Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures [1st ed. 2022] 9789811908477, 9789811908484, 9811908478

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Shuncong Zhong Walter Nsengiyumva

Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures

Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures

Shuncong Zhong · Walter Nsengiyumva

Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures

Shuncong Zhong School of Mechanical Engineering and Automation Fuzhou University Fuzhou, Fujian, China

Walter Nsengiyumva School of Mechanical Engineering and Automation Fuzhou University Fuzhou, Fujian, China

ISBN 978-981-19-0847-7 ISBN 978-981-19-0848-4 (eBook) https://doi.org/10.1007/978-981-19-0848-4 Jointly published with Science Press, Beijing, China The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Science Press. © Science Press 2022 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 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 publishers, 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 publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Foreword

As a researcher in structural integrity, I have been asked many times by engineers this kind of question “how to detect and evaluate the defects in composite structures”. This is a very challenging issue in engineering application of a structural material because engineers cannot use a material without knowing its risk to fail. I am happy to see this new book entitled “Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures” is ready and answers this concern. As the title indicates, the book focuses on the major types of NDT techniques that are used to detect, characterize, and evaluate flaws in fiber-reinforced composite materials and structures. The authors were extremely careful in the choice of the words and terminology to ensure accurate information is conveyed to the readers. Indeed, this is extremely important for beginners and students who are interested in the nondestructive testing and evaluation (NDT&E) of fiber-reinforced composite materials and structures. The authors start by presenting comprehensive background information whereby the different types of defects and damage are accurately described outlining their common location, relative size, cause, and inherent characteristics followed by the description and application requirements for the different NDT techniques. If the guidelines and protocols presented in this book are accurately followed and the different NDT techniques are used appropriately, positive effects on the cost and reliability of fiber-reinforced composite materials and structures can be achieved effectively by evaluating the design prototypes during their development, providing means and feedback for process control during their manufacturing process, and inspecting the final products before and during their service life. This is especially important in the current era of significant technological advancements in the area of Internet of Things (IoT), Big Data, and Industry 4.0 where the complete networking of all industrial platforms is highly recommended to obtain high quality and achieve effective maintainability of the advanced fiber-reinforced composite materials and structures. Consistent with the aforementioned, the present book clearly demonstrates that most NDT techniques are constantly developing by following these developments and adapting to new technologies to introduce the capabilities of the cybersystems into the testing, characterization, evaluation, and maintenance processes of v

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fiber-reinforced composites while equally working to identify possible future NDT needs using new, conventional, and unconventional testing principles to adapt to the constantly developing fiber-reinforced composite designs and satisfy the customers’ requirements. The present book provides the readers with the most up-to-date information, while equally bridging the gap between conventional NDT techniques of the present and the past as well as providing and guiding NDT practitioners to the new NDT opportunities for fiber-reinforced composite materials. In fact, each NDT technique presented in this book provides a detailed description that includes the principles of operation, theoretical analysis, and signal processing techniques involved. Pursuing a rigorous approach with regard to the presentation of the information, the present book establishes a fundamental framework for the NDT of fiber-reinforced composite structures, while equally emphasizing the importance of the techniques’ spatial resolutions, integrated systems analysis, and the significance of the influence stemming from the applicability of NDT and the physical parameters of the test structures in the selection and utilization of adequate NDT techniques. The contributions that make up this book are well presented and can be accurately corroborated by real-time applications/measurements for the different NDT techniques presented. The present book constitutes a good source of information for students, researchers, and engineers aspiring to work with the NDT of fiber-reinforced composite materials and structures. Shanghai, P. R. China January 2022

Shan-Tung Tu Academician of the Chinese Academy of Engineering Professor of East China University of Science and Technology

Preface

Advanced fiber-reinforced composites (FRCs) are constantly gaining popularity in various structural and engineering applications, thanks to their outstanding physical, electrical, and mechanical properties. However, these types of materials are also prone to developing defects and damage ranging from small cracks to big impacts both at the manufacturing and in-service stages of the composite. To this end, a relentless investigation into their structural integrity is crucial as it helps users to determine when to reject, repair, or replace a particular part or component that might be of potential risks to the safety and security of users and the operation of the host structural system as a whole. In this book, we present a series of major nondestructive testing and evaluation (NDT and NDE) techniques for fiber-reinforced composite structures. As interesting as the title of the book might be, one may also want to know why we decided to include the words “testing and evaluation” in a single title instead of just keeping the generic term “testing” which is commonly used in many instances in connection with the word “nondestructive”. To explain where all this comes from, we need to take a deep dive into the original meaning of the individual words of the phrase “nondestructive testing and evaluation”. According to ASTM E1316-17A, the word nondestructive denotes a process of doing something that does not result in damage or harmful effect to the material or structure under test. Similarly, the most appropriate definition of the word “testing” relates to the determination of the presence of certain properties in a component under investigation and to present its inherent conditions or characteristics by direct or indirect means. The word “evaluate”, on the other hand, has a definition that seems to be broader in meaning as it involves a careful examination of the test results and decides of whether the indications or features noted during the examination can be the cause to accept or to reject the material or component consistent with the predefined acceptance criteria. In practice, however, both evaluation and testing have always been used interchangeably in many instances with other expressions such as examination, inspection, and investigation including in some references of the present book. Although all these terms do not necessarily have the same meaning, they all refer to the same technology, the one that is still widely misunderstood or unknown by the general public vii

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and it is even believed that the wrong utilization of these terms contributes in many ways to this misunderstanding. Assuming it is acceptable to take some liberties with all these definitions, we would like to advise that the most suitable definition of NDE, NDT, or NDI would be “an investigative process that does not result in any damage nor changes to the original attributes of the test composite and through which the presence of unpleasant conditions, features or discontinuities can be detected, locate, measured, and then evaluated”. In case this definition is followed and the NDT techniques are used appropriately, the latter can have significant effects on the cost and reliability of composite structures as they can evaluate the design prototypes during the development of composite structures, provide means and feedback for process control during their manufacturing process, and inspect the final product before and during their service stages. In the current era of technological advancements with significant interest in the Internet of Things (IoT), Big Data, and Industry 4.0, the complete networking of all industrial areas is inevitable to ensure the manufacturing of high quality and maintainability of the advanced composite structures. As such, NDT must inevitably follow these developments by adapting to new technologies and introducing the capabilities of the cyber systems into the testing, evaluation, and maintenance processes while equally working to identify possible future NDT needs. Although these NDT techniques are constantly improving with new and unconventional testing principles being introduced into the entire NDT industry, the implacable responsibility for the manufacturing industries to adapt their composite designs to satisfy their customer’s requirements will undoubtedly affect the way testing engineers provide NDT services. Typically, NDT techniques are currently being integrated into quality control schemes involved in composites manufacturing processes, resulting in a significant paradigm shift in the entire industrial quality management and the NDT of composite structures as we know it. It is also believed that some of the classical concepts featuring comparison of components with similar attributes and the statistical analysis of these comparative results will no longer be relevant under these conditions, raising concerns about the prospect of the human factors in the whole industry setup. To this end, it appears that the NDT community must continuously work to produce a highly qualified new generation of NDT engineers capable of making factual decisions based on NDT results and with adequate knowledge about the material properties, the components behavior, and NDT modeling of NDT experiments in accordance with the relevant applications. This new generation of NDT engineers should have a broader overview of conventional and new NDT techniques to rise above the ongoing challenges. Although a considerable amount of overview literature for common NDT techniques is available, the new generation of NDT engineers should know that new methods may, at the present moment, be in experimental and/or development stages (or may not even be presently considered possible NDT candidates) but have the potential for application in many NDT settings. To adequately solve all the future NDT tasks, NDT specialists should have a clear understanding of what is possible and what is not without going into too many details. To present our readers with the most up-to-date information, the present book bridges the gap between conventional NDT techniques of the present and the past and

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provides and guides for the new NDT opportunities for FRC materials. In doing so, a detailed description that includes the principles of operation and theoretical analysis of the most common NDT techniques for FRC structures during their manufacturing and/or in-service stages is presented. To facilitate the understanding and the importance of the different NDT techniques for FRCs, the book first provides some information regarding the defects and material degradation mechanisms observed in FRC structures as well as their general description and most probable causes. Although there are several NDT techniques used to detect and characterize defects and structural damage in FRC structures, they all have their specific advantages, disadvantages, and scopes of application. This means that there is currently no single NDT technique that has proven to be false-negative or false-positive free when it comes to the testing and evaluation of composite structures because even factors such as temperature, stiffness, and mass changes (i.e., due to the installation of sensors) still introduce additional complications than the damage itself. Additional factors such as hostile environment, on-site construction errors, equipment’s spatial resolution, as well as the data collection techniques significantly influence the quality of the measurement data and the size of the smallest detectable defect. As such, the determination of the exact size of defects in composite structures and the selection of appropriate NDT techniques to be used for specific applications remain such a complex practice in the NDT sphere. To provide effective solutions to some of these challenges, the present work was written based on the extensive scientific research and engineering backgrounds of the authors in the NDT and SHM of structural systems from various areas including electrical, mechanical, materials, civil, and biomedical engineering. Pursuing a rigorous approach, the book establishes a fundamental framework for the NDT of FRC structures, while emphasizing the importance of the technique’s spatial resolution, integrated systems analysis, and the significance of the influence stemming from the applicability of the NDT and the physical parameters of the test structures in the selection and utilization of adequate NDT techniques. This book is structured based on the lines of the well-established NDT principles with a particular emphasis on the advanced methods of measurement and data analysis featured in the current state-of-the-art NDT techniques. Although it was not the intent of the book to provide the state-of-the-art mathematical formulations guiding the principles of operation of the different NDT techniques, a brief description of such formulations was introduced in summary for each NDT technique trying to be essential without boring our readers with dispensable digressions. In general, the contributions that make up this book were extracted from the published research works, and others were produced by the engineers in our research group working on advanced optics and NDT techniques for composite structures. To this end, we are sincerely grateful to all those who were involved in the conceptualization, writing, editing of the first draft, and production of the final version of this work as well as the authors of the many excellent publications cited in this work. Although the publication of this book may seem to be the end of this work, the task of keeping up to date with the constantly evolving NDT techniques for composite structures can never be stopped (i.e., additional research can still be regularly conducted to extend and update the present work). In this context, the online version of this book will be

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continuously updated and enlarged at some points in the future to keep the contents up to date and to provide a healthy mixture of classical, new, and conventional NDT techniques for composite structures. Fuzhou, China

Shuncong Zhong Walter Nsengiyumva

Contents

1 Introduction and Background of Fiber-Reinforced Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction and Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Basics of Composites Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Polymeric Matrices and Polymerization Processes . . . . . . . . . . . . . 1.3.1 Thermosets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Thermoplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Reinforcements for Polymer Matrix Composites . . . . . . . . . . . . . . . 1.5 Types of Fiber-Based Reinforcement Architectures . . . . . . . . . . . . . 1.5.1 Woven Fabric Architectures . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Preimpregnated Thermosets . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Non-crimp Fabric Architectures . . . . . . . . . . . . . . . . . . . . . . 1.6 Types of Fibers Reinforcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.1 Glass Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.2 Carbon Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.3 Aramid Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.4 Boron Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.5 Polyethylene Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.6 Ceramic Fibers and Whiskers . . . . . . . . . . . . . . . . . . . . . . . . 1.6.7 Natural Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Design of Polymer-Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . 1.8 Polymer-Matrix Composites Manufacturing Procedures . . . . . . . . . 1.8.1 Hand or Manual Lay-Up Process . . . . . . . . . . . . . . . . . . . . . 1.8.2 Filament Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.3 Pultrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.4 Resin Transfer Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.5 Resin Film Infusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8.6 Resin Liquid Infusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Fiber Metal Laminates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 Sandwich Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11 Properties of Fiber-Reinforced Composites . . . . . . . . . . . . . . . . . . . .

1 1 4 6 7 8 9 11 11 12 12 14 14 15 16 18 19 19 20 21 23 23 25 25 26 27 28 28 29 30 xi

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1.12 Main Types of Deficiency in Polymer-Matrix Composites . . . . . . . 1.12.1 Material Processing Defects . . . . . . . . . . . . . . . . . . . . . . . . . 1.12.2 Manufacturing Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.12.3 In-Service Damage and Material Degradation . . . . . . . . . . 1.13 Trends Towards Novel Technologies . . . . . . . . . . . . . . . . . . . . . . . . . 1.14 General Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Introduction to Nondestructive Testing and Evaluation of Fiber-Reinforced Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Approaching the Nondestructive Testing World . . . . . . . . . . . . . . . . 2.2 Classification of Flaws Vis-À-Vis the NDT Techniques . . . . . . . . . . 2.3 General Hints on Testing Techniques and Procedures . . . . . . . . . . . 2.4 Classification of NDT Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Inspection Requirements for NDT Techniques . . . . . . . . . . . . . . . . . 2.6 Regulations, Standards, and Recommended Practices for NDT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Role of Research and Development in NDT . . . . . . . . . . . . . . . . . . . 2.8 Application of NDT: Impact of Globalization and Operator’s Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8.1 Important Events in Academic and Industrial NDT . . . . . 2.8.2 Important Historical Dates in Standards Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8.3 Qualification and Certification of Personnel for NDT . . . . 2.9 The Need for Reliability and Accurate Statistic Evaluation . . . . . . 2.10 Probability of Detection in DNT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Visual Testing for Fiber-Reinforced Composite Materials . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Important Dates and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Theory and Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 The Object Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 The Light and Test Structure Lighting . . . . . . . . . . . . . . . . . 3.3.3 The Human Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Visual Inspection Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Test Equipment and Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 The Direct Visual-Eyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 The Direct Visual Aids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.3 Remote Visual Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.4 Image Recording and Devices . . . . . . . . . . . . . . . . . . . . . . . 3.5.5 Image and Video Display Units . . . . . . . . . . . . . . . . . . . . . . 3.5.6 Imaging Software and Digital Cameras . . . . . . . . . . . . . . . 3.6 Applications to the Inspection of Composite Structures . . . . . . . . . 3.7 Evaluation and Reporting of Visual Test Results . . . . . . . . . . . . . . .

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Contents

3.8 Advantages and Limitations of Visual Testing . . . . . . . . . . . . . . . . . 3.9 Standards, Codes, and Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Ultrasonic Testing Techniques for Nondestructive Evaluation of Fiber-Reinforced Composite Structures . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Operation and Variants of Ultrasonic NDT . . . . . . . . . . . . . . . . . . . . 4.3 Types of Ultrasonic Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Longitudinal Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Shear Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Rayleigh Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Lamb Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Types of Ultrasonic Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Ultrasound Generation and Coupling with the Composite . . . . . . . 4.6 Ultrasonic NDT Instruments for Composites . . . . . . . . . . . . . . . . . . 4.7 Ultrasonic NDT Methods for Composite Structures . . . . . . . . . . . . 4.8 Applications of Ultrasonic Inspection of Composite Laminates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9 Ultrasonic Inspection of Sandwich Structures . . . . . . . . . . . . . . . . . . 4.9.1 Through-Transmission Ultrasonic NDT . . . . . . . . . . . . . . . 4.9.2 Air-Coupled Ultrasonic Inspection of Sandwich Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.3 Inspection of Composite Sandwich with Perforated Facesheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.4 In-Field Inspection of Honeycomb Sandwich Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Performance Comparison of Ultrasonic NDT . . . . . . . . . . . . . . . . . . 4.11 Current Trends and Prospects of Ultrasonic NDT . . . . . . . . . . . . . . 4.11.1 Sizing of Flaws Using Ultrasonic NDT . . . . . . . . . . . . . . . . 4.11.2 Signal Processing Techniques . . . . . . . . . . . . . . . . . . . . . . . . 4.11.3 Standards for Composite Structures . . . . . . . . . . . . . . . . . . . 4.11.4 Integrated Inspection Systems . . . . . . . . . . . . . . . . . . . . . . . 4.12 Advantages and Limitations of Ultrasonic NDT . . . . . . . . . . . . . . . . 4.13 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Infrared Thermography Testing and Evaluation of Fiber-Reinforced Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction and History of IRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Generalities and Basic Concepts of IRT . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Passive Thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Active Thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Classification and Factors Determining Classes of Active IRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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124 127 128 129 133 133 137 143 144 145 146 146 148 149 154 155 155 168 170 170 172 173 174 175 176 177 178 179 181 183 185 197 197 199 201 203 205

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5.3

Thermal Properties of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Thermal Diffusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Thermal Effusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Infrared Measuring Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Emissivity and Reflected Temperature Measurement . . . . . . . . . . . . 5.6 Established IR Thermographic NDT Techniques . . . . . . . . . . . . . . . 5.6.1 Optical Thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2 Laser Thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Eddy Current Thermography . . . . . . . . . . . . . . . . . . . . . . . . 5.6.4 Microwave Thermography . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.5 Vibrothermography and Ultrasound Thermography . . . . . 5.7 Scanning IR Thermographic NDT . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.1 Line Scanning Thermography . . . . . . . . . . . . . . . . . . . . . . . 5.7.2 Scanning Eddy Current Thermography . . . . . . . . . . . . . . . . 5.8 Applications of IRT-NDT for Composite Structures . . . . . . . . . . . . 5.9 Performance Comparison of IRT and Other NDT Techniques . . . . 5.9.1 Different Heating Functions . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.2 Different Excitation Sources . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.3 Comparison with Other NDTs . . . . . . . . . . . . . . . . . . . . . . . 5.10 Current Trends and IRT Research Prospects . . . . . . . . . . . . . . . . . . . 5.10.1 New Physics and Multiple Physics . . . . . . . . . . . . . . . . . . . 5.10.2 Signal Processing Techniques . . . . . . . . . . . . . . . . . . . . . . . . 5.10.3 Integrated Inspection System . . . . . . . . . . . . . . . . . . . . . . . . 5.10.4 Standards for Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11 Advantages and Limitations of Using IRT . . . . . . . . . . . . . . . . . . . . . 5.12 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Terahertz Testing Technique for Fiber-Reinforced Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction and Background Information . . . . . . . . . . . . . . . . . . . . . 6.2 Terahertz Devices and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Terahertz Pulsed Systems—THz-TDS . . . . . . . . . . . . . . . . 6.2.2 Terahertz Continuous Wave Systems . . . . . . . . . . . . . . . . . . 6.3 Imaging Theory and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Applications of THz Systems to the Inspection of Fibrous Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 THz Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 THz Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Specific Application of THz Systems for the Evaluation of Composite Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Online Process Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Off-Line NDT Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . .

211 211 213 213 214 216 218 218 225 225 227 228 233 234 236 238 246 246 247 248 252 252 253 255 256 257 259 260 273 273 275 275 280 283 284 286 288 290 290 292

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6.5.3

Characterization of In-Service Damage and Material Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Outlook on Possible Future Applications of the THz Systems . . . . 6.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

296 303 304 306

7 Application of Acoustic Emission for the Inspection of Fiber-Reinforced Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Generalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Usage and History of AE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Principle of Operation of an AE System . . . . . . . . . . . . . . . . . . . . . . 7.4 AE Sensors Calibration and Coupling with Test Structure . . . . . . . 7.5 Sources of AE in Composite Materials . . . . . . . . . . . . . . . . . . . . . . . 7.6 Analysis Tools for AE Measurement Signals . . . . . . . . . . . . . . . . . . 7.6.1 Distribution of Signal Hits in the Composite . . . . . . . . . . . 7.6.2 Source Localization of AE Signals in the Composite . . . . 7.6.3 Identification of Wave Features . . . . . . . . . . . . . . . . . . . . . . 7.6.4 Interpretation and Evaluation of AE Signals . . . . . . . . . . . 7.7 Applications in the Testing of Composite Structures . . . . . . . . . . . . 7.8 Combination with Other NDE Techniques and Prospects . . . . . . . . 7.9 Advantages and Limitations of AE Technique . . . . . . . . . . . . . . . . . 7.10 Current Trends and AE Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

315 315 316 319 322 324 327 327 329 331 333 335 340 342 345 347 348

8 Other NDT Methods for Fiber-Reinforced Composite Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Vibration Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Strain Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Electrical Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Ground-Penetrating Radar, Microwave, and Millimeter Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Ground-Penetrating Radar Inspection Technique . . . . . . . 8.5.2 Microwave Testing Technique (MW) . . . . . . . . . . . . . . . . . 8.5.3 Millimeter Waves (MmW) Testing Technique . . . . . . . . . . 8.6 Optical Interferometric Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Radiography and Tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.1 Backscattered X-ray Imaging . . . . . . . . . . . . . . . . . . . . . . . . 8.7.2 X-ray Computed Tomography Imaging . . . . . . . . . . . . . . . 8.7.3 Neutron Imaging Technique . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.4 Nuclear Magnetic Resonance Imaging Technique . . . . . . . 8.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

355 355 356 366 372 375 376 379 380 381 387 388 389 390 392 393 394

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9 Conclusions, Current Developments, and Prospects in the NDT of Fiber-Reinforced Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Current Developments in the NDT of Fiber-Reinforced Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Prospects in the NDT of Fiber-Reinforced Composites . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

407 407 413 416 419

Chapter 1

Introduction and Background of Fiber-Reinforced Composite Materials

1.1 Introduction and Synopsis In recent years, composite materials such as fiber-reinforced composites (FRCs) (also herein referred to as fiber-reinforced polymer-matrix composites, composition materials, fibrous composites, or simply composites), nanocomposites and fiber-metal laminates (FMLs) have become popular for use in aerospace, renewable energy, civil and architecture, marine, and automotive industries. In the early days, these types of materials were used as the fairings or reinforcements for other structures, but they are currently being used as advanced structural components in primary and secondary load-bearing components even in structures where any structural failures would result in catastrophic safety issues [1]. In the aerospace industry, for example, carbon fiber-reinforced polymers (CFRPs) are used in wing planks, sandwich panels, and fuselages; they are also used to strengthen and repair the existing structures of airplanes. Glass fiber-reinforced polymers (GFRPs) are used in thicker structures such as helicopter’s rotor systems in the aerospace industry as well as the piping and storage tanks in the petrochemical industry. Also, nearly all marine hulls are currently made of molded GFRPs or laminate skins bonded to the foams, and in some cases, wood-filled sandwich panels are used [2, 3]. In the construction industry, composites are used as reinforcements to concrete pillars and bridge decks as well as the retrofitting elements for the concrete and masonry structures [4, 5]. In renewable energy and marine industries, hybrid FRCs involving carbon and glass fibers are extensively used in the manufacturing of wind turbine aerofoils and hydrofoils for marine propulsors, respectively [6, 7]. In most of these industries, material development and improvement have always been a key parameter in their evolution, going from wood and metal to composites in search for materials with improved physicomechanical properties such as lightweight, high-temperature stability, corrosion resistance, and many others that could suitably satisfy the application requirements

