Michael Ashby and David Jones Engineering Materials 1 2nd ed PDF

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SECOND EDITION z 'A' AN INTRODUCTION TO THEIF; PROPERTIES & APPLICATIONS Michael F Ashby. David R H Jones zyxwv zyxwvuts Engineering Materials 1 A n lntroduction to their Properties and Applications Ashby and Jones Brydson zyxwvut zyxwvu Other titles of interest Ashby Materials Selection...

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SECOND

EDITION

'A' AN INTRODUCTION TO THEIF; PROPERTIES & APPLICATIONS

Michael F Ashby. David R H Jones

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Engineering Materials 1

A n lntroduction to their Properties and Applications

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Other titles of interest Ashby Ashby and Jones Brydson Charles and Crane Crawford Hull and Bacon Jones Neale Shreir et al. Smallman and Bishop Smith

Materials Selection in Mechanical Design Engineering Materials 2 Plastics Materials, 6th Edition Selection and Use of Engineering Materials, 2nd Edition Plastics Engineering, 2nd Edition Introduction to Dislocations, 3rd Edition Engineering Materials 3 Tribology Handbook, 2nd Edition Corrosion, 3rd Edition Metals and Materials The Language of Rubber

Engineering Materials 1

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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

U T T E R W O R T H E I N E M A N N

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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

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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.

The right of Author name to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988

No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentall to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England WIT 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers

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British Library Cataloguingin 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

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General introduction

1. Engineering Materials and their Properties

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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 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?

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B. The elastic moduli 3. The Elastic Moduli stress and strain; Hooke’s Law; measuring Young’s modulus; data for design

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4. Bonding Between Atoms the types of bonds that hold materials together; why some bonds are stiff and others floppy

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5. Packing of Atoms in Solids how atoms are packed in crystals - crystal structures, plane (Miller) indices, direction indices; how atoms are packed in polymers, ceramics and glasses

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6 . The Physical Basis of Young’s Modulus how the modulus is governed by bond stiffness and atomic packing; the glass transition temperature in rubbers; designing stiff materials man-made composites

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7. Case Studies of Modulus-limited Design the mirror for a big telescope; a stiff beam of minimum weight; a stiff beam of minimum cost

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Contents

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C. Yield strength, tensile strength, hardness and ductility

8. The Yield Strength, Tensile Strength, Hardness and Ductility definitions, stress-strain curves (true and nominal), testing methods, data

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9. Dislocations and Yielding in Crystals the ideal strength; dislocations (screw and edge) and how they move to give plastic flow

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10. Strengthening Methods and Plasticity of Polycrystals solid solution hardening; precipitate and dispersion strengthening; work-hardening; yield in polycrystals

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11. Continuum Aspects of Plastic Flow the shear yield strength; plastic instability; the formability of metals and polymers

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12. Case Studies in Yield-limited Design materials for springs; a pressure vessel of minimum weight; a pressure vessel of minimum cost; how metals are rolled into sheet

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D. Fast fracture, toughness and fatigue 13. Fast Fracture and Toughness where the energy comes from for catastrophic crack growth; the condition for fast fracture; data for toughness and fracture toughness

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14. Micromechanisms of Fast Fracture ductile tearing, cleavage; composites, alloys - and why structures are more likely to fail in the winter

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15. Fatigue Failure fatigue testing, Basquin’s Law, Coffin-Manson Law; crack growth rates for pre-cracked materials; mechanisms of fatigue

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16. Case Studies in Fast Fracture and Fatigue Failure fast fracture of an ammonia tank; how to stop a pressure vessel blowing up; is cracked cast iron safe?

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E. Creep deformation and fracture 17. Creep and Creep Fracture high-temperature behaviour of materials; creep testing and creep curves; consequences of creep; creep damage and creep fracture

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Contents

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18. Kinetic Theory of Diffusion Arrhenius's Law; Fick's first law derived from statistical mechanics of thermally activated atoms; how diffusion takes place in solids

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19. Mechanisms of Creep, and Creep-resistant Materials metals and ceramics - dislocation creep, diffusion creep; creep in polymers; designing creep-resistant materials

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20. The Turbine Blade - A Case Study in Creep-limited Design requirements of a turbine-blade material; nickel-based super-alloys, blade cooling; a new generation of materials? - metal-matrix composites, ceramics, cost effectiveness

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F. Oxidation and corrosion 21. Oxidation of Materials the driving force for oxidation; rates of oxidation, mechanisms of oxidation; data

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22. Case Studies in Dry Oxidation making stainless alloys; protecting turbine blades

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23. Wet Corrosion of Materials voltages as driving forces; rates of corrosion; why selective attack is especially dangerous

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24. Case Studies in Wet Corrosion how to protect an underground pipeline; materials for a light-weight factory roof; how to make motor-car exhausts last longer

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G. Friction, abrasion and wear 25. Friction and Wear 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

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26. Case Studies in Friction and Wear the design of a journal bearing; materials for skis and sledge runners; 'non-skid' tyres

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Final case study

27. Materials and Energy in Car Design the selection and economics of materials for automobiles

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Appendix 1 Examples

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Appendix 2 Aids and Demonstrations

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Appendix 3 Symbols and Formulae

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Index

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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 lo3GN m-2; and are around 102GNmW2 for metals’ - in any real design you need an accurate value, which you can get from suppliers’ specifications; but an order-of-

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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.

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Chapter 1 Engineering materials and their properties

Introduction

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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

Plastic-handled screwdriver A typical screwdriver has a shaft and blade made of a high-carbon steel, a metal. Steel is chosen because its modulus is high. The modulus measures the resistance of the material to elastic deflection or bending. If you made the shaft out of a polymer like polyethylene instead, it would twist far too much. A high modulus is one criterion in

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Engineering Materials 1

Table 1.1 Classes of property Economic

Price and availability Recyclability

General Physical

Density

Mechanical

Modulus Yield and tensile strength Hardness Fracture toughness Fatigue strength Creep strength Damping

Thermal

Thermal conductivity Specific heat Thermal expansion coefficient

Electrical and Magnetic

Resistivity Dielectric constant Magnetic permeability

Environmental Interaction

Oxidation Corrosion Wear

Production

Ease of manufacture Joining Finishing

Aesthetic

Colour Texture Feel

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the selection of a material for this application. But it is not the only one. The shaft must have a high yield strength. If it does not, it will bend or twist if you turn it hard (bad screwdrivers do). And the blade must have a high hardness, otherwise it will be damaged by the head of the screw. Finally, the material of the shaft and blade must not only do all these things, it must also resist fracture - glass, for instance, has a high modulus, yield strength and hardness, but it would not be a good choice for this application because it is so brittle. More precisely, it has a very low fracfure toughness. That of the steel is high, meaning that it gives a bit before it breaks. The handle of the screwdriver is made of a polymer or plastic, in this instance polymethylmethacrylate, otherwise known as PMMA, plexiglass or perspex. The handle has a much larger section than the shaft, so its twisting, and thus its modulus, is less important. You could not make it satisfactorily out of a soft rubber (another polymer) because its modulus is much too low, although a thin skin of rubber might be useful because its friction coefficient is high, making it easy to grip. Traditionally, of course, tool handles were made of another natural, polymer - wood - and, if you measure importance by the volume consumed per year, wood is still by far the most important polymer available to the engineer. Wood has been replaced by PMMA

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because PMMA becomes soft when hot and can be moulded quickly and easily to its fi...