interlaminar shear worries in the region of the bilayer interface circumferential re-participant corner line of the part through opening [13]. Similar burdens registered by the regular balance strategy (EM) are, interestingly, in genuine blunder within the sight of the bilayer interface circumferential reentrant corner line peculiarity emerging out of the inner partthrough opening, and are found to damage the interfacial similarity condition [14,15]. The registered interlaminar shear stress can fluctuate from negative to positive through the thickness of a cross-employ plate in the area of this sort of stress peculiarity. Another examination [16,17] clarified about the auxiliary joining basic for the improvement of composite aviation structures: each basic interconnection implies an aggravation of an enhanced structure bringing about an expansion in general basic weight. The lightweight capability of cutting edge [18e20], elite fiber composite materials is influenced more unequivocally by mechanical attaching strategies than by customary metallic materials because of the low shear and bearing capacities of CFRP materials. Nearby inserting of thin titanium layers into the composite cover in the coupling locale brings about an extensive change in auxiliary productivity of blasted and bolted joints in CFRP structures [21,22]. This change isn’t just evident in the expansion in shear and bearing abilities, yet in addition in the subsequent conceivable outcomes for an outline never again troubled by neighborhood material thickening, unconventionalities and moreover energized nearby bending stresses. This report [23,24] shows trial comes about exhibiting the worthwhile impact of titanium hybridization on particular attributes of CFRP-materials, accordingly demonstrating the mechanical capability of CFRP/titanium half and half materials when utilized as a propelled fortification procedure for exceptionally stacked composite joints [25]. Alaattin Aktas examined [26] the impact of ocean water on the bearing quality conduct of the woven glass fiber composite. The proportion of the edge separation to the stick distance across, and the proportion of the example width to the stick diameter (w/d), were symmetrically shifted amid tests [27,28]. So as to replicate genuine ecological conditions, the example was attached to a ship making voyage in the Sea of Marmara having 2.2% of salt and 22e26
C of surface temperature, with a stainless wire to keep the example in the ocean. The example was kept in the ocean 1, 2, and 4 months. The examinations were done by the ASTM D953. The outcomes demonstrate that the bearing esteems gotten from the example kept in the ocean for 4 months diminished impressively for 1 and 2 months. Be that as it may, the disappointment

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modes for all setups are a similar mode [29]. Another investigation worked for the decrease of the hygroscopic worries at the edges of the half breed composite for the symmetrical and consistent ecological conditions in this paper. We noted amid our pursuit that the hygroscopic anxieties ascertained by the diagnostic strategy are most extreme at both the edges of the overlaid composite, specifically at the principal times of the dampness dispersion [30,31]. Along these lines, a plausible corruption of the overlaid plates can be delivered at the two edges. B. Pradhan and S. K. Panda [32] have given their examination of handle layup and the collaboration of lingering warm burdens and mechanical stacking [33] on the interlaminar lopsided implanted delamination break development conduct in this paper. Two arrangements of full three-dimensional thermoflexible limited component investigations have been performed for the interlaminar circular delaminations, which might be because of assembling abandons or different reasons and are found symmetrically as for the midplane in a semi-isotropic FRP composite overlay layup [34,35]. Contingent on the through-the-thickness area of the implanted curved delaminations, four diverse overlay setups have been considered. Strain vitality discharge rate (SERR) strategies have been utilized to survey the delamination break development qualities at the interfaces [36]. It was discovered that the individual break modes display lopsided and nonself-comparative split development conduct along the delamination front contingent on the area of the interfacial delaminations [37,38]; they employed succession and introduction and thermoflexible anisotropy of the cover. Botelho et al. [24] contemplated that the natural factors, for example, stickiness and temperature, can constrain the uses of composites by diminishing the mechanical properties over some undefined time frame [39,40]. Ecological variables play a vital part amid the construction step and amid the composite’s life cycle. The corruption of composites because of natural impacts is essentially caused by compounds as well as physical harms in the polymer grid, loss of grip at the fiber/lattice interface, and, additionally, diminishment of fiber quality and solidness. Composite’s corruption can be measured by shear tests since shear disappointment is a network overwhelmed property [2].

5.2

Macroscopic behavior of fiber reinforced polymer

Fiber reinforced polymer (FRP) composites are utilized as a part of a wide assortment of applications [41]. Their mechanical properties give extraordinary advantages to the item they are formed into. FRP composite materials have unrivalled mechanical properties including impact resistance, strength, stiffness, flexibility, and ability to carry loads [42]. When planning items out of FRP materials, engineers utilize complex composite material, and programming that ascertains the known properties of the given composite. Ordinary tests used to gauge the mechanical properties of FRP composites include shear stiffness, tensile, and flexible modulus [43]. The two noteworthy parts of an FRP composite material are resin and reinforcement. A cured thermosetting tar with no support is glass-like in nature and appearance,

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however, frequently exceptionally weak [44]. By including a fortifying fiber, for example, carbon fiber, glass, or aramid, the properties are tremendously improved. Also, with strengthening fiber, a composite can have anisotropic properties, meaning that the composite can be designed to have distinctive properties in various ways relying upon the introduction of the fiber support. Aluminum, steel, and different metals have isotropic properties, which means, level with quality every which way [45]. A composite material, with anisotropic properties, can provide extra support toward stresses, and this can make more productive structures at lighter weights. For instance, a pultruded bar having all-fiberglass fortification in a similar parallel way could have rigidity upwards of 150,000 PSI. Though a bar with a similar territory of irregular cleaved fiber would just have rigidity around 15,000 PSI [46]. Another distinction between FRP composites and metals is the response to affect. At the point when metals get affect, they can yield or dent, while FRP composites have no yield point and won’t dent [47]. The following sections discuss the macroscopic properties of the following materials, which includes GFRP under ambient conditions, GFRP exposed to hygrothermal condition in 55
C water baths, GFRP on exposure to moisture, heat (55
C) and alkali (NaOH). The macroscopic analysis that was carried out includes visual observation, tensile testing, fatigue testing, bending or flexural test. Visual observation refers to the visible damages that have resulted in the samples as seen from the naked eyes. It is just a qualitative approach without any weightage to technical inspection. Tensile test, on the other hand, focuses on the ultimate tensile stress of the sample, subjecting it to extension and compression and calculating the stress-and-strain relationship. The resultant graphs are plotted and we are able to infer the maximum amount of force with which the sample can be pushed or pulled without the risk of failure or damage [48]. Fatigue testing is characterized as the procedure of progressive localized permanent structural change happening in a material subjected to conditions that deliver fluctuating stresses and strains at some point that may finish in breaks or cracks after an adequate number of iterations. Generally the material is subject to continuously variable load during its time of operation causing it to fail even before the ultimate tensile limit (UTL) is reached. The endurance limit gives us the measure of such a load [49]. Three-point bending test is performed on the samples so as to get their ultimate flexural load, which is the maximum transverse that the sample can tolerate without permanent damage. The above parameters are the macrostructural properties that will be discussed in the following sections.

5.2.1

Visual observation of failure samples

The visual observations for the samples are limited to our eyes. The following section describes the visual appearance of the sample after their respective treatment.

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The initial samples showed a layer of GFRP that was bonded to their respective additions like epoxy or thermocol. The process of orientation of the fibers can be vaguely deciphered on observation and also on touching the samples. The samples show distinct, well-defined edges and are cut according to the defined ASTM regulations. Next, the visually observable failure effects of the samples due to tensile and the flexural test will be investigated.

5.2.1.1

Hygrothermal conditioned GFRP samples

The first picture, Fig. 5.1(a), shows the effect of tensile test on the GFRP laminates. The broom failure can be clearly visible due to the effect of direct tension, while Fig. 5.1(b) shows the shear failure caused due to bending of the same specimen. The first specimen or the broom failure shows the fibers exposed as a part of the tensile load of failure of the specimen. The resultant dismemberment can be ascribed to the excessive tensile loading, which has caused the interbonding of the GFRP epoxy to break down and result in the exposure of the individual strands, as seen from the images. In the flexural test, the resultant failure is that of shear failure. It is evident that due to bending the layer-by-layer separation has taken place. The shearing of the fibers depicts a separation in the layer of the lamination of the samples and hence results in the separation or the failure. Each type of failure is the characteristic of the specific mechanical damages that are the tensile and the flexural tests.

(a)

Broom failure

(b) Shear failure

Figure 5.1 Hygrothermally treated GFRP samples after (a) tensile test and (b) bending test.

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5.2.1.2

Chemically treated GFRP samples

After the treatment of the GFRP samples with NaOH over a period of 2 months, Fig. 5.2 shows the damaged samples after they have been subjected to the tensile test. There was heavy scale formation on the samples. The condition of aqueous NaOH immersed samples was very bad. The samples had lost their flexibility and we can easily break the samples by bending with hands, like breaking a chip wafer. The epoxy had gone very brittle and the top surface of some samples showed chipping out of the top surface of the epoxy coating. Further concerning the tensile and the flexural treatment of the specimens, they displayed abrupt failure that could be ascribed to an increase in strength of the material as well as its brittle nature. The main cause is evidently due to the degradation effect of the NaOH bath coupled with the temperature exposure that led to a high degree of internal damage to the fibers.

5.2.1.3

GFRP-thermocol sandwich composite

After the treatment of the GFRP-thermocol hygrothermally over a period of 2 months, Fig. 5.3 shows the condition of the samples after the bending test. The crushing of the core is easily visible. It can be inferred that due to the excessive load of the flexural test, at failure certain separation of the layers can be visible. The thermocol layer has reported some amount of detachment from the GFRP layers. This can be attributed to the shear stress experienced by the samples due to the flexural loading.

5.2.2 5.2.2.1

Mechanical performance of FRP composite Prestressed loads of GFRP composite

Fig. 5.4(a) and (b) shows the stress strain diagram of a GFRP sample when it is taken and tested in the ambient condition. The values obtained from the graph can be treated as the reference for calculating the deviation for subsequent treatment. Each line depicts the amount of prestressing that has been done to the sample before testing. Figure 5.2 Chemically treated GFRP samples. Shows abrupt failure

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Core crushing

Figure 5.3 Hygrothermally treated GFRP-thermocol samples.

The effect of prestressing is to replicate an aging process and account for the damages that may be received by the material over the course of time. The green line shows the initial graph without any prestressing, while the subsequent red, blue ,and pink lines show the effect of prestressing at 70%, 50%, and 30% of the initial loads, respectively. Now we shall move on to what happens after hydrothermal treatment to the sample after a period of 2 months.

5.2.2.2

GFRP under ambient conditions

The flexural strength of GFRP laminates exposed to room temperature, subjected to 30% ultimate flexural load and without load, was almost the same after 2 months, whereas the flexural strength of other specimens decreased with time. The flexural modulus of GFRP specimen exposed to natural conditions subjected to without load decreased up to 1 month and increased after 1 month. The flexural strength of CFRP specimen exposed to natural conditions decreased with time, and the trend was the same for all specimens. The tensile strength of GFRP specimens subjected to 70% ultimate tensile load decreased and tensile strength of GFRP specimens subjected to 50% ultimate tensile load and 30% ultimate tensile load was almost the same up to 1 month and then decreased after 1 month.

(a)

(b)

80

Initial peak load of GFRP 30% loading of peak load of GFRP 50% loading of peak load of GFRP 70% loading of peak load of GFRP

20000

70 60

40 30 20 10

Initial peak load of GFRP 70% of initial peak load of GFRP 50% of initial peak load of GFRP 30% of initial peak load of GFRP

Force(N)

Force(N)

15000 50

10000

5000

0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Deformation(mm)

0 0

2

4

6

8

10

Displacement(mm)

Figure 5.4 Initial Graph of GFRP samples, (a) bending test and (b) tensile test.

12

14

16

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5.2.2.3

GFRP under hygrothermal treatment

Fig. 5.5(a) and (b) show the effect of hygrothermal treatment on the flexural strength (Fig. 5.5 (a) and (b)) and flexural modulus of the GFRP samples over a period of 2 months. These graphs are from the two-point bending tests on the hygrothermally exposed GFRP laminates. An average decrease in the values is seen in both cases, however, the slope of the decrease is seen to gradually reduce in the span of time. This can be attributed to the fact that on absorption of water the epoxy resin or the binders have hardened enough to promote better strength. It is expected that due to the absorption of water from its environment, the epoxy resin has filled up the vacant spaces in the cracks and crevices of the GRFP, which were caused due to the prestressing. Similar is the case for the tensile tests as seen from the following diagrams of tensile strength and tensile modulus.

(a)

(b)

450

900

420

850

405

800

Tensile modulus(MPa)

Tensile strength(MPa)

435

390 375 360 345 330 315 300 285 270

GFRP 30% loading GFRP 50% loading GFRP 70% loading

750 700 650 600 550 500

GFRP 30% loading GFRP 50% loading GFRP 70% loading

255 Initial

Month1

Initial

Month2

(c)

150

Flexural modulus(MPa)

Flexural strength(MPa)

165

135 120 105 90 75

45

Month2

(d)

180

60

Month1 Time period

Time period

GFRP 30% loading GFRP 50% loading GFRP 70% loading GFRP without load

Initial

Month1 Time period

Month2

21000 20000 19000 18000 17000 16000 15000 14000 13000 12000 11000 11000 9000 8000 7000

GFRP 30% loading GFRP 50% loading GFRP 70% loading GFRP without load

Initial

Month1

Month2

Time period

Figure 5.5 Graph of GFRP sample showing (a) flexural strength, (b) flexural modulus, (c) tensile strength, and (d) tensile modulus after hygrothermal treatment for 2 months.

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The peak load of both GFRP and CFRP specimens decreased significantly by the second month. The tensile strength and tensile modulus of the specimens showed a decrease. The tensile strength decreased with time. The tensile strength at 70% UTL GFRP decreased after 1 month, whereas for 30%, after the initial decrease at the end of the month, the tensile strength remained almost constant. In the CFRP specimen, tensile strength reduced significantly throughout. The trends in terms of change in ultimate tensile strength under natural conditions also show similar results. The tensile strength appears to be nearly the same after 1 month and 2 months with few exceptions. There is a noticeable change in the magnitude of reduction after 1 and 2 months. The reduction after 1 month is 10%e20% and that after 2 months is 25%e45%. The continuous degradation caused due to hygrothermal loading is the reason behind a reduction in tensile strength over the period in consideration.

5.2.2.4

Effect of GFRP due to chemical treatment

The graphs in Fig. 5.6 depict the effect of an alkali, namely NaOH, moisture, and heat at 55
C after exposure to the GFRP samples for a period of 2 months. Fig. 5.6(a) shows the effect of the alkali after a prestressing of 20%, whereas Fig. 5.6(b) shows the stressestrain plot of the sample under the same conditions after a prestressing of 40%. We can see that there is marginal decrease in the ultimate tensile strength of the samples immersed in water for 1 month compared to that of initial value. Further, after 2 months considerable decrease in tensile strength can be seen easily. But considering the case of samples in NaOH aqueous solution, there is very large reduction in the tensile strength with respect to initial value of the tensile strength. This trend is seen in both the NaOH tanks, which exhibit little change when compared to each other. The decrease in tensile strength in 55
C tank is more than the tank, which is at lower temperature of 45
C, especially in the first months.

(a)

(b)

Figure 5.6 Graph of GFRP sample showing stressestrain relation after 2 months of chemical exposure after (a) 20% and (b) 40% prestressing.

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5.2.2.5

Effect on GFRP-thermocol composites

The drop in maximum flexure load is increasing with increase in bending preloads in both core thickness specimens. The graphs in Fig. 5.7(a) and (b) are showing that the decrease in maximum force in 70% bending preload in both the core thickness specimens is much higher than the other bending preloaded specimen. The drop in maximum flexure load is increasing with respect to time period. These graphs show that the decrease in maximum flexure load for each bending preload specimen of each core thickness is more in 2 months as compared to 1 month. Average drop in force in 8-mm core after 1 and 2 months is 5 and 8.25N, respectively, whereas average drop in force for 16-mm core after 1 and 2 months is 13.33 and 19.33N, respectively. It was observed from the graph in Fig. 5.7 that decrease in strength in 70% bending preload specimen was greater compared to 30% and 50% bending preload specimen in both core thicknesses. Flexural strength decreased due to decrease in maximum applied force. Average decrease in flexural strength in 70% loading in 8-mm core thickness was 16 MPa, whereas in 50% and 30% bending preload it was 11 and 3.35 MPa. In 16-mm core thickness, average decrease in flexural strength for 70% loading was 33.5N, whereas for 50% and 30% bending preload it was 23.23 and 15.86 MPa. It was also observed that degradation in flexural strength was less in without-loaded specimen as compared to bending preloaded specimen. The drop in strength was increasing with increase in bending preloads in 8-m core specimen, whereas in 16-mm core drop was similar. It was further observed that the flexural stress in both the core thicknesses and each bending preload specimen decreases with increasing time. Decrease in flexure strength after 2 months of exposure is greater compared to 1 month and before exposure values.

3.0 Before exposure 1 month 2 month

Flexural strength (MPa)

2.5 2.0 1.5 1.0 0.5 0.0 30% loading

50% loading

70% loading

Samples at various load

Figure 5.7 Flexural strength curves of GFRP-thermocol samples.

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5.3

85

Microscopic results

Microscopic results are the images of the particles that cannot be seen with our naked eyes. For the microscopic analysis we have presented the SEM Images of the samples in the same order as the macroscopic images.

5.3.1

Initial SEM images of GFRP

Fig. 5.8(a) shows the transverse section of the GFRP-laminated samples while Fig. 5.8(b) shows the SEM image of the transverse section of the GPRP samples. The pictures clearly show the undamaged fibers and the epoxy that surround the fibers and maintain circularity.

5.3.2

SEM images of GFRP under hygrothermal treatment

The SEM images after 2 months of hygrothermal treatment display fiber degradation and flaking of the epoxy in case of GFRP. This is ascribed to the hardening of the resin due to water absorption. This is displayed in Fig. 5.9. Protruding damaged fibers can also be seen.

5.3.3

Effect of GFRP under chemical treatment

The SEM image in Fig. 5.10(a) shows the effect of the exposure of the GFRP samples to NaOH for a period of 2 months after an initial prestress of 20% UTL, whereas Fig. 5.10(b) shows the same sample in the same composition with an initial prestress of 40%.

5.4

Apparent moisture diffusivity

The mass flow rate of the moisture in the specimen with respect to time is known as moisture diffusivity.

Figure 5.8 SEM images of GFRP showing (a) transverse section and (b) longitudinal section.

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Figure 5.9 SEM images of GFRP after 2 months of hygrothermal treatment.

Flakes

Figure 5.10 SEM images of GFRP after 2 months of chemical treatment of (a) 20% and (b) 40% prestressed samples.

Moisture can break down a substance from the interior without any apparent surface defect. Thus it is very important to compute the moisture content and the diffusivity of the samples to know the effect of the treatment in the interior due to the absorption of moisture. The apparent moisture diffusivity can be calculated using the following formula:


h D¼p 4Mm

2


 
M2  M1 h h 2 pffiffiffiffi pffiffiffiffi 1 þ þ Le W t2  t1

where, Le, length of the specimen; W, width of the specimen; M1, moisture content at time t1; M2, moisture content at time t2; Mm, %weight gain.

5.4.1

Hygrothermal treatment of GFRP samples

GFRP with hygrothermal treatment was tested for its moisture content according to the nature of prestress and the duration of treatment.

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Fig. 5.11 shows the diffusivity of the different GFRP samples over the period of 50 days graded according to their prestress loads. The CFRP specimen subjected to 70% ultimate flexural load shows sudden increase in percentage weight gain after 6 days, whereas CFRP specimen subjected to 30% ultimate flexural load shows sudden decrease in percentage weight gain after 40 days. The percentage weight gain of GFRP specimen subjected to 70% ultimate flexural load has increased after 6 days, and percentage weight gain of GFRP specimen subjected to 30% ultimate flexural load, 50% ultimate flexural load increased after 30 days. The moisture diffusivity of GFRP specimen and CFRP specimen decreased with time. The percentage weight gain for CFRP specimen subjected to 30% ultimate tensile load was constant after 9 days. The CFRP specimen subjected to 70% ultimate tensile load has decreased in percentage weight gain after 9 days. The CFRP specimen subjected to 30% ultimate tensile load shows constant percentage weight gain up to 30 days and sudden increase after 30 days. Moisture diffusivity of GFRP specimen decreases relatively with time for all specimens, whereas for CFRP specimen decrease is almost the same up to 1 month, and after 1 month moisture diffusivity of CFRP specimen has decreased.

5.4.2

On chemical exposure

The following data (Fig. 5.12) has been received when the similar GFRP samples were exposed to NaOH solution over a period of 2 months. The different amounts of prestressing lead to different amounts of moisture absorption, yet the change in weight is approximately the same. The percentage weight gain (of moisture) was compared for various tanks. It was easily noticed that in water tanks the percentage weight gain increases with respect to time, indicating that more moisture is absorbed after 2 months compared to the first month in almost all the samples with few exceptions. There was marginal increase in weight gain of samples at 55
C temperature as compared to samples at 45
C. In the Figure 5.11 Moisture content of hygrothermally treated GFRP laminate samples.

0.14 0.12

GFRP 30% loading GFRP without load GFRP 70% loading GFRP 50% loading

Diffusivity

0.10 0.08 0.06 0.04 0.02 0.00 0

10

20

30

Time period (days)

40

50

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Apparent diffusivity (mm2/s)

88

2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00

30% loading 50% loading 70% loading Without loading

0

5

10

15

20

25 30 35 Time (days)

40

45

50

55

Figure 5.12 Moisture content of chemically treated GFRP laminate samples as a plot.

NaOH tanks, the percentage weight gain was also higher in second month and also it was higher with respect to water tanks in both the months. The average weight gain in water tanks was 4.73% and 4.54% in Tanks T1 and T4, respectively. But in NaOH tanks, average values were 14.38% and 16.67% in Tanks T2 and T3, respectively. The reason for increase in percentage weight gain is obvious in both tanks as with time the pores of epoxy will loosen due to given hygrothermal load giving way to moisture. But in case of NaOH tanks, the weight gain was higher due to chemical attack on the fiber periphery as seen in SEM images, creating more voids to hold moisture.

5.4.3

Effect on GFRP-thermocol composites

Moisture diffusivity depends upon the mass flow rate of moisture in specimen with respect to time. To better understand results, one specimen of each core thickness and each loading was taken and kept in water at 45
C, and the response of weight gain and moisture diffusivity of each specimen every 3 alternative days was observed (Fig. 5.13). Bending preload does not affect further drop in apparent moisture diffusivity with respect to time. Change in moisture diffusivity is almost the same in all bending preload specimens of both core thicknesses. Graphs show that moisture diffusivity is decreasing with respect to time in both the core thickness specimen and it is almost

% Gain in weight

10.00% 8.00% 6.00% 4.00% 2.00% 0.00% 0%

10%

20%

30%

40%

50%

60%

% UFL

Figure 5.13 Moisture content of GFRP-thermocol laminates.

70%

80%

90%

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constant after 30 days. Decrease in diffusivity is decreasing as time passes. It is relatively high during 3 to 6 days. The reason for decrease in moisture diffusivity is due to increase in percentage weight gain with time.

5.5

Conclusion

Composites are one of the materials that hold a lot of promise in the future. We can see how efficiently they can withstand temperature and moisture contact over 2 months and still produce favorable results. Moreover, there are enormous numbers of composite combinations available besides those that we have discussed. Glass fiber is one of the most commonly used and cheapest available composites, and just by tweaking minor parameters we can bring about remarkable changes. This study can be extended to the analysis of increasing loads applied for cyclic loading. There is lot of scope of improvement in finite element modeling to simulate the results with more accuracy and to study effects of various parameters with respect to time. By changing the fiber orientation, the change in results with respect to current experimentation can be compared.

References [1] Deepa A, Padmanabhan K, Kuppan P. Impact on structural and mechanical properties of composites during machining and cutting: a review. [2] Deepa A, Padmanabhan K, Raghunadh G. Effect of hygrothermal loading on laminate composites. Indian J Sci Technol September 21, 2016;9(34). [3] Ogasawara T, Ishida Y, Kasai T. Mechanical properties of carbon fiber/fullerene-dispersed epoxy composites. Compos Sci Technol September 1, 2009;69(11e12):2002e7. [4] Panigrahi SK, Pradhan B. Through-the-width delamination damage propagation characteristics in single-lap laminated FRP composite joints. Int J Adhesion Adhes March 1, 2009;29(2):114e24. [5] Saito H, Kimpara I. Damage evolution behavior of CFRP laminates under post-impact fatigue with water absorption environment. Compos Sci Technol May 1, 2009;69(6): 847e55. [5a] Elgabbas F, Ei-Ghandour AA, Abdeelrahman AA, Ei-Dieb AS. Different CFRP strengthening techniques for prestressed hollow core concrete slabs: Experimental study and analytical investigation. Compos Struct 2009;92(2):401e11. [6] Okutan B, Karakuzu R. The failure strength for pin-loaded multi-directional fiber-glass reinforced epoxy laminate. J Compos Mater December 2002;36(24):2695e712. [7] Yashiro S, Ogi K. Fracture behavior in CFRP cross-ply laminates with initially cut fibers. Compos Appl Sci Manuf July 1, 2009;40(6e7):938e47. [8] Shi X, Hinderliter BR, Croll SG. Environmental and time dependence of moisture transportation in an epoxy coating and its significance for accelerated weathering. J Coating Technol Res July 1, 2010;7(4):419e30. [9] H€uhne C, Zerbst AK, Kuhlmann G, Steenbock C, Rolfes R. Progressive damage analysis of composite bolted joints with liquid shim layers using constant and continuous degradation models. Compos Struct January 1, 2010;92(2):189e200.

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[10] Elgabbas F, El-Ghandour AA, Abdelrahman AA, El-Dieb AS. Different CFRP strengthening techniques for prestressed hollow core concrete slabs: experimental study and analytical investigation. Compos Struct January 1, 2010;92(2):401e11. [11] O’Higgins RM, McCarthy MA, McCarthy CT. Comparison of open hole tension characteristics of high strength glass and carbon fibre-reinforced composite materials. Compos Sci Technol October 1, 2008;68(13):2770e8. _ Sea water effect on pinned-joint glass fibre composite materials. Compos [12] Aktas¸ A, Uzun I. Struct September 1, 2008;85(1):59e63. [13] Schambron T, Lowe A, McGregor HV. Effects of environmental ageing on the static and cyclic bending properties of braided carbon fibre/PEEK bone plates. Compos B Eng October 1, 2008;39(7e8):1216e20. [14] Kolesnikov B, Herbeck L, Fink A. CFRP/titanium hybrid material for improving composite bolted joints. Compos Struct June 1, 2008;83(4):368e80. [15] Sereir Z, Adda-Bedia EA, Boualem N. The evolution of transverse stresses in hybrid composites under hygrothermal loading. Mater Des May 1, 2011;32(5):3120e6. [16] Woo SC, Choi NS. Analysis of fracture process in single-edge-notched laminated composites based on the high amplitude acoustic emission events. Compos Sci Technol June 1, 2007;67(7e8):1451e8. [17] Liu X, Wang G. Progressive failure analysis of bonded composite repairs. Compos Struct December 1, 2007;81(3):331e40. [18] Nakatani K, Kubo S, Sakagami T, Shiozawa D, Takagi M. An experimental study on the identification of delamination in a composite material by the passive electric potential CT method. Meas Sci Technol November 23, 2006;18(1):49. [19] Kim MG, Kang SG, Kim CG, Kong CW. Tensile response of graphite/epoxy composites at low temperatures. Compos Struct June 1, 2007;79(1):84e9. [20] De Freitas M, De Carvalho R. Residual strength of a damaged laminated CFRP under compressive fatigue stresses. Compos Sci Technol March 1, 2006;66(3e4):373e8. [21] Sereir Z, Adda-Bedia EA, Tounsi A. Effect of temperature on the hygrothermal behaviour of unidirectional laminated plates with asymmetrical environmental conditions. Compos Struct March 1, 2006;72(3):383e92. [22] Pradhan B, Panda SK. The influence of ply sequence and thermoelastic stress field on asymmetric delamination crack growth behavior of embedded elliptical delaminations in laminated FRP composites. Compos Sci Technol March 1, 2006;66(3e4):417e26. [23] Pradhan B, Panda SK. Effect of material anisotropy and curing stresses on interface delamination propagation characteristics in multiply laminated FRP composites. J Eng Mater Technol July 1, 2006;128(3):383e92. [24] Botelho EC, Pardini LC, Rezende MC. Hygrothermal effects on the shear properties of carbon fiber/epoxy composites. J Mater Sci November 1, 2006;41(21):7111e8. [25] Lee J, Soutis C. Thickness effect on the compressive strength of T800/924C carbon fibreeepoxy laminates. Compos Appl Sci Manuf February 1, 2005;36(2):213e27. [26] Wang XW, Pont-Lezica I, Harris JM, Guild FJ, Pavier MJ. Compressive failure of composite laminates containing multiple delaminations. Compos Sci Technol February 1, 2005;65(2):191e200. [27] Hwang WJ, Park YT, Hwang W. Strength of fiber reinforced metal laminates with a circular hole. Met Mater Int June 1, 2005;11(3):197e204. [28] Almeida JP. Analytical and experimental study on the evolution of residual stresses in composite materials. [29] Civgin F. Analysis of composite bars in torsion. Masteral Dissertation. the Graduate School of Natural and Applied Sciences, Dokuz Eylul University; September 2005.

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[30] Raghavendra M, Manjunatha CM, Peter MJ, Venugopal CV, Rangavittal HK. Effect of moisture on the mechanical properties of GFRP composite fabric material. In: International symposium of research students on material science and engineering; 2004. p. 20e2. [31] Ray PK, Bhushan A, Bera T, Ranjan R, Mohanty U, Vadhera S, Ray BC. Mechanical behaviour of hygrothermally conditioned FRP composites after thermal spikes. [32] Zou Z, Reid SR, Li S. A continuum damage model for delaminations in laminated composites. J Mech Phys Solids February 1, 2003;51(2):333e56. [33] Pradhan B, Chakraborthy D. Fracture behavior of FRP composite laminates with embedded two interacting delaminations at the interface. J Reinforc Plast Compos 2002; 21:681e98. [34] Okutan B. The effects of geometric parameters on the failure strength for pin-loaded multidirectional fiber-glass reinforced epoxy laminate. Compos B Eng December 1, 2002;33(8): 567e78. [35] Singh KL, Dattaguru B, Ramamurthy TS, Mangalgiri PD. Delamination tolerance studies in laminated composite panels. Sadhana August 1, 2000;25(4):409e22. [36] Lin HJ, Tsai CC, Shie JS. Failure analysis of woven-fabric composites with moulded-in holes. Compos Sci Technol January 1, 1995;55(3):231e9. [37] de Almeida SF, Neto ZD. Effect of void content on the strength of composite laminates. Compos Struct January 1, 1994;28(2):139e48. [38] Soutis C, Fleck NA, Curtis PT. Hole-hole interaction in carbon fibre/epoxy laminates under uniaxial compression. Composites January 1, 1991;22(1):31e8. [39] Soutis C, Curtis PT, Fleck NA. Compressive failure of notched carbon fibre composites. Proc R Soc Lond A February 8, 1993;440(1909):241e56. [40] Soutis C, Fleck NA. Static compression failure of carbon fibre T800/924C composite plate with a single hole. J Compos Mater May 1990;24(5):536e58. [41] Potter RT. The interaction of impact damage and tapered thickness sections in CFRP. Composites 1985;3(3e4):319e39. [42] Purslow D, Potter RT. The effect of environment on the compression strength of notched CFRP a fractographic investigation. Composites 1984;15(2):112e20. [43] upload.wikimedia.org. [44] https://authors.library.caltech.edu. [45] https://image.slidesharecdn.com/. [46] https://www.researchgate.net/. [47] http://www.argomold.com/. [48] http://ssworxs.com/. [49] https://sc01.alicdn.com/.

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Corrosion detection for natural/ synthetic/textiles fiber polymer composites

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Nor Ishida Zainal Abidin, Mohd Faizul Mohd Sabri, Katayoon Kalantari, Amalina M. Afifi, Roslina Ahmad Department of Mechanical Engineering, University of Malaya

6.1

Introduction

The increment in natural awareness and community concern has led to new environmental regulations and unsustainable utilization of petroleum, driven to considering the usage of environmentally safe materials such as natural fiber. Natural fiber is known as one of the environmentally friendly materials that have great properties compared to synthetic fiber [1,2]. It has been identified that natural fiber reinforced polymer composites (NFPCs) industry sector has grown to U$2.1 billion in 2010 worldwide, and interest in NFPCs industry will keep on developing rapidly around the world. The utilization of NFPCs has extended notably in consumer merchandise as expanding industry sectors at some stage in the last few years. It is illustrated that the NFPCs industry is anticipated to expand 10% globally over 5 years (2011e16) [3]. Natural fibers are defined as fibers that are not manufactured or artificial and are those originated from plants, animals, or minerals [4]. The use of natural fiber from each resource, renewable or unrenewable, such as oil palm, sisal, flax, and jute, in producing composite materials has attained great interest within the last decades. Cellulose fibers in plants can be categorized into bast fibers (jute, flax, ramie, hemp, and kenaf), seed fibers (cotton, coir, and kapok), leaf fibers (sisal, pineapple, and abaca), grass and reed fibers (rice, corn, and wheat), and core fibers (hemp, kenaf, and jute), in addition to all other forms (wood and roots). Fiber-strengthened polymer matrices were given significant consideration in various applications due to the great properties and exceptional advantages of natural fiber over synthetic fibers in terms of its comparatively low weight, low cost, less damage to processing equipment, good relative mechanical properties such as tensile modulus and flexural modulus, better surface finish of molded parts composite, sustainable resources, plentiful, flexibility throughout process, biodegradability, and minimal health risks [5]. NFPCs possessing high specific stiffness and strength can be developed by incorporating tough and lightweight natural fiber into polymer (thermoplastic and thermoset) [6]. Nevertheless, natural fibers are not problem-free and have eminent deficits in properties. Cellulose, hemicelluloses, lignin, pectin, and waxy substances in natural fibers structure are susceptible to moisture absorption from the environment and as such can weaken bindings

Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00006-X Copyright © 2019 Elsevier Ltd. All rights reserved.

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between the fiber and polymer. In addition, the bondings between natural fiber and polymer are challenging since the chemical structures of both fibers and matrix are different, resulting in effective stress transfer at the interface of produced composites. Thus, specific treatments for natural fiber modification are essential. These modifications are normally targeted to the utilization of reagent functional groups which are capable of reacting to the fiber structures and changing their composition, subsequently enhancing the incompatibility between the fibers and polymer matrix because of the reduction of moisture absorption of the natural fibers [7].

6.1.1

Types of fiber-reinforced polymers

Fiber reinforced polymers (FRPs) are composite materials comprised of fibers that have been reinforced to a polymer matrix. The fibers are generally glass, carbon, or aramid, even though other fibers consisting of paper or wood or asbestos have been sometimes used. FRPs are usually used within the aerospace, automotive, marine, and creation industries. Normally, polymers can be classified into two classes, thermoplastics and thermosets. The structure of thermoplastic matrix substances includes oneor two-dimensional moleculars, therefore these polymers will be inclined to become softer at an elevated heat range and roll back their properties during cooling. In contrast, thermoset polymers represent highly cross-linked polymers through curing process using heat, or using heat and pressure, and/or light irradiation. This structure contributes excellent properties to thermoset polymers such as high flexibility for tailoring desired ultimate properties, great strength, and modulus [4,8]. Examples of common thermoplastics used for biofibers are polyethylene [9], polypropylene (PP) [10], and polyvinyl chloride (PVC); the most utilized thermosetting matrices are phenolic, polyester, and epoxy resins [11]. In general, most composites have a weak and less stiff matrix and are reinforced with strong and stiff fibers. The purpose of reinforcement is to make a strong and stiff, yet light component. Generally, commercial composite has thermosetting polymers matrices such as epoxy or polyester resins strengthened with glass or carbon fibers. At times, thermoplastic polymers are favorable because they are able to be molded after primary production. There are other composites using metal or ceramic as matrices. These metal or ceramic composites are nonetheless in the developmental stage, with issues of high costs yet to be overcome [12]. In addition, the intention of adding the fibers are very complex, for instance, enhancements can be sought in creep, wear, fracture toughness, thermal stability, and others [13].

6.1.2

Applications of FRPs

The expanded applications of NFPCs are developing very fast in diverse engineering fields. The numerous sorts of natural fibers reinforced polymer composite have acquired great attention in different automotive applications by many automotive companies, including German auto companies (BMW, Audi Group, Ford, Opel, Volkswagen, Daimler Chrysler, and Mercedes), Malaysian national carmaker PROTON, and Cambridge industry (a car enterprise in the United States). As well, the

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applications of natural fiber composites have additionally been used in civil infrastructure industry, sports, aerospace, and others, for instance in panels, window structures, decking, and bicycle bodies [14]. As mentioned before, FRPs are utilized without exception in advanced engineering structures, with their utilization extending to planes, helicopters, and spacecraft and also chemical processing equipment, buildings, and bridges. FRPs are evolving at a remarkable rate as these materials are used more and more in their current applications and are becoming established in comparatively new applications such as biomedical devices and civil constructions. The basic element driving the increment of the composite usage over the recent years is due to the production of new advanced forms of FRP materials. This comprises the developments in high-performance resin systems and advanced characteristics of reinforcement, such as carbon nanotubes and nanoparticles [15]. The FRP composites are increasingly being considered as an enhancement to and/or substitute for infrastructure components or systems that are constructed of traditional civil engineering materials, namely concrete and steel. FRP composites are lightweight, not corrosive, exhibit high specific strength and specific stiffness, are easily constructed, and can be tailored to satisfy performance requirements. Due to these advantageous characteristics, FRP composites have been included in new construction and rehabilitation of structures through their use as reinforcement in concrete, bridge decks, modular structures, formwork, and external reinforcement for strengthening and seismic upgrade [16]. The applicability of FRP reinforcements to concrete structures as a substitute for steel bars or prestressing tendons has been actively studied in numerous research laboratories and professional organizations around the world.

6.1.3

Properties of FRPs

The properties of natural fiber composite are different from each other according to previous studies because of different kinds of fibers, sources, and moisture conditions. The performance of NFPCs relies on a variety of factors, like mechanical composition, microfibrillar angle [17], structure [11], defects [18], cell dimensions [19], physical properties [8], chemical properties [20], and also the interaction of a fiber with the matrix [21]. FRP reinforcements offer a number of advantages such as corrosion resistance, nonmagnetic properties, high tensile strength, light weight, and ease of handling. However, they generally have a linear elastic response in tension up to failure (described as a brittle failure) and a relatively poor transverse or shear resistance. They also have poor resistance to fire and when exposed to high temperatures. They lose significant strength upon bending, and they are sensitive to stress-rupture effects. Moreover, their cost, whether considered per unit weight or on the basis of force carrying capacity, is high in comparison to conventional steel reinforcing bars or prestressing tendons. From a structural engineering viewpoint, the most serious problems with FRP reinforcements are the lack of plastic behavior and the very low shear strength in the transverse direction. Such characteristics may lead to premature tendon rupture, particularly when combined effects are present, such as at shearcracking planes in reinforced concrete beams where dowel action exists. The dowel action reduces residual tensile and shear resistance in the tendon. Solutions and

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limitations of use have been offered and continuous improvements are expected in the future. The unit cost of FRP reinforcements is expected to decrease significantly with increased market share and demand. However, even today, there are applications where FRP reinforcements are cost-effective and justifiable. Such cases include the use of bonded FRP sheets or plates in repair and strengthening of concrete structures, and the use of FRP meshes or textiles or fabrics in thin cement products. The cost of repair and rehabilitation of a structure is always, in relative terms, substantially higher than the cost of the initial structure. Repair generally requires a relatively small volume of repair materials but a relatively high commitment in labor. Moreover, the cost of labor in developed countries is so high that the cost of material becomes secondary. Thus the higher the performance and durability of the repair material, the more cost-effective is the repair. This implies that material cost is not really a concern in repair and that the fact that FRP repair materials are costly is not a constraining advantage [22]. The characteristics and performance of NFPCs are dependent on several factors such as the hydrophilic nature of the natural fiber [5] and the fiber loading [23]. Generally, in order to obtain NFPCs with good properties, high fiber loading is required [24]. It is noticeable that the rise in fiber content causes improvement in the tensile properties of the composites [14]. The process parameters used are another crucial factor that significantly impacts the properties and surface characteristics of the composites. Therefore, appropriate process techniques and parameters should be precisely chosen in producing composite with excellent properties [11]. The chemical composition of natural fibers represented by the percentage of cellulose, hemicellulose, lignin, and waxes also has a great impact on the characteristics of the composite.

