mechanical engineering questions ( material composite science )

Composite Materials

 

 

 

Krishan K. Chawla

Composite Materials

Science and Engineering

Third Edition

With 278 Illustrations

 

 

Krishan K. Chawla Department of Materials Science and Engineering University of Alabama at Birmingham Birmingham, AL 35294, USA kchawla@uab.edu

ISBN 978-0-387-74364-6 ISBN 978-0-387-74365-3 (eBook) DOI 10.1007/978-0-387-74365-3 Springer New York Heidelberg Dordrecht London

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Cover illustration: Fan blades made of carbon fiber/epoxy composite in the GEnx jet engine. [Courtesy of General Electric.]

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�A no bhadr�ah ˙ kratavo yantu viṡvatah

˙Let noble thoughts come to us from every side Rigveda 1-89-i

Dedicated affectionately to A, K3, and N3

 

 

 

Preface to the Third Edition

Since the publication of the second edition of this book, there has been a spate of

activity in the field of composites, in the academia as well as in the industry.

The industrial activity, in particular, has been led by the large-scale use of

composites by aerospace companies, mainly Boeing and Airbus. It would not be

far off the mark to say that the extensive use of carbon fiber/epoxy resin composites

in Boeing 787 aircraft and a fairly large use of composites in Airbus’s A 380 aircraft

represent a paradigm shift. Boeing 787 has composites in the fuselage, windows,

wings, tails, stabilizers, etc., resulting in 50% in composites by weight. Neverthe-

less, it should be pointed out that in reality, the extensive use of composites in

aircraft is a culmination of a series of earlier steps over the decades since mid-1960s.

Besides the large-scale applications in the aerospace industry, there have been

impressive developments in other fields such as automotive, sporting goods, super-

conductivity, etc.

All of this activity has led to a substantial addition of new material in this edition.

Among these are the following: Carbon/carbon brakes, nanocomposites, biocom-

posites, self-healing composites, self-reinforced composites, fiber/metal laminate

composites, composites for civilian aircraft, composites for aircraft jet engine,

second-generation high-temperature superconducting composites, WC/metal par-

ticulate composites, new solved examples, and new problems. In addition, I have

added a new chapter called nonconventional composites. This chapter deals with

some nonconventional composites such as nanocomposites (polymer, metal, and

ceramic matrix), self-healing composites, self-reinforced composites, biocom-

posites, and laminates made of bidimensional layers.

Once again, I plead guilty to the charge that the material contained in this edition

is more than can be covered in a normal, semester-long course. The instructor of

course can cut the content to his/her requirements. I have always had the broader

aim of providing a text that is suitable as a source of reference for the practicing

researcher, scientist, and engineer.

Finally, there is the pleasant task of acknowledgments. I am grateful to National

Science Foundation, Office of Naval Research, Federal Transit Administration,

Los Alamos National Laboratory Sandia national Laboratory, Oak Ridge National

vii

 

 

Laboratory, Smith International Inc., and Trelleborg, Inc. for supporting my

research work over the years, some of which is included in this text. Among the

people with whom I have had the privilege of collaborating over the years and

who have enriched my life, professional and otherwise, I would like to mention,

in alphabetical order, C.H. Barham, A.R. Boccaccini, K. Carlisle, K. Chawla,

N. Chawla, X. Deng, Z. Fang, M.E. Fine, S.G. Fishman, G. Gladysz, A. Goel,

N. Gupta, the late B. Ilschner, M. Koopman, R.R. Kulkarni, B.A. MacDonald,

A. Mortensen, B. Patel, B.R. Patterson, P.D. Portella, J.M. Rigsbee, P. Rohatgi,

H. Schneider, N.S. Stoloff, Y.-L. Shen, S. Suresh, Z.R. Xu, U. Vaidya, and

A.K. Vasudevan. Thanks are due to Kanika Chawla and S. Patel for help with the

figures in this edition. I owe a special debt of gratitude to my wife, Nivi, for being

there all the time. Last but not least, I am ever grateful to my parents, the late

Manohar L. and Sumitra Chawla, for their guidance and support.

Birmingham, AL, USA Krishan K. Chawla

March, 2011

Supplementary Instructional Resources

viii Preface to the Third Edition

exercises and PowerPoint Slides of figures suitable for use in lectures are available

to instructors who adopt the book for classroom use. Please visit the bookWeb page

at www.springer.com for the password-protected material.

An Instructor’s Solutions Manual containing answers to the end-of-the-chapter

 

 

Preface to the Second Edition

The first edition of this book came out in 1987, offering an integrated coverage of

the field of composite materials. I am gratified at the reception it received at the

hands of the students and faculty. The second edition follows the same format as

the first one, namely, a well-balanced treatment of materials and mechanics aspects

of composites, with due recognition of the importance of the processing.

The second edition is a fully revised, updated, and enlarged edition of this widely

used text. There are some new chapters, and others have been brought up-to-date in

light of the extensive work done in the decade since publication of the first edition.

Many people who used the first edition as a classroom text urged me to include

some solved examples. In deference to their wishes I have done so. I am sorry that it

took me such a long time to prepare the second edition. Things are happening at a

very fast pace in the field of composites, and there is no question that a lot of very

interesting and important work has been done in the past decade or so. Out of

necessity, one must limit the amount of material to be included in a textbook.

In spite of this view, it took me much more time than I anticipated. In this second

edition, I have resisted the temptation to cover the whole waterfront. So the reader

will find here an up-to-date treatment of the fundamental aspects. Even so, I do

recognize that the material contained in this second edition is more than what can be

covered in the classroom in a semester. I consider that to be a positive aspect of the

book. The reader (student, researcher, practicing scientist/engineer) can profitably

use this as a reference text. For the person interested in digging deeper into a

particular aspect, I provide an extensive and updated list of references and

suggested reading.

There remains the pleasant task of thanking people who have been very helpful and

a constant source of encouragement to me over the years: M.E. Fine, S.G. Fishman,

J.C. Hurt, B. Ilschner, B.A. MacDonald, A. Mortensen, J.M. Rigsbee, P. Rohatgi,

S. Suresh, H. Schneider, N.S. Stoloff, and A.K. Vasudevan. Among my students and

post-docs, I would like to acknowledge G. Gladysz, H. Liu, and Z.R. Xu. I am

immensely grateful to my family members, Nivi, Nikhil, and Kanika. They were

ix

 

 

patient and understanding throughout. Without Kanika’s help in word processing and

fixing things, this work would still be unfinished. Once again I wish to record my

gratitude to my parents, Manohar L. Chawla and the late Sumitra Chawla for all they

have done for me!

Birmingham, AL, USA Krishan K. Chawla

February, 1998

x Preface to the Second Edition

 

 

Preface to the First Edition

The subject of composite materials is truly an inter- and multidisciplinary one.

People working in fields such as metallurgy and materials science and engineering,

chemistry and chemical engineering, solid mechanics, and fracture mechanics have

made important contributions to the field of composite materials. It would be an

impossible task to cover the subject from all these viewpoints. Instead, we shall

restrict ourselves in this book to the objective of obtaining an understanding of

composite properties (e.g., mechanical, physical, and thermal) as controlled by their

structure at micro- and macro-levels. This involves a knowledge of the properties of

the individual constituents that form the composite system, the role of interface

between the components, the consequences of joining together, say, a fiber and

matrix material to form a unit composite ply, and the consequences of joining

together these unit composites or plies to form a macrocomposite, a macroscopic

engineering component as per some optimum engineering specifications. Time and

again, we shall be emphasizing this main theme, that is structure–property

correlations at various levels that help us to understand the behavior of composites.

