Materials - Engineering, Science, Processing and Design - M. Ashby, et al., (Elsevier, 2007) WW.pdf

11.4 The physics of friction and wear 3 231 11.4 The physics of friction and wear 3 231

11.5 Design and selection: materials to manage friction and wear 235

11.6 Summary and conclusions 240

11.7 Further reading 241

11.8 Exercises 241

11.9 Exploring design with CES 243

Chapter 12 Agitated atoms: materials and heat 245

12.1 Introduction and synopsis 246

12.2 Thermal properties: definition and measurement 246

12.3 The big picture: thermal property charts 249

12.4 Drilling down: the physics of thermal properties 251

12.5 Manipulating thermal properties 257

12.6 Design to exploit thermal properties 258

12.7 Summary and conclusions 268

12.8 Further reading 269

12.9 Exercises 270

12.10 Exploring design with CES 271

12.11 Exploring the science with CES Elements 272

Chapter 13 Running hot: using materials at high temperatures 275

13.1 Introduction and synopsis 276

13.2 The temperature dependence of material properties 276

13.3 Charts for creep behavior 281

13.4 The science: diffusion and creep 284

13.5 Materials to resist creep 293

13.6 Design to cope with creep 296

13.7 Summary and conclusions 304

13.8 Further reading 305

13.9 Exercises 305

13.10 Exploring design with CES 308

13.11 Exploring the science with CES Elements 308

Chapter 14 Conductors, insulators and dielectrics 311

14.1 Introduction and synopsis 312

14.2 Conductors, insulators and dielectrics 313

14.3 Charts for electrical properties 317

14.4 Drilling down: the origins and manipulation of electrical properties 320

14.5 Design: using the electrical properties of materials 331

14.6 Summary and conclusions 338

14.7 Further reading 338

14.8 Exercises 339

14.9 Exploring design with CES 341

14.10 Exploring the science with CES Elements 343

Contents vii

Chapter 15 Magnetic materials 345

15.1 Introduction and synopsis 346

15.2 Magnetic properties: definition and measurement 346

15.3 Charts for magnetic properties 351

15.4 Drilling down: the physics and manipulation of magnetic properties 353

15.5 Materials selection for magnetic design 358

15.6 Summary and conclusions 363

15.7 Further reading 363

15.8 Exercises 364

15.9 Exploring design with CES 365

15.10 Exploring the science with CES Elements 366

Chapter 16 Materials for optical devices 367

16.1 Introduction and synopsis 368

16.2 The interaction of materials and radiation 368

16.3 Charts for optical properties 373

16.4 Drilling down: the physics and manipulation of optical properties 375

16.5 Optical design 381

16.6 Summary and conclusions 382

16.7 Further reading 383

16.8 Exercises 383

16.9 Exploring design with CES 384

16.10 Exploring the science with CES Elements 385

Chapter 17 Durability: oxidation, corrosion and degradation 387

17.1 Introduction and synopsis 388

17.2 Oxidation, flammability and photo-degradation 388

17.3 Oxidation mechanisms 390

17.4 Making materials that resist oxidation 392

17.5 Corrosion: acids, alkalis, water and organic solvents 395

17.6 Drilling down: mechanisms of corrosion 396

17.7 Fighting corrosion 401

17.8 Summary and conclusions 404

17.9 Further reading 405

17.10 Exercises 405

17.11 Exploring design with CES 406

17.12 Exploring the science with CES Elements 407

Chapter 18 Heat, beat, stick and polish: manufacturing processes 409

18.1 Introduction and synopsis 410

18.2 Process selection in design 410

18.3 Process attributes: material compatibility 413

18.4 Shaping processes: attributes and origins 414 18.4 Shaping processes: attributes and origins 414

18.5 Joining processes: attributes and origins 423

18.6 Surface treatment (finishing) processes: attributes and origins 426

18.7 Estimating cost for shaping processes 427

18.8 Computer-aided process selection 432

18.9 Case studies 434

18.10 Summary and conclusions 443

18.11 Further reading 444

18.12 Exercises 445

18.13 Exploring design with CES 446

18.14 Exploring the science with CES Elements 447 Chapter 19 Follow the recipe: processing and properties

19.1 Introduction and synopsis 450

19.2 Microstructure of materials 450

19.3 Microstructure evolution in processing 454

19.4 Processing for properties 462

19.5 Case studies 464

19.6 Making hybrid materials 472

19.7 Summary and conclusions 474

19.8 Further reading 475

19.9 Exercises 476

19.10 Exploring design with CES 477 Chapter 20 Materials, processes and the environment

20.1 Introduction and synopsis 480

20.2 Material consumption and its growth 480

20.3 The material life cycle and criteria for assessment 483

20.4 Definitions and measurement: embodied energy, process energy and end of life potential

20.5 Charts for embodied energy 490

20.6 Design: selecting materials for eco-design 493

20.7 Summary and conclusions 497

20.8 Appendix: some useful quantities 498

20.9 Further reading 498

20.10 Exercises 499

20.11 Exploring design with CES 501 Index

Preface

Science-led or Design-led? Two approaches to materials teaching

Most things can be approached in more than one way. In teaching this is especially true. The way to teach a foreign language, for example, depends on the way the student wishes to use it—to read the literature, say, or to find accommodation, order meals and buy beer. So it is with the teaching of this subject.