6.1.4

Importance of inspections

FRP is susceptible to premature failures caused by the formation of cracks and microcracks during their service time exposed to moisture, chemicals, thermal, and ultraviolet effects. Premature failures could lead to catastrophic events that will bring suffer and damage to the affected area. In addition, the evolution of cracks and microcracks could also induce catastrophic material failure. Thus, detection of these cracks and microcracks or diagnostics at an early stage are crucial for ensuring the performance reliability, cost-effectiveness, and safety of FRP structures. It will also provide a solution to an effective repair of the detected cracks and microcracks. These cracks and microcracks form due to mechanical, physical, and chemical reactions and are often difficult to detect and observe with the naked eyes. As in example, “barely visible impact damage” is prone to occur in in-service structures [25] because, regardless of negligible external indications of damage, it can cause a complex pattern of matrix cracks, fiber breakages, and a significant amount of delamination [26,27]. Therefore, a dramatic loss of residual structure’s strength and fatigue life is observed [28].

6.1.5

Corrosion by definition related to FRP and effects to FRP

Natural fiber polymer composites are very sensitive to natural agents such as water. It is critical to use data and information on the impacts of dampness on maintenance of

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mechanical properties of natural fiber reinforced composites during extended service in outdoor applications [29]. It is reported that moisture would cause major degradation of natural fiber reinforced composites in terms of it mechanical properties compared to synthetic fiber reinforced composites due to its higher moisture absorption behavior, and the organic nature of the natural fibers [30,31]. Natural fibers can be pretreated using physical (e.g., corona discharge treatment and ultraviolet radiation) or chemical methods (e.g., using coupling agents and graft copolymerization) in order to improve the natural fiber performance. These pretreatments will promote better interaction between the fiber and the matrix. Since the mechanical properties of fiber reinforced composites is determined by adhesion characteristics of the fiberematrix interface, improving the interaction between fiber and matrix would also improve the mechanical properties. Compatibilizers have been used to enhance mechanical properties of reinforced plastics as the nature of the adhesion between the filler and the matrix interface is improved. Hybridization can also improve the stiffness, strength, and moisture resistant behavior of the composite. Natural fiber is hybridized with stronger and more corrosion-resistant synthetic fiber such as carbon or glass fiber. The advantage of using two or more different types of fiber in hybrid composite is that the weakness of one type of fiber could be complemented by the other type of fiber. This would balance the cost and performance of a proper material design for composite. Recent studies on the mechanical properties of natural and synthetic fiber reinforced polymer matrix hybrid composites are very limited, and mostly the durability issues are not highlighted [32].

6.1.6

Corrosion environment

The most challenging problems for composite materials are its long term stability, performance, and maintenance under critical environments. To understand how these materials behave and perform in harsh environments is very crucial [33,34]. Chemical corrosion of composites is one of the major challenges. There is still lack of understanding of the mechanism involved in fiber damage and degradation during chemical corrosion of a composite. The fibers offered resistance during corrosion by depending on the resin resistance activity, toughness, corrosion crack-propagation and -degradation properties. It has been reported that, upon persistent exposure to a harsh environment, there were dramatic morphological and structural changes of the fibers. An alternative fiber that is chemically inert and resistant to various environments is needed to avoid such circumstances. Recently, basalt has come into the picture to address all of these challenges. Beside its superior mechanical properties, basalt also possesses high chemical stability when exposed to different environmental conditions [35]. Therefore, basalt fibers are being used to counteract chemical corrosion. This section further highlights some work carried out by researchers related to the use of basalt fiber as a chemoresistant material. The rate of corrosion depends on several intrinsic and extrinsic factors like temperature, fiber composition, aging time, solution composition, pH, and size of the fibers. Wei et al. [36] reinforced basalt and glass fibers in an epoxy resin to study composite degradation in seawater. The tensile strength and bending results of seawater-treated composites indicated a decreasing trend based on the time of

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treatment. They observed that basalt fiber reinforced composite (BFRC) expressed anti-seawater corrosion properties similar to glass fiber reinforced polymer (GFRP). They concluded that by effectively lowering the content of Fe2þ ions within the basalt fibers, one can achieve better and improved stability of composites in seawater environments. Reports in the literature have revealed that basalt fibers have higher chemical durability and superior corrosion resistance [37].

6.1.7

Corrosion detection techniques

Besides the high production costs of fiber-reinforced polymer production methods, the main disadvantage of composite materials is they are prone to impairment. Low-speed impacts, such as a device or a tool drop event, or damage caused by a foreign object, such as debris impact, are the most recurring causes of delamination in composite materials [38]. This delamination damage is mostly invisible and this would make them seriously dangerous. Damage could also be caused by the heat from lightning strikes or leaks of hot air from an aircraft engine. Delamination occurs when this heat burns the epoxy resin that holds the fiber and matrix together [39]. Despite all of the impressive characteristics, FRPs are still prone to damage due to many other causes, including overloading, chemical penetration, multiaxial fatigue, or a combination of any/all of the above. Such damage could be evident in various modes, such as delamination, fiber fracture or matrix fracture, fiberematrix debonding, and matrix swelling. Damage usually occurs on the subsurface within the laminated construction of the composite, which makes it hardly visible to inspectors. The possibility of hidden damage growing to critical levels without detection is a crucial cause for concern and cannot be take for granted [40]. Various nondestructive testing (NDT) techniques are available to assess the condition of existing reinforced concrete structures. Currently, the most common NDT techniques in civil engineering include impact-echo approaches, ultrasonic methods, ground penetrating radar techniques, microwave-based methods, among others [41].

6.2

Destructive physical analysis methods

Destructive physical analysis (DPA) is the process of disassembling, testing, and inspecting electronic components to verify the internal design, materials, construction, and workmanship. This process of sample inspection is used to help ensure that electronic components are fabricated to the required standards. DPA is also used effectively to discover process defects for identification of production lot problems.

6.2.1

Optical microscopy

Optical microscopy (OM) as well as scanning electron microscopy (SEM) are classical techniques for composite surface studies. OM is one of the most valuabledbut underutilizeddtools for analyzing FRP matrix composites. An optical technique has

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been used that utilizes optical backscatter from the fibers using a probe that is inserted into a hole drilled through the composite. Even though this is a reliable method for ensuring correct lay-up sequence, this technique is extremely localized (will not be useful to evaluate fiber waviness or regions of fiber misorientation) and requires drilling a hole, which is not usually feasible in actual structures. More than 3 decades since the appearance of fiber reinforced polymer composites (FRPCs), the widespread acceptance of FRPC as a reliable class of engineering materials remains an issue. The reasons for this observed hysteresis between development and application of FRPC can be traced primarily to the inability to effectively model and predict their performance and remaining life as a function of both applied load and evolving surrounding environment [42,43], as, for example, currently accomplished in the case of metal alloys [44]. The resulting uncertainty naturally increases the risk in using this material type and ultimately outweighs their comparative advantages as engineering materials. Failure in laminated polymer composites is driven by various damage mechanisms including matrix cracking, fiber breakage, interfacial debonding, transverse ply cracking, and ply delamination [45]. The number, activation, and interactions of these failure mechanisms is further heavily dependent on prior manufacturing and machining operations, which often result in defects and predamage conditions that ultimately affect the observed failure patterns [46]. In addition, the probable simultaneous activation of two or more of such failure mechanisms and their unpredictable evolution with applied loading, changing material states, and interactions with the overall structural design create the need for the development of effective strategies for early detection of damage initiation, as well as subsequent tracking of its evolution and accumulation [47,48]. Many experiments and theoretical studies have been performed in order to identify the underlying origin and micromechanical causes of such intricate composite failure mechanisms and to propose solutions to improve the resistance against failure [49]. Recent NDT applications for damage monitoring of composites include the use of noncontact optical methods. An improved understanding of the microscopic mechanical behavior is essential for materials, in particular the investigation of the local distribution and arrangement of the plastic microstrains in fiber-reinforced composite materials. Better knowledge of the micromechanics of these materials under load will not only make these materials more useful but also contribute to an improvement in safety and design of the engineering components made from them. Strain-field analysis via digital image correlation (DIC), which is also referred to as photogrammetry, is a powerful tool that can be applied to map lateral strain distributions at different length scales. It allows one to conduct a detailed investigation of complex micromechanical aspects that are associated with the lateral distribution of the strain in heterogeneous materials [49]. Giansane et al. [50] proposed a methodology based on the full-field technique, DIC as a valuable tool to be used as a nondestructive control system for glass fiberreinforced composites (GFRCs). In their work, GFRC flat specimens were manufactured in notched and notch-free geometries, in order to test under fatigue loads the material decaying conditions due to damage occurrence. DIC technique was employed to monitor surface specimens

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under fatigue testing and to evaluate two indicative parameters correlated to damage evolution: stiffness and absorbed mechanical energy. The adopted technique presents an uncertainty in displacement measure of the order of 1 m and about 1.5  104 for strain measurement, allowing to obtain reliable information on heterogeneities that develop and evolve during a test. In particular, applied to fatigue tests on GFRC material, DIC allowed to compute the evolution of two parameters strictly associated with fatigue damage: the stiffness (E) and the absorbed energy calculated as the mechanical hysteresis area of a complete cycle (H). These two indexes give a clear indication about the damage localization and about the process of gradual degradation of the mechanical properties of composite. In work of Godara et al. [49], tensile testing coupled with the novel technique of surface displacement mapping via DIC is utilized for resolving the mechanical behavior and spatial distribution of the plastic microstrains in an epoxy resin reinforced with 35 wt% short borosilicate glass fibers. The DIC method works by correlating the digital images of surface patterns before and after straining. The material exhibited a pronounced mechanical anisotropy at both macro- and mesoscale, which depends on the alignment of the fibers relative to the external load. The underlying microstructure of the material explained formation of strain gradients during evolution of full-field strain fields. The levels of localized strain are higher than the global failure strain of the material. Also, SEM on the fracture surfaces revealed the multiple failure mechanisms of the material as a function of the fiber orientation.

6.2.2

Electromagnetic testing

Thermography testing makes use of infrared (IR) imaging to detect defects within the component. An IR camera records spatial and temporal distribution of the surface temperature after the component has been heated. Defects disturb the heat flow and hence can be detected. Thermal NDT methods generally use active thermography. Here, heat waves are sent using an external or internal source. This allows measurement of depth, thickness, and size of internal flaws. Furthermore, pulse and lock-in thermography are in use for NDT applications globally for flaw imaging [51]. Pulsed thermography is a type of thermography in which a stimulus is applied through a flash pulse, usually xenon lamps. A selection of active sources, various IR camera types, and a range of analytical tools are available for monitoring the responses of the objects to the active-sources [52,53]. X-ray microtomography (X-ray micro-CT) has been applied to a variety of problems in materials research. Application of micro-CT to composite materials has mainly concentrated on metalematrix and ceramicematrix composites [54]. The spatial scale of features in these materialsdfiber diameter, particle size, etc.dare accessible to micro-CT. In metalematrix composites (specifically aluminium and titanium matrices), micro-CT has been utilized to characterize internal features including fiber location and waviness [55], fiber breakage [56], particle distribution [57], particle/matrix decohesion [58], particle fracture [57], local porosity and density [59], void volume [60], and fatigue crack growth [61]. Fiberematrix debonding and fiber pullout were detected in ceramicematrix composites by Baaklini et al. [55]. Geandier et al.

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[62] measured the volume fraction and connectivity of the metallic phase in an aluminaechromium composite.

6.3

Nondestructive evaluation methods

For evaluation of materials or component properties and performance, nondestructive techniques have been used widely. The main characteristic of these methods is evaluation without damage or permanent alteration to the components and materials. There are numerous tests of NDT and inspection methods that can be applied in any deformation, defects and abnormality detection in FRP composites such as penetration of dye, ultrasound, visual inspection, radiographic imaging, and acoustic emission. Inappropriate service environment and design would lead to damage induction in the degradation or corrosion form of the matrix and fiber in the FRP composite. This may be due to failure contribution of materials during it service and should be detected in the early stage, otherwise the cost is high in terms of time, money, and even life in many industries. For prevention of any significant risk, a routine inspection using NDT techniques must be applied in industries. Several NDT methods will be discussed in this section. In order to prevent catastrophic failures and detect damages, structures are inspected regularly, at planned maintenance service interruptions, by using NDT methods. The main NDT methods used for composite materials and structures are: ultrasonic testing, vibration analysis, visual testing, eddy currents testing, radiography, and tomography. Unfortunately, high maintenance costs, especially for safety components, are the main drawback of the nondestructive techniques in common practice. In spite of the high level of reliability reached, today about one-quarter of the costs of the operation of commercial aircraft constitute maintenance and repairs [28].

6.3.1

Visual inspection

Visual inspection is normally the first step of evaluation or inspection of materials performance. This is the easiest, lowest cost, and fastest technique to be used. This method requires other techniques’ support for confirmation of any abnormalities and defects spotted with naked eyes, especially for FRP composites. Visual inspection can be used for any visible defects such as discoloration, surface cracks, deformations, or warping cause by fiber and matrix deformation. However, it is impossible to detect any internal defects experienced by the FRP composite, like cracks or delamination caused by corroded fibers. This technique can be done either by localized or overall appearance based on the need or earlier detected indications like outperform component. Depending on the composite’s appearance, either visualized as a translucent or an opaque fiber and matrix, the detection ability of the defect might be limited.

6.3.2

Chemical

Penetration of dye is applied to highlight cracks and contrast improvement between the underlying material and defects. The literature review demonstrates that the current

102

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

procedures are for inspection of dye penetrant of metals and these are not directly applicable to composite materials. As the name implies, this inspection technique involves the dye’s penetration into the defects of the surface and discontinuities. Many engineers and producers choose this technique due to its cheapness and rapidity. The time of penetration can be as short as 5 min or up to 30 min based on the application. Moreover, dye testing many be used to easily inspect parts with complex shapes. This technique also allows for quick inspection of large volumes and areas. However, there are also drawbacks. One is that the detection may be limited to surface defects. Furthermore, rough surfaces testing by this method might lead to false indications (because the excess penetrant removal is difficult). Safety of workers also is one of the main concerns. Skin irritations can result from penetrant spraying, so appropriate training and handling is necessary for the procedure. Proper selection of penetrant and developer is also important [63].

6.3.3

Sonic and resonance

Eddy current thermography is an emerging NDT method, with combined advantages of thermal wave testing and conventional eddy current. Unlike flash thermography, eddy current thermography does not rely on the material surface conditions under test. A couplant is not required in this method compared to ultrasound. With respect to excitation techniques, eddy current thermography can be categorized into these groups: eddy current pulsed thermography (ECPT) [64], eddy current lock-in thermography [65], and eddy current pulsed phase thermography [66]. ECPT has the advantages of combination of pulsed eddy current and merits of thermography (high and fast resolution), and has been widely applied for detection of damage in metallic alloys. Eddy current testing allows detection of cracks in a large variety of conductive materials, either ferromagnetic or nonferromagnetic, whereas other nondestructive methods like magnetic particle technique are limited to metals with ferromagnetic properties. Moreover, compared with other methods, inspection can be implemented without any direct physical contact between the inspected piece and sensor [67]. Furthermore, a wide range of measurements and inspections may be carried out with the eddy current techniques that are beyond the scope of other methods. Thickness of nonconductive coating measurements [68] and conductivity can be done. Conductivity is related to the heat treatment and composition of the test material. Therefore, the eddy current technique can also be applied to distinguish between alloy compositions and pure materials and also used in hardness determination of the test pieces after heat treatments [69].

6.3.4

Ultrasonic testing

Ultrasonic testing is commonly used in nondestructive analysis. The basis of this method is propagation of waves with high frequency. By using a transducer, waves are transmitted to the tested object. A couplant (gel coat, water, etc,) is necessary to prevent propagation of high-frequency waves in air. They propagate through the material and are reflected by the specimen rear surface. There are two possible methods of

Corrosion detection for natural/synthetic/textiles fiber polymer composites

103

ultrasonic testing: reflection and transmission. If the specimen medium and propagation environment that waves pass through have differences, disturbances of waves take place, which indicate an inclusion presence. This technique enables 3D mapping of the specimen. Inclusion, delamination or debonding are localized in depth with different colors based on the scale applied. This kind of method provides details such as the specimen thickness, presence of inhomogeneous medium, the examined specimen modulus of elasticity, or 3D mapping [70]. In recent decades, engineering applications of acoustic emission (AE) technique had increased exponentially. In this method, the localized energy sources generate transient elastic waves by sudden energy release within a material. AE method began in the early 1950s with Joseph Kaiser [71], who introduced this technique in scientific applications. He was the first who applied electronic instrumentation to audible sound detection produced by deformation under tensile tests on metallic specimens. His investigations resulted in an irreversibility phenomenon observation, which nowadays is known as “the Kaiser effect.” In the middle of the 1950s, Schofield [72] improved the AE source of instrumentation and clarified the method during plastic deformation. In the decade of the 1960s, AE was used in the material engineering field and also as an NDT technique in the aerospace industry. It was applied by the United States Navy to verify the integrity of Polaris rocket motors. In 1963, the first AE test on pressure vessels was carried out by Dunegan; extended improvements in instrumentation helped him in the formation of the world’s first AE equipment company. In the early 1970s, studies began on fiber reinforced composites with the goal of assessing the reliability of AE as a nondestructive test of FRP. Liptai [73] demonstrated the breakdown of the “Kaiser effect” in the structure of glass fiber-reinforced plastic. Nowadays, the discovered effect in composite structures is named the “Felicity effect.” Ten years later, a committee on acoustic emission from reinforced plastics (CARP) was established and provided a code (exists under the appellation CARP code). Currently, the application range of AE is manifold. It has wide range from the damage process study of the insight material during behavior investigation of mechanical on specimens, to integrity inspection of real structures. The acoustic emission method finds one of its significant application fields in materials research. Examples are various: initiation of damage detection, discrimination between mechanisms of damage and their rate of evolution under mechanical loading, and propagation study of cracks under quasi-static and dynamic loading. These applications led to better understanding the relationship between structures and properties. This gave rise to the development of new industrial applications (i.e., large structures): bridges, pressure vessels, aircraft, piping, storage tanks, and a wide range of other composite components [71,74].

6.4

Semi-analytical finite element method

Another technique that has been widely applied for analysis of structures is the semianalytical finite element (SAFE) method. SAFE was introduced first by Wilson [75] in

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

1965 for finite element analysis of axisymmetric structures loaded nonaxisymmetrically. SAFE was developed as an alternative approach to more traditional techniques such as the global matrix method, mostly because of its benefits of solving arbitrary cross-section waveguide problems [76]. In this method, discretization of the waveguide happens in the cross section, while an analytical solution is adopted in the direction of wave propagation. Based on a variational scheme, a linear equation system can be constructed with the unknown frequency and wave number. The unknowns can be solved by routines of standard eigenvalue. SAFE can solve wave propagation problems in waveguides with complex cross sections, such as multilayered laminates [77] and rails [26,76], where obtaining analytical solutions is often difficult. SAFE also has advantages compared to analytical matrix techniques due to it is less prone to missing roots in dispersion curves development. In analysis of composite, SAFE was applied in aerospace structures for modeling the composite wing skin-to-spar bonded joints. Some other applications of the SAFE methods for guided waves in composite plates can also be found [27,78,79]. Generally, composite materials are anisotropic and the damage developed is difficult to observe from the structural surface. Guided wave of ultrasonic method provides a promising solution for detection of composite structure damage and health monitoring. However, due to the anisotropic properties of material, the guided waves excited in composites exhibit directivity behaviors. These complicated behaviors have direction-dependent dispersion curves and mode shapes. The SAFE technique discretizes the waveguide cross section with limited elements and uses analytical formulation along the direction of wave propagation [26]. Thus, it makes SAFE capable of modeling waveguides with arbitrary cross section and material properties [80]. The SAFE technique does not need to be a procedure of root searching like most analytical methods. Therefore, it is efficient and easy in terms of numerical computation.

6.4.1

Modeling, formulation, and governing equation

This subsection provides the mathematical derivation and illustrates the formulation of SAFE for plate structures. Fig. 6.1 demonstrates a model of SAFE setup for propagation of waves in plate structures. To discretize the cross section and describe the mode shapes of guided waves, it can be represented by only 1-D elements. The waves propagate along x direction with wavenumber x at frequency u. The cross section lies in the y e z plane. The harmonic displacement, stress, and strain field components at each point of the waveguide are expressed by u ¼ ½ux uy uz  s ¼ ½ sx

sy

(6.1) sz

syz

sxz

sxy ; ε ¼ ½ εx

εy

εz

εyz

εxz

εxy  (6.2)

Corrosion detection for natural/synthetic/textiles fiber polymer composites

x

105

ω, ξ

y

tion e wav n direc e d i Gu agatio p pro

z

Cross section discretization

Figure 6.1 SAFE model showing cross section discretization and wave propagation in plate structures.

The equation of the constitutive at a point is given by s ¼ C ε, where C is complex valued stiffness matrix. This complex stiffness matrix contributes to complex valued wavenumbers for the modeling of damped guided wave propagation. The stresse displacement relation can be given in matrix form as


 v v v ε ¼ Lx þ Ly þ Lz u vx vy vz

(6.3)

where, 2

1

6 60 6 6 60 6 Lx ¼ 6 6 60 6 6 60 4 0

0 0

3

2

0

7 6 60 0 07 7 6 7 6 7 60 0 07 6 7; Ly ¼ 6 7 6 0 07 60 7 6 7 6 60 0 17 5 4 1 0 1

0 1 0 0 0 0

0

3

2

0

7 6 60 07 7 6 7 6 7 60 07 6 7; Lz ¼ 6 7 6 17 60 7 6 7 6 61 07 5 4 0 0

0 0

3

7 0 07 7 7 0 17 7 7 7 1 07 7 7 0 07 5 0 0

(6.4)

The stiffness matrix is usually defined in a coordinate system based on the fiber orientation, with the 1-axis along the fiber direction, the laminate plane and perpendicular to the fiber direction as the 2-axis, and the 3-axis perpendicular to the laminate plane. Fig. 6.2 demonstrates two cases of lamina stacking for direction angle of fiber. Fig. 6.2(a) gives the 1-2-3 axis coinciding with the x-y-z global coordinate system, which refers to a situation where the stacking angle q is zero. Since the stiffness matrix Clocal is defined in 1-2-3 systems, in this case, the stiffness matrix C in global coordinate equals Clocal, i.e., C ¼ Clocal. Fig. 6.2(b) shows the 1-2-3 axis no longer coincides with the x-y-z global coordinate system, which refers to a situation where the stacking angle q is nonzero. The stiffness matrix C in global coordinate is calculated from Clocal by the coordinate transform

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

z

(a)

(b)

y

z

y

x

θ

er

Dir e

f fib

o tion

c

Dire

cti o

no

f fi

be

r

x

Figure 6.2 Coordinate transformation showing different fiber direction angle.

e ¼ T1 C e local TT C

(6.5)

where, 1 means the inverse of a matrix, and the T as the superscript means the matrix transpose. T is the transformation matrix defined as 2

m2

n2

0

0

0

n2

m2

0

0

0

0

0

1

0

0

0

0

0

m

n

0

0

0

n

m

mn

mn

0

0

0

6 6 6 6 6 6 T¼6 6 6 6 6 6 4

2mn

3

7 2mn 7 7 7 7 0 7 7 7 7 0 7 7 7 0 5

(6.6)

m 2  n2

where, m ¼ cos (q) and n ¼ sin (q). Therefore, the stiffness matrix for the finite element procedure for each layer with a certain stacking direction fiber angle can be defined. By inserting the potential and kinetic energies into Hamilton’s equation, the governing equation can be obtained as Eq. (6.7) [80]. Z

t2

dH ¼

dðF  KÞdt ¼ 0

(6.7)

t1

where, F and K are the strain and kinetic energy, respectively. The strain energy and kinetic energy are given by: F¼

1 2

Z v

e K¼ εT CεdV;

1 2

Z u_ T ru_ dV v

(6.8)

Corrosion detection for natural/synthetic/textiles fiber polymer composites

107

where, V is the volume, r is the density of the material, and the dot represents a time derivative. After integrating, Eq. (6.7) can be expressed as: t2
Z

Z t1

  e þ d εT CεdV v

 € d u rudV dt ¼ 0

Z



T



(6.9)

v

In general, the displacement field can be described as: 2

ux ðx; y; z; tÞ

3

2

Ux ðy; zÞ

3

7 6 7 iðxxutÞ 6 7 6 7 u ðx; y; z; tÞ ¼ 6 4 uy ðx; y; z; tÞ 5 ¼ 4 Uy ðy; zÞ 5e uz ðx; y; z; tÞ

(6.10)

Uz ðy; zÞ

where, i is the imaginary unit, x is the wavenumber, and u is the angular frequency. From Eq. (6.10), it can be seen that at each point, there are 3 of freedom corresponding to x, y, and z direction. The displacement field is assumed to be harmonic along the x propagation direction. And, the amplitude of displacement is a space function. The y-z plane amplitude describes the guided waves’ mode shape, for a plate waveguide as in Fig. 6.1, a straight crested wave field propagating in x direction. Thus, the displacement field is independent of the y coordinate and can be simplified as: 2

Ux ðzÞ

3

7 iðxxutÞ 6 7 u ðx; z; tÞ ¼ 6 4 Uy ðzÞ 5e

(6.11)

Uz ðzÞ

6.5

Summary

Nowadays, fiber reinforced polymer composite materials are being applied increasingly in engineered structures. They are often used for their unique properties such as high specific strength and stiffness, and high corrosion resistance. However, detection of damages in them is relatively complex because of the anisotropy and inhomogeneity of the material exhibited by the fiberematrix ensemble, and constitutes a topic of active study. There are some techniques for evaluation of components or materials; nondestructive ones are an important group of them with numerous applications. The evaluation field of nondestructive tests includes damage identification and characterization on the surface and interior of materials without cutting apart or otherwise altering the material. Nondestructive techniques provide a sample cost-effective testing means for individual investigation or may be used on the whole material for checking in a production quality control system. Several methods are applied in the composite nondestructive field, involving thermographic testing ultrasound, testing, radiographic testing, infrared thermography

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

testing, acousto-ultrasonic testing, shearography testing, electromagnetic testing, optical testing, liquid penetrant testing, and magnetic particle testing. Because the composite tools are mostly applied in critical-safety applications such as construction of aircraft, the NDT of composite materials has become more demanding and crucial. In evaluation of the best technique to be used, some factors like efficiency and safety should be considered. Furthermore, the technique chosen should decrease the costs incurred in the operation. It is based on methods that depend on the use of physical values to determine the materials’ characteristics. In addition, for evaluation and identification of destructive and fault defects, these tests use physical principles.

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Haptic-based virtual reality system to enhance actual aerospace composite panel drilling training

7

Sivadas Chandra Sekaran 1 , Hwa Jen Yap 2 , Kan Ern Liew 3 , Hafeez Kamaruzzaman 4 , Chee Hau Tan 5 , Razman Shah Rajab 6 1 Researcher, Aerospace Malaysia Innovation Centre (AMIC), Kajang, Selangor, Malaysia; 2 Associate Professor, University of Malaya, Kuala Lumpur, Malaysia; 3Deputy Chief Technology Officer, Aerospace Malaysia Innovation Centre (AMIC), Kajang, Selangor, Malaysia; 4Principal Consultant, STRAND Business Consulting Center, Petaling Jaya, Selangor, Malaysia; 5Research Assistant, University of Malaya, Kuala Lumpur, Selangor, Malaysia; 6Chief Executive Officer, Aerospace Malaysia Innovation Centre (AMIC), Kajang, Selangor, Malaysia

7.1 7.1.1

Introduction Drilling carbon fiber-reinforced composites

Carbon fiber reinforced polymers (CFRPs) are increasingly replacing conventional metallic alloys in aircraft structures. This is mainly due to tensile strength carried by CFRP, which falls between 1500 and 3500 MPa, whereas its metallic counterparts such as aluminum and steel only possess tensile strength of 450e600 MPa and 750e1500 MPa, respectively. Moreover, a relatively lower density at around 1.5e2.0 g/cm3 of CFRP makes it the ideal choice for being a major structural component of aircrafts [1]. However, machining CFRP presents its own set of difficulties such as delamination, fiber pullout, and fiber breakout [2]. Due to the unpredictive nature of composites, it is important to avoid these issues from the machining stage itself to guarantee their strength against fatigue and assembly tolerance [3]. Drilling operation of aerospace-grade CFRP, being the major cutting process involved in CFRP machining [4], faces delamination as the major defect mode. Multiple studies have been conducted on identifying the causes and solutions for CFRP during delamination. Several factors that impact the products of drillings are still being investigated. The important variables are identified as thrust force, drilling speed, and feed rate. Among them, increased thrust force is identified to be the significant contributor to tool wear as well as composite’s delamination [4]. Although there are the latest computer numerical controlled (CNC) drilling machines that can set and perform drilling at fix thrust force, drilling speed, and Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00007-1 Copyright © 2019 Elsevier Ltd. All rights reserved.

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feed rate, a great portion of CFRP drilling operation in aerospace industry is still being conducted using hand-held pneumatic drill machines (Fig. 7.1). Despite the existence of automated drilling, larger aircraft panels, especially irregular shaped, that make up parts such as spoilers, fairings, and flight control surfaces, require technicians to use a hand-held drill that enables them to perform drilling from different angles. Most often, the drill feed rate and spindle of the machine can be fixed, however, the driller is still responsible for maintaining the thrust force within the threshold, unique to the material that is being drilled. Exceeding the thrust force during drilling of composites causes significant delamination damage to the final layer of the panel, called fiber pushout. Moreover, when it comes to hand-held drilling, drilling angles play a vital role is producing a perfect hole. Any deviation in angle would cause the hole to be elliptical instead of a perfect circle, which in turn would cause the part’s assembly tolerance to be compromised. Therefore, it is crucial for the driller to perform the drilling, perpendicular to the surface, and maintain the required thrust force throughout the drilling. Since it is impossible to quantitatively teach technicians on drilling at a certain thrust force and perpendicular angle, developing motor skills is vital. In large aerospace manufacturing facilities, technicians are trained in-house to develop basic aerospace composite drilling skills. Performing a proper drilling not only produces a high-quality product close to a set standard but also increases the life of the tool itself. At higher-than-required thrust force and cutting speed, change in geometry of drill bit due to tool wear is clearly observed [3]. Based on the Dreyfus model of skill acquisition as shown in Fig. 7.2, training and acquiring a physical skill would require the technician to go through five stages, namely novice, advanced beginner, competence, proficiency, and expertise. The transition from one stage to another requires theoretical knowledge and experience until it ultimately becomes intuitional at the expert stage [5].

Figure 7.1 Pneumatic drill.

Haptic-based virtual reality system to enhance actual aerospace composite

115

Expertise Proficiency Competence Advanced beginner Novice

Figure 7.2 Dreyfus model of skill acquisition.

Therefore, traditionally, in order to develop expert level drillers, aircraft manufacturing facilities have to train them for an extensive amount of time using actual specimens, which in this case are the carbon fiber composites. Unfortunately, this is a major challenge due to 9e10 years of backlog and the need to lower the operation and manufacturing cost in aircraft manufacturing facilities. To aggravate the situation, the attrition rate among aerospace technicians is high. Most facilities use walk-in interview methods to quickly employ more technician to fill the man power, however, the skill lost due to attrition is not sustained. Training these new employees to perform CFRP drilling will take a tremendous amount of time and resources. With spiking global demand on aircraft, and composites to satisfy the requirement [6], alternative methods need to be adopted to train technicians quicker with lower cost. The traditional training system that measures the performance of a trainee via postprocessed, product-based, qualitative evaluation contributes very little to skill attainment. Current practice in drilling training facility is to allow the trainees to drill a fixed number of holes on CFRPs while being monitored for posture and drill orientation by a supervisor. The drilling quality is then tested by observing the quality of the hole using equipment called a go-no-go gauge as well as visual and tactile inspection on the final layer of the CFRP. Error due to angle will cause the panel to fail the go-nogo gauge test if the hole exceeds a few microns (10
6 m) of tolerance. The error done due to not maintaining the optimal thrust force is observed by overshoot made by the drill bit and physical damage caused on the last layer of the composite panel. Although the evaluation method is effective in predicting the quality of the machined product, they play a minimal role in training the technician. With no information on their mistakes done during (real-time) the task, it will require a much longer time to correct themselves and improve. Besides time, providing structured training to technicians also requires extra costs in terms of materials, instructors, and tools. Technicians should be trained to drill

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several types of panels, starting from flat shaped to curved. Actual aerospace CFRP panels are too expensive for training purposes, therefore most facilities leverage on scrapped CFRP panels or different materials for the training. This irregularity in the training regime compromises the effectiveness of the training since the shape, strength, toughness, and texture of the material greatly differs from the actual panel that the technicians are required to work on in the future.

7.1.2

Virtual reality in training and aerospace industry

With dramatic advancements in computing and graphic processing ability, virtual reality (VR) has become an alternative to traditional training. VR was developed in order to simulate physically nonexisting environments and objects to perform or experience a specific imaginary situation with maximum degree of realism and interactivity [7]. Greater processing power allowed computers to do real-time calculation in a shorter period, whereby enhanced graphic processing allowed visual renderings to be close to real and to be generated in more immersive display hardware such as VR computer assisted virtual environment (CAVE), head mounted display (HMD), and 3D power wall [8]. VR has become an integral part in solving issues in different sectors, including manufacturing, maintenance, visualization, and training [9]. The role of VR in training is enhanced even more since the incorporation of 3D haptic devices into VR hardware. 3D haptic devices are able to feed-in 3D positional data to a VR system and produce force feedback. The users can move objects in a virtual world by physically sensing the forces acting upon them, which includes weight and collisions [10]. The development of this system has opened a new dimension in VR where users can use one more sense (touch) on top of previously mimicable visual and hearing senses. With this, the training and operation done in virtual environment is expected to be much more accurate and the knowledge attained to be more transferable from VR to real world application. This technology has already been used in multiple fields such as military, nuclear, and medicine. Force feedback mechanism-based surgical procedures are being applied in training for minimally invasive procedures via surgical simulations [11]. Being an industry that focusses on accuracy, minimal error tolerance, and fast machining, aerospace adapts and benefits a lot from introducing VR into training processes. Currently, VR has been a vital component in training technicians to perform inspection during aircraft maintenance operations [9]. Based on research done by J. Adams, it was found that human training narrows down to cognitive, perceptual, and motor demands. Although VR is capable of enhancing each of the training aspects, motor demand would receive the highest benefit. However, due to lack of devices that can duplicate fine and dexterous motor movements, it is advised that VR-haptic configuration is used in training that involves much coarser tasks. Machining operation and industrial drillings are some of the training that can easily and accurately be reproduced using VR-haptic configuration [11]. Furthermore, not limiting the uses of natural human haptic motion, i.e., using two hands instead of limiting to one, would improve the training outcome drastically [10].

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117

Having the potential to become a revolutionary new training medium, VR is capable of imitating almost any real-life scenarios in terms of visuals. CFRP drilling environment, materials, and tools can be modeled close to reality in 3D using existing computer aided design (CAD) software such as CATIA, SolidWorks, and AutoCAD. Developing the environment provides more immersion and context to the training where the new recruits can have “preexperience” of their future working environment. Repetitive training in such environments is expected to accelerate the training of new technicians to become experts. As more exposure is already been given to them prior to being in actual environment, they could channel more of their focus on the task itself [8]. The tools and panels that will be used in the VR environment would be experienced through the integration of the haptic device, in this case, a VIRTUOUSE 6D Haptic Arm as shown in Fig. 7.3. The arm, with its holder, duplicates the shape of a pneumatic driller. The arm will be coded to provide feedback that a CFRP panel would cause on a drill machine during drilling operation. In short, a complete CFRP drilling training facility is modeled and presented to the technician electronically whereby none of the panel, tools, nor the environment exist physically. However, the full activity of drilling is done and experienced very close to actual. Moreover, this system can be reset and reused as many times as required without additional cost due to tool wear or material utilization. Using the advantage of a VR haptic to solve problems faced by aerospace-grade CFRP drilling training is completely practical and in line with previously produced studies in terms of effectiveness and skill transferability. Ability to realistically illustrate the environment, accurate haptic force feedback, and real-time performance measurement computation are sufficient to develop an effective training mechanism to train motor skills to technicians (hypothesis 1). However, this hypothesis needs to be tested through a pilot study on whether the system can identify the difference in performance and skill between a novice and an expert driller (hypothesis 2) [12]. The sensitivity of the VR training tool in doing such will further strengthen the first

Figure 7.3 6D Haption VIRTUOSE 6D.

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hypothesis that it could serve effectively as a training tool. This study is designed to investigate the second hypothesis.

7.1.3

Materials and method

The VR system for this research is developed around a customized VR tool developed by the Airbus Group named Realistic Human Ergonomics Analysis (RHEA). This system requires motion trackers, a 3D Power Wall system, 3D projectors, and a haptic device. First, the height and the position of the trainee and the relative position of the haptic arm are detected and fed into RHEA using six Advanced Realtime Tracking (ART) cameras. The positional data is gathered and processed at real time between frame rate-dependent time steps to extract drilling parameters. Barco Digital Light Process Technology projector is used to generate stereo 3D render that imitates the actual surrounding of a drilling training facility. The visuals respond immediately to the user’s position and orientation based on data received via ART cameras. This produces real-time stereo visual that is immersive to perform a training process without noticeable latency. Haption’s VIRTUOUSE 6D haptic arm is integrated with RHEA to provide the end effector’s (drill) position relative to the world. Physics engine embedded in RHEA will calculate the collision forces and translate them into haptic feedback and transfer it to the haptic arm to be experienced by the user as illustrated in Fig. 7.4. RHEA, besides being the middleware that manages the communication between various VR devices, facilitates the VR application creation and running. The VR environment mimicking the actual drilling area was developed in CAD software. The models’ visuals and dimensions are tested, then imported into RHEA. The physics (i.e., weight and collision parameters) are modeled in RHEA based on the actual data acquired experimentally. The panel to be drilled was designed based on actual CFRP panel as shown in Fig. 7.5. The modeled composite panel was configured with two blocks placed inside the panel for each hole. The mechanism of drilling a hole on the panel was simulated by separating the two blocks apart by pushing the drill

Tracking devices

User’s position and orientation Force feedback RHEA

Haptic arm End effector/drill position

3D projector

3D stereoscopic visual

Figure 7.4 Communication and data flow.

