Load-Bearing Fibre Composites

281 Pages·1980·13.467 MB·English

by  Michael R. Piggott and D. M. R. Taplin (Auth.)

INTERNATIONAL SERIES ON THE STRENGTH
AND FRACTURE OF MATERIALS AND STRUCTURES
General Editor. D. M. R. Taplin, D.Sc, D.Phil., F.I.M.
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Frontispiece. A recent application of the first glass-polymer: the 107 m long radome
on top of the Toronto 550 m CN Tower (Courtesy of CN Tower Ltd.)
Load-Bearing Fibre
Composites
BY
MICHAEL R. PIGGOTT
A.R.C.S., Ph.D, P.Eng., F.lnst.P.
Centre for the Study of Materials, University of Toronto
PERGAMON PRESS
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Copyright © 1980 M. R. Piggott
All Rights Reserved. No part of this publication may
be reproduced, stored in a retrieval system or
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electronic, electrostatic, magnetic tape, mechanical,
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permission in writing from the publishers.
First edition 1980
Reprinted 1987
British Library Cataloging in Publication Data
Piggott, M. R.
Load bearing fibre composites (Pergamon
international library: international series
on the strength and fracture of materials and
structures).
1. Fibrous composites
620.1 1 TA418.9.C6 79-40951
ISBN 0-08-024230-8 (Hard Cover)
ISBN 0-08-024231-6 (Flexicover)
Printed in Great Britain by A. Wheaton & Co. Ltd, Exeter
Preface
THIS book is based on a course of lectures given to upper-year and graduate students in
Materials Science and Engineering. It is designed to present a unified view of the whole
field of fibre (and platelet) composites, rather than going into any aspect of the subject in
great detail. The reader who wants to go more deeply into any aspect is referred, at the end
of each chapter, to more specialized texts and reviews, or to key papers. The reader is also
recommended to look at the literature acknowledged in the subtitles to the figures.
The field has developed rapidly over the last 20 years, and no materials scientist should
now be without a working knowledge of it. Metallurgists should be aware of the
competition from reinforced polymers and ceramics, and be able to appreciate their
strengths and weaknesses. Designers need to be able to make a rational choice of which
material to use in any situation. This book is aimed at these audiences, but, in addition,
indicates areas where our knowledge is not as complete as it should be. Thus I hope also to
inspire those inclined to do research.
A mathematical development of the subject cannot be avoided if composites are to be
properly understood and correctly used. The interactions between fibres and matrix are
quite complex, and are the subject of multitudes of erudite papers. Here, however,
complicated mathematical expressions are eschewed, and wherever simple ones can be
used with adequate precision these are presented in their place.
All equations are developed from basic principles, and the reader only needs
acquaintance with algebra, calculus, and geometry at first-year University level.
The author is grateful for assistance from many sources. Pictures and information have
been generously provided, and the author has been kindly received in many laboratories
where important work is going on, and benefited greatly from what he has seen and
discussed there.
The writer is especially grateful for assistance while on study leave at the University of
Bath. The University provided material assistance, and Professor Bryan Harris unstint-
ingly provided original pictures, comments, encouragement, and assistance.
1
Introduction
THIS book describes the basic ideas in the relatively new field of fibre reinforced
composites, and provides recent data on their load-bearing capabilities. In addition,
selected data is given for more traditional materials, so that the reader can see where
composite materials fit into the hierarchy of materials.
The discussion starts at a basic level, and such important properties as strength, creep
resistance, and fatigue resistance are described. The connection between chemical bond
strength and tensile strength is discussed, and the role of imperfections is shown to be of
overwhelming importance.
The reasons for the good mechanical properties of fibres are set forth, and an account is
given of the production of strong and stiff fibres. This is followed by a simplified
description of the mechanics of the processes by which fibres can contribute strength,
stiffness, and toughness to weak matrices. Finally a brief sketch of the properties of
composites is given, classified according to whether the matrix is polymeric, metallic, or
ceramic (including cementitious). Emphasis is given, throughout the book, to load-
bearing properties and this is followed by a discussion of some of the diverse end uses of
these materials.
The term composite has come to mean a material made by dispersing particles, of one
or more materials in another material, which forms a substantially continuous network
around them. The properties of the composite may bear little relation to those of the
components, even though the components retain their integrity within the composite.
The components can be randomly arranged, or organized in some sort of pattern.
