S E C O N D E D I T I O N
Engineering Materials 1
AN INTRODUCTION TO THEIF;
PROPERTIES & APPLICATIONS
Michael F Ashby. David R H Jones
Engineering Materials 1
An lntroduction to their Properties and Applications
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Engineering Materials 1
An Introduction to their Properties and Applications
Second Edition
by
Michael F. Ashby
and
David R. H. Jones
Department of Engineering, University of Cambridge, UK
BUTTERWORTH
EINEMANN
OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS
SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO
Butterworth-Heinemann
An imprint of Elsevier Science
Linacre House, Jordan Hill, Oxford OX2 8DP
225 Wildwood Avenue, Wobum, MA 01801-2041
First published 1980
Second edition 1996
Reprinted 1997, 1998 (twice), 2000,2001,2002
0 1980, 1996, Michael F. Ashby and David R. H. Jones. All rights reserved.
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asserted in accordance with the Copyright, Designs and Patents Act 1988
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British Library Cataloguing in Publication Data
Ashby, Michael E
Engineering materials. 1. an introduction to their
properties and applications. - 2nd. ed.
1. Materials 2. Mechanics
I. Title 11. Jones, David R. H. (David Rayner Hunkin),
1945-620.1’1
ISBN 0 7506 3081 7
Library of Congress Cataloguing in Publication Data
Ashby, Michael E
Engineering materials. 1. an introduction to their properties and
applicationsby Michael F. Ashby and David R. H. Jones - 2nd. ed.
p. cm.
Rev.ed of Engineering materials. 1980.
Includes bibliographical references and index.
ISBN 0 7506 3081 7
1. Materials. I. Jones, David R. H. (David Rayner Hunkin),
1945-. 11. Ashby, M.F. Engineering materials III. Title
TA403.A69 96-1677
620.1’1-dc20 CIP
For information on all Butterworth-Heinemann publications
visit our website at www.bh.com
Typeset by Genesis Typesetting, Rochester, Kent
Printed and bound in Great Britain by MFG Books Ltd, Bodmin, Comwall
Contents
General introduction
1. Engineering Materials and their Properties
3
examples of structures and devices showing how we select the right
material for the job
A. Price and availability
2. The Price and Availability of Materials
15
what governs the prices of engineering materials, how long will supplies
last, and how can we make the most of the resources that we have?
B. The elastic moduli
3. The Elastic Moduli
27
stress and strain; Hooke’s Law; measuring Young’s modulus; data for
design
4. Bonding Between Atoms
36
the types of bonds that hold materials together; why some bonds are
stiff and others floppy
5. Packing of Atoms in Solids
45
how atoms are packed in crystals - crystal structures, plane (Miller)
indices, direction indices; how atoms are packed in polymers, ceramics
and glasses
6. The Physical Basis of Young’s Modulus
58
how the modulus is governed by bond stiffness and atomic packing; the
glass transition temperature in rubbers; designing stiff materials -
man-made composites
7. Case Studies of Modulus-limited Design
66
the mirror for a big telescope; a stiff beam of minimum weight; a stiff
beam of minimum cost
vi Contents
C. Yield strength, tensile strength, hardness and ductility
8. The Yield Strength, Tensile Strength, Hardness and Ductility
77
definitions, stress-strain curves (true and nominal), testing methods,
data
9. Dislocations and Yielding in Crystals
93
the ideal strength; dislocations (screw and edge) and how they move to
give plastic flow
10. Strengthening Methods and Plasticity of Polycrystals
104
solid solution hardening; precipitate and dispersion strengthening;
work-hardening; yield in polycrystals
11. Continuum Aspects of Plastic Flow
111
the shear yield strength; plastic instability; the formability of metals and
polymers
12. Case Studies in Yield-limited Design
119
Materials for springs; a pressure vessel of minimum weight; a pressure
vessel of minimum cost; how metals are rolled into sheet
D. Fast fracture, toughness and fatigue
13. Fast Fracture and Toughness
131
where the energy comes from for catastrophic crack growth; the
condition for fast fracture; data for toughness and fracture toughness
14. Micromechanisms of Fast Fracture
140
ductile tearing, cleavage; composites, alloys - and why structures are
more likely to fail in the winter
146
15. Fatigue Failure
fatigue testing, Basquin’s Law, Coffin-Manson Law; crack growth rates
for pre-cracked materials; mechanisms of fatigue
16. Case Studies in Fast Fracture and Fatigue Failure
155
fast fracture of an ammonia tank; how to stop a pressure vessel blowing
up; is cracked cast iron safe?