The traditional approach to it starts with fundamentals: the electron, the atom, atomic bonding, and packing, crystallography and crystal defects. Onto this is built alloy theory, the kinetics of phase transformation and the development of microstructure on scales made visible by electron and optical microscopes. This sets the stage for the understanding and control of properties at the mil- limeter or centimeter scale at which they are usually measured. The approach gives little emphasis to the behavior of structures, methods for material selection, and design.

The other approach is design-led. The starting point is the need: the requirements that materials must meet if they are to perform properly in a given design. To match materials to designs requires

a perspective of the range of properties they offer and the other information that will be needed about them to enable successful selection. Once the importance of a property is established there is good reason to ‘drill down’, so to speak, to examine the science that lies behind it—valuable because an understanding of the fundamentals itself informs material choice and usage.

There is sense in both approaches. It depends on the way the student wishes to use the information. If the intent is scientific research, the first is the logical way to go. If it is engineering design, the sec- ond makes better sense. This book follows the second.

What is different about this book?

There are many books about the science of engineering materials and many more about design. What is different about this one?

First, a design-led approach specifically developed to guide material selection and manipulation. The approach is systematic, leading from design requirements to a prescription for optimized material choice. The approach is illustrated by numerous case studies. Practice in using it is provided by Exercises.

Second, an emphasis on visual communication and a unique graphical presentation of material properties as material property charts. These are a central feature of the approach, helpful both in understanding the origins of properties, their manipulation and their fundamental limits, as well as providing a tool for selection and for understanding the ways in which materials are used.

Third, its breadth. We aim here to present the properties of materials, their origins and the way they enter engineering design. A glance at the Contents pages will show sections dealing with:

• Physical properties • Mechanical characteristics • Thermal behavior • Physical properties • Mechanical characteristics • Thermal behavior

• Electrical, magnetic and optical response • Durability • Processing and the way it influences properties • Environmental issues

Throughout we aim for a simple, straightforward presentation, developing the materials science as far as is it helpful in guiding engineering design, avoiding detail where this does not contribute to this end.

And fourth, synergy with the Cambridge Engineering Selector (CES) 1 —a powerful and widely used PC-based software package that is both a source of material and process information and a tool that implements the methods developed in this book. The book is self-contained: access to the software is not a prerequisite for its use. Availability of the CES EduPack software suite enhances the learning experience. It allows realistic selection studies that properly combine multiple con- straints on material and processes attributes, and it enables the user to explore the ways in which properties are manipulated.

The CES EduPack contains an additional tool to allow the science of materials to be explored in more depth. The CES Elements database stores fundamental data for the physical, crystallographic, mechanical, thermal, electrical, magnetic and optical properties of all 111 elements. It allows inter- relationships between properties, developed in the text, to be explored in depth.

The approach is developed to a higher level in two further textbooks, the first relating to mechan- ical design 2 , the second to industrial design 3 .

1 The CES EduPack 2007, Granta Design Ltd., Rustat House, 62 Clifton Court, Cambridge CB1 7EG, UK, www.grantadesign.com.

2 Ashby, M.F. (2005), Materials Selection in Mechanical Design, 3rd edition, Butterworth-Heinemann, Oxford, UK, Chapter 4. ISBN 0-7506-6168-2. (A more advanced text that develops the ideas presented here in greater depth.)

3 Ashby, M.F. and Johnson, K. (2002) Materials and Design—The Art and Science of Material Selection in Product Design, Butterworth-Heinemann, Oxford, UK. ISBN 0-7506-5554-2. (Materials and processes from an aesthetic

point of view, emphasizing product design.)

Acknowledgements

No book of this sort is possible without advice, constructive criticism and ideas from others. Numerous colleagues have been generous with their time and thoughts. We would particularly like to recog- nize suggestions made by Professors Mick Brown, Archie Campbell, Dave Cardwell, Ken Wallace and Ken Johnson, all of Cambridge University, and acknowledge their willingness to help. Equally valu- able has been the contribution of the team at Granta Design, Cambridge, responsible for the devel- opment of the CES software that has been used to make the material property charts that are a feature of this book.

Resources that accompany this book

Exercises

Each chapter ends with exercises of three types: the first rely only on information, diagrams and data contained in the book itself; the second makes use of the CES software in ways that use the methods developed here, and the third explores the science more deeply using the CES Elements database that is part of the CES system.

Instructor’s manual

The book itself contains a comprehensive set of exercises. Worked-out solutions to the exercises are freely available to teachers and lecturers who adopt this book. To access this material online please visit http://textbooks.elsevier.com and follow the instructions on screen.

Image Bank

The Image Bank provides adopting tutors and lecturers with jpegs and gifs of the figures from the book that may be used in lecture slides and class presentations. To access this material please visit http://textbooks.elsevier.com and follow the instructions on screen.

The CES EduPack

CES EduPack is the software-based package to accompany this book, developed by Michael Ashby and Granta Design. Used together, Materials: Engineering, Science, Processing and Design and CES EduPack provide a complete materials, manufacturing and design course. For further information please see the last page of this book, or visit www.grantadesign.com.

Chapter 1

Introduction: materials—

history and character

Professor James Stuart, the first Professor of Engineering at Cambridge.

