Introduction and synopsis
The selection strategy
Figure 5.2 illustrates how the Kingdom of Materials can be subdivided into families, classes,
subclasses and members. Each member is characterized by a set of attrributes: its properties. As
an example, the Materials Kingdom contains the family ‘Metals’ which in turn contains the class
‘Aluminium alloys’, the subclass ‘5000 series’ and finally the particular member ‘Alloy 5083 in the
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. The problem is that of identifying
the desired attribute
profile and then comparing it with those of real engineering materials to find the best match. This
we do by, first, screening and ranking the candidates to give a shortlist, and then seeking detailed
for each shortlisted candidate, allowing
a final choice. 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 immensely wide choice is narrowed, first, by applying property limits which screen out the
materials which cannot meet the design requirements. Further narrowing is achieved by ranking
the candidates by their ability to maximize performance. Performance is generally limited not by
a single property, but by a combination of them. The best materials for a light stiff tie-rod are
those with the greatest value of the 'specific stiffness', El p, where E is Young's modulus and p the
density .The best materials for a spring, regardless of its shape or the way it is loaded, are those with
the greatest value of a} I E , where a f is the failure stress. The materials which best resist thermal
shock are those with the largest value of a f I Ea, where a is the thermal coefficient of expansion;
and so forth. Combinations such as these are called material indices: they are groupings of material
properties which, when maximized, maximize some aspect of performance. There are many such
indices. They are derived from the design requirements for a component by an analysis of function,
objectives and constraints. This chapter explains how to do this.
The materials property charts introduced in Chapter 4 are designed for use with these criteria.
Property limits and material indices are plotted onto them, isolating the subset of materials which