Fiber-Reinforced Composite Materials: Characterization and Computational Predictions of Mechanical Performance 3031325575, 9783031325571

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Synthesis Lectures on Mechanical Engineering

V. Tuninetti · C. Medina · A. Salas · I. Valdivia · E. Fernández · M. Meléndrez · G. Pincheira · P. Flores

Fiber-Reinforced Composite Materials Characterization and Computational Predictions of Mechanical Performance

Synthesis Lectures on Mechanical Engineering

This series publishes short books in mechanical engineering (ME), the engineering branch that combines engineering, physics and mathematics principles with materials science to design, analyze, manufacture, and maintain mechanical systems. It involves the production and usage of heat and mechanical power for the design, production and operation of machines and tools. This series publishes within all areas of ME and follows the ASME technical division categories.

V. Tuninetti · C. Medina · A. Salas · I. Valdivia · E. Fernández · M. Meléndrez · G. Pincheira · P. Flores

Fiber-Reinforced Composite Materials Characterization and Computational Predictions of Mechanical Performance

V. Tuninetti Department of Mechanical Engineering Universidad de La Frontera Temuco, Chile

C. Medina Department of Mechanical Engineering University of Concepción Concepción, Chile

A. Salas Department of Mechanical Engineering University of Concepción Concepción, Chile

I. Valdivia Department of Mechanical Engineering University of Concepción Concepción, Chile

E. Fernández Department of Aerospace and Mechanical Engineering University of Liège Liège, Belgium

M. Meléndrez Advanced Nanocomposites Research Group Hybrid Materials Laboratory Department of Materials Engineering University of Concepción Concepción, Chile

G. Pincheira Department of Industrial Technologies University of Talca Curicó, Chile

P. Flores Plastic Omnium—New Energies Genk, Belgium

ISSN 2573-3168 ISSN 2573-3176 (electronic) Synthesis Lectures on Mechanical Engineering ISBN 978-3-031-32557-1 ISBN 978-3-031-32558-8 (eBook) https://doi.org/10.1007/978-3-031-32558-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

1 Introduction to Fiber-Reinforced Composites Materials . . . . . . . . . . . . . . . . . . . 1.1 Organization of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Out-of-Plane Compressive Fatigue Behavior of Unidirectional Glass Fiber-Reinforced Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Manufacturing of Fiber-Reinforced Composite . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Methodology for Determining the Influence of IFSS Out-of-Plane Fatigue Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Micro- and Macro-Scale Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Analysis and Prediction of Failure in FRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Failure Criteria for Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Material Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Experimental Setup and Sample Manufacturing for Failure Model Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Computational Failure Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Impact of the Multiwall Carbon Nanotubes on the Transverse Compressive Strength and Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Investigated FRC Engineered with MWCNT . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Experimental Monotonic and Cyclic Compression Testing . . . . . . . . . . . . . . 4.3 Microcracking Analysis and Characterization of Material Degradation . . . 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 4 5 9 11 11 13 16 19 20 22 23 23 28 29 31 33 34 35 43 44

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Contents

5 Mechanical Performance of FRC Manufactured from Recycled Carbon Fibers with Grown CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Recycled Carbon Fibers and CNT Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Mechanical Performance of Recycled Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49 51 53 55 58

Symbols and Abbreviations

Abbreviations CFRP CNC CNT COMP DIC exp exp() FE FEA FEM FRC FRP IFSS MW RTM RVE TENS UD

Carbon fiber-reinforced plastic Computer numerical control Carbon nanotube Compression Digital image correlation Experimental Exponential function Finite Element Finite Element Analysis Finite Element Method Fiber-reinforced composite Fiber-reinforced plastic Interfacial shear stress Multiwall Resin Transfer Moulding Representative volume element Tensile Unidirectional

Characters ρcrack σc Ad Bd E c,sat εp

Crack density Compressive stress Damage parameter Crack rate parameter Macroscopic damage saturation level Accumulated plastic deformation vii

viii

C As E F L g,i t T

Symbols and Abbreviations

Damage rate parameter Cross-sectional area Young’s Modulus Axial load Length of cracks Time Temperature

1

Introduction to Fiber-Reinforced Composites Materials

The most commonly used composite materials in engineering are glass or carbon fibers embedded in a matrix, usually a thermoset or thermoplastic polymer. In industry, there are several standard fiber reinforcement configurations for composite materials, including unidirectional fiber fabrics, fabrics consisting of unidirectional fiber yarns joined crosswise by a smaller number of filaments, and non-crimp fabrics (NCF), fabrics consisting of unidirectional fiber yarns joined by stitching yarns (Poe et al. 1999). Fiber composites are widely used in different technological industries given their high mechanical properties and low density. Applications for these materials are commonly found in passengers or freight transport systems, such as aircraft fuselages, vehicle frames, and sports implementation. However, it is also possible to find high-performance applications for these materials, for example, Park et al. developed and studied the mechanical performance of a heavy-duty hybrid carbon phenolic hemispherical bearing, Li et al. simulated the fatigue failure of a compressor blade, Bakaiyan et al. analyze multilayers thick wall composites pipes and Yeong et al. show the potential and applications in additively manufactured fiber-reinforced composites, in particular, the case of unmanned aerial vehicles (UAVs). Nowadays, new thick applications are not a technical limitation and the composite behavior on the through-thickness is of great interest to engineering and science. Several researchers have studied the numerical and experimental behavior of fiber composite materials, but few of them have documented the out-of-plane performance. Yu et al. presented in this study a method to predict the fatigue life of honeycomb sandwich structures validated by experiments. In this line, Toubal et al. provided an experimental investigation of fatigue damage mechanisms and evolution in thick carbon/epoxy laminate, using three non-destructive techniques, obtained a good description of the different mechanisms involved during their damage process. Hivet et al. studied the effect of mesoscopic out-of-plane defect on the fatigue behavior of a GFRP, concluding that defects in

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 V. Tuninetti et al., Fiber-Reinforced Composite Materials, Synthesis Lectures on Mechanical Engineering, https://doi.org/10.1007/978-3-031-32558-8_1

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1 Introduction to Fiber-Reinforced Composites Materials

the transverse direction (comparing with the longitudinal direction) have a major negative influence on the fatigue life of such a composite. Fujimoto et al. compared the out-ofplane fatigue life between a four-point-bending method (curved section) and the flatwise tension method, observed a much longer fatigue life in the four-point-bending test. Different authors have also performed studies and developed techniques to improve fatigue life on the composites. Sorensen and Goutianos proposed a micromechanical model for predicting the fatigue limit and increased the initial value of the interfacial sliding shear stress. Al-Haik and Boroujeni found growing MWCNT in carbon fiber could improve the fatigue life by 150% due to the protection of fiber against the matrix cracks. The numerical analysis of Llorca et al. explains the failure modes experienced by composites submitted to off-axis loading and concluded that unidirectional composites fail under modes linked to the mechanical features of the matrix and the fiber/matrix interface. According to this, to improve the out-of-plane mechanical performance of a composite, the matrix, and the fiber/matrix interface must be investigated. Shao et al. analyzed the tensile and fracture behavior in five different matrices and the interfacial shear stress (IFSS) of those matrices with carbon fiber reinforcement. They show that the IFSS depends on the matrix features and that this parameter is the most influential on the improvement of the fatigue resistance (1 × 106 cycles and load stress ratio 0.1). Godara et al. achieved an IFSS enhancement by including CNTs on an epoxy glass fiber-reinforced composite. They analyzed three techniques: sizing the fiber with CNTs, mixing the CNTs on the resin, and a combination of both. The three techniques improve the IFSS, being the first one the most performant. In this context, this book provides a comprehensive experimental micro- and macromechanical campaign performed to understand the out-of-plane fatigue, its main associated failure modes, and the influence of IFSS on the composite behavior. Regarding damage of fiber reinforced plastics, several theories or approaches have been developed to predict crack initiation, propagation, and degradation of materials based on linear elastic and elastoplastic fracture mechanics. The selection of these approaches depends mainly on the characteristics of cracks, types of applied loads, and mechanical properties of the analyzed materials. These include most notably the fracture modes, cohesive zone models (CZM), energy release rate concept, stress intensity factor, and the J-integral (Gao and Fan 2020; Gzaiel et al. 2021; Hu et al. 2020; Huang et al. 2020; Jin and Sun 2012; Kang et al. 2021). The several existing failure models for composites are applied depending on the damage conditions and composite features, some include damage of elementary ply (Ladeveze and Ledantec 1992), continuum damage modeling (Van Der Meer and Sluys 2009), elastoplastic progressive failure (Chen et al. 2012), anisotropic damage (Matzenmiller et al. 1995), and strain rate damage dependency (Lomakin et al. 2021). Classical failure criteria for the evaluation of these composite materials considering only the transverse isotropy implies an overestimation of their out-of-plane strength and consequently could generate a non-conservative design of a part and even—depending on the case of application—a catastrophic failure during its operation. According to this, given the similarity in the configuration and the orthotropic behavior of materials

1 Introduction to Fiber-Reinforced Composites Materials

3

reinforced with both unidirectional fiber fabrics and NCF-reinforced fabrics, the out-ofplane failure initiation in tensile is assessed in this book for unidirectional fiberglass fabric reinforced composites through the implementation of the failure criteria for orthotropic NCF-reinforced composites proposed by Molker et al. (2016). First, the failure criteria are programmed as a module coupled to a commercial FEM package, then tests are simulated and, finally, the results obtained are compared and validated with the respective experimental tests. To increase the out-of-plane mechanical resistance of composite materials, the properties of both the matrix and fiber–matrix interface should be improved. Several authors (Bilisik et al. 2020; Chandrasekaran et al. 2011; Kamarian et al. 2020; Ma et al. 2015; Medina et al. 2017) considered adding carbon nanoparticles to composite materials and concluded that this approach improves the properties of the fiber–matrix interface and toughness of the matrix. For instance, Boroujeni and Al-Haik (2019) obtained 20% and 50% increases in tensile strength and fatigue life, respectively, of a carbon FRC with carbon nanotubes (CNTs) compared with the original material. Zhou et al. (2014) assessed the addition of 2 wt.% of carbon nanofibers in the epoxy matrix, obtaining around 15% higher values of the compressive strength, through-hole compressive strength, and interlaminar shear strength. Improvements in damage resistance have also been reported (Grabowski et al. 2017; Han et al. 2018) however, the matrix material may not always exhibit the same performance with the addition of CNTs; low-stiffness resins benefit noticeably, while high-stiffness resins are minimally affected (Ci and Bai 2006). In addition, the agglomeration and concentration of CNTs in the epoxy resin directly affect the improvement of mechanical properties, e.g., epoxy–CNT mixtures exhibit an increased viscosity, which can cause issues during FRC manufacturing with resin transfer molding (RTM) or vacuum resin injection molding (VRIM) due to reduced resin–fiber interactions. According to this agglomeration phenomenon, several authors have reported that 0.5% weight of CNTs dispersed in the epoxy matrix provides optimal strength and that higher concentrations are associated with a decrease in resistance compared to the conventional for fiberreinforced plastic (FRP) (Ayatollahi et al. 2017; Han et al. 2018; Moghimi Monfared et al. 2018). The incorporation of single-wall (SW) CNTs has been studied and variation has also been reported in the composite mechanical properties such as Young’s modulus and tensile strength with 0.3 optimal wt. percentage by Maghsoudlou et al. (2019). Interfacial shear strength (IFSS), a characteristic property depending on physicochemical interactions between the surface of additives and the epoxy resin provides the resistance of the fiber–matrix interface. The computation of IFSS is performed using a variety of tests and configurations, (Li et al. 2019) e.g., the push-out nanoindentation test, microdroplet, the single fiber fragmentation test, the Ioscipescu test. González and Llorca (2007) show that the increase in IFSS directly influences the transverse compressive strength. Medina et al. (2017) incorporated a master batch of MWCNTs in a glass FRC and improved the IFSS by 24%. Later, Bedi et al. (2018) found an increase of the IFSS by grafting CNTs onto the surface of carbon fiber. Godara et al. (2010) determined

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1 Introduction to Fiber-Reinforced Composites Materials

the properties of the fiber–matrix interface for different methods applied to incorporate the CNTs in the FRC and concluded that significant improvement of IFSS can be achieved in composites modified with CNTs. The aforementioned finding was later confirmed by Ma and Zhang (2014) and Qian et al. (2021), particularly when depositing functionalized CNTs for uniform distribution and stronger bonding between the epoxy matrix and CNTs. In this context, Chap. 4 of this book focuses on the effect of adding multiwall MWCNTs to a glass FRC, and its consequence on mechanical compressive strength and damage resistance in the out-of-plane and transverse direction of the material. In addition, according to the increased demand for polymer matrix composites worldwide and the consequent increase in waste generated at the end of the product service life, a recycling process of the carbon fiber-reinforced composite material applied via thermolysis to obtain the carbon fibers, followed by the growth of CNTs on their surface using the Poptube technique is provided in Chap. 5.