Chapter contents

1.1 Materials, processes and choice

1.2 Material properties

1.3 Design-limiting properties

1.4 Summary and conclusions

1.5 Further reading

1.6 Exercises

2 Chapter 1 Introduction: materials—history and character

1.1 Materials, processes and choice

Engineers make things. They make them out of materials. The materials have to support loads, to insulate or conduct heat and electricity, to accept or reject magnetic flux, to transmit or reflect light, to survive in often-hostile sur- roundings, and to do all this without damage to the environment or costing too much.

And there is the partner in all this. To make something out of a material you also need a process. Not just any process—the one you choose has to be com- patible with the material you plan to use. Sometimes it is the process that is the dominant partner and a material-mate must be found that is compatible with it. It is a marriage. Compatibility is not easily found—many marriages fail— and material failure can be catastrophic, with issues of liability and compensa- tion. This sounds like food for lawyers, and sometimes it is: some specialists make their living as expert witnesses in court cases involving failed materials. But our aim here is not contention; rather, it is to give you a vision of the mate- rials universe (since, even on the remotest planets you will find the same ele- ments) and of the universe of processes, and to provide methods and tools for choosing them to ensure a happy, durable union.

But, you may say, engineers have been making things out of materials for centuries, and successfully so—think of Isambard Kingdom Brunel, Thomas Telford, Gustave Eiffel, Henry Ford, Karl Benz and Gottlieb Daimler, the Wright brothers. Why do we need new ways to choose them? A little history helps here. Glance at the portrait with which this chapter starts: it shows James Stuart, the first Professor of Engineering at Cambridge University from 1875 to 1890 (note the cigar). In his day the number of materials available to engineers was small—a few hundred at most. There were no synthetic polymers—there are now over 45 000 of them. There were no light alloys (aluminum was first established as an engineering material only in the 20th century)—now there are thousands. There were no high-performance composites—now there are hun- dreds of them. The history is developed further in Figure 1.1, the time-axis of which spans 10 000 years. It shows roughly when each of the main classes of materials first evolved. The time-scale is nonlinear—almost all the materials we use today were developed in the last 100 years. And this number is enormous: over 160 000 materials are available to today’s engineer, presenting us with a problem that Professor Stuart did not have: that of optimally selecting from this huge menu. With the ever-increasing drive for performance, economy and efficiency, and the imperative to avoid damage to the environment, making the right choice becomes very important. Innovative design means the imaginative exploitation of the properties offered by materials.

These properties, today, are largely known and documented in handbooks; one such—the ASM Materials Handbook—runs to 22 fat volumes, and it is one of many. How are we to deal with this vast body of information? Fortunately another thing has changed since Prof. Stuart’s day: we now have digital informa- tion storage and manipulation. Computer-aided design is now a standard part

1.1 Materials, processes and choice 3

Neanderthal

John F. George W. man

Julius

Henry VIII

of England

Victoria

Roosevelt

Kennedy Bush

10000BC 5000BC

1980 1990 2000 2010 Al–Lithium alloys

Metals Glassy metals Gold Copper Iron Cast iron Aluminum Titanium

Alloy steels

Microalloyed steels Magnesium Super-alloys Zirconium Metal foams

PTFE High-temperature Polymers &

polymers: PEEK elastomers

PE PMMA PP

Polyesters High-modulus polymers

Ceramics & Flint

Technical ceramics: glasses

Glass

Stone

Cement Refractories

silica Cermets

cerams

Pyro-

Al 2 O 3 , SiC, Si 3 N 4 , PSZ etc

Kevlar-FRP Ceramic composites Hybrids

Straw-Brick

Paper

GFRP CFRP Metal–matrix composites

10000BC 5000BC

Figure 1.1 The development of materials over time. The materials of pre-history, on the

left, all occur naturally; the challenge for the engineers of that era was one of shaping them. The development of thermochemistry and (later) of polymer chemistry enabled man-made materials, shown in the colored zones. Three— stone, bronze and iron—were of such importance that the era of their dominance is named after them.

of an engineer’s training, and it is backed up by widely available packages for solid modeling, finite-element analysis, optimization, and for material and process selection. Software for the last of these—the selection of materials and processes—draws on databases of the attributes of materials and processes, doc- umenting their mutual compatibility, and allows them to be searched and dis- played in ways that enable selections that best meet the requirements of a design.

If you travel by foot, bicycle or car, you take a map. The materials landscape, like the terrestrial one, can be complex and confusing; maps, here, are also a good idea. This text presents a design-led approach to materials and manufacturing

4 Chapter 1 Introduction: materials—history and character

processes that makes use of maps: novel graphics to display the world of mate- rials and processes in easily accessible ways. They present the properties of materials in ways that give a global view, that reveal relationships between properties and that enable selection.

1.2 Material properties

So what are these properties? Some, like density (mass per unit volume) and price (the cost per unit volume or weight) are familiar enough, but others are not, and getting them straight is essential. Think first of those that have to do with carrying load safely—the mechanical properties.

Mechanical properties

A steel ruler is easy to bend elastically—‘elastic’ means that it springs back when released. Its elastic stiffness (here, resistance to bending) is set partly by its shape—thin strips are easy to bend—and partly by a property of the steel itself: its elastic modulus, E. Materials with high E, like steel, are intrinsically stiff; those with low E, like polyethylene, are not. Figure 1.2(b) illustrates the consequences of inadequate stiffness.

