Chapter4 - Materials selection charts 4.1 Introduction and...

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Materials selection charts 4.1 Introduction and synopsis Material properties limit performance. We need a way of surveying properties, to get a feel for the values design-limiting properties can have. One property can be displayed as a ranked list or bar-chart. But it is seldom that the performance of a component depends on just one property. Almost always it is a combination of properties that matter: one thinks, for instance, of the strength- to-weight ratio, σ f / ρ , or the stiffness-to-weight ratio, E/ which enter lightweight design. This suggests the idea of plotting one property against another, mapping out the fields in property-space occupied by each material class, and the sub-fields occupied by individual materials. The resulting charts are helpful in many ways. They condense a large body of information into a compact but accessible form; they reveal correlations between material properties which aid in checking and estimating data; and they lend themselves to a performance-optimizing technique, developed in Chapter 5, which becomes the basic step of the selection procedure. The idea of a materials selection chart is described briefly in the following section. The section after that is not so brief: it introduces the charts themselves. There is no need to read it all, but it is helpful to persist far enough to be able to read and interpret the charts fluently, and to understand the meaning of the design guide lines that appear on them. If, later, you use one chart a lot, you should read the background to it, given here, to be sure of interpreting it correctly. A compilation of all the charts, with a brief explanation of each, is contained in Appendix C of this text. It is intended for reference - that is, as a tool for tackling real design problems. As explained in the Preface, you may copy and distribute these charts without infringing copyright. 4.2 Displaying material properties The properties of engineering materials have a characteristic span of values. The span can be large: many properties have values which range over five or more decades. One way of displaying this is as a bar-chart like that of Figure 4.1 for thermal conductivity. Each bar represents a single material. The length of the bar shows the range of conductivity exhibited by that material in its various forms. The materials are segregated by class. Each class shows a characteristic range: metals, have high conductivities; polymers have low; ceramics have a wide range, from low to high. Much more information is displayed by an alternative way of plotting properties, illustrated in the schematic of Figure 4.2. Here, one property (the modulus, E, in this case) is plotted against another (the density, ) on logarithmic scales. The range of the axes is chosen to include all materials, from the lightest, flimsiest foams to the stiffest, heaviest metals. It is then found that data for a given class of materials (polymers for example) cluster together on the chart; the sub-range associated with one material class is, in all cases, much smaller than thefull range of that property. Data for
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Chapter4 - Materials selection charts 4.1 Introduction and...

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