Haptic-based virtual reality system to enhance actual aerospace composite

119

Figure 7.5 CFRP panel used in actual drilling training.

bit through (Fig. 7.6). Each individual block can move 5 mm sideways. Friction caused by the contact between the moving blocks and the panel was transformed to signals, which was then sent to the haptic arm to produce feedback force to the drillers. Figs. 7.7 and 7.8 shows the hardware setup of the system and the virtual environment, respectively. Two rounds of data collection session were done. The second round of data collection was introduced after addressing the comments from the participants on the realism of the drilling experience (beta test). The overall outlook and VR drilling practice was conserved, but the force feedback mechanism was tuned to be more qualitatively realistic to that of a 3-mm CFRP flat panel. More industrially experienced drillers were observed in round 2. The changes and the added realism of round 2 were pretested by same expert drillers participated in round 1 and also expert drillers from round 2. For the first round, data collection was carried out on two groups, namely experienced and novices. All experienced drillers have had a minimum of 3 months of professional drilling exposure on an aerospace manufacturing shop floor, while the novices had none of such exposure. There were 20 volunteers, whereby 11 were categorized as experienced, with the remaining 9 as novices. During the VR drilling, each driller was required to perform three runs of drilling on a VR CFRP panel with a thickness of 3 mm. For each run, there were 20 pilot holes of 5 mm in diameter on the panel, and these holes would then be enlarged upon drilling. RHEA records the position and orientation of each virtual object starting from the first hole until the 20th hole. The data is presented in Cartesian coordinates and quaternions as presented in Fig. 7.9. Further analysis was required to translate these data into drill parameters. Postprocessing was required on the data. A C-sharp (C#)-based program was developed to automatically read and compute the data presented by RHEA into drill parameters as shown in Fig. 7.10.

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Block being moved horizontally during drilling

L1

L0

5mm drilled hole

L2

Original position of the blocks before drilling

Figure 7.6 Mechanism of hole creation.

ART device Drill / end effector

Barco projection (rear)

Haptic arm

Active 3D goggles

Figure 7.7 VR hardware setup.

Haptic-based virtual reality system to enhance actual aerospace composite

Personal protective equipment (PPE)

121

Virtual CFRP panel Virtual drill

Figure 7.8 Virtual drilling environment.

Time

Coordinates x

y

z

Quaternion q1

q2

q3

q4

1.32493

1.83844 1.40498 5.5546

0.720911

0.692874

–0.00498925

0.0137175

1.40832

1.83875 1.40439 5.55458

0.720586

0.693218

–0.00510971

0.0134036

1.51684

1.83918 1.40383 5.55449

0.720414

0.693406

–0.00518238

0.012864

1.61691

1.83964 1.404

5.55508

0.720469

0.693337

–0.00549033

0.0133506

1.70031

1.8399

1.40408 5.55599

0.720562

0.693229

–0.00597062

0.0137143

1.75038

1.84002 1.40421 5.55656

0.720689

0.693085

–0.00636934

0.0141554

2.2424

1.84156 1.40418 5.5558

0.720931

0.692843

–0.00615586

0.0137382

2.53431

1.84184 1.40439 5.55566

0.721208

0.692544

–0.00648841

0.0141607

Figure 7.9 RHEA output.

Seven parameters were computed, which were the offset of the drill tip from the supposed hole (R), maximum overshoot of the drill bit after penetrating the panel (D), maximum drilling angle throughout the drilling process (t), time taken to penetrate the 3-mm panel (s), distance between the two blocks that indicates the maximum opening of the blocks after drilling (L) (which is a proxy for angle), and instantaneous velocity and acceleration of the drill bit, V and A, respectively. From the data collected for V, the average of all forward instantaneous velocities was calculated for each hole. As for the data collected for A, maximum of all absolute instantaneous acceleration was determined for each hole. The parameters are as illustrated in Fig. 7.11. The variable L was introduced in round 2 to act as a proxy for variable t as t was exhibiting large and dynamic fluctuations. L is the distance between two moving blocks as shown in Fig. 7.12. The block reacts proportionally to the drill bit penetration angle. The horizontal entrance angle of the drill bit can be computed from the distance traveled by each block at a given frame rate.

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Hole ID

Time

hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04 hole04

7.02675 7.11856 7.202 7.31025 7.41875 7.56051 7.71901 7.76907 7.84401 7.89414 8.03589 8.14426 8.17764 8.22764 8.26095 8.29432 8.31945 8.33607 8.36107

R 0.6638258 0.5503175 0.4744265 0.4432119 0.4166499 0.4073103 0.3993671 0.4463766 0.4493912 0.4440135 0.3204689 0.2334031 0.2301717 0.2609572 0.2914973 0.3133007 0.4320351 0.4214813 0.4008908

D 0.2723389 0.9522705 1.252319 1.272339 1.282227 1.302246 1.312256 1.302246 1.312256 1.322266 1.372192 1.422363 1.422363 1.432373 1.442383 1.442383 1.602295 1.892334 2.002319

T 2.235459 2.26062 2.229409 2.202115 2.155313 2.151496 2.124122 2.187222 2.223344 2.221494 2.20407 2.251337 2.328349 2.388844 2.451602 2.49975 2.575647 2.561014 2.535358

Figure 7.10 Computed drill parameters.

7.2

Results

Statistical analysis was carried out on SPSS (Ver. 23, IBM Corp). A general linear model (GLM) with repeated measures analysis was carried out on the data in a 2 (novice and experienced) by 3 (runs) configuration, whereby the dependent variables are the 7 performance parameters, namely R (offset of drill tip from supposed hole), D (overshoot of drill tip), t (the angle of drill bit), s (time for penetration of drill), L (displacement separation of blocks within panel), V (forward instantaneous velocity of drill tip), and A (absolute instantaneous acceleration of drill tip). The raw data produced from the data extraction stage contains 60 data points per test subject (20 holes  3 runs). Before feeding the data into SPSS, the results of each run for each person were reduced into means. Thus, the 5 named performance parameters above are actually means for each run. In round 1, the test subjects consist of 9 novices and 9 experienced. A box plot is produced for each performance parameter to illustrate the distribution of the performance data for each test subject. The results of the GLM with repeated measures on R did not show a significant effect on expertise between the novice and the experienced. There were no significant differences between the two groups (F(1,16) ¼ 0.107, P ¼ .748). There were also no significant difference between runs (F(1,16) ¼ 2.404, P ¼ .141), which means test subjects did not differ in terms of performance between subsequent runs. There were also no significant interactions between runs and expertise levels (F(1,16) ¼ 0.809, P ¼ .382).

At t = t0

3mm

At t = t1

A B 10mm

Max D

Reference vector, U t = cos–1

D = AB

tn

t0

t

Driller’s vector, V →→

⎛ U.V ⎞ → → ⎝ |U|.|V|⎠

t0

s = t1 – t0

tn At x = 1.82 At x = 1.807

dL1

dL2

L = dL1 + dL2

Haptic-based virtual reality system to enhance actual aerospace composite

3mm

Instanteneous velocity and acceleration are calculated at every timestamp when the drill bit travels from x = 1.82 to x = 1.807

Figure 7.11 Drill parameters. 123

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Reference vector, U tn

t

Driller’s vector, V t0

tn

t0

dL1

dL2

Figure 7.12 Illustration of variable L inside the CFRP panel.

With parameter D, GLM with repeated measures showed a nonsignificant effect on expertise level between the novice and the experienced (F(1,16) ¼ 3.249, P ¼ .09). There was also no significant difference in between runs (F(1,16) ¼ 1.561, P ¼ .229). There was also no significance in the interaction between expertise and runs (F(1,16) ¼ 0.257, P ¼ .619). GLM with repeated measures carried out on t also revealed no significant difference between the novice and the experienced (F(1,16) ¼ 0.030, P ¼ .865) with also no significant difference between runs (F(1,16) ¼ 0.578, P ¼ .458). There was also no significance in the interaction between expertise and runs (F(1,16) ¼ 0.029, P ¼ .866). The rest of the mentioned performance parameters, namely, L, s, V, and A, were only measured in round 2 as improvements to round 1. Round 2 data collection sought to improve the results obtained from round 1. Improvements were made to the VR system so that the system becomes more skilldemanding; in turn, the performance levels between novices and the experienced will be more distinguished. R was dropped in round 2 data collection as it was perceived that the presence of pilot holes will aid both groups of experienced and novices, resulting in insignificant performance difference between both groups. The results of GLM with repeated measures on the overshoot D improved where there is a significant difference between novices and experienced (F(1,16) ¼ 8.728, P ¼ .008), with no significant difference between runs (F(1,16) ¼ 0.095, P ¼ .762). There was also no significance in the interaction between expertise and runs (F(1,16) ¼ 0.111, P ¼ .743). GLM with repeated measures carried out on t, however, did not have significant difference between novice and experienced (F(1,16) ¼ 0.631, P ¼ .438), while there was significance between runs (F(1,16) ¼ 4.675, P ¼ .045). There was no significance in the interaction between expertise and runs (F(1,16) ¼ 1.751, P ¼ .203). GLM with repeated measures were carried on s, which results in a significant difference between novice and experienced (F(1,16) ¼ 19.835, P ¼ .000), with no

Haptic-based virtual reality system to enhance actual aerospace composite

125

significance between runs (F(1,16) ¼ 0.059, P ¼ .811). The interaction between expertise and runs also reveals no significance (F(1,16) ¼ 0.000, P ¼ .990). With the new parameter L, GLM with repeated measures carried out revealed a significant difference between novice and experienced (F(1,16) ¼ 4.307, P ¼ .053), with no significance between runs (F(1,16) ¼ 0.014, P ¼ .909). There was also no significance in the interaction between expertise and runs (F(1,16) ¼ 0.866, P ¼ .364). GLM with repeated measures were also carried out on two new parameters, namely V and A. For V, there was a significant difference between novice and experienced (F(1,16) ¼ 14.198, P ¼ .001), with no significance between runs (F(1,16) ¼ 0.329, P ¼ .573). There was also no significance in the interaction between expertise and runs (F(1,16) ¼ 0.158, P ¼ .695). For A, there is also a significant difference between novice and expert (F(1,16) ¼ 6.320, P ¼ .022). There were also no significant differences between runs (F(1,16) ¼ 1.239, P ¼ .280) and interaction between expertise and runs (F(1,16) ¼ 0.111, P ¼ .743) (Table 7.1).

7.3

Discussion

For the parameter R, there were no significant differences between novice and experienced group. It suggests that this parameter is basic, and no in-depth training is needed to acquire it since both groups are being aided by the presence of pilot holes. As mentioned, this parameter was dropped in the round 2 data collection. The other parameters, D, L, s, V, and A exhibit similar significance results when tested for significance. In other words, there are significant differences between experienced and novice for these parameters. Hence, the system is able to detect differences in performance between novice and experts, which means hypothesis 1 is valid. If improvements were made to the system, it could be used to train novice drillers (hypothesis 2). The drilling assessment and training criteria can be based on parameters D, L, s, V, and A individually or combinations of them. On the other hand, there were no significant differences between runs for parameter D, s, V, and A. This describes that there were no improvements on the skills measured using those parameters from the first run to the third run. This interprets that the parameters recoded for a driller throughout the three runs were consistent without significant fluctuation. On the other hand, this result also means that the three runs done on the same day would not be sufficient to improve the performance of a driller significantly. Therefore, when planning for a training VR module, more runs over a longer period of time should be conducted to identify the general learning curve. Through that, a conservative training duration and ideal number of runs per session can be observed and used as guide in developing the complete VR training module. Finally, there were also no significant interactions between runs and expertise. This suggests that the parameters recorded are reliable in reflecting upon and unique to an individual’s performance.

126

Overall

Maximum Overshoot, D (mm)

Maximum Angle, t

( )

Time to penetrate, s (seconds)
3

Block Distance, L (10

mm)
2

Maximum acceleration, A (ms )
1

Average velocity, V (ms )

Run 1

Run 2

Run 3

Mean

St. Error

Mean

St. Dev

Mean

St. Dev

Mean

St. Dev

Novice

51.600

1.830

51.720

2.600

51.340

2.310

51.740

1.660

Experienced

44.310

1.660

44.660

7.770

44.170

7.870

44.080

7.410

Novice

6.940

0.540

6.370

1.550

6.850

1.630

7.600

2.170

Experienced

6.380

0.460

6.110

0.950

6.610

1.930

6.410

1.920

Novice

3.720

0.470

3.900

1.350

3.540

1.690

3.730

1.350

Experienced

6.550

0.430

6.720

3.000

6.370

2.560

6.570

1.380

Novice

10.000

0.000

9.700

0.430

9.630

0.230

9.640

0.200

Experienced

9.000

0.000

9.430

0.200

9.430

0.200

9.500

0.230

Novice

41.320

1.910

42.580

7.080

41.000

5.120

40.390

2.800

Experienced

34.850

1.730

35.880

8.710

33.970

7.990

34.700

7.300

Novice

0.038

0.002

0.037

0.004

0.004

0.008

0.038

0.004

Experienced

0.029

0.002

0.030

0.006

0.029

0.009

0.028

0.006

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 7.1 Results of round 2

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127

In terms of direct impact on physical drilling work, the overshoot D is the visually detectable parameter where the depth of penetration can be measured by taking the distance between tip of the drill bit and the last layer (rear) of the virtual panel. The more the overshoot, the more damage it will inflict on the panel via: 1. Pushing out the final layer during drilling. 2. Hitting the drill chuck to the front layer of the panel.

These two defects are most common in current drilling training and also in the actual drilling process on the assembly floor. Via additional analysis, the parameter D can be further mathematically differentiated into V and finally A. Since only the time, s parameter, is the manipulated variable, the results showed that both V and A could also be quantitative criteria that could be used to validate and assess a technician’s performance. The instantaneous acceleration of the drill can be used as the variable that represents the thrust force since force is directly proportional to acceleration. Parallel to our literature review, managing this thrust force to be within drilling threshold would prevent defects. On top of that, an actual accelerometer could be added to the haptic hardware system. The values obtained from the accelerometer could be compared with the values obtained from unity for further validation. Besides being a factor for computing V and A, parameter s alone could reflect on the total time used to drill a hole. Less time for a fixed spindle and drilling speed would practically mean that the driller is less focused and rushing the task, which in turn would easily lead to defects. However, longer than required time for drilling a hole would result in undesired results as well, such as a larger or imperfect hole, which would cause the product to fail under the assembly tolerance criteria. Therefore, maintaining the drill time per hole within threshold is expected to reduce fiber pushout issue. Although this study has proven that VR technology is capable of assisting physical training, its effectiveness only extends to the training of coarse physical activities. Moreover, the result of round 1 also has proven that the activity should require certain “difficulty,” which in this case is the hardness of drilling process. The difficulty is vital in promoting the trainees to exhibit more skills, which in turn will be captured and trained using the VR system. In this research, the minimal difficulty level was obtained via trial and error. Based on current findings, it might be safe to say that any activities that are not physically skill-demanding, such as carrying an item, using a torque wrench, and switching on and off a machine, are not be suitable to be trained using this VR system.

7.4

Conclusion

The VR aerospace manufacturing training system devised to recreate drilling of a CFRP panel is able to differentiate the performance of a novice and an expert. It was statistically proven that this VR system can be used to train beginners to learn aerospace-grade CFRP drilling, which could significantly reduce the cost of training.

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Each of the parameters D, s, V, A, and L, tracked from the performance of the participants, showed a significant difference between groups of different level of skill, suggesting that these parameters are unique and trainable through VR.

Acknowledgments This study was the product of collaborative effort from Aerospace Malaysia Innovation Center (AMIC), MARA Aerospace and Technologies (M-AeroTech), University of Malaya (UM), National University of Malaysia (UKM), supported by University of Malaya UMRG Top Down Programme (Grant No. RP027-2014AET).

References [1] Gilpin A, Tool A. Tools solutions for machining composites [Internet]. Elsevier Ltd Reinf. Plast. 2009;53(6):30e3. https://doi.org/10.1016/S0034-3617(09)70260-7. [2] Park K, Beal A, Kim DD, Kwon P, Lantrip J. Tool wear in drilling of composite/titanium stacks using carbide and polycrystalline diamond tools. Elsevier B.V. Wear [Internet] 2011;271(11e12):2826e35. https://doi.org/10.1016/j.wear.2011.05.038. [3] Lin C, Chen K. Drilling carbon fiber-reinforced composite material at high speed. Wear 1996;194:156e62. [4] Wang H, Sun J, Li J, Li W. Investigation on delamination morphology during drilling composite laminates. Int J Adv Manuf Technol 2014:257e66. [5] Dreyfus SE. The five-stage model of adult skill acquisition. Bull Sci Technol Soc 2004;24: 177e81. [6] Holmes M. Aerospace looks to composites for solutions. Elsevier Ltd. Reinf Plast [Internet] 2017;61(4):237e41. https://doi.org/10.1016/j.repl.2017.06.079. [7] Training ACP, Improves S, Reliability P. White paper immersive virtual reality plant. [8] Mastaglio T, Ph D, Bhatt J, Christensen N. A fully immersive virtual reality training system for rocket fuel mixing operations, vol. 56; 2017. p. 1e9. [9] Kovar J, Mouralova K, Ksica F, Kroupa J, Andrs O, Hadas Z, et al. Virtual reality in context of industry 4.0. [10] Baradaran H, Stuerzlinger W. A comparison of real and virtual 3D construction tools with novice users. 2005. p. 1e5. [11] Adams RJ, Klowden D, Hannaford B. Virtual training for a manual assembly task. Hapticse 2001;2(2):1e7. [12] Cormier J, Ph D, Kozhenikov M, Ph D, Donoff RB, Karimbux NY, et al. Performance of dental students versus prosthodontics residents on a 3D immersive haptic simulator. J Dent Educ 2014;78(4):630e7.

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8

Chhugani Tushar 1 , Routray Ralish 1 , M. Rajesh 1 , M. Manikandan 2 , R. Rajapandi 3 , V.R. Kar 4 , Kandasamy Jayakrishna 1 1 School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India; 2Department of Mechanical Engineering, Amrita College of Engineering and Technology, Nagercoil, Tamil Nadu, India; 3National Institute of Technology Karnataka, Mangalore, India; 4Department of Mechanical Engineering, NIT Jamshedpur, Jharkhand, India

8.1

Introduction

By definition, structural health monitoring (SHM) is a method that analyzes the health of a structure by evaluating it for any damage, and also it provides a short- or long-term solution for the damage caused. It is a multidisciplinary technology meant for the development and implementation of methods that can inspect and detect the damage by integrating with structures [1]. It also includes many methods to diagnose and forecast the damage. Applying SHM systems can determine the damage that occurs inside the material, its accurate position, the speed of the damage propagation, and, finally, it can make a forecast of the residual life of the structure [2]. SHM emerged from the field of smart structures and now it is used in disciplines such as structural dynamics, materials and structures, fatigue and fracture, NDT and evaluation, sensors and actuators, microelectronics, signal processing, and many more [1]. SHM is based on an NDTE method. It has been around for more than two decades. It is mostly confined to institutions and industries dealing with aerospace and civil engineering. But the numerous amount of publications, conferences and workshops, and research projects in many universities and R&D centers prove that it has gained quite a lot of attention, not only in engineering but also in the world of physics, electronics, computer sciences, etc. Over the years there have been many new innovations in the SHM world. Many new techniques have been introduced. Many new methods have been proposed but not yet worked upon. In SHM the health of the structure is directly related with the health of the system. Tandon and Choudhary [3] reviewed different vibration and acoustic techniques such as sound pressure, sound intensity, and acoustic emission (AE) for damage detection in rolling element bearings. Staszewski et al. [4] used both active and passive approaches for impact damage detection in aerospace composite structures. The active method was based on guided ultrasonic waves that didn’t require any signal processing,

Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00008-3 Copyright © 2019 Elsevier Ltd. All rights reserved.

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whereas the second method heavily relied on signal processing but didn’t require any sophisticated instruments. Mba and Rao [5] presented a comprehensive review on the application of AE technology to condition monitoring and assessment of rotating machinery. Loutas et al. [6] used three different online monitoring techniques to condition monitoring of rotating machinery and gear boxes specifically. They found out that by integrating vibration monitoring, AE technique and oil debris monitoring, the reliability and diagnostic capacity of the monitoring scheme can be increased. Wang et al. [7] assessed gear damage monitoring using three different wellestablished techniques: phase and amplitude demodulation, beta kurtosis, and wavelet transform. They found out that beta kurtosis is a very reliable time-domain diagnostic technique and that diagnosis based only on dominant meshing frequency residual is not a suitable gear monitoring technique. The evolution of many new materials in the form of composites with unique properties, ceramics, polymers, and super alloys has dramatically changed the smart materials industry. With the help of these new smart materials, accurate and precise sensors and actuators are being manufactured that make the job of SHM very easy. For example, there are now sensor patches, e.g., PZT patches, PVDF films, and magnetostrictive materials like Terfenol-D, in the form of thick films or in particle form that bond with the structure and have given rise to a new concept of smart structure [8]. Balaji and Sasikumar [9] used graphene oxideebased monitoring technique for polymer matrix composites. Montalvao et al. [10] have reviewed many different vibration-based techniques to diagnose and prognose composite-based structures. Kalamkarov et al. [11] used a modified pultrusion technique to embed FabryePerot fiber-optic sensors in glass fiber-reinforced polymer (GFRP) rods aiming it to use as a long-term SHM system in various infrastructures and concrete reinforcements. Results showed that if these sensors are embedded rather than attached to the surface, they can play a dual role of reinforcing the elements with its mechanical properties as well as strain monitoring with its sensors. Lugovtsova and Prager [12] used guided ultrasonic waves to detect cracks in metal matrix composite pressure vessels. Leng and Asundi [13] used extrinsic FabryePerot interferometer (EFPI) and fiber Bragg grating (FBG) sensors to monitor the cure process of carbon fiber reinforced polymer (CFRP) composite laminates in real time. They found out that both embedded EFPI and FBG sensors can be used to monitor the cure process, and both of them show excellent correlation during the monitoring test. Murukeshan et al. [14] also used FBG sensors for monitoring the cure process of composites under 3- and 4-point bending conditions. Lau et al. [15] have used FRP and optical fiber sensors in concrete repair and structural health monitoring. Ciang et al. [16] used fiber optic strain monitoring and/or damage detection system to obtain global load conditions on a wind turbine and built-in sensors to accurately locate the damage up to submicron size crack. But the limitations of the method were the number of sensors to be used and signal noise during the operation of wind turbine. In this chapter, recent trends in SHM having great potential for wide field applications have been discussed.

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131

Benefits of implementation of structural health monitoring

Conventional methods are inefficient as compared to SHM. SHM considers to replace conventional methods in terms of mechanisms used to modify responses in changed circumstances. Thus, the use of SHM leads to increase in reliability, utility, and reduction of maintenance cost. There are other prominent benefits such as life extension of aging structures, integration of design of structure including its complete management, damage identification of structure, changing the older inspection to condition-based maintenance [17], which helps to use the talents of engineers to look ahead to public safety [18]. SHM is used for inspecting old buildings regarding the faltering strength, to study the environmental factors that may have been skipped during building process, checking the older pipes for corrosion, looking for safety concerns, focusing not only on industry purposes but also on public safety. Modern instruments now work efficiently in studying and analyzing the prominent information regarding the integrity of structures, tracking the geotechnical information regarding the foundation of structures. Life of SHM has increased along with reduction of cost. Harm to the environment is reduced at an efficient cost. Fig. 8.1 shows the influence of SHM on the quality of the structures. The cost of maintenance of a structure with SHM is less as compared to that without SHM, and also the reliability of the structure with SHM is greater compared to that of the structure without SHM [17]. SHM sets a basis in supplying recommendations for analyzing structures. It helps in giving different terminologies for evaluation and provides data free of subjectivity. SHM has a great impact on the maintenance of structures [19], and it has earned a better position among all the monitoring methods by reducing the need of maintenance. Using SHM technologies can reduce maintenance and give better result-oriented analysis of structures, which enhances safety. SHM system can be a key factor to appraise the condition of a defect going through a critical size. This method will help avoid any damage from being unobserved. SHM has come up with many important real-life benefits. Fig. 8.2 shows one of the major benefits of SHM, which includes validation of evaluation process of subsequent Structure with SHM Structure without SHM

Quality of the structure

Reliability

Cost of maintenance Lifetime of the structure

Figure 8.1 Variation in the quality of the structure (with and without SHM) with their lifetime.

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Figure 8.2 Validating the evaluating process for subsequent permit trucks.

permit trucks. SHM is one of the empowering advances to revolutionize the future design, improvement, and maintenance of aircrafts. The utilization of SHM may decrease upkeep costs by more than 40%, by savings in dismantling and examination times, and development in aircraft [20]. Thus SHM has a huge scope in various fields of engineering, and its use will help in a very effective maintenance of systems.

8.3

Challenges for structural health monitoring

Tedious design work is required for each SHM system. The systems have to depend on sensors. Though there are many benefits of using SHM, in a few aspects it still lags behind. In SHM there is no continuous monitoring, so normalization of the data becomes tough. On the other hand, it becomes an arduous act to collect the necessary details required for the maintenance. Some other important challenges are: • • • • • • •

Along with a huge amount of data collected for SHM, the heterogeneity of data makes the process quite complicated [21]. It is hard to discover an SHM process that can perform its operation with high reliability. Unjustified operations result in high costs for systems [22e24]. Cost of SHM should not cross the gained benefits, neither should it be prohibitive so that mass adaption degrades. Lack of confidence could not assure the proper benefits of SHM, especially in case of maintainability of modern aircrafts. Detection of local damage, as SHM finds it easy to detect global damage while it may skip local damages. Before implementing SHM, return on investment has to be kept in mind as it becomes difficult to ensure whether the entire amount invested in implementing SHM will be returned or not [25].

Nowadays, composites are prone to the damages that are scarcely visible and mainly caused by impacts or excessive loads. Neglecting these defects will result in degradation of structural reliability. At present, there are some aspects that will act as obstacles in implementation process of SHM for composite structures. These may include proper system installation, exact calibration, and appropriate connections

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500 400

Colder

Hotter

300 200

76F 78F 80F 82F 85F 87F 90F 92F 96F 100F

Lines representing different temperatures

100 0 –100 –200 –300 –400 –500 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300

Time(seconds) Figure 8.3 Variation of signals with changes in temperature.

within the system [26]. Another major challenge for SHM is its installation at difficultto-access positions as the process of sensing needs different parameters, such as supply of power, transmitting of data, and servicing at inaccessible positions [27]. Sometimes the signals produced by sensors get disturbed due to environmental conditions such as temperature changes, as shown in Fig. 8.3.

8.4

Testing using nondestructive analysis

Nondestructive testing is a wide gathering of examination procedures utilized as a part of science and innovation industry to assess the properties of a material, segment, or framework without causing any harm to them [28]. NDT techniques use endless supplies of electromagnetic radiation, sound, and other flag transformations to inspect a wide assortment of articles (metallic and nonmetallic, nourishment items, relics, and ancient pieces), structures, and condition of articles without damaging them. The increasing load on highway bridges due to increasing heavy goods vehicle traffic, aging, and problems with the durability of structures may lead to obstructions of traffic with ensuing severe economic damage. Effective and reliable condition assessment tools are an important part of the ongoing efforts to evaluate and maintain bridge structures. Currently in many countries the inspection and assessment of bridges are carried out every one or two years. But the evaluation is usually carried out visually. Therefore damages are identified only after they are visible to naked eyes. This costs many lives every year due to bridges collapsing. These can be avoided by using nondestructive testing. This method provides a great amount of information that cannot be deduced by the naked eye.

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A wide assortment of NDT techniques assume significant roles in testing of composite materials [29]. Numerous techniques are used in the composite NDT field, including ultrasonic testing [30], thermographic testing [31], infrared thermography testing [32], and radiographic testing [33]. Basic plastics and composites, for example, fiber-fortified polymers (FRPs) speak to a wide class of materials finding expanded use in scaffold and interstate-related applications. These materials offer vital preferences, including erosion protection, and furthermore, formability. Glass-reinforced FRP material are not conductive and eddy current and the electromagnetic ultrasonic procedures, which depend on excitation of Foucault currents in a conductive material, have extremely constrained application [34]. Customary methods based on NDT techniques are improper and regularly deceptive when connected to anisotropic and inhomogeneous composite materials. In propelled innovation applications, for example, aviation, and with mechanical accentuation on financial aspects and well-being, it is basic to utilize down-to-earth NDT techniques for composites [35]. Harm can emerge in composite materials amid material preparation, manufacture of the part or in-service exercises among which delamination, splits, and porosity are the most widely recognized imperfections. The geometry, physical and material properties of the part being tried are imperative factors in the appropriateness of a method. Prabhakar et al. have surveyed these NDT strategies and analyzed them regarding attributes and appropriateness to composite parts [36]. Pelivanov et al. [37] used NDT of fiber-reinforced composites with a new fiber-optic pumpeprobe laserultrasound system. A present-day challenge is the requirement for quality checks of composite parts, together with substructures collected utilizing cement holding, cocuring, and thermoplastic welding. Groves reviewed present and rising NDT and SHM advances and portrayed some of the difficulties in examination and observing of composite air ship structures. Location and characterization of deformities and harm that emerges in marine composites is basic, to guarantee the protected and solid operation of marine vessels and structures. Ibrahim [38] reviewed the kinds of imperfections happening in marine composites, and presented the idea of NDT to examine structures for blemishes without decreasing their future value. Shokrieh and Mohammadi [39] discussed the main methods that fall into the category of nondestructive methods of residual stress measurement. Crane [40] analyzed radiography method in ensuring a base quality in composite materials and used customary film-based radiography and the new strong state radiography, since customary radiographic methods are not applicable to composites. In this way a strategy that permits consistent well-being checking of the composite progressively and in situ would be to a great degree valuable. Ultrasonic guided wave-based SHM innovation is a standout amongst the most noticeable choices in nondestructive evaluation and NDT strategies. To examine the practicality of guided wave-based SHM for composite funnels, engendering attributes of guided waves in an epoxy cross breed carbon/glass strands pipe are methodically considered utilizing finite element (FE) re-creation and trials [41]. Newman analyzed shearography NDT for composites. Shearography is noncontact and offers high throughput. By applying small pressure changes, for example, warm, interior weight, outer vacuum loads, ultrasonic or sonic vibration, or microwave

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energy inactive assembling imperfections such as fiber spanning, voids, broken fibers, poor fiber compaction, etc. [42]. Acoustic emission monitoring can also play a very effective role in enhancing safety, ensuring availability, and reducing the repair/refurbishment costs of bridges, thus playing an important role in monitoring of composites. A well-established and accepted application of ground-penetrating radar (GPR) is the accurate condition assessment of bridge decks as well as other reinforced-concrete structures. GPR does not require the removal of existing asphalt. Allahyari et al. [43] used four specimens of steel-concrete composite slabs (two DHC and two DHLC) to assess some of the main dynamic characteristics, such as, damping ratio, natural frequencies, and frequency response functions, by means of NDT with hammer excitation. They found out that FEM model can be used for structural performance prediction and damage detection of composite decks with reliable accuracy. Lorenzi [44] used inspection-computer interface for the nondestructive monitoring of infrastructure. The demand for this method will most likely increase since maturing infrastructure will require consistent assessments for ideal arranging of repair and intervention, so as to guarantee performance and human security. Nesvijski [45] has developed dry point contact transducers as a plan for emerging applications. Gracia [46] has aimed to characterize the internal structural configuration along with the stone quality of cultural heritage buildings by using ground penetrating radar and seismic tomography. Busch and Soltani [47] used time-of-flight spectroscopy method for NDT analysis of material detachment from polychromatic ally glazed terracotta artwork. This method can be used to find out and measure air gaps that are present under the glass detachments. Some NDT techniques include: Visual Testing: It is the most established and common NDT method. It is normally the initial phase in examination procedure to review an assortment of item frames, including casting, forgings, machined segments, and welded components, as per NDT training at a test center. Eddy Current: It is one of numerous electromagnetic testing techniques utilized as a part of NDT, utilizing electromagnetic induction to identify and portray surface and subsurface imperfections in conductive materials. Magnetic Particle Inspection: It is an NDT process for recognizing surface and shallow subsurface discontinuities in ferromagnetic materials. Three types of electric currents are used for this process: alternating current (AC), full wave direct current DC [48], and half wave DC. Liquid Penetrant Inspection: This is a cheaper examination strategy that is used to find surface-breaking defects in all nonpermeable materials. This method can be used for testing both ferrous and nonferrous materials. Industrial Radiography: NDT technique where many sorts of fabricated segments can be inspected to check the inner structure and integrity of the specimen. This technique can be performed using either X-rays or gamma rays. Its applications include security inspection at airports, inspections of welds and products. Ultrasonic Testing: Ultrasonic waves are used for testing the specimen. These waves are made to propagate through the object. A typical case of this method of inspection is the estimation of thickness of the object, for instance, to screen pipework corrosion. This technique is frequently performed on steel and different metals and amalgam.

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8.4.1

Limitations of present-day NDT techniques

Every structural material is naturally ‘imperfect’ if one examines at sufficient sensitivity [49]. It is very important to properly recognize and quantify the defects in order to find the most appropriate solution to them and prevent the equipment or structures from further damage. Huge number of impediments of the present-day testing methods are based on the difficulty of describing defects in an adequately quantified manner with the goal that thresholds can be easily set. Halmshaw [49] concluded that there exist some limitations of radiography such that radiography on film or radioscopy consisting of an image generated by computer will produce a “picture” of flaws that can’t be measured accurately for through-thickness and height. Similarly, for ultrasonic testing, length and height are easy to be measured but the nature of flaws is not easy to determine. Some other major limitations of NDT [50] over destructive tests are: • • •

•

• • • • •

NDT measurements are generally qualitative; there is no direct reliability of measurements to get verified There is a need for proper experience and skilled judgment For visual inspection NDT: • There is a requirement of good luminescence • It is only suitable for inspection of surface defects • Good eyesight is required [51]. • Cannot recognize nearer wavelengths • Difficulties in recognizing brightness contrasts For liquid inspection NDT • Can’t be used for porous material • Precleaning and postcleaning operations are mandatory • Irrelevant indications will come if inspecting irregular surface • Penetrant should have the ability to wet the surface • Very small surface discontinuities, for instance, machining marks will become apparent in a very short span of time Ultrasonic testing (imaging) is one dimensional, so the defects that are present parallel to the axis of the test will not be visible [52] For magnetic particle inspection NDT there is a sudden decrease of sensitivity observed with the depths below the surface to be inspected. It is applicable to only those materials that can conduct electric current and influence magnetic lines of flux. In eddy current testing, as the direction of flow of the eddy current is parallel to the surface of the exciting field, this leads to skipping of some lamellar discontinuities parallel to the surface. In hydrostatic testing, unintended damage can be caused to the system as a result of excess pressure application. For radiography, costly equipment is required for interpretation of images. Safety precautions are a must for this technique.

Another limitation of NDT is that accurate sizing of cracks by using NDT techniques is costly [53]. In terms of detection of cracks and their sizing, there are

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certain limitations as sometimes during fatigue assessments, use of postulated cracks is being done. Ultrasonic methods are also time-consuming and complicated. There are some limitations of NDT with regard to damage tolerance. Damage tolerance approach is mainly concerned with the crack propagation and makes use of crack mechanisms to set up the capacity of the structure to work securely for a predefined timeframe. NDT characterizations of damages in the composites present in modern aircrafts are very difficult. DumpstereShafer data fusion approach is used to make use of NDT, with these limitations: • •

The results obtained were poor if the systems were in relative disagreement. Small changes resulted in prominent variation in output decision.

8.5

Comparison between NDT and SHM

Nowadays, NDT can be performed at the manufacturing stage itself because of the innovative advances being made. For example, during additive layer manufacturing only, it is used with the goal that the potential defects are found during manufacturing stage as opposed to being repaired later [54]. SHM utilizes the data accumulated at the manufacturing stage, and the flaws within the within the tolerance are located using NDT. SHM is a general point whereas NDT is used for introductory assessment. Sandeep et al. [55] researched the importance of NDT in reinforced concrete structures, which includes defect identification such as honeycombs and voids. GPR is also efficient and is mainly used for thickness measurement and reinforcement. Research work is underway in Council of Scientific & Industrial ResearchStructural Engineering Research Centre (CSIR-SERC) for effectiveness of Advanced NDT for studying different features in RCC and PSC structure. SHM is mainly focused on condition-based maintenance. It is becoming more important nowadays in various engineering fields such as mechanical, civil, and aerospace. NDT methods are basically localized in nature [56] whereas SHM deals in a broader perspective. NDT requires a prior knowledge of the location of damage whereas there is no such requirement in SHM. SHM is a better and modified version of NDT [57]. Table 8.1 shows the main differences between NDT and SHM. SHM is able to save costs and maintain a proper level of safety taking into consideration the working conditions [59]. SHM is used widely by the aerospace community, especially in a situation of aging of aircrafts [60]. Researchers [61] found out that making use of SHM will be able to reduce the total cost of maintenance by 30% for an aircraft fleet. NDT is slowly moving toward a more lasting attachment and continuous monitoring, and hence toward SHM [62]. Smaller and cheaper sensors have been brought into the process of changing from NDT to SHM [63]. In NDT there is also a requirement of external probes [64]. Disassembling is done in SHM only when required, thus helping to reduce costs as compared to NDT, which requires high cost for maintaining of the system. Thus SHM due to its many advantages over NDT has a good future in the engineering field.

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Table 8.1 Comparison between NDT and SHM NDT

SHM

Advanced sensing technology is used

The sensing technology is in developing stage

Manual operator intervention is required [31]

Automatic or semiautomatic

More reliable and, hence, there is no need of analyzing statistically

Reliability can’t be predicted and there is no need of analyzing statistically

Smaller time scope

Larger time scope

Risk is used as a tool

Risk is not used as a tool

Requirement of labor and cost is more [58]

Less requirement of labor and cost

Evaluation process is offline

Evaluation is online

Maintenance is required from time to time

Maintenance is condition based

No permanent sensors are used to provide continuous information about the state of the structure

Makes use of permanently integrated sensors [57] for providing continuous or on-demand information regarding the state at high-stress locations

Accuracy of diagnosis depends totally on the resolution of the measurements taken [57]

Resolution of measurements is not the only key for accuracy of diagnosis

8.6

Structural health monitoring

SHM is a process of damage detection and avoidance of important resources, for instance, pillars, bridges, buildings, stadiums, pipelines, etc. Different types of detectors are being researched for examination of structural integrity. This is where SHM technologies come into the picture, as they are not only effective for damage detection but also very efficient in providing lifetime prediction of the structure. The greatest challenges in designing an SHM system are knowing what changes to look for and how to identify them [65]. This calls for new age innovative techniques. There are many emerging SHM technologies available now and many are being researched. For instance, Kessler and his team [65] managed to determine the presence of damage easily and accurately by monitoring the transmitted waves with PZT using Lamb wave technique. This brings us to a very important and widely used “piezoelectric materialsebased sensors and actuators.”