The steel ruler bends elastically, but if it is a good one, it is hard to give it a permanent bend. Permanent deformation has to do with strength, not stiffness. The ease with which a ruler can be permanently bent depends, again, on its shape and on a different property of the steel—its yield strength, σ y . Materials with large σ y , like titanium alloys, are hard to deform permanently even though their stiffness, coming from E, may not be high; those with low σ y , like lead, can be deformed with ease. When metals deform, they generally get stronger (this is called ‘work hardening’), but there is an ultimate limit, called the tensile strength, σ ts , beyond which the material fails (the amount it stretches before it breaks is called the ductility). Figure 1.2(c) gives an idea of the consequences of inadequate strength.

So far so good. One more. If the ruler were made not of steel but of glass or of PMMA (Plexiglas, Perspex), as transparent rulers are, it is not possible to bend it permanently at all. The ruler will fracture suddenly, without warning, before it acquires a permanent bend. We think of materials that break in this way as brittle, and materials that do not as tough. There is no permanent defor- mation here, so σ y is not the right property. The resistance of materials to

cracking and fracture is measured instead by the fracture toughness, K 1c . Steels are tough—well, most are (steels can be made brittle)—they have a high K 1c . Glass epitomizes brittleness; it has a very low K 1c . Figure 1.2(d) suggests conse- quences of inadequate fracture and toughness. We started with the material property density, mass per unit volume, symbol ρ. Density, in a ruler, is irrelevant. But for almost anything that moves, weight carries a fuel penalty, modest for automobiles, greater for trucks and trains, greater still for aircraft, and enormous in space vehicles. Minimizing weight has

1.2 Material properties 5

(a)

All as it should be

(b)

Not stiff enough (E too low)

(c)

Not strong enough

(σ y too low)

(d) Not tough enough

(K 1c too low)

(e)

T oo dense (ρ too high)

Figure 1.2 Mechanical properties.

much to do with clever design—we will get to that later—but equally to choice of material. Aluminum has a low density, lead a high one. If our little aircraft were made of lead, it would never get off the ground at all (Figure 1.2(e)).

These are not the only mechanical properties, but they are the most import- ant ones. We will meet them, and the others, in Chapters 4–11.

Thermal properties

The properties of a material change with temperature, usually for the worse. Its strength falls, it starts to ‘creep’ (to sag slowly over time), it may oxidize, degrade or decompose (Figure 1.3(a)). This means that there is a limiting tem- perature called the maximum service temperature, T max , above which its use is impractical. Stainless steel has a high T max —it can be used up to 800°C; most polymers have a low T max and are seldom used above 150°C.

6 Chapter 1 Introduction: materials—history and character

(a) High service temperature T max

Low service temperature T max

(b) High expansion coefficient α

Low expansion coefficient α

(c) High conductivity λ

Low conductivity λ

(d) High T-diffusivity a

Low T-diffusivity a

Figure 1.3 Thermal properties.

Most materials expand when they are heated, but by differing amounts depending on their thermal expansion coefficient, α. The expansion is small, but its consequences can be large. If, for instance, a rod is constrained, as in Figure 1.3(b), and then heated, expansion forces the rod against the con- straints, causing it to buckle. Railroad track buckles in this way if provision is not made to cope with it.

Some materials—metals, for instance—feel cold; others—like woods—feel warm. This feel has to do with two thermal properties of the material: thermal conductivity and heat capacity. The first, thermal conductivity, λ, measures the rate at which heat flows through the material when one side is hot and the other cold. Materials with high λ are what you want if you wish to conduct heat from one place to another, as in cooking pans, radiators and heat exchang- ers; Figure 1.3(c) suggests consequences of high and low λ for the cooking ves- sel. But low λ is useful too—low λ materials insulate homes, reduce the energy consumption of refrigerators and freezers, and enable space vehicles to re-enter the earth’s atmosphere.

1.2 Material properties 7

These applications have to do with long-time, steady, heat flow. When time is limited, that other property—heat capacity, C p —matters. It measures the amount of heat that it takes to make the temperature of material rise by a given amount. High heat capacity materials—copper, for instance—require a lot of heat to change their temperature; low heat capacity materials, like polymer foams, take much less. Steady heat flow has, as we have said, to do with ther- mal conductivity. There is a subtler property that describes what happens when heat is first applied. Think of lighting the gas under a cold slab of material with

a bole of ice-cream on top (here, lime ice-cream) as in Figure 1.3(d). An instant after ignition, the bottom surface is hot but the rest is cold. After a while, the middle gets hot, then later still, the top begins to warm up and the ice-cream first starts to melt. How long does this take? For a given thickness of slab, the time is inversely proportional to the thermal diffusivity, a, of the material of the slab. It differs from the conductivity because materials differ in their heat capacity—in fact, it is proportional to λ/C p .

There are other thermal properties—we’ll meet them in Chapters 12 and 13—but this is enough for now. We turn now to matters electrical, magnetic and optical.