In Part I, after an introduction (Chap. 1), fabrication and properties of the various

types of reinforcement are described with a special emphasis on microstructure–

property correlations (Chap. 2). This is followed by a chapter (Chap. 3) on the three

main types of matrix materials, namely, polymers, metals, and ceramics. It is

becoming increasingly evident that the role of the matrix is not just that of a binding

medium for the fibers but it can contribute decisively toward the composite

performance. This is followed by a general description of the interface in

composites (Chap. 4). In Part II a detailed description is given of some of the

important types of composites (Chap. 5), metal matrix composites (Chap. 6),

ceramic composites (Chap. 7), carbon fiber composites (Chap. 8), and multifilam-

entary superconducting composites (Chap. 9). The last two are described separately

because they are the most advanced fiber composite systems of the 1960s and

1970s. Specific characteristics and applications of these composite systems are

brought out in these chapters. Finally, in Part III, the micromechanics (Chap. 10)

and macromechanics (Chap. 11) of composites are described in detail, again

emphasizing the theme of how structure (micro and macro) controls the properties.

xi

 

 

This is followed by a description of strength and fracture modes in composites

(Chap. 12). This chapter also describes some salient points of difference, in regard

to design, between conventional and fiber composite materials. This is indeed of

fundamental importance in view of the fact that composite materials are not just any

other new material. They represent a total departure from the way we are used to

handling conventional monolithic materials, and, consequently, they require uncon-

ventional approaches to designing with them.

Throughout this book examples are given from practical applications of

composites in various fields. There has been a tremendous increase in applications

of composites in sophisticated engineering items. Modern aircraft industry readily

comes to mind as an ideal example. Boeing Company, for example, has made

widespread use of structural components made of “advanced” composites in 757

and 767 planes. Yet another striking example is that of the Beechcraft Company’s

Starship 1 aircraft. This small aircraft (eight to ten passengers plus crew) is

primarily made of carbon and other high-performance fibers in epoxy matrix. The

use of composite materials results in 19% weight reduction compared to an

identical aluminum airframe. Besides this weight reduction, the use of composites

made a new wing design configuration possible, namely, a variable-geometry

forward wing that sweeps forward during takeoff and landing to give stability and

sweeps back 30� in level flight to reduce drag. As a bonus, the smooth structure of composite wings helps to maintain laminar air flow. Readers will get an idea of the

tremendous advances made in the composites field if they would just remind

themselves that until about 1975 these materials were being produced mostly on

a laboratory scale. Besides the aerospace industry, chemical, electrical, automobile,

and sports industries are the other big users, in one form or another, of composite

materials.

This book has grown out of lectures given over a period of more than a decade to

audiences comprised of senior year undergraduate and graduate students, as well as

practicing engineers from industry. The idea of this book was conceived at Instituto

Militar de Engenharia, Rio de Janeiro. I am grateful to my former colleagues there,

in particular, J.R.C. Guimarães, W.P. Longo, J.C.M. Suarez, and A.J.P. Haiad, for

their stimulating companionship. The book’s major gestation period was at the

University of Illinois at Urbana-Champaign, where C.A. Wert and J.M. Rigsbee

helped me to complete the manuscript. The book is now seeing the light of the day

at the New Mexico Institute of Mining and Technology. I would like to thank my

colleagues there, in particular, O.T. Inal, P. Lessing, M.A. Meyers, A. Miller,

C.J. Popp, and G.R. Purcell, for their cooperation in many ways, tangible and

intangible. An immense debt of gratitude is owed to N.J. Grant of MIT, a true

gentleman and scholar, for his encouragement, corrections, and suggestions

as he read the manuscript. Thanks are also due to R. Signorelli, J. Cornie, and

P.K. Rohatgi for reading portions of the manuscript and for their very constructive

suggestions. I would be remiss in not mentioning the students who took my courses

on composite materials at New Mexico Tech and gave very constructive feedback.

A special mention should be made of C.K. Chang, C.S. Lee, and N. Pehlivanturk

for their relentless queries and discussions. Thanks are also due to my wife,

xii Preface to the First Edition

 

 

Nivedita Chawla, and Elizabeth Fraissinet for their diligent word processing; my

son, Nikhilesh Chawla, helped in the index preparation. I would like to express my

gratitude to my parents, Manohar L. and Sumitra Chawla, for their ever-constant

encouragement and inspiration.

Socorro, NM, USA Krishan K. Chawla

June, 1987

Preface to the First Edition xiii

 

 

 

About the Author

Professor Krishan K. Chawla received his B.S. degree fromBanaras HinduUniversity

and his M.S. and Ph.D. degrees from the University of Illinois atUrbana-Champaign. He has taught and/or done research work at Instituto Militar de Engenharia, Brazil;

University of Illinois at Urbana-Champaign; Northwestern University; Université

Laval, Canada; Ecole Polytechnique Federale de Lausanne, Switzerland; the New

Mexico Institute of Mining and Technology (NMIMT); Arizona State University;

German Aerospace Research Institute (DLR), Cologne, Germany; Los Alamos

National Laboratory; Federal Institute for Materials Research and Testing (BAM)

Berlin, Germany; and the University of Alabama at Birmingham. Among the honors

he has received are the following: Eshbach Distinguished Scholar at Northwestern

University, U.S. Department of Energy Faculty Fellow at Oak Ridge National Labo-

ratory, Distinguished Researcher Award at NMIMT, Distinguished Alumnus Award

from Banaras Hindu University, President’s Award for Excellence in Teaching at the

University of Alabama at Birmingham, and Educator Award from The Minerals,

Metals and Materials Society (TMS). In 1989–1990, he served as a program director

xv

 

 

for Metals and Ceramics at the U.S. National Science Foundation (NSF). He is a

Fellow of ASM International.

Professor Chawla is editor of the journal International Materials Reviews.

Among his other books are the following: Ceramic Matrix Composites, Fibrous

Materials, Mechanical Metallurgy (coauthor), Metalurgia Mecânica (coauthor),

Mechanical Behavior of Materials (coauthor), Metal Matrix Composites

(coauthor), and Voids in Materials (coauthor).

xvi About the Author

 

 

Contents

Preface to the Third Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Preface to the Second Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Preface to the First Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Part I

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Reinforcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.2 Fiber Spinning Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.1.3 Stretching and Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Glass Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.1 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.2 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.3 Properties and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Boron Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3.1 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3.2 Structure and Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3.3 Residual Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.4 Fracture Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.3.5 Properties and Applications of Boron Fibers . . . . . . . . . . . . . . . 22

2.4 Carbon Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.4.1 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.4.2 Structural Changes Occurring During Processing . . . . . . . . . . 31

2.4.3 Properties and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.5 Organic Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.5.1 Oriented Polyethylene Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.5.2 Aramid Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

xvii

 

 

2.6 Ceramic Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

2.6.1 Oxide Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

2.6.2 Nonoxide Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.7 Whiskers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

2.8 Other Nonoxide Reinforcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

2.8.1 Silicon Carbide in a Particulate Form . . . . . . . . . . . . . . . . . . . . . . . 65

2.8.2 Tungsten Carbide Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2.9 Effect of High-Temperature Exposure on the Strength

of Ceramic Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

2.10 Comparison of Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3 Matrix Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3.1.1 Glass Transition Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

3.1.2 Thermoplastics and Thermosets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

3.1.3 Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

3.1.4 Molecular Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

3.1.5 Degree of Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

3.1.6 Stress–Strain Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

3.1.7 Thermal Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

3.1.8 Fire Resistance or Flammability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

3.1.9 Common Polymeric Matrix Materials . . . . . . . . . . . . . . . . . . . . . . . . 80

3.2 Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

3.2.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

3.2.2 Conventional Strengthening Methods . . . . . . . . . . . . . . . . . . . . . . . . 93

3.2.3 Properties of Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.2.4 Why Reinforcement of Metals? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

3.3 Ceramic Matrix Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

3.3.1 Bonding and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

3.3.2 Effect of Flaws on Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

3.3.3 Common Ceramic Matrix Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 101

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

4 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 4.1 Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4.1.1 Effect of Surface Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

4.2 Crystallographic Nature of Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4.3 Interactions at the Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4.4 Types of Bonding at the Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

