ENGINEERED INTERFACES IN FIBER REINFORCED COMPOSITES JANG-KYO KIM & YIU-WING MA1 c f t ENGINEERED INTERFACES IN FIBER REINFORCED COMPOSITES ENGINEERED INTERFACES IN FIBER REINFORCED COMPOSITES Jang-Kyo Kim Department of Mechanical Engineering Hong Kong University of Science and Technology Clear Water Bay, Hong Kong Yiu-Wing Mai Centre for Advanced Materials Technology and Department of Mechanical & Mechatronic Engineering University of Sydney, NSW 2006, Australia 1998 ELSEVIER Amsterdam Lausanne * New York * Oxford - Shannon * Singapore Tokyo ELSEVIER SCIENCE Ltd The Boulevard, Langford Lane Kidlington, Oxford OX5 IGB, U.K. Library of Congress Cataloging-in-Publica~on Data Kim, Jang-Kyo. Engineered interfaces in fiber reinforced composites / Jang-Kyo Kim and Yiu-Wing, Mai. 1st ed. p. cm. Includes index. ISBN 0-08-042695-6 (hardcover) 1. Fibrous composites. I. Mai, Y. W., 1946- . 11. Title. TA418.9.C6K55 1998 620,1'18 DC21 97- 5 2002 CIP First edition 1998 ISBN 0-08-042695-6 0 1998 Elsevier Science Ltd All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical photocopying, recording or otherwise, without permission in writing from the publishers. Q The paper used in this publication meets the requirements of ANSUNIS0 239.48-1992 (Permanence of Paper). Printed in The Netherlands It is a pleasure to write the foreword to this book. This work emphasizes for the first time in one volume how interfaces in fibrous composites can be defined, measured, improved and optimized. Many practitioners of composites technology will find in this book the information they have been seeking to match fiber and matrix at the interface, thereby obtaining the best mix of properties in the final application. Composites engineering is a relatively young field in which the test methods and measurement techniques are not yet fully developed. Even more important, the ideas linking the properties of composites to the interface structure are still emerging. This book not only reviews the historic and pragmatic methods for studying composites; but it also presents the most recent theories and fundamental tests of interface properties. This allows the reader to find the true framework of theory to fit his/her observations. The fact that two brittle materials can be brought together to give a tough product is the proof that interfaces are critical to composite properties. However, the complexities of this process depend on the raw materials, on the surface chemistry of the components, on the fabrication procedures, on the chemistry of hardening, and on the damage and corrosion sustained in use. A wide view of material science, chemistry, mechanics, process engineering and applications experience is necessary to focus successfully on the role of the interface. The authors have demonstrated such a global view in this volume. I have known Professor Mai for over 20 years. He is a foremost authority on fracture mechanics of composite materials, having studied polymer composites, cement, ceramic and natural composite systems, in the US, Britain, Australia and Hong Kong. In particular, he has made memorable contributions to the understanding of cracks and to the crack-inhibiting effects seen in fibrous composites. He has previously coauthored two books on fracture. Professor Kim originally worked in the composites industry and has returned during the past 10 years to study interface mechanisms more closely. He is currently working in the Hong Kong University of Science & Technology. In summary, the topic of engineered interfaces in composites is an important one, critical to the advance of the composites industry. Many practitioners from a range of disciplines are seeking the information which can be found in this book. The authors display the wide experience and theoretical knowledge necessary to provide a critical view of the subject. I strongly recommend this volume to the composite expert and student alike. Kevin Kendall Keele University, UK May 1997 V PREFACE The study and application of composite materials are a truly interdisciplinary endeavor that has been enriched by contributions from chemistry, physics, materials scicncc, mcchanics and manufacturing cnginecring. The undcrstanding of thc interface (or interphase) in composites is the central point of this interdisciplinary effort. From the early development of composite materials of various nature, the optimization of the interface has been of major importance. While there are many reference books available on composite materials, few of them deal specifically with the science and mechanics of the interface of fiber reinforced composites. Further, many recent advances devoted solely to research in composite interfaces are scattered in different published literature and have yet to be assembled in a readily accessible form. To this end this book is an attempt to bring together recent developments in the field, both from the materials science and mechanics perspective, in a single convenient volume. The central theme of this book is tailoring the interface properties to optimize the mechanical performance and structural integrity of composites with enhanced strength/stiffness and fracture toughness (or specific fracture resistance). It deals mainly with interfaces in advanced composites made from high performance fibers, such as glass, carbon, aramid, ultrahigh modulus polyethylene and some inorganic (e.g. B/W, A1203, Sic) fibers, and matrix materials encompassing polymers, metals/ alloys and ceramics. The book is intended to provide a comprehensive treatment of composite interfaces in such a way that it should be of interest to materials scientists, technologists