Electrical, magnetic and optical properties

We start with electrical conduction and insulation (Figure 1.4(a)). Without electrical conduction we would lack the easy access to light, heat, power, con- trol and communication that—today—we take for granted. Metals conduct well—copper and aluminum are the best of those that are affordable. But con- duction is not always a good thing. Fuse boxes, switch casings, the suspensions for transmission lines all require insulators, and in addition those that can carry some load, tolerate some heat and survive a spark if there were one. Here the

property we want is resistivity, ρ e , the inverse of electrical conductivity κ e . Most plastics and glass have high resistivity (Figure 1.4(a))—they are used as insulators—though, by special treatment, they can be made to conduct a little.

Figure 1.4(b) suggests further electrical properties: the ability to allow the passage of microwave radiation, as in the radome, or to reflect them, as in the passive reflector of the boat. Both have to do with dielectric properties, partic-

ularly the dielectric constant ε D . Materials with high ε D respond to an electric field by shifting their electrons about, even reorienting their molecules; those with low ε D are immune to the field and do not respond. We explore this and other electrical properties in Chapter 14. Electricity and magnetism are closely linked. Electric currents induce magnetic fields; a moving magnet induces, in any nearby conductor, an electric current. The response of most materials to magnetic fields is too small to be of practical value. But a few—called ferromagnets and ferrimagnets—have the capacity to trap a magnetic field permanently. These are called ‘hard’ magnetic materials because, once magnetized, they are hard to demagnetize. They are used as permanent magnets in headphones, motors and dynamos. Here the key property is the rema- nence, a measure of the intensity of the retained magnetism. A few others—‘soft’

8 Chapter 1 Introduction: materials—history and character

ON

OFF

(a) Low resistivity ρ e High resistivity ρ e

(b) Low dielectric response

High dielectric response

(c) ‘Hard’ magnetic behavior

Soft magnetic behavior

(d) Refraction

Absorption

Figure 1.4 Electrical, magnetic and optical properties. magnet materials—are easy to magnetize and demagnetize. They are the materi-

als of transformer cores and the deflection coils of a TV tube. They have the capacity to conduct a magnetic field, but not retain it permanently (Figure 1.4(c)). For these a key property is the saturation magnetization, which measures how large a field the material can conduct. These we meet again in Chapter 15.

Materials respond to light as well as to electricity and magnetism—hardly surprising, since light itself is an electromagnetic wave. Materials that are opaque reflect light; those that are transparent refract it, and some have the ability to absorb some wavelengths (colors) while allowing others to pass freely (Figure 1.4(d)). These are explored in more depth in Chapter 16.

Chemical properties

Products often have to function in hostile environments, exposed to corrosive flu- ids, to hot gases or to radiation. Damp air is corrosive, so is water; the sweat of your hand is particularly corrosive, and of course there are far more aggressive environments than these. If the product is to survive for its design life it must be made of materials—or at least coated with materials—that can tolerate the sur- roundings in which they operate. Figure 1.5 illustrates some of the commonest of these: fresh and salt water, acids and alkalis, organic solvents, oxidizing flames

1.3 Design-limiting properties 9

(a) Fresh water

(b) Salt water

(c) Acids and alkalis

(d) Organic solvents

(e) Oxidation

(f) UV radiation

Figure 1.5 Chemical properties: resistance to water, acids, alkalis, organic solvents,

oxidation and radiation. and ultraviolet radiation. We regard the intrinsic resistance of a material to each

of these as material properties, measured on a scale of 1 (very poor) to 5 (very good). Chapter 17 deals with the material durability.

1.3 Design-limiting properties

The performance of a component is limited by certain of the properties of the materials of which it is made. This means that, to achieve a desired level of performance, the values of the design-limiting properties must meet certain targets—those that fail to do so are not suitable. In the cartoon of Figure 1.2, stiffness, strength and toughness are design limiting—if any one of them were too low, the plane won’t fly. In the design of power transmission lines electrical resistivity is design limiting; in the design of a camera lens, it is optical quality and refractive index.

Materials are chosen by identifying the design-limiting properties and apply- ing limits to them, screening out those that do not meet the limits (Chapter 3).

10 Chapter 1 Introduction: materials—history and character

Processes, too, have properties, although we have not met them yet. These too can be design limiting, leading to a parallel scheme for choosing viable processes (Chapters 18 and 19).

1.4 Summary and conclusions

Engineering design depends on materials that are shaped, joined and finished by processes. Design requirements define the performance required of the mate- rials, expressed as target values for certain design-limiting properties. A mate- rial is chosen because it has properties that meet these targets and is compatible with the processes required to shape, join and finish it.

This chapter introduced some of the design-limiting properties: physical properties (like density), mechanical properties (like modulus and yield strength) and functional properties (those describing the thermal, electrical, magnetic and optical behavior). We examine all of these in more depth in the chapters that follow, but those just introduced are enough to be going on with. We turn now to the materials themselves: the families, the classes and the members.

1.5 Further reading

The history and evolution of materials

A History of Technology (1954–2001) (21 volumes), edited by Singer, C., Holmyard, E.J., Hall, A.R., Williams, T.I. and Hollister-Short, G. Oxford University Press, Oxford, UK. ISSN 0307-5451. (A compilation of essays on aspects of technology, including materials.)

Delmonte, J. (1985) Origins of Materials and Processes, Technomic Publishing Company, Pennsylvania, USA. ISBN 87762-420-8. (A compendium of information about materials in engineering, documenting the history.)