4.4.1 Mechanical Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

4.4.2 Physical Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4.4.3 Chemical Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4.5 Optimum Interfacial Bond Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

xviii Contents

 

 

4.5.1 Very Weak Interface or Fiber Bundle

(No Matrix) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

4.5.2 Very Strong Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

4.5.3 Optimum Interfacial Bond Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4.6 Tests for Measuring Interfacial Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4.6.1 Flexural Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4.6.2 Single Fiber Pullout Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

4.6.3 Curved Neck Specimen Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

4.6.4 Instrumented Indentation Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

4.6.5 Fragmentation Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

4.6.6 Laser Spallation Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Part II

5 Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 5.1 Processing of PMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

5.1.1 Processing of Thermoset Matrix Composites . . . . . . . . . . . . . . . . 138

5.1.2 Thermoplastic Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5.1.3 Sheet Molding Compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

5.1.4 Carbon Fiber Reinforced Polymer Composites . . . . . . . . . . . . . . 154

5.2 Interface in PMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

5.2.1 Glass Fiber/Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

5.2.2 Carbon Fiber/Polymer Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5.2.3 Polyethylene Fiber/Polymer Interface . . . . . . . . . . . . . . . . . . . . . . . . 163

5.3 Structure and Properties of PMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

5.3.1 Structural Defects in PMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

5.3.2 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

5.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

5.4.1 Pressure Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

5.5 Recycling of PMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

6 Metal Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 6.1 Types of Metal Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

6.2 Important Metallic Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

6.2.1 Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

6.2.2 Titanium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

6.2.3 Magnesium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

6.2.4 Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

6.2.5 Intermetallic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

Contents xix

 

 

6.3 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

6.3.1 Liquid-State Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

6.3.2 Solid State Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

6.3.3 In Situ Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

6.4 Interfaces in Metal Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

6.4.1 Major Discontinuities at Interfaces in MMCs .. . . . . . . . . . . . . . . 214

6.4.2 Interfacial Bonding in Metal Matrix Composites . . . . . . . . . . . . 214

6.5 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

6.5.1 Modulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

6.5.2 Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

6.5.3 Thermal Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

6.5.4 High Temperature Properties, Creep, and Fatigue . . . . . . . . . 236

6.6 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239

6.6.1 Electronic-Grade MMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

6.6.2 Recycling of Metal Matrix Composites . . . . . . . . . . . . . . . . . . . . . 243

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

7 Ceramic Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 7.1 Processing of CMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

7.1.1 Cold Pressing and Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

7.1.2 Hot Pressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

7.1.3 Reaction Bonding Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

7.1.4 Infiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

7.1.5 Directed Oxidation or the Lanxide™ Process . . . . . . . . . . . . . . 256 7.1.6 In Situ Chemical Reaction Techniques . . . . . . . . . . . . . . . . . . . . . . 259

7.1.7 Sol–Gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

7.1.8 Polymer Infiltration and Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

7.1.9 Electrophoretic Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

7.1.10 Self-Propagating High-Temperature Synthesis . . . . . . . . . . . . . 267

7.2 Interface in CMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

7.3 Properties of CMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

7.4 Toughness of CMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

7.4.1 Crack Deflection at the Interface in a CMC . . . . . . . . . . . . . . . . 282

7.5 Thermal Shock Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

7.6 Applications of CMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

7.6.1 Cutting Tool Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

7.6.2 Ceramic Composite Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

7.6.3 Other Applications of CMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

8 Carbon Fiber/Carbon Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 8.1 Processing of Carbon/Carbon Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

8.1.1 High Pressure Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

8.2 Oxidation Protection of Carbon/Carbon Composites . . . . . . . . . . . . . . . . 297

8.3 Properties of Carbon/Carbon Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

xx Contents

 

 

8.3.1 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

8.3.2 Frictional Properties of the Composites . . . . . . . . . . . . . . . . . . . . . . 301

8.3.3 Ablative Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

8.4 Applications of Carbon/Carbon Composites . . . . . . . . . . . . . . . . . . . . . . . . . . 302

8.4.1 Carbon/Carbon Composite Brakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

8.4.2 Other Applications of Carbon/Carbon Composites . . . . . . . . . . 305

8.4.3 Carbon/SiC Brake Disks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

9 Multifilamentary Superconducting Composites . . . . . . . . . . . . . . . . . . . . . . . . 309 9.1 The Problem of Flux Pinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

9.2 Types of Superconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

9.3 Processing and Structure of Multifilamentary

Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

9.3.1 Niobium–Titanium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

9.3.2 A15 Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

9.3.3 Ceramic Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

9.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

9.4.1 Magnetic Resonance Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

Part III

10 Micromechanics of Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 10.1 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

10.2 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

10.2.1 Prediction of Elastic Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

10.2.2 Micromechanical Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

10.2.3 Halpin-Tsai Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

10.2.4 Transverse Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

10.3 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

10.3.1 Expressions for Coefficients of Thermal

Expansion of Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354

10.3.2 Expressions for Thermal Conductivity

of Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358

10.3.3 Electrical Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

10.3.4 Hygral and Thermal Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362

10.3.5 Thermal Stresses in Fiber Reinforced Composites . . . . . . 364

10.3.6 Thermal Stresses in Particulate Composites . . . . . . . . . . . . . 367

10.4 Mechanics of Load Transfer from Matrix to Fiber . . . . . . . . . . . . . . . . 371

10.4.1 Fiber Elastic–Matrix Elastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372

10.4.2 Fiber Elastic–Matrix Plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

10.5 Load Transfer in Particulate Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

Contents xxi

 

 

11 Macromechanics of Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 11.1 Elastic Constants of an Isotropic Material . . . . . . . . . . . . . . . . . . . . . . . . . . 388

11.2 Elastic Constants of a Lamina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

11.3 Relationships Between Engineering Constants

and Reduced Stiffnesses and Compliances . . . . . . . . . . . . . . . . . . . . . . . . . 396

11.4 Variation of Lamina Properties with Orientation . . . . . . . . . . . . . . . . . . 398

11.5 Analysis of Laminated Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400

11.5.1 Basic Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

11.5.2 Constitutive Relationships

for Laminated Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404

11.6 Stresses and Strains in Laminate Composites . . . . . . . . . . . . . . . . . . . . . . 413

11.7 Interlaminar Stresses and Edge Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417

12 Monotonic Strength and Fracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 12.1 Tensile Strength of Unidirectional Fiber Composites . . . . . . . . . . . . . 421

12.2 Compressive Strength of Unidirectional Fiber Composites . . . . . . 422

12.3 Fracture Modes in Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

12.3.1 Single and Multiple Fracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

12.3.2 Debonding, Fiber Pullout,

and Delamination Fracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

12.4 Effect of Variability of Fiber Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432

12.5 Strength of an Orthotropic Lamina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440

12.5.1 Maximum Stress Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441

12.5.2 Maximum Strain Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443

12.5.3 Maximum Work (or the Tsai–Hill) Criterion . . . . . . . . . . . . 443

12.5.4 Quadratic Interaction Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447

13 Fatigue and Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 13.1 Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

13.1.1 S–N Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

13.1.2 Fatigue Crack Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458

13.1.3 Damage Mechanics of Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464

13.1.4 Thermal Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

13.2 Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

13.3 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

14 Designing with Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 14.1 General Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

14.2 Advantages of Composites in Structural Design . . . . . . . . . . . . . . . . . . . 486

14.2.1 Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486

14.2.2 Simplicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486

14.2.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486

14.2.4 Longevity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486

xxii Contents

 

 

14.3 Some Fundamental Characteristics

of Fiber Reinforced Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

14.4 Design Procedures with Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

14.5 Hybrid Composite Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

15 Nonconventional Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 15.1 Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

15.1.1 Polymer Clay Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

15.2 Self-Healing Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

15.3 Self-Reinforced Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501

15.4 Biocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

15.5 Laminates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

15.5.1 Ceramic Laminates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506

15.5.2 Hybrid Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

Appendix A Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

Appendix B Fiber Packing in Unidirectional Composites . . . . . . . . . . . . . . . . 519

Appendix C Some Important Units and Conversion Factors . . . . . . . . . . . . 521

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533

Contents xxiii

 

 

Part I

 

 

Chapter 1

Introduction

It is a truism that technological development depends on advances in the field of

materials. One does not have to be an expert to realize that the most advanced

turbine or aircraft design is of no use if adequate materials to bear the service loads

and conditions are not available. Whatever the field may be, the final limitation on

advancement depends on materials. Composite materials in this regard represent

nothing but a giant step in the ever-constant endeavor of optimization in materials.