1.1

Organization of the Book

This introduction chapter has presented the latest advances and research on the mechanical performance of FRPs in context with the subjects covered in the following chapters of this book. Chapter 2 describes a comprehensive experimental micro- and macromechanical campaign performed to understand the out-of-plane fatigue, and provides the main associated failure modes found and the influence of IFSS on the composite behavior. In Chap. 3,1 the Molker failure criteria are implemented and applied to determine the initiation of out-of-plane failure in unidirectional fiberglass fabric composites. The model is assessed by comparing numerical and experimental results of out-of-plane failure in a corrugated panel. In addition, several failure criteria used in unidirectional fiber-reinforced composite that consider transverse isotropy are evaluated. Chapter 42 focuses on the addition of multiwall carbon nanotubes for fabricating the FRP and the assessment of transverse compressive strength and damage resistance. Chapter 53 presents the recycling process of a carbon fiber-reinforced plastic applied by pyrolysis to obtain carbon fibers followed by grown CNTs over their surfaces using the Poptube technique. The effect of the presence of the grown CNTs on the laminate resistance is investigated. Each chapter provides overall conclusions including research challenges for further studies.

1 Based on Valdivia et al. (2021). 2 Based on Salas et al. (2022). 3 Based on Medina et al. (2021).

References

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References Ayatollahi MR, Barbaz Isfahani R, Moghimi Monfared R (2017) Effects of multi-walled carbon nanotube and nanosilica on tensile properties of woven carbon fabric-reinforced epoxy composites fabricated using VARIM. J Compos Mater 51(30):4177–4188. https://doi.org/10.1177/002 1998317699982 Bedi HS, Tiwari M, Agnihotri PK (2018) Quantitative determination of size and properties of interphases in carbon nanotube-based multiscale composites. Carbon 132:181–190. https://doi.org/10. 1016/j.carbon.2018.02.059 Bilisik K, Karaduman N, Sapanci E (2020) Short-beam shear of nanoprepreg/nanostitched threedimensional carbon/epoxy multiwall carbon nanotube composites. J Compos Mater 54(3):311– 329. https://doi.org/10.1177/0021998319863472 Boroujeni AY, Al-Haik M (2019) Carbon nanotube – Carbon fiber reinforced polymer composites with extended fatigue life. Compos Part B Eng 164(November 2018):537–545. https://doi.org/ 10.1016/j.compositesb.2018.11.056 Chandrasekaran VCS, Advani SG, Santare MH (2011) Influence of resin properties on interlaminar shear strength of glass/epoxy/MWNT hybrid composites. Compos A Appl Sci Manuf 42(8):1007–1016. https://doi.org/10.1016/j.compositesa.2011.04.004 Chen JF, Morozov EV, Shankar K (2012) A Combined elastoplastic damage model for progressive failure analysis of composite materials and structures. Compos Struct 94(12):3478–3489 Ci L, Bai JB (2006) The reinforcement role of carbon nanotubes in epoxy composites with different matrix stiffness. Compos Sci Technol 66(3–4):599–603. https://doi.org/10.1016/j.compscitech. 2005.05.020 Gao R, Fan S (2020) Research on the propagation characteristics of fatigue cracks on rail surfaces. Int J Appl Mech 12(10):2050121 Godara A, Gorbatikh L, Kalinka G, Warrier A, Rochez O, Mezzo L, Luizi F, van Vuure AW, Lomov SV, Verpoest I (2010) Interfacial shear strength of a glass fiber/epoxy bonding in composites modified with carbon nanotubes. Compos Sci Technol 70(9):1346–1352. https://doi.org/10.1016/ j.compscitech.2010.04.010 González C, Llorca J (2007). Mechanical behavior of unidirectional fiber-reinforced polymers under transverse compression: microscopic mechanisms and modeling. Compos Sci Technol 67(13):2795–2806. https://doi.org/10.1016/j.compscitech.2007.02.001 Grabowski K, Zbyrad P, Uhl T, Staszewski WJ, Packo P (2017) Multiscale electro-mechanical modeling of carbon nanotube composites. Comput Mater Sci 135:169–180. https://doi.org/10.1016/ j.commatsci.2017.04.019 Gzaiel M, Triki E, Barkaoui A, Chafra M (2021) Finite element study of mixed fracture: velocitydependent insertion of pointed blades into soft material. Int J Appl Mech 13(01):2150003 Han L, Li K, Sun J, Song Q, Wang Y (2018) Reinforcing effects of carbon nanotube on carbon/ carbon composites before and after heat treatment. Mater Sci Eng A 735(August):10–18. https:/ /doi.org/10.1016/j.msea.2018.08.025 Hu J, Ji C, Chen S, Li S, Wang B, Zhou Z (2020) Novel mathematical-statistical models for the distribution of fatigue life and residual strength for fiber reinforced polymer composites. Int J Appl Mech 12(09):2050104 Huang J, Li J, Pan X, Xie T, Hua W, Dong S (2020) Numerical investigation on mixed mode (I-II) fracture propagation of CCBD specimens under confining pressure. Int J Appl Mech 12(10):2050111 Jin Z, Sun C (2012) Fracture mechanics. Academic Press, New Delhi

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Kamarian S, Bodaghi M, Isfahani RB, Song J-i (2020) A comparison between the effects of shape memory alloys and carbon nanotubes on the thermal buckling of laminated composite beams. Mech Based Des Struct Mach 1–24. https://doi.org/10.1080/15397734.2020.1776131 Kang J, Liu S, Wang C (2021) Drying-induced pressure rise and fracture mechanics modeling of the sphagnum Ladevez P, Ledantec E (1992) Damage modelling of the elementary ply for laminated composites. Compos Sci Technol 43(3):257–267 Li W, Chen W, Tang L, Jiang Z, Huang P (2019) A general strength model for fiber bundle composites under transverse tension or interlaminar shear. Compos A Appl Sci Manuf 121(January):45– 55. https://doi.org/10.1016/j.compositesa.2019.03.009 Lomakin E, Fedulov B, Fedorenko A (2021) Strain rate influence on hardening and damage characteristics of composite materials. Acta Mech 232(5):1875–1887 Ma C, Liu HY, Du X, Mach L, Xu F, Mai YW (2015) Fracture resistance, thermal and electrical properties of epoxy composites containing aligned carbon nanotubes by low magnetic field. Compos Sci Technol 114:126–135. https://doi.org/10.1016/j.compscitech.2015.04.007 Ma PC, Zhang Y (2014) Perspectives of carbon nanotubes/polymer nanocomposites for wind blade materials. Renew Sustain Ener Rev 30:651–660. https://doi.org/10.1016/j.rser.2013.11.008 Maghsoudlou MA, Barbaz Isfahani R, Saber-Samandari S, Sadighi M (2019) Effect of interphase, curvature and agglomeration of SWCNTs on mechanical properties of polymer-based nanocomposites: experimental and numerical investigations. Compos B Eng 175(June):107119. https:// doi.org/10.1016/j.compositesb.2019.107119 Matzenmiller A, Lubliner J, Taylor RL (1995) A constitutive model for anisotropic damage in fibercomposites. Mech Mater 20(2):125–152 Medina C, Fernandez E, Salas A, Naya F, Molina-Aldereguía J, Melendrez MF, Flores P (2017) Multiscale characterization of nanoengineered fiber-reinforced composites: effect of carbon nanotubes on the out-of-plane mechanical behavior. J Nanomater 2017:1–9. https://doi.org/10.1155/ 2017/9809702 Medina C, Vial J, Canales C, Flores P, Jaramillo A, Tuninetti V, Sanhueza JP, Rojas D (2021) Ultrafast carbon nanotubes growth on recycled carbon fibers and their evaluation on interfacial shear strength in reinforced composites. Sci Rep 11:5000. https://doi.org/10.1038/s41598-021-84419-y Moghimi Monfared R, Ayatollahi MR, Barbaz Isfahani R (2018) Synergistic effects of hybrid MWCNT/nanosilica on the tensile and tribological properties of woven carbon fabric epoxy composites. Theor Appl Fract Mech 96:272–284. https://doi.org/10.1016/j.tafmec.2018.05.007 Molker H, Wilhelmsson D, Gutkin R, Asp LE (2016) Orthotropic criteria for transverse failure of non-crimp fabric-reinforced composites. J Compos Mater 50(18):2445–2458 Poe CC, Dexter HB, Raju IS (1999) Review of the NASA textile composites research. J Aircr 36(5):876–884 Qian WM, Vahid MH, Sun YL, Heidari A, Barbaz-Isfahani R, Saber-Samandari S, Khandan A, Toghraie D (2021) Investigation on the effect of functionalization of single-walled carbon nanotubes on the mechanical properties of epoxy glass composites: Experimental and molecular dynamics simulation. J Market Res 12:1931–1945. https://doi.org/10.1016/j.jmrt.2021.03.104 Salas A, Oñate A, Escudero B, Medina C, Tuninetti V, Meléndrez M (2022) Effect of 0.5% CNT reinforcement of a glass fiber composite on strength and cyclic damage induced by transverse and out-of-plane compressive loads. J Compos Mater 56(18):2895–2906. https://doi.org/10.1177/002 19983221106522 Valdivia I, Canales C, Tuninetti V, Flores P, Medina C (2021) Numerical prediction of failure in unidirectional fiber reinforced composite. Int J Appl Mech 13(06):2150073. https://doi.org/10.1142/ S1758825121500733

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Van Der Meer FP, Sluys LJ (2009) Continuum models for the analysis of progressive failure in composite laminates. J Compos Mater 43(20):2131–2156 Zhou Y, Jeelani S, Lacy T (2014) Experimental study on the mechanical behavior of carbon/epoxy composites with a carbon nanofiber-modified matrix. J Compos Mater 48(29):3659–3672. https:/ /doi.org/10.1177/0021998313512348

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Out-of-Plane Compressive Fatigue Behavior of Unidirectional Glass Fiber-Reinforced Composite

Fiber composites are widely used in different technological industries given their high mechanical properties and low density. Applications for these materials are commonly found in passengers or freight transport systems, such as aircraft fuselages, vehicle frames, and sports implementation. However, it is also possible to find high-performance applications for these materials, for example, Park et al. developed and studied the mechanical performance of a heavy-duty hybrid carbon phenolic hemispherical bearing, Li et al. simulate the fatigue failure of a compressor blade, Bakaiyan et al. analyze multilayers thick wall composites pipes and Yeong et al. show the potential and applications in additively manufactured fiber-reinforced composites, in particular the case of unmanned aerial vehicles (UAVs). Nowadays, new thick applications are not a technical limitation and the composite behavior on the through-thickness is of great interest to engineering and science. Properties such as fatigue and fracture could show poor performance in this direction, mainly explained by the fiber-matrix interface (micro-scale) and delamination behavior (macro-scale). Because of this, the understanding of these phenomena through thickness is relevant, however little experimental information can be found and therefore few numerical models can be developed. It remains an unexplored field of study, allowing new research and preventing lack of knowledge limit the development of new performant mechanical structures and machine elements. Particularly, in the out-of-plane mechanical behavior and application of complex loads (multi-direction). Several researchers have studied the numerical and experimental behavior of fiber composite materials, but few of them have documented the out-of-plane performance. Yu et al. presented in this study a method to predict the fatigue life of honeycomb sandwich structures validated by experiments. In this line, Toubal et al. provided an experimental investigation of fatigue damage mechanisms and evolution in thick carbon/epoxy laminate, using three non-destructive techniques, obtained a good description of the different

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 V. Tuninetti et al., Fiber-Reinforced Composite Materials, Synthesis Lectures on Mechanical Engineering, https://doi.org/10.1007/978-3-031-32558-8_2