Tylecoate, R.F. (1992) A History of Metallurgy, 2nd edition, The Institute of Materials, London, UK. ISBN 0-904357-066. (A total-immersion course in the history of the extraction and use of metals from 6000 BC to 1976, told by an author with forensic talent and love of detail.)

1.6 Exercises

Exercise E1.1 Use Google to research the history and uses of one of the following materials:

• Tin • Glass • Cement

1.6 Exercises 11

• Titanium • Carbon fiber.

Present the result as a short report of about 100–200 words (roughly half a page).

Exercise E1.2 What is meant by the design-limiting properties of a material in a given appli- cation?

Exercise E1.3 There have been many attempts to manufacture and market plastic bicycles. All have been too flexible. Which design-limiting property is insufficiently large?

Exercise E1.4 What, in your judgement, are the design-limiting properties for the material for the blade of a knife that will be used to gut fish?

Exercise E1.5 What, in your judgement, are the design-limiting properties for the material of an oven glove?

Exercise E1.6 What, in your judgement, are the design-limiting properties for the material of an electric lamp filament?

Exercise E1.7

A material is needed for a tube to carry fuel from the fuel tank to the carbure- tor of a motor mower. The design requires that the tube can bend and that the fuel be visible. List what you would think to be the design-limiting properties.

Exercise E1.8

A material is required as the magnet for a magnetic soap holder. Soap is mildly alkaline. List what you would judge to be the design-limiting properties.

Exercise E1.9 The cases in which most CDs are sold have an irritating way of cracking and breaking. Which design-limiting property has been neglected in selecting the material of which they are made?

Exercise E1.10 List three applications that, in your judgement, need high stiffness and low weight.

Exercise E1.11 List three applications that, in your judgement, need optical quality glass.

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Chapter 2

Family trees: organizing materials and processes

Chapter contents

2.1 Introduction and synopsis

2.2 Getting materials organized: the materials tree

2.3 Organizing processes: the process tree 18

2.4 Process–property interaction

2.5 Material property charts

2.6 Computer-aided information management for materials and processes

2.7 Summary and conclusions

2.8 Further reading

2.10 Exploring design using CES

2.11 Exploring the science with CES

14 Chapter 2 Family trees: organizing materials and processes

2.1 Introduction and synopsis

A successful product—one that performs well, is good value for money and gives pleasure to the user—uses the best materials for the job, and fully exploits their potential and characteristics.

The families of materials—metals, polymers, ceramics and so forth—are introduced in Section 2.2. What do we need to know about them if we are to design products using them? That is the subject of Section 2.3, in which distinc- tions are drawn between various types of materials information. But it is not, in the end, a material that we seek; it is a certain profile of properties—the one that best meets the needs of the design. Each family has its own characteristic profile—the ‘family likeness’—useful to know when deciding which family to use for a given design. Section 2.2 explains how this provides the starting point for a classification scheme for materials, allowing information about them to

be organized and manipulated. Choosing a material is only half the story. The other half is the choice of a process route to shape, join and finish it. Section 2.3 introduces process fami- lies and their attributes. Choice of material and process are tightly coupled: a given material can be processed in some ways but not others, and a given process can be applied to some materials but not to others. On top of that, the act of processing can change, even create, the properties of the material. Process families, too, exhibit family likenesses—commonality in the materials that members of a family can handle or the shapes they can make. Section 2.3 intro- duces a classification for processes that parallels that for materials.

Family likenesses are most strikingly seen in material property charts, a cen- tral feature of this book (Section 2.5). These are charts with material properties as axes showing the location of the families and their members. Materials have many properties, which can be thought of as the axes of a ‘material–property’ space—one chart is a two-dimensional slice through this space. Each material family occupies a discrete part of the space, distinct from the other families. The charts give an overview of materials and their properties; they reveal aspects of the science underlying the properties, and they provide a powerful tool for materials selection. Process attributes can be treated in a similar way to create process–attribute charts—we leave these for Chapter 18.

The classification systems of Sections 2.2 and 2.3 provide a structure for computer-based information management, introduced in Section 2.6. The chapter ends with a summary, further reading and exercises.

2.2 Getting materials organized: the materials tree

Classifying materials

It is conventional to classify the materials of engineering into the six broad fam- ilies shown in Figure 2.1: metals, polymers, elastomers, ceramics, glasses and

2.2 Getting materials organized: the materials tree 15

Steels Cast irons Al alloys

Metals

Cu alloys Zn alloys Ti alloys

Aluminas

PC, PS, PEEK PE, PP, PET,

Silicon carbides

PA (nylons)

Silicon nitrides

Segmented structures

Epoxies

Lattices and

foams

Soda glass Borosilicate glass

Neoprene Isoprene

Glasses Butyl rubber

Elastomers

Silica glass Glass-ceramics

Natural rubber

Silicones EVA

Figure 2.1 The menu of engineering materials. The basic families of metals, ceramics,

glasses, polymers and elastomers can be combined in various geometries to create hybrids.

hybrids—composite materials made by combining two or more of the others. There is sense in this: the members of a family have certain features in common: similar properties, similar processing routes and, often, similar applications. Figure 2.2 shows examples of each family.

