Materials Engineering Science Processing Design Past Paper PDF

2 Chapter 1 Introduction: materials-history and character Chapter contents 1.1 Materials, processes and choice 2 1.2 Material properties 4 1.3...

2 Chapter 1 Introduction: materials-history and character Chapter contents 1.1 Materials, processes and choice 2 1.2 Material properties 4 1.3 Design-limiting properties 12 1.4 Summary and conclusions 13 1.5 Further reading 13 1.6 Exercises 13 1.1 Materials, processes and choice Engineers make things. They make them out of materials, and they shape, join and finish them using processes. 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 surroundings and to do all this without damage to the environment or costing too much. There is also a partner in all this. To make something you also need a process. And not just any process-it has to be compatible with the material you plan to use. Sometimes the process is the dominant partner and a compatible material-mate must be found-it is a marriage. Compatibility is not easily found-many marriages fail-and material failure can be cata- strophic, with issues of liability and compensation. This sounds like food for lawyers, and sometimes it is: some lawyers make their living as expert witnesses in court cases involving failed materials. But our aim here is not contention; rather, it is to give a vision of the universe of materials (since even on the remotest planets you will find the same elements) 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 lsambard Kingdom Brunei, 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. 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 twentieth century)-now there are thousands. There were no high-performance composites-now there are hundreds 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 non-linear-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 Digitised by the Department of Library Services, University of Pretoria, 2020  1.1 Materials, processes and choice 3 Before Common Era Common Era 10000 5000 1000 1500 1800 1900 1940 1960 1980 1990 2000 2010 I A -Lithium alloys C-steels Aluminu(Tl Titanium Zirconium Glassy metals Gold Copper Tin Iron Castiron Metals Alloy steels Superalloys Microalloyed steels Bronze Magnesium :. Metal foams -.....,._....... s_hape-mamot')( alloys · Nario-structures High temperature Nylon PC Acrylics PTFE polymers: PEEK Polymers & Wood Fibres Rubber Bakelite PS Epoxies High modulus elastomers Skins Glues polymers PE PMMA PP Polyesters Conducting polymers Diamond-like carbon Ceramics & , Flint Glass :Portland Carbon fibers Stone : Cement R~fractqries glasses Pottery ; ' : cement Technical ceramics: AI,03. SiC, Si3N4, PSZ : , Carbon nano-tubes Kevjar-FRP Ceramic ~mposite~ Hybrids Straw-Brick Paper ~FR~FRP Meta~matrix comP!'Sites 10000 5000 0 1000 1500 1800 1900 1940 1960 1980 1990 2000 2010 Date 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 thermo-chemistry, 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. selecting the best one. 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 just one of many. How are we to deal with this vast body of information? Fortunately another thing has changed since Professor Stuart's day: we now have digital information storage and manipulation. Computer-aided design is now a standard part of an engineer's training, and it is backed up by widely- availa ble packages for solid modelling, finite-element analysis, optimisation and material and process selection. Software for the last of these-the selection of materials and processes- draws on data bases of the attributes of materials and processes, documents their mutual compatibility and allows them to be searched and displayed in ways that enable selections that best meet the requirements of a design. Digitised by the Department of Library Services, University of Pretoria, 2020 4 Chapter 1 Introduction: materials-history and character 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 processes that makes use of such maps: novel graphics to display the world of materials 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 properlies A steel ruler is easy to be 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 also on a different property of the steel-its yield strength, ay. Materials with large ay, like titanium alloys, are hard to deform permanently even though their stiffness, coming from E, may not be high; those with low ay, 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, a15 , beyond which the material fails (the amount it stretches before it breaks is called the ductility). The hardness, H, is closely related to the strength, ay. High hardness gives scratch resistance and resistance to wear. Figure 1.2(c) gives an idea of the consequences of inade- quate strength. So far so good. There is one more property, and it is a tricky one. If the ruler were made not of steel but of glass or of PMMA (Plexiglas, or 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 deformation here, so ay is not the right property. The resistance of materials to cracking and fracture is measured instead by the fracture toughness, Ktc· Steels are tough-well, most are (steels can be made brittle)-and they have a high Ktc· Glass epitomises brittleness; it has a very low Ktc. Figure 1.2(d) suggests the consequences of inadequate fracture toughness. Digitised by the Department of Library Services, University of Pretoria, 2020  1.2 Material properties 5 (a) All as it should be Not stiff enough (E too low) (c) Not strong enou gh (cry too low) i Not tough enough (K1c too low) Figure 1.2 Mechanical properties. We started with the material property density, mass per unit volume, symbol p. Density, in a ruler, is irrelevant. But for almost anything that moves, weight carries a fuel penalty, which is modest for automobiles, greater for trucks and trains, greater still for aircraft and enormous in space vehicles. Minimizing weight has much to do with clever design (we will get to that later) but equally with 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)). Digitised by the Department of Library Services, University of Pretoria, 2020 6 Chapter 1 Introduction: materials-history and character Example 