8.6.1

SHM for polymer composites including metal matrix composites

In recent years, the use of composites has increased exponentially. Almost all industries are using composite materials to manufacture different parts or build different structures. Due to its light weight and high strength the composites have become a

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necessity for many industries. Unlike conventional materials, composites are made up of two or more different materials. Now these may sound like alloys, but they are not. Alloys also comprise multiple materials, but the properties of the alloy resemble that of the base material. For example, steel has many different materials in it but it is not a composite as the properties of steel resemble that of iron. But in composite materials, the properties of the composite are totally different from that of the constituent materials. Another interesting property of composites is that they are anisotropic. This means the properties of composites change in the direction of applied changes. This is the main source of problems in monitoring the health of a structure made of composites as the damage is difficult to predict. This is why continuous monitoring is needed in case of a structure that is made of composites. This calls for advanced techniques in structural health monitoring, such as used by Schulte and Baron [66] who used the piezo resistivity method for the structural health monitoring to identify internal damage of CFRP laminates. They used the variation in electrical resistivity of CFRP specimens under different loading as the basis for SHM. Permanent deviation from the measured resistivity would mean damage in the structure. A similar kind of technique was used by Balaji and Sasikumar [9], who embedded nickel-based alloys such as karma and nichrome into the composites to monitor the damage induced in the composites. The piezo resistivity response of the nickel alloys was used to predict the damage state of the composites, under real-time loading. Under gradually increased loading, if the increase in resistivity of the structure is minimal then the damage caused is nothing more than development of micropores. But if there is any abrupt increase in the resistivity of the structure, then it indicates damage in the structure. Another widely used composite is glass fiberereinforced polymer composite, particularly in aviation industry due to its light weight. Balaji and Sasikumar [67] used reduced graphene oxide (rGO)-based SHM for polymer composites. They embedded rGO-coated glass fibers into GFRP composites and measured the piezo resistivity of the structure under different loading conditions. Based on the deviation of fractional change from mean elastic response, they identified whether there is any serious damage in the structure or just the development of microcracks. The electrical resistance change method is a promising method for SHM of composites. In their work, Joo et al. [68] created Cu electrodes on CFRP substrates by flash light sintering of Cu nano-ink and then characterized the fabricated Cu electrodes using scanning electron microscope and X-ray photoelectron spectroscopy to sense the damage caused in the composite. Li et al. [69] quantitatively evaluated the damage caused in CFRP using a noncontact electromagnetic sensor. They used the EM sensor along with coupled spiral inductors to precisely detect the location, depth, and width of subsurface defects. In the field of metal matrix composites, Montinaro et al. [70] used flying laser spot thermography to detect and characterize disbands on fiber metal laminate hybrid composites. They used the method on a Glass Reinforced Aluminum Laminate (GLARE) sample which is used extensively in aerospace industry. By recording the changes in standard deviation in temperature of the monitored area, they were able to detect position, size, and sometimes the shape of the defect. Lugovtsova and Prager [71] used guided ultrasonic waves to detect the main damage in metal composite pressure vessels.

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8.7 8.7.1

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Emerging SHM technologies Piezoelectric effect

In 1880 Jacques and Pierre Curie discovered piezoelectric effect while carrying out some experiments using quartz crystals [72]. This probably makes piezoelectric materials the oldest type of smart materials ever known [72]. Simply put, piezoelectric effect means generation of electricity by a body if the shape of the material is changed by applying some mechanical force and vice versa. The change in shape can be related to the change in the crystal structure of the material. Piezoelectric materials have both ordered and disordered crystal structure. The ordered structure is equivalent to the polarization of the molecules and the disordered structure is equivalent to the nonpolarized form of the material. Now if a mechanical force is applied to the material to change its shape, the molecules reorganize themselves creating a voltage difference that generates an electric field as shown in Fig. 8.4(a). This is piezoelectric effect. One of the biggest benefits of the piezoelectric materials is that the time lag between shape change and generation of electric field is very, very small, or in other words, the effect is instantaneous.

(a) Mechanical force

V

Induced voltage

(b) Induced mechanical deformation Applied voltage

Figure 8.4 (a): Mechanical forceeinduced electrical voltage [72]. (b) Electric voltageeinduced mechanical deformation [72].

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Natural piezoelectric materials: • • • • • • • • • • • • • •

Quartz e SiO2, mineral composed of silicon and oxygen in SiO4 tetrahedron framework Berlinite e AlPO4 (aluminium phosphate), occurs as a rare mineral at high-temperature metasomatic deposits Rochelle Salt e KNaC4H4O6$4H2O (potassium sodium tartrate), the first material discovered to exhibit piezoelectric properties Tourmaline group minerals e a crystalline boron-silicate (cyclosilicate) mineral containing elements such as Al, Fe, Mg, Na, Li, and K Sucrose e C12H22O11 (common table sugar) Topaz e Al2SiO4 (F, OH)2, silicate mineral of aluminium and fluorine Lead titanate e PbTiO3, lead salt of titanic acid Bone e bones also exhibit piezoelectric properties due to presence of collagen Silk e natural protein fiber Tendon e collection of connective tissue that connects bones to muscle Wood due to its piezoelectric texture Enamel Dentin DNA

Artificial materials: • • • • • • • • •

Langasite e La3Ga5SiO14 Lithium niobate e LiNbO3, Lithium tantalate e LiTaO3 Gallium orthophosphate e GaPO4 Barium titanate e BaTiO3 Potassium niobate e KnbO3 Sodium tungstate e Na2WO3 Cadmium sulphate Lead zirconium titanate (PZT) e the most commonly used piezoelectric material

Ionic polymers can show piezoelectric effect but in order to do that they have to be wet [72]. After that an electric current is passed through it to produce a change in its crystalline structure, and hence its shape as shown in Fig. 8.4(b).

8.7.1.1

Piezoelectric coefficients

Piezoelectric coefficient is the measure of volume change when a piezoelectric material is subjected to an electric field [73]. Some common piezoelectric coefficients are given in Table 8.2.

8.7.1.2

Piezoelectric sensor used in SHM

This sensor works in two ways. SHM systems can be “active” or “passive.” In passive systems, the sensor only “listens” to noises originating from the crack propagation or to the variations in frequencies responded to by the structure [74]. However, in “active” systems, the structure generates sound waves in the material. When high-frequency sound pulses (emerging from the sensor) hit the material with

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 8.2 Piezoelectric coefficients [73] Piezoelectric crystals

Coefficient (d) [10L12 m/volt]

Electromechanical conversion factor (K)

Quartz

2.3

0.1

Polyvinylidene fluoride, PVDF

18

e

PbNb2O6

80

e

Barium titanate

100e190

0.49

PbZrTiO6

250

e

Rochelle Salt

350

0.78

PZT

480

0.72

Output of electrical energy Where, d ¼ sP where, P is polarization and s is stress, K2 ¼ input of mechanical energy or

Output of mechanical energy input of electrical energy .

different impedance (density and acoustic velocity), they are partly reflected or absorbed. Thus by analyzing the transmission between these two transducers, it can be detected whether there is a flaw in the structure or not. This includes complex signal processing and analysis. Typically monolayer piezo transducers providing low power, high sensitivity, and wide frequency range are used for SHM applications, for example, NCE51 material.

8.7.2

Acousto-ultrasonics method

Acousto-ultrasonics (AU) is derived from “acoustic-emission monitoring” and “ultrasonic characterization.” It was first invented by Alex Vary at Lewis research center of the National Aeronautics and Space Administration, in Ohio [75,76]. The method uses monitoring and analyzing the acoustic signals received from an insonified test material. It is a highly sophisticated and advanced method using digital signal processing and pattern recognition algorithms. AU considers the entire ultrasonic response, in time as well as in frequency, of the entirely insonified material. First, a number of measurements are repeated a number of times on many identical specimens. After many feature extraction processes on all the waveform data, templates are created for each feature. Then these templates are tested by examining the same specimen, and the system is tested for its feature recognition capability. If the reliability is not up to the mark, it has to be changed, and then everything will be repeated until it is reliable for feature recognition [75e77]. Then comes the inspection part; the inspection system contains all the transducers and a mechanical subsystem to move the parts and to place the transducers in contact with the parts. It also contains an ultrasonic pulser/receiver unit, and a switch to control everything, DSP boards, and a computer as shown in Fig. 8.5. After all the parts are in place, all the transducers are

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143

Active sensing

Actuator

Damage

Sensor

Passive sensing

Sensor

Damage

Figure 8.5 A typical AE/AU system consists of signal detection, amplification, data acquisition, processing, and analysis.

brought into contact. Then using the switches, waveform data are acquired and processed for the following: • • •

Signal enhancement Feature extraction Signal classification

Overall, each feature is compared against the respective template and a final decision is reached whether or not there is any damage [75e80].

8.7.2.1

Applications

AU method is useful for the monitoring and testing of structures having repeatable shapes or components [77]. The method has very short inspection time. The method can be used for very complex material systems. It has also been used for strength prediction of composites, wood, titanium, and other adhesive bonds. It has also been used for corrosion detection in riveted aluminium plates. And it has been used for tire damage evaluation, thermal-oxidative damage detection, and rocket motor propellant bond-line adhesion.

8.7.3

Acoustic emission testing

Acoustic emission means generation of transient elastic waves due to a sudden change in the stress distribution of a material. When a structure is exposed to external impetus

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

(such as temperature or pressure), energy is released from inside of the material in the form of stress wave, which transmits to the surface. This is then caught by the sensor. Detection and evaluation of AE signals can be very valuable in determining the origin and importance of discontinuity in a material. AE testing (AET) is different from other NDTs in two ways. The first difference is that, instead of supplying energy to the structure, it just listens for the energy released by the structure. The second difference is that AET deals with changes in the material rather than stagnant processes. This is particularly useful in differentiating between a developing and a static crack in the object. But AET fails to give correct results because of two things. The first is that it can only qualitatively determine how much damage is contained inside the structure. For quantitative results, we have to use other techniques. The second is that AE processes are generally loud, which contributes to extra noise to the signals, which may give inaccurate results.

8.7.3.1

Mechanism

When a stress is exerted on a material, a strain is induced in the material as well. Depending on the magnitude of stress and strain produced, the effect may lie in the elastic or plastic region, i.e., the material may return to its original state or the deformation may become permanent. So when the material goes into plastic deformation or it is loaded near or at its yield stress, detectable acoustic emission takes place. On the microscopic level, as the deformation occurs, atomic planes slip through the movement of various dislocations. These movements and dislocations release energy in the form of elastic waves, which are called acoustic waves [81]. These waves are detected by a sensor. The signal that is detected by the sensor is very low in intensity because when an AE wave passes through a material, its elastic and kinetic energies are absorbed and converted into heat. Also its amplitude decreases by 30% (50% for 3D structures) every time the distance doubles from its source. Also due to geometric discontinuities and structural boundaries, wave scattering takes place, which further decreases the signal intensity. Therefore the AE wave is passed through a multitude of amplifiers, band filters, and transducers to increase the signal strength, filter out unnecessary noises and high frequency waves, and convert the AE wave into electric signal, respectively, before it reaches the measurement circuit [82] as shown in Fig. 8.6. Here it is compared with a threshold voltage value that has been preprogramed by the operator. Now each time the threshold voltage is exceeded, the circuit will release a digital pulse. This will continue for the predefined amount of time. After that the data is read into the computer and the circuit is reset.

8.7.3.2 • • • • •

Applications

Weld monitoring Pickup truck integrity evaluation Pressurized jumbo-tube trailer evaluation Bridges Aerospace structures

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145

Storage Display

Data buffers

Micro-computers

Printer

Keyboard

Sensors

Main Preamplifiers amplifier with with filters filters

Measurement circuitry

Figure 8.6 Diagram of a four-channel acoustic emission testing system. • • •

Glass-fiber reinforced structures such as fan blades Material research Detection and location of high-voltage partial discharges in transformers

8.7.4

Electrical-mechanical impedance method

The electrical-mechanical impedance (EMI) method stands out from all other methods due to its usage of light and small-sized transducers and the shear simplicity [83]. They are like stickers or patches that are patched on the structure to be monitored. This allows monitoring of a large area of the structure without altering its mechanical properties by using a large number of sensors. This makes is useful to monitor and evaluate structures that are aerodynamic and need as little weight as possible, such as aircraft.

8.7.4.1

Mechanism

EMI almost always uses piezoresistors (mostly PZT ceramic). However, recently other transducers have been used that are made up of microfiber composites and piezoelectric diaphragms [83,84]. In this method, the PZT transducer is excited by a sinusoidal waveform with low amplitude, at frequencies ranging from 30 to 1000 kHz, depending on the structure and type of application. Lower frequencies cover a large sensing area whereas higher frequencies are used to exactly point out the position of damage. The advantage of high frequency is that due to its very small wavelength it can detect really small fractures and holes, which can be very useful in cases like aircraft [83]. The PZT patch is attached to the structure. The measurement system has a data acquisition device (DAQ) and a computer running software [83]. The DAQ excites the transducer by sending an appropriate signal that acquires the response signal. The excited and responded signals are processed using fast Fourier transform in the

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

PZT

Measurement system

ZE

Structure

Figure 8.7 Basic experimental configuration of the EMI method.

frequency domain on the computer. It gives the electrical impedance signature on the appropriate frequency range. In the EMI method as shown in Fig. 8.7, the transducer acts as both actuator and a sensor, and due to its piezoelectric effect an interaction happens between the electrical impedances of the transducer and the mechanical impedance of the structure. Hence there is a relation between these two impedances. So, if there is any variation in the mechanical impedance of the structure, due to structural damage such as shock, cracking, corrosion, etc., this will cause a variation in the electrical impedance of the transducer. Finally, the electrical impedance of the transducer when the structure was healthy is compared with the electrical impedance of the transducer measure, and based on the variation the health of the structure is determined.

8.8 8.8.1 • • • • •

Civil engineering

Short- and long-term monitoring of bridges, tunnels. Monitoring of high-pressure water pipe. Building and increasing life cycle of roads and foundations. Measurement of loads to be applied and lifetime calculation.

8.8.3 • •

Aerospace

SHM technology helps in increased availability of aircraft by on-the-spot evaluation Effective assessment of actual damage events. Reduced cost of lifecycle and total ownership. Reduced logistics. Increased safety and reliability.

8.8.2 • • • •

Industrial applications of SHM

Railway

Continuous, stand-alone monitoring of vehicle loads Long-term measurements of rails, railroad switches, and ties.

Maintenance and monitoring of composites

8.8.4 • •

Wind energy

Professional structural monitoring of on- and offshore wind power plants. Measurement of tower, foundation, rotor blades.

8.8.5 • •

147

Oil and gas

Efficient measuring of offshore applications. Using measurement technology to monitor risers, pipes, cranes, foundation structures, pipelines, etc.

8.9

Conclusion

Advanced technology and various sensors used in SHM for composite materials for various applications have been reviewed, and it has been emphasized that due to complexity in design of composites, damage is difficult to predict. This cannot be done by conventional methods as they are inefficient as compared to SHM. In this manner, the utilization of SHM leads to increase in reliability, utility and reduces the cost for maintenance. SHM developed from the field of massive structures and now it encompasses various disciplines, which include basic elements, materials and structures, weakness and crack, NDT and assessment, sensors and actuators, microelectronics, flag handling, and many more. In this chapter, advantages, benefits, industrial applications, emerging technologies, and solution domain of SHM have been explained. Detailed review of state of practices used in SHM, challenges for SHM, factors affecting the measurement data and the real life examples of incidents that have happened due to lack of proper monitoring have been carried out in order to have in-depth understanding of SHM and for its use to prevent massive damage of structures in the coming years. Limitations of present-day NDT techniques have been found, and a comparison has been made between NDT and SHM. Due to many advantages of SHM over NDT, SHM has a bright future in the engineering field. Methods used in NDT are generally localized in nature whereas SHM deals in a wider perspective. Also, NDT requires earlier observation of the area of damage while there is no such prerequisite in SHM. Further applicability of SHM uses technologies of piezoelectric sensors such as AU, AE. and EMI.

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Synthetic/natural fiber properties of fire-designated zone of an aircraft engine: a structural health monitoring approach

9

A.R. Abu Talib 1 , I. Mohammed 2 1 Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; 2Department of Mechanical Engineering, Hassan Usman Katsina Polytechnic, Katsina, Nigeria

9.1 9.1.1

Introduction Background

With the continuous need of composite materials in aerospace industries, weight of the composite materials, design flexibility, specific strength and stiffness, corrosion resistance, fatigue endurance, thermal resistance, and wear resistance are the main factors that affect the performance of the materials in the composite, these factors have caused the U.S. Federal Aviation Authority (FAA), and aerospace industries sectors assisted by researchers, to be engaged in improving the composite structures to suit modern aircraft by the use of highly efficient performing materials in composite fabrication. In the methods of designing composites of the fire-designated zone of an aircraft engine, the search for more efficient materials in aero-engine applications have been always encouraging by continual research in the field of materials science, thermal and combustion analysis, and heat transfer. The main factors that lead to the invention of composites on aircraft components are the needs of low density, less weight, corrosion resistance, impact resistance, high strength, and stiffness, which reduce fuel consumption and have good thermal behaviors [1e4]. The most recent composites used on aerospace industries are fiber metal laminates (FMLs), whereby the composites contain fine sheets of metal alloy, such as aluminium alloy, magnesium alloy, and titanium alloy; the composites are the fiber-reinforced polymers such as carbon fiber, glass fiber, aramid fiber, and natural fibers [5,6]. The composite was first developed 1945 in building a structure at Fokker aircraft, whereby the composites were proved to be useful in aerospace industry due to their importance in fracture toughness improvement [7]. The first aramid-reinforced fiber metal laminates were developed in the Delft University of Technology in 1978, using aluminium alloy 2024-T3 as the metal and epoxy as the polymer [8]; this composite, however, has a brittleness characteristic, which limits its function on the primary structure of an aircraft. Therefore, carbon fiber and glass fiber composites of fiber metal laminates Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00009-5 Copyright © 2019 Elsevier Ltd. All rights reserved.

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were developed, which had increased mechanical properties compared to the aramid fiber metal laminates [9,10]. The most common characteristics of fiber metal laminates are: good fatigue resistance, low density, reduced weight, high strength and stiffness, high moisture resistance, high fire resistance, less material degradation, etc. [11]. Due to its attractiveness in the aerospace industry, FMLs now have many applications in this industry sector. For the past decades, the fundamental study of FMLs has lagged behind in the industries and in academic research due to lack of understanding its importance. This has affected the feasibility studies for the product to be fabricated in large quantities. Nevertheless, nowadays the production of such products has started and yielded a positive impact in the aerospace industry. Much scientific research has now been published in this area, and the results show not only interest but also scientific understanding of the advantages of these composites. Currently, the FML composites are one of the most interesting areas focusing on in the aerospace industries; the composites consist of metals and reinforced fibers (synthetic and natural), with an excellent polymer [12,13]. Various companies are engaged on production and marketing the FML composites, such as Fokker Aerostructure BV in Papendrecht, Netherlands; AGY Holding Corp in South Carolina, USA; GKN Aerospace in the Redditch, United Kingdom; Lockheed Martin Aeronautics in Texas, USA; Premium Aerotec in Augsburg, Germany, etc. Various companies produced the metal alloy used in fabricating the fiber metal laminates, such as Kaiser Aluminium in California, USA; Aluminum Corp. of China in Hong Kong, China; Alcoa Inc. in Pittsburg, USA; UC Rusal in Moscow, Russia; Dubal Aluminum Co. in Dubai, United Arab Emirates, etc. Among the companies that produce synthetic and natural fibers are Narsingh Dass & Company Private Limited in Delhi, India and KEFI Malaysia Sdn Bhd, Kenaka Corporation in Malaysia, among others. The natural fibers were mainly derived from plants, animals, and mineral resources; these also have the advantage of environmental benefits and are cost-effective. FML composites were fabricated using different metal alloys and by various fiber-reinforced composites that include both synthetic and natural fibers. The composites can be tailored to different forms in order to develop novel solutions to today’s complications. The significant applications of FMLs in aerospace industry are deemed to be virtually unlimited; among its advantages in different areas are as fire insulation of the material, light weight, kinetic energy absorption, high strength, high stiffness, and thermal resistance, amongst others. It is to be noted that FML composites have been used in aircraft by Fokker aircraft since 1945, introduced as bonding in the F-27 aircraft, but the usage was not in a modern form. Likewise, in 1978 a new composite that was brittle was introduced by the Department of Aerospace in the Delft University of Technology; nowadays there is a recent development in the FMLs that has introduced composites with flexible blankets for high-temperature applications that are up to 1100
80 C for fire protection and thermal resistant in aerospace systems. Nevertheless, weight-saving characteristics of FMLs and good thermal and mechanical properties make it one of the best materials nowadays to be used in various aircraft components, including as a component in firedesignated zone of an aircraft engine, due to its protection against high temperature, corrosion, fatigue crack, and wear [9,10,14]. There are various component parts in

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an aircraft that are required to be protected against fire; some of these components are fire resistant, such as components carrying flammable fluid, components that convey the flammable fluid, fittings and each line in the fire-designated area; whereas the other components are fireproof, such as flammable fluid tanks and support in the firedesignated zone. In the authors’ knowledge, there is lack of information on the analysis of FML composites using synthetic and natural fiber in a fire-designated zone of an aircraft engine. The motivation is to achieve less thickness and lower weight composites application in aircraft engine. Hence, there is a need to perform the feasibility study of thin layer of FML composites for potential application of aircraft engine. Small numbers of layers of either synthetic or natural fibers or the sandwich of two with a metal alloy, when bonded together by a highly adhesive epoxy, are regarded as one composite, which consists of either only one type of fiber with metal alloy or the combination of synthetic with natural. The combination of natural fiber with synthetic fiber produces a composite with high stress and stiffnessetoeweight ratio, excellent fatigue properties, crack growth resistance, and corrosion resistance, and reduces the risk of health hazard [15]. Such composites protect the materials from absorbing water, which is mostly encountered by natural fibers, solving the ecological and environmental degradation problem caused by synthetic fibers [16e18]. The main factor to consider in this chapter is the process of preventing hazards on the composites used and the ability of the composite to withstand a high-temperature application while considering the mechanical and thermal properties of the composites. All of these conditions are achieved by setting the operational conditions in such a way that all the materials are used when bonded together by an excellent binder. This study aims to achieve a thin-layered FML composite that can withstand a hightemperature application for a certain time with excellent mechanical and thermal properties. The study also looks at the properties of the polymer used in binding the metal alloy with synthetic and natural fiber composites with their fabrication process. The experimental results of these composites are also presented based on their mechanical, thermal, and fire behavior, and impact loading velocity properties of the composites under research.

9.1.2

Scientific gap

The FML composites are considered to be accepted in aerospace industry due to their manufacturing process, whereby the composite consists of metal alloy (normally aluminium alloy), synthetic fiber, natural fiber, and polymer. The cost of fabricating this composite is low, especially the oldest method (hand layup method) used in fabricating composites. The present shift in the aerospace material is currently moving toward the evolution of new techniques tailoring the composites into different forms in order to solve the problems at hand in the aircraft components. Apart from the use of FML composites on fire-designated zone of an aircraft engine, these composites have applications in other parts of an aircraft such as in the aircraft wings, wing ribs, doors, tail cone, cowling, nose section, fuselage, amongst others [19]. A study was conducted by Sinmazçelik, Avcu [8] that reported that stress will be distributed in the fiber

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composite even if the metal alloy cracks, therefore the composite will not fail, and it will prevent the crack growth in the metal in which the fiber forms a bridge in between the polymer and the fiber. The problems of corrosion of metal, water absorption of the composite by natural fibers, and fatigue crack of synthetic fibers were solved by combining the properties of all the three materials and also the property of the polymer used as binder; aluminium alloy produces excellent impact resistant and the synthetic fibers yield high strength and stiffness [20,21]. In the search for fireproof materials in aerospace industry and renewable energy systems, strong efforts are being made by developing a strong composite of FML and using an inexhaustible material (natural fiber) in developing the composite; therefore, these composites are now finding their way into structural insulation at a faster rate than any other composite in aerospace industry. The solution thus far comprises carbon and glass fiber-reinforced polymer, aluminium and titanium alloy that has been used in different components of aircraft that mostly are flammable and cause fire hazards [22e25]. Metal laminate is now used in the said composite either by applying it on the front and rear face of the composite that serves as the cover to the composite or by alternating it in the fiber layers, along with an excellent polymer as the organic binder developed as fire blanket and for thermal insulation of the structure to fire-designated zones of an aircraft engine and other components of an aircraft that are related to the aircraft body. These materials also can be used in other industries such as automobile and steel industries [26]. Moreover, a study was conducted to develop lightweight composite having both good mechanical and thermal properties with good fire resistance behavior. It is very useful to note that despite the expansions and applications of the FML composites in the aerospace industry, the automobile and steel and other industries benefit from these composite technologies to a great extent owing to their having superior fire protection behavior and good thermal performance and improved mechanical properties. The research on FMLs was motivated by the FAA and U.S. National Aeronautics and Space Administration; this study presents an innovative way to apply the FMLs in aeronautical study, which uses a smaller number of layers in the composites. Various studies were conducted on the different types of FML composites that show some restriction on the use of composite in aircraft applications, such as brittleness properties, delamination of the metal from the composites, the hardness of the structure, inadequate polymer distribution in the composite amongst other factors. Sadighi et al. [9] and Fan et al. [10] carried out a study on fiberglass and carbon fiber metal laminates to reconstitute the mechanical properties of the fiber metal used using aramid metal laminates that are used only on the secondary structure of aircraft, with results that found improved properties of the composites, allowing the composites to be used also on the primary structure of the aircraft. The need for understanding of the effect of the volume ratio of the fiber with polymer and weight ratio of metal with fiber still exists. Also, the thicknesses of the composites with a number of layers of fibers and metal alloys sheets were not yet confirmed, whether the metal alloy should only be at the outer layers of the composites or if it should intersect with fiber layers. A new technique of fabricating FML panels was also introduced by Skhabovskyi and Batista et al. [27] in which the panel was reinforced with metal pins deposited by

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157

CMT welding. This technique improved the mechanical properties of the composites. Likewise, the techniques reduced the crack growth rate of the composites from onetenth to almost one-hundredth of the monolithic aluminium alloy [18,28]. The study of thermal degradation of the composite was carried out by Tranchard and Samyn et al. [29], where the internal pressure, thermal expansion that induces the crack formation, gas migration through the materials, and thermal delamination of the composite were reported, and they developed a model that can prevent the composite from delamination in a fire test. Also reported was the degradation of the epoxy resin used in the composites fabrication, in which the degradation differs from one form to another, which depends mostly on the chemical composition of the hardener, the content used, width of the contents, and the orientation of the fibers in the laminate composite [30,31]. Panthapulakkal and Sain [32] were among the few researchers that studied the hybridization of synthetic/natural fiber reinforced composites. In their study, properties of hemp/fiberglassepolypropylene composites for automobile interior parts based on water absorption were resolved. Whereas the properties of banana/glass fibere reinforced hybrid composite based on mechanical and interfacial properties showed an increased impact strength [33]. Also, thermal decomposition of polyphenylene sulphide (PPS) matrix in a composite of carbon fiber laminate in the flame fire was studied by Petit and Vieille et al. [34], which shows that the PPS redistributed in the carbon fiber at high temperature. Kenaf and flax fiber were among the natural fibers that were being used globally in industry that includes aerospace industry, automobile industry, and furniture industry, among others; the kenaf fiber has higher strength than the other natural fibers [35]. The fire safety of the aircraft cabin was conducted by FAA tests in 1986 using a Bunsen burner test of coupon, methane gas as used in test [36], and new materials were developed in an aircraft cabin by Lyon [37] and Pulvino [38] to meet the fire safety standard of a commercial aircraft. Also, burn-through fire test was conducted for full-scale fuselage in 1999 to identify the roots of external fire to the fuselage for postcrash fire [39]; the statistical analysis shows that there is an improvement in fire hardening of the fuselage [40]. The finding of these researchers led the FAA to introduce a modern approach for testing aircraft material in a pool of fire [41]. The first composite fire test response was conducted by Dodds and Gibson et al. [42], by using furnace and thermal modeling on different aircraft components using carbon fibere reinforced composite, which later proved to be flammable. This led to Mouritz [43] developing a fire test using a fire-resistant polymer and flame retardant epoxy, which reduces the problem of flammability of the composite. Different types of reinforced fibers and polymer were used in fire response test, and in that relation different results were obtained, which led to the introduction of fiber metal laminate composites, which had much better properties than the traditional materials used, as shown by experimental and numerical methods [44e46]. Nowadays, carbon fiber, glass fiber, and aramid fiber reinforce the most common types of synthetic fibers used in aerospace industries [47,48]. In contrast to the research study undertaken by other researchers, this study concentrates on achieving composite laminates with a thin-layered thickness of less than

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4 mm to serve as components in fire-designated zone of an aircraft engine that comprises metal alloy synthetic and natural fibers due to their nonhazardous characteristics. The main reason for this is that no research investigations have been carried out previously on the thin-layered carbon fiber with natural fiber and metal laminate on the fire-designated zone of an aircraft engine. Again, carbon fiberereinforced epoxy is flammable in nature, therefore, it will be very hazardous to use it in fire-designated zone of an aircraft engine [42] and using thick composite can add more weight and consume more space in the zone [24]. In general, there is an argument that requires answers as to whether (1) the existing types of natural fibers in the composites can be considered for high-temperature applications, (2) can a natural fiber be used only on fiber metal laminate composites used in fire-designated zone of an aircraft engine, and (3) can the type of epoxy resin/hardener bind the metal alloy together with the reinforced fiber without delamination during the fire test? The investigation study uses standard techniques to find a solution to the technological inquiries through its targeted objectives in the techniques of fabricating the FMLs of a fine sheet of aluminium alloy and a thin layer of synthetic and natural fibers. It is advantageous to note that at this stage, if the targeted composite has been achieved, the FML composite can be used in a fire designated zone of an aircraft engine.

9.1.3 9.1.3.1

Objectives Aim

To study the feasibility of using fiber metal laminates of aluminium alloy 2024-T3 with a sandwich of synthetic and natural fiber in fire-designated zone of an aircraft.

9.1.3.2

Specific objectives

The specific objectives of the study are as follows: 1. To assess the suitability of FML composites of the sandwich between synthetic and natural fibers, using aluminium alloy 2024-T3 as a metal alloy. 2. To determine the mechanical, thermal properties, fire behavior, and impact loading of the sandwich composites using an ISO 2685 propane-air burner according to the standard. 3. To examine the burn-through responses of the composites and their burn-through time under fire test.

9.1.4

Study questions

Question 1: Does the FML composite of aluminium alloy 2024-T3 sandwich with synthetic and natural fiber-reinforced composite withstand a high-temperature application?

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159

Question 2: What is the maximum time that each sandwich composite can withstand the high temperature according to ISO 2685 standard? Question 3: How does the layering pattern affect the performance of the composites in the high-temperature application?

9.1.5

Significance of study

FML composite, being the most qualitative fire retardant and thermal insulator material and among the lightest materials use in fire-designated zone of an aircraft engine, has been chosen and considered as the possible candidate for future use in aerospace industry; it is applicable in aeronautics where weight and space savings are essential. Previous investigations have addressed the development and fabrication of the fiberreinforced polymer composites for fire flexible blankets and thermal insulation; such composites use multilayered fibers. The investigation presents modern techniques for developing a fiber-metal laminate composite of a small number of layers of fibers and a metal alloy using hand layup techniques. This study mainly proves the hypothesis of whether the fibers in FML composite can be sandwich together between the synthetic fiber and natural fiber and can act as fireproof material, to validate the proposed methodology. Implementation of this composite into fire-designated zone of an aircraft engines depends on the position of the component in the zone. The application of such composites can range from component to component in the firedesignated zone for improved performance and reduced deterioration in protecting polymer matrix for enabling weight savings, fire blanket, and thermal insulation.

9.1.6

Scope of study

The study was based on the performance FMLs of a sandwich of synthetic and natural fiber with aluminium alloy 2024-T3 in a fire-designated zone of an aircraft engine, to establish the behavioral performance of these sandwich structural materials in a pool-fire scenario, using ISO 2685 standard by using the propane-air burner. The mechanical and thermal properties of these composites were evaluated, and the impact behaviors of these composites were also evaluated and analyzed.

9.2 9.2.1

Methodology Development of a novel composite

A comprehensive flowchart that can be used in developing a novel fiber metal composite for the application under consideration is presented in Fig. 9.1. The flowchart demonstrates the action responsible for the development level of the composite to its commercialization. A clarified form of the flowchart that was applicable to the current investigation study is modified as seen in Fig. 9.2. The study was of the need of a composite that will serve as the component in the fire-designated zone of an aircraft engine; in this study, it is fiber-metal laminate composites. The main specification of the

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Composite conformity check with specification No

Preparation of composite specimens Conform? Yes Realisation of composite on “Real” component

No

Feasible? Yes Composite quality in service

No

Need for composite

Satisfaction? Yes

Initial choice of composite process pre/post-fabrication method Feasibility test on small sample

Feasible?

No

Yes Composite fabrication Properties of composite

Production of composite

Figure 9.1 Comprehensive flowchart in developing novel composite.

composite in this study was to obtain a composite with a small number of layers (thin thickness) that comprises aluminium alloy with synthetic and natural fibers that are bonded together with an excellent adhesive epoxy. The test was based on a labscale sample that consists of arranging the fibers and sheet of aluminium alloy in a layering form in the specified mold. Characteristics of the developed composites were based on improved characteristics of each material used. Fig. 9.3 illustrates the research activities in a systematic form based on the technological availability in accordance with the achievable specific objectives of the study.

9.2.2

Fiber-metal laminate composite fabrication

The composite was fabricated using three types of materials, including a metal alloy (aluminium alloy 2024-T3), a synthetic fiber (woven carbon fiber), and natural fiber (kenaf and flax), with epoxy resin and hardener. Table 9.1 shows the dimensions of the material used in fabricating the composite. The composite was fabricated using the material dimensions of Table 9.1; two different types of laminate composites were fabricated in a mold using hand layup techniques. The two composites fabricated included carbon fiber kenafereinforced aluminium laminate (CAKRALL) and carbon fiber flaxereinforced aluminium laminate (CAFRALL). The thickness of the two composites was almost 3.5
0.2 mm;

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

Start

Need for composite (FMLs) Composite specifications: AA 2024–T3, CF, and NF Initial choice for composite process (FMLs) Feasibility test on lab-scale samples Feasible?

No

Yes Composite structure study Yes Conformity witht specification

No

End

Figure 9.2 Clarified flowchart in developing novel FMLs composite.

Assessment of FMLs composite for fire designated zone of an aircraft engine (objective I)

Deduction of FMLs properties (objective II)

Analysis of burn-through responses of the composite in fire designated zone of an aircraft engine (objective III)

Figure 9.3 Order of research activities in accordance to specific objectives.

161

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Table 9.1 Material specifications and their dimensions Materials

Aluminium alloy 2024-T3

Woven carbon fiber

Kenaf

Flax

Dimensions (mm)

0.3

0.2

1.4

0.6

No. of Layers

4

9

1

2

Table 9.2 shows the number of layers of each material in each composite, whereby aluminium alloy was used only on the front and rear face of each composite. Each fiber in the composites was arranged accordingly, and an epoxy resin/hardener was applied at each layer; the volume of resin and hardener were in the ratio of 2:1, respectively. Also, the weight ratio of fiber/metal to the polymer used was 2:3, respectively. The fabricated composites were compressed using a compression machine until its curing time (24 h) at room temperature and postcure for an hour in an oven at 80 C. Fig. 9.4 shows a sketch of the composite arrangements.

9.2.3

ISO 2685 propane-air burner assembling

The ISO 2685 standard specifies the test conditions that are relevant to all components, equipment, and structures located in zones designated as “fire zones” and built to fulfill the lowest level for fire resistance as stated in ISO 2685 standard. The burner was used to test the materials that can be used as a component in fire-designated zone of an aircraft engine, which includes the flammable tanks, support lines, etc. The firedesignated zone of an aircraft engine is defined by the standard as the region of an aircraft that is the compartment containing the main engines and auxiliary power units, designated in accordance with the requirements of the approving authorities, with a flame having the characteristic of 1100
80 C temperature and 116
10 kW/m2 heat flux density.

9.2.3.1

Test facility

The burner settings were carried out at the Propulsion Laboratory, Department of Aerospace Engineering; Faculty of Engineering at Universiti Putra Malaysia, in Malaysia. Table 9.2 Number of material layers in the composite Materials layers (mm) Composites

Aluminium alloy 2024-T3

Carbon fiber

Kenaf

Flax

CAKRALL

2

4

1

e

CAFRALL

2

5

e

2

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

(a)

163

Al-alloy Carbon fibre Kenaf Carbon fibre Al-alloy

(b)

Al-alloy Carbon fibre Flax Carbon fibre Flax Carbon fibre Al-alloy

Figure 9.4 Fibre metal laminate of synthetic and natural fibers.

9.2.3.2

Apparatus

The burner was constructed according to ISO 2685 standard; all the tubes were fabricated according to the standard as indicated in Fig. 9.5. All the components and equipment in constructing the burner were installed according to the specifications of the standard, and the circumstances on the fabricated burner were almost the same as the conditions experienced in the normal aircraft engine. The burner consisted of a small chamber where propane gas and air are first mixed, a plenum chamber where combustion takes place, a burner plate that consists of 373 copper tubes that carry the mixed gas and air, which are cooled by air, 332 holes that supply cool air to the burner head, four holes that are equally spaced at the surface of the burner that supplies the cool air to the burner (secondary air), and a burner plate placed at the top of the burner that contains copper tubes and holes.

9.2.3.3

Fire test rig preparation for ISO 2685 burner

The test rig was prepared by assembling the burner on its stand, the temperature measuring device at their housing, and also a heat flux measuring device. A propane gas valve, air valve, and compressed air were all connected to the burner, and manometers used to measure the pressure of gases (propane, primary air, and secondary air) were connected to their tubes as indicated in Fig. 9.6. Also, the sample stand was fabricated and placed at the test rig.

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Figure 9.5 Configuration of propane burner (ISO2685, 1998).

9.2.4

ISO 2685 burner calibration

The study proceeded with the next step after rig arrangement by burner calibration, which is the first step before samples are tested. This technique should give the details of the temperature and heat flux to be used in the test with the specified distance according to the standard used. The technique was conducted by igniting the burner by adjusting the propane gas, air, and compressed air to the standard values as stated by ISO 2685 standard.

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Figure 9.6 Burner assembly.

9.2.4.1

Test facility

The burner calibration was carried out at the Propulsion Laboratory, Department of Aerospace Engineering; Faculty of Engineering Universiti Putra Malaysia ,in Malaysia.

9.2.4.2

Materials

The burner was made from copper that is connected with mild steel fittings, according to the specified standard. In addition, the thermocouple and heat flux housing were made from copper and their stand from mild steel.

9.2.4.3

Apparatus

Seven R-type thermocouples were installed in their housing at a distance of 1 inch from one thermocouple to another, a heat flux meter (SBG01) was also installed in its housing. The temperature and heat flux measuring devices were connected to a data logger in different channels that read the output of the result obtained during the calibration process. Plastic tubes were installed in the main tubes that supplied gas, primary air, and secondary air; the pressure of each gas was read using manometers connected to these tubes. Water was flowing in and out of the thermocouple housing to cool the housing plate; likewise, the water was flowing in and out of heat flux meter (SBG01) to maintain the temperature of the sensor. Thermometers were used to measure the inlet and outlet temperature of the flowing water in the heat flux meter.

9.2.4.4

Procedure

The burner calibration was conducted according to ISO 2685 standard, whereby the propane gas was turned on first and the burner was ignited using a gas lighter, and

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Temperature calibration

Heat flux calibration

Figure 9.7 Burner calibration during test.

then primary air and secondary air were introduced to the burner. The gas, primary air, and secondary air were all set to their corresponding pressures by adjusting their valves; propane gas was set to 440 Pa, primary air 4265 Pa, and secondary air 2940 Pa. The flame was allowed to settle for at least 5 min before starting calibration. The calibration was started by temperature calibration, whereby the thermocouple housing was placed in a pool of fire at a 3-inch distance between the burner face and thermocouple tips and 1 inch above the center line of the burner; in order to account for the buoyancy, the calibration was done at least for 2 min and the result was recorded by data logger. The same thing applied to the heat flux meter, and the results for 2 min were recorded as shown in Fig. 9.7. The temperatures resulted in the range between 1100
80 C, and heat flux between 116
10 kW/m2 according to the standard; this result was obtained by adjusting the gas and air valves.

