Finite Element Models for the Viscoelasticity of Open-Cell Polyurethane Foam

Finite Element Analysis (FEA) was used to model viscoelastic effects in the compression of polyurethane (PU) foam. Modelling of the impact compression of foam cubes predicted that a compressed region propagated through the sample, causing step increases in the stress traces. Modelling of head impacts on gymnastic ‘crash’ mats, made of PU chip foam, revealed that the majority, but not all, of the energy losses could be attributed to the polymer viscoelasticity. The Gibson-Ashby micromechanics model was used to show that the hysteresis in a viscoelastic foam could be higher than in the parent polymer. A wet Kelvin foam, with a better foam geometry, was then used for viscoelastic analysis. The lattice of uniform-sized cells, in a Body Centred Cubic array, were compressed in the [111] direction. With the PU treated as a linear viscoelastic material, the predicted hysteresis in the cyclic stress-strain curve was slightly smaller than that measured for a PU flexible foam.