Strictly speaking, the idea of composite materials is not a new or recent one.

Nature is full of examples wherein the idea of composite materials is used.

The coconut palm leaf, for example, is essentially a cantilever using the concept

of fiber reinforcement. Wood is a fibrous composite: cellulose fibers in a lignin

matrix. The cellulose fibers have high tensile strength but are very flexible (i.e., low

stiffness), while the lignin matrix joins the fibers and furnishes the stiffness. Bone is

yet another example of a natural composite that supports the weight of various

members of the body. It consists of short and soft collagen fibers embedded in a

mineral matrix called apatite. Weiner andWagner (1998) give a good description of

structure and properties of bone. For descriptions of the structure–function

relationships in the plant and animal kingdoms, the reader is referred to Elices

(2000) and Wainwright et al. (1982). In addition to these naturally occurring

composites, there are many other engineering materials that are composites in a

very general way and that have been in use for a very long time. The carbon black in

rubber, Portland cement or asphalt mixed with sand, and glass fibers in resin are

common examples. Thus, we see that the idea of composite materials is not that

recent. Nevertheless, one can safely mark the origin of the distinct discipline of

composite materials as the beginning of the 1960s. It would not be too much off the

mark to say that a concerted research and development effort in composite materials

began in 1965. Since the early 1960s, there has been an increasing demand for

materials that are stiffer and stronger yet lighter in fields as diverse as aerospace,

energy, and civil construction. The demands made on materials for better overall

performance are so great and diverse that no one material can satisfy them.

This naturally led to a resurgence of the ancient concept of combining different

materials in an integral-composite material to satisfy the user requirements.

K.K. Chawla, Composite Materials: Science and Engineering, DOI 10.1007/978-0-387-74365-3_1, # Springer Science+Business Media New York 2012

3

 

 

Such composite material systems result in a performance unattainable by the

individual constituents, and they offer the great advantage of a flexible design;

that is, one can, in principle, tailor-make the material as per specifications of an

optimum design. This is a muchmore powerful statement than it might appear at first

sight. It implies that, given the most efficient design of, say, an aerospace structure,

an automobile, a boat, or an electric motor, we can make a composite material that

meets the need. Schier and Juergens (1983) surveyed the design impact of

composites on fighter aircraft. According to these authors, “composites have

introduced an extraordinary fluidity to design engineering, in effect forcing the

designer-analyst to create a different material for each application as he pursues

savings in weight and cost.”

Yet another conspicuous development has been the integration of the materials

science and engineering input with the manufacturing and design inputs at all

levels, from conception to commissioning of an item, through the inspection during

the lifetime, as well as failure analysis. More down-to-earth, however, is the fact

that our society has become very energy conscious. This has led to an increasing

demand for lightweight yet strong and stiff structures in all walks of life. And

composite materials are increasingly providing the answers. Figure 1.1 makes a

comparison, admittedly for illustrative purposes, between conventional monolithic

materials, such as aluminum and steel, and composite materials (Deutsch 1978).

This figure indicates the possibilities of improvements that one can obtain over

conventional materials by the use of composite materials. As such, it describes

vividly the driving force behind the large effort in the field of composite materials.

Glass fiber reinforced resins have been in use since the early twentieth century.

Glass fiber reinforced resins are very light and strong materials, although their

stiffness (modulus) is not very high, mainly because the glass fiber itself is not very

stiff. The third quarter of the twentieth century saw the emergence of the so-called

advanced fibers of extremely high modulus, for example, boron, carbon, silicon

Fig. 1.1 Comparison between conventional monolithic materials and composite materials [from Deutsch (1978), used with permission]

4 1 Introduction

 

 

carbide, and alumina (Chawla 1998, 2005). These fibers have been used for

reinforcement of resin, metal, and ceramic matrices. Fiber reinforced composites

have been more prominent than other types of composites for the simple reason that

most materials are stronger and stiffer in the fibrous form than in any other form.

By the same token, it must be recognized that a fibrous form results in reinforcement

mainly in fiber direction. Transverse to the fiber direction, there is little or no

reinforcement. Of course, one can arrange fibers in two-dimensional or even three-

dimensional arrays, but this still does not gainsay the fact that one is not getting the

full reinforcement effect in directions other than the fiber axis. Thus, if a less

anisotropic behavior is the objective, then perhaps laminate or sandwich composites

made of, say, two different materials would be more effective. A particle reinforced

composite will also be reasonably isotropic. There may also be specific nonmechan-

ical objectives for making a fibrous composite. For example, an abrasion- or

corrosion-resistant surface would require the use of a laminate (sandwich) form,

while in superconductors the problem of flux-pinning requires the use of extremely

fine filaments embedded in a conductive matrix. In what follows, we discuss the

various aspects of composites, mostly fiber reinforced composites, in greater detail,

but first let us agree on an acceptable definition of a composite material. Practically

everything in this world is a composite material. Thus, a common piece of metal is a

composite (polycrystal) of many grains (or single crystals). Such a definition would

make things quite unwieldy. Therefore, we must agree on an operational definition

of composite material for our purposes in this text. We shall call a material that satisfies the following conditions a composite material:

1. It is manufactured (i.e., naturally occurring composites, such as wood, are

excluded).

2. It consists of two or more physically and/or chemically distinct, suitably

arranged or distributed phases with an interface separating them.

3. It has characteristics that are not depicted by any of the components in isolation.

References

Chawla KK (1998) Fibrous materials. Cambridge University Press, Cambridge

Chawla KK (Feb., 2005) J Miner, Metals Mater Soc 57:46

Deutsch S (May, 1978). 23rd National SAMPE Symposium, p 34

Elices M (ed) (2000) Structural biological materials. Pergamon Press, Amsterdam

Schier JF, Juergens RJ (Sept., 1983) Astronautics Aeronautics 21:44

Wainwright SA, Biggs WD, Currey JD, Gosline JM (1982) Mechanical design in organisms.

Princeton University Press, Princeton, NJ

Weiner S, Wagner HD (1998) Annu Rev Mater 28:271

References 5

 

 

Problems

1.1. Describe the structure and properties of some fiber reinforced composites that

occur in nature.

1.2. Many ceramic-based composite materials are used in the electronics industry.

Describe some of these electroceramic composites.

1.3. Describe the use of composite materials in the Voyager airplane that circled

the globe for the first time without refueling in flight.

1.4. Nail is a fibrous composite. Describe its components, microstructure, and

properties.

1.5. Discuss the use of composite materials in civilian aircraft, with special atten-

tion to Boeing 787 and Airbus A380 aircraft.

6 1 Introduction

 

 

Chapter 2

Reinforcements

2.1 Introduction

Reinforcements need not necessarily be in the form of long fibers. One can have

them in the form of particles, flakes, whiskers, short fibers, continuous fibers, or

sheets. It turns out that most reinforcements used in composites have a fibrous form

because materials are stronger and stiffer in the fibrous form than in any other form.

Specifically, in this category, we are most interested in the so-called advanced

fibers, which possess very high strength and very high stiffness coupled with a very

low density. The reader should realize that many naturally occurring fibers can be

and are used in situations involving not very high stresses (Chawla 1976; Chawla

and Bastos 1979). The great advantage in this case, of course, is its low cost.