9.2.5

Properties of the composites

The composite undergoes four types of properties tests that show the ability of the materials to be used in fire-designated zone of an aircraft engine, which includes mechanical properties, thermal properties, fire response behavior, and velocity impact test.

9.2.5.1

Mechanical properties test

In the mechanical test, three properties undergo the testing, including tensile, compression, and flexural tests. The tests were carried out using a universal testing machine. The first test conducted on the composite was a tensile test. This test was performed according to ASTM standard [49], in which the samples were cut 120 mm  20 mm  3.5 mm. The samples of each composite were held in a fixture during loading whereby the load was applied to it until failure. The main properties of each composite laminate were found and recorded accordingly; the crosshead speed of the test was at 1 mm/min.

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Compression test was the second test conducted on the composites, carried out using ASTM standard [50], in which samples were cut 12.75 mm  12.75 mm  3.5 mm. The samples of each composite were held in a fixture during the loading, the compression load was applied until failure, and the crosshead speed was sustained at 1 mm/min; the properties of each composite were obtained and recorded accordingly. The last test conducted was a flexural test, and the test was carried out according to ASTM standard [51], in which the samples were cut 67.2 mm  20 mm  3.5 mm whereby the length was determined by Eq. (9.1) as reported by Ning and Cong et al. [52]. L ¼ 16d þ 20% of 16d

(9.1)

Where, L is the length of the cut, d is the thickness of the sample. In the test, three point loading techniques were used in which two supports were located at 10% of the whole length of the samples from the end of each composite, whereas the load was applied at the top center of each sample, also crosshead speed was maintained at 1 mm/min, and the properties of each composite were determined and recorded accordingly. The equation used in calculating the flexural stress and flexural strain of the composites was reported by Properties [53] as indicated in Eqs. (9.2) and (9.3): bf ¼ 3uL=2bd2

(9.2)

 εf ¼ 6Dd L2

(9.3)

Al-Darwish and Hurley et al. [54] reported the equation used to find the flexural strength during the composites fracture as in Eq. (9.4): d ¼ 3uI=2bd2

(9.4)

Where, d ¼ Flexural Strength in MPa, u ¼ Maximum load applied to the specimen in Newton, I ¼ Distance between two supports in millimeters, b ¼ Width of the specimen in millimeters, d ¼ Height of the specimen in millimeters, L ¼ Support span in mm, D ¼ Maximum deflection of the center of the beam in mm, bf ¼ Flexural stress in N/mm2, and 3f ¼ Flexural strain in %.

9.2.5.2

Thermal properties test

In the thermal test, two properties tests were conducted, which are thermogravimetry analysis (TGA) and dynamic mechanical analysis (DMA). TGA was conducted on both composites to know the thermal degradation of each composite by thermo-balance coupled to a mass spectrometer, using an average sample size of 80 mg. The test was performed from ambient temperature to 800 C using the heating rate of 5, 10, 20, and 40 C/min by using nitrogen gas as the fluid in the

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experimental test. The test was conducted in order to substantiate the carbonization-innitrogen procedure on the thermoset polymer. The percentage weight residue of each composite was determined, and the average weight of the samples was found to be 0.785 N with a gas flow rate of 50 mL/min. DMA was performed according to an ASTM standard [55] whereby each composite was cut 35 mm  10 mm  3.5 mm; the experiment was conducted using a bending mode of multiple frequencies with a 6N dynamic force that is oscillating at fixed frequency and amplitude of 15.0 MinOscF. The relaxation spectra recorded temperature of the experiment from ambient to 200 C, at a heating rate of 5.00 C/min to 200.00 C from ambient temperature. DMA test was conducted to substantiate the storage modulus, loss modulus, and tan d of each composite.

9.2.5.3

Fire response test

The fire test was conducted on the prepared composite samples of 300 mm  300 mm  3.5 mm. After the propane-air burner was calibrated according to the standard [56], three K-type thermocouples were used at the rear face of composite plates, where the first thermocouple was placed at the center of the plate, the second one was placed at 1 inch above the center of the plate, and the last one was placed 1 inch right of the center of the plate. The composite was placed at a distance of 3 inches from the burner face and underwent the fire test for 15 min, while the rear face temperature was recorded using a data logger. The composites that withstood the standard flame temperature and heat flux for the 15 min were termed as fireproof composite, while the composites that failed before 15 min and after 5 min were termed as fire-resistant composites, and the composites that failed before 5 min were termed as neither fireproof nor fire resistant composite. The thermal conductivity of the two FML composites was determined using the relationship of Eq. (9.5): K¼

9.2.5.4

W D A  DT

(9.5)

Velocity impact test

The velocity ballistic impact tests of the two composite samples were conducted, so as to evaluate the impact strength and specific perforation energy of each composite against the external and internal forces on fire-designated zone of an aircraft engine. The size of the samples used in the test was 100 mm  100 mm  3.5 mm; the target area of the plate was the center of the plate of around 15
2 mm. The test used a flat cylindrical mild steel projectile of 5 g, 8.5-mm diameter and 13.5-mm length, and the distance between the target and the projectile was 14 inches in the gun tunnel of 20mm bore. A high-speed video camera was used to measure and record the movement of the projectile, and helium gas was used as the fluid that pressurized the projectile in the barrel to the target. Different sets of the experiment were conducted using different pressure, so as to achieve the required velocity that the composite can withstand. The

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

169

average absorption energy of each composite was evaluated using the absorption energy equation [57], as in Eq. (9.6):
1 EAbs ¼ mp V2i  V2r 2

(9.6)

Where, EAbs ¼ Energy absorption by the specimen (J); mp ¼ Mass of projectile (kg); Vi ¼ Impact velocity (m/s); Vr ¼ Residual velocity (m/s). The impact strength and specific perforation energy were obtained from Eqs. (9.7) and (9.8), respectively [58]: Impact strength ¼

QI A

Specific perforation energy ðspeÞ ¼

(9.7) P$E  A W

(9.8)

Where, QI ¼ Impact Energy; A ¼ Area of the plate; P$E ¼ Potential Energy, and W ¼ Weight of the plate (composite). The impact energy is the difference between the potential energy impact before and after hitting the sample.

9.3 9.3.1

Results and discussion Burner calibration results

As indicated in Section 9.2, the burner had been calibrated according to ISO 2685 standard and the results are shown in Tables 9.3 and 9.4 for temperature and heat flux calibration, respectively. The temperature value of the calibration was in accordance with the standard with a minimum value of 1057.09 C and the maximum value of 1169.56 C, likewise, the heat flux values with least value of 108.17 kW/m2 and the highest value of 124.60 kW/m2 as the standard range stated [56]. The values obtained in burner calibration were in coincidence with the values obtained by some researcher in their studies as those obtained by Kao and Tambe et al. [59]. The fluctuation of the calibration results was based on the difference of the ambient air in the test experimental room and also the rise and drop of the propane and air gases that occurred during the experiment.

9.3.2

Properties of the composite results

The set of results obtained from the experimental test are presented in this section, and results included are based on the performance of the composite that can be applied to the real fire-designated zone of an aircraft engine; the results include the mechanical properties, thermal properties, velocity impact properties, and fire response behavior of the composites under study.

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Table 9.3 Temperature calibration results S/No

9.3.2.1

Time (S)

Temperature (8C)

1

5

1117.95

2

10

1098.22

3

15

1109.65

4

20

1098.45

5

25

1057.09

6

30

1105.98

7

35

1096.45

8

40

1111.98

9

45

1137.98

10

50

1145.98

11

55

1129.76

12

60

1109.78

13

65

1076.87

14

70

1065.98

15

75

1098.02

16

80

1106.23

17

85

1145.65

18

90

1169.56

19

95

1098.66

20

100

1095.34

21

105

1106.64

22

110

1118.09

23

115

1102.65

24

120

1087.98

Mechanical properties results

The results of three mechanical properties were obtained in this study, including tensile, compression, and flexural test results. The results were analyzed and discussed accordingly. The first test conducted on the mechanical test was a tensile test; the average results of five samples of the two composites under test are shown in Table 9.5. The two composites considered include CAKRALL and CAFRALL. The composites use the same thickness of 3.5
0.2 mm, with a different number of layers of the materials.

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171

Table 9.4 Heat flux calibration S/No

Time (S)

Heat flux (kW/m2)

1

5

108.173

2

10

110.64

3

15

111.64

4

20

112.60

5

25

114.23

6

30

112.60

7

35

115.78

8

40

113.77

9

45

111.46

10

50

116.36

11

55

117.35

12

60

119.22

13

65

117.88

14

70

110.33

15

75

113.98

16

80

115.88

17

85

119.38

18

90

122.62

19

95

124.19

20

100

124.60

21

105

118.98

22

110

119.86

23

115

115.88

24

120

109.68

From the results shown in Table 9.5, it is noticed that the CAFRALL extended more than CAKRALL before failing; also the results show that both composites withstood a longer extension before they fell, and a higher load of almost 26.5 kN for CAFRALL and 22 kN for CAKRALL. This characteristic was in agreement with those reported by Tamilarasan and Karunamoorthy et al. [4], that uses a synthetic fiber (carbon fibere reinforced aluminium alloy 6061-T6) sandwich laminate and the maximum load obtained was 40 kN with a maximum extension of 9.25 mm, which is almost in comparison with the sandwich laminate of synthetic and natural fibers.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 9.5 Average tensile test results Load (N) Extension (mm)

CAKRALL

CAFRALL

0

0

0

1

3828.77

2550.43

2

7453.07

5171.11

3

10,002.23

7874.32

4

13,045.43

10,760.56

5

17,302.65

13,950.88

6

20,003.56

18,057.65

7

22,039.07

22,008.45

8

21,701.03

25,487.11

9

e

26,461.4

10

e

26,045.11

The average results of some of the properties of the tensile test were shown in Fig. 9.8; this result was in agreement with the result obtained in Tamilarasan and Karunamoorthy et al. [4], in which the tensile test was conducted based on pure synthetic fiber. The result shows that CAFRALL produces higher properties than CAKRALL. The FML composites that consist of natural fiber yield the same result but with less

Maximum load (N) Modulus (automatic) (MPa) Tensile stress at maximum load (MPa) Tensile strain at maximum load (%)

30000 25000

Properties

20000 15000 10000 5000 0 Cakrall

Cafrall Composite

Figure 9.8 Average tensile stress properties.

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

173

applied load and stresses, when compared with synthetic fiber alone and synthetic fiber combined with natural fiber, as in the case of this study [60]. In analysis of the results, modulus of elasticity of the composites was found by stress/strain curve of Fig. 9.9 The curves of the two composites show an elastic characteristic at the beginning to around 300 MPa, and the characteristic changes to plastic above 300 MPa, whereby the sample of the composite deformed and led to the samples’ failure. The gradient of the stress/strain curves results in the modulus of elasticity of the composites. From the curves, CAFRALL produces the highest modulus of elasticity of 4400 MPa and CAKRALL 3750 MPa. The difference obtained in the tensile strength of the two composites was due to the interfacial cohesion that exists in between the fibers and polymer matrix and also the diffusion that arises in between the fiber and polymer matrix; this result was reported in a tensile that uses different types of polymers [61e63]. The more the load applied to the composites, the less the stresses. Also, the thickness of the composite affects the stresses on the composites. These results were reported by Zikre and Bhatt [60]. The second mechanical properties test was compression test, and the results of the test are shown in Table 9.6, whereby the average result of the two composites for the test is included. From Table 9.6 it is clearly observed that CAFRALL withstands a higher load than CAKRALL; as the compressive extension increases, also the load increases until it reaches the maximum extension, and then the composite fell. Fig. 9.10 shows the average load of each composite, and the result was in agreement with the result obtained by Zikre and Bhatt [60], where synthetic fiber (glass fiber) was used in the experiment.

450

Cakrall Cafrall

400

Tensile stress (MPa)

350 300 250 200 150 100 50 0 0

5

10 15 Tensile strain (%)

Figure 9.9 Tensile stress with tensile strain curve.

20

25

174

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 9.6 Average compressive test result Load (N) Extension (mm)

CAKRALL

CAFRALL

0

0

0

0.5

5109.43

9632.08

1.0

10,976.92

16,843.45

1.5

15,278.06

18,453.96

2.0

18,078.65

21,450.08

2.5

22,678.98

29,564.76

3.0

27,987.04

40,453.9

3.5

38,567.03

64,987.45

Maximum load (N) Compressive strength (MPa)

100000

Properties

80000

60000

40000

20000

0 Cakrall

Cafrall Composites

Figure 9.10 Compressive test properties.

From the graph of Fig. 9.10, the highest compressive strength and maximum load were applied to it during the test. Table 9.7 shows the result of compressive stress and compressive strain, whereby CAFRALL recorded the highest value of compressive stress. The result indicates a consistent increase in stress as percentage strain increases until ultimate failure. The compressive failure of the composites was observed by premature failure initiation by end crushing and cracking as reported by Botelho and Almeida et al. [64] in their study of hygrothermal conditioning on the elastic properties of CARALL laminate. Later, delamination occurs on the compressed composites; this phenomenon was reported in Jin and Batra [65] in strength of centrally cracked metal/ fiber composite laminate.

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

175

Table 9.7 Average compressive stress with compressive strain test result Stress (MPa) Strain (%)

CAKRALL

CAFRALL

0

0.0

0.0

4

26.5

31.0

8

50.5

62.0

12

82.2

85.4

16

103.9

107.3

20

123.1

133.0

24

152.5

162.8

28

180.0

201.6

29

173.0

190.0

The last test conducted in the mechanical test of the composite was flexural test; the results of the test are presented in Table 9.8, whereby the results are almost like that of tensile and compression test in that the load increases as the flexural extension increases until it reaches the peak extension, and then the composite fell and the flexural load dropped. From the results obtained and the observed behavior of the composites was the earlier delamination and breakage of the metal alloy before falling of the composites. The variation of the flexural strength of the composites was due to the strength of the composites material used in fabricating the composites; this result was also reported in Tamilarasan and Karunamoorthy et al. [4]. A maximum flexural load of 2.45 kN at a flexural extension of 11-mm fracture of the composites at the edge of the samples Table 9.8 Average flexural test results Load (N) Extension (mm)

CAKRALL

CAFRALL

0

0

0

0.5

453

215

1.0

789

452

1.5

970

654

2.0

1100

775

2.5

1060

799

3.0

780

756

176

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

during the test caused the main failure in the composite. Fig. 9.11 shows the average maximum load, maximum stress, modulus of elasticity, and flexural strength, in which the maximum flexural strength was recorded by CAKRALL. Flexural modulus and flexural strength were the main properties used to find the properties of the materials used in the test, the properties that include elasticity of the materials, fracture resistance, and the degradation of the materials at the edge. The results found during the test were in agreement with the results obtained in Manhart and Kunzelmann et al. [66] in new composite restorative materials. Flexural stress and strain were obtained by using Eqs. (9.2) and (9.3), respectively, and the results are shown in Table 9.9. CAKRALL produces the highest flexural stresses in all the strain with 362.46 MPa and 256.98 for CAFRALL. There is improvement in the flexural strength of the composite owing to an organized interface between the matrix and the fibers that give a better stress transfer between them, which is in agreement with the results reported in Tawakkal and Talib et al. [67]. Basically, the composites from all the three tests indicate a premature failure from the bottom and top layers of each sample of the composites (metal alloy sheet), aluminium alloy delaminates from the reinforced fibers and breaks before fibers in the composites break as in the case of tensile test; these events were also reported by Reyes and Kang [68], where aluminium alloy 2024eT3 and glass fiber reinforcements with polymer polypropylene composite materials were used. Also, Rajkumar and Krishna et al. [69] report the same kind of failure in the experiment of FML hybrid of synthetic fibers (carbon and glass fiber) fiber. The same behavior was observed in the compression test, whereby the composite delaminate during the test due to adhesive debonding of the link between fibers and polymer matrix, which results in crack noise from the matrix and later the composites turned out to be plastic; Remmers and De Borst [70] experienced the same effect in the study of compression test. The metal

Maximum load (N)

1200

Maximum stress (MPa) Modulus of elasticity (GPa)

1000

Flexural strength (MPa)

Properties

800 600 400 200 0 Cakrall

Cafrall Composites

Figure 9.11 Average flexural test result.

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

177

Table 9.9 Flexural stress with flexural strain test result Stress (MPa) Strain (%)

CAKRALL

CAFRALL

0

0

0

1

78

50

2

134

87

3

178

128

4

220

167

5

261.76

223.98

6

300.87

256.98

7

362.46

235.98

8

350

e

alloy delaminates and breaks before the reinforced fibers break in the flexural test, which shows the same characteristics with two other properties. The tests were both tensile and compression tests at the same time by using the three-point bending test techniques.

9.3.2.2

Thermal properties results

Two properties were studied on the synthetic/natural fiber sandwich FML composites, which are thermogravimetric analysis and dynamic mechanical analysis; their results are presented in this section. The TGA was conducted on the composites to evaluate the percentage of residual mass left after the composites have undergone the test. Nitrogen gas was used as the fluid during the test. The results of the thermal properties and analysis and their TGAs and DTGs are presented in Table 9.10 for the two composites under study. The TGA and DTG results for the two composites show almost the same values for the percentage weight loss and their derivative values at a temperature lower than 330 C. From the results, as the temperature increases above 330 C, there is a rapid mass loss in all the composites, and early degradation was noticed at a lower heating rate. At a temperature of 150 C, at around 2 wt% was noticed that represents the total degradation, which is mostly due to the low boiling additive in the composites and loss of moisture contents in the composites; this event was observed by Brazier and Nickel [71], Sircar and Lamond [72], and Yang and Kaliaguine et al. [73]. The degradation of greater than 15 wt% was observed at a temperature of 330 C for both composites, almost 50 wt% at around 390 C, while the total degradation of almost 60e70 wt% was noticed around 750e800 C for the CAFRALL and CAKRALL, respectively. The percentage residue of each composite was obtained at the end of the experiment,

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 9.10 Thermogravimetry analysis and derivative weight test results

S/No.

Temperature (8C)

Weight (%)

Derivative weight (%/min)

CAKRALL

CAFRALL

CAKRALL

CAFRALL

1

30

e

e

e

e

2

90

99.72

99.69

0.1231

0.1260

3

150

97.95

97.89

0.4637

0.4353

4

210

95.09

95.20

0.5002

0.4811

5

270

91.10

91.15

0.9821

1.0730

6

330

82.33

81.76

3.1260

2.8810

7

390

43.72

52.61

3.6280

3.0200

8

450

32.89

43.02

0.7462

0.7747

9

510

30.64

40.68

0.1802

0.1665

10

570

30.00

40.10

0.0647

0.0579

11

630

29.70

39.83

0.0398

0.0345

12

690

29.48

39.63

0.0357

0.0337

13

750

29.29

39.44

0.0215

0.0286

14

810

e

e

e

e

where CAKRALL recorded 29.22% CAFRALL produces a residue of 39.32%, which indicates that CAFRALL can withstand more thermal load than CAKRALL. This results show that a composite that was sandwiched between natural and synthetic fiber decomposes earlier than the composite that has synthetic fiber only. This clearly shows that the synthetic fiber (carbon fiber) retarded the degradation of the matrix during the experiment; these results were in agreement with the results obtained with composites of synthetic fiber with thermoplastic matrices [74,75]. The experiment was conducted to evaluate the volume content of polymer used and the fiber in the two composites. The thermal decomposition of the composites occurs due to moisture that escapes by the melting and drying techniques in the experimental tests. The greatest decomposition of the composites was noticed around a range of temperature between 330 and 390 C, likewise, the compound’s great degradation. The DTG peak temperature for the two composites was obtained around 390 C, which is the most essential value in the thermogravimetric analysis programmed temperatures in the experiment. Carbon fiber was the main residual component that was left after the analysis with a small part of metal alloy (aluminium alloy), whereas the matrix (epoxy) experienced a complete degradation during the experimental test. Therefore, in the analysis polymer matrix shows a higher thermal stability than the fibers. The thermal stability of each composite depends on the cohesion of polymer matrix with the reinforced fibers (composition and morphological factors).

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

179

The second thermal properties test was DMA, and the test was conducted using nitrogen gas as the fluid in the experiment. The test was conducted so that the change in stiffness of each FML composite as the function of temperature would be examined; the results of this study are shown in Table 9.11, where the storage modulus, loss modulus, and tan d of the composites are analyzed. The results of storage modulus, loss modulus, and the measure of the damping within the system (tan d) were evaluated and analyzed accordingly. The glass transition temperatures of the two FML composites ranges between 69 and 72 C as observed from the test. The test was performed from ambient temperature to 200 C and also from different temperature ranges; these differences in temperature and frequencies yielded various responses to different transitions and relaxation action of epoxy polymer in the FML composites, and the outcomes present detailed characteristics of fiberematrix interfaces on the composites under investigation. A significant fall of the FML composites in the storage modulus was observed in a temperature range from 60 to 75 C; this temperature range was conformable to glass transition temperature of the two composites. Around 75 to 85 C, the stiffness of the composites was moved around to all the sections of the composites than softening point of the matrix; as observed from Table 9.11, there was a little change in storage modulus for the two FML composites at that temperature range. This result was in agreement with the results obtained on influence of carbon nanotubes on the thermal, electrical, and mechanical properties of poly (ether ether ketone)/glass fiber laminates and DMA studies of vapor grown carbon nano fiber [74,76]. The rapid fall of all the composite at a temperature of less than 100 C from the two composites indicates that the composite lacks corporation of polymer matrix with the reinforced fibers, which will enhance the stiffness of polymer matrix; therefore more fiber in the composites will increase the stiffness of the FML composites, this result was coincident with the results reported by Joseph and Mathew et al. [77] and Samal and Mohanty et al. [78]. The storage modulus of the composites was affected by the composition of the material used and the morphology of the composites, which in turn affected the elastic energy storage of the composites. Fig. 9.12 shows the varying storage moduli at different temperatures of 30, 60, 90, and 120 C. More cohesion forces, effective stress, uniform loads, interfacial adhesion, and internal porosity were observed in the composites that consist of only synthetic fiber than in the hybrid composites of natural/synthetic fibers [86]. The different glass transition temperatures of the FML composites has to do with the interfacial interaction of the polymer matrix with the reinforced synthetic and natural fibers; this is in agreement with Warrier and Godara et al. [79]. The main cause of lower glass transition temperature in this study is that no addition of any binder to thermoset polymer was used. CAKRALL recorded the highest value of tan d at a lower temperature and the least value at a higher temperature, as indicated in Table 9.11, which also allows additional polymer matrix mobility than CAFRALL. From the study, it is observed that the result of tan d was affected by the shear stress concentration and elastic properties of the fibers in coincidence with viscoelastic energy dissipation in the polymer matrix of the composite. It is also observed from Table 9.11 that the peak value differences in temperature for loss modulus were negligible with

180

Storage modulus (MPa)

Loss modulus (MPa)

Tan d

Temp. (8C)

CAKRALL

CAFRALL

CAKRALL

CAFRALL

CAKRALL

CAFRALL

30

e

e

e

e

e

e

50

4894

3743

325.3

245.4

0.0665

0.0656

70

786.1

690.7

410.0

320.7

0.5233

0.4651

90

265.9

252.6

24.2

28.0

0.0909

0.1108

110

259.9

240.1

9.7

12.3

0.0373

0.0513

130

267.1

242.3

8.0

9.9

0.0300

0.0409

150

273.2

242.8

7.4

9.3

0.0271

0.0385

170

271.5

246.0

7.4

9.6

0.0271

0.0392

190

253.3

252.8

6.5

10.6

0.0254

0.0418

210

e

e

e

e

e

e

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 9.11 Dynamic mechanical analysis properties test results

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

E30 degree celcius E60 degree celcius E90 degree celcius E120 degree celcius

6 Stroage modulus (GPa)

181

5 4 3 2 1 0 Cakrall

Cafrall Composites

Figure 9.12 Storage modulus at different temperatures.

CAKRALL having the highest value and the transition peak values that correspond to epoxy relaxation.

9.3.2.3

Burn-through time response result

The fire test results were conducted using propane-air burner, the two composites were burned at an average time of 15 min whereby one of the composites withstood a 15 min flame temperature of 1100
80 C and a heat flux of 116
10 kW/m2 (fireproof), while the other composite failed before 15 min (fire resistant) as indicated in Fig. 9.13. The average burn-through time responses of each composite was recorded using a data logger.

Burn-through time (s)

1000 800 600 400 200 0

Cakrall

Cafrall Composites

Figure 9.13 Burn-through time of the composites.

182

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

It is observed that the front sheet of aluminium alloy melted at about 90 s, the epoxy and the first layer of fiber formed a fire barricade and a layer carbonized on its thickness, as the fire continued to impinge on the fiber delamination of the fibers, this led to layer separation, which allowed air to pass through the layers and serve as an insulator to the composites. The carbonized epoxy and air that pass through fiber layers prevented the rear facing aluminum alloy sheet from flame, therefore the temperature at the rear face aluminium was lower than the temperature of the front face of the composites during the test as measured by K-type thermocouples. The temperature increased as the burn-through time increased; this type of result obtained was also observed in Hooijmeijer and Vlot [80]. The rear face temperatures of the composites at different burn-through times in three locations are presented in Table 9.12; the thermocouple was placed at the center of the each composite, 1 inch above the center and 1 inch to the right of the center. It is observed from the two composites that around 125 s the flame become more severe on the samples plate, which resulted in flaring and smoke, which was due to the nature of the materials used (the fibers and epoxy). At around 300 s a red glow was noticed in the center of the composites while the smoke and flaring weakened Table 9.12 Result of the rear face temperature of the composite Temperature (8C) CAKRALL Time (s)

Center

100

above

CAFRALL 100

right

Center

100 above

100 right

60

95.76

90.83

88.98

87.15

69.77

68.61

120

177.43

159.94

154.10

130.85

113.01

112.54

180

225.09

214.09

210.54

219.07

179.23

170.88

240

294.77

279.09

274.03

269.10

231.63

220.67

300

358.08

350.87

344.94

312.08

283.48

275.77

360

432.77

419.71

412.77

349.06

324.88

318.98

420

543.05

538.98

530.92

392.10

369.08

360.54

480

653.99

641.64

633.78

437.34

415.99

410.75

540

769.03

751.91

745.93

498.56

468.09

463.09

600

857.52

832.74

830.09

554.99

524.88

513.60

660

e

e

e

592.87

570.88

564.09

720

e

e

e

657.98

624.34

615.87

780

e

e

e

708.65

690.55

683.88

840

e

e

e

752.98

741.48

736.05

900

e

e

e

802.67

788.86

781.44

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

183

and a further decrease in flaring was observed after 500 s, The CAFRALL was very hot and no sign of burn was observed, but some signs of burn-through were observed in CAKRALL, which burned at 622 s. The CAFRALL withstood the 900-s burnthrough time. Eq. (9.5) was used to determine the thermal conductivity of the fiber metal laminates and the results are shown in Table 9.13 where CAKRALL recorded the highest thermal conductivity.

9.3.2.4

Ballistic impact results

The results of the impact and residual velocities of the two composites are evaluated and analyzed in this section. The impact velocity of the test at initial was 214
2 m/s, which proves very high velocity was used in that test; the projectile penetrated all the composites and later a lower velocity of 33.46 and 33.51 m/s were used on the CAKRALL and CAFRALL, respectively, and the composites absorbed the impact and withstood the velocity. In the first test, there was much damage to composites at the rear face as the projectile passed through it. The damage noticed at the rear face was due to the fiber fractures and ceasing of projectile at the exit of the FML composites, which resulted in negligible or no stress existing between the fibers and the polymer matrix; in the absence of stress, shear stress exists on the polymer matrix that leads to the composites’ failure, and this event was also reported by Day and Rodrigez [81]. The enormity of frictional force affects the performance of energy absorption due to the impact of the projectile to the composites under study, [82]. The great damage and delamination of the composites observed in the two composites was due to the thickness and the mode of composite fabrication (unidirectional) that led to fragmentation along the fibers’ axis. The shapes of the image formed after impact at the rear face were not circular, but the front face maintained the circular shape, which is in agreement in Gorham and Field [83] and Rogers and Sidey et al. [84], with a small deflection at the front face as observed in Choi [85]. The results of the two impacts and residual velocity are shown in Table 9.14, whereby in the low-velocity test CAKRALL produces higher residual velocity, but in the high-velocity test, CAFRALL produces high residual velocity. The energy absorption of the two composites was evaluated using Eq. (9.6) and the values are shown in Fig. 9.14 for a high-velocity test of the two composites under study. CAKRALL absorbed more energy than CAFRALL; this result was in coincident with the results obtained by Ref. [86] in the investigation of the ballistic impact test Table 9.13 Thermal conductivity of the composites Composites

Thermal conductivity (W/mK)

CAKRALL

1.674

CAFRALL

1.365

184

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Table 9.14 Impact and residual velocity High velocity (m/s)

Low velocity (m/s)

Composite

Before impact

After impact

Before impact

After impact

CAKRALL

213.58

140.78

33.46

3.34

CAFRALL

215.03

158.31

33.51

2.02

High velocity energy absorption (J) Low velocity energy absorption (J)

70

Energy absorption (J)

60 50 40 30 20 10 0 Cakrall

Cafrall Composities

Figure 9.14 Energy absorption of the composites.

of FMLs of synthetic fiber and synthetic with natural fiber, whereby high absorption energy was found on the FMLs of synthetic fiber. As the projectile hit the target, the damaged fiber was pulled out from matrix in the composite, which resulted in energy dissipation, therefore the impact energy from these two composites was determined, which led to the evaluation of impact strength using Eq. (9.7). The results of impact strength for the higher and lower velocity are shown in Table 9.15. The results indicate that CAKRALL has the highest impact strength, and also the higher value was found from the low-velocity test; this result was in agreement with the results reported by Razali and Sapuan et al. [87] where various percentage loads of Roselle fiber were investigated based on its impact strength. Among the factors Table 9.15 Impact strength of the high and low velocity and specific perforation energy test Impact strength (kJ/m2) Composite

Low velocity

High velocity

Specific perforation energy (Jm2/kg)

CAKRALL

9.81  103

6.3

13.04

4.7

8.85

CAFRALL

3

4.91  10

Synthetic/natural fiber properties of fire-designated zone of an aircraft engine

185

that affected the impact strength of these composites were stress concentration at the end of fiber, poor interfacial bonding that occurs between the matrix and fiber, good agglomeration of the fiber that affects the stress transfer between fiber and the matrix, and poor adhesion; these factors were in agreement with the factors reported by Yang and Kim et al. [88] in the study of change in impact strength with a filler content of rice husk with polypropylene composites. Also, the specific perforation energy of the two composites for high velocity are presented in Table 9.15, by using Eq. (9.8), with CAKRALL having the highest specific perforation energy.

9.4

Conclusion

The main conclusions are: 1. To determine the mechanical, thermal properties, fire behavior, and impact loading of the sandwich composites using an ISO 2685 propane-air burner according to the standard. 2. To examine the burn-through responses of the composites and their burn-through time under fire test. a. The results obtained from the tests show that the sandwich fiber metal laminate composites are suitable to be used in fire-designated zones of an aircraft engine, more especially CAFRALL, which proved to be fireproof composite. The use of natural fiber in the aerospace industry is now advancing, which in turn will replace the use of synthetic fiber, due to its health benefits, reduce weight, abundance, and cost-effectiveness. Therefore it is used in a sandwich form with synthetic fibers, promoting the effective use of it in terms of the properties of the composites. The findings from the analysis show that the two composites considered that were sandwiched from synthetic and natural fibers had the ability to reduce health risks produced by synthetic fibers through skin irritation and inhaling the fibers. b. The properties of the two types of fiber metal laminates of metal alloy (aluminium alloy) with synthetic (carbon fiber) and natural fibers (kenaf and flax) were successfully analyzed. From the properties considered in this study, the two composites could be considered to be used in a fire-designated zone of an aircraft engine, but CAKRALL can only be used in some components that are fire resistant, since it did not withstand 15 min in a flame fire as indicated by the standard; the other composite was proved to be a fireproof composite, therefore can be used in all the components of fire-designated zones of an aircraft engine. In terms of mechanical properties, CAFRALL withstands more tensile and compression stress; for thermal properties, it has a higher percentage of weight residue. But the CAKRALL has more impact resistance and flexural stress with highest storage modulus. c. Out of these two composites, viz. carbon fiber kenafereinforced aluminium laminate and carbon fiber flaxereinforced aluminium laminate, CAFRALL is the first choice in terms of fire protection in the fire zone as it has the properties of the fireproof composite, where the composite can withstand a flame temperature of 1100
80 C and 116
10 kW/m2 heat flux.

Conclusively, it can be noted that the two composites under study can be used in the fire-designated zone of an aircraft engine, but CAKRALL with only fire-resistant components. Therefore, introducing these composites in the fire-designated zone of an

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aircraft engine will be of use to the industry due to the main properties of the composites. It is recommended for future studies to consider this work to be a benchmark when developing new composites with other types of synthetic fibers and natural fibers, and also to use a thicker fiber and aluminium alloy sheet than the one used in this study so as to reduce the number of layers used in the composites; this will reduce the tendency of fiber delamination. From this feasibility study, it can be recommended that the fiber metal laminate composites can be used in fire-designated zones of an aircraft engine straight away without any improvements in their structural properties and that only more adhesive polymer should be used. The extent of fire protection by fiber metal laminate composites into the fire-designated zone of an aircraft engine has been used and evaluated using the ISO 2685 standard as described experimentally by Abu Talib, Neely, Ireland, and Mullender [89]. Physical properties such as microstructure, thermal conductivity, and thermal diffusivity of the developed composites should be determined, and the numerical methods of this study should be done using different types of metal alloys, synthetic fibers, and natural fibers in future work.

Acknowledgments This study was carried out in Faculty of Engineering in Propulsion Laboratory, Strength of Material Laboratory, Aerodynamic Laboratory and Biocomposite Laboratory in INTROP Universiti Putra Malaysia. The authors would like to thanks University Putra Malaysia for providing the financial support through the IGPS Grant Scheme (No. 9538500). Supportive members of this study are Associate Professor Ir Dr Mohamed Thariq Hameed Sultan, Dr. Syamimi Saadon of Department of Aerospace engineering, and Assistant Engineers Mohamad Nasir Johari of Propulsion Laboratory and Saffairus Salih of Aerodynamic Laboratory Universiti Putra Malaysia.

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A.R. Abu Talib 1 , M.I. Nadiir Bheekhun 2 1 Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; 2Aerospace & Communication Technology Research Group, Faculty of Information Sciences and Engineering, Management and Science University Shah Alam, Malaysia

10.1

Introduction

10.1.1 Background With the continuous rise in energy consumption across the globe, proliferating energy charges due to the limited supply of fossil fuels plus global warming and climate issues, both governments and industry sectors supported by academia are pursuing alternative or improved thermal systems by employing high-performance insulative materials. In aeronautics, the quest for ever-more-efficient aero-propulsion systems has been constantly motivated by perpetual developments in a wide range of fields, including turbine design, combustion analysis, heat management, and material sciences. The key drivers behind these evolutions are the demands for reduced weight and increased fuel efficiency while simultaneously reducing the emissions and noise levels [1]. On the other hand, silica aerogel, invented in 1931 by Samuel Kistler [2], is an illustrious nanostructured material considered as the world’s paramount thermal insulator and second lightest solid after being surpassed by graphene aerogel in 2013. By definition, an aerogel is a gel in which the liquid phase has been replaced by a gas in such a way that the solid network is being retained with only slight or no shrinkage in the gel [3]. Consequently, an open-cell network consisting of nanosized pores is developed, thereby unfolding further numerous exceptional characteristics. Within the class of aerogels, silica aerogel, which is the porous nanostructured form of silica dioxide, exhibits the most fascinating properties, such as low thermal conductivity (w0.012 W/mK), ultralow bulk density (w0.003 g/cm3), optical transparency in the visible spectrum (w99%), high specific surface area (w1000 m2/g), low dielectric constant (w1.0e2.0), low refractive index (w1.05), and low sound velocity (100 m/s). Due Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00010-1 Copyright © 2019 Elsevier Ltd. All rights reserved.

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to its attractiveness and being the first aerogel invented, silica aerogel is often referred to as simply aerogel. This chapter consequently adopts this shortened term as well. Since early on, fundamental studies on aerogels have significantly lagged behind in academia, and hence industries, due to its high manufacturing cost. This further hampered its feasibility for integration into the commercial market. However, considerable enhancement started taking place at the dawn of this millennium. Numerous scientific reports have been published since then, showing not only enthusiasm for but also increased scientific understanding of these nanostructures. Nowadays, one of the most focused areas of work is the modeling of the thermal behavior of granularand fibre-based silica aerogel and its composites [4e10]. On the market level, largescale production of the material has been achieved by various producers such as the North Americanebased industrials Cabot Corporation and Aspen Aerogels, and Nano Hi-Tech in China, which is considered to be the third player, followed by Korean JIOS, amongst others. In contrast to these companies, which produce synthetic-based aerogels using usually tetraethyl orthosilicate (TEOS) as the precursor of silica, there are also other manufacturers that synthesize ecological aerogels from rice husk ash, such as the Malaysian-established Maerotech SDN BHD and the Spanish Green Earth Aerogel Technologies (GEAT). These biologically derived aerogels have both potential cost and environmental benefits. It is noteworthy to mention also that silica aerogels can be prepared in different forms, such as in monolith, granules, and fine powder. As a result, aerogels are often required to be tailored specifically to develop innovative solutions to today’s problems. Their applications are considered to be almost illimitable, tracing their way in including as thermal and acoustical insulation, for kinetic energy absorption, and for electronic, optical, chemical, and biomedical uses amongst others. As far as aerospace is concerned, aerogels have been used extensively by the U.S. National Aeronautics and Space Administration (NASA) since 1992 for astronautical missions in the form of hypervelocity particle capture and thermal insulator, both in the shape of blocks. Recently, Aspen Aerogel has introduced aerogel-based flexible blankets for high temperature use up to 650
C for both thermal and fire protection in aerospace systems. However, due to the constraints of weight and space saving, blankets are not encouraged by aero-engine manufacturers. Instead, micrometer-thick thermally sprayed coatings are opted for to protect against high temperature, wear, and corrosion, such as thermal barrier coatings (TBCs) in gas turbines [11]. Longo reported that some gas turbine engines have nearly 5500 parts that are thermally sprayed to enhance their performance, reliability, and durability [12]. Thermal spray processes are extensively used because virtually all materials, including metals, superalloys, and ceramics, can be deposited, but it is considerably challenging to retain the characteristics of the nanostructured materials during deposition, which in turn gives rise to a series of study. To date, no analysis has been carried out, and as a result, the feasibility to achieve a single-layered aerogel-based thermally sprayed coating is being established. A singlelayered thermally sprayed coating is regarded as one that consists of either only one type of particle or an agglomeration of different ceramics particles that is deposited directly onto a substrate. The coating system does not comprise any bond coat nor is part of a multilayered or graded coating. While this work concentrates on realizing

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a single-layered aerogel-based adhering coating, it also targets the coating to consist of a bimodal microstructure. A bimodal microstructure is one that includes molten particles acting as a binder that upholds the coating integrity, whereas the semi-molten ones contain nanostructures residing in them [13]. This is attained by setting the operational conditions in such a way that some of the particles are completely melted and some only partially. Such coating microstructure is intrinsically related to thermal spraying of delicate nanostructured materials [14], thus making aerogel eligible to be taken into consideration for thermal spraying, whether by conventional or suspension approach. This study hence intends to achieve a single-layered aerogel-based thermally sprayed coating by firstly ascertaining the suitable aerogel powders for the application of plasma spraying, a versatile type of the thermal spray technology. The study further considers the possibility of employing an optimized agglomerated spray-dried aerogelbased powder as feedstock to allow or improve deposition. The practicability of spraying aerogel powders using the conventional atmospheric plasma spraying (APS) and the advanced suspension plasma spraying (SPS) in order to achieve an adhering coating on glass substrate is then carried out, after which a microstructural analysis of the resulting aerogel-based plasma-sprayed coatings is done.