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2 Out-of-Plane Compressive Fatigue Behavior of Unidirectional Glass …

mechanisms involved during their damage process. Hivet et al. studied the effect of mesoscopic out-of-plane defect on the fatigue behavior of a GFRP, concluding that defects in the transverse direction (comparing with longitudinal direction) have a major negative influence on the fatigue life of such a composite. Fujimoto et al. compared the out-ofplane fatigue life between a four-point-bending method (curved section) and the flatwise tension method, observed a much longer fatigue life in the four-point-bending test. On the other hand, different authors have presented studies and techniques to improve fatigue life on the composites. Sorensen and Goutianos proposed a micromechanical model for the prediction of the fatigue limit, which shows it can be increased by increasing the initial value of the interfacial sliding shear stress. Al-Haik and Boroujeni found grow in MWCNT in carbon fiber could improve fatigue life by 150% due to the protection of fiber against matrix cracks. The numerical analysis of Llorca et al. explains the failure modes experienced by composites submitted to off-axis loading. It can be concluded that on unidirectional composites the failure modes are linked to the mechanical features of the matrix and the fiber–matrix interface. According to this, to improve the out-of-plane mechanical performance of a composite, the matrix and the fiber–matrix interface must be strengthening. This can be achieved by fiber sizing by matrix doping or by matrix curing kinetics modification. Shao et al. analyzed the tensile and fracture behavior in five different matrices and the interfacial shear stress (IFSS) of those matrices with carbon fiber reinforcement. They show that the IFSS depends on the matrix features and that this parameter is the most influential on the improvement of the fatigue resistance (1 × 106 cycles and load stress ratio 0.1). Godara et al. achieved an IFSS enhancement by including CNTs on an epoxy glass fiber-reinforced composite. They analyzed three techniques: sizing the fiber with CNTs, mixing the CNTs on the resin, and a combination of both. The three techniques improve the IFSS, being the first one the most performant. In this chapter, the influence of IFSS over the out-of-plane fatigue behavior of an epoxy–glass fiber-reinforced composite is analyzed. The use of MWCNTs is also included in order to improve the IFSS, and out-of-plane compressive static and compression–compression fatigue tests (under two stress ratios) are performed to assess the influence of this nanoreinforcement on the fiber–matrix interface and over the out-of-plane composite mechanical performance. Then, to analyze the differences in the mechanical response, a top-down experimental campaign is applied to identify the MWCNT’s influence on the matrix mechanical behavior as well as its influence on the fiber–matrix interface and the consequent effect on the macroscopic mechanical performance. The chapter is outlined as follows: Sect. 2.2 is focused on a description of specimens’ preparation and the micro/ macro-analysis methodology applied. Push-out tests, out-of-plane static compressive tests, and out-of-plane fatigue compressive/compressive tests at two loading ratios performed on both a glass fiber reinforced composite (FRC) and an MWCNT-engineered FRC (nFRC) are also described. Qualitative and quantitative analyses of the results observed from experimental tests described in Sect. 2.2 are presented systematically in Sect. 2.3. The

2.2

Methodology for Determining the Influence of IFSS Out-of-Plane …

11

effects of the MWCNTs on both the fiber/matrix interface and the matrix are quantified including the main conclusions in Sect. 2.4.

2.1

Manufacturing of Fiber-Reinforced Composite

The reference material (FRC: Fiber-Reinforced Composite) consists of an epoxy resin (L20 with EPH 161 hardener from Momentive, USA) reinforced with a 220 g/m2 unidirectional E-glass fibers woven (Interglass style 92,145 from PD-Interglass, Germany). The laminate is a 10 mm thick plate with 58 plies arrangement in the same direction, i.e., [0°]58 manufactured by RTM on an aluminum-sealed mold of 250 mm × 150 mm × 10 mm cavity. The resin is transferred into the mold for resin injection at 4 bar at room temperature. The curing takes place at room temperature for 24 h and a post-curing at 100 °C for 15 h. This leads to a composite with 47.6 (3) % fiber volume fraction, 0.5 (0.1) % void volume fraction, 1.82 (0.04) g/m3 density (according to ASTM D3171-15, method 1), and 54 (4) Barcol Hardness. The MWCNTs doped composite material (nFRC: nanohybridized Fiber-Reinforced Composite) is manufactured in the same way but, in this case, the resin in the liquid state, prior to the resin transfer into the mold, is modified by mixing a carbon nanotubes masterbatch (Epocyl NC E128-02 from Nanocyl, Belgium). The masterbatch diluted in resin yields an MWCNT concentration of 0.5 wt.% in the matrix. The dilution starts with mechanical stirring for 15 min at 1500 RPM, then in an ultrasound bath at 30 °C for 15 min. Then, this blend is mixed with the hardener by mechanical stirring for 5 min at 1500 RPM and degassed in an ultrasound bath for 15 min before the resin transfer to the mold at 4 bar at 30 °C. The resulting composite has 48.0 (0.5) % fiber volume fraction, 0.5 (0.3) % void volume fraction, 1.83 (0.01) g/m3 density (according to ASTM D3171-15, method 1), and 56 (3) Barcol Hardness. The specimen geometry is a 10 mm × 10 mm × 10 mm cube as selected by Kim et al.’s study. This study concludes that for unidirectional composites there is no significant effect whether the specimen is cylindrical or cubical, nor the scalar effect in an order of magnitude. The cubes are cut with a diamond saw-cutting disc installed in a milling machine and subsequently polished. The cut surface quality was verified via optical microscopy rejecting the delaminated samples.

2.2

Methodology for Determining the Influence of IFSS Out-of-Plane Fatigue Behavior

According to Llorca et al., the dominant damage mechanisms that control compressive strength are the decohesion at the interface and shear band formation in the matrix. They found that the composite properties under transverse compression were mainly controlled

12

2 Out-of-Plane Compressive Fatigue Behavior of Unidirectional Glass …

by interface strength and the matrix yield strength in uniaxial compression. They also assigned a secondary role on the composite mechanical behavior to the matrix friction angle, the interface fracture energy, and the thermoelastic residual stresses. This information is used to design an experimental campaign to assess the influence of the MWCNTs on the composite mechanical behavior. First, the push-out test is used to evaluate the effect of the MWCNTs on the fiber–matrix interfacial shear strength (IFSS). Second, uni-axial compression tests are performed on epoxy resin and on MWCNTs doped epoxy resin in order to evaluate changes in the yield strength. These two tests take into consideration the main dominant damage mechanisms defined by Llorca et al. In addition, the effect of the MWCNTs on the matrix is evaluated for the glass transition temperature, tensile strength, tensile elastic modulus, shear strength, fracture resistance (modes 1 and 2), and tensile–tensile fatigue. Computation of interfacial shear strength It is well-known nanoparticles can modify interfacial zone and materials properties. In this line, an increase in the IFSS is generated through the use of MWCNTs to later evaluate the influence on the composite compressive out-of-plane fatigue behavior. The push-out test is selected to compute the IFSS. In this test, a diamond flat conical tip of 5 µm diameter is placed in a Hysitron TI 950 triboindenter instrument to push out the fibers out of the sample. The test is controlled in displacement at 50 nm/s. The 200 µm thickness samples were wire cut from the composite plate and then polished with a sequence of silicon carbide papers of 1000, 2000, and 4000, and finished with polishing pastes of 0.3 and 0.1 µm until the sheet thickness is in within 20–40 µm range. The samples are placed on a metallic support with a central groove to carry out the fiber push out. The IFSS is computed as follows: I F SS =

P , π · D·t

(2.1)

where P is the maximum applied load, D is the fiber diameter, and t is the sheet thickness. Testing the influence of the MWCNTs on the epoxy matrix mechanical behavior The glass transition temperature (T g ) is measured by a dynamical mechanical analysis (DMA) using a Perkin Elmer DMA 7E equipment set a temperature ramp of 3 °C/min from room temperature up to 200 °C and 1 Hz frequency. The tensile, shear, and fracture tests are performed in an Instron 8801 servohydraulic machine with a 10 kN load cell, while the compressive tests are performed in the same machine with a 100 kN load cell. The tensile test is performed according to the ASTM D 638 using a Type IV specimen shape at a cross-head speed of 5 mm/min. The compressive specimens are 10 mm × 10 mm × 20 mm at a cross-head specimen of 3 mm/min, the test follows the ASTM D695 standard. In both loading conditions, the strains during the test are computed with an optical system based on digital image correlation (Aramis, GOM

2.3

Micro- and Macro-Scale Analysis

13

Gm). The punch tool shear test (according to ASTM D732) is performed at 1.25 mm/min with 4 mm thickness specimens. The fracture toughness is measured on single-edge notch bending (SENB) specimens of 10 mm thickness. Mode 1 follows the ASTM D 5045 standards while mode 2 is performed according to Ayatollahi et al. The notch on the sample came directly from the mold and no intervention to the sample is required. Finally, tensile/tensile fatigue tests are performed at load stress ratio R = 0.67. Out-of-plane compressive fatigue testing The tests are carried out on an Instron 8801 servo-hydraulic testing machine with a 100 kN load cell. The specimens are compressed between two lubricated steel plates without a self-alignment fixture to diminish the strength measurement scatter. The stress is computed from the force measured from the load cell and the initial specimen cross section. The material strength is computed using the maximal load registered during the test. The reference quasi-static tests are performed up to rupture at 1.2 mm/min and the axial strain during deformation is measured by a 2 mm strain gauge. The compression– compression fatigue tests are performed at 5 Hz at 90, 86, 80, and 76% of the compressive strength for a stress ratio of R = 2 and at 90, 87, 80, 70, 65, and 60% of the compressive strength for a stress ratio of R = 10. Each test is repeated at least five times.

2.3

Micro- and Macro-Scale Analysis

The effect of the MWCNTs on the IFSS parameters is analysed on this section. Table 2.1 shows the average IFSS of 20 samples per material configuration. In average the IFSS increase in 12.2% with the inclusion of MWCNTs. Table 2.2 summarizes the results obtained from the analysis of to influence of the MWCNTs on the epoxy matrix mechanical behavior. It can be seen that the MWCNTs increase the compressive elastic modulus and the cycles up to failure under fatigue. On the contrary, the inclusion of MWCNTs decreases the tensile and compressive yield strength and the mode 1 fracture toughness. The observed changes on the T g , on tensile elastic modulus, on the tensile and shear strength, and on mode 2 fracture toughness are neglected. Table 2.1 Interfacial shear strength (SD: Standard deviation)

Material

IFSS MPa (SD)

FRC (reference material)

54.7 (5.0)

nFRC

61.4 (5.8)

Relative difference %

+12.2

14

2 Out-of-Plane Compressive Fatigue Behavior of Unidirectional Glass …

Table 2.2 Mechanical behavior of the neat resin and the MWCNTs doped epoxy resin Test

Variable

Neat epoxy

MWCNTs epoxy

DMA

Glass transition temperature, T g °C

114.6 (±0.5)

115.1 (±0.7)

+0.4

Tensile test

Tensile elastic modulus GPa

3.34 (±0.09)

3.37 (±0.09)

+0.9

Tensile yield strength MPa

49.5 (1.91)

46.8 (3.09)

−5.4

Tensile strength Mpa

73.7 (±0.6)

72.7 (±0.4)

−1.3

Compressive elastic modulus Gpa

3.07 (±0.06)

3.42 (±0.05)

+11.5

Compressive yield strength Mpa

61.3 (±2.9)

58.6 (5.0)

−4.4

Compression test

Relative difference (%)

Shear test

Shear strength MPa 54.0 (±1.6)

53.4 (±2.0)

−1.1

Fracture test

KIC MPa · m1/2

1.24 (±0.12)

1.19 (±0.07)

−4.0

KIIC MPa · m1/2

2.61 (±0.26)

2.57 (±0.10)

−1.5

The results for the quasi-static tests are summarized in Table 2.3 and the specimens are represented in Fig. 2.1. It can be seen that the MWCNT-doped composite experienced a rise in the measured static mechanical properties. The results for the compressive–compressive fatigue tests are plotted in terms of the compressive strength S and the number of cycles N (S − N Diagram) in Fig. 2.2. The experimental points are adjusted by linear regression and the coefficient of determination is shown. Figure 2.2a shows the results for the R = 10 case and Fig. 2.2b for R = 2. It can be remarked for R = 10 case that nFRC the stress amplitude is about 7 MPa over the FRC, while the slopes remain the same between both materials. In the case where R = 2, an increment in the stress amplitude and the slope is observed (about 20%). Table 2.3 Quasi-static compressive elastic modulus and strength (SD: Standard deviation) of the composite material

Material

Elastic modulus GPa

Strength MPa

FRC (reference material)

8.73 (0.17)

203.5 (5.3)

nFRC

9.27 (0.15)

215.2 (9.6)

Relative difference %

+6.2

+5.7

2.3

Micro- and Macro-Scale Analysis

15

Fig. 2.1 Compression specimens a before testing and b after testing. (nFRC in black)

Fig. 2.2 S − N curves. a R = 10 and b R = 2

The failure angle, as observed in Fig. 2.3, is different in terms of the loading condition but it is independent of the presence of MWCNTs. The difference can be noticed whether is static or dynamic, but remains steady in the in the cycles range under study. Figure 2.4 presents scanning electron microscopy (SEM) images of the failed surfaces after the fatigue test, where (a) is the FRC surface, while (b) belongs to the nFRC. The images put on evidence that at failure the nFRC composite has torn resin adhered at the fiber surface, while, on the FRC, the fibers seem completely unstuck from the matrix. This phenomenon is observed also in the quasi-static case.