The vegetable kingdom is, in fact, the largest source of fibrous materials. Cellulosic

fibers in the form of cotton, flax, jute, hemp, sisal, and ramie, for example, have

been used in the textile industry, while wood and straw have been used in the paper

industry. Other natural fibers, such as hair, wool, and silk, consist of different forms

of protein. Silk fibers produced by a variety of spiders, in particular, appear to be

very attractive because of their high work of fracture. Any discussion of such fibers

is beyond the scope of this book. The interested reader is directed to some books

that cover the vast field of fibers used as reinforcements (Chawla 1998; Warner

1995). In this chapter, we confine ourselves to a variety of man-made

reinforcements. Glass fiber, in its various forms, has been the most common

reinforcement for polymer matrices. Aramid fiber, launched in the 1960s, is much

stiffer and lighter than glass fiber. Kevlar is Du Pont’s trade name for aramid fiber

while Twaron is the trade name of aramid fiber made by Teijin Aramid. Gel-spun

polyethylene fiber, which has a stiffness comparable to that of aramid fiber, was

commercialized in the 1980s. Other high-performance fibers that combine high

strength with high stiffness are boron, silicon carbide, carbon, and alumina. These

were all developed in the second part of the twentieth century. In particular, some

ceramic fibers were developed in the last quarter of the twentieth century by

K.K. Chawla, Composite Materials: Science and Engineering, DOI 10.1007/978-0-387-74365-3_2, # Springer Science+Business Media New York 2012

7

 

 

some very novel processing techniques, namely, sol-gel processing and controlled

pyrolysis of organic precursors.

The use of fibers as high-performance engineering materials is based on three

important characteristics (Dresher 1969):

1. A small diameter with respect to its grain size or other microstructural unit. This

allows a higher fraction of the theoretical strength to be attained than is possible

in a bulk form. This is a direct result of the so-called size effect; the smaller the

size, the lower the probability of having imperfections in the material. Figure 2.1

shows that the strength of a carbon fiber decreases as its diameter increases

(de Lamotte and Perry 1970). Although this figure shows a linear drop in

strength with increasing fiber diameter, a nonlinear relationship is not uncom-

mon. Figure 2.1 should be taken only as a general trend indicator.

2. A high aspect ratio (length/diameter, l/d), which allows a very large fraction of the applied load to be transferred via the matrix to the stiff and strong fiber (see

Chap. 10).

3. A very high degree of flexibility, which is really a characteristic of a material

that has a low modulus or stiffness and a small diameter. This flexibility permits

the use of a variety of techniques for making composites with these fibers.

Next we consider the concept of flexibility, and then we describe the general

fiber spinning processes.

2.1.1 Flexibility

Flexibility of a given material, as pointed out in the attributes above, is a function of

its elastic stiffness and dimensions of the cross-section. Intuitively, one can easily

visualize that the higher the stiffness of material, the less flexible it will be. But

when we think of flexibility of a fiber or thread, we wish to know to what arbitrary

Fig. 2.1 Decrease in strength (sf) of a carbon fiber with increase in diameter [from

de Lamotte and Perry (1970),

used with permission]

8 2 Reinforcements

 

 

radius we can bend it before it fails. We can treat our single fiber to be an elongated

elastic beam. Let us subject this fiber of Young’s modulus, E and diameter, d to a bending moment,M, which will bend it to a radius, R. For such an elastic bending of a beam, we define flexural rigidity as MR. From elementary strength of materials, we have the following relationship for a beam bent to a radius R:

M

I ¼ E

R ;

or,

MR ¼ EI ;

where E is the Young’s modulus of the material and I is the second moment of area or moment of inertia of its cross-section. For a beam or a fiber of diameter, d, the second moment of area about an axis through the centroid of the beam is given by

I ¼ pd4/64. Now, we can define flexibility of the fiber (i.e., the elastic beam under consideration) as the inverse of flexural rigidity. In other words, flexibility of a fiber

is an inverse function of its elastic modulus, E, and the second moment of area or moment of inertia of its cross-section, I. The elastic modulus of a material is generally independent of its form or size and is generally a material constant for

a given chemical composition (assuming a fully dense material). Thus, for a given

composition and density, the flexibility of a material is determined by its shape, or

more precisely by its diameter. Substituting for I ¼ pd4/64 in the above expression, we get,

MR ¼ EI ¼ Ep d 4

64 ;

or, the flexibility, being equal to 1/MR, is

Flexibility ¼ 1 MR

¼ 64 Epd4

; (2.1)

where d is the equivalent diameter and I is the moment of inertia of the beam (fiber). Equation (2.1) indicates that flexibility, 1/MR, is a very sensitive function of diameter, d. We can summarize the important implications of Eq. (2.1) as follows:

• Flexibility of a fiber is a very sensitive inverse function of its diameter, d. • Given a sufficiently small diameter, it is possible to produce, in principle, a fiber

as flexible as any from a polymer, a metal, or a ceramic.

• One can make very flexible fibers out of inherently brittle materials such as

glass, silicon carbide, alumina, etc., provided one can shape these brittle

materials into a fine diameter fiber. Producing a fine diameter ceramic fiber,

however, is a daunting problem in processing.

2.1 Introduction 9

 

 

To illustrate this concept of flexibility, we plot the diameter of various materials in

fibrous formwith flexibility (1/MR) equal to that of highly flexible filament, namely, a 25-mm-diameter nylon fiber as function of the elastic modulus, E. Note that given a sufficiently small diameter, it is possible for a metal or ceramic to have the same

degree of flexibility as that of a 25-mm-diameter nylon fiber; it is, however, another matter that obtaining such a small diameter in practice can be prohibitively expensive.

2.1.2 Fiber Spinning Processes

Fiber spinning is the process of extruding a liquid through small holes in a spinneret

to form solid filaments. In nature, silkworms and spiders produce continuous

filaments by this process. There exists a variety of different fiber spinning

techniques; some of the important ones are:

Wet spinning. A solution is extruded into a coagulating bath. The jets of liquid freeze or harden in the coagulating bath as a result of chemical or physical

changes.

Dry spinning. A solution consisting of a fiber-forming material and a solvent is extruded through a spinneret. A stream of hot air impinges on the jets of solution

emerging from the spinneret, the solvent evaporates, and solid filaments are left

behind.

Fig. 2.2 Fiber diameter of different materials with flexibility equal to that of a nylon fiber of diameter equal to 25 mm. Note that one can make very flexible fibers out of brittle materials such as glass, silicon carbide, alumina, etc., provided one can process them into a small diameter

10 2 Reinforcements

 

 

Melt spinning. The fiber-forming material is heated above its melting point and the molten material is extruded through a spinneret. The liquid jets harden into solid

filaments in air on emerging from the spinneret holes.

Dry jet–wet spinning. This is a special process devised for spinning of aramid fibers. In this process, an appropriate polymer liquid crystal solution is extruded

through spinneret holes, passes through an air gap before entering a coagulation

bath, and then goes on a spool for winding. We describe this process in detail in

Sect. 2.5.2.

2.1.3 Stretching and Orientation

The process of extrusion through a spinneret results in some chain orientation in the

filament. Generally, the molecules in the surface region undergo more orientation

than the ones in the interior because the edges of the spinneret hole affect the near-

surface molecules more. This is known as the skin effect, and it can affect many other properties of the fiber, such as the adhesion with a polymeric matrix or the ability to

be dyed. Generally, the as-spun fiber is subjected to some stretching, causing further

chain orientation along the fiber axis and consequently better tensile properties, such

as stiffness and strength, along the fiber axis. The amount of stretch is generally

given in terms of a draw ratio, which is the ratio of the initial diameter to the final

diameter. For example, nylon fibers are typically subjected to a draw ratio of 5 after

spinning. A high draw ratio results in a high elastic modulus. Increased alignment of

chains means a higher degree of crystallinity in a fiber. This also affects the ability of

a fiber to absorb moisture. The higher the degree of crystallinity, the lower the

moisture absorption. In general, the higher degree of crystallinity translates into a

higher resistance to penetration by foreign molecules, i.e., a greater chemical

stability. The stretching treatment serves to orient the molecular structure along

the fiber axis. It does not, generally, result in complete elimination of molecular

branching; that is, one gets molecular orientation but not extension. Such stretching

treatments do result in somewhat more efficient packing than in the unstretched

polymer, but there is a limit to the amount of stretch that can be given to a polymer

because the phenomenon of necking can intervene and cause rupture of the fiber.