© Science Press 2022 S. Zhong and W. Nsengiyumva, Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures, https://doi.org/10.1007/978-981-19-0848-4_1

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of each of aforementioned industries [8, 9]. In particular, though, the weight to resistance ratio has always been a factor of great concern in industries such as automotive, marine, sports, energy, construction, and aerospace. In the aerospace industry, for example, the utilization of composite materials has gone through an extensive revolutionary history since the first flight by the Wright brothers (i.e., using a wood-and-fabric biplane) in 1903 to the time when the aircraft became a common means of transportation in the mid-1950s. It is noted that aerospace composites development started at the beginning of the twentieth century (i.e., a period known as the pioneering phase) when Aluminum was not yet available at reasonable prices. At that time, wood (which is indeed a composite material provided by the natural world) was perceived to be the only viable material that could be used for the manufacturing of structures intended for use in flying machines [9]. In particular, wood was considered the cheapest and the most readily available substance that could easily be tailored into the desired shape and strong enough to withstand flight loads and turbulences. Inspired by the development of the military aviation industry at the beginning of the twentieth century, Hugo Junkers pioneered the first all-metal plane in 1915 [10] and in the 1930s the structural revolution started whereby wood-based structures were being replaced by metal-based structures (i.e., mostly Aluminum at that time) [8, 11]. The aforementioned structural revolution was marked by the manufacturing of all-metal fuselages that were used in Boeing 247D (i.e., manufactured in 1933) and DC-3 (i.e., manufactured in 1935) aircraft systems [12]. Research continued during World War II and significant developments were achieved in the 1960s and 1970s. In the 1960s, for example, advanced composite materials were first introduced in military aviation, and in the 1970s, the same materials were introduced in civil aviation [8]. However, the use of composite materials was initially limited to the fabrication of secondary wing and tail components such as the rudder and wing trailing edge panels, involving directional reinforcement. Revolutionary expansions of using composites in the aerospace industry took place in the 2000s with the production of two big airplanes being the main milestone of these key signs of progress. These two big airplanes are the Airbus A380 (i.e., manufactured in 2005) and the Boeing Dreamliner (i.e., manufactured in 2009). In these two airplanes, composite materials are intentionally used in the primary load-carrying structure to prove their efficacy and strength, and today, most aircraft structures have their major parts made of composite materials including the A400 whose chassis is made almost entirely of composites. In the automotive industry, composite materials have been used in the manufacturing of caps and steering wheels since the 1940s. However, if one was to pinpoint the exact date indicating the birth of composites in the automotive industry, then the 1953 GM Motorama auto show at the Waldorf-Astoria Hotel in New York City would be the best choice. It was at this event when the Chevrolet Corvette was first unveiled followed by the presentation of the stylish convertible, polo white with red interiors six months later. The Corvette body featured GFRP composites and was indeed the first production car to use structural polymer composites, establishing a significant milestone for the automotive industry [13]. At this time, the use of GFRPs was already becoming a popular method for constructing lightweight vehicles. In 1957,

1.1 Introduction and Synopsis

3

the Lotus Elite was manufactured using several GFRP moldings to create monocoque composites. At this time, engineers started using the same new method (i.e., use of several glass-fiber-reinforced plastics molding to manufacture monocoque composites) to produce other sporting products such as fishing poles/rods, tennis racquets, spars/shafts for kayak paddles, windsurfing masts, and boards, hockey sticks, as well as kites and bicycle handlebars among others [14]. The method of using composites to manufacture molding for cars, especially cars designed for speed and fuel-saving efficiency, mainly derived from the development of natural composites. The latter are composite materials obtained by combining plant fibers (i.e., particularly plants that have a makeup of fibers) and resins. In particular, natural composites present many interesting properties and their ability to sway under the force of wind gives them a unique combination of strength and flexibility and a key feature enabling them for use in many industrial applications. Inspired by the same natural concept of trees, composites manufacturers have discovered that the same results, if not better, could also be produced by using other components. To this end, composites have been used and experimented with for many years and significant improvements have been recorded, hence the ability to make stronger lightweight materials that are currently used in cars, boats, and aerospace equipment to list but a few. In the early 1990s, for example, numerous super-cars such as Jaguar XJ220 and Bugatti EB110 were manufactured using a variety of composite materials in their body structures [15]. The Jaguar XJ220, for example, was manufactured using an aluminum mixture while the Bugatti EB110 was manufactured using a combination of aluminum, carbon panels, and steel. In the construction and architecture industries, the combination of different materials in building and construction has been around since ancient history. The literature indicates that one of the earliest examples of using composite materials for the manufacturing of structural systems in construction can be traced back to around 3400 B.C. when Mesopotamians glued wood strips at different angles to create plywood [16]. As the technology continued to grow, new materials and methods of making composites were developed for different applications. However, composite materials were generally utilized in structural engineering as fairings and reinforcements for the existing structures and were not utilized to their full potential until recently. This is particularly because the engineering capabilities of making advanced composites were not at a satisfactory level yet and their properties were poorly understood in the engineering world compared to their metallic counterparts. In recent years, however, the shift has been gradually moving toward their applications in primary load-bearing structures, thanks to the continued development of science and technology which has enabled engineers to better understand the properties and improved designs for advanced composite materials. It is noted that the term advanced materials refer to materials based on new fiber and resin systems having greater strength and stiffness properties than those of conventional glass and carbon fiber-based composites materials. Indeed it is now clear that the use of composite materials has enjoyed successful advancements over the years and continue to grow to this date-driven primarily by several factors such as the transition from simple natural composites to advanced natural and man-made composites (i.e., also referred to as synthetic composites) as

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well as the advancements registered in the development of polymeric resins since the 1930s, along with the development of carbon fibers in the 1960s which laid the groundwork for today’s manufacturing of fiber-reinforced composite materials [17]. Advancements and expertise in specialized molding processes have also allowed composites manufacturers to be able to tailor some of the unique properties of the composite materials to fulfill certain requirements not to mention their favorable strength to weight ratios which are indeed the key parameters for aerial craft systems that have to operate against the gravity force [9]. Government regulations in certain countries such as the United States and the European Union also continue to impose tighter restrictions on vehicle emissions and demand the improvement in materials and transportation safety, and one way to meet these requirements by the automobiles manufacturing industries is by using advanced lightweight materials such as plastics and composites that have the highest potential in this regard [13]. In summary, the above overview outlining the chronological development of materials for different industries from wood to the generation of advanced fiberreinforced composite materials highlights the continuing trend of increasing exploitation of advantageous composite materials physicomechanical properties such as the strength-to-weight, temperature corrosion resistance, and many others. However, since the selection of materials for a particular component in a structure depends on the various design and operation criteria, it is necessary to consider all the pros and cons that go along with the application of interest before choosing the raw materials for the fabrication of specific components. In particular, some of the important criteria to be considered taking into account the above-mentioned prerequisites include physical (i.e., density, temperature resistance, etc.), mechanical (i.e., tensile strength, modulus, toughness, etc.), environmental (i.e., biodegradability, toxic level, etc.), and cost (i.e., raw material cost, manufacturing cost, etc.). Although composite materials have had tremendous success in structural engineering for many years, their manufacturing process may often lead to a complex product that yet entails serious technical hitches in the complete understanding of its performance. A lot has been done since the appearance on the market of the first made-made or synthetic composite materials, but the world of composites remains an open endeavor that still needs to be fully explored.

1.2 Basics of Composites Materials In general, composite materials are obtained by combining two or more basic materials with different physical or chemical properties (often referred to as constituent materials or material constituents) to generate a relatively homogeneous material with improved properties compared to those of its original material constituents. Two main categories of materials constituent viz. the matrix (also referred to as the binder) and reinforcement are generally used. In the context of this book, a composite material denotes a material consisting of a combination of filaments (fibers) in a common matrix. The basic premise of these materials is that the combination of

1.2 Basics of Composites Materials

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materials does not alter their individual properties (both good and poor properties), but rather provides support to one another to overcome their limitations and achieve the desired properties [18]. It is certainly this design freedom that gives composites their greatest advantages, uniqueness as well as distinct attributes allowing their material constituents to remain flexible and be able to play different roles for a common goal [8]. In general, the fibers provide the essential axial high strength and stiffness of the composite material, with a low density that gives the significant benefits of exceptionally high specific properties. However, fiber is brittle and requires support against premature fracture to allow the composite to meet its design specifications. Thus, a matrix is added to the composite to perform other critical functions such as maintaining the reinforcements (i.e., fibers in this case) in the proper orientation and location, protecting them from abrasion and environmental effects, helping to transfer or distribute stresses between the adjacent fibers and particles, avoiding the propagation of fractures, and contributing to electrical conductivity as well to the thermal stability of the composite. In addition, the matrix properties generally determine the resistance of the composite to most of the damaging and/or material degradative processes that eventually cause the failure of the structure including impact damage, delamination, water absorption, chemical attack, and high-temperature creep. Thus, the matrix is typically the weak link in fiber-reinforced composite structures. The resulting structure provides an increase in the damage tolerance and toughness of the brittle fibers with minimal loss of their beneficial mechanical and physical properties. Several types of materials are generally used as the matrix for composites including polymers, cement, ceramics, and metals, with some of these materials being predominantly used in some applications than in others. In civil engineering, for example, cement is widely used in concrete-based products whereby additional materials such as sand, stones, and steel serve as reinforcement embedded in the form of particles or metal rods (i.e., reinforced concrete). Although ceramic matrix composites (CMCs) are extremely brittle, they are particularly appreciated for their resistance to high temperature, low densities, thermal conductivities, and environmental effects such as corrosion and oxidation [19]. As a result, their primary use is for thermal protection systems (TPSs), for example, as carbon-reinforced silicon carbide (C/SiC) or silicon carbide reinforced silicon carbide (SiC/SiC), which are used where oxidation resistance and high-temperature capability are critical [20], especially for thermal protection in the aerospace field. Substantial advances have been made in the thermal barrier coating (TBC) of gas turbine blades and vanes [21]. Also, metal matrix composites (MMCs) are composed of a metal matrix (e.g., Magnesium, Titanium as well as Aluminum and their alloys) and a reinforcement/filler material (e.g., silicon carbide and graphite) which provide them with excellent mechanical performance compared to their base metals and their polymer-matrix-based counterparts. Although MMCs have lower toughness than their base metals, they present many enhanced properties such as specific strength, stiffness and moduli, low density and high-temperature resistance, lower coefficients of thermal expansion, and, in some cases, better wear resistance. In general, a magnesium matrix is used for the manufacturing of parts for gearboxes, compressors, and engines such as casings its composites help to offset the

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fuel costs associated with flight and help reduce carbon emissions (i.e., magnesium has the highest strength-to-weight ratio of all the structural metals with 36% lighter per unit volume than Aluminum and 78% lighter than iron) [22, 23]. Instead, a titanium matrix is mainly used for the manufacturing of turbine engine components (fan blades, actuator pistons, synchronization rings, connecting links, shafts, and discs) [24, 25]. Moreover, in the aerospace field, Al-SiC, Al-B, Mg-C, Al-C, Al-Al2 O3 continuous and discontinuous reinforcements are widely used for frames, reinforcements, and aerial joining elements [8, 26]. It is worth mentioning that commercial applications for metal matrix composites are generally sparse due to their high cost. However, because they are considered enablers for future hypersonic flight vehicles, metal matrix composites remain important materials in the aerospace industry [27]. In particular, polymers present a relatively low-processing cost and several advantageous physicomechanical properties and they are by far the most commonly used type of matrix in non-civil products. Reinforcements are often made of glass or carbon fibers and they are often referred to as fiber-reinforced polymers-matrix (FRP) composites. If classified by matrix, fiber-reinforced composite structures are classified as thermoplastic (e.g., long and short fiber-reinforced thermoplastics) and thermoset (e.g., paper composites panels) composites. Advanced thermoset polymer matrix systems usually incorporate aramid fiber and carbon fiber in an epoxy resin matrix. Indeed, fiber-reinforced composites have generally been considered the most market-dominant types of composites among the other composite materials in the aircraft industry and will be treated in greater detail consistent with the line of work and the subject-matter being discussed in this book.

1.3 Polymeric Matrices and Polymerization Processes Generally speaking, the term “polymer” denotes a substance or material consisting of very large molecules, or macromolecules, composed of many repeating subunits (i.e., small molecules called monomers, or “mers”). The latter are bonded together through a chemical reaction process referred to as “polymerization”. However, each monomer must have at least two reaction sites or functional groups for the polymerization process to take place. It is noted that two types of polymerization are usually identified depending on the process by which the polymers are formed. The two polymerization processes are addition polymerization and condensation polymerization. In the former, several mers react in the presence of a catalyst to form the polymer without the formation of byproducts, and in the latter, the polymeric chain development is followed by the elimination of small molecules such as water (H2 O), methanol (CH3 OH) and many others [28]. Interestingly, their consequently large molecular mass, relative to small molecule compounds, produces some of the unique physical properties including toughness, high elasticity, viscoelasticity, and a tendency to form amorphous and semi-crystalline structures rather than crystals. Indeed different polymers present different properties and these can even be used

1.3 Polymeric Matrices and Polymerization Processes

7

Fig. 1.1 General comparison of the main characteristics of thermoplastic and thermoset polymeric matrices used in fiber-reinforced composites— (Source Darrel R. Tenney, NASA Langley Research Center) [31]

to classify them into different groups [29]. Two main groups are generally considered viz. thermosets and thermoplastics depending on their behavior under heating or cooling conditions [29, 30], properties of these two types of polymers are significantly different. To this end, the general characteristics of each matrix type are shown in Fig. 1.1; however, recently developed matrix resins have begun to change this picture, as noted below.

1.3.1 Thermosets Prior to polymerization, thermosets behave like low-viscosity resin and cure gradually at a relatively low temperature (i.e., between 20 to 200 °C). As soon as they are fully cured, thermosets cannot be reprocessed by reheating or otherwise. In their fully cured form, molecules in polymeric thermosets are cross-linked to connect the entire matrix in a three-dimensional network (a process called curing), and because of this property, these types of polymers are generally referred to as cross-linked polymers [32]. Thermoses, because of their three-dimensional crosslinked structure, tend to have high dimensional stability, high-temperature resistance, and good resistance to solvents. Recently, considerable progress has been made in improving the toughness and maximum operating temperatures of thermosets. Also, molecules of fully cured thermosetting polymers are permanently insoluble and infusible. Typical examples of thermosetting polymers include unsaturated epoxies, polyesters, vinyl esters, polyamides, and phenolics among others [8, 30]. As mentioned earlier, cured epoxy resins remain reasonably stable even in hostile environments (i.e., high resistance to corrosive chemical attack) and are excellent adhesives having low shrinkage during curing (polymerization) and present no emission of volatile gases. Indeed the aforementioned characteristics confer to these materials with high mechanical properties and high corrosion resistance (coupled with a quite simple curing process), making them the most popular amongst advanced composite resin matrices. However, epoxies are generally expensive, they cannot be stored for a long time and so are mainly used in high technology areas.

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In contrast to epoxy resins, thermosetting polyesters are relatively cheaper, easily accessible, and used in a wide range of fields. In addition, liquid polyesters can be stored at room temperature for a longer time (the storage time usually varies between several months and sometimes even years), and with the mere addition of a catalyst can cure within a short time; the cured polyester can be rigid, or flexible, as the case may be, and transparent. Thermosetting polyesters are commonly used in fiber-reinforced plastics. They are mainly used in the automotive and naval fields. Phenolics represent the first truly synthetic plastic (i.e., commercialized in 1905) obtained combining formaldehyde and phenol. They are water and solvent resistant, can be used as electrical insulators (i.e., they are extensively used in circuit boards), but are generally brittle even if they can be strengthened, to a certain extent, by fillers. Nowadays, they have been practically superseded by modern plastics such as epoxy or polyester resins. Phenolic and amino resins are another group of polymermatrix resins as well as polyamides, which are relative newcomers to the advanced composite industry and have not been studied to the extent of the other resins.

1.3.2 Thermoplastics Currently, thermoplastics only represent a relatively small portion of all the composites in the entire FRCs industry. They are typically supplied as nonreactive solids (no chemical reaction occurs during processing) and require only heat and pressure to form the finished part. Unlike the thermosets, the thermoplastics can usually be reheated and reformed into another shape if desired. Thermoplastics are also linear polymers, which are composed of chainlike molecules (i.e., long and discrete molecules) that may be some high-viscosity resins with varying degrees of crystallinity. These types of resins are also referred to as engineering plastics and they include some polyesters, polypropylene (PP), polyvinylchloride, polyetherimide (PEI), polyamide-imide, polyether ether ketone (PEEK), polystyrene, and polyphenylene sulfide (PPS) as well as liquid crystal polymers among others. Regarding their properties, most thermoplastics can be dissolved in certain liquids, and as indicated earlier, they soften or melt upon heating above their melting/processing temperature (i.e., typically from 100 to 400 °C) for additional processing, and after forming, they are cooled to become amorphous, semicrystalline, or even crystalline solids (i.e., the degree of crystallinity has a strong effect on the final matrix properties). Unlike the curing process of thermosetting resins, the processing of thermoplastics is reversible, and if desired, the resin can be formed into another shape by simply reheating the original product to its process temperature. Although thermoplastics are generally inferior to thermosets in terms of properties and resistance to environmental corrosion (i.e., particularly in hightemperature strength and chemical stability), they are generally more resistant to cracking and impact damage [33, 34]. However, it should be noted that the recently

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developed high-performance thermoplastics, such as PEEK, which have a semicrystalline microstructure, exhibit excellent high-temperature strength and solvent resistance than previously measured with other types of thermoplastics. In terms of their applications, thermoplastics have recently become of great interest to engineers and researchers in material science for their ductility and highprocessing speed as well as for the greater flexibility and choice of manufacturing techniques. Their processing can be selected depending on the scale and rate of production required as well as the size of the component. In addition, thermoplastic composites can be easily repaired and/or remanufactured because their transition to the softened phase can be accomplished any number of times by simply heating them making them extremely useful especially in the areas where structural repairs are of great concern. Thermoplastics offer great promise for the future from a manufacturing point of view, because it is easier and faster to heat and cool a material than it is to cure it. This makes thermoplastic matrices attractive to high-volume industries such as the automotive and aerospace industries. Currently, thermoplastics are used primarily with discontinuous fiber-reinforced composite materials such as chopped/short glass or carbon fiber/graphite. However, there is great potential for high-performance thermoplastics reinforced with continuous fibers also known as long FRCs. While thermoplastics could be used to replace thermosets-based structures in the next generation of fighter aircraft systems [35–37], thermoset matrices (i.e., especially epoxy) are generally preferred for aeronautical applications due to their lightweight requirements. In addition, even though the use of thermoplastics is emerging as the total composite percentage in airplanes is currently increasing, thermosets still lead the way in these types of structures and are expected to remain so for many years to come.

1.4 Reinforcements for Polymer Matrix Composites Generally speaking, fiber-reinforced composite materials consist of a variety of short or continuous fibers (reinforcements) bound together either by a polymer matrix which can be a thermoset or thermoplastic type of polymer (binder). Fiberreinforced composite materials are generally designed to transfer the loads between the individual fibers in the polymer matrix and present several advantages such as lightweight, good abrasion resistance, good corrosion resistance, high stiffness as well as high strength in the direction of their reinforcements. Generally speaking, the reinforcement of a composite material indicates all the methods used to improve its mechanical properties it also indicates the materials used to provide the composites with additional strength and flexural stiffness. Indeed the choice of reinforcement material is particularly important because it provides most of the composite’s strength and stiffness and determines some of the most important mechanical properties of the finished structure such as its load-carrying capacity, strength, impact resistance, and flexural stiffness. Several types of materials are usually used as reinforcements for fiber-reinforced composite materials and they can be introduced in the polymer

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Fig. 1.2 Types of reinforcement materials: a fibers, b whiskers (discontinuous reinforcement), c particles and d fabrics

matrix as particles (i.e., also known as discontinuous reinforcement), short fibers or whiskers, fabrics, and continuous fibers as illustrated in Fig. 1.2. Particles or discontinuous reinforcements are generally cubic pieces of material, while whiskers are single stretched crystals and fibers are single filaments of significantly small diameter/length ratio that are aligned following one main direction or randomly dispersed into the polymer matrix. It is noted that fibers are the most commonly used types of reinforcements because the resultant composite (i.e., fibrous composite or polymer matrix composite) often comes with small and scarce defects compared to other types of composites and the fact that fibers can be oriented following the direction of the main tensile stresses does provide additional strength to the composite [8]. Interestingly, fibers with a small diameter offer great flexibility to manufacturers during the fabrication processes. High-performance composites are generally manufactured with continuous fibers, while composites in less-demanding applications can be made with cheaper short fibers in an aligned array or with a random orientation. Generally speaking, fiber-reinforced composites usually contain about 60% reinforcing fibers of the total volume of the composite, and glass fibers, graphite, and aramid are among the commonly used fibers. The fibers that are commonly found and used within fiber-reinforced composite materials include fiberglass, graphite, and aramid. Glass fibers present a relatively low stiffness at the same time exhibit a competitive tensile strength compared to other types of fibers. The cost of glass fibers is also dramatically lower than that of other types of fibers which is probably why glass fibers are the most widely used type of fibers in fiber-reinforced composite materials. The reinforcing fibers have their highest mechanical properties along their lengths rather than their widths. Thus, the reinforcing fibers may be arranged and

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oriented in different forms and directions to provide different physical properties and advantages based on the application [7, 8]. In general, proper selection of the fiber type, fiber volume fraction, fiber length, and fiber orientation is very important, since it influences the density, tensile strength and modulus, compressive strength and modulus, fatigue strength as well as fatigue failure mechanisms, electrical and thermal conductivities and ultimately the cost of the composites.