16

2 Out-of-Plane Compressive Fatigue Behavior of Unidirectional Glass …

Fig. 2.3 Failure angle at different loading conditions

Fig. 2.4 SEM images of failed surfaces after fatigue testing. a FRC and b nFRC

The results presented in this section put on evidence an improvement in the mechanical behavior of the composite under out-of-plane compressive load. The increment is seen on the compressive elastic modulus, on the compressive strength, and on the fatigue endurance represented by the cycle’s number at two stress ratio. In addition, the failed surfaces after fatigue testing show that the nFRC has torn resin adhered to the fibers which can be due to a modification of the fiber/matrix interface as a consequence of the addition of MWCNTs.

2.4

Conclusions

An experimental micro/macromechanical campaign was performed to investigate the outof-plane fatigue, its main associated failure modes, and the influence of IFSS on the composite behavior. Using this approach, the influence of the MWCNTs on the fiber/

2.4

Conclusions

17

matrix IFSS was assessed by comparing the mechanical response of the FRC and an MWCNT-engineered nFRC under out-of-plane loadings. Push-out test shows a 12% higher value of IFSS for the nFRC. Out-of-plane static compressive tests give an increase of 6% for both the elastic module and the strength of the nFRC. Out-of-plane fatigue compressive/compressive tests at R = 2 show around a 20% increase in both the stress amplitude and the slope for nFRC. These results correlated with microanalysis of SEM images allow understanding the associated fatigue mechanisms, the influence of IFSS on out-of-plane fatigue response, and the connections between micro- and macro-level phenomena.

3

Analysis and Prediction of Failure in FRP

The study of damage has been of a great interest to different scientific communities, and several theories or approaches have been generated to predict crack initiation, propagation, and degradation of materials based on linear elastic and elastoplastic fracture mechanics. The selection of these approaches depends mainly on the characteristics of cracks, types of applied loads, and mechanical properties of the analyzed materials. These include most notably the fracture modes, cohesive zone models (CZM), energy release rate concept, stress intensity factor, and the J-integral (Gao and Fan 2020; Gzaiel et al. 2021; Hu et al. 2020; Huang et al. 2020; Jin and Sun 2012; Kang et al. 2021). UD-FRC exhibits fully orthotropic behavior. It has also been shown that NCFreinforced composites are orthotropic from an elasticity perspective (Bru et al. 2016), and from a failure perspective (Marklund et al. 2014). Accordingly, other non-reinforced composites, as is the case with pre-impregnated unidirectional sheets that are transversely isotropic, composites reinforced with unidirectional fiber fabrics and reinforced with NCFs have three orthogonal main axes along which their properties are defined. These characteristics must be considered in order to use these composites in a design process, and also failure initiation must be potentially predictable for all directions, especially in the out-of-plane direction, where these materials generally exhibit lower strength properties. Several existing failure models for composites are applied depending on the damage conditions and composite features, which include damage of elementary ply (Ladevez and Ledantec 1992), continuum damage modeling (van der Meer and Sluys 2009), elastoplastic progressive failure (Chen et al. 2012), anisotropic damage (Matzenmiller et al. 1995), and strain rate damage dependency (Lomakin et al. 2021). The use of the classical failure criteria to evaluate these composite materials considering only the transverse isotropy therefore overestimates the out-of-plane strength, and consequently could generate a non-conservative design of a part and even—depending on the case of application—a

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 V. Tuninetti et al., Fiber-Reinforced Composite Materials, Synthesis Lectures on Mechanical Engineering, https://doi.org/10.1007/978-3-031-32558-8_3

19

20

3 Analysis and Prediction of Failure in FRP

catastrophic failure during its operation. Accordingly, given the similarity in the configuration and the orthotropic behavior of materials reinforced with both unidirectional fiber fabrics and NCF-reinforced fabrics, in this chapter, the out-of-plane failure initiation in tensile is analyzed for unidirectional fiberglass fabric reinforced composites through the implementation of the failure criteria for orthotropic NCF-reinforced composites proposed by Molker et al. (2016). First, the failure criteria are programmed as a module coupled to a commercial FEM package, then tests are simulated and, finally, the results obtained are compared and validated with the respective experimental tests. The choice to develop the failure criteria as a module lies in its independence as a central module; this allows the input of stress data from another source or finite element software to evaluate failure in the modeled material.

3.1

Failure Criteria for Composite Materials

Failure criteria are used to predict the conditions under which a material fails when it is subjected to a multiaxial stress state. Tsai and Wu (1971) proposed the first failure criterion for composite materials based on the observed anisotropy, while later Hashin (1980) developed a failure criterion considering various failure modes. Subsequently, more refined criteria have been proposed based on the physical behavior of different failure modes (He et al. 2019; Zhang et al. 2021). Twelve failure criteria for theoretically and experimentally testing composite materials under multiaxial stress were presented in 2013 through the World Wide Failure Exercises II (Kaddour and Hinton 2013). In that research, two criteria were validated with more accurate results, those of Puck and Schürmann (2004) and LaRC05 (Pinho et al. 2012), both of which apply to unidirectional materials with transverse isotropy. These failure criteria generally consider three failure modes: intralaminar matrix failure by compression and tension, fiber failure by tension, and kink or fiber splitting failure by compression in the fiber direction. Certain failing criteria for NCF-reinforced orthotropic materials have been developed, including those criteria proposed in 2001 by Juhasz et al. (2001). These criteria are characterized by requiring too many variables to which the model is sensitive; consequently, the use of these criteria remains relatively uncommon. Another example of such criteria is those proposed by Molker et al. (2016), which are based on the models established by LaRC05 and operate at the layer-scale level. In the present study, the failure modes of the LaRC05 criteria are used as a basis, with two further failure modes being added in the out-of-plane direction to model the failure of NCF-reinforced composites. The first additional transverse failure mode, known as interbundle failure, is related to the stress load outside the plane and is observed at the interface between the impregnated fiber tow and the surrounding matrix. The fracture plane is perpendicular to the thickness

3.1

Failure Criteria for Composite Materials

21

direction (Bundle I). The strength for this mode is significantly lower when compared to the transverse failure mode within the fabric, known as intrabundle failure (Bundle II). For NCF-reinforced composites, the interbundle failure mode is evaluated in a fracture plane that is perpendicular to the thickness direction and acts on the matrix interface (MI). The failure initiation considers the shear strengths S T,MI , S L,MI , and the out-of-plane strength Z T . This failure index is called FI M,MI .  F I M,M I =

τT ,M I ST ,M I

2

 +

τ L,M I SL,M I

2

 +

σ N ,M I ZT

2 = 1, i f σ N ,M I > 0.

(3.1)

As shown in Eq. (3.1), the failure mode between fiber tows is only evaluated using this equation as long as the normal stress out-of-plane is positive. If a high level of shear stress outside the plane is acting on the material in combination with a low compression stress, the model assumes that interbundle failure cannot occur. For convenience, and given that the failure occurs within the matrix, per Eq. (3.1), the shear strengths S T,MI and S L,MI in Eq. (3.1) are assumed to be equal to interlaminar shear strength (ILSS). ST ,M I = SL,M I = I L SS.

(3.2)

The second additional transverse failure mode, called intrabundle failure and denoted by FI M,B , is evaluated using the LaRC05 criteria for matrix failure including in situ effects as denoted by the superscript is  F I M,B =

τT ,B STis − h T σ N ,B

2

 +

τ L,B SLis − h L σ N ,B

2

 +

σ N ,B YTis

2 = 1,

(3.3)

In Eq. (3.3), the transversal shear strength S T is based on the transverse compression strength Y C , and the fracture angle for pure transverse compression (α 0 ) as given in Eq. (3.4):   cos(α0 ) 2 ST = YC cos(α0 ) sin(α0 ) + , tan(2α0 )

(3.4)

Y T denotes the tensile strength in the plane, and the friction parameters hT and hL are related to the longitudinal shear strength S L , and to the transversal shear strength S T : hL hT = . ST SL

(3.5)

The in situ values for the strength parameters S T , S L , and Y T are obtained according to Pinho et al. (2012). Finally, the general transverse failure index FI M is determined as the highest of the FI M,MI and FI M,B indices for all potential fracture angles α for the present stress state.

22

3 Analysis and Prediction of Failure in FRP

 F I M = max F I M,M I ,

max

α=[0◦ ,180◦ ]



F I M,B



 .

(3.6)

Out-of-plane failure initiation investigated in the unidirectional fiberglass-fabric reinforced composites is based on coupled model developed comprising two parts: the first, a finite element simulation through a commercial code (FEM package), and the second, a programming part from which failure indexes are obtained for all failure mechanisms, in particular, transverse out-of-plane failure mechanisms based on the results obtained in material coordinates from the numerical simulation. The ACP module for composite materials of the calculation software ANSYS® was used as the commercial model and simulation tool for the FE simulation. Failure criteria were programmed in the MATLAB environment and required, as inputs, the computed values of the stress tensor (σ1 , σ2 , σ3 , σ23 , σ13 , and σ12 ) in each integration point of the discretized composite piece. The results are then directly exported from the FE software interface and included the location given as structural coordinates (X, Y, and Z) with the corresponding stresses given as material coordinates (1, 2, and 3). Consequently, the developed program computes the matrix FI with the different failure indexes for each failure mechanism projecting the highest failure index for the integration point.

3.2

Material Characterization

For the numerical simulation of the unidirectional fiber fabric-reinforced composite, material characterization was carried out according to ASTM D792, and ASTM D2734 standards. Table 3.1 shows the material and fabrication characteristics for the material characterization specimens. Quality control indicated an average fiber volume percentage of 52.7% for the fabricated specimens, with a standard deviation of 2.4%. Table 3.2 shows the elastic and strength properties determined for the FE model of the validation piece reinforced with unidirectional fabric. For the coupled model the ILSS and the fracture angle for pure transverse compression, α 0 , were experimentally determined by mechanical tests, which complied with ASTM Table 3.1 Manufacturing characteristics of ASTM D2584 specimens Type of fabric

Unidirectional fiberglass fabric, planar density 220 g/m2

Type of matrix

EPO 200 epoxy resin matrix with hardener 813 in a 2:1 ratio

Laminate configuration For samples N1, N2, and N3, the laminate was [90]18 ; for samples N4, N5, and N6, the laminate was [0]36 Manufacturing method

Vacuum infusion (working pressure − 1 bar)

Specimen thickness

The thickness of specimens N1, N2, and N3 was 3 mm, and 6 mm for specimens N4, N5, and N6

3.4

Computational Failure Analysis

23

Table 3.2 Elastic and mechanical properties for unidirectional glass fiber fabric-reinforced composite E1 (GPa)

E2 (GPa)

E3 (GPa)

G12 (GPa) G13 (GPa) G23 (GPa) ν12 = ν12 = 0.75 × ν23

30.47

9.59

8.39

3.98

XT (MPa) YT (MPa) YC (MPa) ZT (MPa) 478.0

49.4

−153.4

21.0

3.38

2.78

0.3

ZC (MPa) S12 (MPa) Density (g/cm3 ) −218.9

43.0

1.846

D2344/D2344M and ASTM D3410/D3410M standards, respectively. The tests indicated an average value of 26.02 MPa for the ILSS, with a standard deviation of 3.47 MPa; the fracture angle for pure transverse compression α 0 was indicated as having a value of 57.8° with a standard deviation of 2.68°.