10.1.2 Scientific Gap Aerogels are considered to be established in terms of their manufacturing process, thereby now having an array of synthesis methods that have overcome, to a considerable extent, the high manufacturing cost arising from the source of silica or drying method. The current trend is now inclining toward the development of innovative solutions by tailoring their properties or synthesizing composites of theirs to specifically address practical problems. Beneath the aerospace dome, applications of aerogel are still emerging, and amongst are thermal insulation systems that include mainly aerogel-doped flexible blankets operating from cryogenic to extreme temperatures for astronautical applications. These are typically composites of silica aerogel particles dispersed in a reinforcing fiber matrix that turns the brittle aerogel into a durable and flexible insulating mat [15]. A study was conducted to evaluate the sustainability of these blankets during hydrogen-fueled flights at hypersonic speeds ranging from Mach 5 to Mach 9 in harsh thermal environments [16]. Fesmire tackled a common problem to both space launch applications and cryogenic propulsion test facilities of providing suitable thermal insulation for complex cryogenic piping, tanks, and components that cannot be vacuum-jacketed or otherwise be broad-area-covered by the use of a layered composite insulation system made of aerogel blankets [17]. A benzoxazine organic-inorganic hybrid aerogel blanket was recently developed in a one-pot sol-gel synthesis to obtain an easy-to-handle, light, superinsulating material for space applications [18]. There has been only one feasibility study that has been carried out on aerogel for aeronautical integration, wherein the developed product is again an aerogel-based blanket. The work, accomplished by Aspen Aerogel, consisted of evaluating the flammability of the blanket to protect components such as pipes, wires, and electronic accessories within the fire zones of aero-engines. The fact of being an

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inorganic and inflammable material with a constant operating temperature ranging from 273 to 650
C and a high melting point of 1400
C upholds silica aerogel as an excellent firewall. The investigation consisted of a thickness of 7-mm mat subjected to a flame at a temperature 1100
C for at least 15 min. The temperature on the rear side of the wrapped system did not exceed 150
C. The product, Pyrogel 6350, has yet to be integrated for aeronautical applications. The thermal loss that happens when the blankets are under constant vibration and gravitational stress through repeated thermal cycles is still under study. In the quest for renewable energy systems, substantial efforts are being made toward green architecture, and consequently, aerogels are making their way into building insulation at a faster rate than any other applications. Existing solutions comprise aerogel-based blankets, powdered aerogel-doped paints, translucent aerogel pellets, and aerogel fillers in vacuum-packed insulation sheets [19e22]. A mortar-applied coating that consisted of a mineral and/or organic hydraulic binder, an insulating filler comprising of hydrophobic silica aerogel, a structuralizer, and additives was developed for thermal insulation of buildings’ multilayer exterior wall structures [23]. Furthermore, a study was conducted to develop an ultra-lightweight cementitious composite having both excellent mechanical and thermal insulating properties [24]. It is worthwhile to mention that along with the aforementioned expansions and applications, the oil and gas industry remains the undisputed sector that benefits from these aerogel technologies to the largest extent due to their compatibility of having superior thermal performance and improved chemical/pressure resistance and large-scale usage coupled with assembly cost savings and ease of use. Motivated by global CO2 emission and space-saving restrictions, this study proposes a novel route to implement aerogel in aeronautics as a micro-thick singlelayered thermally sprayed coating. It can be anticipated that limited related studies have been conducted due to the restrictions that exist in aerogel powders such as irregular particle shape, inadequate particle size distribution, ultralow density, possibility of the nanostructures to be annealed in plasma, and unknown adhering ability. A study was carried out by Zulkifli and colleagues [25] to reconstitute aerogel particles, more precisely, the Malaysian-made Maerogel (now Hamzel) using a secondary ceramic, soda-lime, via spray-drying, but the study did not highlight whether the process can be optimized for thermal spraying. There is still lack of understanding of the effect of the formulation and granulation variables on the granulometric properties and morphology of the reconstituted spray-dried aerogel. It is also not confirmed whether spray-drying aerogel is a repeatable process and whether the process is economically feasible, in other words, whether or not the yield of the aerogel-based spray-dried powder is sufficient enough to carry out APS in an efficient manner. Although a patent was filed on applying the thermal spray technology on aerogel to coat a missile surface that would act as an insulation layer, aerogel was not thermally sprayed directly onto the substrate (steel) but instead there was an alloy bond coat (NiCrAlY) in between the substrate and the upcoming aerogel layer, which means that the coating system was not a single-layered one [26]. In addition, the silica aerogel being deposited was not on its own but rather as agglomerated ceramic particles. The patent employed plasma spraying, which is a technique that can literally deposit any powder onto

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any substrate, but retaining the microstructural characteristics of the starting material so that the coating can exhibit comparable properties is challenging [26]. The appropriateness of plasma spraying aerogel particles was not explained, and the claims did not prove whether the coating has a skeletal microstructure similar to that of the starting aerogel. The patent did not establish whether a critical aspect of bimodal microstructure was achieved. A bimodal microstructure is one that consists of partially molten particles and fully molten particles in order to retain the properties being possessed by the feedstock. Another shortcoming was that coating developed had a thickness of 1.5e2.0 mm. The patent was filed 9 years ago and as yet no further studies have been done after it depicted a significant discontinuity of study during which aerogel particles have been modified in terms of their granulometric properties by the manufacturers due to new methods of synthesis. The granulometric properties are considered to be the most critical characteristics to be known when considering thermal spraying technique. Ecological aerogels were not available at that time. Recently, another multilayer concept was proposed by Jin and colleagues [27] whereby yttria stabilized zirconia (YSZ) and silica aerogel were used in conjunction to improve the thermal insulation capability of an aerospace-graded titanium alloy (Tie6Ale4V or TC4) using the APS technique. However, the aerogel was not thermally sprayed but rather adhered onto the surface of the alloy using an organic glue as a transitional layer or bond coat between the substrate TC4 and the upcoming plasmasprayed YSZ. On the other hand, SPS of aerogel has not been reported anywhere so far. In contrast to the works reported herein, this study focuses on achieving a singlelayered aerogel-based coating for various reasons. The first reason is that no such investigation has been carried out previously. Secondly, multilayered coating systems such as TBCs in aero-engines usually experience delamination in between the two different layers of the coating due to thermal shocks as the coating system consists of different deposited materials that have their own thermal expansion coefficients [11]. Hence, to prevent any delamination in the aerogel-based coating system, if intended to be used as a thermal insulative coating, it is foreseen that a singlelayered coating would be more suitable. Also, the thickness of a multilayered coating system is often of millimeter order, greater than 1.0 mm rather than of micrometer order. Overall, it can be argued that there are still no answers whether (1) the existing types of aerogel powders can be considered for thermal spray applications, more precisely APS and SPS, (2) aerogel can be practicably deposited as a single-layered coating using the opted thermal spray technology, and (3) the morphology of the deposited aerogel-based coating consists of any microstructural characteristics resembling that of aerogel. This study uses a systematic approach to find remedies to these technological queries through its targeted objectives in the process of achieving a single-layered aerogel-based plasma-sprayed coating on glass in order to obtain a clearer microscopy view of the microstructures of the aerogel layer. It is worthwhile to mention at this stage that if the targeted coating has been achieved on glass, the same spraying conditions can be tried on aerospace-graded substrates.

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Objectives

10.1.3.1 Aim To study the feasibility of depositing aerogel using plasma spraying.

10.1.3.2 Specific objectives The specific objectives of the study are as follows: 1. To evaluate the suitability of as-received aerogel powders for atmospheric plasma spraying and suspension plasma spraying based on their physical properties 2. To deduce the optimum spraying conditions for surface adhesion of a single-layered aerogelbased plasma-sprayed coating 3. To examine the microstructure features of the developed single-layered plasma-sprayed coatings

10.1.4

Study questions

Question 1: Are the commercialized available aerogel powders suitable for plasma spraying, more precisely, for atmospheric plasma spraying and suspension plasma spraying? Question 2: Can aerogel be transported into the plasma stream and achieve an adhering coating using APS and SPS? Question 3: How is the microstructure of single-layered aerogel-based plasmasprayed coating?

10.1.5

Significance of study

Silica aerogel, being the most superior thermal insulator and one of the lightest materials, has been perpetually regarded as a potential candidate for future superinsulation systems in aeronautics where weight and space savings are fundamental. Previous studies have addressed particularly the development and evaluation of aerogel-based flexible blankets for such protection uses or multilayered coatings. This study presents a new concept whereby a single-layered aerogel-based micro-thick coating is developed using the thermal spray technology, more precisely, plasma spraying, thereby opening a panoply of prospects. This investigation principally validates the hypothesis whether aerogel powders can be successfully deposited directly as a single-layered coating onto a substrate and provides an insight how to achieve such a validation by the proposed methodologies. The microscopic analysis of the deposited aerogel furthermore brings an understanding on how the particles are affected when propelled into a plasma. Implementation of such an aerogel-based coating into aero-engines depends on the type of substrate. Typically, applications can range from adjusting thermal responses for improved performance and reduced deterioration to protecting polymer matrix composites, enabling weight savings for equivalent level of fire protection.

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10.1.6 Scope of study The study starts with the philosophy that aerogel has ultralow density and lowest thermal conductivity of any other solids, which are two crucial factors when considering thermal insulation systems for aeronautical applications due to load and space constraints. The work targets to achieve a single-layered aerogel-based coating using thermal spray, more precisely, atmospheric plasma spraying and suspension plasma spraying. It devises a methodical approach to accomplish the coating by: 1. Characterizing the physical properties of all of the six commercialized powdered aerogels available on the market and assessing their suitability for APS and SPS based on the evaluated characteristics. 2. Conducting a parametric investigation to predict the optimum spraying conditions using the suitable types of aerogel with respect to the APS and SPS. The experiment consists of altering the spraying conditions such as the gun nozzle diameter, power, volumetric rates of plasma gas and carrier gas, and concentration of aerogel in a systematic manner to overcome the challenges encountered until an adhering aerogel-based coating is achieved or up to the technological limitations. 3. The analysis of the thermally sprayed coatings being developed is limited to a microstructural observation using a scanning electron microscope (SEM).

10.2

Methodology

10.2.1 Development of a novel coating A generic flowchart that can be considered while developing a novel coating for a given application has been presented by Pawlowski [14], as shown in Fig. 10.1. It reflects the steps to be undertaken from the study and development level up to the commercialization. A simplified version that was relevant to this present study was adjusted as per Fig. 10.2. The latter depicts the need for a coating that is, in this case, an aerogel-based coating. The coating specification in quest in this study was to obtain an adhering single-layered coating with the aerogel’s network being undamaged. The initial choice for the coating process was primarily silica aerogel using thermal spraying, more precisely APS and SPS. The feasibility test on lab-scale samples consisted of depositing different types of aerogel feedstock using the two aforementioned deposition techniques on glass substrates. The feedstock was considered to as-received (pure) aerogel powders. Characterization of the developed thermally sprayed coatings was limited to microscopic analysis in this study. Fig. 10.3 exemplifies the targeted tasks in a systematic manner based on the technological availability with respect to time in order to achieve the specific objectives of this study.

10.2.2 Assessment of aerogel powders for plasma spraying Rice husk ash-derived aerogels, namely Hamzel, GEAT 0.125 and GEAT CDZ, waterglass-synthesized AeroVa, and TEOS-produced Enova Aerogels IC3100 and

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Need for a coating

Coating’s conformity check with specification

No Conform ?

Preparation or modification of coating specification

Initial choice of: coating process coating material pre-/post spray method

Yes

Pealisation of coating on “Real” pieces

Feasibility test on small samples

Yes No

No Feasible?

Feasible?

Yes

Coating quality in service

Microstructure study

Satisfactory?

Properties study

No

Yes

Production of coatings

Figure 10.1 Generic flowchart in developing a novel coating [14].

IC3110 were supplied from Maerotech Sdn Bhd, Green Earth Aerogels Technologies, JIOS Aerogel, and Cabot Corporation, respectively. All the materials were used asobtained with no further treatment. A preliminary investigation was conducted to obtain and reaffirm the relevant physical properties of the aerogels in order to recognize their suitability for plasma spraying. It was also of prime importance to have a microscopic visual of the microstructural features of the aerogels in order to compare with the ones residing inside the singlelayered aerogel-based thermally sprayed coatings yet to be developed. The evaluation of which type of aerogel powder was suitable for which plasma spraying processes was deduced by analyzing the physical properties obtained and

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Start

Need for a coating (aerolgel-based coating)

Coating specification: (single layered adhering coating, skeletal structure retained)

Initial choice for: coating process (APS/SPS) coating material (Aerogel) pre-spray method spraydying for YSZ-aerogel

Feasibility tests on lab-scale samples

No Feaslble?

Yes Microstructure study Yes Conform with specification

No

End

Figure 10.2 Simplified flowchart in developing a novel single layered aerogel-based coating.

comparing them with the requisites of plasma spray feedstock found in the notable textbook by Pawlowski [14]. Furthermore, the granule size distribution and flowability are two essential characteristics to be looked into when considering powder injection, APS in this case, whereas in the case of SPS, the flowability is not of prime importance [28]. These factors were taken into consideration to carry out the evaluation.

10.2.3 Atmospheric plasma spraying of as-received aerogels Depositing silica aerogel via the conventional APS was anticipated to be extremely difficult if not impossible due to their poor flowability that could be expected because

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Assessment of of aerogel powders for APS and SPS (Objective I)

Deduction of spraying conditions for surface adhesion of a single layered aerogel-based plasma sprayed coating (Objective II)

Analysis of the microstructure features of the developed single layered plasma sprayed coatings (Objective III)

Figure 10.3 Order of research activities in accordance to specific objectives.

of the low density and unfavorable irregular morphology arising from the sol-gel preparation method. There was also the possibility of absorption of gases through the aerogel powder and tendency to agglomerate. Nevertheless, this study also considered this technique to investigate the extent of these hypotheses. Out of the six aerogels, GEA 0.125 aerogel was found to be most suitable based on the evaluation test made in the previous Section 10.3.2 to assess the feasibility of depositing aerogel via plasma spraying. More details on the aerogel evaluation results are provided in the next section.

10.2.3.1 Test facility Atmospheric plasma spraying was carried out at the Laboratory of Science of Ceramic Processing and Surface Treatments (SPCTS), University of Limoges in France.

10.2.3.2 Apparatus A 40 kW plasma torch SG-100 of Praxair (Indianapolis, USA) was mounted on an ABB 6 axis robot and used for deposition. This gun operates in a number of modes ranging from subsonic to Mach I, allowing a range of gas velocity levels. The parts are designed by the manufacturer to be self-aligning, which can be assembled without difficulty while maintaining concentricity, i.e., the center line of the electrode on the axis of the nozzle (anode). The more precise the alignment is, the more centered the arc is within the nozzle. The plasma torch was set up by the laboratory technician to ensure good concentricity and hence proper gas flow patterns. Normal SG-100 operations require a minimum flow of 47.3 standard liter per minute (SLPM or slpm) for water cooling system. The standard liter per minute is the default volumetric rate unit used by thermal spray systems whereby the standardized condition is room temperature and pressure [14]. The powder feed pipe was set under an angle of 85 degrees to the axis of the torch. At this angle, the injection of the feedstock into the

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ABB robot

SG-100 torch Tube for air cooling substrate holder

Tube for air cooling gun system Substrate holder equipped with vacuum sucker system

Figure 10.4 Overview of plasma spraying setup.

plasma stream right on the face of the nozzle was observed, which allowed optimal trajectory of the particle in the plasma jet. An overview picture of the plasma spray gun and its installation is shown in Fig. 10.4.

10.2.3.3 Substrate preparation In thermal spray feasibility studies, normally glass can be used as substrate for effective and efficient microscopic assessment, in particular, unlike stainless steel, which needs to be polished to have a mirror-like surface prior to microstructural analysis [29]. In consequence, glass substrates having diameter of 25 and 1.5 mm thick were adhered onto stainless steel coins of similar diameter and thickness of 3 mm using a vacuum-sucked system equipped in the substrate holder. The glass substrates were washed with ethanol and dried in the oven at 40
C prior to use to eliminate any risk of contamination.

10.2.3.4 Deposition process Four spraying parameters were considered in the APS, namely the nozzle diameter, power, total plasma gas volumetric rate, and carrier gas flow rate according to Table 10.1 to spray GEA 0.125 aerogel onto glass substrate fixed at a distance of 100 mm, a typical distance for a feasibility study as recommended by Pawlowski [14]. In principle, if a material for which there are no operating parameters yet, then the possibility of beginning with parameters for a comparable powder, in terms of thermal characteristics, specific gravity, and powder sizing for which settings are available can be considered. In this study, this idea could not be implemented due to the fact that no other existing material close to silica aerogel has been thermal sprayed so far. Hence, a parametric study was required. The process variables were rather margined between a

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Table 10.1 APS operating conditions

Test

Nozzle diameter (mm)

1e6

0.3, 0.5, 2, 4

Power (kW)

Plasma gas Ar D H2 (slpm)

Carrier gas Ar (slpm)

25, 30, 35, 40

45 þ 5

7.0e9.0

minimum value and a maximum value basis to observe the phenomena occurring and acted accordingly based on the observations made from each precedent session to overcome the problem encountered in a systematic manner in order to achieve an adhering coating. The nozzles available for the SG-100 gun of Praxair (Indianapolis, USA) which had internal diameters of 0.3, 0.5, 2, and 4 mm were all selected. The plasma, which consisted of a combination of argon and hydrogen, was maintained at a volumetric flow rate of 45 þ 5 slpm, which fell in the typical total flow rate of 40e50 slpm [30]. The same article highlighted that each working gas has its own function such as argon, being the primary gas, stabilizes the arc inside the nozzle while hydrogen as the secondary gas enhances the heat transfer to the particles due to its high thermal conductivity. The total plasma gas flow, particle size, and density affect the powder particle velocity at the time of impact, which is one of the two critical factors to optimize during deposition. The temperature of the powder particle at the time of impact with the substrate is governed primarily by the net energy of the plasma stream and the total gas flow through the plasma gun. Gas flow determines velocity, which in turn determines particle dwell time (time the particle is resident in the plasma stream) [31]. The plasma gun is primarily an electrical device. As described by Pawlowski [14], increasing electrical power input normally results in an increase in net energy or enthalpy (energy in the plasma stream). Usually, an increase in electrical power input is indicated when the powder particles are not sufficiently melted before impinging on the substrate. However, circumstances may exist where the same effect can be accomplished by reducing total arc gas flow while increasing the current input. Reduced total arc gas flow decreases gas velocity thus lengthening the particle dwell time. If the particles are overheated to the point of vaporization, decrease the electrical power input, or in some cases, increase the total arc gas flow. In other words, the second factor is the velocity of the inflight particles. Based on this information, the power was set within the range of 25e40 kW. It is worth mentioning that attempting such tedious trials is constantly being done by the thermal spray community, while developing a novel coating, because the online diagnosis system to monitor the actual temperature and velocity of inflight particles, thereby obtaining the optimum parameters is not readily available in research centers. Another important aspect of the plasma spray operation is obtaining maximum heat transfer efficiency from the plasma to the powder particles. There is a pronounced temperature gradient along the length and across the width of the plasma, so powder particles should be entrained in the core of the plasma, not in the cooler outer regions [32]. Insufficient powder carrier gas flow will not push the powder up into the core, and too

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much carrier gas will push the powder through the core. Either too much or too little powder carrier gas results in an inferior deposit, because of insufficient heating or overheating of the powder particles. Improper carrier gas settings can also cause powder “spitting” from the gun face. The optimum powder feed condition is reached when most of the powder injected into the plasma stream is carried along its axis [28]. Consequently, the carrier gas flow rate was made to alter from 7.0 to 9.0 slpm with an increment increase of 0.2. The minimum flow of gas was used to provide feeding of the powder, thereby starting an ejection of the silica aerogel powders in the injector/ nozzle. The increase also had to ensure continuity of the powder feeding and prevent clogging of the powder in the pipe transporting the powder. To enhance the flowability of the powders and to prevent clogging, the possibility of drying the powders prior to injection was considered as well.

10.2.4 Suspension plasma spraying of as-received aerogels The study continued to its second option thereby attempting to deposit silica aerogel using a more advanced technique, known as suspension plasma spraying (SPS). This technique can spray nanosized to submicron-sized particles dispersed into a suspension, which together form the feedstock. Both GEA 0.125 aerogel and Enova IC3100 aerogel had been considered to study the feasibility of depositing aerogel using SPS due to their smaller granule size distribution compared to the other silica aerogels.

10.2.4.1 Test facility SPS was also carried out at the SPCTS, University of Limoges in France.

10.2.4.2 Materials Biologically derived GEA 0.125 aerogel and synthetically produced Enova IC3100 aerogel were used for this part of the feasibility study. Commercialized ethanol (C2H5OH) and phosphoric ester, Beycostat C213 (CECA, France), were provided by SPCTS. The former was the liquid medium while the latter acted as dispersant, as employed in previous studies [33e35].

10.2.4.3 Apparatus The same combination of SG-100 Praxair torch-ABB 6 axis robot used for APS was employed for SPS except that the feeding system was connected to an external injector at an angle of 85 degrees to the axis of the torch for the same reason of obtaining optimum trajectory of the suspension into the plasma jet.

10.2.4.4 Substrate preparation Similar cleaned glass substrates were opted for the SPS sessions of both GEA 0.125 and Enova IC3100 aerogels as in APS for the same reason of allowing clearer microstructural analysis.

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Table 10.2 SPS operating conditions

Test 7e19

Type of aerogel

Aerogel content (wt%)

GEA 0. 125, Enova IC3100

1.33, 3.33, 6.66

Dispersant (wt%) 0, 0.5

Nozzle diameter (mm)

Power (kW)

0.3, 0.5, 2, 4

30, 35, 40

Plasma gas Ar D H2 (slpm) 45 þ 5

10.2.4.5 Deposition process Six spraying design parameters were taken into consideration, namely, type of aerogel, suspension concentration, nozzle diameter, power and total plasma gas flow rate to plasma spray GEA 0.125 and Enova IC3100 aerogels with values as illustrated in Table 10.2. The glass substrates were placed 100 mm away from the gun, being an appropriate reference distance for a feasibility study [14]. Two types of aerogels were selected for SPS based on their superior physical properties for SPS compared to others as discussed in Section 10.2.4. Aerogel-based suspensions were prepared at three different concentrations, 1.33, 3.33, and 6.66 wt% with the aforementioned alcohol solvent (C2H5OH). The possibility of using the dispersant Beycostat to increase the fluidity of the suspension was looked into with a fixed amount of 0.5 wt% [32]. Ultrasonic-aided mixing of suspension was carried out where necessary. All four nozzle sizes available (0.3e 4 mm) for the SG-100 gun were taken into account. High power was applied ranging from 30 to 40 kW. Plasma gas, Ar þ H2, was kept at 45 þ 5 slpm, which is within the typical range of 40e50 slpm [14].

10.2.5

Microstructural analysis of developed plasma-sprayed coatings

The microstructures of the as-formed splats and coatings on the glass substrates were observed using SEM (Philips XL 30) and analyzed accordingly.

10.3 10.3.1

Results and discussion Appraisal of aerogel powders

10.3.1.1 Physical properties of aerogels The first attempt undertaken in this study was characterizing and evaluating silica aerogel powders produced by different producers for APS and SPS. Table 10.3 shows the physical properties found experimentally. They are being discussed herewith to have a better understanding of the selection of the appropriate aerogels for plasma spraying.

Product Property

Hamzel

GEA 0.125

GEA CDZ

Enova IC3100

EnovaIC3110

AeroVa aerogel

Granule size range (mm)

10e500

2e300

20e700

2e40

100e700

1e20

Median particle size (mm)

200

81

220

9

514

8

*Uniformity

3.171

1.993

2.121

1.590

2.745

0.988

150

129

131

124

123

60

192

159

167

171

173

82

1.28

1.23

1.27

1.38

1.41

1.37

Specific surface area (m /g)

712

859

857

915

916

950

Primary particle diameter (nm)

10e20

2e6

2e6

2e4

2e4

2e5

Mean pore diameter (nm)

30

20

20

20

20

10

*Thermal conductivity at 25
C (W/mK)

0.098

0.015

0.015

0.012

0.012

0.020

Surface chemistry

Hydrophilic

Hydrophilic

Hydrophilic

Hydrophobic

Hydrophobic

Hydrophobic

3

Apparent density (kg/m ) 3

Tapped density (kg/m ) Hausner ratio 2

Aerogel-based thermally sprayed coatings for aero-propulsion systems

Table 10.3 Physical properties of commercialized silica aerogel powders

Values rounded to the nearest integer except for * because of relatively small differences. Hausner ratio ¼ tapped density/apparent density. Hausner ratio < 1.24 indicates that the powder (aerogel) has good flowability.

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The granule size for each type of the aerogel powder differed largely from a minimum of 1 mm to a maximum 700 mm. Rice-husk-derived Hamzel had a granule size range of 10e500 mm with median particle size as 200 mm, ecological GEA aerogels could be obtained in 2e300 mm and 20e700 mm having median granule size of 81 and 220 mm for GEA 0.125 and GEA CDZ, respectively. Cabot synthetically derived aerogels consist of two series of granules ranging from 2 to 40 mm and 100 to 700 mm. Their median particle sizes were 9 and 514 mm, respectively. Enova IC3110 consisted of the biggest aerogel powders. Waterglass-based AeroVa aerogel had the smallest particle size of 1 mm and has a distribution up to 20 mm with a median particle size of 8 mm. The granule size range depends mainly on both the chemical formulation and drying method and is tailored according to demands and applications. Wei and colleagues controlled the particle size by altering the pH and the disperse solution volume. In turn, the gelation time required was changed [7]. The uniformity or polydispersity index (PDI) of the powders of aerogels, which gives an indication of the heterogeneity of sizes of the granules, was also quoted from the particle size analysis and was found to be in the range of 0.988e3.171 for the six types of aerogel. AeroVa had the least uniformity whilst Hamzel consisted of the most uniform granules although its particle size distribution was relatively wide. Owens and colleagues attested that the sol-gel process, as employed to synthesize aerogels, usually produces powders with high PDI [36]. The apparent densities provided in Table 10.3 had been obtained according to ASTM: B417-13, which is the hall flowmeter technique due to the inappropriate measurements recorded by the pycnometer in the sense that it provided the skeletal density as studied by Woignier and Phalippou [37]. This property was not of interest and hence neglected. The lightest aerogel powder was the one prepared by JIOS aerogel as AeroVa, which had a density of only 60 kg/m3 compared to the one synthesized by Maerotech Sdn Bhd, which was the heaviest silica aerogel with a density of 150 kg/m3. The other aerogels weighed almost the same with densities of 123, 124, 129, and 131 kg/m3 for Enova IC3110, Enova IC3100, GEA 0.125, and GEA CDZ, respectively. The tapped density values ranged from 82 to 192 kg/m3 for AeroVa and Hamzel, respectively. Tapped densities had been observed to be 159, 167, 171, and 173 kg/m3 for GEA 0.125, GEA CDZ, Enova IC3100, and Enova IC3110, respectively. The apparent density is predominantly dictated by the source of the silica and its concentration during the gel preparation [38]. It could be noticed that rice-husk ash-derived aerogels such as Hamzel, GEA 0.125, and GEA CDZ were relatively heavier than the synthetically produced aerogels Enova IC3100, Enova IC3110, and AeroVa. Besides, the lesser the concentration, the lighter is the aerogel, but it can be anticipated that the skeletal structure of the gel will be weaker. It would be worthwhile to mention that the drying conditions affect the density as well, to some extent, as experienced by Smith and colleagues with the use of subcritical drying method [39]. The one-step continuous sol-gel method and ambient drying technique employed by JIOS could be the reason for the extremely low density compared to others. The tapped density of powdered silica aerogel has been reported less often because of its minimal importance in accordance to applications. Heley and colleagues used another synthetic

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precursor, namely silicon tetrachloride (SiCl4), and investigated the effect of drying. It was found that the tapped density is 10 times lower when supercritical drying was employed compared to that of oven-dried aerogels. It was also reported that accelerating the hydrolysis process during the gel preparation by increasing the ratio of H2O: SiCl4 for a fixed time of aging did not affect the tapped density [40]. In this study, tapped density, which is the density obtained after a series of standard taps, was used to calculate the Hausner ratio, which in turn dictated the flowability of the aerogel powders, a paramount factor when considering the conventional thermal spray processes, such as APS, as described in the forthcoming section. Similar to bulk density, tapped density is affected by the particle size distribution, particle shape, and cohesiveness as these are the parameters that are actually being altered during the mechanical taps. AeroVa aerogel exhibited the highest specific surface area with a value of 950 m2/g, followed by Enova aerogels with 916 and 915 m2/g for IC3110 and IC3100, respectively. GEA aerogels had comparable values of 857, 859, and 895 m2/g, whilst Hamzel had the least specific surface area (SSA) of 712 m2/g. SSA is defined as the surface area of the solid particles divided by the mass of the solid particles [41]. The density of the aerogel was found to affect the SSA, which can in turn be controlled by the source of the silica and concentrations. Reducing the density was claimed to increase the SSA [42]. Even though the above aerogels are derived from only one precursor, it would be wise to mention that adding coprecursors such as Perfluoroalkylsilane can result in a higher SSA (w1100 m2/g) as reported by Zhou and colleagues [43]. Conversely, Satha and his team argued in a more specific manner through their findings that the specific surface area of the aerogels does not depend directly on the bulk density but rather on the primary particle size of the silica particles and necks between them [44]. A correlation of the bulk density and SSA could be nonetheless observed amongst the six types of aerogels in the sense that Hamzel showed the least SSA with the highest density and AeroVa had the largest SSA being the lightest. The primary particle diameter, which corresponds to the diameter of the silica particle, was of nano-order with GEA, Enova, and AeroVa aerogels possessing practically the same size of silica particle as 2e6 nm, 2e4 nm, and 2e5 nm, respectively. Hamzel is the only one that stands alone with values ranging from 10 to 20 nm. Heley and colleagues [40] claimed that the primary particle size is correlated with the hydrolysis and condensation processes during gelation through the pH. When the pH was increased using ammonia, the SSA was decreased due to an increase in the primary particle size. The relatively bigger primary particle size in Hamzel could be because of insufficient mixing as witnessed by Heley and colleagues [40] while preparing aerogels with silicon tetrachloride (SiCl4). The next nanostructure characteristics, namely mean pore diameter, also followed the same pattern with values of the GEAT and Cabot aerogels closer to each other compared to the Malaysian-made aerogel, Hamzel. The smallest mean pore diameter, 10 nm, was revealed by AeroVa followed by Enova and GEA aerogels having similar value of 20 nm, and lastly Hamzel, possessing a value of 30 nm. The first observation would be that all the aerogels have mesopores as their diameters fall within the range of 2e50 nm, as dictated by the International Union of Pure and Applied Chemistry. The pore size is affected by the concentration

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of the aging solution and aging time. Smitha and colleagues [45] found that an increase in the precursor concentration in the aging solution resulted in an increase in the pore size and SSA. Furthermore, a more uniform pore size distribution can be obtained by the preparation of catalyzed gels as proposed by Suh and colleagues [46]. The manipulation of the pore size of these powdered aerogels could have also been done through the drying solvent as claimed by Scherer and his team [47]. Thermal conductivity value ranged from 0.012 W/mK shown by Enova aerogels to 0.098 W/mK for Hamzel. GEA aerogels showed a value of 0.015 W/mK and AeroVa, almost close with a heat conductivity of 0.020 W/mK. The thermal conductivity of aerogels was already demonstrated at the time of invention by its inventor, Kistler [3]. It was shown that the thermal conductivity was in the order of 0.02 W/mK under ambient conditions. The low heat conductivity is mainly due to the high porosity and nanometer pore size. The passage of thermal energy through a material takes place through three mechanisms: solid conductivity, gaseous conductivity, and radiative transmission. The summation of these three modes gives the total heat conductivity of the material. Fricke [48] determined that for porous superinsulation materials, such as aerogels, both the solid conductivity and the gas conductivity were proportional to the density. Considering the densities of the six aerogels under study, a similar trend could be observed thereby with an increasing in the apparent density, the thermal conductivity augmented as well. This could further be explained by the smaller fraction of the silica particles present as the density decreases. Besides, the gaseous thermal conduction is literally suppressed since even at ambient conditions, the mean free path of the gas molecules is comparable with the pore size, that is, the larger the pore size, the weaker is the gaseous heat conductivity [49]. The surface chemistry of the aerogel powders alternated from being hydrophilic to hydrophobic nature. The rice-husk-derived aerogels, viz. Hamzel, GEA 0.125 and GEA CDZ were found to be able to mix with water whilst the synthetically prepared aerogels were observed to be hydrophobic. The nature of the surface chemical groups of aerogels can be anticipated to be strongly dependent on the conditions used during synthesis. For example, if liquid alcohol is used during the drying process, the surface will consist mainly of alkoxy (-OR) groups, which makes it hydrophobic. However, if carbon dioxide is used, the surface will primarily be covered with hydroxyl (-OH) groups [7]. It is known from the manufacturers that Hamzel and GEA aerogels are produced under CO2 supercritical drying conditions and ambient drying, respectively, hence accounting for their hydrophilic nature. Enova and AeroVa aerogels are thought to be prepared using carbon dioxide as well, but followed with an additional step, called methylation, which converts the hydroxyl groups (-OH) to nonpolar (-OR) groups.

10.3.1.2 Suitability of aerogels for plasma spraying As pointed out in the previous chapter, there is no specific technical or standard method to be used to evaluate the suitability of a feedstock for thermal spraying. However, two properties that have significant impact on the coating process and quality of the coating are granule size range and flowability of the feedstock [28]. Flowability

Aerogel-based thermally sprayed coatings for aero-propulsion systems

209

Table 10.4 Ranking of silica aerogel powders for plasma spraying Product Spray process

Hamzel

GEA 0.125

GEA CDZ

EnovaIC3100

EnovaIC3110

AeroVaAerogel

APS

No

Yes

No

No

No

No

SPS

No

Yes

No

Yes

No

Yes

APS, Atmospheric plasma spraying; No, Considered to be unsuitable for APS/SPS; SPS, Suspension plasma spraying; Yes, Considered to be suitable for APS/SPS.

applies only for conventional plasma spraying, in other words, APS, and can be calculated using the Hausner ratio, which equals the fraction of tapped density over apparent density. A Hausner ratio 1200
C) and that is why SPS cannot melt the particles, but previous studies have shown that SPS using a 40 kW gun power have been able to melt YSZ having melting point of 2800
C [32]. However, the YSZ granules were smaller than 1 mm. This indicates that the melting point is not of prime importance when dealing with plasma spraying, but rather the granule size as pointed out earlier. A study further attests to this fact that the melting point depends on the characteristics of the plasma stream, which is dictated by the spraying conditions set by the gun [31].

10.4

Conclusions

Our concluding remarks are as follows: 1. The physical properties of six types of marketable silica aerogel powders comprising the ricehusk ash-derived aerogels, namely, Hamzel, GEA 0.125 and GEA CDZ, and TEOSproduced Enova Aerogels IC3100, IC3110, and AeroVa were successfully evaluated. The ranking process that depicts which aerogel powders are suitable for APS and SPS was based on the granulometric and morphological properties, in principle. The findings showed that

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only GEA 0.125, Enova IC3100, and AeroVa could be considered eligible for carrying out plasma spraying. The Spanish-made aerogel GEA 0.125 with a granule size range of 2e300 mm but irregular shapes was thought to have a negligible chance of success for APS but could have a greater chance than its concurrent Malaysian-made aerogel, Hamzel, as the latter had a much larger particle size range of 10e500 mm. In addition, the morphological characteristics of GEA 0.125 constituting a specific surface area of 859 m2/g, primary particle diameter of 2e6 nm, and mean pore size of 20 nm were superior than Hamzel. The granules of Hamzel were hygroscopic in nature, resulting in high degree of agglomeration and cohesiveness that would definitely cause clotting when being thermally sprayed. APS of GEA 0.125 was thought to have a better characteristics after being reconstituted optimally into spherical particles via the intermediate step of spray-drying. SPS of the hydrophilic GEA 0.125 was considered to be possible in the sense that it had a median particle size of 81 mm. However, the hydrophobic US-made Enova IC3100 with a narrower particle size range and smaller median particle size of 9e40 and 9 mm, respectively, with better nanocharacteristics than GEA 0.125, was assumed to have a greater chance for SPS. AeroVa, which had almost comparable properties as Enova IC3100, was assumed to be suitable for SPS only but could not be investigated experimentally because of lack of availability. The particles of GEA CDZ (20e700 mm) and Enova Aerogels IC3110 (100e700 mm) were relatively too big to be considered for any thermal spray applications. 2. Out of four choices, viz. APS of as-received GEA 0.125 aerogel, APS of reconstituted spraydried GEA 0.125 aerogel-based powder, SPS of as-received GEA 0.125 aerogel, and SPS of Enova IC3100 aerogel, this study demonstrated that the first option, that is, APS of asreceived GEA 0.125 aerogel, is feasible even though its granulometric properties and morphology were not in accordance with the requisites as an air plasma spraying feedstock. An adhering aerogel-based coating with a bimodal microstructure was achieved when the spraying parameters were optimized via a nozzle diameter of 4 mm powered at 30 kW with a plasma gas rate of 45 þ 5 slpm and carrier gas flow rate of 8.1 slpm. The singlelayered aerogel-based coating had a thickness ranging within 77.9 to 132.0 mm. It can therefore be argued that it is not essential to tailor aerogel particles via spray-drying for deposition via APS. SPS of GEA 0.125 did not take place due to agglomeration of the particles prior to spray. The last alternative of spraying Enova IC3100 aerogel particles using SPS has been indicated as encouraging and possible, yet could not be achieved, possibly because the Enova IC3100 granules were too big to be melted by the plasma stream using the available 40 kW plasma gun. Another interpretation would be that a gun with a power higher than 40 kW is needed with appropriate spraying conditions.

In a nutshell, it can be concluded that a single-layered aerogel-based thermally sprayed coating can be achieved but only with selected commercialized aerogel powders. The feedstock can be pure aerogel particles without any further treatment such as spray-drying. The developed micro-thick single-layered coating using as-received GEA 0.125 aerogel subjected to APS has a bimodal microstructure. The singlelayered aerogel-based coating has therefore the possibility of exhibiting the fascinating physical properties of aerogel such as low heat conductivity. Last but not least, while the success of this work in realizing a bimodal microstructure aerogel-based coating via APS has already opened a new arena of promises for aerogel as a high-performance thermal insulation system for aero-propulsion applications, the realization of SPSdeposited aerogel coating can bring forward even more captivating outcomes.

