Hysteresis and energy loss in flexible polyurethane foams

The behaviour of open cell flexible polyurethane foams in energy management applications, such as automotive seating, where static and dynamic comfort are the main functional attributes, is governed by two material properties: the effective stiffness; and energy lost through hysteresis under the conditions that prevail in use. In most applications the foam is subject to combined stresses (tension, shear and compression) and in some characterization procedures, such as indentation-force-deflection and ball rebound, these conditions prevail. However to elucidate the mechanisms responsible for behaviour a simple deformation regime is normally employed, in particular, simple compression. The imposed static strains may be large (40–70%) which means in practical terms that we are dealing with a highly non-linear static and dynamic mechanical situation. This is manifest by the fact that under high intensity harmonic excitation a flexible foam-based vibration isolating system exhibits behaviour similar to that of classical chaos [1].