5.1 Introduction and Synopsis

This chapter sets out the basic procedure for selection, establishing the link between material and function (Figure 5.1). A material has attributes: its density, strength, cost, resistance to corrosion, and so forth. A design demands a certain profile of these: a low density, a high strength, a modest cost and resistance to sea water, perhaps. It is important to start with the full menu of materials in mind; failure to do so may mean a missed opportunity. If an innovative choice is to be made, it must be identified early in the design process. Later, too many decisions have been taken and commitments made to allow radical change: it is now or never. The task, restated in two lines, is that of

Figure 5.1: Material selection is determined by function. Shape sometimes influences the selection. This chapter and the next deal with materials selection when this is independent of shape.

1. identifying the desired attribute profile and then

2. comparing it with those of real engineering materials to find the best match.

The first step in tackling it is that of translation, examining the design requirements to identify the constraints that they impose on material choice. The immensely wide choice is narrowed, first, by screening-out the materials that cannot meet the constraints. Further narrowing is achieved by ranking the candidates by their ability to maximize performance. Criteria for screening and ranking are derived from the design requirements for a component by an analysis of