1.5 Types of Fiber-Based Reinforcement Architectures As indicated earlier, fiber reinforcements are the most commonly used reinforcements for composites materials and they will indeed be the primary focus for this book.

1.5.1 Woven Fabric Architectures The reinforcing medium can be also produced in the form of woven fabric by directly interlacing either separate bundles of fibers or tows, combining warp (0◦ ) and weft (90◦ ) in a regular pattern or weave style (i.e., formed by weaving). Examples of woven fabric architectures are illustrated in Fig. 1.3 and their preference of usage in the manufacturing of fiber-reinforced composite is solely based on the compromise between ease of handling during the manufacturing process, drapability (i.e., the ability to form the fabric into a three-dimensional geometry), and mechanical performance to list but few. In the compact plain weave (Fig. 1.3a), individual warp fiber passes alternately under and over each weft fiber, making it one of the most stable types of woven fiber (due to the intertwined weave structure), but also one of the most difficult weaves to drape. In addition, this type of weave does produce composites with significantly reduced in-plane strength and stiffness because of its high level of fiber crimp (i.e., misalignment of fibers from the plane of the fabric),

Fig. 1.3 Some woven fabric types: a plain-woven fabric, b twill woven fabric, and c satin woven fabric

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which produces resin-pockets zones (i.e., resin-rich and/or resin-starved areas) in the composite and their limited performance attributes. As opposed to compact plain weave, the twill weave (Fig. 1.3b) presents one or more warp fibers alternately weave over and under two or more weft fibers in a regular repeated manner with the visual effect of a straight or broken diagonal ‘rib’ to the fabric. Although this type of weave presents slightly less stable composites, the twill weave features superior wet out and drape features and it does provide composites with improved mechanical properties, reduced crimp, and a smoother surface compared with the plain weave. Satin weaves (Fig. 1.3c) are fundamentally twill weaves modified to produce fewer intersections of warp and weft. The ‘harness’ number used in their designation (typically 4, 5, and 8) is the total number of fibers crossed and passed under before the fiber repeats the pattern. In most cases, a 5harness satin weave is the most preferred drapeable example, and it features a weave pattern of reduced intertwining which provides improved in-plane mechanical properties at the expense of stable handling. In addition, the asymmetry that needs to be considered in the satin weaves entails that one face of the fabric involves fibers running predominantly in the warp direction while the other face has fibers running predominantly in the weft direction.

1.5.2 Preimpregnated Thermosets Either reinforcing unidirectional (UD) fiber tapes or woven fabrics can be preimpregnated with a partially cured thermoset resin to produce what is known as a ‘prepreg’. These compounds form the basis of most high-performance structural composite components and have been used for many decades. The material needs storage at low temperatures to prevent the resin from prematurely curing, which could start at room temperature. The material, supplied in rolls or tape, is cut into pieces and laid up in a mold manually or robotically, then, it must be vacuum bagged before the start of the curing process, which generally takes place in an autoclave, under a controlled pressure/temperature cycle. The resulting structures made from prepreg materials can show significant performance benefits compared to those made from alternative continuous fiber materials, with an increase of stiffness and strength up to 30% [38]. These advantages are to be ascribed to a high nominal fiber volume fraction of 60% and the use of high-grade, fully impregnated resins. Adversely, the production of such materials is expensive in terms of equipment required, storage, and skilled labor costs.

1.5.3 Non-crimp Fabric Architectures In stricter terms, non-crimp fabrics (NCFs) arose from the challenge to create reinforcements that combine unidirectional fibers with integrity, ease of handling, and

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Fig. 1.4 Typical examples of stitching in non-crimp fabric architecture: a and b unidirectional type of stitching and b biaxial type of stitching

drape of textile fabrics [39]. As indicated in Fig. 1.4, these types of fibers differ from woven fabrics by a stitching material (polyester yarn) that is introduced to bind several unidirectional fiber layers to avoid misalignments (i.e., individual layers of reinforcement in weft yarns are placed on top of each other and are then fixed by knitting yarns in a warp-knitting process). The benefit of overlaying straight tows joined by stitching is that the formation of tow crimps may be avoided, which results in improved strength, stiffness, and fatigue life. Apart from their unique properties, NCFs are often obtained through different manufacturing processes and different bonding solutions. As with woven fabrics, a variety of assembly styles are available [8] involving different mass per unit area (i.e., defined by the number of unidirectional layers in the fabric and the number of individual fibers contained in a single tow) and stitching mechanisms. An additional advantage of such a process is that the manufacturers can control the angle in which fibers are placed (i.e., from a wide range of approximately 20° to −20° against the direction of production) to obtain the best possible semi-finished textile products for composite applications. Ideally, the fibers are laid down at angles that correspond to the load state of the final composite part, of which, quadraxial NCF has been found to achieve quasi-isotropic behavior in the material by numerous researchers [40]. The stitching properties such as pattern, length, and tension of the yarn have a great influence on the properties of the textile. This is in particular valid for mechanical properties (i.e., drapability) of dry textiles. Also, the manufacturing process is very simple (i.e., uses conventional stitching machinery) and the absence of the tow crimps may result in composites of higher mechanical performance as compared to woven fabrics. Although the mechanical properties of NCFs greatly depend on the fiber material used, NCFs do generally exhibit a significant reduction in performance when compared to those derived from pre-pregs mainly because of lower fiber volume fractions and the use of lower performance resins suitable for infusion. Interestingly, the fiber over resin percentage may be enhanced by reducing the tow spacing and by improving the stitching architecture as well the overall manufacturing process, leading to a product of comparable performance but cheaper than those from pre-pregs.

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1.6 Types of Fibers Reinforcements Generally, fibers are the principal constituents in fiber-reinforced composite materials and constitute about 60% of the total volume of the composites [41, 42]. There are currently many types of fibers used for the manufacturing of composite materials (both natural and synthetic fibers) whose primary role is to provide the required strength and stiffness to the composite. While carbon and glass fibers are the most commonly used reinforcements in fiber-reinforced composite materials, aramid fibers (e.g., Kevlar and Twaron), as well as boron fibers, have also been extensively used in composites and each of these two types of fibers offers some beneficial properties such as excellent toughness and compressive strength, respectively [42]. Additional types of fibers include but are not limited to silicon carbide, spectra, high-modulus polyethylene (PE), poly-p-phenylene-2,6-benzobisoxazole (PBO), and alumina as well as natural fibers such as sisal, kenaf, banana, bamboo, and hemp. Some of these fibers are used in dedicated applications, others for general applications, and the best type of fibers to be used for a particular application is determined based on several factors including the required strength, stiffness, corrosion resistance, and the available budget [41]. The following sections present thorough details of the most important fibers that are used for the manufacturing of composite materials.

1.6.1 Glass Fibers Glass fibers constitute the vast majority of all fibers used in the composites industry. Glass fiber was first discovered in the 1890s and made commercially available in the early 1930s; it was first used as insulation material in electrical, thermal, and acoustic uses. Then, it achieved great popularity during the 1950s when it was considered as a good substitute for asbestos fibers whose health hazards were becoming apparent. Today fiberglass is the dominant reinforcement fiber in composite construction, accounting for about 90% of worldwide consumption. This is simply because it has good strength-to-weight characteristics, can be easily processed, and sells at a low price. Glass filaments are relatively easily produced by extruding molten glass, which is obtained by blending quarry products (e.g., sand, kaolin, limestone, and colemanite) at about 1600 °C; then, the formed liquid is passed through micro-fine bushings and simultaneously cooled to produce the fiber filaments of a diameter generally ranging between 5 and 24 mm. The filaments are drawn together into a strand (i.e., closely associated) or roving (i.e., more loosely associated), and coated with a ‘size’, or binder; this is to provide an efficient filament cohesion and to minimize the degradation of the filament strength that would otherwise be caused by filament-to-filament abrasion. The size may be temporary, as in the form of a starchoil emulsion that is subsequently removed by heating and replaced with a glassto-resin coupling agent known as a finish. On the other hand, the size may be a

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compatible treatment that performs several necessary functions during the subsequent forming operation and which, during impregnation, acts as a coupling agent to the resin being reinforced [43]. Consistent with the aforementioned, it is noted that different types of glass including A-glass (i.e., alkali glass), C-glass (i.e., chemical glass), E-glass (i.e., electrical glass), R-, S-, T-glass (i.e., structural glass), M-glass (i.e., modulus glass) and D-glass (i.e., dielectric glass) can be produced by varying the ‘recipe’ (i.e., the addition of chemicals to silica sand). While the present book does not detail the characteristics and properties of each of the above types of glasses (i.e., interested readers are referred to Ref. [8] for more details), it is important to indicate that electrical and structural glasses are, by far, the most common types found in composites (even though the type of fibers to be used is selected based on the application) because of their good combination of mechanical properties, chemical resistance, and insulating properties. In particular, however, electrical glass looks more attractive from the cost point of view, while structural glass offers better mechanical performance than electrical glasses. Apart from the selection of the type of the fibers to be used in a given application itself, the proper selection of the fiber volume fraction, fiber length, and fiber orientation are equally important since it influences the characteristics of the composite laminate’s density, tensile strength, and modulus, compressive strength and modulus, fatigue strength as well as fatigue failure mechanisms, electrical and thermal conductivities and cost to list but a few. Additional information regarding the types of characteristics of the different types of glass fibers is provided in Ref. [41] and readers are directed to this specific study for more details.

1.6.2 Carbon Fibers Carbon has the highest strength and highest price of all reinforcement fibers available for composites manufacturing today. These fibers were first produced in the United Kingdom in the early 1960s, even if Edison had much earlier used them in lighting lamps. The most common method of making long carbon fibers is the oxidation and thermal pyrolysis of an organic precursor, poly-acrylonitrile (PAN). Through heating at correct conditions (2500–3000 °C), the non-carbon constituents evaporate away with a resulting material having a 93–95% carbon content. Indeed the properties of carbon fibers largely depend on the raw material and the manufacturing process the relative amount of exposure at high temperatures 500–3000 °C results in greater or less graphitization of the fiber. Higher degrees of graphitization usually result in a stiffer fiber (higher modulus) with greater electrical and thermal conductivity values. The size, or thickness, of carbon tows, is measured in “k” or thousands of filaments. A 3 k tow contains 3000 filaments and a 12 k has 12,000 filaments. Carbon fibers exhibit substantially better strength and stiffness values than all the other types for fiber reinforcement, outstanding temperature performance, and high electrical and thermal conductivities. The impact damage tolerance in pure carbon composite products can be from relatively low to very poor and greatly depends on

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the processing method. Notwithstanding the aforementioned, carbon fibers represent the best reinforcement to use particularly when the weight of the composite (i.e., final product) is important because of the significant advantages retained by them their high stiffness-to-weight ratio, high strength, corrosion resistance, fatigue resistance, as well as their high-energy absorption on impact. The fact that the properties of the final product can be tailored to meet the design requirements is another added advantage. Consistent with the aforementioned, carbon fibers are often praised for their lightweight (i.e., ability to produce lightweight structures) and designers can choose between stiff and strong fibers depending on the composite part being produced. Another major advantage is that their thermal expansion is zero; this means that unlike metals, which expand when heated, carbon fibers remain in their basic form with remarkable benefits in specific projects where thermal stability is required. In addition, the material can resist very high temperatures (1000 °C), being practically limited only by the matrix. If properly designed and conceived, carbon fiber composite structures do not suffer any fatigue issues. Finally, carbon fibers are permeable to X-ray and do not corrode, which is a huge concern with metals. The material also has some disadvantages that need to be taken into consideration when planning a project. As an example, carbon fibers are fairly expensive compared to other reinforcements even if their price is steadily decreasing due to the progress of production technology. Also, carbon fiber is an electric conductor and, as such, can reflect radio waves, which can be a disadvantage in some cases. In addition, carbon fibers are brittle and material breakage can create debris, which can fly in multiple directions with safety implications. The handling of carbon fibers may often be difficult and some protection is necessary due to their intrinsic brittleness. The sized material must be appropriately chosen since it must provide consistent handling without swelling residues on the processing equipment and obstacles to the penetration of the resin matrix into the fiber bundle. However, a different blend of physical characteristics optimized for the fiber shape and surface texture is required due to the differences in commercial carbon fiber surface features (i.e., striated, round, smooth, or kidneyshaped, etc.). Finally, the size of the materials should equally be compatible with the resin matrix to facilitate some of the most important precursor steps such as their solubility and/or reactivity with the formulated resin which allows the resin matrix to better impregnate the fiber bundle and form an adequate fiber-matrix interface.

1.6.3 Aramid Fibers Aramids constitute a family of generic organic material made from polyamides and they belong to the broad family of nylons (often referred to as aliphatic polyamides) though they are a little bit different from the common nylons. The main difference between these two families (aramids and common nylons) is that common nylon is an aliphatic polyamide containing amide-carboxylic bonds only while aramids have an aromatic ring structure in addition to these amide-carboxylic bonds

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providing them with higher tensile strength and thermal resistance than the aliphatic polyamides. Aramids are grouped into meta-aramid (e.g., Nomex®) and para-aramid (e.g., Kelvar®) depending on the position of the amide group (i.e., meta-position or para-position) and both of these two groups have aromatic chains between their amide groups which provide the resultant fibers with unique properties [44]. Aramid fibers present several properties including very high performance, very high chemical and thermal stability (attributed to the presence of the aromatic rings and the added strength of the amide linkages), high toughness, and exceptional tensile strength and modulus. The aramid fibers have high energy absorption during failure, which makes them ideal for impact and ballistic protection. Because of their low density, they offer a high tensile strength-to-weight ratio, and high modulus-to-weight ratio, which makes them attractive for aircraft and body armor. Conversely, they have relatively poor shear and compression properties, which requires an exceptionally careful design particularly when they are to be used in structural applications that involve bending or compression. Aramid was introduced by DuPont in the 1960s (Kevlar is the registered trade name of DuPont aramid) as a result of research on aliphatic polyamide fibers. Aramid fibers were introduced in the marketplace for the first time in the 1970s as a replacement for asbestos for tire reinforcements. As indicated in the previous paragraphs, their chemical structure presents the aromatic benzene rings along the polymeric backbone; and indeed, their name derives from the blend of aromatic polyamide. These types of fibers present good strength and modulus, with compression and shear strength similar to E-glass, but low density and UV resistance. Additionally, Aramid fiber has superior toughness and resistance to impact and heat (up to 500 °C) related damage, which makes it ideal for use in armor, as well as the military and ballistic applications in the manufacturing of safety equipment such as ballisticrated body armor fabric and ballistic composites, helmets and bulletproof vests, as well as the fabrication of firefighting protection devices/equipment. Aramid fiberreinforced composites are also used in aerospace and military applications, marine cordage, and marine hull reinforcement. In the aerospace industry, in particular, aramid fiber-reinforced composites are used to produce the underside of airplanes (these are the structural body of the airplane that protects it against the stone strikes during the landing and takeoff process) and the underside of the race cars to give them additional strength and resistance to impact damage. In general, structures that are exposed to impact, abrasion, and/or heat damage are the best candidate structure that could benefit from the combined resistance properties of aramid fiber-reinforced composites. Although the utilization of aramid fiber-reinforced composite is highly beneficial, the manufacturing of aramid fiber-reinforced composite materials and structural systems is particularly difficult than glass and carbon fibers-based composites due to several drawbacks including (1) aramid fibers are extremely tough and their fabrics prove significantly difficult to cut through using conventional tools, (2) aramid fabrics are hard to wet out compared with glass fibers and carbon fibers, (3) orthophthalic polyester is almost impossible to adhere to these types of fibers, and (4) transformation of raw composites into finished products (e.g., making components, trimming off the edges, polishing the finished structures, etc.) are also demanding.

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In this context, studies suggest that a good compromise may be the use of carbonaramid hybrid reinforced composites, which share the high strength and stiffness of carbon and the impact protection of aramid. Apart from carbon-aramid hybrid reinforced composites, there are certainly other possible combinations that can be considered for hybrid composites with aramid fibers and users should investigate these possibilities based on the applications and the properties required. Aramid fibers are extensively used in both composites and nanocomposites applications, in diverse industrial sectors including aerospace, sports, automotive, and marine. The use of aramid fibers depends upon the specific type and fiber form and their physico-mechanical properties. Typically, meta-aramid fibers such as Nomex® are generally known for, their excellent thermal resistance, good textile characteristics and poor mechanical properties, hence they are commonly used in protective clothing, reinforced belts, hoses, industrial coated fabrics, etc. Para-aramid fibers such as Kevlar® and its various subtypes (e.g., Twaron®, etc.) possess excellent mechanical properties. Continuous filament yarns and rovings of para-aramids are used in composite pressure vessels, rocket motor casings, sporting goods, rope, and cable, etc. Fabrics are used in facings of sandwich constructions in aircraft systems and helicopters, boat hulls, etc. Staple fibers are used in automotive applications such as brake and clutch linings, gaskets, etc. In addition to glass, carbon, and aramid fibers, there are other reinforcing fibers used in fiber-reinforced composite materials; notable among these are boron and extended chain polyethylene fiber. Ceramic fibers and whiskers are commonly used in MMCs and CMCs. A unique class of reinforcing fibers is natural fibers, and it has become a subject of active research now. Aramid fibers can be chopped into staple form to make felt for applications such as chain saw-protective garments, or they may be blended with other fibers for other end uses. Aramid fiber is lyotropic. It is solution-spun and it melts at a lower temperature than a thermotropic liquid crystal fiber.

1.6.4 Boron Fibers Generally speaking, boron fibers are high-performance fibers with tensile strength and a tensile modulus. They are produced through the chemical vapor deposition process from a mixture of hydrogen and boron tri-chloride on fine diameter tungsten filaments at high temperatures (i.e., around 1200 °C). In order to get the boron filaments, boron is continuously deposited on a tungsten wire core, as well as on a glass or graphite filament core. In case the tungsten is used to collect boron filaments, the latter is rather fat with an overall diameter ranging from 100–140 µm because the tungsten wire core is very fine with a nominal diameter of 13 µm. Boron fibers are characterized by la ow density (i.e., 2.6 g/cm3 ), a high tensile strength (i.e., ranging from 3100–4200 MPa), and a high modulus of elasticity (i.e., of up to 360 GPa). Although boron fibers can be formed into resin-impregnated tapes for hand lay-up and filament winding processes, these types of fibers are extremely stiff (e.g., five times stiffer than the usual glass fibers), and hence, they are difficult to weave, bend, braid or

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twist. Another advantage of boron fibers relates to their high compressive properties, and their ability to retain their mechanical properties at high temperatures of up to 800 °C. With that said, applications of boron fibers have been limited to their use in experimental aircraft and spacecraft systems due to the high cost associated with these fibers. Additional applications of boron fibers involve their use as reinforcements for MMCs and for some boron/epoxy composites used to manufacture certain sports equipment.

1.6.5 Polyethylene Fibers Extended chain polyethylene (PE) fibers are a type of high-performance organic fibers with some extraordinary properties, though not as popular as glass, carbon, or aramids. These fibers are produced by solid-state extrusion and gel spinning methods, and they possess a combination of favorable and unfavorable properties. Notable among the favorable properties are very low density (0.97 g/cm3 ), high tensile modulus (130 GPa) and high tensile strength (2700 GPa), outstanding fatigue and impact resistance, excellent environmental resistance, etc. However, they are poor in creep, compression, and transverse directional properties. Further, these fibers have a very low service temperature, and untreated extended chain PE fibers do not have good compatibility with the resin matrix. Typical applications of these fibers include bulletproof vests, military helmets, ropes, cables, nets, surgical gloves, sports goods, etc. Gel-spun polyethylene fibers are ultra-strong, high-modulus fibers that are based on simple and flexible polyethylene molecules. They are called high-strength, light-weight polyethylene fibers, high-modulus polyethylene (HMPE) fibers, highperformance polyethylene fibers, or sometimes extended chain polyethylene fibers. The gel-spinning process uses physical processes to make available the high-potential mechanical properties of the molecule. Due to low density and good mechanical properties, the performance on a weight basis is extremely high. The chemical nature of polyethylene remains in the gel-spun fiber and this can both be positive and a limitation: abrasion and fatigue properties are very high but the melting point is limiting certain application areas. Nowadays the versatile HMPE fibers are widely used in a wide range of industries like military and law enforcement (ballistic protection), marine, offshore, commercial fishing, sports, forestry, protective clothing, aviation, and medical, and a variety of composite applications.

1.6.6 Ceramic Fibers and Whiskers Ceramic fibers both with oxide as well as non-oxide compositions are also used in composites. Typical examples of oxide fibers are alumina (Al2 O3 ), alumina-silica (Al2 O3 -SiO2 ), and zirconite (Zr-Al2 O3 ), whereas silicon carbide (SiC) is a typical non-oxide fiber. Ceramic fibers are available in continuous as well as discontinuous

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forms, and due to their high-temperature stability, they are commonly used as reinforcements for MMCs and CMCs in high-temperature applications. Whiskers are single-crystal fibers with nearly zero defects. They are very short in length, but their aspect ratios are very high. Whiskers, owing to their nearly perfect crystal alignment and defect-free structure, possess high mechanical properties compared to their bulk forms.