E. Creep deformation and fracture
17. Creep and Creep Fracture
169
high-temperature behaviour of materials; creep testing and creep curves;
consequences of creep; creep damage and creep fracture
Contents vii
18. Kinetic Theory of Diffusion
179
Arrhenius's Law; Fick's first law derived from statistical mechanics of
thermally activated atoms; how diffusion takes place in solids
19. Mechanisms of Creep, and Creep-resistant Materials
187
metals and ceramics - dislocation creep, diffusion creep; creep in
polymers; designing creep-resistant materials
20. The Turbine Blade - A Case Study in Creep-limited Design
197
requirements of a turbine-blade material; nickel-based super-alloys,
blade cooling; a new generation of materials? - metal-matrix composites,
ceramics, cost effectiveness
F. Oxidation and corrosion
21. Oxidation of Materials
211
the driving force for oxidation; rates of oxidation, mechanisms of
oxidation; data
22. Case Studies in Dry Oxidation
219
making stainless alloys; protecting turbine blades
23. Wet Corrosion of Materials
225
voltages as driving forces; rates of corrosion; why selective attack is
especially dangerous
24. Case Studies in Wet Corrosion
232
how to protect an underground pipeline; materials for a light-weight
factory roof; how to make motor-car exhausts last longer
G. Friction, abrasion and wear
25. Friction and Wear
241
surfaces in contact; how the laws of friction are explained by the
asperity-contact model; coefficients of friction; lubrication; the adhesive
and abrasive wear of materials
26. Case Studies in Friction and Wear
250
the design of a journal bearing; materials for skis and sledge runners;
'non-skid' tyres
viii Contents
Final case study
27. Materials and Energy in Car Design
261
the selection and economics of materials for automobiles
Appendix 1 Examples
273
Appendix 2 Aids and Demonstrations
290
Appendix 3 Symbols and Formulae
297
Index
303
General introduction
To the student
Innovation in engineering often means the clever use of a new material - new to a particular application, but not necessarily (although sometimes) new in the sense of ‘recently developed’. Plastic paper clips and ceramic turbine-blades both represent attempts to do better with polymers and ceramics what had previously been done well with metals. And engineering disasters are frequently caused by the misuse of materials. When the plastic tea-spoon buckles as you stir your tea, and when a fleet of aircraft is grounded because cracks have appeared in the tailplane, it is because the engineer who designed them used the wrong materials or did not understand the properties of those used. So it is vital that the professional engineer should know how to select materials which best fit the demands of the design - economic and aesthetic demands, as well as demands of strength and durability. The designer must understand the properties of materials, and their limitations. This book gives a broad introduction to these properties and limitations. It cannot make you a materials expert, but it can teach you how to make a sensible choice of material, how to avoid the mistakes that have led to embarrassment or tragedy in the past, and where to turn for further, more detailed, help. You will notice from the Contents list that the chapters are arranged in groups, each group describing a particular class of properties: the elastic modulus; the fracture toughness; resistance to corrosion; and so forth. Each such group of chapters starts by defining the property, describing how it is measured, and giving a table of data that we use to solve problems involving the selection and use of materials. We then move on to the basic science that underlies each property, and show how we can use this fundamental knowledge to design materials with better properties. Each group ends with a chapter of case studies in which the basic understanding and the data for each property are applied to practical engineering problems involving materials. Each chapter has a list of books for further reuding, ranked so that the more elementary come first. At the end of the book you will find sets of examples; each example is meant to consolidate or develop a particular point covered in the text. Try to do the examples that derive from a particular chapter whilesthis is still fresh in your mind. In this way you will gain confidence that you are on top of the subject. No engineer attempts to learn or remember tables or lists of data for material properties. But you should try to remember the broad orders-of-magnitude of these quantities. All grocers know that ’a kg of apples is about 10 apples’ - they still weigh them, but their knowledge prevents them making silly mistakes which might cost them money. In the same way, an engineer should know that ’most elastic moduli lie between 1 and lo3 GN m-2; and are around 102GNm W2fo r metals’ - in any real design you need an accurate value, which you can get from suppliers’ specifications; but an order-of magnitude knowledge prevents you getting the units wrong, or making other silly, and possibly expensive, mistakes. To help you in this, we have added at the end of the book a list of the important definitions and formulae that you should know, or should be able to derive, and a summary of the orders-of-magnitude of materials properties.