Generally the arrangement will have a large effect on the properties. Further, they can
have roughly spherical shapes, e.g. stones in concrete, or can have some very distinctive
shape such as the iron carbide laminae found in some steels, or long thin fibres, such as the
cellulose fibres in wood. The particle shape also has a very profound effect on the
properties of the composite. Since this book is concerned with fibre-reinforced
composites, it will largely restrict itself to the discussion of two-component composites,
where one of the components is a long thin fibre or whisker, and where the composite has
to have good load-carrying capacity. Also reinforcement by thin platelets will be
described.
Fibre-reinforced materials have been used by man for a very long time. The first to be
used were naturally occurring composites, such as wood, but man also found out, long
ago, that there were advantages to be gained from using artificial mixtures of materials
with one component fibrous, such as straw in clay, for bricks, or horse hair in plaster of
paris for ceilings.
Recently, with the advent of cheap and strong glass fibres, and with the discovery of a
1
2 Load-Bearing Fibre Composites
number of new fibre-forming materials with better properties than anything available
heretofore, the interest in fibre reinforced materials has increased rapidly, and is still
accelerating. Fibre-reinforced polymers are replacing metals in a whole host of situations
where load-carrying capacity is important. More efficient aircraft, turbine engines, and
cars can be produced with fibre composites, and worthwhile new applications for these
materials are being found almost every day.
In order to appreciate the potential benefits to be gained from fibre reinforced
materials, however, it is necessary first to be aware of what can be done with more
traditional materials. The introductory section of this text therefore starts with a review of
the important properties required for load-bearing materials, and discusses traditional
materials in this context. Then follows a brief statement of the basic ideas of isotropic
elasticity theory, and its extension to non-isotropic cases of interest for fibre
reinforcement.
1.1. Conventional Materials
The important properties for load-bearing materials fall naturally into two groups:
mechanical and non-mechanical. Mechanical properties include stiffness, strength,
ductility, hardness, toughness, fatigue, and creep. These will be discussed first. Non-
mechanical properties, which will be discussed later, include density, temperature
resistance, corrosion resistance (including stress corrosion and hydrogen embrittlement),
and cost.
1.1.1. Stijfness
The stiffness of a material, or its resistance to reversible deformation under load, is a
very important mechanical property. In order to characterize the effect of the load on the
material, it is normally converted to a stress, that is the force per unit area of cross-section
acted on by the load. Expressed in these terms, it is then an indication of the forces
experienced by the individual atoms in the material as a result of applying the load. The
response of the atoms is to change their positions slightly. Hence, under a stretching, or
tensile stress, the atoms move apart in the direction of the force. The distance moved,
divided by the original distance, is called the strain.
The movement of the atoms under the action of applied loads can be observed and
measured using X-ray diffraction or neutron diffraction. The combined movement of the
atoms constituting the whole specimen being stressed can usually be measured quite easily
using a sensitive distance gauge.
A stiff material is one which deforms very little. Young's modulus is normally used as a
measure of this, and is defined as the ratio of the tensile stress to the strain produced. The
unit normally used for Young's modulus is the Pascal (Pa). (These units are described in
Appendix B.) Other stiffness parameters will be discussed in the second part of this
chapter.
Few materials are isotropic, and so measurements of Young's modulus made in
different directions will give different results. These differences are particularly marked
with single crystals of non-metals. However, polycrystalline metals, polymers, ceramics
are generally sufficiently isotropic that they can be assigned a single value for the Young's
Introduction 3
modulus. Young's moduli range from about 1 TPa for a favourable direction in a
diamond crystal through 100 GPa for a metal, 1 GPa for a polymer, and 10 MPa for a
rubber. Table 1.1 gives the moduli of a representative selection for metals, ceramics, and
polymers. The modulus of well-made specimens of crystalline elements or compounds is a
measure of the deformability of the bonds between the atoms, and hence will be the same
for different samples of the material. This is not quite the case with metal alloys, while with
some polymers quite large variations between the modulus of different specimens of
chemically similar materials can be found.
1.1.2. Strength
The strength of a material is the stress required to break it. It differs markedly from the
modulus, in being determined as much by the method of manufacture and previous
history of the specimen, as by the nature of the atoms and their arrangement. Pure,
annealed, metals, and non-fibrous polymers are weak in both tension and compression.
Ceramics and hard materials are generally much weaker in tension than in compression.
Some polymer fibres are very strong in tension, while modern alloys, developed for use as
structural members, are generally strong in both tension and compression. Typical tensile
strengths are given in Table 1.1 for a variety of materials, excluding fibres and wires, some
properties of which are given in Table 3.1.
