Chapter3 - Engineering materials and their properties 3.1...

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Engineering materials and their properties 3.1 Introduction and synopsis Materials, one might say, are the food of design. This chapter presents the menu: the full shopping list of materials. 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: brings out their flavour, so to speak. The classes of materials - metals, polymers, ceramics, and so forth - are introduced in Section 3.2. But it is not, in the end, a material that we seek; it is a certain profile of properties. The properties important in thermo-mechanical design are defined briefly in Section 3.3. The reader confident in the definitions of moduli, strengths, damping capacities, thermal conductivities and the like may wish to skip this, using it for reference, when needed, for the precise meaning and units of the data in the selection charts which come later. The chapter ends, in the usual way, with a summary. 3.2 The classes of engineering material It is conventional to classify the materials of engineering into the six broad classes shown in Figure 3.1 : metals, polymers, elastomers, ceramics, glasses and composites. The members of a class have features in common: similar properties, similar processing routes, and, often, similar applications. Metals have relatively high moduli. They can be made strong by alloying and by mechanical and heat treatment, but they remain ductile, allowing them to be formed by deformation processes. Certain high-strength alloys (spring steel, for instance) have ductilities as low as 2%, but even this is enough to ensure that the material yields before it fractures and that fracture, when it occurs, is of a tough, ductile type. Partly because of their ductility, metals are prey to fatigue and of all the classes of material, they are the least resistant to corrosion. Ceramics and glasses, too, have high moduli, but, unlike metals, they are brittle. Their ‘strength’ in tension means the brittle fracture strength; in compression it is the brittle crushing strength, which is about 15 times larger. And because ceramics have no ductility, they have a low tolerance for stress concentrations (like holes or cracks) or for high contact stresses (at clamping points, for instance). Ductile materials accommodate stress concentrations by deforming in a way which redistributes the load more evenly; and because of this, they can be used under static loads within a small margin of their yield strength. Ceramics and glasses cannot. Brittle materials always have
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Engineering materials and their properties 21 Fig. 3.1 The menu of engineering materials. a wide scatter in strength and the strength itself depends on the volume of material under load and
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Chapter3 - Engineering materials and their properties 3.1...

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