1.1 Design requirements (1) You are asked to select a material for the teeth ofthe scoop of a digger truck. To do so you need to prioritize the materials properties that matter. What are they? Answer. The teeth will be used in a brutal way to cut earth, scoop stones, crunch rock, often in unpleasant environments (ditches, sewers, fresh and salt water and worse) and their maintenance will be neglected. These translate into a need for high hardness, H, to resist wear, and high fracture toughness, Kt c, so they don't snap off. Does the cost of the material matter? Not much-it is worth paying for good teeth to avoid expensive downtime. These are not the only mechanical properties, but they are the most important ones. We will meet them, and others, in Chapters 4-11. Thennal 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) and it may oxidize, degrade or decompose (Figure 1.3(a)). This means that there is a limiting temperature called the maximum service temperature, T max , above which its use is impractical. Stainless steel has a high T max and can be used up to 800 °C; most polymers have a low T max and are seldom used above 150 °C. Most materials expand when they are heated, but by differing amounts depending on their thermal expansion coefficient, a. 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 constraints, causing it to buckle. Railroad track buckles in this way if provision is not made to cope with it. Bridges have expansion joints for the same reason. Digitised by the Department of Library Services, University of Pretoria, 2020  1.2 Material properties 7 (a) High service temperature Tmax Low service temperature Tmax ~--~~ §!--~- ~- ~--~ ~-~ &~~~&~ &~~~&~ I I I I (b) High expansion coefficient a Low expansion coefficient a ~~~~~~ ~ ~R~\r&F I I I I (C) High conductivity A Low conductivity A ~ ~~&~~~ D DD I I (d) High T-diffusivity a Low T-diffusivity a Figure 1.3 Thermal properties. 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, A, measures the rate at which heat flows through the material when one side is hot and the other cold. Materials with high A are what you want if you wish to conduct heat from one place to another, as in cooking pans, radiators and heat exchangers; Figure 1.3(c) suggests consequences of high and low A for the cooking vessel. But low A is useful too-low A materials insulate homes, reduce the energy consumption of refrigerators and freezers and enable space vehicles to re-enter the earth's atmosphere. These applications have to do with long-time, steady heat flow. When time is limited, the other thermal property matters-heat capacity , Cp. It measures the amount of heat that it takes to make the temperature of material rise by a given amount. High-heat capacity mate- rials-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 thermal conductivity. There is a subtler property that describes what happens Digitised by the Department of Library Services, University of Pretoria, 2020 8 Chapter 1 Introduction: materials-history and character when heat is first applied. Think of lighting the gas under a cold slab of material with a ball 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 bit, the middle gets hot, then later still, the top begins to warm up, and only then does the ice cream start 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 ca- pacity, in fact, diffusivity is proportional to A/Cp. Example 1.2 Design requirements (2) You are asked to select a material for energy-efficient cookware. What material properties are you looking for? Answer. To be energy-efficient the pan must have a high thermal conductivity,.A, to transmit and spread the heat well, and it must resist corrosion by anything that might be cooked in it, including hot salty water, dilute acids (acetic acid, vinegar) and mild alkalis (baking soda). There are other thermal properties, which we'll meet in Chapters 12, 13, and 17, but these are enough for now. We turn now to matters electrical, magnetic and optical. Electrical, magnetic and optical properties Start with electrical conduction and insulation (Figure 1.4(a)). Without electrical conduction we would lack the easy access to light, heat, power, control and communication that we take for granted today. Metals conduct well-copper and aluminum are the best of those that are affordable. But conduction is not always a good thing. Fuse boxes, switch casings and the suspensions for transmission lines all require insulators that must also carry some load, tolerate some heat and survive a spark if there is one. Here the property we want is resistivity, Pe, the inverse of elec- trical conductivity, Ke. Most plastics and glass have high resistivity (Figure 1.4(a)); they are used as insulators, although with 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 it, as in the passive reflector of the boat. Digitised by the Department of Library Services, University of Pretoria, 2020  1.2 Material properties 9 ON G OFF (a) Low resistivity Pe High resistivity Pe (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. Both have to do with dielectric properties, particularly the dielectric constant eD· Materials with high eD respond to an electric field by shifting their electrons about, even reorienting their molecules; those with loweD 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 ferro- magnets and ferri-magnets-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 remanence, a measure of the intensity of the retained magnetism. A few others-'soft' magnetic materials-are easy to magnetize and demagnetize. They are the materials of transformer cores and the deflection coils of an old TV tube. They have the capacity to conduct a magnetic field, but not to retain it permanently (Figure 1.4(c) ). For these a key property is the Digitised by the Department of Library Services, University of Pretoria, 2020 10 Chapter 1 Introduction: materials-history and character 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. Example 1.3 Design requirements (3) What are the essential and the desirable requirements of materials for eyeglass (spectacle) lenses? Answer. Essentials: The optical qualities are paramount, so a material with optical quality transparency