Introduction and synopsis
Foams can be made out of almost anything: metals, plastics, ceramics, glasses, and even composites. Their properties depend on two separate sets of parameters. There are those which describe the geometric structure of the foam – the size and shape of the cells, the way in which matter is distributed between the cell edges and faces, and the relative density or porosity; these are described in Chapter 2. And there are the parameters which describe the intrinsic properties of the material of which the cell walls are made; those we describe here.
The cell wall properties which appear most commonly in this book are the density, ρs, the Young's modulus, Es, the plastic yield strength, σys, the fracture strength, σfs, the thermal conductivity, λs, the thermal expansion coefficient, αs, and the specific heat Cps. Throughout, the subscript ‘s’ indicates a property of the solid cell wall material while a superscripted ‘*’ refers to a property of the foam itself. Thus, E*/Es means ‘the foam modulus divided by that of the cell wall material’; this is also referred to as ‘the relative modulus’.
It is helpful to start with an overview of the properties of solid materials, which have values which lie in certain characteristic ranges. A perspective on these is given by material property charts (Ashby, 1992), of which Figs. 3.1 and 3.2 are examples. The first shows Young's modulus, Es, plotted against the density, ρs. Metals lie near the top right: they have high moduli and high density.