Figure 2.3 illustrates how the families are expanded to show classes, sub- classes and members, each of which is characterized by a set of attributes: its properties. As an example, the Materials universe contains the family ‘Metals’, which in turn contains the class ‘Aluminum alloys’, sub-classes such as the ‘6000 series’ within which we find the particular member ‘Alloy 6061’. It, and every other member of the universe, is characterized by a set of attributes that include not only the properties mentioned in Chapter 1, but also its processing characteristics, the environmental consequences of its use and its typical appli- cations. We call this its property profile. Selection involves seeking the best match between the property profiles of the materials in the universe and that required by the design. As already mentioned, the members of one family have certain characteristics in common. Here, briefly, are some of these.

Metals have relatively high stiffness, measured by the modulus, E. Most, when pure, are soft and easily deformed, meaning that σ y is low. They can be made strong by alloying and by mechanical and heat treatment, increasing σ y , but they remain ductile, allowing them to be formed by deformation processes. And,

broadly speaking, they are tough, with a usefully high fracture toughness K 1c . They are good electrical and thermal conductors. But metals have weaknesses too: they are reactive; most corrode rapidly if not protected.

Ceramics are non-metallic, inorganic solids, like porcelain or alumina—the material of spark-plug insulators. They have many attractive features. They are stiff, hard and abrasion resistant, they retain their strength to high temperatures,

16 Chapter 2 Family trees: organizing materials and processes

Figure 2.2 Examples of each material family. The arrangement follows the general pattern

of Figure 2.1. The central hybrid here is a sandwich structure made by combining stiff, strong face sheets of aluminum with a low-density core of balsa wood.

and they resist corrosion well. Most are good electrical insulators. They, too, have their weaknesses: unlike metals, they are brittle, with low K 1c . This gives ceramics a low tolerance for stress concentrations (like holes or cracks) or for high contact stresses (at clamping points, for instance). For this reason it is more difficult to design with ceramics than with metals.

Glasses are non-crystalline (‘amorphous’) solids, a term explained more fully in Chapter 4. The commonest are the soda-lime and borosilicate glasses famil- iar as bottles and Pyrex ovenware, but there are many more. The lack of crys- tal structure suppresses plasticity, so, like ceramics, glasses are hard and remarkably corrosion resistant. They are excellent electrical insulators and, of course, they are transparent to light. But like ceramics, they are brittle and vulnerable to stress concentrations.

2.2 Getting materials organized: the materials tree 17

Sub-class Member Attributes

Density Al 6061

Cu alloys

Mechanical props. Density

Al alloys

Thermal props. Mechanical props.

• Polymers

Ti alloys

Electrical props. Thermal props.

Optical props. Electrical props.

• Elastomers

Ni alloys

Corrosion props. Optical props.

• Hybrids

Zn alloys

Corrosion props.

-- specific Documentation

-- general – specific – general

Material records

Figure 2.3 The taxonomy of the kingdom of materials and their attributes. Computer-

based selection software stores data in a hierarchical structure like this.

Polymers are organic solids based on long chains of carbon (or, in a few, sil- icon) atoms. Polymers are light—their densities ρ are less than those of the lightest metals. Compared with other families they are floppy, with moduli E that are roughly 50 times less than those of metals. But they can be strong, and because of their low density, their strength per unit weight is comparable to that of metals. Their properties depend on temperature so that a polymer that is tough and flexible at room temperature may be brittle at the ⫺4°C of a household freezer, yet turn rubbery at the 100°C of boiling water. Few have useful strength above 150°C. If these aspects are allowed for in the design, the advantages of polymers can be exploited. And there are many. They are easy to shape (that is why they are called plastics): complicated parts performing sev- eral functions can be molded from a polymer in a single operation. Their prop- erties are well suited for components that snap together, making assembly fast and cheap. And by accurately sizing the mold and pre-coloring the polymer, no finishing operations are needed. Good design exploits these properties.

Elastomers—the material of rubber bands and running shoes—are polymers with the unique property that their stiffness, measured by E, is extremely low (500–5000 times less than those of metals) and their ability to be stretched to many times their starting length yet recover their initial shape when released. Despite their low stiffness they can be strong and tough—think of car tires.

Hybrids are combinations of two (or more) materials in an attempt to get the best of both. Glass and carbon-fiber-reinforced polymers (GFRP and CFRP) are hybrids; so, too, are sandwich structures, foams and laminates. And almost all the materials of nature (wood, bone, skin, leaf) are hybrids—bone, for instance, is a mix of collagen (a polymer) with hydroxyapatite (a mineral). Hybrid

18 Chapter 2 Family trees: organizing materials and processes

components are expensive and they are relatively difficult to form and join. So despite their attractive properties the designer will use them only when the added performance justifies the added cost. Today’s growing emphasis on high performance and fuel efficiency provides increasing drivers for their use.

2.3 Organizing processes: the process tree

A process is a method of shaping, joining or finishing a material. Casting, injec- tion molding, fusion welding and electro-polishing are all processes; there are hundreds of them (Figures 2.4 and 2.5). It is important to choose the right process-route at an early stage in the design before the cost-penalty of making changes becomes large. The choice, for a given component, depends on the material of which it is to be made, on its shape, dimensions and precision, and on how many are to be made—in short, on the design requirements.