1.6.7 Natural Fibers Interest in natural fibers as an alternate class of fibers for composites has been growing and a considerable amount of work has been published especially in recent years. Natural fibers are generally obtained from three broad sources of natural fibers viz. plants, animals, and minerals. Plant fibers are the most abundant types of fibers and depending on the specific part of a plant that is used, these types of fibers are categorized into bast fiber (i.e., fibrous material from the phloem of the plant or simply the fibrous vascular tissue of a plant), seed fiber (i.e., fibers collected from the seeds of the plant), leaf fiber (i.e., typically sisal and abaca, leaf-fibers are hard fibers that are rich in lignin content and mainly used for cordage or producing rope), fruit fiber (i.e., fibers that are contained in the fruits of the plant), and stalk fiber (i.e., natural fibers that are taken from the stalk of a plant). Similarly, animal fibers can also be categorized into wool or hair, silk fiber, and avian fiber. Natural fibers such as jute, sisal, flax, hemp, silk, bamboo, and coir possess several favorable characteristics viz. inexpensive, abundant, and renewable, are lightweight, with low density, high toughness, and are biodegradable. Additionally, these types of fibers are also biocompatible and environment-friendly [45]. Looking at their advantageous properties, natural fibers such as jute have the potential to be used as a replacement for traditional reinforcement materials in composites for applications that require a high strength-to-weight ratio and further weight reduction. However, mechanical properties of natural fibers (e.g., batch-to-batch inconsistency and other fiber quality considerations, high flammability and thermal instability, performance limitations including the tensile strength, impact strength, and low thermal resistance; as well as odor and fogging, etc.) are generally poor and non-uniform compared to their synthetic counterparts (i.e., glass, carbon, and aramid fibers, etc.) [45–47]. In addition to the aforementioned, natural fibers also present relatively variable dimensions (i.e., difficult to control), poor compatibility with most of the matrix systems as well as a higher susceptibility to rotting and hydrophilic character makes them vulnerable to high moisture absorption feature are the other two of their major drawbacks that affect the properties of natural fiber polymer composites, making them unusable in applications where these drawbacks might be a challenge. Although their disadvantages far outweigh their advantages, further applications for natural fibers are currently being explored in cutting-edge technologies. To this end, some of the present applications of natural fibers are in the automotive industry (i.e., the two most important natural fibers are flax and hemp). The latter are used to build

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automotive interiors such as car seats and dashboards and they help in a drastic weight reduction of a car (i.e., enabling the car’s capabilities of producing fewer emissions and enhancing its fuel efficiency). Natural fibers are also an essential element of materials used in the construction industry (e.g., particularly in the manufacturing of doors, windows, etc.) [45, 48]. While the application of natural fibers in structural engineering continues to grow, it is important to understand the individual properties and growing conditions of widely used plant-based fibers, to use them effectively in composites applications. Consistent with the aforementioned, the manufacturing of natural fibers-based composites requires the manufacturers to maintain the uniformity of the final product to evaluate the properties of the composites effectively. For example, any addition of hydrophilic natural fibers (e.g., kenaf) to hydrophobic plastic (e.g., polypropylene (PP)) will result in a composite with poor properties due to non-uniform fiber dispersion in the matrix and an inferior fiber-matrix interface. However, this problem can be overcome by the suitable selection of a compatibilizer (e.g., maleated PP) or by surface treating the fibers. As a result, in recent years, natural fiber usage has substantially increased in injection molding due to the good mechanical properties of the products and an increased cost-performance ratio in comparison to common injection-molded synthetic plastics [49, 50]. The limited thermal stability of natural fibers, which leads to degradation during processing beyond 200 °C, also restricts the use of mass manufacturing methods and this has to be overcome by improving the precision of effective manufacturing techniques. Another challenge in composites manufacturing using plant-based natural fibers is to maintain a high aspect ratio in order to achieve superior thermal, mechanical, and functional properties. Composite processing methods, such as extrusion and injection molding, have a significant influence on fiber aspect ratio and fiber length retention [51–53]. It is important to disintegrate fiber bundles into individual fibers without damaging them. Effective dispersion and distribution of fibers within the matrix material by avoiding agglomerates can create homogeneous composites with enhanced properties. It has been found that the reduction in fiber length associated with extrusion or injection molding may not affect the tensile properties significantly due to improvements in fiber orientation along the polymer flow direction and increased fiber dispersion through dimensional changes [50].

1.7 Design of Polymer-Matrix Composites Generally speaking, all of the advanced fiber-reinforced composite materials are designed materials and this surely underscores the premises of their usefulness in structural engineering. In particular, their properties can be optimized for specific applications based on the spectrum of matrix and reinforcement materials available. Typically, advanced composites can be designed to have zero coefficient of thermal expansion and the fact that they can be reinforced with combinations of different fiber materials (i.e., hybrid fiber-reinforced composite materials) and geometries

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to maximize performance and minimize cost is another added advantage [2, 54]. Until recently, the design opportunities for fiber-reinforced composite materials were still in their infancy and it was only from the 1980s where the research in composite materials started taking it to the next level making available so many types of composite designs that are available today [55, 56]. Although there has been tremendous progress in terms of design and manufacturing (i.e., thanks to the current technological advancements), the research suggests that a lot still needs to be done with some of the most advanced designs only starting to emerge now. Particularly, the enormous design flexibility of advanced composites is obtained at the cost of a large number of unfamiliar design variables. In fact, composites are more accurately characterized as customized structures, rather than customized materials. Although the engineering properties of the homogeneous resins and fibers can be easily determined, the properties of each type of fiber-reinforced composite material depend on the composition (i.e., material constituents), fiber length, and geometry as well as the nature of the interphase. The categories of mechanical and physical properties used to characterize fiber-reinforced composites stem from the long engineering experience with metals which may or may not provide adequate results given the distinct nature of the materials involved. A major and quite frankly urgent need to obtain better designs in advanced composites technology is a better capability for modeling the structures’ property relationships including the relationship of the materials with the stages of design and the optimization and with the mechanics of materials. There are many excellent textbooks available detailing the mechanics of fiber-reinforced composite materials such as those presented in Refs. [57, 58]. As such, the present book will not provide any theoretical formulations or derivations of mechanics of composite materials, and interested readers are directed to these specific books for more details on the mechanics of fiber-reinforced composite materials. It is also noted that even despite the lack of adequate knowledge related to materials’ mechanics, experience to date has shown that designers and manufacturers can produce quite reliable fiber-reinforced composite structures. This is probably because, in the face of uncertainty, designers tend to overdesign; that is, they are conservative in their use of material, to avoid any possibility of material failure. It is also important to indicate that fiber-reinforced composite structures are extensively tested before use, to prevent any potential problems that might arise during the in-service stage are detected during these preliminary tests (i.e., the testing). Thus, both the flexibility and advanced properties of fiber-reinforced composite materials themselves have been proven that they can be easily used to fabricate fiber-reinforced composite structures that are reliable and meet all the design criteria. However, both overdesign and empirical testing are costly and drive up the prices of fiber-reinforced composites. Thus, a principal benefit of enhanced modeling capability will be to help make advanced composites more cost-competitive [59, 60].

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1.8 Polymer-Matrix Composites Manufacturing Procedures In general, composites are produced through a wide range of manufacturing processes (e.g., hand lay-up, pultrusion, filament winding, resin transfer molding, etc.). However, the choice of the manufacturing procedure does often depend on the type of reinforcements (dry or pre-impregnated) and matrices (thermosetting or thermoplastics polymers) being used as well as the application of the resultant composite. Additional factors determining the type of the manufacturing process include the low cost of the manufacturing technologies as well as the ability to enhance the signature properties of the resultant structure through the manufacturing process. In the past, the manufacturing of composites was primarily dominated by the prepreg philosophy with the primary challenge for designers and manufacturers being to obtain composite materials of high performance to compete with, or surpass, metals. However, the production of composites with pre-pregs is expensive and, the attention is currently moving toward the use of new types of reinforcements and cheaper manufacturing processes. To this end, composites obtained from dry fabrics and liquid infusion processing are being considered for applications in various structural components instead of prepreg-based composites. As an example, non-crimp fabrics-bases composites, involving a multiaxial reinforcement (made up of multiple plies of dry fibers layered on top of each other or stitch-bonded with a polyester thread), represent a typical good compromise between the fiber strength and costs. Although there are several types of composites manufacturing procedures, the basic steps involved in each of them include (1) the curing process for thermosetting polymer matrices or the thermal processing for thermoplastic polymer matrices, as well as (2) the finishing process. (3) the impregnation of the fibers with the resin, (4) the formation of the composite structure, However, these steps may occur separately or continuously depending on the process. The following sections present a brief description of some of the most important composites manufacturing processes.

1.8.1 Hand or Manual Lay-Up Process Also called manual lay-up, hand layup is the simplest and oldest open molding method for fabricating composite materials with prepregs [8, 27]. It is based on the superimposition of pre-impregnated fibers or laminas of thickness in the range 0.125– 0.30 mm [8]. At first, dry fibers or laminas in the form of woven, knitted, stitched, or bond fabrics are partially impregnated with resin, and then manually placed in the mold, and a brush is used to apply the resin matrix on the reinforcing material. The raw material is generally found in rolls or tapes, from which laminas or fibers of given dimensions are cut. The cutting operations are generally automated unless the number of pieces or parts required does not justify the cost of programming an automated cutter or in case the latter is not available at all. However, if hand cutting is

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to be selected, templates to facilitate the cutting operation may have to be fabricated. Also, if the lay-up requires any contour of the plies, the contour will equally have to be factored into the templates [27]. Automated materials cutting involves the use of both reciprocating knives and ultrasonically driven ply-cutting methods. In the former process, a carbide blade reciprocates up and down (similar to a saber saw) while the lateral movement is controlled by a computer-controlled driven head. To allow the blade to penetrate the prepreg (or any other material being cut), the bed supporting the prepreg consists of nylon bristles that allow the blade to penetrate during the cutting operation. The reciprocating knife cutter is usually less efficient because only one out of five plies can be cut during a single pass. The latter operates similarly but the mechanism is a chopping rather than a cutting action. Instead of a bristle bed that allows the cutter to penetrate the prepreg being cut, a hard plastic bed is used with the ultrasonic method. It is worth mentioning that most of the modern automated ply-cutting equipment is fast and produces high quality cuts. In addition, many of these systems have automated ply-labeling systems in which the ply identification label is placed directly on the prepreg release paper. A typical ply label will contain both the part number and the ply identification number. This makes sorting and kitting operations much simpler after the cutting operations are completed to avoid any possible confusion among the different parts, sizes, and destinations. As soon as the cutting operation is complete, obtained pieces are layered on top of each other following the predefined placement, shape, and stacking pattern; the latter entails the orientation of fibers and the composite thickness. Prior to performing ply or fibers collation, the tool should have either been coated with a liquid mold release agent, or covered with a release film to avoid the part being stuck to the mold. If the surface is going to be painted or adhesively bonded after cure, some lay-ups also require a peel ply on the tool surface (usually nylon, polyester, or fiberglass fabrics coated with release agents or not). However, it is important to thoroughly characterize the peel ply material that is bonded to a composite surface, particularly if that surface is going to be structurally adhesively bonded in a subsequent operation. Prior to placing a ply onto the lay-up, the operator should make sure that all of the release paper is removed and that there are no foreign objects on the surface. Once the laminas are stacked, a vacuum bag is used to remove entrapped air (or any other gases developed during the process) and an autoclave cycle curing is performed (i.e., the heating cycle required to start and complete the cure of the resin matrix). In general, the curing cycle involves cooperation between the levels of temperature and the vacuum to achieve two main purposes: (1) reach the correct viscosity of the resin for its uniform distribution throughout the laminate, and (2) eliminate the entrapped air or other developed gases to remove voids and porosity (i.e., voids and porosity). This procedure allows the manufacturing of fiber-reinforced composite materials with optimal physical and thermomechanical properties, but it requires skilled personnel. In particular, voids and porosity should be minimized at all costs when manufacturing reinforced composite materials because if not addressed, they severely affect the mechanical properties and limit the lifespan of the composite by degrading its matrixdominated properties (e.g., interlaminar shear strength, longitudinal compressive strength, and transverse tensile strength).

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1.8.2 Filament Winding Filament winding is a mature fiber-reinforced composite materials manufacturing technique that is primarily used to obtain open-end (cylinders) or closed-end structures (pressure vessels or tanks) [42]. At a minimum, this type of manufacturing process requires a tow of pre-impregnated fibers and a rotating male mandrel whereby the tow is continuously wound onto the mandrel following the desired design (i.e., pattern or angle) by dedicated software to obtain a specific fiber orientation and quantity that fulfills the design requirement of the composite [12]. Polar, helical, and hoop are generally the three dominant winding patterns used in filament winding systems [27]. According to Ref. [42], this technique has been in continuous use since the mid-1940s and the most commonly used filaments are glass and carbon and these are continuously impregnated in a bath with resin as they get wound onto the rotating male mandrel (the choice of the mandrel type and design is determined by the function of the design and size of the part to be built). After the winding process is completed (i.e., once the mandrel is completely covered to the desired thickness), and depending on the resin system that is used and its cure characteristics the wet wound mandrel is put in a vacuum bag and then in an oven (preferred method) or sometimes in an autoclave or simply placed under radiant heaters for curing [27]. The hollow composite is obtained after the resin system has completely cured and the mandrel is removed. However, some products such as gas bottles require the mandrel to remain in place as an integral part of the finished product forming a liner to prevent gas leakage or as a barrier to protect the composite from the fluid to be stored when in service. The controlled variables for winding are fiber type, resin content, wind angle, as well as the tow or bandwidth and thickness of the fiber bundle. The angle at which the fiber is wound has an effect on the properties of the resultant composite. A high angle “hoop” will provide circumferential strength, while lower angle patterns (either polar or helical) will provide greater longitudinal/axial tensile strength. The procedure is completely automated and can be used to fabricate almost any body of revolution, such as cylinders, shafts, spheres, and cones [27]. It also allows the easy fabrication of pressure vessels, aircraft bodies, power transmission poles, bicycle rims, pipes, and many more.

1.8.3 Pultrusion As the name indicates, pultrusion is a portmanteau word combining “pull” and “extrusion”. However, in pultrusion, the materials get pulled while they get pushed in extrusion. Pultrusion consists in pulling the raw materials into a shaped die to obtain the composite, including the fiber impregnation pre-form and curing. Generally speaking, the process begins with racks, or creels, holding rolls of fiber mats or doffs of fiber rovings. The raw fiber is then pulled off the racks and guided through a resin impregnation system (the resin can also be injected directly into the die depending

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on the pultrusion system). After that, the pre-impregnated fibers are guided through a series of tooling (pre-form), which organize the fiber into the correct shape, while the excess resin is squeezed out of the composite. In its final step, the obtained composite passes through a heated steel die where an exothermic reaction takes place to cure the resin matrix. At this stage, the component is continuously dragged and exits a hot mold with a constant cross-sectional area, and the final product gets cooled by forced air convection fed into and then cut into small pieces consistent with the application being considered. Pultrusion is generally cost-effective particularly when it comes to the production of high volumes of constant cross-section composite parts. The earliest reports regarding the “birth” pultrusion pertain to the patent developed by J. H. Watson regarding the manufacture of a string or the like in 1944. A latter patent filed by M.J. Meek in the 1950s focused on the method for fabricating a glass rod. In 1952, Rodger White filed his patent pertaining to the manufacturing of articles from thermosetting materials. These were followed by W. R. Brandt Goldsworthy in 1953, with his patent entitled “Apparatus for producing elongated articles from fiber-reinforced plastic material” who is considered the initiator of pultrusion to this day. Parallel to the work of Goldsworthy, who concentrated his work on unsaturated polyester resins, Ernst Kühne developed a quite similar process in 1954 based on epoxy resin. To date, the invention, development, and issuance of patents continue in the field of pultrusion with the latest system being developed and patented by Thomas GmbH + Co. Technik + Innovation KG in 2008 (Germany).

1.8.4 Resin Transfer Molding Resin transfer molding (RTM) [13] represents a viable alternative to prepregs and autoclave-based composites production. It involves a rigid two-sided mold set that forms both surfaces of the panel. The molds are typically constructed from aluminum or steel, but composite molds are sometimes used. The two sides fit together to produce a mold cavity. The detailed process of the RTM starts with the reinforcement mat, or woven roving, being draped in the bottom half of the mold. Then, the top section of the mold is closed and catalyzed, low-viscosity resin (i.e., heated resin) is pumped into the mold using a high-pressure pump, displacing the air and venting it to the edges of the mold until the latter is filled up. After that, the mold is clamped off and the resin matrix is allowed to cure until the composite is completely formed. One of the main advantages of the RTM is that the dimensions of the obtained composites solely depend on the shape of the molds and their distinguishing features. Also, the reinforcement materials are first placed into the cavity of the mold set and closed before the introduction of the matrix material, which makes the process easier to handle and relatively clean. In this context, the obtained composites do generally have a good surface finish on both sides, suggesting that they are generally ready to be used without further treatments. This process is cheaper than most of the other composites manufacturing processes and it is currently gaining popularity in many different types of industries including the aerospace industry where components such

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as door pillars, stringers, stiffened panels, rudder tips, and ribs are manufactured by RTM. Although the above indicates the general operation of the RTM process, the technique does include numerous variants which differ in the mechanisms of how the resin is introduced to the reinforcement in the mold cavity. These variations include everything from the RTM methods used in out of autoclave composite manufacturing for high-tech aerospace components to vacuum infusion (that is primarily used in the boat building industry) to vacuum-assisted resin transfer molding (VARTM). RTM can be performed at either ambient or elevated temperatures and is suitable for the manufacturing of high-performance composite components in medium volumes.

1.8.5 Resin Film Infusion The resin film infusion (RFI) is a composite manufacturing process that was developed by NASA and the Long Beach division of McDonnell-Douglas (Boeing). The development was driven by the desire to obtain 3D reinforcement for damage-tolerant wing design for commercial aircraft and be able to use an adequate prepreg resin system for the matrix resin and be able to overcome the challenges related to the requirement of minimum viscosities observed in conventional RTM when using prepreg resin systems. It is noted that the minimum viscosities required in conventional RTM are usually greater than 500 cps which are too high to be able to successfully inject and fill the stitched preform during injection [42]. Unlike conventional RTM, the RFI process requires just one male or female mold of the desired shape to manufacture the composite [14]. In this mold, dry fabrics are first laid up interleaved with the layers of pre-catalyzed semisolid resin films supplied on a release film/paper, and then the entire lay-up is placed in a vacuum bag where it is vacuumed to remove the air entrapped in the dry fabrics. Finally, the vacuumed lay-up is placed in an oven or autoclave to thermally cure the resin matrix, while the vacuum is continuously applied. In this process, the temperature is first increased to reduce the resin viscosity to a level when it is fluid enough to flow into the fabric layers of the composite under the applied pressure and once the infusion is complete the pressure and temperature are raised to consolidate and fully cure the composite. Understanding the compaction and permeability of the preform as well as the viscosity and kinetics of the resin system is the towards a successful RFI process. In addition, the design of the preform and its placement within the tooling, as well as the design of the tooling itself and its dimensional control are critical for this process. A special cure cycle must be developed to achieve the correct time–temperature-viscosity profile to ensure complete saturation of the preform. A variant of this process is resin liquid infusion (RLI), in which a liquid resin instead of a solid resin is placed or injected into the bottom of the tooling prior to loading the preform. This process allows to reach high specific strength and it is suited for making relatively large structures such as stiffened skins and rib-type structures [61] but it is not generally used for manufacturing of structural parts.

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1.8.6 Resin Liquid Infusion In the composite manufacturing industry, the resin liquid infusion is a composite molding process whereby the liquid resin is drawn into a stack of dry fabrics with the aid of vacuum pressure to impregnate the laminate. This process is technically similar to the RFI with the only difference being the fact that only dry fibers are placed in the mold (i.e., single-sided mold), which is enclosed in a vacuum bag and connected to the resin supply source and the vacuum pump. In this context, the liquid resin with hardener infuses into the reinforcing fibers, thanks to the vacuum pumping the resin matrix into the mold. In the field of composites, this method is particularly effective because it helps to obtain fibrous composites with a limited amount of voids (i.e., the voids in an evacuated stack of porous material are filled with liquid resin). Curing and de-molding steps follow the impregnation process to finish the product. One of the main advantages of this process is that it is capable of producing large and thin parts in a relatively inexpensive way. In the composite manufacturing industry, resin liquid infusion technology is generally used to manufacture composite parts for aerospace primary and secondary load-bearing structures such as engine fan blades, composite wing boxes, spoiler components, etc.

1.9 Fiber Metal Laminates As their name indicates, fiber metal laminates (FMLs) are laminated materials consisting of thin layers of metal sheet and unidirectional fiber layers embedded in an adhesive system. FMLs are particularly attractive for the aerospace industry because they combine the high bearing strength and impact resistance features of the metals with the excellent fatigue characteristics, high strength, and stiffness, as well corrosion resistance of fiber-reinforced polymers. All these properties and advantages combined enable these types of materials to overcome most of the disadvantages of single monolithic materials sheets. FMLs were conceived at the Delft University of Technology in the Netherlands, starting from the end of the 1970s (i.e., with the first patent on FMLs filed in the USA in 1981 [62]), with the idea to increase the fatigue performance of aluminum alloys. Although fiber-reinforced polymers were widely studied at that time, they were still very expensive, and, against this background, researchers wanted to combine the metals and fiber-reinforced polymers to obtain a kind of material with intermediate metallic and fiber-reinforced polymers features. A significant improvement was observed when aramid fibers were introduced into the adhesive layers, and with that, the first FMLs were called aramid fiberreinforced aluminum laminates (ARALL) [8]. Four different types of ARALL were first investigated passing transitioning to carbon-fiber-reinforced aluminum laminates (CARALL) and then to glass-fiber-reinforced aluminum laminates (GLARE). All these types of materials present different properties and interested readers are directed to ref. for more information.