and practising engineers, as well as graduate students and their supervisors in advanced composites. We hope that this book will also serve as a valuable source of reference to all those involved in the design and research of composite interfaces. The book contains eight chapters of discussions on microstructure-property relationships with underlying fundamental mechanics principles. In Chapter 1, an introduction is given to the nature and definition of interfaces in fiber reinforced composites. Chapter 2 is devoted to the mechanisms of adhesion which are specific to each fiber-matrix system, and the physico-chemical characterization of the interface with regard to the origin of adhesion. The experimental techniques that have been developed to assess the fiber-matrix interface bond quality on a microscopic scale are presented in Chapter 3, along with the techniques of measuring interlaminar/intralaminar strengths and fracture toughness using bulk composite laminates. The applicability and limitations associated with loading geometry and interpretation of test data are compared. Chapter 4 presents comprehensive theoretical analyses based on shear-lag models of' the single fiber composite tests, with particular emphasis being placed on the interface debond vii [...]... definition of interface that is in general use in this book However, for analytical purposes in micromechanics the interface is still conveniently considered to be infinitely thin and the properties of the mating fiber and matrix are isotropic and homogeneous 1 2 Engineered interfaces in fiber reinforced composites Thermal, chemical, mechanical Surface layer Fig I 1 Schematic illustration of the components... composites in many engineering applications has made the issue of interfuce (or more properly termed, interphase (Drzal et al., 1983)) a major focus of interest in the design and manufacture of composite components A classic definition of the interjiuce in fiber composites is a surface formed by a common boundary of reinforcing fiber and matrix that is in contact with and maintains the bond in between... each fiber- matrix system (Kim and Mai, 1991) Adhesion in general can be attributed to mechanisms including, but not restricted to, adsorption and wetting, electrostatic attraction, 5 6 Engineered interfaces i fiber reinforced composites n chemical bonding, reaction bonding, and exchange reaction bonding (Kim and Mai, 1993), which are schematically shown in Fig 2.1 and discussed in the following sections... ‘non-wetting’ and ‘wetting’ If the liquid does not form a droplet, i. e 8 = O”, it is termed ‘spreading’ and the relationship given by F q (2.2) becomes invalid In this case, the i equilibrium is expressed by an inequality Ysv - Yst > YLV (2.3) Vapor ‘A Fig.2.2 Contact angle, I ) , and surface energies, yLv, ysL and ysv for a liquid drop on a solid surface 8 Engineered interfaces in jber reinforced composites... nanoscopic nature of interfaces in most useful advanced fiber composites requires the characterization and measurement techniques to be of ultrahigh magnification and resolution for sensible and accurate solutions In addition, cxperiments have to be carried out in a well-controlled environment using sophisticated testing conditions (e.g in a high vacuum chamber) There are many difficulties often encountered... Carbon Fibers 186 Polymeric Fibers 196 Aramid Fibers 196 Ultrahigh Modulus Polyethylene (UHMPE) Fibers 201 Inorganic Fibers 205 Introduction 205 Selection of Coating Materials and Coating Techniques 206 Carbon Fibers 210 Boron Fibers 214 Silicon Carbide (Sic) Fibers 216 Alumina (A1,OJ Fibers 223 References 228 Interface Mechanics and Fracture Toughness Theories 239 Interface-related Fracture Toughness... in various ways, creating inevitable interfaces In fiber composites, both the fiber and the matrix retain their original physical and chemical identities, yet together they produce a combination of mechanical properties that cannot be achieved with either of the constituents acting alone, due to the presence of an interface between these two constituents The growing number of uses for fiber reinforced. .. interface optimization of a given combination of fiber and matrix, various chemical-physical and thermodynamic-mechanical principles along with previous experience are invaluable sources of information to design the interface qualitatively A number of potential solutions have been suggested to improve specific properties of the composites, particularly the interface bond quality for efficient stress... Theories 239 Introduction 239 Fiber- Matrix Interface Debonding in Mode I1 Shear 242 Post-debond Friction 243 Stress Redistribution 243 Fiber Pull-out 243 Total Fracture Toughness Theories 245 Fracture of Ductile Fibers and Ductile Matrices 247 Toughness Theories for Short and Randomly Oriented Fiber Composites 247 Introduction 247 Fiber Pull-out Dominant Fracture Mechanisms 248 Matrix Dominant Fracture... by introducing the spreading pressure The Young-Dupre equation is then modified to Although the discussion of wettability presented above has focused on the thermodynamics between the fiber surface and the liquid resin, real composite systems consist of an extremely large number of small diameter fibers embedded in a matrix Adding to the issue of proper wetting of fiber surfaces by the resin, a key . ENGINEERED INTERFACES IN FIBER REINFORCED COMPOSITES JANG-KYO KIM & YIU-WING MA1 c f t ENGINEERED INTERFACES IN FIBER REINFORCED. are combined with matrix materials of similar/ dissimilar natures in various ways, creating inevitable interfaces. In fiber composites, both the fiber and