3.3

Experimental Setup and Sample Manufacturing for Failure Model Validation

The manufacturing process of validation specimen reinforced with unidirectional fabric is performed by vacuum infusion (Fig. 3.1a) with fabrication characteristics given in Table 3.3. The experimental configuration and the geometry are shown in Fig. 3.1b and c. Note that the geometry represents a section of a corrugated panel of the composite material Molker et al. (2017).

3.4

Computational Failure Analysis

NCF-reinforced composite orthotropic failure criteria were programmed as a subroutine, called from the main program with flow charts shown in Fig. 3.2. The ANSYS® ACP software module for composite materials was used as the simulation tool for the FE simulation. The model for the validation sample (Fig. 3.1c) was discretized with 264,040 hexahedral elements, with one element equaling the thickness of the tested fabric (Fig. 3.3a). The element choice relies on the reduced number of elements for the same volume of nodes, compared to tetrahedral elements. In addition, they allow for a more structured meshing with acceptable quality parameters and more accurate results. The quality of the mesh was verified by means of the following parameters: aspect ratio, Jacobian ratio, and skewness with values of 1.8, 1.0, and 0.009, respectively, giving a global elements quality of 0.9. The load on the validation piece was applied through a rigid horizontal surface at the top of the sample by frictionless contact. The fixation of the model was of the sliding type and located at the base of the piece. The model was solved as a static load case with the option of large deformations.

24

3 Analysis and Prediction of Failure in FRP

Fig. 3.1 a Manufacturing process of validation specimens. b Validation test configuration. c Geometry of validation specimen and boundary conditions for the FE simulation Table 3.3 Characteristic of the test specimen for experimental and numerical tests

Type of fabric

Unidirectional fiberglass fabric; planar density 220 g/m2

Type of matrix

EPO 200 epoxy resin matrix with hardener 813 at a 2:1 ratio

Laminate configuration

[90]18

Manufacturing method

Vacuum infusion with 1 bar of working pressure

Specimen thickness

3 mm

Post-curing

16 h at 70 °C

Test speed

1 mm/min (quasi-static)

3.4

Computational Failure Analysis

Fig. 3.2 Flow chart of the main program (left), and failure subroutine (right)

25

26

3 Analysis and Prediction of Failure in FRP

Fig. 3.3 a Isometric view of the simulated fiberglass sample. b Sample loaded at 790 N showing the maximum stresses predicted in the transverse out-of-plane direction (direction 3)

For experimental validation, five tests were performed using the manufactured specimens. The results indicate an average critical load or average transverse failure initiation load of 676.4 N with a standard deviation of 63.5 N. The occurrence and location of damage can be detected by measuring the strain ratios by fiber optic strain sensors based on Bragg gratings (Matveenko et al. 2021); however, in the present work, an optical camera recorded multiple images during testing and the onset of failure was determined for the moments that cracks were clearly recognizable to the human eye. The finite element simulation results showed that the highest stresses in the transverse out-of-plane direction for the applied critical load are at approximately 19° with respect to the vertical, in the central zone of the thickness of the composite, and that they reach a maximum value of 17.8 MPa (Fig. 3.3b). The coupled model developed in the current study indicated an average critical load or load of transverse failure initiation due to outof-plane stresses (interbundle failure) of 790 N for the analyzed validation piece (Fig. 3.4). For validation of the coupled model for unidirectional fabric, the results indicated that both numerical modeling and experiments show failure initiation to be mainly attributable to out-of-plane stresses (interbundle failure); these stresses are at an approximate angle of 20° with respect to the vertical central line and in the central zone of the cross section of the validation specimen (Fig. 3.5). Nevertheless, the values of the critical load, i.e., the failure initiation load obtained by the coupled model and the experimental validation differ by 16.8%. The results of the critical load of failure initiation obtained for the validation specimen for different commonly used failure criteria are presented in Table 3.4. The failure criteria for NCF-reinforced composites Molker et al. (2016) into unidirectional fiber fabric-reinforced composites compared with the experimental data shows a 17% error in the failure load prediction. Since the Molker criterion is based on LaRC05, the code developed here was modified to consider transverse isotropy, and thus to obtain the critical load for failure initiation. The results of critical load for the other failure criteria,

3.4

Computational Failure Analysis

27

Fig. 3.4 Results obtained by the coupled model developed for the analyzed piece showing the transversal failure initiation

Fig. 3.5 Validation samples at failure. a Failure location in the half of the thickness of the specimen (t/2) and b several failure initiation points

presented in Table 3.4, were obtained directly from the ACP (Post) module for composites of ANSYS® software. The experimental validation results of the coupled failure model, when applied to unidirectional fiberglass fabric reinforced composites, indicate that although the failure initiation both for the numerical and experimental results was mainly due to out-of-plane stresses (interbundle failure), and despite their similar geometrical location (Fig. 3.5), their values differ by 16.8% with respect to the critical load. Analysis of the results indicates that this small discrepancy may be due to the configuration difference between NCFs and unidirectional fiber fabrics. The unidirectional fiber tows are joined by the fiber tows of fewer filaments (unidirectional fiber fabrics) rather than by stitching yarn (NCF). This may cause additional stress concentrators that would justify the decrease in strength seen in the experimental tests with respect to the numerical tests. The results presented in

28

3 Analysis and Prediction of Failure in FRP

Table 3.4 Critical load results for different failure criteria and prediction errors compared with the measured value of 676.4 N Failure criteria

Critical initial failure load in the Load prediction percentage validation specimen (N) error (%)

Tsai and Wu (1971)

1100

62.6

Hashin (1980)

1290

90.7

Puck and Schürmann (2004)

1200

77.4

LaRC05 (Pinho et al. 2012)

1580

133.5

Implemented Molker et al. (2016)

790

16.8

Table 3.4 indicate that Molker’s criteria overestimate the out-of-plane strength in unidirectional fiber fabric-reinforced composites. However, compared to the criteria applicable today that include transverse isotropy, the error found is much smaller. This demonstrates the importance of considering the orthotropy of the materials, especially when failure is analyzed in the out-of-plane direction.

3.5

Conclusions

The orthotropic failure criterion proposed by Molker and programmed in MATLAB as a module and was coupled to a constitutive model available in a FEM package using the stress tensor. In addition, the characterization of specimens reinforced with unidirectional fiber fabric was carried out and the necessary parameters for Molker’s orthotropic failure criterion were determined. The experimental validation of the model performed on the geometry proposed by Molker to generate an out-of-plane failure of the material showed that the failure initiation was due to out-of-plane stresses (interbundle failure) and in a similar geometrical location. However, the critical load value found by the coupled model was 16.8% higher than the experimental critical load value. Additionally, a comparison of Molker’s criterion was performed with different established criteria that consider transverse isotropy for the same experimental setup, showing a 60% overestimation of the out-of-plane strength of the material. Finally, this study demonstrates that the use of Molker’s criteria developed for non-crimp fabric-reinforced materials offers a significant improvement in predicting out-of-plane failure in unidirectional fiber fabric-reinforced materials compared to commonly used conventional criteria.

References

29

References Bru T, Hellström P, Gutkin R, Ramantani D, Peterson G (2016) Characterisation of the mechanical and fracture properties of a uni-weave carbon fibre/epoxy non-crimp fabric composite. Data Brief 6:680–695 Chen JF, Morozov EV, Shankar K (2012) A Combined elastoplastic damage model for progressive failure analysis of composite materials and structures. Compos Struct 94(12):3478–3489 Gao R, Fan S (2020) Research on the propagation characteristics of fatigue cracks on rail surfaces. Int J Appl Mech 12(10):2050121 Gzaiel M, Triki E, Barkaoui A, Chafra M (2021) Finite element study of mixed fracture: velocitydependent insertion of pointed blades into soft material. Int J Appl Mech 13(01):2150003 Hashin Z (1980) Failure criteria for unidirectional fiber composites. J Appl Mech 47(2):329–334 He G, Liu Y, Bammann DJ, Francis DK, Chandler MQ, Horstemeyer MF (2019) A multiphase internal state variable model with rate equations for predicting elastothermoviscoplasticity and damage of fiber-reinforced polymer composites. Acta Mech 230(5):1745–1780 Hu J, Ji C, Chen S, Li S, Wang B, Zhou Z (2020) Novel mathematical-statistical models for the distribution of fatigue life and residual strength for fiber reinforced polymer composites. Int J Appl Mech 12(09):2050104 Huang J, Li J, Pan X, Xie T, Hua W, Dong S (2020) Numerical investigation on mixed mode (I-II) fracture propagation of CCBD specimens under confining pressure. Int J Appl Mech 12(10):2050111 Jin Z, Sun C (2012) Fracture mechanics. Academic Press, New Delhi Juhasz J, Rolfes R, Rohwer K (2001) A new strength model for application of a physically based failure criterion to orthogonal 3D fiber reinforced plastics. Compos Sci Technol 61(13):1821– 1832 Kaddour A, Hinton M (2013) Maturity of 3D failure criteria for fibre-reinforced composites: comparison between theories and experiments: Part B of WWFE-II. J Compos Mater 47(6–7):925– 966. https://doi.org/10.1177/0021998313478710 Kang J, Liu S, Wang C (2021) Drying-induced pressure rise and fracture mechanics modeling of the sphagnum Ladevez P, Ledantec E (1992) Damage modelling of the elementary ply for laminated composites. Compos Sci Technol 43(3):257–267 Lomakin E, Fedulov B, Fedorenko A (2021) Strain rate influence on hardening and damage characteristics of composite materials. Acta Mech 232(5):1875–1887 Marklund E, Asp LE, Olsson R (2014) Transverse strength of unidirectional non-crimp fabric composites: multiscale modelling. Compos B Eng 65:47–56 Matveenko V, Kosheleva N, Serovaev G (2021) Damage detection in materials based on strain measurements. Acta Mech 232(5):1841–1851 Matzenmiller A, Lubliner J, Taylor RL (1995) A constitutive model for anisotropic damage in fibercomposites. Mech Mater 20(2):125–152 Molker H, Wilhelmsson D, Gutkin R, Asp LE (2016) Orthotropic criteria for transverse failure of non-crimp fabric-reinforced composites. J Compos Mater 50(18):2445–2458 Molker H, Gutkin R, Asp LE (2017) Implementation of failure criteria for transverse failure of orthotropic non-crimp fabric composite materials. Compos A Appl Sci Manuf 92:158–166 Pinho S, Darvizeh R, Robinson P, Schuecker C, Camanho P (2012) Material and structural response of polymer-matrix fibre-reinforced composites. J Compos Mater 46(19–20):2313–2341

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3 Analysis and Prediction of Failure in FRP

Puck A, Schürmann H (2004) Failure analysis of FRP laminates by means of physically based phenomenological models. In: Hinton MJ, Kaddour AS, Soden PD (Eds) Failure criteria in fibre-reinforced-polymer composites. Elsevier Ltd., pp 264–297 Tsai SW, Wu EM (1971) A general theory of strength for anisotropic materials. J Compos Mater 5(1):58–80 van der Meer FP, Sluys LJ (2009) Continuum models for the analysis of progressive failure in composite laminates. J Compos Mater 43(20):2131–2156 Zhang Y, Ping X, Wang C, Xiao Z, Yang J, Chen M (2021) A new computational approach for threedimensional singular stress analysis of interface voids. Acta Mech 232(2):639–660

4

Impact of the Multiwall Carbon Nanotubes on the Transverse Compressive Strength and Damage