Raw materials

Primary shaping Casting

Composite Special methods:

forming: methods:

Sand

Hand lay-up Rapid prototype Die

Filament winding Lay-up Investment

Blow molding

Drawing

Slip casting

RTM Electro-form

Heat treat:

Cut, turn, plane

Quench, temper,

drill, grind

age-harden

Joining Surface treatment

Heat bonding

Texturing

Metallizing

Snap fits,

Friction bond

Finished product

Figure 2.4 The classes of process. The first row contains the primary shaping processes;

below lie the secondary processes of machining and heat treatment, followed by the families of joining and finishing processes.

2.3 Organizing processes: the process tree 19

Raw materials

Primary shaping

Crucible Laser

Mould

Granular polymer

Sand mould

Mirror Laser

Parting beam workpiece line cavity Mould

Sand casting Injection molding Photosensitive resin Rolling Special

cyanide Insulation

Heat treatment

Joining Surface treatment

covering Flux

Deposit electrode Component Oil Workpiece

Evolved Slag gas shield

Core wire

Fabric wheel

atoms Evaporated evaporant Heated

Heater

metal Weld

Weld

pool

Parent metal

Sulphuric acid electrolyte

Rotation

Spray gun Shoulder

spray Dispenser Paint Aluminum Weld

Glue line

Anodized surface

Friction-stir weld Cathode Adhesives Anodizing (lead or aluminum)

plate

Anode

Compressed air

Painting

Finished product

Figure 2.5 Examples of the families and classes of manufacturing processes. The arrangement follows

the general pattern of Figure 2.4.

20 Chapter 2 Family trees: organizing materials and processes

The choice of material limits the choice of process. Polymers can be molded, other materials cannot. Ductile materials can be forged, rolled and drawn but those that are brittle must be shaped in other ways. Materials that melt at mod- est temperatures to low-viscosity liquids can be cast; those that do not have to

be processed by other routes. Shape, too, influences the choice of process. Slender shapes can be made easily by rolling or drawing but not by casting. Hollow shapes cannot be made by forging, but they can by casting or molding.

Classifying processes

Manufacturing processes are organized under the headings shown in Figure 2.4. Primary processes create shapes. The first row lists six primary forming processes: casting, molding, deformation, powder methods, methods for forming com- posites, special methods including rapid prototyping. Secondary processes modify shapes or properties; here they are shown as ‘machining’, which adds features to an already shaped body, and ‘heat treatment’, which enhances sur- face or bulk properties. Below these come joining and, finally, surface treat- ment. Figure 2.5 illustrates some of these; it is organized in the same way as Figure 2.4. The merit of Figure 2.4 is as a flow chart: a progression through a manufacturing route. It should not be treated too literally: the order of the steps can be varied to suit the needs of the design. The point it makes is that there are three broad process families: those of shaping, joining and finishing.

To organize information about processes, we need a hierarchical classifica- tion like that used for materials, giving each process a place. Figure 2.6 shows part of the hierarchy. The Process universe has three families: shaping, joining and surface treatment. In this figure, the shaping family is expanded to show classes: casting, deformation, molding etc. One of these—molding—is again expanded to show its members: rotation molding, blow molding, injection molding and so forth. Each process is characterized by a set of attributes: the materials it can handle, the shapes it can make, their size, precision and an eco- nomic batch size (the number of units that it can make most economically).

The other two families are partly expanded in Figure 2.7. There are three broad classes of joining process: adhesives, welding and fasteners. In this figure one of them—welding—is expanded to show its members. As before each member has attributes. The first is the material or materials that the process can join. After that the attribute list differs from that for shaping. Here the geome- try of the joint and the way it will be loaded are important, as are requirements that the joint can or cannot be disassembled, be watertight and be electrically conducting.

The lower part of the figure expands the family of finishing processes. Some of the classes it contains are shown; one—coating—is expanded to show some of its members. Finishing adds cost: the only justification for applying a finish- ing process is that it hardens, or protects, or decorates the surface in ways that add value. As with joining, the material to be coated is an important attribute but the others again differ.

We return to process selection in Chapters 18 and 19.

2.4 Process–property interaction 21

Size range Minimum section

Resin casting

Minimum batch size

Cost model

Blow molding

Documentation

A process record

Figure 2.6 The taxonomy of the kingdom of process with part of the shaping family

expanded. Each member is characterized by a set of attributes. Process selection involves matching these to the requirements of the design.

Joint geometry Joint geometry

Size range Size Range

Fasteners

e-beam

Section thickness Section thickness

Hot gas

Relative cost ... Relative cost ...

Hot bar...

Supporting information

Heat treat

Paint/print

Purpose of treatment

Surface

Anodize

Purpose of treatment

Coating thickness

treatment

Coat

Coating thickness

Powder coat

Polish

Surface hardness Surface hardness

Metallize...

Relative cost ...

Texture ...

Relative cost ...

Documentation Supporting information

Process records

Figure 2.7 The taxonomy of the process kingdom again, with the families of joining and

finishing partly expanded.

2.4 Process–property interaction

Processing can change properties. If you hammer a metal (‘forging’) it get harder; if you then heat it up it gets softer again (‘annealing’). If polyethylene—the stuff

22 Chapter 2 Family trees: organizing materials and processes