2.2 Glass Fibers

Glass fiber is a generic name like carbon fiber or steel or aluminum. Just as different

compositions of steel or aluminum alloys are available, there are many of different

chemical compositions of glass fibers that are commercially available. Common

glass fibers are silica based (~50–60 % SiO2) and contain a host of other oxides of

calcium, boron, sodium, aluminum, and iron, for example. Table 2.1 gives the

compositions of some commonly used glass fibers. The designation E stands for

2.2 Glass Fibers 11

 

 

electrical because E glass is a good electrical insulator in addition to having good strength and a reasonable Young’s modulus; C stands for corrosion and C glass has a better resistance to chemical corrosion than other glasses; S stands for the high silica content that makes S glass withstand higher temperatures than other glasses.

It should be pointed out that most of the continuous glass fiber produced is of the E

glass type but, notwithstanding the designation E, electrical uses of E glass fiber are only a small fraction of the total market.

2.2.1 Fabrication

Figure 2.3 shows schematically the conventional fabrication procedure for glass

fibers (specifically, the E glass fibers that constitute the workhorse of the resin

reinforcement industry) (Loewenstein 1983; Parkyn 1970; Lowrie 1967). The raw

materials are melted in a hopper and the molten glass is fed into the electrically

Fig. 2.3 Schematic of glass fiber manufacture

Table 2.1 Approximate chemical compositions of some glass fibers (wt.%)

Composition E glass C glass S glass

SiO2 55.2 65.0 65.0

Al2O3 8.0 4.0 25.0

CaO 18.7 14.0 –

MgO 4.6 3.0 10.0

Na2O 0.3 8.5 0.3

K2O 0.2 – –

B2O3 7.3 5.0 –

12 2 Reinforcements

 

 

Fig. 2.4 Glass fiber is available in a variety of forms: (a) chopped strand, (b) continuous yarn, (c) roving, (d) fabric [courtesy of Morrison Molded Fiber Glass Company]

Fig. 2.5 Continuous glass fibers (cut from a spool) obtained by the sol–gel technique [from Sakka (1985), used with permission]

 

 

heated platinum bushings or crucibles; each bushing contains about 200 holes at its

base. The molten glass flows by gravity through these holes, forming fine continu-

ous filaments; these are gathered together into a strand and a size is applied before it is a wound on a drum. The final fiber diameter is a function of the bushing orifice

diameter; viscosity, which is a function of composition and temperature; and the

head of glass in the hopper. In many old industrial plants the glass fibers are not

produced directly from fresh molten glass. Instead, molten glass is first turned into

marbles, which after inspection are melted in the bushings. Modern plants do

produce glass fibers by direct drawing. Figure 2.4 shows some forms in which

glass fiber is commercially available.

The conventional methods of making glass or ceramic fibers involve drawing

from high-temperature melts of appropriate compositions. This route has many

practical difficulties such as the high processing temperature required, the immis-

cibility of components in the liquid state, and the easy crystallization during

cooling. Several techniques have been developed for preparing glass and ceramic

fibers (Chawla 1998). An important technique is called the sol–gel technique

(Brinker and Scherer 1990; Jones 1989). We shall come back to this sol–gel

technique at various places in this book. Here we just provide a brief description.

A sol is a colloidal suspension in which the individual particles are so small (generally in the nm range) that they show no sedimentation. A gel, on the other hand, is a suspension in which the liquid medium has become viscous enough to

behave more or less like a solid. The sol–gel process of making a fiber involves a

conversion of fibrous gels, drawn from a solution at a low temperature, into glass

or ceramic fibers at several hundred degrees Celsius. The maximum heating

temperature in this process is much lower than that in conventional glass fiber

manufacture. The sol–gel method using metal alkoxides consists of preparing an

appropriate homogeneous solution, changing the solution to a sol, gelling the sol,

and converting the gel to glass by heating. The sol–gel technique is a very

powerful technique for making glass and ceramic fibers. The 3M Company

produces a series of alumina and silica-alumina fibers, called the Nextel fibers,

from metal alkoxide solutions (see Sect. 2.6). Figure 2.5 shows an example of

drawn silica fibers (cut from a continuous fiber spool) obtained by the sol–gel

technique (Sakka 1985).

Glass filaments are easily damaged by the introduction of surface defects.

To minimize this and to make handling of these fibers easy, a sizing treatment is

given. The size, or coating, protects and binds the filaments into a strand.

2.2.2 Structure

Inorganic, silica-based glasses are analogous to organic glassy polymers in that they

are amorphous, i.e., devoid of any long-range order that is characteristic of a

crystalline material. Pure, crystalline silica melts at 1,800 �C. However, by adding some metal oxides, we can break the Si–O bonds and obtain a series of amorphous

14 2 Reinforcements

 

 

glasses with rather low glass transition temperatures. Figure 2.6a shows a two-

dimensional network of silica glass. Each polyhedron consists of oxygen atoms

bonded covalently to silicon. What happens to this structure when Na2O is added to

the glass is shown in Fig. 2.6b. Sodium ions are linked ionically with oxygen but

they do not join the network directly. Too much Na2O will impair the tendency for

glassy structure formation. The addition of other metal oxide types (Table 2.1)

serves to alter the network structure and the bonding and, consequently, the

properties. Note the isotropic, three-dimensional network structure of glass

(Fig. 2.6); this leads to the more or less isotropic properties of glass fibers. That

is, for the glass fiber, Young’s modulus and thermal expansion coefficients are the

same along the fiber axis and perpendicular to it. This is unlike many other fibers,

such as aramid and carbon, which are highly anisotropic.

2.2.3 Properties and Applications

Typical mechanical properties of E glass fibers are summarized in Table 2.2.

Note that the density is quite low and the strength is quite high; Young’s modulus,

however, is not very high. Thus, while the strength-to-weight ratio of glass

fibers is quite high, the modulus-to-weight ratio is only moderate. It is this latter

characteristic that led the aerospace industry to other so-called advanced fibers

(e.g., boron, carbon, Al2O3, and SiC). Glass fibers continue to be used for rein-

forcement of polyester, epoxy, and phenolic resins. It is quite cheap, and it is

available in a variety of forms (see Fig. 2.4). Continuous strand is a group of

Table 2.2 Typical properties of E glass fibers

Density

(g/cm3) Tensile strength (MPa) Young’s modulus (GPa)

Coefficient of thermal

expansion (K�1)

2.55 1,750 70 4.7 � 10�6

Fig. 2.6 Amorphous structure of glass: (a) a two-dimensional representation of silica glass network and (b) a modified network that results when Na2O is added to (a). Note that Na

+ is

ionically linked with O2� but does not join the network directly

2.2 Glass Fibers 15

 

 

individual fibers; roving is a group of parallel strands; chopped fibers consists of

strand or roving chopped to lengths between 5 and 50 mm. Glass fibers are also

available in the form of woven fabrics or nonwoven mats.

Moisture decreases glass fiber strength. Glass fibers are also susceptible to what

is called static fatigue; that is, when subjected to a constant load for an extended

time period, glass fibers can undergo subcritical crack growth. This leads to failure

over time at loads that might be safe when considering instantaneous loading.

