A Numerical Investigation of the Dynamic Behaviour of Functionally Graded Foams

Two Finite Element models approximating the dynamic behaviour of functionally graded foam materials (FGFMs) have been developed under free weight drop impact and Kolsky wave propagation conditions. The FGFM is modeled by discretising the material into a large number of layers through the foam thickness. Each layer is described by a unique constitutive cellular response, which is derived from the initial relative density, ρ∗, unique to that layer. Large strain uniaxial compressive tests at strain rates of 0.001, 0.01 and 0.1/s were performed on expanded polystyrene (EPS) and ALPORASⓇ Aluminium (Al) foam and their σ − ε response was used as input to a modified constitutive model from the literature. Simulations were then performed on both uniform and graded specimens. For both impact and wave propagation conditions it is found that under certain conditions an FGFM can outperform a uniform foam of equivalent density in terms of reducing peak accelerations imparted from an impact, or mitigating stress wave magnis through increased plastic deformation. These properties provide significant insight into the hypothesised behaviour of FGFMs and elucidate the potential for the future use in the design of next generation cushioning structures.