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Glass fiber laminate aluminum reinforced (GLARE) is a type of aluminum fiber metal laminate composite, in which unidirectional S-2 glass fibers are embedded in FM-34 epoxy structural film adhesive and is normally available in six different standards grades [27]. Exceedingly improved impact resistance and mechanical properties make Glare one of the most successful FML for advanced aerospace structural applications. Unlike classical composites, Glare generally undergoes plastic deformation under impact with the indenting of the external aluminum layer (which is easily noticed by the naked eye), making it easier to diagnose and take necessary action concerning the replacement and/or repair. In addition, the growth rate of fatigue cracks, which is normally high in aluminum sheets, is significantly reduced in the presence of glass fibers. As a most important feature, Glare has good burn-through resistance and it is therefore considered one of the optimal materials for the fabrication of parts that are most exposed to heat burns during the service stage. However, Glare is extremely expensive and users should examine its benefits based on the application. In modern aircraft, Glare is mainly used for the fabrication of the aircraft fuselage (e.g., Airbus A380) and the leading edge of the tail surfaces. Gunnink and Vlot [62] provided additional information regarding the historical overview, manufacturing process, properties, and the application of FLMs, and interested readers are directed to their study for more information.

1.10 Sandwich Structures Sandwich structures are panels of lightweight core (whose material may be lowdensity foam or honeycomb) sandwiched between two relatively thin but hard and strong sheets of laminates. The literature indicates that various types of core and skins can be used depending on the performance and the application requirements of the final part. However, since sandwich structures must generally be strong and lightweight at the same time, a good solution has always been to insert a cellular structure like an open or closed-cell foam, honeycomb core (both metallic and nonmetallic configured in hexagonal, flexible-core, or over-expanded shapes), balsa wood, as well as syntactic between two thin composite laminates. Although the honeycomb cores are generally more expensive and difficult to fabricate than any other sandwich cores, the former provides superior performance to the structure and this may be the reason why many commercial applications use foam cores, while aerospace applications use the higher performance and are more expensive honeycomb cores (e.g., Nomex® which is a honeycomb core made from aramid paper). It is also noted that the foam materials are normally much easier to work with than their honeycomb counterparts. Typically, the foam core sandwich assemblies can be bonded together with supported film adhesives but the more common case is to use either liquid/paste adhesives or do wet lay-up of the skin plies directly on the foam surface. Quite recently, foam cores with dry composite skins are being impregnated and bonded with liquid molding techniques, such as RTM or VARTM. In contrast to the foam core sandwich assemblies, supported film adhesives are normally used to

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bond composite structural honeycomb assemblies. While the foam and honeycomb cores are the most commonly used sandwich cores, the materials for the sandwich cores should be carefully selected as they determine the performance and properties of the final parts. Consistent with the aforementioned, it is noted that aluminum cores have the best combination of strength and stiffness, followed by the non-metallic honeycomb cores and then polyvinyl chloride (PVC) foam. However, the in-service experience with aluminum honeycomb cores has not always been good. Apart from the fact that aluminum honeycomb cores have durability problems, they are also susceptible to moisture migrating into the assemblies and causing corrosion of the aluminum core cells. There is also a problem with liquid water, which upon entering the core (through exposed edges, such as panel edges, closeouts, door and window sills, attachment fittings, or almost any location that the skin and core bond terminate) causes the degradation of the skin-to-core bond strength, the fillet bond strength, and the node bond strength and the failure of the structure may follow. It is also very difficult to make major repairs to honeycomb assemblies. In recent years, the shift is being directed towards the development of new materials (e.g., metal foams) that can be used as the core in sandwich structures against the aforementioned background. A metal foam is a cellular structure consisting of solid metal with gas-filled pores comprising a large portion of the volume as their signature characteristic. Interestingly, this foam is generated by forced expansion of a precursor, which is generally an aluminum alloy mixed with a percentage of foaming agent. As a great advantage, a metal foam is generally much lighter but it maintains nearly all the mechanical characteristics of the initial solid material. As such, metal foams present good stiffness and strength-to-weight ratios as well as the ability to absorb both the impact energy and the electromagnetic waves. Also, metal foams can be used as acoustic insulators (i.e., soundproofing materials that prevent the sound from entering or exiting an enclosed space by creating some barrier between the interior and the exterior areas) and/or thermal insulator (i.e., reduction of heat or thermal energy transfer between objects of differing temperature), owing to the air pockets contained in their structures.

1.11 Properties of Fiber-Reinforced Composites In general, the properties of fiber-reinforced composite materials depend on the matrix, the reinforcements (i.e., types of fibers), and their direction in the composite, as well as the quality of the interphase between the different laminates or plies. Consequently, there are many variables to consider when designing a fiber-reinforced composite material to ensure the optimization of the aforementioned parameters. Individual material constituents (i.e., matrix and reinforcement) are generally not considered alone, composites designers should also consider their relative proportions, the geometry of the reinforcement, and the nature of the interphase. Each of these variables must be carefully controlled to produce a structural material system

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optimized for the conditions for which it is intended to be used. Typically, the use of continuous-fiber reinforcement confers a directional character, called isotropy, to the properties of fiber-reinforced composite materials. It is noted that fiber-reinforced composite materials are strongest when stressed (i.e., when the load is applied) parallel to the direction of the fibers (0°, axial, or longitudinal, direction) and weakest when stressed perpendicular to the fibers (90°, transverse direction). In practice, however, most structures are subjected to complex loads, necessitating the use of fibers oriented in several directions (e.g., 0◦ , ±45◦ , and 90◦ ). While the above loading frameworks are generally not practical, fiber-reinforced composite materials users should know that these types of composites are most efficiently used in applications that can take advantage of the inherent anisotropy of the materials. Users of fiberreinforced composite materials should also know that when discontinuous fibers or particles are used as reinforcement, the properties tend to be more isotropic because these reinforcements tend to be randomly oriented. In this case, fiber-reinforced composite materials will be partially deprived of their continuous fiber’s full strength although they can be produced more cheaply by using the technologies developed for unreinforced plastics viz. extrusion, injection molding, and compression molding. A typical example of such materials is the sheet molding compound (SMC) which is widely used in the automotive industry [63]. Longer fibers in SMCs result in better strength properties than standard bulk molding compounds (BMC) products. Although SMCs require extensive disposal of the hazardous chemicals (i.e., one of their negative effects) [64], they have proved themselves to be versatile reinforced plastics in the areas of painted and unpainted (e.g., bumpers, fenders, exterior and interior panels, structural elements, and more recently, high-temperature underhood parts) automotive componentry [65]. Consistent with the aforementioned, fiber-reinforced composite materials offer the best combination of cost and performances, and often exhibit comparable and in some cases better properties than the traditional metallic materials. Indeed, the low densities of the fiber-reinforced composite materials along with their HT-to-weight and modulus-to-weight ratios make them suitable candidates to be used as replacements to some metallic materials in exigent applications requiring high-performance (HP), lightweight structures [66]. However, the complexity of advanced composites can complicate any efforts to compare their properties with those of conventional materials. In general, however, properties such as specific strength are relatively easy to compare although advanced composite materials tend to have higher specific strength and stiffness than metals. Also in many cases, properties such as the composite’s toughness and surface hardness that are easily defined in metallic structures are less easily defined in their advanced composites counterparts. While the dynamics of crack propagation and failure are relatively well understood in metals, for example, toughness can be defined relatively easily. On the contrary, toughness in advanced composite structures is a joint complicated function of the matrix, fiber, as well as the interphase and the reinforcement geometry. Additionally, both the shear and compression properties of advanced composites are poorly defined. Another result of the complexity of fiber-reinforced composite materials is that the mechanical properties are highly interdependent and should not be analyzed independently. As an

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example, the matrix cracking associated with the development/application of shear stresses may result in a loss of stiffness. Impact damage can seriously reduce the compressive strength and the load-bearing capacity of fiber-reinforced composites. This is because the compressive and shear properties of advanced composites can be seen as strongly related to the toughness of the matrix on the one hand and the strength of the interfacial bond between matrix and fiber on the other.

1.12 Main Types of Deficiency in Polymer-Matrix Composites As the name indicates, composite materials are made of two or more basic materials (referred to as material constituents) and manufactured through complex processes based on temperature, pressure, and chemical reaction modifications to obtain materials with improved properties. In this context, it should be expected that the final product may be affected by types of anomalies of different sizes and significance that could subsequently affect its performance or utilization altogether. Apart from the abnormalities that can arise during the manufacturing process, the in-service environment or mode of utilization of the final composite components can also be responsible for damage formation and/or material degradation. In the following discussion, the defects that are mostly present in composites will be grouped into two categories: manufacturing defects and in-service failures.

1.12.1 Material Processing Defects As the name indicates, material processing defects occur in fiber-reinforced composite materials during the preparation and processing of their material constituents [18, 67]. Generally speaking, most of these types of defects are caused by improper or poor storage conditions, poor quality control, as well as the prefabrication process, material mishandling, and batch certification processes that lead to raw materials or prepreg variations and/or damage [67–69]. Although these types of material processing defects are susceptible to occur in nearly all the types of fibrous composites, some of these defects are more prevalent in some of the composites than they are in others. Typical materials processing defects include but are not limited to variations/damaged filaments such as broken filaments, knots, splices, split tow, fiber separation, as well as hollow and interrupted fibers [18]. In other types of composites such as metal-matrix composites and fiber-metal laminates defects such as improper/poor storage, poor quality control, and over-aged materials are often observed [69, 70]. One of the most dominant consequences related to material processing defects is that they significantly reduce the interfacial strength between the matrix and the different material constituents of the composite of interest, which

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further degrades its overall stiffness and strength making it unable to fulfill its functional requirements [71]. Although the variation in different constituents is often seen as a component manufacturing defect that is controlled by the curing process, research suggests that it may also be a material processing defect under some circumstances suggesting that parameters setup, the manufacturing environment, and the integrity of the individual materials should all be controlled to obtain the desired fiber-reinforced composite materials [18]. Additional materials processing defects in fiber-reinforced composite materials include fiber misalignment, marcelled fibers, wrong matrix percentage vis-à-vis other constituents, fiber miscollination as well as over-aged prepreg among others [18, 68]. If and when these defects occur in the composite, they significantly degrade its properties [32] and its response to various loading conditions will significantly differ from its original design specifications and ultimate failure may follow. The above constitute just the basic information regarding the material processing defects occurring in fiber-reinforced composite materials and interested readers are encouraged to read [17, 18, 41] for more information and descriptions of other types of material processing defects.

1.12.2 Manufacturing Defects As the name indicates, manufacturing defects are those accidental defects (i.e., unintended defects) that occur in the composites during the manufacturing process as a result of poor handling and/or control of the materials as well as the contaminated environment. Several types of defects may occur during the fabrication of fiberreinforced composites, the most common being a fiber/ply misalignment, broken fibers, resin cracks or transversal ply cracks, voids, porosity, slag inclusions, nonuniform dispersion of fiber/resin volume ratio throughout the entire composite, disbonded interlaminar regions, the presence of sections with kissing bonds, incorrect cure and mechanical damage around machined holes and/or cuts to list but a few. While the effective performance of fiber-reinforced composites depends on the correct alignment of reinforcements (i.e., in this case the fibers) vis-à-vis the stress/strain direction within the composite. In fact, in the presence of fiber misalignment, the loading of the fibers may change from straight tension/compression loading to shear loading of the weaker interface. This may result in a considerable drop in the composite mechanical properties, and hence, the composite will be unable to fulfill its functional requirements [17]. Also, different types of inclusions such as dirt and debris may unintentionally contaminate the matrix, causing areas of local stress concentrations in the composite, and delamination may follow (i.e., this kind of delamination is susceptible to occur in the composite during the manufacturing process or the in-service stages). In retrospect, we note that the strength of CFRP components is strongly dependent on both the volume percentage of the resin matrix and the fiber volume fraction/percentage in the composite also known as content in fiber volume fraction. In fact, the presence of regions of fibers unsupported by the polymeric matrix can induce

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1 Introduction and Background of Fiber-Reinforced …

local stress concentrations throughout the composite (i.e., also known as the notch effect) followed by severe degradation of its strength and stiffness which shortens its service life span. Amongst the procedures that allow adequate production and subsequently the quality of the resulting composite, the most important is probably the resin curing process. Indeed the curing process has to be optimized in order to get appropriate components responding to the structural design requirements. Otherwise, resin curing is dependent on the temperature rate increase, the temperature and duration of the curing plateau, the time at which pressure is applied, and the postcuring temperature and pressure. If something unexpected occurs in the process, the typical consequence could be incomplete or inappropriate chemical reactions, uneven wetting of the fibers, incorrect fiber-volume ratio as well as the formation of local matrix-rich pockets or matrix-starved regions. Moreover, the vacuum pressure, if not suitable, could affect the degassing of contaminants with only partial removal of the gases developing during chemical reactions. This may induce the formation of voids (or porosity) within the matrix, between the plies, or at the fiber/matrix interface. The detrimental effects of porosity have been known since 1978 when it was found that there was a decrease of the interlaminar shear strength by about 7% for each 1% of voids up to a total void content of about 4% [72, 73]. The decrease of other properties for the first 1% of voids is reported as high as 30% (flexural strength), 9% (torsional shear), 8% (impact strength), and 3% (tensile properties). Although the above-described defects are found to be the most recurrent ones; research indicates that some other types of defects are peculiar to some specific manufacturing processes than others. In particular, composites manufactured using pre-impregnated layers may entail some specific defects due to the improper storage of pre-pregs such as out-of-date resin because of exposure to ambient temperature (higher than that required for correct storage), wrinkled surface because of an uneven position, which may result in, resin-rich regions within the laminate, accumulation of debris, resulting in slag inclusions and broken, or damaged, fiber tows resulting in reduced strength of the final composite laminate. Also, some defects can occur in the laminate during the processing in the autoclave, mainly because of the presence of the regions with inadequate cured resin following the use of incorrect pressure/temperature during the curing process. The application of non-uniform pressure on a certain surface of the composite will often lead to the lack of bonding between adjacent layers in the composite and this may often trigger additional damage (i.e., delamination, fiber breakage, etc.) and materials degradation (i.e., moisture ingress and high-temperature degradation). Additionally, other manufacturing techniques such as RTM often come with the formation of porosity due to the volatilization of dissolved gases in the polymeric resin, the mechanical entrapment of gas bubbles, or in some cases the evaporation of the mold-release agents/coatings. In particular, excessive void formation is often influenced by several factors including the resin properties, the molding temperature, as well as both the injection and the external pressure that are applied during the curing process. Indeed, whatever the type of the manufacturing defect is in the composite, it may result in slight, severe, or even variations of the material properties from those of its intended design which, in turn, leads to the reduction of its originally predicted life

1.12 Main Types of Deficiency in Polymer-Matrix Composites

35

span. In fact, these variations may act as sites for the initiation of fatigue damage or may facilitate the growth of a fatigue crack during cyclic loading. A comprehensive assessment of the quality of a composite structure prior to installation into service is hence as important as the monitoring of its levels of damage accumulation during the service [19]. And this probably justifies the increasing attention of many scientists and engineers toward the development of effective NDT techniques capable of detecting and characterizing the different defects at their incipient initiation, as well as adequate design procedures that can reliably achieve zero-growth thresholds for any type of defect. In theory, though, most of the manufacturing defects may be avoided and the overall quality of a composite component or material itself may be increased with the implementation of certain procedures and manufacturing practices combined with the use of specific instrumentation. As an example, the use of computer-controlled tapelaying machines may assure the construction of a prepreg stack for autoclaving to very high standards of quality and repeatability. Similarly, errors of control in pressing can be avoided to some extent by the use of automated autoclaves with pressure– temperature cycles carefully programmed from detailed chemical knowledge of the gelation and viscosity characteristics of the resin in use [74]. Of course, this entails extra costs that remain an industrial decision. In light of the aforementioned, one may want to know the real consequences of the presence of a defect in any composite structure. While this may seem to be a fair question to know the answer to, it is not easy to answer because many parameters should be defined before a plausible answer is provided. Among them, the size and orientation of the defects vis-à-vis the loading direction may play an important role in determining their effect on the life of the composite structure [75]. This corroborates the findings by Harris, who, in his compressive book on engineering composite materials [74] indicated that a few isolated spherical pores (i.e., having a diameter of a micron or so in diameter), do generally not have significant effects on any of the material physical properties, and may not, therefore, affect its tensile or flexural mechanical performance. In contrast, a considerable distribution of innocuouslooking pores can markedly reduce the interlaminar shear strength of a material and, by providing sites for accumulation of moisture, may also considerably decay the electrical or dielectric performance of the material. Similarly, minor delamination between the adjacent plies of the complex laminate may have no effect on the tensile strength of the material, but such defects are frequently injurious to the compression performance of laminates and, as indicated previously, can rapidly grow to eventually damage the composite under cyclic loading conditions. As a result, it is important to use the most effective NDT techniques to detect the defects and establish the critical size with a case-by-case approach owing to the specific type of composite and the specific service conditions for any given application in order to avoid unpredictable failure catastrophes [17]. The application of NDT would also help to localize and characterize flaws at their incipient initiations and allow engineers to take an appropriate course of action which could ultimately save resources, eliminate unplanned breakdown, and provide a timely window for repair-maintenance activities when the composites are in service.

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1 Introduction and Background of Fiber-Reinforced …

1.12.3 In-Service Damage and Material Degradation In-service damage or material degradation of composite structures are caused by several types of unpleasant events in the service environment including but not limited to impact strikes [20], prolonged exposure to harsh conditions (i.e., high temperatures, exposure to corrosive chemicals, UV lights, etc.). In aerospace and marine composite structures, for example, impact strike is quite a devastating type of damage and is considered to be the most ubiquitous type of damage in aerospace composite structures. To this end, the weakness of composites to impact strikes is well known in CFRP, especially to low-energy impacts that cause subsurface damage in the form of delamination, cracks, and/or fiber breakage. In general, these types of impacts show little visual evidence on the external surface (i.e., other than a small dent) that it happened, and hence the name of barely visible impact damage (BVID) [18, 76]. The latter is particularly a serious type of damage because it transfers its impact energy into the affected area of the composite in pyramidal fashion (i.e., in a form of tree root of matrix cracks and propagating delamination) leading to significant degradation of the composite’s structural properties (e.g., residual strength, composite compactness, etc.). In this case, the composite becomes defenseless and ultimately vulnerable to almost any subsequent damage including out-of-plane deflections and ply buckling when subjected to shear and compressive loading. Indeed these initial cracks and delamination often continue to grow slowly under alternating or fluctuating stress leading to significant losses of stiffness and ultimately catastrophic failures may follow. Consistent with the aforementioned, delamination in CFRP laminates is particularly devastating because it significantly reduces the composite’s strength (i.e., particularly the compression strength). In more severe cases (i.e., if undetected and unrepaired timely), this kind of defect often results in a significant reduction of the as-designed load capability, and in some cases, the inability to withstand design limit load may follow [8]. In contrast, when it comes to the safety issues in aircraft structures, the structure must be able to ‘withstand reasonable loads without failure or excessive structural deformation after an impact damage’ for the ‘operational life of the aircraft’ or ‘until the damage is detected and fixed’ [77, 78]. Indeed one of the most viable solutions toward the production of efficient fiber-reinforced composite materials is that designers should have sufficient knowledge about the failure mechanisms of the final product to ensure their products can effectively respond to their design functions without their structural integrity being compromised. In the Griffith model [79–81], for example, it is indicated that the fracture of homogeneous materials is based on the stress-intensity factor (K) and the associated strain-energy release rate (G), which is mainly related to the material fracture toughness (i.e., material resistance to crack propagation) [74]. In real fiber-reinforced composite material, however, both the microstructural inhomogeneity and anisotropy of the material are the key parameters causing the failure mechanism to be very complex as they involve a combination of micro-failure events (i.e., these micro-cracks can often give rise to high levels of fracture energy). In fact, unlike homogeneous materials such as metals,

1.12 Main Types of Deficiency in Polymer-Matrix Composites

37

composites have crack-stopper ability inherent in both types of interfaces at a microscopic level (i.e., crack-stoppers between the fibers and the polymeric matrix) and a macroscopic level (i.e., crack-stoppers between separate laminas). This gives rise to complex fracture mechanisms involving the breaking of fibers and matrix, delamination between fibers and matrix, and a combination of crack deviations alongside the interfaces (i.e., all of these taking place in the composite at both the micro and macroscopic levels). In practice, the fracturing of a composite is driven by three factors: the matrix, the reinforcement, and the interface; it is important to consider the types of matrix and fibers, their mutual volumes, and the type of bonds in between them, meaning the curing process. In general, the toughness of a composite is derived from its material constituents, and the relative contributions of the individual materials depend not only on the physical and mechanical characteristics but also on how they interact with one another. To this end, it is generally difficult to predict the toughness of all composites based on the characteristics of their material constituents [74], because even the procedures based on classical fracture mechanics cannot be purely applied in the design of the composites without any prior modifications and expect to obtain the properties as estimated. To put this into perspective, when fibers are incorporated into the polymeric matrix, their separate phases do not necessarily contribute to the composite’s structural toughness in an additive fashion, as the breadth of this interaction largely depends on the level of constraint that is set consistent with their properties differences [72, 74]. As such, it is rather difficult, or even impossible, to predict the way a composite will fail. To overcome this difficulty, scientists generally prefer to establish the damage threshold load (DTL) under impact in specific industries such as the aircraft industry to be able to predict the behavior of the composite during its service life [82]. Although the establishment of the DTL has been the line of work for many years and there is a significant amount of numerical simulation and experimental testing data, there is still no clear methodology to unambiguously establish this DTL threshold more reliably. This is particularly because establishing an accurate DTL mainly depends on many factors, including but not limited to the effective material mechanical characteristics as well as the geometry of both the target [83, 84], and the impactor [85]. In addition, the high variability of the composite’s mechanical properties is the main cause of porosity that is induced in the final product during the manufacturing processes (i.e., can be reduced by accurately controlling the manufacturing parameters but not fully removed) [86]. It is largely believed that properties variability is the main reason causing composite structures to display a large variety of damaging ways when subjected to impact (i.e., produce deformations in a small zone surrounding the point of impact while others completely deform the entire structure) [87]. Also, a major portion of the impact energy is transferred to the plate in some cases, while most of the impact energy may be returned to the impactor in others [8], or the effect of indentation is almost negligible in some cases while it absorbs a significant portion of the impact energy (i.e., it must be adequately modeled in the analysis) in the others. Apart from the establishment of the DTL threshold, it must be pointed out that impact threats are also quantified in terms of impact energy in the aircraft industry,