is the first requirement. It is also essential that it can be molded or ground with precision to the required prescription. And it must resist sweat and be suf- ficiently scratch-resistant to cope with normal handling. Desirables: A high refractive index and a low density allow thinner, and thus lighter, lenses. Given that, a material that is cheap allows either a lower cost for the consumer or a greater profit-margin for the maker. Chemical properties Products often have to function in hostile environments, being exposed to corrosive fluids, hot gases or 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 and ultra-violet radiation. We regard the intrinsic resistance of a material to each of these as material Digitised by the Department of Library Services, University of Pretoria, 2020  1.2 Material properties 11 Fresh water Salt water l:j""""""l NITRIC ACID.. r Acids and alkalis Organic solvents Oxidation UV radiation Figure 1.5 Chemical properties: resistance to water, acids, alkalis, organic solvents, oxidation and radiation. properties, measured on a scale of 1 (very poor) to 5 (very good). Chapter 17 deals with the material durability. Environmental properties Making, shaping, joining and finishing materials consumes nearly one-third of global energy demand. The associated emissions are already a cause for international concern, and demand for material extraction and processing is likely to double in the next 40 years. It's important, therefore, to understand the environmental properties of materials and to seek ways to use them more sustainably than we do now. Chapter 20 introduces the key ideas of material life-cycle analysis, material efficiency and material sustainability, all of which are central to the way we will use materials in the future. Digitised by the Department of Library Services, University of Pretoria, 2020 12 Chapter 1 Introduction: materials-history and character Example 1.4 Design requirements (4) What are the essential and the desirable requirements of materials for a single-use (disposable) water bottle that does minimal environmental harm? Answer. Essentials: The health aspects come first: the material of the bottle must be non-toxic and able to be processed in a way that leaves no contaminants. The bottle will meet its intended use only if its material and the process used to shape it are cheap. Desirables: The material of the bottle should be recyclable and, if possible, biodegradable. It makes handling easier if the material is not brittle, although glass bottles are widely used. 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, and those that fail to do so are not suitable. In the cartoon graphic of Figure 1.2, stiffness, strength and toughness are design- limiting-if any one of them are too low, the plane won't fly. In the design of power trans- mission 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, applying limits to them, and screening out those that do not meet the limits (Chapter 3 ). Processes, too, have properties, although we have not met them yet. These can be design-limiting as well, leading to a parallel scheme for choosing viable processes (Chapters 18 and 19). Digitised by the Department of Library Services, University of Pretoria, 2020  1.6 Exercises 13 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 materials, expressed as target values for certain design-limiting properties. A material 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, such as 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 proceed with. We turn now to the materials themselves: the families, classes and members. 1. 5 Further reading The history and evolution of materials Delmonte, J. (1985). Origins of Materials and Processes. Pennsylvania, USA: Technomic Publishing Company. ISBN 87762-420-8. (A compendium of information about materials in engineering, documenting the history.) Hummel, R. (2004). Understanding Materials Science: History, Properties, Applications (2nd ed.). New York, USA: Springer Verlag. ISBN 0-387-20939-5. Singer, C., Holmyard, E. J., Hall, A. R., Williams, T. 1., & Hollister-Short, G. (Eds.), (1954-2001). A History of Technology, 21 volumes. Oxford, UK: Oxford University Press. ISSN 0307-5451. (A compilation of essays on aspects of technology, including materials.) Tylecoate, R. F. (1992). A History of Metallurgy (2nd ed.). London, UK: The Institute of Materials. 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 a love of detail.) 1.6 Exercises Exercise E1.1 Use a search engine such as Google to research the history and uses of one of the following materials: Tin. Glass. Cement. Titanium. Carbon fibre. Present the result as a short report of about 100-200 words (roughly half a page). Digitised by the Department of Library Services, University of Pretoria, 2020 14 Chapter 1 Introduction: materials-history and character Exercise E1.2 What is meant by the design-limiting properties of a material in a given application? 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 judgment, are the design-limiting properties for the material of the blade of a knife that will be used to gut fish? Exercise E1.5 What, in your judgment, are the design-limiting properties for the material of an oven glove? Exercise E1.6 What, in your judgment, 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 carburetor of a motor-powered mower. The design requires that the tube be flexible, and that the fuel be visible. List what you think would 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 judgment, need high stiffness and low weight. Think of things that must be light (as they are moved, perhaps rapidly) but must not be too 'bendy'. Exercise E1.11 List three applications that, in your judgment, need optical-quality glass. Think of products that rely on distortion-free imaging. Exercise E1.12 List three applications that you think would require high thermal conductivity. Think of things that you have to get heat into or out of. Exercise E1.13 List three applications that you think would require low thermal expansion. Think of things that will lose accuracy or won't work if they distort. Digitised by the Department of Library Services, University of Pretoria, 2020