Experimental techniques and identification of nonlinear and viscoelastic properties of flexible polyurethane foam

Identification of the vibrational behavior of polyurethane foams used in automotive seats is described. The dynamic system consists of a rigid block mounted on a 3' cube of foam material, which serves as the only flexible component. When constrained to undergo linear unidirectional motion, the dynamic system is modeled as a single degree of freedom system, governed by an integro-differential equation. In addition to a relaxation kernel representing the linear viscoelastic behavior of the foam, the model includes a polynomial type stiffness to account for the foam's strain-based nonlinearities. The relaxation kernel is assumed to be of an exponential type. Experimental methodologies for obtaining repeatable, accurate measurements of the system's response to an impulse and to single frequency harmonic base excitations are described. Analysis methods are then investigated for extracting the relevant linear, nonlinear, and viscoelastic parameters. Characterization of these foam properties as functions of compression level is also presented.