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1 Introduction and Background of Fiber-Reinforced …

though this is technically not a reliable method because even at the same impact energy, a small mass and a large mass impactor exhibit completely different responses [31]. In a recent study, Meola and co-authors in [88] provided a solution to this drawback, after conducting a series of studies [89–91], by proposing a relationship that links the damaged area to the impact energy and the dimension of the impactor dimension. They indicated that it is possible to visualize the thermal effects that develop under the low-energy impact, and which may supply information about initiation and propagation of the impact damage [85]. Since the kinetic energy passes from the impactor nose to the target during the impact event and because such energy is partly dissipated as heat, the detection of thermal signatures developing under the impact damage is important for the understanding of failure modes in fibrous composites. They also demonstrated that any form of damage (delamination and/or fiber breakage) is generally followed by some form of heat dissipation, which is expressed by the appearance of hot spots/areas on the surface of the material. In this context, infrared thermography is generally perceived as one of the most beneficial and unique tools to detect such damage (see details in chapter five of the book). Although most of these damage and material degradation processes occur in the composite as a result of natural phenomena, some of them may also occur following initial damage that leads the way. Liquid water normally enters the core through exposed edges, such as panel edges, closeouts, door and window sills, attachment fittings, or almost any location where the skin and core bond terminates. The majority of the damage is often found at the edges of panels [92]. Adhesive bond degradation will lower the skin-to-core bond strength, the fillet bond strength, and the node bond strength. Node bond degradation can reduce the core shear strength so that the assembly fails prematurely by core failure. In addition, water will enter the assembly through any puncture in the facesheets. Since some honeycomb assemblies contain extremely thin skins, water has been known to pass through the skins and then condense on the cell walls of the sandwich structures, making them vulnerable to disbonds and continuous material degradation. Interconnected microcracks in thin skin honeycomb panels can also allow water ingression [92, 93]. Although absorbed moisture affects the properties of any composite assembly, it is the presence of liquid water in the cells that do the majority of the damage. Many field reports blame water ingression on “poor” sealing techniques. While there is a great deal of truth to the statement that good sealing practices are important, it is the author’s opinion that it is just a matter of time before water will find its way into the core of most honeycomb designs and initiate the damage process. Table 1.1 summarizes the most important types of defect or damage occurring in composites as well as their location, relative size, cause, and inherent characteristics. In summary, Table 1.1 indicates that there are many different types of defects some of the defects will likely occur at certain stages of the lifecycle of the composite than others. Although most of these defects are known to the designers, their prediction is still very difficult, especially in multidirectional laminates whereby a mixture of unidirectional failure modes is susceptible to occur under different loading spectra. In addition, the failure mode is determined by postmortem examination of the fracture surface in the majority of defects cases. Some of the defects posing the most problems

Characterization of the defect or the damage

It is a macroscopic type of defect and is usually manifested in different forms including but not limited to variation in fiber distribution across the composite, fiber marcelling, kinks, fracture, misalignment, fiber/matrix debonds, as well as damaged filaments, and many others

It is a macroscopic defect occurring in the composite in a form of an inter-laminar crack, separation of the layers or plies, or cracks between the plies of laminate parallel to the fibers

Types of the defect or damage

Fiber faults or damaged filaments

Delamination

Impact damage, matrix and free edge cracking, incorrect impregnation of fibers, and compressive and tensile loading

Generally caused by broken filaments, knots, splices, and split tow-in raw fibers. Inadequate cure temperature, pressure, and resin propagation. Also, improper yarn spacing, fiber kinking, poor prefabrication control and handling, excessive local shear transfer stresses in short fibers, deviation from the adequate winding patterns, and washout of fiber due to the excessive resin flow are among the additional possible causes

Common causes of defect or the damage

In-plane matrix, between the plies and within the laminate itself. It occurs during the manufacturing of in-serve stages of the composite

These types of defects or damage are often identified in the raw fibers and in the in-plane matrix between the plies of the laminate or within the laminate itself. They occur at the different stages of the life of the composite (material processing, composite manufacturing, and in-service stages) with some of them being predominant in some stages than in others

Locations of the defects or damage and time of occurrence

(continued)

It significantly reduces the overall stiffness of the composite and both its compressive and shear strengths cannot be guaranteed

Separation at the fiber-matrix interface, induced matrix cracking, loss of shear transfer, and degradation of the overall tensile and compressive strengths as well as the stiffness of the composite. Fiber faults can also cause continuous fiber damage, formation of undesired shapes of the parts and ply buckling during the layup process, loss of tensile strength, accelerated fiber buckling, and premature failure

Major effects of defect or the damage on the composite

Table 1.1 Summary of the different types of defect or damage occurring in composites as well as their location, relative size, cause, and inherent characteristics

1.12 Main Types of Deficiency in Polymer-Matrix Composites 39

Characterization of the defect or the damage

It is often a macroscopic defect that is characterized by matrix breakage. It may also induce a localized through-the-thickness cracking and fiber breakage depending on the properties of the matrix

It is a macroscopic defect/damage that is characterized by a change in the mechanical properties of the composite. Obsolescence and reduction in the overall performance of the affected structure

It is a macroscopic defect and is characterized by a matrix crack on the corners of the structure or delamination between plies. The defect may be perpendicular or trans-laminar to the ply

Types of the defect or damage

Matrix cracking

Impact damage

Corner crack

Table 1.1 (continued)

Projectile impact damage, inter-laminar failure, free edge, static or cyclic tensile loading, compression, micro-cracks, voids, and geometric boundaries

It is generally caused by projectile impact, collision with a different object or structure, mechanical stress, and long-term mechanical bending cycles

It is usually caused by over-stressing of the matrix through various loading conditions, thermal expansion, defective cure cycle, impact damage, and geometric discontinuity

Common causes of defect or the damage

It is usually formed at the corners or areas of geometric discontinuity of the composite while in-service. It may happen during component manufacturing

It is susceptible to occur/appear anywhere in the composite and it often occurs during the in-service stage

It often occurs in the interface planes of continuous fiber composites and their laminates during the manufacturing and in-serve stages of the composite

Locations of the defects or damage and time of occurrence

(continued)

Reduce stiffness and strength, affect the compression and shear strength of the components

It often leads to wear and tear, fracture of fiber layers, fissure in the matrix, fiber/matrix layout, and degradation of the compactness of the material

Localized stress concentrations, reduction of stiffness and shear strength, fiber fracture, and delaminations

Major effects of defect or the damage on the composite

40 1 Introduction and Background of Fiber-Reinforced …

Characterization of the defect or the damage

They are all macroscopic defects and as the name indicates, the first two are cuts and scratches on the surface of the composite while notches are characterized by uneven surface finishing

It is a macroscopic defect/damage that is often considered to be a localized lamina or ply delamination

A defect whose size variable (microscopic or macroscopic) and is characterized by the inclusion of foreign materials such as the peel ply and backing paper at the interface level between the plies

The moisture is absorbed into the laminate through the matrix. Usually contained in the outer plies. Composite parts performance is compromised

Types of the defect or damage

Cuts, scratches, and notches

Blistering

Contamination or artificial inserts

Moisture ingress

Table 1.1 (continued)

The in-plane matrix between the plies of the laminate or within the laminate itself. It happens during the composite manufacturing stage

Usually on the surface of the composite but susceptible to occur anywhere in the lamina or ply. Usually in-service but may happen during the component manufacturing

Notches are often observed on the surface, corners, and edges of the composite while scratches and cuts are on the surface of the composite. They may happen during the component manufacturing and in-service stages

Locations of the defects or damage and time of occurrence

Caused by water diffusing into the Usually contained in the outer composite part over time plies and take place during the in-service stage of the composite

It is caused by poor process control and materials mishandling

Expansion of trapped gases within the lamina, localized heating of the matrix, chemical attack, and moisture ingress

They are all caused by poor release procedures, composite mishandling, solid particle erosion, and low-speed projectile impact

Common causes of defect or the damage

(continued)

The degradation of the resin properties (softening and stiffness), reduction of fiber interface bond strength, and swelling

Affect the thermo-mechanical properties of the structures

Degrade the resin matrix, and damage the fiber/matrix interface. Reduces the compression, stiffness, and shear strengths

They all lead to a significant reduction in the static strength of the composite through the severity depending on their sizes, the application, and the direction of the load being applied

Major effects of defect or the damage on the composite

1.12 Main Types of Deficiency in Polymer-Matrix Composites 41

Characterization of the defect or the damage

Size is variable and porosity may be microscopic or macroscopic and these are small and randomly dispersed air bubbles within the laminate. A cluster of several microscopic voids. Usually, the size is not as important as the concentration

These may be microscopic or macroscopic defects indicating localized or distributed trapped gases, volatiles materials in the resin

Usually indicated by the variation of the thickness or the density of the composite. It may be a macroscopic or microscopic variation

Types of the defect or damage

Porosity

Voids

Composite size or thickness variations

Table 1.1 (continued)

Within the laminate often takes place during the component manufacturing

Locations of the defects or damage and time of occurrence

Generally caused by the Within the laminate and it is a inconsistencies in the resin composite manufacturing defect content of the laminate or the adhesive layer in the bonded joints as well as the presence of voids, or porosity within the composite

Generally caused by poor process They are often located within the control, defective resin matrix of the laminate during the propagation, trapped air within the manufacturing process resin and filament bundle, residual solvent carrier, chemical reaction products, volatilization of low-molecular-weight components, and organic inclusions

Poor material or process control, over-aged material, air/moisture entrapment in the prepreg, or an autoclave malfunction

Common causes of defect or the damage

(continued)

Excessive peel stresses in bonded joints, degrade the bond joint efficiency. Their effect on the laminate is similar to that caused by the prepreg variability

Voids degrade the mechanical and thermal properties. Reduces the modulus and fatigue resistance. Reduces the interlaminar shear, longitudinal, transverse flexural, tensile, and compressive strength

Porosity causes localized stress, strength, and fatigue concentrations. Degradation of tensile, interlaminar, compression, and bearing properties at high temperatures

Major effects of defect or the damage on the composite

42 1 Introduction and Background of Fiber-Reinforced …

Not true defects but rather a result of microscopic residual stresses within the laminate

Thermal stresses

Solid particle erosion/abrasion The recurrent impact from solid particles that remove the external matrix/fibers from the target surface. It is often a macroscopic type of damage

It is a type of macroscopic damage Usually over-aged material or and is characterized by the exposure to excessive heat or formation of multiple cracks in the severe ultraviolet radiations matrix following all directions. Common in nonstructural matrix materials

Matrix crazing

Interface planes of the continuous fiber composites and their laminates

The surface of the composite structure and takes place during the in-service stage of the composite

Locations of the defects or damage and time of occurrence

The repeated impact of solid particles in the air/liquid, wear, and abrasion

It is usually located at the free edges, geometric discontinuity, and the surface of the part

High and periodic thermal stresses In components after removal from from the curing process. Improper autoclave or during cool down. cool-down rates Component manufacturing and in-service of the parts

Use/contact of undesirable solvents on the outer ply of the structure

A kind of macroscopic blister that causes the surface of the composite to swell

Surface swelling

Common causes of defect or the damage

Characterization of the defect or the damage

Types of the defect or damage

Table 1.1 (continued)

(continued)

Particle erosion reduces the local strength and stiffness and causes asymmetry and out-of-plane stresses of the composite

Affect the component’s structural performance. Thermoelectric damage and potential surface blisters

Localized stress concentrations, fiber fracture, and/or delaminations

Localized breakdown of the matrix, loss of fiber/matrix stiffness, and shear transfer

Major effects of defect or the damage on the composite

1.12 Main Types of Deficiency in Polymer-Matrix Composites 43

These are macroscopic through-the-thickness cracks that are often followed by fibers breakage

Separations in a secondary adhesive bond or sandwich facing. Typical delaminations between bonded structures. It is generally a macroscopic defect or damage

Also known as the “furring” of the Poor process control during the fibers—They are considered as manufacturing process prepreg deviations

Translaminar cracks

Unbond or debond

Pills or fuzz balls

Poor process control/fitting, impact damage, thermal spikes, overload, or freeze/thaw cycle during in-service

Generally caused by extreme overload or impact damage

It is a type of macroscopic damage Lightning strikes, local overheat, that is usually confused with other battle damage (laser), surface surface damage types of defects damage, chemical attack

Surface oxidation

Common causes of defect or the damage

Characterization of the defect or the damage

Types of the defect or damage

Table 1.1 (continued)

Within the laminate, through-hole areas during the manufacturing stage of the composite

Between bonded structures and usually takes place during the composite manufacturing and in-service of the parts

Within the laminate during the in-service stage

The surface of the composite during its in-service stage

Locations of the defects or damage and time of occurrence

(continued)

The affected area becomes susceptible to localized high peel stresses

Reduce the local stiffness of the structure

Same effects as fracture and hole stress concentrator

The disintegration of structural integrity and matrix properties

Major effects of defect or the damage on the composite

44 1 Introduction and Background of Fiber-Reinforced …

A matrix-dominated failure. Size, shape, and frequency must be monitored for critical components

This is a macroscopic type of damage that is characterized by the opening up of existing edge delamination of the entire delaminated area during loading. These are also cracks between the different plies of the composite

Creep

Corner splitting

Caused by edge impact or out-of-plane stresses. Usually, a maintenance or part installation-related damage

Plastic deformations that are caused by continuous loading at elevated temperatures

It is a macroscopic type of damage Often caused by impact damage or that is characterized by local inelastic collision with moving indentations or surface dents. objects Often a sign of further internal damages: delaminations, fiber breakage, or matrix cracks

Composite crushing

Common causes of defect or the damage

Characterization of the defect or the damage

Types of the defect or damage

Table 1.1 (continued) Major effects of defect or the damage on the composite

Tensile, shear, and compressive strength are reduced. In-plane extension leads to out-of-plane deflections and buckling

(continued)

Edges and corners, as well as the Affect the shear strength of the geometric discontinuity of the part component that takes place during the in-service stage of the composite

Sustained heat damage areas occurring during the service life of the composite

It is the exterior of the part, matrix Affect tensile, Compression, and cracks and, fiber breakage that shear strength of the part takes place during the in-serve stage of the composite as a result of impact damage or inelastic collision

Locations of the defects or damage and time of occurrence

1.12 Main Types of Deficiency in Polymer-Matrix Composites 45

This is a macroscopic type of damage and its common features include edge splitting, free-edge, and delaminations

A type of macroscopic matrix cracks parallel to the fiber axis in the corner radius of the part. The resulting delamination runs longitudinally

It is a defect of variable size (maybe microscopic or macroscopic) and is characterized by the presence of a foreign body or resin-starved area in the matrix

Usually a macroscopic defect and is characterized by tolerance error

Edge damage

Corner radius delaminations

Nonuniform agglomeration of hardener agents

Mismatched parts

Produce several micro-cracks and micro-voids, progressive failure of the part, matrix cracks, and holes

Major effects of defect or the damage on the composite

This type of defect/damage mostly Affect the composite part’s appears on the edges of the compressive and shear strengths component and occurs during the composite manufacturing and in-serve stage of the parts

Usually located at the joint between the pin (fastener) and the edge of the hole and it takes place during the service life of the composite

Locations of the defects or damage and time of occurrence

Poor control process

Poor control process and material mishandling

Especially in the joint areas and takes place during the component manufacturing

The in-plane matrix between the plies of the laminate or within the laminate itself and usually takes place during the component manufacturing

(continued)

Affect the load response of the laminate and often lead to fitting errors

Significant degradation of the local matrix properties

Predominantly caused by Corners of the structure, and at the Induces significant out-of-plane manufacturing errors while giving geometric discontinuity areas. stresses into the component a particular shape to the composite Usually in-service but may happen during the component manufacturing

Components mishandling and high out-of-plane normal or shearing stresses at the proximity of the free edges

A type of macroscopic damage Improper installation, loose that features fiber fractures, fastener, joint overload, free delamination, and matrix cracking edges, uneven distributions of fiber strength, and imperfect fiber/matrix interface bonding

Bearing surface damage

Common causes of defect or the damage

Characterization of the defect or the damage

Types of the defect or damage

Table 1.1 (continued)

46 1 Introduction and Background of Fiber-Reinforced …

Characterization of the defect or the damage

It is a microscopic type of defect and is characterized by the under cure of over cure cycles causing additional stresses to the composite

These are macroscopic indentations observed at the point of interaction between the composite and a moving object but no fibers are broken

It’s a macroscopic type of defect that is characterized by laminate dimensional tolerance errors

Types of the defect or damage

Over/under cured

Dents

Excessive ply overlap

Table 1.1 (continued)

Takes place when the ply is not adequately trimmed during the assembly process

Tool impressions and design variance

Improper curing process - Too long/short in time and/or too high/low cure temperature

Common causes of defect or the damage

In the plies of the laminate and take place during the composite manufacturing stage

Usually on the surface of the part and take place during the composite manufacturing and in-serve

A matrix defect and takes place during the composite manufacturing stage

Locations of the defects or damage and time of occurrence

(continued)

Often leads to warping which and high peel stresses

Partial loss of stiffness

Localized laminate’s strength, poor stiffness response, poor shear transfer in the fiber/matrix interface

Major effects of defect or the damage on the composite

1.12 Main Types of Deficiency in Polymer-Matrix Composites 47

Characterization of the defect or the damage

This is a macroscopic type of damage that is characterized by the presence of repair or rework patches. Loss of mechanical properties. Criticality depends on the repair and the application

This is a microscopic type of defect and is either characterized by over-aged prepreg or prepreg variation

It is a macroscopic defect that is characterized by bent or twisted out of shape, typically as a result of the effects of heat or dampness

Types of the defect or damage

Reworked areas

Faulty prepreg

Composite warping

Table 1.1 (continued)

Residual thermal stresses that remain in the laminate after fabrication of the composite or parts assembly mismatch

In the former case either B-staged prepreg resin has aged or the final cure does not provide adequate fiber/matrix adhesion and volatile evacuation while in the latter case The preset material property levels prior to cure are exceeded

Repeated repair

Common causes of defect or the damage

This type of defect is usually located at the joints areas and takes place during the in-service stage of the composite

This type of defect is located in the in-plane matrix between the plies of the laminate or within the laminate itself and occurs during the component manufacturing stage of the composite

Areas of repair, either resin-filled or patched and takes place during the in-service

Locations of the defects or damage and time of occurrence

(continued)

Part may not fit at the next assembly or, if fitted, out-of-plane stresses may be induced

Faulty prepreg often causes a significant reduction of the laminate’s strength and stiffness as well as the potential increase in voids and porosity

Degradation of structural performance, localized strength, and loss of stiffness

Major effects of defect or the damage on the composite

48 1 Introduction and Background of Fiber-Reinforced …

Fastener holes defects, Fastener removal and re-installation, The hole elongation, The hole wear, Improper fastener installation and seating, Over-torqued fasteners, Missing fasteners, Hole exit-side damage, High drill-bit feed, fastener pull-through, incorrect installation of interference-fit fasteners, resin-starved bearing surface; and titled countersink holes

Faulty holes

Tilted countersink and interference-fit fasteners, delamination, hole-exit damage. Reworking a hole by removing or reinstalling the fastener. Overloading of the composite or bearing failure. The movement of the fastener in the hole. Improper installation, seating, and size of fasteners, under/over-torqued fasteners. Absence of a fastener in a through-hole. Over-torqued fasteners and pull-through of the fasteners. Drill burn, repair, penetration damage, countersink tear-out, fastener pull-through, hole exit damage, miss-drill, tilted holes, resin-filled holes, and free edge porosity. Improper drill-bit feed that causes delamination on the backside of the composite

Wrong materials used in the Poor control process fabrication of the component are a blueprint error

Wrong materials

Common causes of defect or the damage

Characterization of the defect or the damage

Types of the defect or damage

Table 1.1 (continued)

These are macroscopic types often located in and around the through-hole areas of the composite and they generally take place during the service life of the composite

Anywhere in the structure: between plies of the laminate or within the laminate itself and usually takes place during the composite manufacturing stage

Locations of the defects or damage and time of occurrence

(continued)

Local ply damage, affect the tensile strength, compression strength, and the structure’s loading response. The wear and tear of the hole especially when the fastener shank pulls through the hole. Reduces joint efficiency, and produces a stress concentration for through-holes. Local crushing of the outer plies. Tension, compressive and shear strengths are affected. The criticality of the damage depends on the size of the inflicted damage. Stress concentration, fiber breakage. Severity depends on the size of the drill burn, shape, and location of the hole. Effect on the part depends on the severity of the damage but is most delicate in tension and compression loading modes

Inadequate stress and stiffness characteristics and thermally warped panel due to layup error

Major effects of defect or the damage on the composite

1.12 Main Types of Deficiency in Polymer-Matrix Composites 49

Characterization of the defect or the damage

This is a condition that occurs when any of the plies of the laminate is missing or the ply is not correctly trimmed during the assembly stage

N/A

Types of the defect or damage

Missing plies

Ply underlap or gap

Table 1.1 (continued)

Too short ply size

Usually Caused by the mishandling of components

Common causes of defect or the damage

Usually located at the interface between two consecutive plies

Usually located at the interface between the plies. Usually takes place during the composite manufacturing stage

Locations of the defects or damage and time of occurrence

Affect the stiffness load response, uneven strength distribution

Incorrect staking sequence of plies, asymmetric laminate, out-of-plane shape, unevenly distributed stress

Major effects of defect or the damage on the composite

50 1 Introduction and Background of Fiber-Reinforced …

1.12 Main Types of Deficiency in Polymer-Matrix Composites

51

to composite structures’ integrity during the in-service stages of their lifetime are the intralaminar matrix cracking, delaminations, debonds, holes, and moisture ingress.