TABLE 1.1. Strength and Stiffness of Various Materials
Young's Modulus Tensile
Material (GPa) strength
(MPa)
Metals:
Aluminium (pure, annealed) 71 60
High-strength aluminium alloy
(Al-Zn-Cu-Mg) 71 650
Iron (cast) 152 360
High-strength iron alloy (marag-
ing steel) 212 2000
Magnesium alloy (Mg-Zn-Zr) 45 340
Titanium alloy
(Ti-Zn-Al-Mo-Si) 120 1400
Tungsten 411 1800
Zirconium alloy
(zircaloy 2) 97 590
Ceramics:
Alumina (high density) 400 280
Concrete 50 3.5
Glass (sheet) 70 70
Polymers:
Epoxy resin 2.5 60
Polycarbonate 2.5 65
Polyethylene (branched) 0.2 10
Rubber (natural) 0.018 32
Rubber (fluorocarbon) 0.002 7
Wood (Douglas fir) 14 34t
Wood (white pine) 7.6 lot
t Parallel to the grain; long-term strength is half this.
4 Load-Bearing Fibre Composites
1.13. Ductility, Hardness, and Toughness
Figure 1.1 shows the stress-strain curves obtained with two different metals. Here the
stress is calculated using the initial cross-section of the material, and the strain is
calculated directly from the change in length of the test section.
600 STEEL
Region Near Origin
Strain Axis Expanded
^400
Έ
υ 0.1 0.2 0.3
Strain
FIG. 1.1. Stress-strain curves for steel and brass.
With many metals the curve has three regions. First there is a linear region where the
material is elastic, and on removal of the stress at any part of the line, the line is retraced
back to the origin. The slope of the line gives the Young's modulus of the material.
The second part of the curve starts at the end of the linear region (this point is the yield
point) and, except for a slight fall at the yield point in the case of some steels, the stress
increases monotonically up to a maximum value, called the ultimate tensile strength.
In the third region the stress decreases monotonically with increasing strain until final
failure occurs. In this region the specimen is not deforming uniformly; a region of reduced
cross-section is formed (necking is occurring) and failure occurs at the neck.
(The maximum stress at the neck can be considerably greater than the stress in the rest
of the material, so that the true stress at failure is much greater than that indicated by the
stress axis. These two failure stresses are sometimes distinguished by calling the lower
stress the engineering stress at failure, and the higher stress the true stress at failure. The
true stress-true strain curve does not have a peak; the true stress increases monotonically
to the failure point.)
In the second and third regions of the stress-strain curve, removal of the stress does not
result in the material retracing the curve back to the origin. Instead, with decreasing stress
the strain decreases at a rate governed by the modulus of the material, i.e. the line has a
slope which is the same as that near the origin (Fig. 1.2). When the stress has been reduced
to zero, there is now still some strain remaining. This strain is the origin of ductility. A
stress-strain curve which is entirely linear up to the breaking-point, such as that obtained
with a hard steel (e.g. razor blade steel) indicates very little ductility. A curve with a very
long region after the yield point indicates a material with great ductility. With pure metals,
the length of the specimen can sometimes be doubled before failure occurs.
Introduction 5
Strain —►
Residual Strain
FIG. 1.2. Residual strain in a metal.
Figure 1.3 shows the stress-strain curves for two polymers. It can be seen that they
show similar features to those observed with metals, except that yielding is generally not
so sharp, and the curve does not usually have a monotonically decreasing region. Some
polymers (e.g. polyesters) have very little ductility. Note that the yield point, or elastic
limit, occurs at much higher strains for plastics than it does for most metals.
60 POLYCARBONATE
40
CO
20
POLYETHYLENE
0.5 1.0 1.5
Strain
FIG. 1.3. Stress-strain curves for polyethylene and polycarbonate.
Hardness is a measure of the resistance of materials to plastic deformation, and hence
ductile materials are usually not hard. It is determined by indenting the material with a
hard ball, or a diamond with a pyramid-shaped tip, and measuring the size of indentation
for a given load. Diamonds will leave indentations in glass, showing that plastic
deformation, albeit small, is possible with glass. Table 1.2 gives some typical hardness
values. They can be converted to roughly equivalent yield stresses in MPa by multiplying
by three. Note, however, that with any metal or alloy, a wide range of hardness values can
usually be obtained by heat treatment and cold working.