The composite material properties, which are directly related to the matrix and the reinforcement, are in general significantly weaker in the thickness direction than in the lamina plane directions. Several studies focused on the prediction of transverse crack formation in FRCs (Huchette et al. 2015; LLorca et al. 2011; Raimondo et al. 2010; Valdivia et al. 2021) demonstrate that once the material damage process is initiated, cracks extend through the matrix due to shear stresses, and subsequently propagate through the laminates by intralaminar damage, generating the final failure of the composite upon reaching a critical level of damage. Damage modeling in FRCs requires the prediction of the different coexisting failure modes, such as fiber–matrix interface decohesion, matrix cracking, delamination, and fiber cracking (Burgarella et al. 2019; Kästner et al. 2016; Maragoni and Talreja 2019; Prasad et al. 2016). In particular, the transverse tensile strength is governed by the fiber–matrix decohesion interface failure, which leads to the formation of cracks perpendicular to the loading axis. By contrast, the resistance to transverse compression is dominated by the formation of a shear band of plastic deformation located in the matrix. To better test and understand this shear effect, virtual modeling is presented as an innovative and powerful strategy (González and LLorca 2007; Naya et al. 2017; Totry et al. 2008; Zhuang et al. 2018a, b); however, this strategy depends largely on the prediction accuracy of the constitutive models of the material components. In general, these models can be characterized from experimental mechanical testing, and assessed by comparing the prediction of the macroscopic behavior obtained from a representative volume element (RVE) with different experimental conditions. Recent studies have shown a good correlation between the predictions and the experiments (Burgarella et al. 2019; Chevalier et al. 2019; Sun and Hallett 2018); however, discrepancies have been observed between the model of the interface and actual damage. Chevalier et al. (2019) performed an in situ study of the decohesion phenomena in a composite material and compared the

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 V. Tuninetti et al., Fiber-Reinforced Composite Materials, Synthesis Lectures on Mechanical Engineering, https://doi.org/10.1007/978-3-031-32558-8_4

31

32

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

digital image correlation (DIC) measurements with the results obtained from the constitutive models available in the literature, concluding that this behavior could not be predicted by the considered laws, and adding that new experimental techniques should be improved or developed to accurately characterize the interface properties in the transverse direction. Burgarella et al. (2019) and Salavatian and Smith (2014) proposed that cracks produced by transverse loads must pass through the plies and, therefore, may initiate a micro-delay affecting their connections. These approaches imply the challenge of understanding the microscale behavior of FRCs and require comprehensive experimental studies to determine the transverse properties of the composite laminates. To increase the out-of-plane mechanical resistance of composite materials, the properties of both the matrix and fiber–matrix interface should be improved. Several authors (Bilisik et al. 2020; Chandrasekaran et al. 2011; Kamarian et al. 2020; Ma et al. 2015; Medina et al. 2017) considered adding carbon nanoparticles to composite materials and concluded that this approach improves the properties of the fiber–matrix interface and toughness of the matrix. For instance, Boroujeni and Al-Haik (2019) obtained 20% and 50% increases in tensile strength and fatigue life, respectively, of a carbon FRC with carbon nanotubes (CNTs) compared with the original material. Zhou et al. (2014) assessed the addition of 2 wt.% of carbon nanofibers in the epoxy matrix, obtaining around 15% higher values of the compressive strength, through-hole compressive strength, and interlaminar shear strength. Improvements in damage resistance have also been reported (Grabowski et al. 2017; Han et al. 2018); however, the matrix material may not always exhibit the same performance with the addition of CNTs; low-stiffness resins benefit noticeably, while high-stiffness resins are minimally affected (Ci and Bai 2006). In addition, the agglomeration and concentration of CNTs in the epoxy resin directly affect the improvement of mechanical properties, e.g., epoxy–CNT mixtures exhibit an increased viscosity, which can cause issues during FRC manufacturing with resin transfer molding (RTM) or vacuum resin injection molding (VRIM) due to reduced resin–fiber interactions. According to this agglomeration phenomenon, several authors have reported that 0.5% weight of CNTs dispersed in the epoxy matrix provides optimal strength and that higher concentrations are associated with a decrease in resistance compared to the conventional for fiber-reinforced plastic (FRP) (Ayatollahi et al. 2017; Han et al. 2018; Moghimi Monfared et al. 2018). The incorporation of single-wall (SW) CNTs has been studied and variation has also been reported in the composite mechanical properties such as Young’s modulus and tensile strength with 0.3 optimal wt. percentage by Maghsoudlou et al. (2019). The interfacial shear strength (IFSS) is a function of physicochemical interactions between the surface of additives and the epoxy resin. It provides the resistance of the fiber–matrix interface and its measurement is performed using a variety of tests and configurations (Li et al. 2019), e.g., the push-out nanoindentation test, microdroplet, the single fiber fragmentation test, and the Ioscipescu test. González and LLorca (2007) show that the increase in IFSS directly influences the transverse compressive strength. Medina et al.

4.1

Investigated FRC Engineered with MWCNT

33

(2017) incorporated a master batch of MWCNTs in a glass FRC and improved the IFSS by 24%. Later, Bedi et al. (2018) found an increase in the IFSS by grafting CNTs onto the surface of carbon fiber. Godara et al. (2010) determined the properties of the fiber–matrix interface for different methods applied to incorporate the CNTs in the FRC and concluded that significant improvement of IFSS can be achieved in composites modified with CNTs. The aforementioned finding was later confirmed by Ma and Zhang (2014) and Qian et al. (2021), particularly when depositing functionalized CNTs for uniform distribution and stronger bonding between the epoxy matrix and CNTs. In the present chapter, the effect of adding multiwall (MW) CNTs to a glass FRC is examined, focusing on mechanical compressive strength and damage resistance in the out-of-plane and transverse direction. The monotonic and cyclic compression tests performed make it possible to determine the crack density and the elastic modulus degradation for damage evolution characterization of both investigated materials: FRC and FRC reinforced with 0.5 wt% MWCNTs.

4.1

Investigated FRC Engineered with MWCNT

The materials used in this study are a unidirectional E-type glass fiber (Interglass 92415, FK 144, R&G GmbH, Germany) of planar density 220 g/m2 with a configuration of 95% warp and 5% weft, Bisphenol-A/F epoxy resin (YD-114F, KUKDO CHEMICAL CO, Korea), hardener (KH-813, KUKDO CHEMICAL CO., Korea) with a 2:1 weight ratio of resin to hardener, and a masterbatch with a 20% dilution of MWCNTs (NanoCyl NC E128-02, Belgium). A 190 mm × 100 mm plate is manufactured with the laminate [0]58 so that the volume fraction of fiber v f equals 50%. Resin transfer molding (RTM) is selected as the manufacturing process. Fifty-eight unidirectional fiberglass fabrics are added to the previously waxed rectangular mold, then the resin is introduced into a chamber, and subsequently the mold inlet is connected at pressures of 1–5 bar for 30–40 min. For CNT-based FRC, the nanotube masterbatch is diluted in the resin and sonicated until fully dispersed, followed by the addition of the hardener (Fig. 4.1a). Quality control was performed according to ASTM D 792 and ASTM D 2584, with properties given in Table 4.1. To ensure the required geometric tolerances of the specimens, the cutting process was performed with a computer numerical control (CNC) machine and ten flutes flat milling cutter (Amana) with a diameter of 1/8 inch. The parameters used include a spindle speed of 10,000 rpm, a feed rate of 2000 mm/min, and a depth of cut of 1 mm per pass. All samples are cooled with water. The composite plate and cutting path are shown in Fig. 4.1b. The sanding and polishing of each sample were done with a Minitech 250 SP1 (Hagen, Germany) using sandpaper starting at 320 grit up to 1200 grit, then using a 3-micron polishing compound and 1-micron alumina powder, the step-by-step shown in Fig. 4.1c. The microstructure was captured with an Optika microscope (Bergamo, Italy).

34

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

Fig. 4.1 Manufacturing process, geometry, and configuration of the compression test samples. a RTM process with 10 mm thickness composite, b Composite plate with toolpath cutting, c Example of sanding/polishing samples of 10 × 10 × 10 mm, and d Compressive load through-thickness direction and perpendicular-to-laminate direction

Table 4.1 Properties of the composite material Matrix

Volume fraction of fiber ν f (%)

Density (g/ cm2 )

Reinforcement

Laminate

Thickness (mm)

Epoxy

49.39 ± 1.42

1.86 ± 0.012

Glass fiber UD

[0]58

10

Epoxy + 0.5% CNT

49.55 ± 2.26

1.83 ± 0.076

4.2

Experimental Monotonic and Cyclic Compression Testing

The compressive properties were examined with cubic specimens shown in Fig. 4.1a. Two directions were considered: 0° (through-thickness direction, ethickness ) and 90° (perpendicular-to-laminate direction, eweft ). The compression test procedure was based on ASTM D695. To determine the maximum compressive strength σc,max and elastic modulus E c,0 , five monotonic compression tests were performed for each sample configuration (loading directions 0° and 90°) and for composite materials of CNT-based and CNT-free matrices. The tests were carried out at a speed of 1 mm/min. A Teflon film was applied on the sample surface in contact with the compression plates, and the deformation was measured with a strain gauge 2 mm in length and a resistance of 120 . For both static and cyclic tests, a Zwick/Roell machine with a load cell of 100 kN was used. Figure 4.1b and c show the configuration of the specimen for each test. For cyclic compression tests, the load cycles were performed by controlling the displacement of the head in increments of 0.05 mm until fracture and at a crosshead speed of 1 mm/min, neglecting the small variation of the strain rate and its effect on the mechanical behavior (Tuninetti et al. 2020).

4.3

4.3

Microcracking Analysis and Characterization of Material Degradation

35

Microcracking Analysis and Characterization of Material Degradation

The damage evolution is determined by the calculation of degradation of the elastic modulus. Different strategies can be applied to determine the elastic modulus during each load cycle according to the particular case. Figure 4.2a illustrates the strategy proposed by Wuang et al. (2019), while the methodology applied hereafter is according to Bru et al. (2017) and Garoz et al. (2017) depicted in Fig. 4.2b and c. The crack density ρcrack measurements are performed according to the Burgarella approach (Burgarella et al. 2019) with the assumption of negligible crack width (i.e., aspect ratio equal to 0). In a micrograph analysis of an FRC, four types of composite failures as depicted in Fig. 4.3 can be quantified by measuring and tracking the approximate length of each crack (Eq. 4.1). n L g,i , (4.1) ρcrack = i=1 As where As is the area of measurement, n is the number of cracks, and L g,i is the length of visible cracks. It is worth noting that in order to obtain a representative measurement of the crack density, only the effective cracks are quantified with n. Figure 4.3, for instance, shows only two effective or visibly propagating cracks: L g,1 and L g,2 . Figure 4.4 graphs the experimental stress–strain relationship of the samples under monotonic compression, and Table 4.2 provides the computed initial elastic moduli and maximum initial stress resistance for each loading configuration. According to the results, the specimens including CNT showed an improvement in their mechanical properties for the through-thickness direction (0°), with an increase of 11% and 13% in the initial elastic modulus E c,0 and maximum stress resistance σc,max , respectively. In addition, for the 90° direction, no effect of the CNT was found on the macroscopic mechanical behavior of the FRC. This phenomenon can be attributed to the higher rigidity of the material in the 90° direction compared to the 0° direction. This is consistent with the results reported in

Fig. 4.2 Different strategies for computation of initial and current elastic moduli

36

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

Fig. 4.3 Failures present in damage FRC and methodology for crack measurement

Ci and Bai (2006) where the addition of nanoparticles in high-stiffness resins had negligible effects on the strength of the material. Additionally, from the stress–strain results, anisotropic mechanical behavior of the composite material in the transverse direction is also found (San Juan et al. 2016). The stress–strain behavior of the tested materials under cyclic compression is shown in Fig. 4.5. The influence of CNTs on the mechanical behavior of the material shows the same trend as in the monotonic tests, with an increased ultimate strength and elastic modulus. This effect is more noticeable in specimens loaded at 0°. The high stiffness of the material in the 90° direction yields a maximum deformation 50% lower than in the 0°

(a)

(b)

Fig. 4.4 Stress versus strain curves in the monotonic transverse compression tests. Compression a through-thickness and b perpendicular-to-laminate direction

4.3

Microcracking Analysis and Characterization of Material Degradation

37

Table 4.2 Initial elastic moduli and compression resistance of the investigated composite materials in monotonic compression tests Compressive loading direction 0°

90° FRC + CNT

FRC

Variation (%)

FRC

FRC + CNT

Variation (%)

E c,0

(GPa)

7.05 ± 0.59

7.84 ± 0.52

+11.2

11.34 ± 0.64

11.74 ± 0.49

+3.5

σc,0

(MPa)

