Modeling the dynamic crushing of closed-cell polyethylene and polystyrene foams

The geometry of low-density, closed-cell, polyethylene, and polystyrene foams was modeled, considering the shape of the cell edges, using a Kelvin foam model. Typical regions in a foam product, undergoing uniaxial compressive impact, were considered using periodic boundary conditions, for two directions in the body-centered cubic lattice. Finite element analysis predicted yield stresses, when 24% of the polymer was in the cell edges, which were 18% smaller than those of the equivalent dry foam of the same relative density of 0.046. The face deformation mechanisms were almost the same as in the dry foam. However, plastic hinges needed to form across some cell edges before the foam yielded. The cell gas contribution to the compressive stress in polyethylene foams could be calculated, to a good approximation, by considering isothermal compression of gas in a single cell with zero lateral expansion. The model explained why the lateral expansion of polystyrene foams after compressive yield is small, but significant for polyethylene foams.