1.13 Trends Towards Novel Technologies As indicated in the earlier sections of this chapter, fiber-reinforced composite materials have superior properties compared to their conventional structural material systems. Fiber-reinforced composite materials have already gained a very high standard and their application in the aerospace and other important industries is now prevalent than ever before, however, there is still room for further improvements. Recent improvements are now aiming toward the manufacturing and cost reduction during material processing as well as the development of new applications and the improvement of the overall material properties among others. In particular [94], the development of new materials with improved properties such as resistance to high temperature and corrosive substances, high stiffness coupled with high lightness, have always been the primary objective in the aeronautical field. Also, the exploitation of novel materials, in both military and civil aviation as well as space aeronautics, has allowed achieving results that were impossible to obtain with traditional materials [11]. The development of new materials is constantly progressing, and now, in the era of composites, it is rather easy to create a new material by simply adding a new ingredient to the basic recipe or by changing the curing parameters. In particular, presently (in which words like smaller, younger, mini, and thinner are imperative) the word nano is dominating the composite materials world than ever before. Indeed, nanotechnology allows control of matter and processes at the nanoscale (i.e., 1–100 nm) [95, 96] with the possibility to create materials of desired characteristics. The use of nanoparticles (e.g., carbon nanotubes, graphene, etc.) as filler in polymer resins (i.e., to generate composites known as nanocomposites) have been under investigation in the last decade and they resulted in tremendous improvements in mechanical properties [64, 94, 97]. In general, nanocomposites present the advantage of a high aspect ratio with the main effect of an increase in the reinforcement surface area. This offers the possibility to obtain the same performance properties achieved with traditionally filled resins but with a smaller filler volume fraction. In addition, nano-filled resins often exhibit other novel beneficial characteristics on the macroscale properties of the composite such as improved thermal and electrical conductivity, reduced flammability, higher stiffness, strength, and resistance to wear and damage. Also, the nanocomposites used as the matrix for fiber-reinforced polymers (FRPs) have the potential to improve overall mechanical properties as interlaminar shear stress, first-ply failure, impact, and compression after impact to list but a few. In electrical and electronic engineering, for example, the fact that polymer matrix (i.e., an insulator by nature) can be turned into an electrically conductive material is particularly beneficial because this additional function can be used for both the electric charge ablation and the sensing of the composite deformation and/or

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damage. In particular, their potential for strain sensing applications with electrical conductivity methods can be used to investigate the response of the electromechanical deformation and its subsequent effect on fiber-reinforced composite materials with embedded nanoparticles when subjected to mechanical load. Consistent with the aforementioned, an integration of nanoparticles into the matrix of fiber-reinforced composite materials not only improves the overall properties but also has the capabilities of turning the entire composite material into a multifunctional material whereby superior mechanical properties are combined with excellent sensing capabilities. Composite materials can even better compete with conventional structural materials, respectively, are an even more attractive alternative [94]. Although nanocomposites present improved properties with potentially more applications compared to their simple fiber-reinforced composite materials counterparts, the manufacturing of nanocomposites has raised the challenge of obtaining uniform dispersion of the nanoscale fillers throughout the polymeric resin matrix. Several solutions have been proposed to solve this problem including the use of centrifuge machines, microfluidization, and the formation of 3D nanoparticles before infusing them with the polymer resin. A useful application of nanotechnology has been devised in the enhancement of fiber-reinforced polymer composites. This may include three approaches the modification of the polymeric matrix, modification of the reinforcements, and the incorporation of a macroscopic arrangement of a nanometerdimensional material among others [98–100]. Of course, the new material may be created by pursuing one, or more, of the three approaches described above. In particular, a novel technology that is still under development consists of the production of reinforcements (glass or carbon) infused with carbon nanostructures to be used as pre-pregs for the fabrication of composites [101–103]. This technology is known as carbon-enhanced reinforcement and it is expected to deliver improved composite in-plane shear strength and greater interlaminar shear strength. In recent years, there has also been a growing interest in hybrid composites materials particularly for the manufacturing of strong and highly resistant structures such as airplane wings and wind turbine blades, thanks to their ability to provide the manufacturers with greater control of the properties while equally enabling the achievement of a more favorable balance between the composites’ inherent advantages and disadvantages [104]. This is particularly because the advantages of one reinforcement could complement the disadvantages of another reinforcement through hybridization in order to obtain a cost-effective hybrid composite with desirable properties through the appropriate material selection. Given the fact that fiber reinforced polymers are predominantly made of synthetic fibers (i.e., glass fiber, carbon fibers), an emerging field of high-performance natural fibers, especially bast fibers (i.e., including flax, hemp, and jute), is gaining significant interest. Therefore, the industry includes many significant innovations at every stage of the manufacturing of composites, which extends from the fibers and their precursors or preforms through to the manufacturing processes and their associated industries. There are currently many innovations taking place in the market across the value chain, most of which are focusing on performance improvement and cost benefits in the composites industry [50, 105]. Under the same umbrella, the hybridization of natural fibers and synthetic fibers is

1.13 Trends Towards Novel Technologies

53

particularly an attractive area in the transport industry as it provides composites with a lightweight characteristic in comparison to non-hybrid synthetic fiber-reinforced composites, which is due to the significantly lower density of natural fibers. Indeed this characteristic is one of the criteria that is particularly important in transportation sectors to reduce fuel dependence and energy consumption without any significantly deteriorating the safety measures.

1.14 General Conclusions In this chapter, a general introduction and background information pertinent to the manufacturing of fiber-reinforced composite materials is provided outlining the general analysis and properties of these types of materials. However, the information provided in this chapter is only the summary that was gathered to help the readers as the starting point to understand the thematic subject of this book as illustrated in the title, suggesting that some of the details pertinent to the design and manufacturing of FRCs might be missing. This is particularly because the primary aim of this book is to provide the readers with information regarding the usefulness of the different NDT techniques in the detection and characterization of the different types of defects and damage in fiber-reinforced composite materials (i.e., including their design, development, flaws evaluation methodologies and the subsequent maintenance of the material). It is also acknowledged that some of the figures used herein, which refer to well-known facts, were presented following some inspirations from some of the NDT websites, and books and references have been provided when appropriate. In this framework, this chapter has to be regarded as an introduction to composite materials because it only provides the basic information outlining the main types of composite materials with varying types of matrix and reinforcement, as well as some general hints on the fabrication processes that help to obtain high-quality composites. A section is also dedicated to the description of the main types of defects that may arise in composites, either during the processing of the raw materials, the manufacturing of the composite as well as during its service. Particularly, we believe that such a description will help readers to understand the main weaknesses of composites and provide a viable source of information to the engineers on what they should expect when using composite structures in their designs. The chapter ends with a look at the environmental impact and the future research trend of the existing composite material, which appears to be dominated by nanotechnology, and the apparent move towards the development of environment-friendly and sustainable composite materials. The next chapter entirely focuses on the approaches of assessing the material’s soundness in a nondestructive fashion. In particular, a general introduction to NDT is provided outlining its usefulness in the detection and characterization of the different flaws and damage. In doing so, the main intention is to ensure our readers have relevant information regarding the semantics pertinent to the available testing techniques and methodologies for fiber-reinforced composite materials in the NDT community. Also, this will be thoroughly discussed in the chapters of this book, we believe that

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the information provided in the next chapter may also be used as the basis for the engineers or NDT practitioners to establish, through a general comparison, whether an NDT technique may be used alone, in place of, or integrated with, some other techniques to obtain reliable and accurate testing and/or evaluation results. Important Disclosure In this chapter, some figures referring to well-known facts and popular THz systems were arranged traying inspiration from NDT websites and sources were also referenced when appropriately.

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71. N. Pham-Thanh, H. Van Tho, Y.J. Yum, Evaluation of cavitation erosion of a propeller blade surface made of composite materials. J. Mech. Sci. Technol. 29(4), 1629–1636 (2015). https:// doi.org/10.1007/s12206-015-0334-4 72. W. Jiang, Z. Huang, Y. Wang, B. Zheng, H. Zhou, Voids formation and their effects on mechanical properties in thermoformed carbon fiber fabric-reinforced composites. Polym. Compos. 40(S2), E1094–E1102 (2019). https://doi.org/10.1002/pc.24876 73. M. Mehdikhani, L. Gorbatikh, I. Verpoest, S.V. Lomov, Voids in fiber-reinforced polymer composites: a review on their formation, characteristics, and effects on mechanical performance. J. Compos. Mater. 53(12), 1579–1669 (2019). https://doi.org/10.1177/002199831877 2152 74. B. Harris, Engineering Composite Materials. (IOM 1999) 75. K.O. Kim, Effects of manufacturing defects on the device failure rate. J. Korean Stat. Soc. 42(4), 481–495 (2013). https://doi.org/10.1016/j.jkss.2013.02.003 76. M.S. Sohn, X.Z. Hu, J.K. Kim, L. Walker, Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement. Compos. Part B Eng. 31(8), 681–691 (2000). https://doi.org/10.1016/S1359-8368(00)00028-7 77. C. Bouvet, S. Rivallant, Damage tolerance of composite structures under low-velocity impact, in Dynamic Deformation, Damage and Fracture in Composite Materials and Structures, (Elsevier, 2016), pp. 7–33. https://doi.org/10.1016/B978-0-08-100080-9.00002-6 78. C. Bouvet, S. Rivallant, J.J. Barrau, Low velocity impact modeling in composite laminates capturing permanent indentation. Compos. Sci. Technol. 72(16), 1977–1988 (2012). https:// doi.org/10.1016/j.compscitech.2012.08.019 79. C. Kuhn, R. Müller, A continuum phase field model for fracture. Eng. Fract. Mech. 77(18), 3625–3634 (2010). https://doi.org/10.1016/j.engfracmech.2010.08.009 80. Z. Yu, J. Liu, X. Wei, Unraveling crack stability and strain localization in staggered composites by fracture analysis on the shear-lag model. Compos. Sci. Technol. 156, 262–268 (2018). https://doi.org/10.1016/j.compscitech.2017.12.035 81. W. Tan, E. Martínez-Pañeda, Phase field predictions of microscopic fracture and R-curve behaviour of fibre-reinforced composites. Compos. Sci. Technol. 108539 (2020). https://doi. org/10.1016/j.compscitech.2020.108539 82. D.J. Elder, R.S. Thomson, M.Q. Nguyen, M.L. Scott, Review of delamination predictive methods for low speed impact of composite laminates. Compos. Struct. 66(1), 677–683 (2004). https://doi.org/10.1016/j.compstruct.2004.06.004 83. W.J. Cantwell, Geometrical effects in the low velocity impact response of GFRP. Compos. Sci. Technol. 67(9), 1900–1908 (2007). https://doi.org/10.1016/j.compscitech.2006.10.015 84. G.A. Schoeppner, S. Abrate, Delamination threshold loads for low velocity impact on composite laminates. Compos. Part Appl. Sci. Manuf. 31(9), 903–915 (2000). https://doi. org/10.1016/S1359-835X(00)00061-0 85. C. Meola et al., New perspectives on impact damaging of thermoset- and thermoplastic-matrix composites from thermographic images. Compos. Struct. 152, 746–754 (2016). https://doi. org/10.1016/j.compstruct.2016.05.083 86. L. Liu, B.-M. Zhang, D.-F. Wang, Z.-J. Wu, Effects of cure cycles on void content and mechanical properties of composite laminates. Compos. Struct. 73(3), 303–309 (2006). https:// doi.org/10.1016/j.compstruct.2005.02.001 87. S. Abrate, Modeling of impacts on composite structures. Compos. Struct. 51(2), 129–138 (2001). https://doi.org/10.1016/S0263-8223(00)00138-0 88. C. Meola, G.M. Carlomagno, Impact damage in GFRP: New insights with infrared thermography. Compos. Part Appl. Sci. Manuf. 41(12), 1839–1847 (2010). https://doi.org/10.1016/j. compositesa.2010.09.002 89. C. Meola, G.M. Carlomagno, Infrared thermography to evaluate impact damage in glass/epoxy with manufacturing defects. Int. J. Impact Eng. 67, 1–11 (2014). https://doi.org/10.1016/j.iji mpeng.2013.12.010 90. C. Meola et al., Impact damaging of composites through online monitoring and nondestructive evaluation with infrared thermography. NDT E Int. 85, 34–42 (2017). https:// doi.org/10.1016/j.ndteint.2016.10.004

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Chapter 2

Introduction to Nondestructive Testing and Evaluation of Fiber-Reinforced Composites

2.1 Approaching the Nondestructive Testing World In recent years, structural systems have been dominated by the utilization of longer, bigger, and significantly complex fiber-reinforced composites including but not limited to wind turbine blades (WTB), bridge decks, helicopter rotor systems, marine hulls, and airplane wing-spars. Such an increase in size and complexity further complicates their manufacturing processes, the likelihood of developing defects, and strict utilization and maintenance requirements [1–3]. To avoid sudden failures, users and manufacturers always seek to inspect composite structures during their lifecycle by using adequate NDT techniques to ensure their functional design is not compromised [4]. Among these NDT techniques, visual inspection remains the most common testing method to reveal the truth and provide answers to questions such as “what is inside?”, “is there damage?”, “must be anyhow rejected?” and “does it need to be repaired?” etc. However, vision alone does not provide answers to all these questions because it only works in a small band of frequencies in the electromagnetic spectrum called visible light (i.e., with the wavelength λ varying between 400 and 700 nm). In fact, human eyes are only capable of detecting defects of certain sizes (only on the surface of an illuminated component), unless such a component is transparent under the visible light (e.g., glass, some plastic, etc.) [5]. As a result, the inspection of composites for possible defects through vision remains limited because the vast majority are not transparent in the visible band. Interestingly, it is possible to find at least one physical phenomenon (the inspection parameter) that will interact with and be influenced by the test specimen through some physical principles and be able to see through the material and visualize the details in its interior that would otherwise be invisible to the human naked eye. In material engineering, product safety assurance, in-line diagnostics, quality control, health monitoring, as well as security testing, and structural engineering, methods using such physical principles to view the interior of the material without altering its original attributes are referred to as nondestructive testing (NDT). Recent © Science Press 2022 S. Zhong and W. Nsengiyumva, Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures, https://doi.org/10.1007/978-981-19-0848-4_2

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studies [1] suggest that NDT is a very broad and interdisciplinary area that encompasses all the techniques and methods that are used for the testing and evaluation of structural systems or components to ensure they continuously perform their functional design more reliably and cost-effectively [6]. In this context, NDT practitioners and engineers generally define the conditions and types of nondestructive tests and conduct them to locate, characterize and evaluate the material or structural system’s conditions and flaws that might be threatening the structural integrity of components and systems causing them to fail prematurely while in-service (e.g., the train derail, plane crash, reactor failure, pipelines burst, pole breaking, etc.) and a variety of less visible, but equally troubling events. Since NDT allows inspection of components or materials without interfering with their structural attributes, NDT techniques provide several benefits including the excellent balance between quality control and cost-effectiveness [4, 7, 8]. The application of NDT techniques for the test and evaluation of fiber-reinforced composite materials has been an area of continued growth for over 60 years now and its growth does not only depend on its practical demands but also its interdisciplinary nature. While there are many published studies on this topic, the semantics are quite complex in most of the studies and often may be misleading. As such, we believe that it is important to first establish the correct semantics to be used in this book before starting the discussion on NDT of fiber-reinforced composite materials, which is indeed our primary focus in this book. In reality, some terms NDT, nondestructive evaluation (NDE), and nondestructive inspection (NDI) are often used interchangeably including in some references in this book. Although these three terminologies appear to be synonymous, a deeper analysis reveals that fundamental differences exist between the three terms viz. inspection, testing, and evaluation [4]. To this end, one may want to know the correct terminology to use and what would be its significance/meaning in the NDT context. Generally speaking, inspection indicates the action of observing the material or the structure critically and carefully to ensure it is in line with the predefined standards. That is, inspection mainly involves the sense of sight where the individual involved uses his or her eyes to carefully observe the material of interest. In many engineering applications, for example, the inspection of structural systems involves the use of measurement systems, test instruments, and gauges to obtain some features and/or characteristics of the test material/structural system which are then compared to some predefined requirements and standards to determine whether the material or structural system is in line with these targets. According to ISO/IEC 17,020 standard, inspection denotes an examination of the features of the products, structural systems, processes, services, or installations and/or their designs to ensure their conformity with specific requirements based on professional judgment or with general requirements [9]. Usually, inspection is performed by accredited bodies and may be followed by legal action if protocols are not properly followed. Conversely, the word testing mainly indicates the action of doing something with our hands and minds to ascertain if the material presents certain features. Finally, evaluation entails the expression of an opinion based on experience or measurements. That is, evaluation is closely linked with testing because, in the sequence of events, the former follows the latter (i.e., the output of a test may either be a matter of evaluation to appraise the health

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status of the test structure or help in decision-making about the replacement, repair or removal from the process). This indicates that evaluation is inherently a theoretically informed approach to interpret the results obtained from tests and give a meaning that is easily understandable and summarizes the main takeaway from these results. Interestingly, all these terminologies preserve their original meanings when coupled with the word nondestructive. In general, NDI mainly entails the use of vision testing that results in either discarding the test component or calling for advanced NDT techniques if macroscopic nonconformities are observed. The purpose of using these advanced NDT techniques is to help engineers and technicians to gain more information that will aid in the decision-making process pertaining to the replacement, repair, or removal from the process. NDT involves a set of physical actions performed on the composite material to verify or understand its characteristics, properties, behavior, integrity, composition, or any other features in ways that do not impair its future usefulness and serviceability. It is noted that the results of such tests deserve particular attention since they will be critically analyzed and evaluated for a subsequent conclusion or decision-making process. NDE represents the action of making the final judgment based on both the inspection and testing results [10]. As an example, when buying a “watermelon” from the market, the buyer first glances over the melons, picks one of them then looks it over for possible flaws (i.e., visual inspection). After that, he/she tests the melon’s ripeness by tapping the surface and listening for the hollow response (i.e., acoustic testing). Finally, he/she evaluates the quality of the melon through the hallow response relative to his/her preset NDE criteria (e.g., “The melon must look good, compact and be ripe enough”) and decides whether to pay for it or choose another one. Quite surprisingly though, all of the melons (even the flawed ones) will eventually be sold, and this reveals a common problem with NDE: inspectors’ standards often change relative to their individual perceptions. Hence, standard guidelines are crucial to establish the baseline for the accept-or-reject criteria or any other types of evaluation and decision-making processes. In this book, the term NDT is used to indicate that the investigations being referred to involve the tests, while the term NDE denotes the use of both the tests and evaluation of the test results. It is also important to know what we are looking for when applying the testing and evaluation to a material system. As indicated earlier NDT seeks to determine the quality of the test structure (i.e., during the manufacturing process) [11–14], investigate the structural integrity (i.e., in-service stage of the composite) [15–17]. In doing so, NDT engineers and practitioners seek to locate internal and surface discontinuities (e.g., defects, flaws, damage, etc.) and determine their features (e.g., size, shape, flaw orientation within the structural system vis-à-vis the loading orientation, etc.) [4, 11, 14, 18] and material/structural properties (e.g., optical, dielectric, conductivity, etc.) [19, 20]. This leads us to another set of terminologies viz. defect, flaw and damage. A flaw denotes an imperfection or discontinuity in material or structural systems that may be detectable by an NDT technique but is not necessarily rejectable. In contrast, defects are the types of laws with an aggregate size, shape, orientation, location, or properties surpassing the pre-specified or predefined acceptance criteria and are therefore rejectable. In most cases, “flaw” and “defect” are used

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interchangeably, including in some of the references in this paper. In this context, we will try and make that distinction in the present book based on the detailed information in the respective publications to ensure accurate information is conveyed to our readers. As opposed to flaws and defects, the damage denotes an observable change that happens to the material or a structural system to alter their geometric properties which ultimately affects their structural performance when in-service (i.e., damage also includes material degradation) [10]. At the end of any nondestructive test, inspectors should be able to answer whether the component is defectious, needs some repair, or should be discarded. Indeed a straightforward answer to any of these questions cannot be delivered by just looking at the component of interest because several factors must be taken into consideration consistent with the intended use and the acceptance criteria (which may be different depending on the industry and the intended application). In general, however, several factors related to the defects in structural systems (e.g., the presence of the defects, their nature, sizes, etc.) play an important role in the decision-making process regardless of the industry, the destination, and the acceptance criteria. In some cases no defect (not even a very small defect) is acceptable, and the part must be discarded if any defect is detected while in other cases, the part passes the control stage even if it contains defects at a certain level less than the predefined threshold.

2.2 Classification of Flaws Vis-À-Vis the NDT Techniques Etymologically, the word defect comes from the Latin word “defectus” and this word in Latin means weakness, deficiency, or lack of something for wholeness or completeness. Although the literature indicates that the size of the defect presents a significant effect on its criticality and the validity of the structure thereof, there is no clear classification of defects based on their relative sizes with respect to the size of the structure of interest except the general designation as microscopic and macroscopic defects, also known as micro defects and macro defects, respectively. Indeed this classification does not provide any measurements whatsoever but rather a random judgmental comparison between the relative sizes of the defects in the composite of interest and sometimes the individual perception of the estimation of its size. Generally speaking, microdefects affect the material crystalline structure and this is probably the reason why they are called crystalline defects. Interestingly, these types of defects may be beneficial or adverse to the functional design of the structure depending on the application. In semiconducting materials, for example, crystalline defects may provide the material with specific properties that can also be exploited for technological purposes. In particular, defects in the crystal structure such as semiconductors may include either spoiled atoms or irregularities in the atoms’ alignment and may be classified as point defects or linear defects. The former constitutes a group of local imperfections in the crystal such as the missing atoms or presence of holes (i.e., in semiconducting compound materials) and the presence of

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Fig. 2.1 Illustration of the crystalline defects: a the normal crystal, b a crystal with some missing atoms, and c a crystal with some interstitial defects

an interstitial impurity (e.g., the carbon atoms that are mixed with the iron atoms to form the steel) as depicted in Fig. 2.1. Conversely, macrodefects are generally large and undesirable types of defects occurring in the composites. They represent discontinuities in the material which may form at different stages of the composite’s life cycles during manufacturing or in-service. As indicated in the first chapter of this book, incorrect manufacturing processes often lead to the formation of defects such as voids, porosity, cracks, foreign inclusions, and many others. In-service material degradation occurs mainly due to fatigue cycles, impact damage, and adverse environmental conditions that cause corrosion and overall decay of the composite. Usually, macroscopic defects are classified as open defects (e.g., cracks, delamination, holes, etc.), gas-filled defects (e.g., porosity, voids, etc.), solid slag inclusions (backing film, dust, foreign material fragments), and the lack/excess, of adhesives in bonded joints or any other adhesive-based structures. A typical example illustrating some of the above-mentioned macroscopic defects is presented in Fig. 2.2. The first scheme shows a general flaw that may be either a solid slag inclusion or a gas-filled volume. It is noted that this example is presented for illustration purposes and might depict the like of the real defects as the latter generally tend to be of complex geometry with jagged edges. The second scheme represents a discontinuity in the adhesive film between two surfaces of a junction. This type of discontinuity is generally very thin and small since it is located over either narrow or large surfaces but in a spotty fashion.