120.0 ± 10.01

135.63 ± 9.46

+13.1

112.16 ± 11.36

109.93 ± 7.87

−2.0

direction. The addition of nanotubes in fatigue applications indicates a positive effect of increasing the service life of the composites, a phenomenon attributed to the fact that the CNTs act as energy dissipators, limiting the propagation of cracks in the matrix and at the fiber–matrix interface (Chang 2010; Jangam et al. 2018). In the present study, damage is induced to the composite at low loading cycles. Similar trends to fatigue studies with a decreasing effect of damage in both loading conditions investigated are characterized hereafter. The results of the cyclic tests were used to determine the damage evolution through the degradation of the elastic modulus in each cycle based on the methodology proposed by Bru et al. (2017). This methodology was finally applied due to the similarity of the behavior found in the cyclic test curves with those reported in the reference. Figure 4.6 shows the results obtained for each direction and type of specimen. The mathematical expression for calculating the evolution of the elastic modulus (E c ) and characterizing damage (Eq. 4.2) was adapted from Medina et al. (2014)

(a)

(b)

Fig. 4.5 Representative curves of stress versus strain for the cyclic transverse compression tests. a Out-of-plane and b transverse directions

38

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

(a)

(b)

Fig. 4.6 Damage evolution in load–unload compression tests at low cycles

E c (ε p ) = E c,sat + (E c,0 − E c,sat )exp(−Cε p ),

(4.2)

where E c,sat is the saturation level, ε p is the accumulated plastic deformation per cycle, and C is the damage rate parameter. The values of the parameters for each type of specimen and loading direction are given in Table 4.3. The stagnation of elastic modulus degradation is reached at a lower level of plastic deformation for specimens loaded at 0° compared to specimens subjected to 90° loading. Moreover, specimens with CNTs show a higher damage rate (C) and earlier saturation than specimens without CNTs. These results of the elastic modulus degradation agree with other reported studies (Audibert et al. 2018; Bru et al. 2017; Medina et al. 2014). Although the loading conditions differ from those of the present work (pure shear, bending, and cyclic tension), the general behavior follows the same pattern: degradation occurs exponentially and saturates at a certain level of plastic deformation. Figure 4.7 shows the failures occurring in the composites during the monotonic tests. In specimens loaded at 0°, a typical failure due to shear stress is found, with a resulting crack at a failure angle close to 45°. In addition, specimens loaded at 90° show an increase in the failure angle with a value close to 50°. The increase in failure angle is assumed to Table 4.3 Characteristic parameters for the degradation of the elastic modulus 0°

90°

FRC

FRC + CNTs

FRC

FRC + CNTs

E c,0

(GPa)

7.05

8.75

11.34

11.02

E c,sat

(GPa)

4.63

6.24

5.77

7.26

C

12

24

8

11

R2

0.903

0.871

0.894

0.903

4.3

Microcracking Analysis and Characterization of Material Degradation

39

be the result of the CNTs that modify the internal friction of the material, delaying crack propagation. A summary of these results is provided in Table 4.4. In the specimens loaded at 90°, two failures are observed: one is shear stress-induced (90° failure type 1) and the other was a V-crack failure (90° failure type 2), with the latter having a higher resulting failure angle. These observations imply that more damage mechanisms affect the material behavior in this direction. Opelt et al. (2018) identified failure modes in compression specimens oriented at 90° and obtained results similar to those of the present study. Another important consideration affecting the failure response in the composites is the fibers supporting the laminate fabric, which are oriented in the weft direction. San Juan et al. (2016) also investigated this factor and concluded that a unidirectional laminate is not isotropic in its transverse direction. The resulting composite failure during cyclic transverse compression tests at 0° and 90° loading directions are shown in Fig. 4.8 and Fig. 4.9, respectively. The 0° specimens with CNT-free matrix had more failures along the fiber–matrix interface. On the other hand, specimens with CNTs showed improved resistance to the interface crack formation

Fig. 4.7 Failure planes at the macroscopic level during cyclic compression

Table 4.4 Comparison of failure angles at the macroscopic level

Loading direction

Failure angle (°) FRC

FRC + CNTs

0°

43 ± 6

48 ± 8

90°

50 ± 9

52 ± 11

40

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

during the load cycles (Fig. 4.8). This is consistent with the hypothesis and previously reported results given in the introduction of this chapter, and could be explained as follows: The compressive deformations in the loading directions produce transverse positive strains of constituents, the magnitude of which is governed by the Poisson’s coefficient. As the composite components differ in properties, with Poisson’s coefficient being lower for the fiber (Shan and Chou 1995; Zhuang et al. 2018a, b), differential transverse deformation generates a shear stress locally at the matrix–fiber interface. With the addition of CNTs, a proportion of these nanoaggregates localize in the matrix during resin curing, while others embed at the fiber–matrix interface, strengthening the bond and delaying or limiting failure mechanisms during loading, such as debonding, matrix cracking, and interface and intralaminar crack propagation. From the previous study reported by Medina et al., the IFSS interfacial strength indicated an increase of about 24%. This increase is also observed in the failure mechanisms induced by cyclic loading, where the composite material with CNTs shows a reduction of debonding and crack propagation. In the 90° loaded composite specimens, the failure occurs immediately in the first load cycle by forming very visible cracks (Fig. 4.9). In this case, the cracking phenomena are dominated by intralaminar failure (interphase or matrix crack). Here, the crack follows

Fig. 4.8 Comparison of failures under cyclic transverse compression at 0°

4.3

Microcracking Analysis and Characterization of Material Degradation

41

Fig. 4.9 Comparison of failures under cyclic transverse compression at 90°

a parallel path with the loading direction, causing longitudinal cracks along each strand that propagates between laminates until final failure. To quantify the effect of CNTs on the composite failure resistance, the crack density vs. stress level is determined. The results shown in Fig. 4.10 validate the conclusions drawn from the image analysis related to the reduction in crack propagation in specimens with CNTs. The experimental evolution of crack density over compressive stress is described by the model given in Eq. (4.3) with its respective identified parameters (Table 4.5). This damage model only provides a trend of crack density with the compressive stress value. The prediction accuracy of this phenomenological damage model is adjusted within the error bars of the experimental data, which makes it possible to conclude that the proposed model accurately predicts crack density. However, other studies including local variables or phenomena, for instance, interlaminar and intralaminar shear strength-based models with intrabundle failure are required for a more accurate modeling (Valdivia et al. 2021). ρcrack (σc ) = Ad exp(Bd σc ).

(4.3)

From data shown in Fig. 4.10 and Table 4.5, it is noted that CNTs reduce the crack propagation for both the 0° and 90° loading directions. In addition, the efficiency of CNT

42

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

Fig. 4.10 Evolution of the crack density in a out-of-plane and b transverse compression. c Comparison of model prediction for the investigated materials and loading directions. The different stages of local failure phenomena as a function of crack density for d 0° and e 90° loadings Table 4.5 Parameters identified for the crack density evolution model 0°

Ad

90°

FRC

FRC + CNT

Reduction (%)

FRC

FRC + CNT

Reduction (%)

6.54E−6

4.37E−7

−93%

8.43E−4

1.041E−4

−87%

+30%

0.024

0.042

+75%

0.92

0.98

Bd

0.057

0.076

R2

0.99

0.99

Reduction ρcrack (%)

30 [6–42]%

41 [2–54]%

4.4

Conclusions

43

on the cracking behavior at 90° is also dependent on the compressive stress, and at stresse levels higher than 100 MPa, the effect of the presence of CNT is reduced compared to its effect at lower stress levels. The reduced fiber–matrix decohesion for the 0° direction observed from the image analysis is confirmed by the overall 30% reduction in crack propagation for the specimens with CNTs. This reduction at 0° directions attributed to carbon nanotubes could be considered negligible if the error bars obtained in the determination of crack densities for each type of material were included in the analysis, as the scatter is generally associated with the manufacturing process and the uncertainties or stochastic nature of the constituent materials (Toft et al. 2013). The effect of the CNTs is most noticeable in the 90° loaded specimens, with a reduced crack propagation by up to 40%. This differential effect of CNTs on the crack density and damage as a function of stress level can be correlated with the failure mechanism depending on the compressive loading directions. The 0° loaded samples mainly fail by interphase debonding, while 90° loaded specimens exhibit matrix or interphase cracking. Therefore, these observations provide evidence that stronger links between the matrix–fiber interphase at intralaminar levels occur by increasing the resistance of the composite under transversal compressive loadings. These results are in good agreement with the evolution of the elastic moduli from the macroscale measurements, indicating that the CNTs improve the mechanical properties in the evaluated transversal loading directions. Figure 4.10d and e schematically show the difference between the types of failure, distinguishing two characteristic points: failure initiation (above the yield point) and crack propagation. As previously noted, the main difference between specimens loaded at 0° and 90° is in the nature of the failure initiation. The 0° specimens predominantly present debonding and microcracks, which accumulate up to a critical level of damage; here, the effect of the nanotubes is much more important since increasing the IFSS interfacial strength enhances the resistance to debonding. In this condition, the effect of CNTs is significant as the specimens resist or withstand more microcrack accumulations (this is seen in a near horizontal slope between 30 and 60 MPa). CNTs increase this resistance up to about 80 MPa, decreasing the slope and reducing the propagation speed. On the other hand, the 90° specimens do not show accumulation of debonding damage, but cracks in the matrix and within the strands. For this condition, crack formation follows the same direction of the applied load and propagates more rapidly, with the effect of the CNT being a reduced crack propagation speed. The CNT effect does not considerably improve the slope between 15 and 60 MPa.

4.4

Conclusions

In this chapter, the effect of adding 0.5 wt% CNT in the epoxy matrix of a unidirectional glass FRC fabricated by resin transfer molding was experimentally assessed by transverse and out-of-plane monotonic and cyclic compressive loadings. The degradation

44

4 Impact of the Multiwall Carbon Nanotubes on the Transverse …

properties such as elastic modulus, strength, crack density, and crack propagation have been determined and analyzed, yielding the following findings: • Including 0.5% of CNTs in the glass matrix of a glass FRC produces an increase of 11% in the initial elastic modulus and 13% in the compressive strength for transverse 0° direction. • The stagnation of elastic modulus degradation is reached at a lower plastic deformation for composites with 0.5% CNTs. This was because the CNTs improved the mechanical properties of the matrix and fiber-matrix interface. The cumulative plastic deformation was also lower in the specimens with CNTs: a 20% reduction in the 0° direction and almost 30% reduction in the 90° direction. • At the microscopic scale, image analysis showed that the CNTs improved the properties of the interface; particularly in the 0° loading direction, where minor decohesion failure was observed. This result was confirmed by a 30% reduction in computed crack density. • Image analysis in the 90° loading direction shows failure dominated by microcracks formed by intralaminar failure with no decohesion of the interface. The CNTs here present stronger effect on the crack density with a reduction of 41%. This delay in the crack propagation is confirmed by the increase in the failure angle, assumed to be the product of the variation of internal friction of the material caused by the addition of CNTs. The proposed damage model for crack density prediction as a function of stress level for low cyclic loads shows acceptable accuracy; however, more advanced models including the identified failure phenomena and the different composite directions is a current interest. Finally, it is worth mentioning that research progress on damage properties and decohesion is still need it, and statistical techniques to describe all possible failure scenarios are required for the formulation and identification of a constitutive law or adequate material model for multi-objective application conditions.

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5

Mechanical Performance of FRC Manufactured from Recycled Carbon Fibers with Grown CNTs

This chapter presents the impact of the addition of carbon nanotubes (CNTs) on the interlaminated resistance of recycled fibers. The recycling process of the composite material is applied here by pyrolysis to obtain the carbon fibers followed by a growing of CNTs over their surfaces using the Poptube technique. The global demand for products manufactured of polymer matrix composite materials has increased during recent years, together with the increase in waste generated at the end of the product lifetime. Besides, the recycling of polymer matrix composites is a complex process because they are composed of two or more materials, usually a matrix and fibers, both of different characteristics and properties. The processes that exist today are focused on recovering the fiber either chemically or thermally, or crushing the entire material, known as mechanical recycling (Dauguet et al. 2015; Pickering 2006). Chemical and thermal recycling, which seek to recover matrix fibers such as hydrolysis, pyrolysis, or fluidized bed process (Oliveux et al. 2015; Pickering 2006; Pimenta and Pinho 2011), have the potential to reuse the fibers to make a new part, while mechanical recycling only allows the crushed material to be used as reinforcement for other parts. The processes that are focused on recovering the fibers are more interesting from the point of view of recycling and subsequent reuse of the fibers, however, they present certain problems. Both thermal and chemical recyclings have been shown to generally decrease the mechanical properties of the fibers, by up to 85% in certain cases (Jiang et al. 2009; Kim et al. 2017; Oliveux et al. 2015; Pickering 2006). The decrease in properties, in general, is attributed to the interface, which can be affected by carbonization, matrix residues, or oxidation in the fibers (Meyer et al. 2009). Besides, it is possible that during the recycling process, due to the use of high temperatures, the fiber sizing is lost, as well as the protective coating of the fibers during handling, transport, and manufacturing, which at the same time improves the interface between fiber and matrix (Paipetis and Galiotis 1996; Yumitori et al. 1994).

