Abstract
A finite element, functionally graded foam model (FGFM) is proposed, which is shown to provide more effective energy absorption management, compared to homogenous foams, under low energy impact conditions. The FGFM is modelled by discretising a virtual foam into a large number of element layers through the foam thickness. Each layer is described by a unique constitutive cellular response, which is derived from the initial foam density, ρ, unique to that layer. Large strain unixial compressive tests at a strain rate of 0.001 s-1 are performed on expanded polystyrene (EPS), and their σ −ε response is used as input to a modified constitutive model from the literature. It is found that under low energy impacts an FGFM can outperform a uniform foam of equivalent density terms of reducing peak accelerations, while performing almost as effectively as uniform foams under high energy conditions. These novel materials, properly manufactured, could find use as next generation helmet liners in answer to recent, more rigorous equestrian helmet standards, e.g. BS EN 14572:2005.