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Future studies may consider this work to be a benchmark while developing an aerogel-based thermally sprayed coating as it filters the appropriate aerogel powders suitable for plasma spraying as well as it gives a generic pathway to achieve a single-layered adhering coating. Through this feasibility study, it can be recommended that aerogel particles can be air plasma sprayed directly without any modifications in their granulometric properties and morphology using spray-drying. Physical properties such as thermal conductivity and thermal diffusivity of the developed single-layered aerogel-based coating shall be evaluated. As far as suspension plasma spraying of Enova IC3100 aerogel is concerned, a more powerful gun, above 40 kW, shall be employed to melt the particles and thus further exploration can be carried out. Applying an aerogel-based thermally sprayed coating onto polymer matrix composites in aero-engines for thermal and fire protection can be now experimented by employing GEA 0.125 aerogel coupled by the sprayable conditions of APS deduced. The level of fire protection by aerogel onto these substrates can be evaluated using the ISO 2685 standard as described experimentally by Abu Talib and his team [57].

Acknowledgments This study was carried out in collaboration with one of the leading scholars in thermal spray, Professor Lech Pawlowski, from the Laboratory of Science of Ceramic Processing and Surface Treatments at the University of Limoges, France. His tremendous effort and profound investigation made this research feasible. Dr. Stefan Kozerski, another team member at the same laboratory, contributed remarkably in the development of the aerogel-based coating. The authors would also like to thank the government of Malaysia for having offered an FRGS grant (No. 5524611) to investigate the physical properties of aerogel, as part of this research.

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A. Hamdan 1,2 , M.T.H. Sultan 1, 2,3 , F. Mustapha 1 1 Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; 2Aerospace Manufacturing Research Centre (AMRC), Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; 3Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), University Putra Malaysia, Serdang, Malaysia

11.1

Structural health monitoring application

Structural health monitoring (SHM) can be divided into two aspects: passive and active responses. Condition monitoring is one of the passive response techniques in SHM. The periodical response received will be compared with the normal (default) data to determine if there are any changes in the signal, which take effect from the structural integrity [1e6]. Meanwhile, the active response will monitor the signal and feedback to the system if the signal changes from the natural signal. There are several methods available for detecting damage or failure in a structure via SHM: acoustic emission, ultrasonic, fiber optic, laser Doppler vibrometer, and thermal imaging [7]. Additionally, a new smart material (piezoelectric material) offers a better technology, which not only detects damage but acts as an actuator and electricity harvester as well [8,9]. In this system, the sensor will react if several situations occur, such as a curvature of the structure, a strain state in the sensors, damage in the structure, and a failure mode of the structure called buckling [10]. The active composite fiber sensor acts as well as SHM as it offers the following advantages: low weight, low cost compared to other devices, and easy installation [11]. SHM aspires as the new tool to assess structural performance and identify damage at an early stage via examinations and structural response of the sensors [12,13]. In approaching the fourth industrial revolution era, the new materials’ innovations are its smart criteria, such as capability of self-sensing and monitoring, self-diagnosis and prognosis, and adaptive response to increase structural reliability and safety, minimize maintenance costs, and extend service life [14]. In civil engineering, research on SHM composite bridges employs fiber Bragg gratings (FBGs) as the sensors [15,16]. FBG sensors can be remotely monitored, hence, offering better safety to the SHM system and the workers [12,17]. This activity is vital Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00011-3 Copyright © 2019 Elsevier Ltd. All rights reserved.

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to assess the structural integrity, maintenance scheduling, and design codes validation [18,19]. Research on SHM, especially in the aerospace industry, has been undergoing intensive exploration. There are also other fields that apply SHM, such as civil engineering, architecture, mechanical systems, and marine. FBG is a sensor being applied in the aerospace sector. One of the advantages of FBG is that it can be embedded into a composite structure. It can give real-time health performance on the structure, especially for aircraft [20,21]. The information on damage severity and structure survivability can be assessed easily. As well, a few items in the aircraft structure can also be easily monitored such as dynamic strain [22], impact damage [23], in-flight wing loads [24], flap deformations and cracking detection [25], composite landing gear [20], as well as pressure value in the hydraulics system [26]. Fig. 11.1 shows the key parts of an aircraft that need serious attention. The application of SHM in wind turbine systems is raising serious attention nowadays [27e29]. By using SHM, designers are able to reduce rotor weight and drive trains by replacing conservative design assumptions with automatic state awareness and control measures. Furthermore, manufacturers could use the health information about loads and damage correlated with these loads to improve wind turbine designs, manufacturing and quality control processes, and shipping and installation methods [7]. The SHM system occurs in real time or in an online situation. Hence, it also has advantages over damage detection, damage localization, damage assessment, and life prediction [30] compared to the nondestructive testing (NDT) that is conducted offline [31]. A comparison of NDT features is tabulated in Table 11.1. The selection of NDT and SHM system depends on efficiency, cost, and safety factors of the usage [32]. In the ultrasonic guided wave approach, the lower excitation frequencies show a more consistent performance across all sensor paths of the 9-m CX-100 wind turbine rotor blade in a controlled fatigue-test environment [31,35]. In addition, a Lamb wave is also able to determine the undamaged, damaged, and repaired characteristics of carbon fiberereinforced plastic (CFRP) laminate [36].

Vertical stabiliser Horizontal stabiliser Trailing edge

Elevator Fuselage

Flaps Aileron

Leading edge

Engine

Figure 11.1 Key parts of a typical aircraft to be monitored [17].

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Table 11.1 Nondestructive test (NDT) comparison NDT types

Advantages

Shearography Testing. Laser optical method [33,34]

• Stress concentration failure can easily visualize. • Easy to operate the machine. No need for skilled technician to handle the machine.

Acousto-Ultrasonic [33]

• The combination method of acoustic and ultrasonic testing. • Verify the severity of internal imperfections and inhomogeneity in a composite.

Acoustic Emission [32,33]

• The sensor has high sensitivity and the imperfection analysis can be inspected effectively. • It can differentiate between developing and stagnant defects. • No need to disassemble and clean a specimen.

Electromagnetic Testing [33]

Apply magnetism and electricity to detect and evaluate fractures, faults, corrosion or other conditions of materials.

Radiographic Testing [33]

Visualize the delamination of composite in 3D image. The delamination orientation must not perpendicular to the X-ray beam.

Thermography testing [33]

It can inspect a large surface; suitable for thin parts.

Ultrasonic Testing [33]

It can scan and inspect the defect very fast, present good resolution image, and capable of detecting flaws.

CFRP is replacing aluminium alloys as the primary and secondary structures for newly developed aircraft such as Boeing 787 and Airbus A350. Research reported that the intervals of interest were selected using Morlet wavelet analysis to evaluate the condition structural index and the amplitude based assessment for each condition. Next, the results were characterized using principal component analysis (PCA) to distinguish the characteristics: undamaged, damaged, and repaired [36]. On the other hand, Mustapha et al. [37] employed outlier analysis (OA) as a statistical approach to detect damage. The experiments were performed on a hollow cylinder-like structure, under both damaged and undamaged conditions. The scattering of an ultrasonic-guided wave was investigated and characterized using PCA. It was found to be a reliable, practical, and simple approach [37]. FBG is one of the most practical sensors employed in the aerospace industry [4e6]. Rytter [38] suggested a hierarchical process in SHM, starting with damage detection, location detection, damage assessment, and life prediction. OA [37,39e41] and Morlet wavelet analysis [36] were grouped and employed at the damage detection level. Besides multilayer perceptron, a neural network was employed in damage location detection [42e44]. Yang et al. [45] reported that the inner product vector can locate the delamination of a composite beam correctly. In addition, acoustic

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wave propagation, continuous sensors, and wavelet analysis were employed to optimize the actuator and sensor locations [46]. Todoroki et al. [47] reported on damage assessment and life prediction that employed an electric resistance change method. The results reported that delamination size identifications with response surfaces were successfully performed for the cross-ply and quasi-isotropic laminates. Hamdan et al. [3] reported that sensor location from the natural axis may influence the signal quality of the SHM system. Table 11.2 summarizes the three sensors in SHM for various applications. A comparison of several techniques for SHM has been conducted by several researchers. Lamb wave propagation, frequency response functions, and time seriesebased method were utilized to analyze the wind turbine blades [46]. Research showed that the time series analysis was the most effective and reliable method to detect damage, while Lamb wave testing could locate and detect the onset of damage. On the other hand, Ghoshal et al. [48] proposed that the resonant comparison algorithm is much more suitable for application in real-time monitoring in turbine blades as compared to the transmittance function, operational deflection shape, and wave propagation methods. This experiment employed a piezoceramic patch and a scanning laser Doppler vibrometer to translate the signal into damage detection via certain algorithms. From the previous discussion, the applications of SHM were penetrated strategically in various engineering sectors. It has already gained serious attention from all researchers around the world, especially with the coming of the fourth industrial revolution. However, there is still a need to explore the applications of SHM in wind turbine, especially at the damage detection level. Table 11.2 The advantages of SHM sensors in certain applications Structural health monitoring

Advantages

Applications

Guided ultrasonic waves [4]

Generate small amplitude attenuation over long distances

Function as detection and localization of representative damage in aircraft structures

Fiber Bragg Gratings [4e6]

Immune to electromagnetic interference, and have good resistance to chemicals and the environment. Can be located on the surface of the structure or embedded inside the structure

Function as detection and localization of representative damage in aircraft structures

Piezoceramic transducer [49,50]

• Function as sensors and actuators • Suitable to integrate in the structure • Work with several types of data and can isolate various propagation modes of Lamb waves

Generating and monitoring Lamb waves

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11.2

231

Application of biocomposites, fiber-reinforced composites, and hybrid composite

A composite utilizes fiber as a reinforcement for resin matrix. The fiber will provide a greater strength, while the resin acts as a binder for the fiber [51]. Anisotropic composite has special material properties compared to the isotropic material. The understanding of natural fiber, hybrid, and synthetic composites can give a good understanding of how SHM research was done on composite materials. Research conducted on biocomposites revealed several types of natural fiber, including flax [52,53], bamboo [54], pineapple [55], jute [56,57], and kenaf [57e59] (used as a reinforcement for biodegradable plastics). The studies examined molding conditions and interfacial bonding [59], and showed positive results. Studies on the mechanical properties of several natural fibers, such as tensile strengths, flexural strengths, and charpy impact, also showed comparable results to the properties of glass fibers [57]. The potential of natural fiber to penetrate into the global market is promising and has increased year by year. In 2010, the global market for natural fiber was estimated at USD 2.1 billion and projected to increase 10% annually until 2016 [60] for a wide range of industries including aerospace, construction, civil, and sports and leisure [51,61]. The potential in the automotive industry is convincing [62]. Some car parts can be replaced with natural fibers such as the bumper beams and dashboards. Research on the mechanical properties of kenaf has been progressing extensively. Table 11.3 reports on the tensile and flexural results of kenaf fiber from previous findings. Several research studies on kenaf polymer have already been conducted such as analysis studies on the effect of chemical treatment [63e65], the fiber type [58,66], weave orientation [67,68], and resin application [57,59,69]. Besides that, the effect of warp and weft density (Fig. 11.2) can be further analyzed to understand the mechanical properties of the structures. Yarn size and woven orientation influence the performance of stacked layer orientation in woven kenaf composites [70]. Natural fiber can provide better performance in terms of lightweight, great insulation, and fireproof properties. In addition, natural fiber has abundant availability and good processing characteristics such as biodegradable, corrosion resistant, low cost, and nonabrasive [71]. As an example, kenaf core block is being used in house construction [72]. The main enemies for natural fiber are moisture absorption [61] and temperature [32]. They can deteriorate the strength of the natural fiber structure if it is exposed to moisture and high temperature for a long time. The determination of the mechanical properties of structures is very important especially for SHM system integration. It will show the manufacturability, performance, and longevity of the structures [32]. The damage failure in the composite is mainly due to several factors, including matrix cracking, fiber fracture, fiber debonding, and fiber pullout [32]. Therefore, researchers can focus on obtaining better mechanical properties, including optimizing the interfacial bond between the fiber and resin by means of fiber treatment. A hybrid composite is a combination of synthetic and natural fibers or more than two different materials in the fiber reinforcement of a composite. Evaluation of

232

Table 11.3 Mechanical properties of kenaf composite from previous research [70] Tensile strength (MPa)

Tensile modulus (GPa)

Sample

Reinforce form

Resin

Woven kenaf (Untreated NaOH) [63]

Woven (552 g/m2)

Epoxy

51.28

2.74

24

1.1

Woven kenaf (Treated NaOH) [63]

Woven (552 g/m2)

Epoxy

21.76

0.72

27

2.9

Woven kenaf (Yahaya et al., 2014) [65]

Woven (552 g/m2)

Epoxy

6.64

3.27

16.46

0.5

Kenaf/polyester [73]

Short fiber (mold compress process)

Polyester

e

e

26.5

1.3

Treated kenaf [74]

Unidirectional 10 vol%

Epoxy

e

e

58

6.8

Treated kenaf [74]

Unidirectional 30 vol%

Epoxy

e

e

12.4

14.4

Treated kenaf [74]

Unidirectional 40 vol%

Epoxy

e

e

16.4

18.15

Warp woven kenaf [68]

Woven (617 tex)

Epoxy

47.8

e

e

e

Weft woven kenaf [68]

Woven (617 tex)

Epoxy

11.1

e

e

e

Kenaf/polypropylene [57]

Random mat 40 vol%

Polypropylene

27

2

e

e

Kenaf/corn starch [69]

Randomly distributed short (10 mm) 60 vol% fiber

Corn starch

e

4.8

e

e

Untreated kenaf [64]

Unidirectional

Epoxy

235.13

5.57

e

e

Treated kenaf [64]

Unidirectional

Epoxy

301.64

6.74

e

e

Woven kenaf [70]

Woven

Epoxy

127.47

7.5

94.3

8.1

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Flexural modulus (GPa)

Maximum stress (MPa)

Structural health monitoring of biocomposites, fibre-reinforced composites, and hybrid

233

Warp direction

Figure 11.2 Completed woven kenaf [70].

hybridization effect on the mechanical performance of short banana/sisal hybrid fiberreinforced polyester composites found that the tensile properties of the natural fiber were improved with the addition of banana fibers [75]. The hybridization of kenaf and pineapple leaf fiber reinforcement has improved the tensile and flexural strength compared to the original composite [76]. One study reported that the hybrid kenaf/fiber glass showed potential as an alternative to the existing glass mat thermoplastic products [77]. Previously, most of the studies performed on the woven form of textile composites were on synthetic, rather than the natural, fiber [68]. Several studies on material effectiveness have been carried out involving materials such as piezoelectric ceramics with a microstructure texture containing a template of SrTiO3 [78], multilayer piezocomposite composed of layers of carbon/epoxy, Lead zirconate titanate (PZT) ceramic and glass/epoxy [79] (Fig. 11.3), piezoelectric polymer polyvinylidene fluoride [80], ionically conductive ionic polymer transducer [80], and aluminium nitride as a piezoelectric material [79,81]. The studies reported that the PZT performance depended on the distance of the PZT layer from the neutral axis of the structure as shown in Fig. 11.3. The situation was related to the bending theory.

11.3

Issues of SHM

In the fourth industrial revolution, engineers and designers move forward to innovate a smart environment where machines are digitized, can learn, and make a decision. In civil engineering, the application of carbon nanotubes can give a different perspective to the sensors. However, very little work has been done on the development of cement-based sensors using carbon nanotubes [12]. A new goal for the new sensor is to create a sensor capable of interacting with the environmental factors such as temperature, chloride, and moisture. Besides that, it can respond to different stress conditions (axial, bending, and shear) [12].

234

(b)

(c) Glass/epoxy (39×14×0.09 mm3)

Glass/epoxy (39×14×0.09 mm3)

Glass/epoxy (39×14×0.09 mm3) Carbon/epoxy (37×12×0.1 mm3) PZT ceramic (37×12×0.25 mm3) Moment arm: 1.064m×10–4 Bending stiffness: 0.893 N-m2×10–2 0.62 mm Glass/epoxy layer 0.53 mm Glass/epoxy layer 0.44 mm Carbon/epoxy layer Neutral axis 0.34 mm Piezoceramic layer 0.321 mm 0.09 mm Glass/epoxy layer Bottom 0.0 mm

Carbon/epoxy (37×12×0.1 mm3) PZT ceramic (37×12×0.25 mm3) Moment arm: 1.470m×10–4 Bending stiffness: 1.320 N-m2×10−2 0.62 mm Glass/epoxy layer 0.53 mm Carbon/epoxy layer 0.43 mm Neutral axis Glass/epoxy layer 0.364 mm 0.34 mm Piezoceramic layer 0.09 mm Glass/epoxy layer Bottom 0.0 mm

Carbon/epoxy (37×12×0.1 mm3) PZT ceramic (37×12×0.25 mm3) Moment arm: 1.875m×10−4 Bending stiffness: 1.967 N-m2×10−2 0.62 mm Carbon/epoxy layer 0.52 mm Glass/epoxy layer Neutral axis 0.43 mm Glass/epoxy layer 0.403 mm 0.34 mm Piezoceramic layer 0.09 mm Glass/epoxy layer Bottom 0.0 mm

Figure 11.3 Geometry of multilayer piezocomposite [79]. Investigation on three different layup configuration affecting the energy harvester performance of piezocomposite generating element (PCGE)(PCGE-a, PCGE-b, and PCGE-c).

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

(a)

Structural health monitoring of biocomposites, fibre-reinforced composites, and hybrid

235

In terms of mechanical properties of composite materials, damage in the structure will directly increase the resistance in SHM sensing system [82]. Therefore, the damage can be easily detected. Studies on natural fiber usually employ random orientation and compressed mat [68]. It is also noted that less attention has been devoted to evaluate the mechanical properties of woven kenaf thermoset composite at various orientations. Therefore, the research on kenaf at various orientations may help to provide good information and data on the signal pattern and operational signal of SHM in biocomposite fiber.

11.3.1 New trends of SHM as an energy harvester The application of SHM sensor as an enhancement in energy harvesting could become a new challenge. The piezoelectric material can act as a micro energy harvester or wind vibration energy harvester [9]. The technology is focused on as an alternative to the conventional battery, or proposed as a lifelong battery for low-power subsystems or devices [9,83]. This concept was applied in the unmanned aerial vehicle technology [84] and showed promising results. It will be based on mechanical vibration, structure dimension [85], mechanical stress and strain, thermal energy from furnaces, heaters and friction sources, unsteady airflow phenomena [86], sunlight or room light, the human body, and chemical or biological sources that can generate mW or mW level power [83,87e93]. Furthermore, a piezoelectric cantilever was proposed as a micromachined Si proof mass in a low-frequency vibration application [94]. A few factors may influence and improve the performance of energy harvesters such as optimizing power conditioning circuitry [8] by using different beam shapes [95] and multilayer structures [96]. Preliminary research was conducted on a complete autonomous sensing unit that incorporated SHM and power harvesting technologies into a single, self-powered device [8]. Anton et al. [93] reported that a multisource energy harvester was capable of powering a wireless impedance device sensor node through solar and vibration energy harvesting. The experiment was conducted on a full-scale wind turbine during bright, cloudy, mild wind, and high wind conditions. It sufficiently charged its input capacitor (0.1 F) to 3.6 V in an acceptable time period. In addition, macro-fiber composite (MFC) offers better performance in terms of flexibility and suitable for bonding on large and vibrating structures [97]. It also has high electromechanical coupling coefficient [98e100]. Fig. 11.4 shows the woven kenaf composites turbine blade: (a) embedded MFC location and (b) bonded MFC located at the same place as the embedded MFC. In general, studies on the self-energized wind turbine need to be undertaken and experimentally validated, especially in terms of user-friendliness. Several aspects, such as the sensor bonding technique, natural frequency of the structure, and material of the turbine blades need to be highlighted. The amount of harvested energy obtained is influenced by several factors, such as the piezoelectric material, proof mass of the beam or structures, material properties of the beam or structures, gap of the interdigitated electrodes, overlapping effect of the resonance frequencies, beam length [98], bonded technique for MFC [3], operating mode of the piezoelectric conversion [101], lateral beam or structures deflection, output voltage across the resistor, and dissipated power [102].

236

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Figure 11.4 Woven kenaf composites turbine blade (a) embedded MFC location, (b) bonded MFC located at the same place as the embedded MFC [3].

11.4

Conclusion

SHM is gaining some serious attention from structural engineers in the respective sectors. The application increasingly is bringing positive impacts to industries in terms of cost reduction, less maintenance time, and better life prediction. The development of smart SHM integration system will bring into the industries a new chapter of adapting to the fourth industrial revolution. The material science of composites for SHM has been widely explored by previous researchers. SHM performance and capability to integrate with composite material were explored. A few factors were involved such as structural integrity of the composite, material properties, sensor performance, and fiber quality. In general, SHM performs well in composite material structural integrity systems. Natural fiber composite is expected to become the new approach due to its advantages. Several SHM issues and its potential as an energy harvester were highlighted as well. Further investigation and product enhancement should be conducted on SHM to provide a sustainable energy system and produce smart health monitoring systems.

Acknowledgments The authors gratefully acknowledge the Aerospace Manufacturing Research Centre for providing the facilities to complete this research. This work was supported by a grant (GP-IPB 9490602) from Universiti Putra Malaysia.

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[73] Roslan MN, Ismail AE, Hashim MY, Zainulabidin MH, Khalid SNA. Modelling analysis on mechanical damage of kenaf reinforced composite plates under oblique impact loadings. Appl Mech Mater 2014;465e466:1324e8. [74] Reza M, Jamaludin MY, Abdul Rahman MS, Mehdi R. Characteristics of continuous unidirectional kenaf fiber reinforced epoxy composites. Mater Des 2014;64:640e9. [75] Idicula M, Joseph K, Thomas S. Mechanical performance of short banana/sisal hybrid fiber reinforced polyester composites. J Reinf Plast Compos 2010;29:12e29. https:// doi.org/10.1177/0731684408095033. [76] Aji IS, Zainudin ES, Abdan K, Sapuan SM. Mechanical properties and water absorption behavior of hybridized kenaf/pineapple leaf fibre-reinforced high-density polyethylene composite. J Compos Mater 2012. https://doi.org/10.1177/0021998312444147. [77] Davoodi MM, Sapuan SM, Ahmad D, Ali A, Khalina A, Jonoobi M. Mechanical properties of hybrid kenaf/glass reinforced epoxy composite for passenger car bumper beam. Mater Des 2010. https://doi.org/10.1016/j.matdes.2010.05.021. [78] Jeong SJ, Lee DS, Kim MS, Im DH, Kim IS, Cho KH. Properties of piezoelectric ceramic with textured structure for energy harvesting. Ceram Int 2011:5e14. [79] Tien CMT, Goo NS. Use of a piezocomposite generating element in energy harvesting. J Intell Mater Syst Struct 2010;21:1427e36. [80] Farinholt KM, Pedrazas NA, Schluneker DM, Burt DW, Farrar CR. An energy harvesting comparison of piezoelectric and ionically conductive polymers. J Intell Mater Syst Struct 2009;20:633e42. [81] Elfrink R, Kamel TM, Goedbloed M, Matova S, Hohlfeld D, Andel YV, et al. Vibration energy harvesting with aluminum nitride-based piezoelectric devices. J Micromech MicroEng 2009;19. Paper No. 094005. [82] Swait TJ, Jones FR, Hayes SA. A practical structural health monitoring system for carbon fibre reinforced composite based on electrical resistance. Compos Sci Technol 2012;72: 1515e23. [83] Sodano HA, Park GH, Leo DJ, Inman DJ. Electric power harvesting using piezoelectric materials. Center for Intelligent Material Systems and Structures Virginia Polytechnic Institute and State University 2003. [84] Anton SR, Inman DJ. Vibration energy harvesting for unmanned aerial vehicles, Proc. SPIE 6928. Active and Passive Smart Structures and Integrated Systems, 2008 18 April 2008;692824. https://doi.org/10.1117/12.774990. [85] Akaydın HD, Elvin N, Andreopoulos Y. Wake of a cylinder: a paradigm for energy harvesting with piezoelectric materials. Exp Fluids 2010;49:291e304. [86] Liu J, Fang H, Xu Z, Mao X, Shen X, Chen D, et al. A MEMS-based piezoelectric power generator array for vibration energy harvesting. Microelectron J 2008;39: 802e6. [87] Heung SK, Joo-Hyong K, Jaehwan K. A review of piezoelectric energy harvesting based on vibration. Int J Precis Eng Manuf 2011;12:1129e41. [88] Flynn AM, Sanders SR. Fundamental limits on energy transfer and circuit considerations for piezoelectric transformers. IEEE Trans Power Electron 2002;17:8e14. [89] Erturk A, Inman DJ. An experimentally validated bimorph cantilever model for piezoelectric energy harvesting from base excitations. Smart Mater Struct 2009;18. [90] Sodano HA, Inman DJ, Park GH. Comparison of piezoelectric energy harvesting devices for recharging batteries. J Intell Mater Syst Struct 2005;16:799e807. [91] Sodano HA, Inman DJ, Park GH. A review of power harvesting from vibration using piezoelectricmaterials. Shock Vib Dig 2004;36:197e205.

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Fracture surface morphologies in understanding of composite structural behavior

12

A. Deepa, Kandasamy Jayakrishna, G. Rajiyalakshmi School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India

12.1

Introduction

A. Mukherjee has described the auxiliary scale tests on the synergistic impacts of dampness, temperature, alkalinity, and anxiety on the execution and sturdiness of glass fiber-reinforced polymer (GFRP) sheet fortified remotely on concrete [1e9]. This section portrays the miniaturized scale basic examinations to discover the nature, quantum, and instrument of crumbling in the molded sheets. Micrographic examinations were completed utilizing a filtering electron magnifying lens (scanning electron microscope; SEM) to image the adjustments in the microstructure. Alternate tests are vitality dispersive X-beam examination (EDX) and inductively coupled plasma mass spectrometry (ICP-MS) to decide the substance changes in the composite. In another study [10] an arrangement of quickened maturing and regular habitat tests was completed to assess execution of glass strands strengthened polymer (GFRP) sheets fortified on concrete in tropical conditions. Plain solid shafts were thrown and remotely strengthened by holding with E-glass GFRP sheets. The pillars were inundated in a 60
C water shower for shifting lengths. The curiosity of the analysis was that the natural presentation was given while they were subjected to benefit loads. This heap helped in leaking sheet presented to heated water under focused-on condition. In this way field condition fundamentally the same as tropical atmosphere was mimicked. The stacked examples were additionally subjected to common weathering for 6 months and a year’s time. The sheets were expelled from the examples and the rigidity and modulus were resolved to survey the debasement, assuming any. In the initial part of this chapter, the basic level examinations are examined. An arrangement of quickened maturing and regular habitat tests has been completed to assess execution of GFRP strengthening bar in a tropical domain [11e13]. Shafts were thrown with the GFRP fortifying bars as interior fortification. They were submerged in a 60
C water shower for changing lengths. The curiosity of the examination was that the natural introduction was given to the pillars while they were subjected to benefit loads. These heaps kept the air out for strengthening bars to stay presented to high temp water. Subsequently, a field domain fundamentally the same as a tropical atmosphere was made. The stacked examples were additionally subjected to normal weathering for 18 and 30 months’ time. The strengthening bars were expelled from Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00012-5 Copyright © 2019 Elsevier Ltd. All rights reserved.

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the examples and researched at both auxiliary and miniaturized basic scale to survey the debasement, assuming any. In another investigation, the basic scale tests on the synergistic impacts of dampness, temperature, alkalinity, and feeling of anxiety on the execution and strength of GFRP strengthening bars in concrete have been discussed. In this part, examinations on smaller scale auxiliary investigations, completed to discover the nature, quantum, and component of decay in the molded strengthening bars, are accounted for [14,15]. Microstructural examinations were completed utilizing a checking electron magnifying instrument (SEM) to imagine the adjustments in the microstructure. Alternate tests that have been completed are vitality dispersive EDX and inductively coupled plasma mass spectrometry to decide the compound changes in the composite. Pavankiran Vaddadi et al. studied transient hygrothermal stresses [16] instigated in fiber-strengthened composites in detail by using a novel heterogeneous portrayal approach. This approach joins two particular highlights: transient dampness retention investigation of genuine composite materials presented to a sticky domain, as well as point-by-point computational examinations that catch the genuine heterogeneous microstructure of the composite. The last component is done by displaying a uniaxial overlay having more than 1000 individual carbon filaments that are arbitrarily dispersed inside an epoxy grid. Results show that these computational models are fundamental in catching the precise dampness retention procedure of the real example. In the examination, the developments of warm leftover anxieties and dampness-incited stresses inside the dampness and warm uncovered composites have been investigated. It was seen that extensive anxiety fixation is created in the epoxy stage where high fiber thickness or fiber bunching exists and this increases as the dampness content immerses. Expansive anxieties can possibly start epoxy harm or delamination of epoxy and strands. Besides, because of restricting impacts of warm and dampness introduction, bring down burdens are found in the cover when both are thought about at the same time. Another approach [17] has been produced, in light of a reverse examination procedure, to decide basic dampness dissemination parameters for a fiber-fortified composite. This method consolidates two particular highlights: coordinate test perceptions of the weight picked up by a composite material presented to a sticky condition, and profoundly essential computational investigations that catch the genuine heterogeneous microstructure of the composite. The last element was completed by demonstrating more than 1000 singular carbon filaments that are arbitrarily circulated inside an epoxy grid. The viability of this system was constructed by leading an examination on a high-review IM7/997 carbon fiber-fortified epoxy to decide the greatest dampness content at immersion and the diffusivity of epoxy. With the converse investigation, the time span required to evaluate these dampness dissemination parameters could be radically diminished when contrasted with traditional strategies. Along these lines, the setup models were utilized to portray transient dampness ingestion process inside the composite. Here [18,19], it was exhibited that displaying the heterogeneous microstructure of the composite is basic for getting exact dispersion parameters, and an expository model with viable properties does not create remedy

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transient dampness retention conduct. Moreover, the development of stress fields because of dampness-initiated volumetric development was evaluated. It was seen that high anxiety fixations are created in areas of fiber focus. These districts at that point go about as potential disappointment start destinations that can prompt lower harm resilience. In another paper [20e22], a coupled model for strain-assisted dissemination comes from the essential standards of continuum mechanics and thermodynamics, and material properties portrayed utilizing dissemination tests. The proposed technique constitutes a huge advance toward displaying the synergistic bond debasement instrument at the fortified interface between a fiber reinforced polymer (FRP) and a substrate, and for anticipating debond start and engendering along the interface within the sight of a diffusing penetrant at the break tip and at higher temperatures. It is presently wellsuited law with much of the time lacking for depicting dampness dissemination in polymers and polymer composites. Non-Fickian or strange dissemination is probably going to happen when a polymer is subjected to outside anxieties and strains, and lifted temperatures and mugginess. In this paper [23,24], a displaying strategy in light of the essential standards of continuum mechanics and thermodynamics is produced, which permits portrayal of the consolidated impacts of temperature, dampness, and strain on dissemination coefficients as well as on dampness immersion level, from dampness weight pickup information. The exhaustion development of a fiber-fortified composite cover was described under warm cycling utilizing a consolidated trial and computational examination [25e27]. The investigation of break, fruition of the exhaustion tests, uncovers a calculated or crimped split front development with more prominent engendering removes close to the free surfaces/edges. Due to the nonuniform break development over the example thickness, three-dimensional limited component examinations are performed to explore the exhaustion development systems under warm load. From the investigation [28], the vitality discharge rate and, additionally, the blended mode push force factors are ascertained and the varieties of these break parameters are observed to be predictable with the watched break front setup. Utilizing the registered outcomes, the tentatively measured split development rates are additionally associated with the large vitality discharge rate, and a power law type of the weakness law is built up. The pertinent coefficients and, additionally, the limit vitality discharge rate are also decided. The present examination is valuable for not just understanding the weakness delamination components under warm cycling but also for evaluating the edge temperature variety that is expected to drive break development. A particular paper [29e31] portrayed a thermodynamics-based model for viscoelastic composites with harm and delineates its utilization in portrayal of viscoelastic reaction of polymer grid woven texture composites subjected to stacking at high temperatures. The portrayal is directed by a test technique supported by limited component (FE) display. The trial portrayal depends on crawl information acquired under steady heaps of various extents and at various temperatures, and on recuperation information gathered subsequent to emptying. A carbon fiber/polyamide gum woven composite with glass helps to progress the temperatures [32,33].

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12.2 12.2.1

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Microhardness CFRP on hydrothermal treatment

Microhardness tests of specimens were taken in order to estimate the hardness of specimens before and after thermal treatment of the specimens (Fig. 12.1). The load applied was 200 g and VHN values were determined by applying this load by using a calibration distance of 50 units in quantinet software used for image analyzing. The dwell time used during load application was 20 s. An indent is formed in diamond shape, used for calculating VHN2 GFRP on chemical treatment. The microhardness readings of CFRP-laminated specimen decreased when compared to initial reading before hygrothermal treatment due to hardening of epoxy with absorption of water as shown in Fig. 12.1. The microhardness of CFRP specimen subjected to flexural load decreased with time. The microhardness of CFRP specimen subjected to tensile load decreased with time. The microhardness of epoxy specimen also decreased with time.

12.2.2

GFRP on hydrothermal treatment

Microhardness tests of specimens were taken in order to estimate the hardness of specimens before and after thermal treatment of the specimens (Fig. 12.2). The load applied was 200 g and VHN values were determined by applying this load by using a 17 16 15 14

VHN

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Figure 12.1 Vickers hardness number variation with respect to time.

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16 15 14 GFRP 30% loading GFRP without load GFRP 50% loading GFRP 70% loading

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Figure 12.2 Vickers hardness number variation with respect to time.

calibration distance of 50 units in quantinet software used for image analyzing. The dwell time used during load application was 20 s. An indent is formed in diamond shape, used for calculating VHN2 GFRP on chemical treatment. The microhardness readings of GFRP-laminated specimen decreased when compared to initial reading before hygrothermal treatment due to hardening of epoxy with absorption of water. The microhardness of GFRP specimen is same at the end of exposure duration. The micro hardness of GFRP-laminated specimen decreased when compared to initial reading before hygrothermal treatment due to hardening of epoxy with absorption of water. The hardness graph of GFRP specimen is almost same at the end as saturation is attained. But CFRP specimen has not attained saturation as the absorption of moisture for CFRP specimen is slow as compared to GFRP. On comparison of the hardness parameter to the macroscopic parameters like flexural strength a kind of synergistic relationship is observed. A gradual decrease in hardness is observed with the decrease in the flexural strength. The reduction in flexural strength is considerably large in almost all the specimens immersed in water tank as compared to the initial specimen’s flexural strength. There was reduction in microhardness with decrease in strength of the laminate. With the decrease in strength of GFRP specimen, the microhardness values decreased.

12.2.3 Sandwich composites The Vickers hardness test or the 136 degree diamond pyramid hardness test is a microindentation method. The indenter produces a square indentation, the diagonals of

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4.0 3.5

30% loading 50% loading 70% loading Without load

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30 Time (Days)

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Figure 12.3 Vickers hardness number variations with respect to time.

which are measured. Vickers hardness number (VHN) can be found directly from the Vickers hardness testing machine. In that machine, first an indent was made on the surface of the specimen by applying force of 200 g with the help of indenter for dwell time of 20 s. Average of three readings in each specimen at different places was taken (Fig. 12.3). It is observed that the core thickness does not affect so much in change in microhardness (VHN) with respect to time. It is also observed that drop in microhardness (VHN) in 16-mm core thickness specimen is almost constant after 30 days with some exceptions, whereas drop in microhardness (VHN) in 8-mm core thickness after 30 days is less. Change in microhardness (VHN) in different bending preload specimens is not very high with respect to time. These are almost similar with some variations for both the core thickness specimens. It is observed from the graphs that the microhardness (VHN) is constantly decreasing with time. Up to 20 days the drop in VHN is comparatively more. Reason for the decrease in microhardness (VHN) with time may be the hygrothermal environment. The epoxy immerged in the water at 45
C may get softer due to temperature and moisture. Pores of epoxy will loosen, which gives way to moisture, and then the sandwich structure gets softer day by day and hardness decreases.

12.3 12.3.1

Area of fracture and circularity ratio CFRP on hydrothermal treatment

The 70% UFL CFRP bending specimen percentage area fraction is almost the same after 1 month. In case of CFRP tensile specimen, percentage area fraction is almost the same after 1 month.

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The area fraction of specimen is compared for epoxy and for fibers (Fig. 12.4(a) and (b)). From comparison of epoxy fractions it is clearly seen that it is increasing with time as there is considerable increase after 2 months compared to 1 month. But the trend is seen to be opposite in fiber fraction comparison. Here the fiber fraction is increased in the first month and decreased in the second month. All the specimens loaded at different values show the same trend when compared between 1 and 2 months. The reason for such a trend seems to be that fiber in total area is degrading with heat and moisture attack and epoxy seems to have expanded with the above effect, which obviously leaves more area for epoxy as compared to fiber. The circularity of fibers was decreasing when compared to initial specimen. But the circularity of fiber is almost same for CFRP specimen as shown in Fig. 12.4(b).

12.3.2 GFRP on hydrothermal treatment The GFRP bending specimen percentage area fraction for 50% UFL GFRP specimen was decreasing. The GFRP tensile specimen percentage area fraction for 70% UTL GFRP specimen is decreasing after 1 month and is almost same (Fig. 12.5(a)). The area fraction of specimen is compared for epoxy and for fibers. From comparison of epoxy fractions it is clearly seen that it is increasing with time as there is considerable increase after 2 months compared to 1 month. But the trend is seen to be opposite in fiber fraction comparison. Here the fiber fraction is increasing in the first month and decreased in the second month. The entire specimen loaded at different value shows the same trend when compared between 1 and 2 months. The reason for such a trend seems to be that fiber in total area is degrading with heat and moisture attack and epoxy seems to have expanded with above effect, which obviously leaves more area for epoxy as compared to fiber. The circularity of fibers was decreasing when compared to initial (Fig. 12.5(b)). The area fraction is decreasing with respect

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Figure 12.4 (a) Area of fracture variation with respect to time, (b) circularity variation with respect to time.

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Figure 12.5 (a) Area of fracture variation with respect to time, (b) circularity variation with respect to time.

to time in GFRP specimen subjected to 50% ultimate flexural load and 70% ultimate flexural load. The decrease in percentage area fraction of fiber of GFRP specimen subjected to 70% ultimate tensile load is high compared to other specimen area fraction. The area fractions of CFRP specimen subjected to ultimate flexural load are almost the same up to 1 month.

12.3.3

GFRP on chemical treatment

The area fraction of the fiber and epoxy are analyzed using image analysis (Fig. 12.6(a) and (b)). The results showed an increase in area fraction of the epoxy with increasing time showing a marked increase in the second month in all the samples of both water and aqueous NaOH tank. Similarly, the result of fiber area fraction shows a decreasing trend with time, with marked decrease in the second month. This indicates that epoxy is expanding covering more area with fiber degradation and pullout, leading to decrease in fiber in the same area. The change in circularity ratio is seen more pronounced in the fibers of samples immersed in aqueous NaOH, which can be noticed easily in SEM image of samples. The chemical attack on periphery of fibers by NaOH leads to change in shape. Thus change in circularity ratio is evident in case of aqueous NaOH samples, but some samples immersed in water tank had also shown chipping of an edge completely with rest of periphery in quite a circular shape. From comparison of epoxy fractions it is clearly seen that it is increasing with time as there is considerable increase after 2 months compared to 1 month. But the trend is seen to be opposite in fiber fraction comparison. Here the fiber fraction is increasing in the first month and decreased in the second month. All the samples loaded at different values show the same trend when compared between 1 and 2 months. The samples in NaOH tanks have shown more increase in epoxy area fraction after 2 months compared to 1 month and, similarly, more decrease in fiber fraction when comparing

Fracture surface morphologies in understanding of composite structural behavior

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Figure 12.6 (a) Area of fracture variation with respect to time and environment, (b) circularity variation with respect to time and environment.

to that of water tanks. The reason for such a trend seems to be that fiber in total area is degrading with heat and moisture attack and epoxy seems to have expanded with above effect, which obviously leaves more area for epoxy as compared to fiber. In NaOH tank as seen in SEM images, the fiber has also been eaten up at the periphery, which further supports more loss of fiber, leaving more area for epoxy in the composite cross section. The circularity ratio values seem to have gone downward from a ratio of 1.0 especially in NaOH tanks, with more downward trend with increasing time. The reason for such a trend is eating up of fiber by the NaOH attack, leading to change in shape with most damage on outer circumference of fibers.