Glass fiber reinforced resins are used widely in the building and construction

industry. Commonly, these are called glass-reinforced plastics, or GRP. They are

used in the form of a cladding for other structural materials or as an integral part of a

structural or non-load-bearing wall panel; window frames, tanks, bathroom units,

pipes, and ducts are common examples. Boat hulls, since the mid-1960s, have

primarily been made of GRP. Use of GRP in the chemical industry (e.g., as storage

tanks, pipelines, and process vessels) is fairly routine. The rail and road transporta-

tion industry and the aerospace industry are other big users of GRP.

2.3 Boron Fibers

Boron is an inherently brittle material. It is commercially made by chemical vapor

deposition (CVD) of boron on a substrate, that is, boron fiber as produced is itself a

composite fiber.

In view of the fact that rather high temperatures are required for this deposition

process, the choice of substrate material that goes to form the core of the finished

boron fiber is limited. Generally, a fine tungsten wire is used for this purpose.

A carbon substrate can also be used. The first boron fibers were obtained by

Weintraub (1911) by means of reduction of a boron halide with hydrogen on a

hot wire substrate.

The real impulse in boron fiber fabrication, however, came in 1959, when Talley

(Talley 1959; Talley et al. 1960) used the process of halide reduction to obtain

amorphous boron fibers of high strength. Since then, interest in the use of strong but

light boron fibers as a possible structural component in aerospace and other

structures has been continuous, although it must be admitted that this interest has

periodically waxed and waned in the face of rather stiff competition from other so-

called advanced fibers, in particular, carbon fibers.

2.3.1 Fabrication

Boron fibers are obtained by CVD on a substrate. There are two processes:

1. Thermal decomposition of a boron hydride. This method involves low temperatures, and, thus, carbon-coated glass fibers can be used as a substrate.

16 2 Reinforcements

 

 

The boron fibers produced by this method, however, are weak because of a lack

of adherence between the boron and the core. These fibers are much less dense

owing to the trapped gases.

2. Reduction of boron halide. Hydrogen gas is used to reduce boron trihalide:

2BX3 þ 3H2 ! 2Bþ 6HX; (2.2)

where X denotes a halogen: Cl, Br, or I.

In this process of halide reduction, the temperatures involved are very high, and,

thus, one needs a refractory material, for example, a high-melting-point metal such

as tungsten, as a substrate. It turns out that such metals are also very heavy. This

process, however, has won over the thermal reduction process despite the disad-

vantage of a rather high-density substrate (the density of tungsten is 19.3 g/cm3)

mainly because this process gives boron fibers of a very high and uniform quality.

Figure 2.7 shows a schematic of boron filament production by the CVD technique,

and Fig. 2.8 shows a commercial boron filament production facility; each vertical

reactor shown in this picture produces continuous boron monofilament.

Fig. 2.7 Schematic of boron (B) fiber production by halide

decomposition on a tungsten

(W) substrate [from van

Maaren et al. (1975), used

with permission]

2.3 Boron Fibers 17

 

 

In the process of BCl3 reduction, a very fine tungsten wire (10–12 mm diameter) is pulled into a reaction chamber at one end through a mercury seal and out at the

other end through another mercury seal. The mercury seals act as electrical contacts

for resistance heating of the substrate wire when gases (BCl3 + H2) pass through

the reaction chamber, where they react on the incandescent wire substrate. The

reactor can be a one- or multistage, vertical or horizontal, reactor. BCl3 is an

expensive chemical, and only about 10 % of it is converted into boron in this

reaction. Thus, an efficient recovery of the unused BCl3 can result in a considerable

lowering of the boron filament cost.

There is a critical temperature for obtaining a boron fiber with optimum

properties and structure (van Maaren et al. 1975). The desirable amorphous form

of boron occurs below this critical temperature while above this temperature

crystalline forms of boron also occur that are undesirable from a mechanical

properties viewpoint, as we shall see in Sect. 2.3.2. With the substrate wire

Fig. 2.8 A boron filament production facility

(courtesy of AVCO

Specialty Materials Co.)

18 2 Reinforcements

 

 

stationary in the reactor, this critical temperature is about 1,000 �C. In a system where the wire is moving, this critical temperature is higher, and it increases with

the speed of the wire. One generally has a diagram of the type shown in Fig. 2.9,

which shows the various combinations of wire temperature and wire drawing speed

to produce a certain diameter of boron fiber. Fibers formed in the region above the

dashed line are relatively weak because they contain undesirable forms of boron as

a result of recrystallization. The explanation for this relationship between critical

temperature and wire speed is that boron is deposited in an amorphous state and the

more rapidly the wire is drawn out from the reactor, the higher the allowed

temperature is. Of course, higher wire drawing speed also results in an increase in

production rate and lower costs.

Boron deposition on a carbon monofilament (~35-mm diameter) substrate involves precoating the carbon substrate with a layer of pyrolytic graphite. This

coating accommodates the growth strains that result during boron deposition

(Krukonis 1977). The reactor assembly is slightly different from that for boron on

tungsten substrate, because pyrolitic graphite is applied online.

2.3.2 Structure and Morphology

The structure and morphology of boron fibers depend on the conditions of

deposition: temperature, composition of gases, gas dynamics, and so on. While

theoretically the mechanical properties are limited only by the strength of the

atomic bond, in practice, there are always structural defects and morphological

irregularities present that lower the mechanical properties. Temperature gradients

and trace concentrations of impurity elements inevitably cause process

irregularities. Even greater irregularities are caused by fluctuations in electric

power, instability in gas flow, and any other operator-induced variables.

Fig. 2.9 Temperature (T) vs. wire speed (V) for a series of boron filament diameters.

Filaments formed in the gray

region (above the dashed line) contain crystalline regions

and are undesirable [from

van Maaren et al. (1975),

used with permission]

2.3 Boron Fibers 19

 

 

2.3.2.1 Structure

Depending on the conditions of deposition, the elemental boron can exist in various

crystalline polymorphs. The form produced by crystallization from the melt or CVD

above 1,300 �C is b-rhombohedral. At temperatures lower than this, if crystalline boron is produced, the most commonly observed structure is a-rhombohedral.

Boron fibers produced by theCVDmethod described earlier have amicrocrystalline

structure that is generally called amorphous. This designation is based on the charac- teristic X-ray diffraction pattern produced by the filament in the Debye-Scherrer

method, that is, large and diffuse halos with d spacings of 0.44, 0.25, 0.17, 1.4, 1.1, and 0.091 nm, typical of amorphous material (Vega-Boggio and Vingsbo 1978).

Electron diffraction studies, however, lead one to conclude that this “amorphous”

boron is really a nanocrystalline phase with a grain diameter of the order of 2 nm

(Krukonis 1977).

Based on X-ray and electron diffraction studies, one can conclude that amorphous

boron is really nanocrystalline b-rhombohedral. In practice, the presence of micro- crystalline phases (crystals or groups of crystals observable in the electron

microscope) constitutes an imperfection in the fiber that should be avoided. Larger

and more serious imperfections generally result from surpassing the critical temper-

ature of deposition (see Sect. 2.3.1) or the presence of impurities in the gases.

When boron fiber is made by deposition on a tungsten substrate, as is generally

the case, then depending on the temperature conditions during deposition, the core

may consist of, in addition to tungsten, a series of compounds, such as W2B, WB,

W2B5, andWB4 (Galasso et al. 1967). A boron fiber cross-section (100 mmdiameter) is shown in Fig. 2.10a, while Fig. 2.10b shows schematically the various subparts of

the cross-section. The various tungsten boride phases are formed by diffusion

of boron into tungsten. Generally, the fiber core consists only of WB4 and W2B5.

On prolonged heating, the core may be completely converted into WB4. As boron

diffuses into the tungsten substrate to form borides, the core expands from its

Fig. 2.10 (a) Cross-section of a 100-mm-diameter boron fiber. (b) Schematic of the cross-section of a boron fiber with SiC barrier layer

20 2 Reinforcements

 

 

original 12.5 mm (original tungsten wire diameter) to 17.5 mm. The SiC coating shown in Fig. 2.10b is a barrier coating used to prevent any adverse reaction between

B and the matrix, such as Al, at high temperatures. The SiC barrier layer is vapor

deposited onto boron using a mixture of hydrogen and methyldichlorosilane.

