Chapter 6 - The mechanics of foams: refinements

Summary

Introduction and synopsis

In Chapter 5 we described the uniaxial behaviour of isotropic foams. Expressions for their elastic moduli and compressive strengths, and their resistance to tensile fracture and fatigue, were derived assuming that the temperature and rate of loading corresponded to those for which the cell wall properties are tabulated (typically, T = 20°C and ɛ = 10⁻³/s). In practice, foams are often anisotropic. And in many engineering applications, they are loaded in more than one direction and at high rates or different temperatures.

Military, aerospace, automative and packaging applications often require a knowledge of foam properties at high rates of deformation and at temperatures other than room temperature. Here the modulus and failure-mechanism diagrams of Chapter 3 become useful. The moduli and strength of rigid foams (metals as well as polymers) decrease linearly as the temperature rises. Increasing the strain-rate does not affect their moduli but increases their strength. Semirigid foams (those used at a temperature close to the glass temperature T_g of the base polymer) are more complicated: both their moduli and strength increase with strain-rate, sometimes dramatically. Elastomeric foams are different again: their moduli increase slightly with increasing temperature, but are almost independent of strain-rate.

These effects are directly related to the temperature and strain-rate dependence of the cell-wall properties (Chapter 3): they are inherent properties of the material of which the foam is made.