To the lecturer
This book is a course in Engineering Materials for engineering students with no previous background in the subject. It is designed to link up with the teaching of Design, Mechanics and Structures, and to meet the needs of engineering students in the 1990s for a first materials course, emphasising applications. The text is deliberately concise. Each chapter is designed to cover the content of one 50-minute lecture, twenty-seven in all, and allows time for demonstrations and illustrative slides. A list of the slides, and a description of the demonstrations that we have found appropriate to each lecture, are given in Appendix 2. The text contains sets of worked case studies (Chapters 7, 12, 16, 20, 22, 24, 26 and 27) which apply the material of the preceding block of lectures. There are examples for the student at the end of the book; worked solutions are available separately from the publisher. We have made every effort to keep the mathematical analysis as simple as possible while still retaining the essential physical understanding, and still arriving at results which, although approximate, are useful. But we have avoided mere description: most of the case studies and examples involve analysis, and the use of data, to arrive at numerical solutions to real or postulated problems. This level of analysis, and these data, are of the type that would be used in a preliminary study for the selection of a material or the analysis of a design (or design-failure). It is worth emphasising to students that the next step would be a detailed analysis, using more precise mechanics (from the texts given as 'further reading') and data from the supplier of the material or from in-house testing. Materials data are notoriously variable. Approximate tabulations like those given here, though useful, should never be used for final designs.
Chapter 1
Engineering materials and their properties
Introduction
There are, it is said, more than 50,000 materials available to the engineer. In designing a structure or device, how is the engineer to choose from this vast menu the material which best suits the purpose? Mistakes can cause disasters. During World War 11, one class of welded merchant ship suffered heavy losses, not by enemy attack, but by breaking in half at sea: the fracture toughness of the steel - and, particularly, of the welds was too low. More recently, three Comet aircraft were lost before it was realised that the design called for a fatigue strength that - given the design of the window frames - was greater than that possessed by the material. You yourself will be familiar with poorlydesigned appliances made of plastic: their excessive 'give' is because the designer did not allow for the low modulus of the polymer. These bulk properties are listed in Table 1.1, along with other common classes of property that the designer must consider when choosing a material. Many of these properties will be unfamiliar to you - we will introduce them through examples in this chapter. They form the basis of this first course on materials. In this first course, we shall also encounter the classes of materials shown in Table 1.2. More engineering components are made of metals and alloys than of any other class of solid. But increasingly, polymers are replacing metals because they offer a combination of properties which are more attractive to the designer. And if you've been reading the newspaper, you will know that the new ceramics, at present under development world wide, are an emerging class of engineering material which may permit more efficient heat engines, sharper knives, and bearings with lower friction. The engineer can combine the best properties of these materials to make composites (the most familiar is fibreglass) which offer specially attractive packages of properties. And - finally - one should not ignore natural maferials like wood and leather which have properties which - even with the innovations of today's materials scientists - are hard to beat. In this chapter we illustrate, using a variety of examples, how the designer selects materials so that they provide him or her with the properties needed. As a first example, consider the selection of materials for a