12.3.4 Sandwich composites It is clearly seen that area fraction of epoxy is increasing with time as there is considerable increase after 2 months compared to 1 month (Fig. 12.7(a) and (b)). But the trend is seen to be opposite in fiber area fraction. Here the fiber area fraction is decreasing with time. In 8-mm core thickness specimens, drop of fiber area fraction is greater compared to 16-mm core thickness specimens. All specimens subjected to different bending preloads show almost the same trend, with some exceptions, when compared between 1 and 2 month. The reason for such a trend seems to be that fiber in total area is degrading with heat and moisture attack, and epoxy seems to have expanded with the above effect, which leaves more area for epoxy as compared to fiber. The area fraction of the fiber and epoxy were analyzed using image analysis. The results show an increase in area fraction of the epoxy. Increase in area fraction of epoxy was more in second month when compared to first month for all specimens subjected to bending preload. Similarly, the result of fiber area fraction shows a decreasing trend with time with marked decrease in second month as compared to 1 month in all specimens. All specimens loaded at different value show the same trend of decrease in fiber area fraction and increase in epoxy area fraction. This indicates that epoxy was expanding, covering

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

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Figure 12.7 (a) Area of fracture variation with respect to time, (b) circularity variation with respect to time.

more area with fiber degradation and pullout, leading to decrease in fiber in the same area. Fiber area fraction of 8-mm core thickness specimen was slightly higher as compared to 16-mm core thickness specimen. The circularity ratio values seem to have gone downward from a ratio of 1.0, with some exceptions, with time. The reason for such a trend is hygrothermal load, which leads to change in shape with most damage on outer circumference of fibers. The slight change in circularity was observed in the fiber after 2 months, which was easily noticed in SEM image of specimen. There was slight change in circularity of fibers after 1 month in both the core thickness specimens. Change in circularity of fibers was slightly higher in 16-mm core thickness samples after 2 months compared to the 8-mm core thickness samples. There is not much effect of bending preload on circularity of fiber.

12.4 12.4.1

Visual observation and scaling CFRP on hydrothermal treatment

As per the visual observation, the initial specimen had fine shiny epoxy coating. The specimen under bending load has no crack during initial bending. After 1 month of exposure of specimen in water baths, there was heavy visible scale formation on the entire specimen. Surface observation showed that a thin layer of salt is deposited on surface (Fig. 12.8). But still specimen showed considerable strength and flexibility on bending. After 2 months of natural exposure in water bath, degradation observation and scaling was comparatively low as compared to water tank maintained at 45
C (Fig. 12.8). The second month degradation of specimen was more visible as compared to first month observations. For 2 months of hygrothermal exposure in water bath, observations of heavy degradation of fiber and scaling were noted as compared to first month specimen. Clean abrupt failure like cleavage edge was visibly observed as shown. The specimens had lost their flexibility completely and were easily broken by slight bending with hands as compared to 1-month-exposed specimens.

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Figure 12.8 Visual observation of CFRP laminate after 2 months of hygrothermal treatment.

12.4.2 GFRP on hydrothermal treatment As per the visual observation, the initial specimen had fine shiny epoxy coating and the failure of specimen under uniaxial tensile load showed a broom-type failure due to typical sudden and violent release of stored elastic energy. The specimen under bending load showed a crack at the middle of GFRP specimen. Surface observation showed that a thin layer of salt is deposited on the surface (Fig. 12.9). But still specimens showed considerable strength and flexibility on bending. Broom-type failure was visible in GFRP specimen during the tensile test. While in case of bending, hairline cracks were generated at the middle of the specimen. After 2 months of natural exposure in water bath, degradation observation and scaling was comparatively low as compared to water tank maintained at 45
C. The second month degradation of specimen in the tank was more visible as compared to first month observations. For 2 months of hygrothermal exposure in water bath, observations of heavy degradation of fiber and scaling were noted as compared to firstmonth specimens. Clean abrupt failure like cleavage edge was visibly observed. The specimens had lost their flexibility completely and were easily broken by slight bending with hands as compared to 1-month-exposed specimen. Abrupt failure had occurred in specimen during tensile testing. In case of bending, the hairline cracks were clearly visible at the point where load was applied.

12.4.3 GFRP on chemical treatment As per the visual observation, the initial sample had fine shiny epoxy coating and the failure of sample also showed broom-type failure, which is indicative of good load failure. After 1 month there was scale formation on the samples, with water-immersed samples showing heavy scale formation. Surface observation showed that epoxy had lost its shine and outer edges on either side showed little fanning out. But still after bending

Figure 12.9 Visual observation of GFRP laminate after 2 months of hygrothermal treatment.

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Figure 12.10 (a) Visual observation of GFRP after 2 months on water exposure, (b) Visual observation of GFRP after 2 months on chemical exposure.

(by hands), the epoxy showed considerable strength and flexibility. The condition of aqueous NaOH-immersed samples had gone very bad. The samples had lost their flexibility and we could easily break the samples by bending with hands like breaking a chips wafer. The epoxy had gone very brittle and top surface of some samples showed the chipping out of top surface of epoxy coating. After 2 months the same observations were noted, with more scale formation on water-immersed samples and damage, especially on aqueous NaOH-immersed samples (Fig. 12.10(a) and (b)).

12.4.4

Sandwich composites

After 1 month there was some scale formation on the specimen, with water-immersed specimen. Surface observation showed that due to scale formation the epoxy had lost some shine on outer edges on either side. But still after bending (by hands) the epoxy showed considerable strength and flexibility. After 2 months, scale formation throughout the specimen was noted. There was considerable decrease in strength of the composite and its flexibility. As per visual observation of specimen, it was observed that core cracking and delamination occurred in thermocol sheet during the flexural testing. Degradation in thermocol was observed, which is clearly visible in Fig. 12.11(a) and (b); also, core defects like shear cracks and core crushing were observed. These defects affect the strength and durability of sandwich composite structures. Cracking in thermocol and delamination were observed in specimen after 2 month of exposure in the longitudinal view of the specimen. Epoxy cracking was also observed in some portions. Debonding between core (thermocol) and skin (fiber/epoxy sheet) were found and is clearly visible in the above figures. Core cracking with delamination was observed in transverse view of the specimen of 8-mm core thickness (70% loading). Specimen without bending preload did not show any defect in 1 month. Delamination was observed in some portions in all specimens.

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(b)

Heavy scaling on surface

Figure 12.11 (a) Longitudinal view of sandwich composite after 2 months on hygrothermal exposure, (b) Lateral view of sandwich composite after 2 months on hygrothermal exposure.

12.5

Conclusion

This study provides us a comprehensive view of the surface effects of different forms of GFRP composites, namely laminated form, single-layered and sandwich composites. There were different situations and environments that the GFRP were exposed to like the hygrothermal treatment as well as the chemical exposure; each of the cases displayed excessive scaling after an exposure of 2 months. The layers case of the sandwich composites also displayed a lot of disbonded sites. Further, the hardness test has shown to be in agreement with the tensile and flexural test of all the samples. With a decrease in hardness there has been a decrease in the tensile strength as well. The circularity ratios on the area of fraction were also easily deciphered with the help of the graphs and were seen to gradually decrease with the increase in time for the environmental degradation. A lot of improvement may be focused upon in future research by using finite element modeling and allowing a comparison with the experimental models. The study will definitely provide a framework for the implementation of these advanced materials into actual working models.

References www.whitebuffalobeadsandstones.com & www.ia.ucsb.edu. www.wikipedia.com/Composite material.html. www.structsource.com/pdf/composite.pdf. www.emba.uvm.edu/iatridis/me257/Introduction.html. www.engr.sjsu.edu/sgleixner/PRIME/FRP.pdf. www.autospeed.com/composites.html. ww.engineer.tamuk.edu/departments/ieen/faculty/DrLPeel/Courses/Meen3344/ Powerpoint_Files/Chapter_16_avi.ppt. [8] Samborsky DD, Mandell JF. Effects of glass fabric and laminate construction on the fatigue of resin infused blade materials: American Institute of Aeronautics and Astronautics. [9] Mukherjee A, Arwikar SJ, (ACI Structural Journal) Title No. 102-S82. Performance of externally bonded GFRP sheets on concrete in tropical environments Part II: micro structural tests. 2006.

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[10] Chen TL, Bert CW. Design of composite-material plates for maximum uniaxial compressive buckling load: The University of Oklahoma. [11] www.basf-cc.co.in. [12] www.asme.org. Bond DA, Smith PA. Modelling the transport of low-molecular-weight penetrates within polymer matrix composites. [13] Mukherjee A, Arwikar SJ, (ACI Structural Journal) Title No 102-S76. Performance of externally bonded GFRP sheets on concrete in tropical environments Part I: structural scale tests. 2006. [14] Mukherjee A, Arwikar SJ. Performance of glass fibre-reinforced polymer reinforcing bars in tropical environments-Part I: structural scale tests. 2007. [15] Mukherjee A, Arwikar SJ. Performance of glass fibre-reinforced polymer reinforcing bars in tropical environments-Part II: micro structural tests. 2007. [16] Vaddadi P, Singh RP. Transient hygrothermal stresses in fibre reinforced composites a heterogeneous characterization approach. 2003. [17] Vaddadi P, Singh RP. Inverse analysis for transient moisture diffusion through fibrereinforced composites. 2002. [18] Roy S, Liechti KM. Characterization and modelling of strain assisted diffusion in an epoxy adhesive layer. 2005. [19] Vaddadi P, Singh RP. Interlaminar fatigue crack growth of cross-ply composites under thermal cycles. 2007. [20] Ahci E, Talreja R. Characterization of viscoelasticity and damage in high temperature polymer matrix composites. 2006. [21] Zheng-Ming H. Simulation of the mechanical properties of fibrous composites by the bridging micromechanics model. 2000. [22] Ashcroft IA, Abdel Wahab MM, Crocombe AD. Predicting degradation in bonded composite joints using a semi-coupled finite element method. 2003. [23] Asp LE. The effects of moisture and temperature on the interlaminar delamination toughness of a carbon/epoxy composite. 1997. [24] Alvarez V, Vazquez A. Effect of hygrothermal history on water and mechanical properties of glass/vinylester composites. 2005. [25] Benkhedda A, Tounsi A, Adda bedia EA. Effect of temperature and humidity on transient hygrothermal stresses during moisture desorption in laminated composite plates. 2007. [26] Botelho EC, Pardini LC, Rezende MC. Hygrothermal effects on the shear properties of carbon fibre/epoxy composites. 2006. [27] Mishnaevsky Jr LL, Schmauder S. Continuum mesomechanical finite element modelling in materials development. 2001. [28] Mishnaevsky Jr LL. Three-dimensional numerical testing of microstructures of particle reinforced composites. 2004. [29] Mishnaevsky Jr L, Brøndsted P. Micromechanisms of damage in unidirectional fibre reinforced composites with ductile matrix: computational analysis. 2007. [30] www.selecindia.com/products.php?category¼30. [31] www.ab.com/industrialcontrols/products/relays_timers_and_temp_controllers/solid-state_ relays.html. [32] Abaqus User Manual-Getting started with Abaqus-Chapter3-Page3-4. [33] Mallick PK. Fibre reinforced composites (materials, manufacturing and design). 3rd ed. CRC Press.

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A. Sofi Associate Professor, VIT Vellore, Tamil Nadu, India

13.1

Applications of composite materials

13.1.1 Introduction Metals (counting composites) are comprised of particles and are described by metallic holding (i.e., the valence electrons of every iota being delocalized and shared among every one of the molecules). A large portion of the components in the Periodic Table are metals. Cases of compounds are CueZn (metal), FeeC (steel), and SnePb (bind). In view of component show compounds are characterized. The primary classes of combinations are press-based compounds (for structures), copper-based amalgams (for channeling, utensils, warm conduction, electrical conduction, and so on.) and aluminum-based amalgams (for lightweight structures and for metal-grid composites). Amalgams are quite often in the polycrystalline shape. Earthenware production uses inorganic mixes, for example, Al2O3 (for start plugs and for substrates for microelectronics), SiO2 (for electrical protection in miniaturized scale gadgets), Fe3O4 (ferrite for attractive recollections utilized as a part of PCs), silicates (e.g., mud, concrete, glass), SiC (a grating), and so on. The fundamental classes of pottery are oxides, carbides, nitrides, and silicates. Earthenware production is regularly halfway crystalline and incompletely formless. They are comprised of particles (regularly iotas too) and are described by ionic holding (frequently covalent holding, too). Polymers as thermoplastics (e.g., nylon, polyethylene, polyvinyl chloride, elastic) are comprised of particles that include covalent holding inside every atom and van der Waals powers between the particles. Polymers as thermosets (e.g., epoxy, phenolics) are comprised of a system of covalent bonds. Polymers are nebulous, with the exception of a minority of thermoplastics. Because of the holding, polymers are regularly electrical and warm covers. Be that as it may, leading polymers can be acquired by doping and directing polymer-grid composites can be gotten by the utilization of leading fillers. Semiconductors are portrayed by having the most elevated possessed vitality band (the valence band, where the valence electrons dwell vivaciously) full, with the end goal that the vitality hole between the highest point of the valence band and the base of the void vitality band (called the conduction band) is sufficiently little for Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00013-7 Copyright © 2019 Elsevier Ltd. All rights reserved.

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some part of the valence electrons to be left from the valence band to the conduction band by warm, optical, or different types of vitality. Ordinary semiconductors, for example, silicon, germanium, and gallium arsenide, are covalent system solids. Composite materials are multistage materials acquired by counterfeit blend of various materials, in order to accomplish properties that the individual segments, without anyone else’s input, can’t achieve. An example is a lightweight auxiliary composite that is gotten by inserting persistent carbon filaments in at least one introduction in a polymer grid. The filaments give the quality and firmness, while the polymer fills in as the fastener. Another case is solid, which is a basic composite gotten by consolidating (through blending) bond, sand (fine total), rock (coarse total), and alternatively different fixings that are known as admixtures. As a rule, composites are characterized by their lattice material. The principal classes of composites are polymer-framework, concrete network, metal-lattice, carbon-grid, and earthenware framework composites. Polymer-network and concrete-framework composites are the most widely recognized, attributable to the minimal effort of manufacture. Polymer-lattice composites are utilized for lightweight structures (air craft, brandishing merchandise, wheelchairs, etc.), notwithstanding vibration damping, electronic nooks, blacktop (composite with pitch, a polymer, as the network), patch substitution, and so on. Bond network composites as concrete, steel-strengthened solid, mortar or bond glue are utilized for common structures, preassembled lodging, compositional pre-throws, stone work, landfill cover, warm protection, sound ingestion, and so on; carbon-framework composites are imperative for lightweight structures (e.g., space transport) and parts (e.g., aircraft brakes) that need to withstand high temperatures, yet they are generally costly attributable to the high cost of creation. Carbonframework composites experience the ill effects of their inclination to be oxidized (C þ O2 CO2), thereby getting to be vapor. Clay grid composites are better than carbon-framework composites in the oxidation protection; however, they are not also created as carbon-network composites. Metal-lattice composites with aluminium as the grid are utilized for lightweight structures and low-warm development electronic fenced-in areas, however, their applications are restricted by the high cost of manufacture and by galvanic consumption.

13.1.2

Basic applications

Basic applications allude to applications that require mechanical execution (e.g., quality, solidness, and vibration damping capacity) in the material, which might possibly bear the load in the structure. In situations where the material bears the load, the mechanical property necessities are especially stringent. An illustration is a structure in which steel-strengthened solid segments bear the load of the structure and unreinforced concrete design boards cover the substance of the building. Both the sections and the boards serve auxiliary applications and are basic materials, albeit just the segments bear the load of the structure. Mechanical quality and firmness are expected of the boards; however, the prerequisites are more stringent for the sections. Structures incorporate structures, spans, wharfs, interstates,

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landfill cover, bikes, wheelchairs, ships, submarines, apparatus, satellites, rockets, tennis, rackets, angling poles, skis, weight vessels, freight compartments, furniture, pipelines, utility posts, reinforcement, utensils, and clasp. Notwithstanding mechanical properties, an auxiliary material might be required to have different properties, for example, low thickness (light weight) for fuel sparing on account of aircraft and vehicles, for speed on account of race bikes, and for handle ability on account of wheelchairs and reinforcement. Another property that is frequently required is erosion protection, which is attractive for the strength of all structures, especially vehicles and extensions. A moderately new pattern is for a basic material to have the capacity to serve functions other than the auxiliary capacity, with the goal that the material progresses toward becoming multifunctional. A case of a nonbasic capacity is the detecting of harm. Such detecting, likewise, called auxiliary wellbeing checking, is significant for the counteractive action of risks. It is especially essential to maturing aircraft and extensions. The detecting capacity can be accomplished by installing sensors (for example, optical filaments, the harm or strain of which influences the light throughput) in the structure. Nonetheless, the installation more often than not causes corruption of the mechanical properties and the implanted gadgets are expensive and poor in toughness contrasted with the auxiliary material. Another approach to achieve the detecting capacity is to recognize the adjustment in property of the basic material because of harm. Mechanical execution is essential to the choice of a basic material. Attractive properties are high quality, high modulus, high flexibility, high strength, and high limit with regard to vibration damping. Quality, modulus, and pliability can be measured under strain, pressure, or flexure at different stacking rates, as managed by the kind of stacking on the structure. A high compressive quality does not suggest a high elasticity. Weak materials have a tendency to be more grounded under pressure than strain because of the miniaturized scale splits in them. High modulus does not suggest high quality, as the modulus portrays the versatile distortion conduct though the quality depicts crack conduct. Low durability does not infer a low limit with respect to vibration damping, as the damping might be because of slipping at interfaces in the material, instead of being because of the shear of a viscoelastic stage in the material. Auxiliary materials are overwhelmingly metal-based, bond-based, and polymerbased materials, despite the fact that they additionally incorporate carbon-based and fired-based materials, which are important for high-temperature structures. Among the metal-based basic materials, steel and aluminum amalgams are most common. Steel is invaluable in high-quality applications, though aluminum is favorable in low-thickness applications. For high-temperature applications, intermetallic mixes have been developed, despite the fact that they experience the ill effects of their fragility. Metal-network composites are better than the comparing metal-grid in their high modulus, high crawl protection, and low warm development coefficient, however, they are costly a direct result of the preparing cost. Among the bond-based auxiliary materials, concrete is most prevalent. Albeit concrete is an old material, change in long haul solidness is required. Change relates to diminishing the drying shrinkage, as the shrinkage can cause breaks. It likewise relates

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to diminishing the liquid porousness, as water pervading into steel-strengthened cement can cause consumption of the fortifying steel. Besides, it relates to change in solidifying defrost sturdiness, which is the capacity of cement to withstand variations in temperatures lower than 0 C. Among the polymer-based basic materials, fiber-fortified polymers are very common, because of their mix of high quality and less thick. All polymer-based materials experience the ill effects of their failure to withstand high temperatures. This failure can be because of the corruption of the polymer itself or, on account of a polymer-lattice composite, the warm anxiety coming about because of the warm development jumble between the polymer grid and the filaments. Most structures include joints, which might be achieved by welding, brazing, and binding, the utilization of cements. The basic respectability of joints is to the uprightness of the general structure. As structures can corrupt or be harmed, repair might be required. Repair frequently includes the utilization of a repair material, which might be the same as or not the same as the first material. Consumption protection is alluring for all structures. Metals, because of their electrical conductivity, are especially inclined to consumption. Conversely, polymers and earthenware production, because of their poor conductivity, are significantly less inclined to erosion. It is regularly achieved by appending to or inserting in the structure a viscoelastic material, for example, elastic. Upon vibration, shear twisting of the viscoelastic material causes vitality dissemination. The nearness of the viscoelastic material brings down the quality and modulus of the structure when contrasted with basic material due to the low quality and modulus of the viscoelastic material. On account of a composite material being the auxiliary material, adjustment can include the expansion of a filler of a small size, with the goal that the aggregate filler-framework interface region is substantial and slippage at the interface amid vibration gives a component of vitality dispersal.

13.1.3

Electronic applications

Electronic applications incorporate electrical, optical, and attractive applications, as the electrical, optical, and attractive properties of materials are generally administered by electrons. Electrical applications relate to PCs, gadgets, electrical hardware, electronic gadgets, optoelectronic gadgets, thermoelectric gadgets, piezoelectric gadgets, mechanical autonomy, micro machines, ferroelectric PC recollections, electrical interconnections, dielectrics, substrates for thin film and thick film, warm sinks, electromagnetic impedance protecting, links, connectors, control supplies, electrical vitality stockpiling, engine electrical contacts and brushes, electrical power transmission, whirlpool current review, and so on. Optical applications relate to lasers, light sources, optical filaments, safeguards, reflectors and transmitters of electromagnetic radiation of different wavelengths channels, low-observable or stealth flying machines, radomes, transparencies, optical focal points, photography, photocopying, optical information stockpiling, holography, shading control, and so forth. Attractive applications relate to transformers, attractive chronicle, attractive PC recollections, attractive protecting, attractively suspended trains, apply autonomy, micro machines, attractive molecule investigation, attractive vitality stockpiling, magnetostriction,

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magnetorheological fluids damping that is controlled by an attractive field, attractive reverberation imaging, mass spectrometry, and so forth.

13.1.4 Thermal applications Warm applications allude to applications that include warmth exchange, regardless of whether by conduction, convection, or radiation. Warmth move is required in warming structures, warming associated with mechanical procedures, for example, throwing and toughening, in cooling and modern materials, cooling of hardware, expulsion of warmth created by compound responses, for example, the hydration of concrete, evacuation of warmth produced by erosion or scraped spot as in a brake and in machining, expulsion of warmth created by the impingement of electromagnetic radiation, expulsion of warmth from mechanical procedures, for example, welding, and so on.

13.1.5 Electrochemical applications Electrochemical applications include those relating to electrochemical responses. An electrochemical response includes an oxidation response in which electrons are created, and a diminishment response in which electrons are expended. The terminal that discharges electrons is known as the anode, the terminal that gets electrons is known as the cathode. Materials required for electrochemical applications incorporate the anodes, current gatherer, conductive added substance, electrolytes, and cell holder. An electrolyte can be strong as long as it is an ionic conductor. The interface between the electrolyte and an anode ought to be close and incredibly influences cell execution. The capacity to revive a cell is represented by the reversibility of the cell responses.

13.1.6 Environmental applications Natural applications are those identifying with assurance of the environment from contamination. Protection can include evacuation of a contamination or decrease in the measure of poison created. Poisoning can be lessened by changing the materials and procedures utilized as a part of industry, by utilizing biodegradable materials, by utilizing materials that can be reused, or by changing the vitality sources from nonrenewable energy sources to batteries, power modules, sun-based cells, or potentially hydrogen. Along these lines, composites with biodegradable polymer matrices are appealing.

13.1.7 Biomedical applications Biomedical applications are those including the finding and treatment of conditions, illnesses and handicaps, and also their counteractive actions. Carbon is an especially biocompatible material that can be utilized for implants. Composites with biocompatible polymer lattices are additionally utilized for inserts. Composites with biodegradable polymer networks are utilized for pharmaceuticals.

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13.2 13.2.1

Literature review Materials

13.2.1.1 Polymer matrix composites To enhance the viable warm, mechanical, and electrical properties of unadulterated polymeric materials, polymer framework composites (PMCs) have been fabricated. The upsides of PMCs, for instance, light weight, straightforwardness of process, great quality, and multi-helpfulness, have been demonstrated in aeronautics applications. PMCs will be, as it were, abused in the exponentially rising industry of versatile equipment, and likewise, in the imperativeness, auto, aeronautics, wearing stock and system divisions. The introduction of significantly conductive fillers in thermally ensuring polymers is required to enhance the general warm and mechanical properties of the consequent polymer network composites by means of a multiple polymers for large scale [1]. As the usage of fiber-fortified composites in the avionic business builds, an consideration of the plan of such vehicles should fundamentally consider the cost of generation. Plan for manufacturability is quickly turning into a backbone in lightweight air and space vehicle generation. Mechanized assembling will be the standard for future generations of substantial aviation structures made of cutting-edge lightweight multimaterials that will produce life-cycle cost decreases. Fiber-reinforced polymer composite (FRPC) materials contain a polymer network that encompasses fiber tows containing framework material, a great comprehension of the lattice state amid the curing procedure and which impacts resulting mechanical exhibitions. The condition of the lattice amid curing is changed by the nearness of filaments and their coatings, and relies upon the points of interest of the cure cycle. Inner anxieties are created in a curing network because of shrinkage, responses with the fiber surfaces, and warmth discharge amid compound responses. A fiber tow is a composite that contains the isotropic curing grid and the transversely isotropic strands. Given immense computational power, the perfect approach is to display the individual filaments and the network inside the tows (Fig. 13.1). Additionally, rather than conventional

(a)

(b)

Discrete fiber-matrix tows

Matrix

No real boundary between tow and surrounding matrix

Homogenized tows

Matrix

Boundary between tow and surrounding matrix

Figure 13.1 Cross-section of a textile composite showing tows with its: (a) constituent fibers and matrix, and (b) homogenized tows Royan et al. [2].

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unidirectional covers, tows regularly have higher fiber volume portions (for the most part in the range 0.65e0.75) that require fine work refinement, which again increases computational cost. In this manner, a proper homogenization plot must be produced that takes the “net” impact of curing for the tow, with the end goal that both the proportionate solidness and nonmechanical strains of the tow are considered [2].

13.2.1.2 Polymer nanocomposite (PNC) Polymer nanofiber strengthened polymer nanocomposite (PNC). The mechanical execution of fiber-reinforced polymer (FRP) composite material basically relies upon the mechanical property of the fiber, its uniform scattering in the polymer framework, and solid interfacial holding between the filaments and the encompassing lattice. To get superior polymer nanofiber fortified PNCs, correspondingly, the latest research endeavors have been dedicated to upgrading these attributes. Parameters of electrospun polymer nanofibers, for example, measure, perspective proportion, arrangement, stacking, structure, and posttreatment assume a critical part in execution of last PNCs [3]. Joining biobased/sustainable polymers with inexhaustible fortifications could address the property-execution hole amongst inexhaustible and oil-based polymers. Nano-cellulose can be acquired through two methodologies: bottom up by biosynthesis or top down of plant materials [4]. Polymer-framework composites (PMCs) assume a noteworthy part in engineering applications. They are promising materials in the gadgets business, the car business, and in aerospace engineering and aeronautics. In autos and airplanes, PMCs have supplanted metals and metallic amalgams because of their lower production costs and their light weight. PMCs are comprised of a polymer network that is fortified with another material to enhance the properties of the composites. They are usually carbon-based materials for their light weight. PMCs are comprised of a polymer network that is strengthened with another material keeping in mind the end goal to enhance the properties of the composites. Usually carbon-based materials such as carbon nanotubes (CNTs), graphite or graphene, are utilized [5]. Coming from the viscoelastic conduct of the polymer grid, the successful material properties of keen composites with polymer lattice progress toward becoming time-subordinate [6]. Common strands are made for the most part out of cellulose, hemicellulose, and lignin, with the synthesis fluctuating as indicated by the kind of plant and geographic area. Their low thickness, simple handling, minimal effort, plenitude, and biodegradability make them perfect for use as natural filler in polymer grids [7].

13.2.1.3 Concrete matric composites Concrete-based composite materials (CBCMs) with unrivalled mechanical quality and amazing toughness are constantly alluring in reasonable applications. The joining of CNTs as nano inclusions for the advancement of electrically conductive bond-based composites opens an immense scope of conceivable outcomes for checking of solid structures [8]. As a sort of basic material, bond is generally utilized as a part of development, street and extension field, inferable from its low value, high compressive

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quality, and toughness and also its dependability. As of late, such a significant number of sorts of nano carbon materials, for example, graphene oxide (GO), carbon nanotube, and carbon fiber, have successfully enhanced work on the properties of concrete. In spite of the fact that the expansion of GO can expand the properties of concrete, the lethal disadvantage of GO altered bond is the low smoothness, henceforth synthetically functionalized graphene oxide (GOM) was orchestrated by the substance response of polyether amine with graphene oxide (GO) to enhance its ease property [9]. CNT bondebased composites are drawing expanding consideration when compared with other materials. These composites show strain-detecting abilities giving quantifiable varieties of their electrical properties under connected mechanical distortions. This interesting property, together with the likeness between these composites and auxiliary cement, recommends the likelihood of creating disseminated strain-detecting frameworks with significant changes in the cost-viability of expansive scale solid structures [10]. Large-scale deformity-free bonds (MDF), a sort of polymerconcrete composite, are described by amazingly high mechanical properties. Their flexural qualities are 20e30 times higher than those of customary concrete glues, about equivalent to that of normal steel. Composite macro defect free (MDF) polymer-bond composites were delivered by utilizing the proper material, for example, calcium aluminate concrete, PVA, glycerol and water. The accompanying advances required for making MDF: (1) parts premixing utilizing a planetary blender (2) high-shear blending (calendering), (3) warm squeezing at 80 C and 5 MPa for 10 min, and (4) stockpiling of the last example for 24 h in the stove at a similar temperature [11]. The two most regular material frameworks consolidating the high elasticity and firmness of these strands are FRPs and strain solidifying bond composites (among which are textile reinforced concrete or mortar). While FRP utilizes a natural grid (more often than not an epoxy sap), concrete composites utilize an inorganic, bond-based network to impregnate the filaments. As a result, bond composites have a superior imperviousness to fire, higher porousness, and better warm similarity with concrete [12]. Steel strands are usually utilized as a part of bond-based materials for some applications, for example, floors, basic components, repairs, and so on. The chloride-initiated consumption by means of entrance of seawater may turn into a hazard for execution of the steel fiber fortified bond-based composites. The erosion of steel fiber is significantly less harmful as contrasted and conventional steel rebar fortification [13]. Concrete-based composite materials can be utilized as electromagnetic protecting material for protecting the electromagnetic radiation, by doped protecting medium included into the bond framework. Silica rage and colloidal graphite can be utilized as a part of nickel fiber bond-based composites as a means of enhancing the dispersity of nickel fiber and the electromagnetic impedance protecting viability (SE) of concrete-based composites. Cement based composites of different sorts and inceptions ended up plainly normal segments of bond composite plan in recent decades. Cement based composites are add to the hydration procedure of cementitious materials, affecting the idea of the hydration items and, thusly, the strength of the last composites. Concrete containing composite blends can be enhanced altogether by taking the measures of active and inactive parts in the cement based composite PA into account [14].

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13.2.1.4 Carbon-network composites Carbon strands are delivered by warm treatment of antecedent filaments while at strain in no less than two phases alleged adjustment and carbonization. Carbon filaments are intended to be utilized basically in a grid and help exchanging the anxiety far from network keeping in mind the end goal to enhance mechanical properties of composite structures. Amid carbonization process the majority of the noncarbonaceous components are expelled and just carbon stays in filaments. Showing carbon molecules brings about a solid and synthetically stable fiber. To enhance holding vitality amongst fiber and grid, a surface treatment is regularly required paying little mind to the kind of antecedent utilized [15]. Ceaseless carbon fiber polymer-network composites are principally of one of a few structures. These structures incorporate multidirectional fiber overlays (made by the stacking and combination of fiber laminae, with the strands in every lamina being either unidirectional or woven and the quantity of filaments stacked along the thickness of every lamina regularly extending from 25 to 50), unidirectional fiber poles (made by pultrusion), and multidirectionally twisted fiber tubings (made by fiber winding). In these structures, the composite is exceptionally anisotropic, with the quality, modulus, electrical conductivity, and warm conductivity being all substantially higher in the fiber course of the composite than alternate bearings. For superior basic applications, the covers include nonwoven strands, with the end goal that the filaments are unidirectional in every lamina and the fiber headings in various laminae are not all the same [16]. CNTs are known to have a phenomenal arrangement of electrical, mechanical, warm, organic, and synergist properties that have pulled in light of a legitimate concern for scientists for conceivable applications in various present and future applications [17].

13.2.1.5 Metal-framework composites Oxide-fiber/nickel-based framework composites can’t be utilized at temperatures higher than around 1200 C; consequently, composites with networks of higher softening focuses are required. Metal-framework composites have a reasonable significance gave vital coatings ought to be produced since mechanical properties of the composites, for example, quality, harm resilience and crawl protection are adequately high at room temperature and high temperatures up to no less than 1300 C. Metal network composites showed up on the specialized skyline because of the development of high entropy amalgams (HEAs). In reality, a one-of-a-kind mix of physical, concoction, and mechanical properties of HEAs make them almost perfect lattices for sinewy composites [18]. Main parts of aircrafts are including metal framework composites in their parts due to their superb mechanical and physical properties, prompting diminishment in the heaviness of basic segments and vitality prerequisite for driving. Segments made by cross bred metal grid composite (in view of aluminum combination strengthened with single-divider and multidivider carbon nanotubes, graphene, and clay particles) required optional operations to improve the dimensional resistance and surface wrapup. Half-and-half metal matrix composites (MMCs) were shaped by fortifying

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the base framework with more than one fortification having distinctive properties. Those composites that have a blend of at least two fortification particles improve the mechanical properties of the composite [19]. Aluminum in its composite shape is as of now being utilized for the assembling of different motor parts. The impediment of aluminum is that it is inclined to scratches and spaces effortlessly. Therefore the need of, aluminum-based composite will be wear safe in nature by the expansion of reasonable fortifications in characterized extent. Aluminum framework composites have phenomenal capacity to manage pliable and additionally, compressive, powers. Fly fiery debris is delivered as a result from the consuming of pummeled coal in control age plants. It is harmful to people if breathed in through the air. Aluminum and fly fiery debris were blended with the assistance of an extraordinarily composed stirrer, at a rotational speed of 100 rpm with the guide of the rotor for uniform blending. The liquid blend of aluminum with fly fiery debris was poured in the predefined pit of required measurements. After resulting cooling and optional machining forms, distinctive examples were made for testing of wear and coefficient of rubbing [20]. Metal-frame composites join the upsides of the metal lattices (high flexibility) and the encased fortification particles (high quality and hardness), yielding the coveted mechanical execution in administrations [21]. MMC coatings are one of the sorts of defensive coatings that are of extraordinary interest because of their exceptional mix of hardness, quality, and durability. In MMC coatings, the hard strengthening particles that are normally produced using pottery are disseminated inside a malleable network. The blend of the hardness of the strengthening particles with the durability of a bendable grid has brought about high protection from wear in MMC coatings [22]. MMCs manufactured by means of blend throwing are found to have characteristic deformities, for example, porosity that destroys the mechanical properties [23]. Particle-fortified metal lattice composites (PMMCs) have the astounding properties of framework materials and fortifying phase, and have incredible quality-to-weight proportion when contrasted with conventional metals and combinations, particularly under the state of moderately high temperature [24]. Metal grid composites display higher quality-to-weight proportion, hardness, firmness, wear protection, and so on when contrasted with ordinary metals and combinations [25]. Optical micrographs are shown in Fig. 13.2.

13.2.1.6 Ceramic-matrix composites Artistic materials have high quality and modulus at raised temperature. Yet, their utilization as basic parts is seriously restricted due to their fragility. Ceramic-matrix composites, by fusing strands in earthenware lattices, be that as it may, misuse their alluring high temperature quality as well as lessen the penchant for cataclysmic disappointment. At the point when the composite material is subjected to a worry along the fiber course, a basic worry at which the composites display initially proof of network splitting is characterized as fibre metal matrix composites. The FMCS is considered as the most extreme reasonable outline worry for fiber-strengthened CMCs for parts subjected to oxidizing condition [27].

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Figure 13.2 Optical micrographs of (a) A359 aluminum alloy and (b) A359 with 20 vol% SiC particles MMC. By Li et al. [28].

The utilization of clay materials has altogether expanded in different applications because of the superlative qualities they display contrasted to metals and polymers. The worthwhile qualities of earthenware materials are low thickness, high grating strength, hardness, and unbending nature. The solid earthenware production downside is their low break durability, which causes fragile cracks [28]. Ultra-high temperature ceremic composites have been broadly utilized as a part of warm security frameworks and drive frameworks in aviation applications because of their high liquefying temperature, great substance, and physical dependability under high temperature. Fiber-fortified artistic lattice composites are a standout amongst the most encouraging contenders for auxiliary applications in industry, for example, airplanes, aviation, and military ventures [29]. Ceremic composites are utilized as a part of high-temperature aviation applications (>800 C) in light of their capacity to keep up quality and durability in these situations. These are multisegment frameworks made out of clay fiber, a feebly bound interface, and artistic lattice. The filaments fortify the CMC, giving structure and quality. The earthenware grid is the greater part of the ceremic composites that requires properties, for example, low thickness, high mechanical quality, and high warm and concoction security. The interface among fiber and network is critical to the usefulness of the ceremic composites CFR-CMC and gives vitality in retaining components, for example, break redirection and debonding which mean to dodge cataclysmic disappointment of the CMC. Interphase coatings are connected to clay filaments keeping in mind the end goal to debilitate the interfacial bond amongst framework and fiber. Boron nitride (BN), pyrolytic carbon (PyC), silicon nitride (Si3N4), silicon carbide (SiC) and mixes of those have been thoroughly investigated as interphase materials [30]. While trying to influence critical change in the practical qualities of brake cushions for airplanes and vehicles, distinctive materials have been utilized, and this has come about in the improvement of various sorts of brake cushions. An iron millscale (IMS) molecule strengthened earthenware network composite (CMC) was created by the

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

powder metallurgy technique and described for brake cushions [31]. The ceremic composites display unmistakable practices at worries above and underneath the grid splitting anxiety, which is related with the beginning of lattice breaking and with the arrangement of hysteresis circles that outcomes from network splitting and frictional slipping of the strands crossing over grid splits [32]. Fired lattice composites are promising contenders for aviation applications, for example, the hot segment segments of turbine motors, because of their low thickness and great high-temperature quality. The damage in a fiber-strengthened clay grid composite can happen in a few diverse ways. A few illustrations incorporate transverse as well as longitudinal lattice splits, interfacial debonding, fiber breakage, and delamination among others. Comprehension, recognizing, and observing diverse sorts of damage are fundamental in accomplishing ideal execution from parts made out of clay grid composites [33]. Ceremic matrix composites (CMCs) begins from better oxidation conduct when thought about than nonoxide based variations (e.g., SiCf/SiC or SiCf/Al2O3), which is especially evident at temperatures over 800 C [34].

13.3 13.3.1

Material properties Mechanical properties

At give neither electrospun nano fibers expected high quality and modulus accessible, nor is the mechanical conduct of single electrospun nano fiber surely knew. It is outstanding that quality and modulus of fortifying filaments are essential components to decide mechanical execution of resultant polymer composite materials. In any case, most electrospun nano fibers regularly have elasticity