Metal Foams: A Design Guide

56 Metal Foams: A Design Guide
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.
5.2 Formulating a property profile
The steps are as follows:
1. Function. Identify the primary function of the component for which a material
is sought. A beam carries bending moments; a heat-exchanger tube
transmits heat; a bus-bar transmits electric current.
2. Objective. Identify the objective. This is the first and most important quantity
you wish to minimize or maximize. Commonly, it is weight or cost;
but it could be energy absorbed per unit volume (a compact crash barrier);
or heat transfer per unit weight (a light heat exchanger) – it depends on
the application.
3. Constraints. Identify the constraints. These are performance goals that
must be met, and which therefore limit the optimization process of step 2.
Commonly these are: a required value for stiffness, S; for the load, F, or
moment, M, or torque, T, or pressure, p, that must be safely supported; a
given operating temperature implying a lower limit for the maximum use
temperature, Tmax, of the material; or a requirement that the component be
electrically insulating, implying a limit on its resistivity, R.
It is essential to distinguish between objectives and constraints. Asan
example, in the design of a racing bicycle, minimizing weight might be the
objective with stiffness, toughness, strength and cost as constraints (‘as light
as possible without costing more than $500’). But in the design of a shopping
bicycle, minimizing cost becomes the objective, and weight becomes
a constraint (‘as cheap as possible, without weighing more than 25 kg’).
4. Free variables. The first constraint is one of geometry: the length, ℓ, and
the width, b, of the panel are specified above but the thickness, t, is not – it
is a free variable.
5. Lay out this information as in Table 5.1.
6. Property limits. The next three constraints impose simple property limits;
these are met by choosing materials with adequate safe working temperature,
which are electrical insulators and are non-magnetic.
7. Material indices. The final constraint on strength (‘plastic yielding’) is
more complicated. Strength can be achieved in several ways: by choice of
material, by choice of area of the cross-section, and by choice of crosssection
shape (rib-stiffened or sandwich panels are examples), all of which