## 51 Background

A property profile is a statement of the characteristics required of a material if it is to perform well in a given application. It has several parts. First, it identifies simple property limits which are dictated by constraints imposed by the design: requirements for electrical insulation or conduction impose limits on resistivity; requirements of operating temperature or environment impose limits on allowable service temperature or on corrosion and oxidation resistance. Second, it identifies material indices which capture design objectives: minimizing weight, perhaps, or minimizing cost, or maximizing energy storage. More precisely, a material index is a grouping of material properties which, if maximized or minimized, maximizes some aspect of the performance of an engineering component. Familiar indices are the specific stiffness, E/p, and the specific strength, oy/p, (where E is Young's modulus, oy is the yield strength or elastic limit, and p is the density), but there are many others. They guide the optimal selection of established materials and help identify potential applications for new materials. Details of the method, with numerous examples are given in Ashby (1999). PC-based software systems that implement the method are available (see, for example, CES, 1999).

Section 5.2 summarizes the way in which property profiles, indices and limits are derived. Section 5.3 gives examples of the method. Metal foams have particularly attractive values of certain indices and these are discussed in Section 5.4. A catalogue of indices can be found in the Appendix at the end of this Design Guide.

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