2.3.2.2 Morphology

The boron fiber surface shows a “corn-cob” structure consisting of nodules

separated by boundaries (Fig. 2.11). The nodule size varies during the course of

fabrication. In a very general way, the nodules start as individual nuclei on the

substrate and then grow outward in a conical form until a filament diameter of

80–90 mm is reached, above which the nodules seem to decrease in size. Occasion- ally, new cones may nucleate in the material, but they always originate at an

interface with a foreign particle or inclusion.

2.3.3 Residual Stresses

Boron fibers have inherent residual stresses that have their origin in the process of

chemical vapor deposition. Growth stresses in the nodules of boron, stresses

induced by the diffusion of boron into the tungsten core, and stresses generated

by the difference in the coefficient of expansion of deposited boron and tungsten

boride core, all contribute to the residual stresses and thus can have a considerable

influence on the fiber mechanical properties. The residual stress pattern across the

transverse section of a boron fiber is shown in Fig. 2.12 (Vega-Boggio and Vingsbo

1978). The compressive stresses on the fiber surface are due to the quenching action

involved in pulling the fiber out from the chamber (Vega-Boggio and Vingsbo

1978). Morphologically, the most conspicuous aspect of these internal stresses is

the frequently observed radial crack in the transverse section of these fibers. The

crack runs from within the core to just inside the external surface. Some workers,

however, doubt the preexistence of this radial crack (Krukonis 1977). They think

that the crack appears during the process of boron fiber fracture.

Fig. 2.11 Characteristic corncob structure of boron fiber [from van Maaren et al. (1975), used with permission]. The fiber diameter is 142 mm

2.3 Boron Fibers 21

 

 

2.3.4 Fracture Characteristics

It is well known that brittle materials show a distribution of strengths rather than a

single value. Imperfections in these materials lead to stress concentrations much

higher than the applied stress levels. Because the brittle material is not capable of

deforming plastically in response to these stress concentrations, fracture ensues at

one or more such sites. Boron fiber is indeed a very brittle material, and cracks

originate at preexisting defects located at either the boron-core interface or the

surface. Figure 2.13 shows the characteristic brittle fracture of a boron fiber and

the radical crack. It is worth pointing out here that the radial crack does not extend

all the way to surface of the fiber. This is because the surface layer of boron fiber is

in compression; see Fig. 2.12. The surface defects in boron fiber stem from the

nodular surface that results from the growth of boron cones. In particular, when a

nodule coarsens due to an exaggerated growth around a contaminating particle,

a crack can result from this large nodule and weaken the fiber.

2.3.5 Properties and Applications of Boron Fibers

Many researchers have investigated the mechanical properties of boron fibers

(Krukonis 1977; Vega-Boggio and Vingsbo 1978; Galasso et al. 1967; Galasso

and Paton 1966; DeBolt 1982; Wawner 1967; DiCarlo 1985). Due to the com-

posite nature of the boron fiber, complex internal stresses and defects such as

voids and structural discontinuities result from the presence of a core and the

deposition process. Thus, one would not expect boron fiber to show the intrinsic

Fig. 2.12 Schematic of residual stress pattern

across the transverse section

of a boron fiber [from

Vega-Boggio and Vingsbo

(1978), used with permission]

22 2 Reinforcements

 

 

strength of boron. The average tensile strength of boron fiber is 3–4 GPa, while

its Young’s modulus is between 380 and 400 GPa.

An idea of the intrinsic strength of boron can be obtained in a flexure test

(Wawner 1967). In flexure, assuming the core and interface to be near the neutral

axis, critical tensile stresses would not develop at the core or interface. Flexure tests

on boron fibers lightly etched to remove any surface defects gave a strength of

14 GPa. Without etching, the strength was half this value. Table 2.3 provides a

summary of the characteristics of boron fiber (DiCarlo 1985). Commercially

produced 142-mm-diameter boron fiber shows average tensile strength of 3.8 GPa. The tensile strength and fracture energy values of the as-received and

some limited-production-run larger-diameter fibers showed improvement after

chemical polishing, as shown in Table 2.3. Fibers showing strengths greater than

4 GPa had their fracture controlled by a tungsten boride core, while fibers with

strengths of 4 GPa or less were controlled by fiber surface flaws. The high-

temperature treatment, indicated in Table 2.3, improved the fiber properties by

putting a permanent axial compressive strain in the sheath.

Boron has a density of 2.34 g/cm3 (about 15 % less than that of aluminum).

Boron fiber with a tungsten core has a density of 2.6 g/cm3 for a fiber with 100 mm diameter. Its melting point is 2,040 �C, and it has a thermal expansion coefficient of 8.3 � 10�6 �C�1 up to 315 �C.

Boron fiber composites are in use in a number of US military aircraft, notably the

F-14 and F-15, and in the US space shuttle. They are also used for stiffening golf

shafts, tennis rackets, bicycle frames, and in making repair patches for PMCs. A big

advantage of boron fiber over other high performance fibers is their relatively better

properties in compression. This stems from their larger diameter. A commercial

product called Hy-Bor uses a mix of carbon and boron fibers in an epoxy matrix,

Fig. 2.13 Fracture surface of a boron fiber showing

a characteristically brittle

fracture and a radial crack

2.3 Boron Fibers 23

 

 

wherein improved properties in compression result because of the boron fibers. One

big obstacle to the widespread use of boron fiber is its high cost compared to that of

other fibers.

2.4 Carbon Fibers

Carbon is a very light element with a density equal to 2.268 g/cm3. Carbon can exist

in a variety of crystalline forms. Our interest here is in the so-called graphitic

structure wherein the carbon atoms are arranged in the form of hexagonal layers.

The other well-known form of carbon is the covalent diamond structure wherein the

carbon atoms are arranged in a three-dimensional configuration with little structural

flexibility. Another form of carbon is Buckminster Fullerene (or Bucky ball), with a

molecular composition of C60 or C70. One can also have carbon nanotubes, which

are nothing but drawn out version of Buckyballs. Carbon in the graphitic form is

highly anisotropic, with a theoretical Young’s modulus in the layer plane being

equal to about 1,000 GPa, while that along the c-axis is equal to about 35 GPa. The graphite structure (Fig. 2.14a) has a very dense packing in the layer planes.

The lattice structure is shown more clearly with only lattice planes in Fig. 2.14b.

As we know, the bond strength determines the modulus of a material. Thus, the

high-strength bond between carbon atoms in the layer plane results in an extremely

high modulus while the weak van der Waals-type bond between the neighboring

layers results in a lower modulus in that direction. Consequently, almost all

processing techniques of carbon fiber have the goal of obtaining a very high degree

of preferred orientation of hexagonal planes along the fiber axis.

Carbon fibers of extremely high modulus can be made by carbonization of

organic precursor fibers followed by graphitization at high temperatures.

The organic precursor fiber, that is, the raw material for carbon fiber, is generally a

special textile polymeric fiber that can be carbonized without melting. The precursor

fiber, like any polymeric fiber, consists of long-chain molecules (0.1–1 mm when

Table 2.3 Strength properties of improved large-diameter boron fibers

Diameter (mm) Treatment

Strength Relative fracture

energyAveragea (GPa) COVb (%)

142 As-produced 3.8 10 1.0

406 As-produced 2.1 14 0.3

382 Chemical polish 4.6 4 1.4

382 Heat treatment plus polish 5.7 4 2.2 a Gauge length ¼ 25 mm bCoefficient of variation ¼ standard deviation/average value Source: Reprinted with permission from J Metals 37, No. 6, 1985, a publication of The Metallur- gical Society, Warrendale PA