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 V. Tuninetti et al., Fiber-Reinforced Composite Materials, Synthesis Lectures on Mechanical Engineering, https://doi.org/10.1007/978-3-031-32558-8_5

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Fig. 5.1 POPTUBE process scheme, decomposition of a carbon source, and growth of microwaveassisted CNTs

The progress in nanotechnology and in particular in the synthesis of carbon nanotubes (CNTs) has demonstrated through studies that the addition of these nanostructured materials in a matrix strengthens the interface of composite materials (Medina et al. 2016, 2017; Yu et al. 2014), and improves both, the interlaminar and fracture resistance (Fan et al. 2008; Joshi and Dikshit 2012; Wang et al. 2017; Zhou et al. 2016). However, other studies report problems of agglomeration and difficulty when adding them to the matrix due to their high levels of viscosity, which results in a decrease in properties (Borowski et al. 2015; Karapappas et al. 2009; Sánchez et al. 2013; Siddiqui et al. 2011; Zeinedini et al. 2018). The solution to these issues is found if the CNTs are grown on the fibers. Besides, according to Gorbatikh et al. (2011), a 30% improvement in interfacial cut resistance (IFSS) is obtained compared to when the CTN is dispersed in the matrix. Several methods are used for growing CNTs on surfaces such as fibers such as chemical vapor deposition (CVD), however, this technique is complex and expensive due to the use of reactors, gases, and high temperatures (Al Aiti et al. 2018). Another technique that allows these nanostructures to grow is using microwaves, where the growth of CNTs is achieved simply and quickly, with ferrocene in the presence of some conductive material and irradiating with a microwave oven (Algadri et al. 2017; Bajpai and Wagner 2015; Hazarika et al. 2017; Zhan et al. 2017). A schematic flowchart of the process can be seen in Fig. 5.1. In order to improve the interface of the recycled carbon fiber and the matrix affected during the recycling processes, CNTs on the surface of recycled carbon fiber were incorporated in this research using a microwave-assisted physical method, assessing the effect of the presence of the CNT of a laminate in its resistance to fracture in mode I. The material used for this study was Tenax® HTA 40/200 tex (3 k) carbon fiber, which was impregnated with a KUKDO® YD-114F epoxy resin and KH-813 hardener. It was first manufactured by a vacuum-assisted process, virgin fiber laminates, from which the fibers are then recovered through the recycling process. The process selected for recycling is pyrolysis. A Thermolyne® oven was used and the process was carried out at two approximate temperatures at the steam temperature (500 and 700 °C) to analyze the effect of the pyrolysis temperature on the fibers. For the growth of the CNTs, the recycled fibers were impregnated with a 0.5 molar solution of

5.1

Recycled Carbon Fibers and CNT Growth

Table 5.1 Summary of the recycling process samples

Table 5.2 Summary of the DCB test samples

Sample State

51

Process condition

MTV

Recycled Pyrolysis process up to vapors temperature

M500

Recycled Pyrolysis process up to 500 °C

M700

Recycled Pyrolysis process up to 700 °C

MV

Virgin

–

Sample

State

MC

Recycled with CNTs

MR

Recycled (unmodified)

MV

Virgin

ferrocene/toluene (ferrocene, 98% purity supplied by ALDRICH® ) for 30 min, followed by microwave irradiation on the fibers with a 700 (W) microwave oven at a frequency of 2.45 (GHz) for 25 s. For the characterization of the composite material and the growth of the CNTs from the microwave-assisted physical method, the Raman spectroscopy and Scanning Electron Microscopy (SEM) technique was used, both to study the effect of the recycling temperature and for the CNT growth over fiber. Finally, to study the interlaminar resistance, the double beam cantilever (DCB) test was performed according to the parameters established in the ASTM D5528-13 standard, using a Zwick/Roell brand Z005 universal testing machine. The results of the recycled fibers with CNT were compared with virgin fibers and recycled fibers without CNT, performing five tests for each sample according to the standard. Tables 5.1 and 5.2 show a summary of the samples to be investigated.

5.1

Recycled Carbon Fibers and CNT Growth

From the SEM images, important information on the effect of pyrolysis temperatures on the recycled material can be obtained. For the temperature of vapors (around 350 °C), there is still a significant amount of resin between the carbon fiber fabric, as can be seen in Fig. 5.2a, b. These resin residues disappear when the pyrolysis temperature rises to 500 °C, as shown in Fig. 5.2c, d. Finally, when the pyrolysis temperature rises to 700 °C, there are no traces of resin, but certain fibers have superficial damage (Fig. 5.2e, f), similar to that reported by Pimenta and Pinho (2011). Based on the results obtained from the Raman spectroscopy technique, the previous observations are complemented. The samples analyzed by Raman were the MT 500, MT 700, and MV, and the results can be seen in Fig. 5.3. In all samples, the D (~1350 cm−1 ) and G (~1580 cm−1 ) bands can be observed, which is characteristic of graphite or

52

5 Mechanical Performance of FRC Manufactured from Recycled Carbon …

Fig. 5.2 SEM images of the pyrolyzed CF (a–b) until obtaining vapors (onset of matrix decomposition) showing resin residues on its surface, up to 500 °C (c–d) and 700 °C (e–f)

graphene, or both structures (Meyer et al. 2009; Zhao and Wagner 2004; Zhao et al. 2016). There are no peaks associated with the presence of resin, but only those related to carbon, which confirms that the fibers are clean or without evidence of impurities. Figure 5.4 shows SEM images of nanotube growth with the microwave-assisted process, where it can be seen that certain fibers experienced CNT growth on their surfaces. In

5.2

Mechanical Performance of Recycled Fibers

53

Fig. 5.3 Raman spectroscopy of virgin and recycled CF at 500 °C y 700 °C

addition, with Raman spectroscopy, it was possible to verify the precedence of the NCTs, since the characteristic bands of these nanostructures appear in the spectrum, adding the G or 2D band (~2700 cm−1 ) (Bokobza and Zhang 2012; Keszler et al. 2004; Zhao and Wagner 2004), as observed in Fig. 5.4f.

5.2

Mechanical Performance of Recycled Fibers

The results of the fracture toughness in mode I obtained by the DCB tests are shown in Fig. 5.5 and in Table 5.3. As can be seen from the results, the recycling process modifies the resistance to fracture in mode I, according to that reported in the literature (Kim et al. 2017; Oliveux et al. 2015; Pickering 2006). The reason for this decrease is due to a poor interface attributed to the loss of sizing that occurs during fiber recycling. This decrease is due to a poor interface attributed to the loss of sizing that occurs during fiber recycling, as reported by Douguet et al. (2015), Paipetis and Galiotis (1996), Yunitori et al. (1994). Regarding the sample with fiber with microwave-assisted growth, a decrease can be seen with respect to the recycled fibers. This result is contrary to what can be found in the literature about the effect of the addition of CNT in virgin fibers on fracture resistance in mode I (Kepple et al. 2008; Wicks et al. 2010; Zhang et al. 2015), therefore the recycling process has a certain negative effect that should be further investigated.

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Fig. 5.4 SEM images of the growth of CNTs via Poptubes on virgin CFs (a–c). Crecimiento de CNTs sobre CF recicladas a 500 °C (d–e). Raman spectrum of recycled CFs 500 °C/CNTs

Fig. 5.5 Fracture toughness in mode I obtained by the DCB tests Table 5.3 Summary of the results of the DCB tests

Sample

GIC (J/m2 )

Variation ()

MV

1052.48 ± 93.40

–

MR

863.61 ± 80.95

– 17.94%

MC

744.09 ± 133.85

– 29.30%

5.3

Conclusions

55

The fracture zones of the virgin fiber sample obtained by SEM (Fig. 5.6a, b) show a type of fragile fracture with a good interface between resin and fibers, as expected. The latter can be best observed in Fig. 5.6b, where the resin is adhered to the fiber. The roughness seen in Fig. 5.6c, is a product of the matrix failure. For the recycled fiber samples shown in Fig. 5.6c, d, a fracture quite different from the previous one is identified. The roughness disappears and the fibers have no resin coating, product of the poor resin-fiber interface (Fig. 5.6c). In Fig. 5.6d, the marked area shows an approach to the fibers where the complete separation between the matrix and the fiber is more clearly noted. Finally, in the test specimens with CNT growth, different results were obtained. In certain areas of the fractures analyzed, no CNTs are observed (Fig. 5.6e). This could be due to the fact that the CNT did not grow in this particular area, or because they separated due to poor adhesion with the fiber at the onset of fracture, similar to that reported by Kim et al. (2015). On the other hand, in certain areas where there was a vast extension of nanoparticles, two phenomena are observed. The first phenomenon consists of areas of CNT with a good interface between fiber and resin (Fig. 5.6f), where the adherence of CNTs is good to both fiber and resin (Kim et al. 2015; Li et al. 2018). However, the second phenomenon appears in areas such as that shown in Fig. 5.7 where the CNTs are presented on the fibers but without major resin residues, which indicates a good adherence of the CNTs to the fibers but not to the resin. The latter may be due to a possible excess of ferrocene over the fibers during the impregnation process, creating a layer that adheres to the fiber, but not in other upper layers, interfering with the interface between fiber and matrix in the subsequent manufacture of the laminate (Fig. 5.8a). During the growth process, combustion was generated on the nanoparticles resulting from microwave irradiation. The equipment used has no control over the irradiation power so this combustion could not be avoided. As a result of this combustion, some areas with a high concentration of nanoparticles remain on the fiber sheets. A scheme of this approach is shown in Fig. 5.8a. The highlighted areas in Fig. 5.8b present a higher concentration of nanoparticles. In areas with the highest concentration of CNT, the nanoparticles easily detach from the fiber. If the interaction between the CNTs and the fiber is poor or the growth is generated by layers, the presence of the CNTs reduces the area of contact between the fiber and the matrix. This hinders the interface, which is reflected in the decrease in mechanical properties. Similar observations have been reported by other researchers (De Riccardis et al. 2006; Kim et al. 2015).

5.3

Conclusions

The microwave-assisted CNT growth process allows nanostructures to grow on the surface of recycled carbon fibers, irradiating the modified fibers with a 700 W microwave oven for 25 s. On the other hand, the process of recycling by pyrolysis at 500 °C allows to eliminate the epoxy matrix and rescues the fibers, and does not produce major structural

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5 Mechanical Performance of FRC Manufactured from Recycled Carbon …

Fig. 5.6 SEM images of the fracture zone of test specimens made with CFV (a–b); fibers with good adhesion are observed. Fracture zone of specimens manufactured with CFR-500 °C (c–d); zones with a poor interface zone are observed. Fracture zone of CFR-500 °C-CNTs specimens (e–f); CNTs adhered to the fiber and matrix are observed

5.3

Conclusions

57

Fig. 5.7 SEM images of CFR-500 °C-CNTs specimens showing no adhesion to the matrix

Fig. 5.8 a Scheme of the possible mechanism of CNT growth by layer, on CFV and CFR via Poptube. b CFC with non-homogeneous growth of CNTs

changes on them. For a temperature of 700 °C, surface pitting occurs in certain strands of the fiber, while a temperature close to 350 °C (vapors temperature), the matrix cannot be completely pyrolyzed. Regarding the mechanical properties, the growth of CNT on recycled carbon fibers decreased the resistance to fracture in mode I. This was found presumably due to the poor adherence of the CNT to the fiber, or due to the agglomeration of CNT layers on the fiber, or both. Further research should focus on the improvement of reclycing process of the unmodified CF and the carbon fiber with CNTs.

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