Fig. 2.2 Typical examples of macroscopic flaws in fibrous composite structures: a flaw buried in the material, and b lack of adhesive between the parts of a joint

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Although the standard terminologies for NDT are internationally standardized, there are currently many types of international standards and some of the terminologies might be slightly different. While it is not our intention to mention all the definitions and terminologies found in the above documents [10, 21], some of the main definitions are worth mentioning. In the context of NDT, “indication” denotes on the one hand the response or evidence from an examination, such as a blip on the screen of an instrument. Indications are classified either as true or false. In the latter case, whereby the former is defined as those indications caused by factors not related to the principles of the testing method or by improper implementation of the method, like film damage in radiography, electrical interference in ultrasonic testing, etc. True indications are further classified as relevant and non-relevant depending on whether they are caused by flaws (delaminations, cracks, etc.) or by known features of the tested object (i.e., gaps, threads, case hardening, etc.). “Interpretation” on the other hand indicates the process of determining if an indication is of a type to be investigated or if a certain action is to be taken. In electromagnetic testing, for example, indications from metal loss are considered flaws because they should usually be investigated, but indications due to variations in the material properties may be harmless and non-relevant. According to ASTM E1316-17a [10], a flaw denotes an imperfection or discontinuity that may be detectable by the NDT technique and is not necessarily rejectable as the part with a flaw can still be used to fulfill its functional design. From the NDT perspective, a flaw is something that can occur in various sizes, shapes, orientations, locations, and can even only be isolated to a tiny portion of the material properties within a material volume. In general, flaws can occur naturally or can be introduced by the material processing or finishing processes. As such, with this wide range of definition and generation possibilities, there is also a lot to take into consideration when understanding NDT capabilities for detecting flaws. As opposed to a flaw, a defect denotes one or more flaws featuring an aggregate size, shape, orientation, location, or properties that do not meet specified acceptance criteria and are therefore rejectable. Although the terms “flaw” and “defect” are often used interchangeably (including in some references of this book), a defect is considered a rejectable flaw by the standard and the content in this book. Also, the term “damage” denotes changes in materials or geometric that affect the performance of the final structural systems. In summary, a complete classification of the different defects occurring in fiberreinforced composites vis-à-vis the different types of NDT techniques has to be performed. This is particularly useful to the NDT engineers because there are several types of fibrous composites and some of the defects/damage types occur in some than in others depending on factors such as the field of application of the material (e.g., transport industry, nuclear plants, chemical plants, architecture, environment, etc.). Also, some defects/damage types are more serious in some types of fibrous composites while their effects are almost negligible. In this context, some defects are particularly serious and must be discovered in time to avoid subsequent catastrophic consequences! Indeed, the most important thing to be considered by most of the NDT inspectors is the use that the material is intended for, or better, whether or not the material is used for the construction of structural parts. In normal circumstances, the

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application also entails the urgency of detecting flaws before it is too late. Once, the inspectors understand the urgency and the importance of detecting hidden defects in specific structures or composite parts, the question is now to determine the safest method and procedures they can use to discover all these buried defects without destroying the material. In the following sections of this chapter, we will try to respond to these questions in general, and in subsequent chapters, specific answers will be provided.

2.3 General Hints on Testing Techniques and Procedures The use of NDT methods to detect, localize and characterize flaws in composite materials has always been an area of continued growth for over 60 years. However, the need for NDT has significantly increased in recent years for many reasons including product safety, in-line diagnostics, quality control, health monitoring, and security testing to list but a few. In addition to its inherent practical demand, significant advancements in NDT are also dependent on its interdisciplinary nature and the areas where it is most indispensable including aerospace engineering, civil engineering, electrical engineering, material science and engineering, mechanical engineering, nuclear engineering, petroleum engineering, and physics among others. This clearly indicates that a single NDT technique cannot in any way be sufficient to accommodate all these areas, suggesting that even in a single area more than one NDT technique may be needed, and hence a synergic integration of several techniques may be required [1], which further reinforces the commonly repeated mantra that not a single NDT technique can identify all types of defects in all types of materials. To this end, the literature indicates that there are at least two dozen NDT methods in use today [1, 7, 22]. In fact, any sensor that can examine the inside of material without interfering with its original attribute (i.e., in a nondestructive fashion) is useful for NDT. In today’s composites manufacturing world, once a composite part is manufactured, additional steps are undertaken to ensure its conformity with the specific standards as established by the customers and ensure there is no buried defect. To this end, the oldest and cheapest method that humans used to confirm the conformity of an artefact’s correctness and/or exquisite design was by using their sensory receptors (i.e., vision, percussion, and auscultation). Among them, the vision (i.e., the visual inspection) is perceived to be the NDT method that every single manufacturer uses automatically to verify the artefact’s correctness, without considering it as a particular NDT method. In other words, inspectors or manufacturers must look at the finished part to check the shape, dimensions and surface conditions then decide whether the part is to proceed to the next stage (e.g., assembly) or reject (i.e., for the parts that are entirely or completely off the design). Similarly, percussion is predominantly used in architecture to examine the areas of disbonding patches or adhesives partly because the bond quality is often accurately estimated by analyzing acoustical response from a carefully controlled knock. However, inspection results obtained by using human senses are almost subjective because, first, they are qualitative, and

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second, they largely depend on the operator’s skill, experience as well as his/her senses’ perception acuity. This is particularly because people often have different levels of perception even when considering the same information and that may affect the quality estimated from the information received by their senses. On the contrary, the results of a serious quality control-related investigation must always be quantitative and of overall validity (i.e., objective), and hence, adequate NDT techniques must be used to ensure the inspection results represent an accurate account of the health state of the test composite. Consistent with the aforementioned, there are currently many different NDT techniques are available to conduct the materials’ control quality during the development and investigate the health status of the resulting structural systems when in service. However, constantly evolving technologies give rise to the development of new materials which also bring about new testing challenges and defects types and orientations that must unequivocally be overcome by NDT engineers and practitioners. In this context, more sophisticated devices are constantly being developed to meet the challenges of the day, which clears the way for possible development of new techniques as well the enhancement of the already existing ones to fit into the testing kits of the materials and systems designs. The existence of different techniques means that many different physical parameters and properties can be exploited for the NDT of materials. In fact, there are NDT techniques that are based either on the optical signal, thermal signal, acoustic signal, or electrical signal [22, 23]. All of these methods are used for the detection of many different types of defects, whether they are shallow or deeply buried within the thickness directions of the composite. Generally, the NDE of composite materials is achieved by sending energy from the testing kits into the test material system and analyzing its response. That is, certain signals (e.g., γ-rays, electric current, electromagnetic waves, heat flux, ultrasound waves, etc.) are delivered to or inside the test fibrous composite material or structural system, then the test instrumentation is used to measure and analyze the reaction of the test material or composite structural system to such an external stimulus (i.e., the analysis is conducted to quantify the changes experienced by the signal after passing through the material) to obtain the information about the quality or soundness of the test composite. In terms of their operation, most NDT techniques operate differently depending on the behavior of the material and its inherent flaws vis-à-vis the energy levels of the input signal. In general, however, when a given input signal strikes the surface of the test composite material, the former may be absorbed, scattered, or reflected by the test material or structural system and in some cases, this energy may even be transmitted through the material depending on its reflectivity, absorptivity, and transmissivity properties vis-à-vis the test signal. That is, the propagation of the signal inside the fiber-reinforced composite material generally depends on its inherent properties (e.g., mechanical, thermal, chemical, acoustical, electrical, optical properties, etc.). Indeed, all these properties combined determine the nature of the input signals used by most NDT techniques in today’s nondestructive inspection practices. In case there are changes in the material’s properties taking place as a direct consequence of inhomogeneities and overall material alteration, these changes can easily be picked up by

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NDT engineers/practitioners by applying certain NDT techniques. It is also possible to assess the variations in one set of material properties by examining changes in another set of properties of the same material, and this is indeed one way of evaluating the material integrity used by several NDT techniques (i.e., indirect measurements). In all these cases, inspectors should choose the most appropriate NDT technique, considering their effectiveness which largely depends on the properties of the host material (e.g., electrical, dielectric, conductivity, thermal, mechanical, optical, etc.) to block or let the test signal travel through the material (i.e., NDT techniques measures the perturbation inflicted by a hidden discontinuity to the propagation of the specific test energy signal). This is particularly because, NDT practitioners must always ensure these two conditions are complied with so that the readable quantity (i.e., caused by the perturbation of the input signal by the material’s inhomogeneity), must be sufficiently large to not be confused with the apparent background noise. After the first step of measuring and acquiring the inspection data the material’s integrity is evaluated by using the most advanced and effective signal processingbased technique (i.e., wavelet transforms, gapped-smoothing method (GSM), Teager energy operator (TEO), etc.) or by combining of techniques (i.e., also known as NDT data fusion) [1]. It is important to indicate that most NDT practices do involve two main aspects viz. the examination of the material degradation in terms of loss of its original characteristics (i.e., thermal or electrical conductivity, material’s hardness, elasticity, variation of the density, etc.) and the presence of defects or damage both superficial (e.g., scratches, loss of optical properties, etc.) and internal (e.g., voids, delaminations, cracks, etc.). A full description of these types of defects, damage, and material degradation is provided in Table 1.1, and readers are referred to this table for more information.

2.4 Classification of NDT Techniques In order to provide a plausible classification of NDT techniques, it is important to establish the right semantics pertinent to NDE and NDT as well as other related terms. According to ASTM E1316-17A [10], NDT is the development and application of technical methods that use the physical measurements to examine the materials or components to detect, locate, measure, and evaluate flaws; to assess integrity, properties, and composition; and to measure geometrical characteristics in ways that do not impair its future usefulness and serviceability [1, 10]. Nondestructive Inspection (NDI) is often used to replace NDT (i.e., they are both considered synonymous) because they are both related to the use of measurements tools per specification and assessment against specified criteria. Consistent with the aforementioned, NDT is generally an interdisciplinary area and it plays a crucial role in assuring that structural components/systems perform their function more reliably and cost-effectively. NDT practitioners and engineers define and implement nondestructive tests that identify and evaluate the health status of the test structures and/or materials to prevent structural component or system failure (e.g., plane crash, reactor failure, the train derail,

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pipelines burst) and a variety of less visible, but equally troubling events. Since NDT allows inspection of components without interfering with their final usage, it provides an excellent balance between quality control and cost-effectiveness [4, 7]. Although nondestructive evaluation (NDE) is a term that is often used interchangeably with NDT, the former is used to describe more quantitative measurements. That is, an NDE facility would not only identify and determine the position of a flaw in a composite material, but it would also be used to obtain detailed information about its size, shape, and orientation. NDT techniques also help to determine the properties of the materials including the dielectric properties, optical properties, fracture toughness, formability, etc., which are generally important for specific applications of composite materials such as the design of electrostatic discharge devices, the manufacturing of electromagnetic shielding systems and/or printed circuit boards (PCB), etc. In some cases, nondestructive characterization (NDC) is often used to replace NDE and they both denote the use of nondestructive measurements for material condition, properties, or state assessment, especially in a quantitative manner. Although both NDT and NDE are often used interchangeably as synonymous by many NDT practitioners (i.e., including in some of the references of this book), readers are encouraged to make a distinction between these two terms in the context of this book according to the above-provided definitions. NDT&E is usually used to designate a combination of both NDT and NDE and the same denomination is adopted in this book. The classification of NDT techniques into different groups is not a trivial action to perform because it involves a number of criteria and considerations which may conflict with one another, resulting in techniques being wrongly classified as belonging to one group or the other. However, a consensus governing the classification of NDT into different groups considers a limited number of considerations. That withstanding, nearly all the available NDT techniques can be classified based on the way a test is conducted, or simply put, based on the relative position of the energy input actuator and/or the measuring sensors vis-à-vis the surface of the composite material being tested. To this end, it is possible to classify all the NDT techniques into the two main groups viz. contact and noncontact NDT. As the name indicates, the former involves all the NDT techniques that require either some kind of direct contact with the surface of the composite material being tested or an indirect contact when a coupling medium is needed while the latter involves all the NDT techniques that do not need any direct contact nor coupling agent between the measuring sensor and composite material being tested [1]. Each of these techniques presents many advantages and depending on the application some techniques may be relevant than others. Another classification is often attempted and this considers the type of output signal involved. That is, the output signal may be thermal energy, sound wave, electromagnetic radiation, X-rays, electric and magnetic fields to list but a few [4]. Indeed this has been proven to be equally an effective way of classifying the different types of NDT as the output signal may be the signature characteristic of each NDT technique. The main signal classes that are often recorded in NDT activities include thermal energy, sound, electricity/voltage, and radiation. Regarding the thermal energy signal, the heat is delivered to the surface of the component or it is generated inside the

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material. Then, the analysis is performed as a result of heat propagation and/or local variations of temperature. Similarly, the sound waves, often ultrasound waves, are exploited for the material evaluation owing to the changes that the signal undergoes and its crossing speed. The use of electric signals involves all the techniques that require the application of a direct electric current to be injected into the test material or structural system or any effects related to the use of electric/magnetic field to investigate the health status of materials or structural systems. In the context of this book, magnetic-based techniques are considered to be part of the electric testing methods also known as electromagnetic testing methods (e.g., eddy current testing, electrical resistance testing, etc.) [23]. Indeed, electric and magnetic fields are different and may exist independently, but they are also interrelated as the magnetic field is produced by moving electric charges. Finally, electromagnetic radiation with short-wavelength such as X-rays,γ -rays, or neutrons (i.e., the wavelength is generally less than 10 nm), are used to penetrate an object of a given material. The radiation collected by the instrument after passing through the test sample is analyzed to look for variations in intensity levels indicating the difference in material properties due to the presence of defects, flaws, or discontinuities. Indeed, the use of the output (reading) signal is quite a general way used by most NDT engineers/practitioners to classify different NDT techniques in different groups and the classification results may not be accurate. Typically, heat can easily be generated into the test material or structure after the introduction of electric current through the Joule effect. Similarly, several types of output signals can also be generated into the test material or structural system after the introduction of sound waves into the material or structural system through the friction effect. In other cases, the classification of different NDT techniques into different groups is solely based on the output signal that is directly under analysis, without any distinction of what caused it. An additional type of classification is also performed based on the safety issues of the workplace, the individuals involved, and emissions that may be harmful to the environment (e.g., toxic gases, ionizing radiation, etc.). That is, the NDT technique may be classified as harmful or harmless to human lives. The former includes all the NDT techniques with potential health risks, including the NDT techniques that use electromagnetic radiation or emit different types of pollutants (e.g., chemicals, radioactive emissions, etc.) during their operation, or they involve the use of mobile mechanical parts, etc. This is particularly the case for X-ray and neutron imaging, which represent the common examples of harmful NDT techniques. Conversely, harmless NDT techniques do not emit any harmful radiation, cannot require the users to be exposed to any hazardous materials/chemicals, and must not include any harmful or unsafely moving mechanical parts in the test setup. It is noted that these classifications do not provide any tangible measure to determine the best NDT technique but rather the starting point in the whole NDT technique selection practices. The optimum selection of the most relevant NDT technique for a particular test is reached by not only choosing the best from each of the aforementioned classes, but also by choosing an NDT technique that can detect the types of defects or damage being considered (i.e., its spatial resolution, sensitivity, and variation in output signal), the available personnel, the type of composite materials being tested, as well as the

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cost of the test equipment involved among others. The technique should also be harmless and noninvasive not only to the environment and personnel but also to the sample itself to guarantee the structural integrity of the sample after the test. Finally, after inspectors are aware of the type of material being tested and the type of defect to discover, the determination of the best possible NDT technique is made through a fair compromise between all the above-discussed considerations.

2.5 Inspection Requirements for NDT Techniques The inspection of composite materials is performed at the different stages of their life cycle (i.e., material processing, manufacturing, and in-service) [1, 24, 25]. At the manufacturing level, NDT determines the part’s fitness to serve consistent with the predefined specific requirements. At the in-service level, NDT determines the health status, monitors the damage mechanism, and helps the inspectors to make informed decisions regarding the structure’s remaining life span, and if necessary recommends a maintenance-repair process and timeline [26, 27]. There are mainly four considerations that should be observed when selecting the most suitable NDT to be used in the inspection. These considerations include (1) the type of damage the structure is to be inspected for, (2) the smallest detectable flaw size, shape, and orientation within the composite, (3) the location of the damage/defect (i.e., accessibility, surface, or internal), and finally (4) the sensitivity, spatial resolution, and the subsequent limitations of the NDT method under consideration. In some cases where in-service composites are involved, the right NDT method for a specific application is the one required by law to guarantee the safety of users and NDT personnel. In case no specific laws is indicating the type of the test method, guidance may still be found by reviewing the best practices published by international standards organizations as listed in Ref. [1]. Similarly, the manufacturer of the structure being tested may also publish a list of NDT standards and best practices for their product. If none of the above provides an answer, it is recommended for the test practitioners to consult a Level III NDT technician who can recommend the best next steps to take though some well-established methods such as ultrasonic testing and infrared thermography might likely be the best methods to envision. Careful consideration of all the aforementioned factors will guarantee reliable inspection results, optimize the composite performance while in-service, and minimize the safety concerns [1, 7, 20]. Figure 2.3 outlines the different steps involved in an NDT test to examine the quality and integrity of composite structures. NDT produces always results in the form of an indication, which is then subjected to an intense interpretation to decide whether it is a false indication (i.e., interpreted to be caused by a condition other than a discontinuity or imperfection), non-relevant (i.e., caused by a condition or type of discontinuity that is not rejectable), or relevant (i.e., caused by a condition or type of discontinuity that requires a subsequent evaluation). If it has been interpreted as relevant, the subsequent evaluation will result in the decision to accept or reject the results consistent with their conformance or

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Fig. 2.3 Illustration of the steps and decision-making process involved in the application of NDT to examine the integrity of thick composites

nonconformance to the already set acceptance criteria or requirements [28–30]. All NDT techniques do not perform in the same way and when referring to the efficacy of the method, some formulations such as flaw detection, flaw localization, and flaw characterization are often used by researchers including the references in this book. In most cases, such denominations are used after the interpretation of the inspection results and each of them presents a different meaning from the NDT terminology standpoint. Flaw characterization indicates the process of determining the size, shape, orientation, location, growth, or other properties, of a flaw based on the NDT response [10]. The flaw detection indicates the action or the process of identifying the presence of a flaw in the structure, while flaw localization denotes the determination of its exact position within the structure, and the flaw quantification denotes the sizing of the flaws. Generally, setting the inspections requirements is not a trivial task given the amount of work and levels of uncertainties involved. A robust and accurate quantitative examination limitation requires NDT personnel to take into account the detection capacities and the tolerances and probability-of-detection (POD) for every NDT, and this means that practitioners must conduct extensive NDT reliability studies to produce a POD curve whereby minimum unfailingly detectable sizes of defects/damage are determined, taking into consideration the operation conditions of the NDT [1, 7]. To date, it is not possible yet to establish a credible chart outlining the limitations of different NDT in terms of the sizes of the damage they can detect unfailingly, other than the inspection goals which are usually set by the operators considering the effect and the criticality of the defects or the damage of a specific size on the composite part consistent with the aforementioned considerations [7, 31]. Also, the literature [1] suggests that there are still no clear inspection requirements that would facilitate the choice of appropriate NDT for a particular structure when considering certain features such as the thickness, size, and the type of flaws. Instead,

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the detection goals are fixed based on the effects of the damage on the composite and the industry where the composite is intended to be used [7, 32], particularly because setting robust and quantitative inspection requirements requires extensive reliability studies on different NDT techniques to establish a probability-of-damage curve and the subsequent minimum unfailing detectable damage size [7, 33]. In particular, the constantly increasing utilization of thick composites and sandwich structures in structural engineering, their quality and performance specifications are becoming increasingly demanding and in some cases difficult to achieve, suggesting that the determination of robust and reliable NDT techniques (particularly for the techniques that are used for the testing of composite structures) is highly warranted. Although there has not been much discussion about reliable NDT techniques to examine different composite structures with regards to all the above-mentioned considerations and acceptance criteria for composite structures as safe to use, the determination of the damage sizes has always been the focus of each NDT technique. As a result, certain threshold values have been established for different inspection practices and the techniques that can achieve the inspection up to these threshold values are generally considered reliable. To this end, in reviewing the NDT of porosity in PMC structures in 2004, the authors in Ref. [32] proposed that a 2% voids in volume be considered the normally acceptable threshold to choose a reliable inspection for general-purpose composites and sandwich structures during the manufacturing process [1, 7, 34]. In order to keep our discussion focused, simple, and convenient, the same limitation is considered in this book. In addition, a technique that is considered capable of examining composite structures is considered herein as being able to reliably detect a localized flaw of approximately