High strain rate compressive response of ultra-high molecular weight polyethylene fibre composites

The mechanisms of deformation during the dynamic in-plane compression of 〖[0^o/〖90〗^o]〗_n (cross-ply) ultra-high molecular weight polyethylene (UHMWPE) fibre composites with polymeric matrices have been investigated for strain rates in the range 0.01 s^(-1) to 4000 s^(-1). The measured strain rate sensitivity was mild for strain rates less than about 100 s^(-1), but increased sharply at higher rates. X-ray computed tomography and optical microscopy revealed that over the range of strain rates investigated here, the deformation mechanism was kinking (micro-buckling) of the plies with a kink band width of about 1 mm. Ply delamination was also observed, but only during softening phase of the response after the peak strength had been attained. To gain a mechanistic understanding of the observed strain rate sensitivity, finite element (FE) simulations were used to model the compression experiments. For these calculations, each specimen ply was explicitly modelled via a pressure-dependent crystal plasticity framework that accounts for the large shear strains and fibre rotations that occur within each ply in the kink band. Calculations were conducted in the limits of perfectly-bonded and completely un-bonded plies. Good agreement between measurements and predictions was obtained when plies were assumed to be perfectly bonded, confirming the hypothesis that ply delamination plays a small role in setting the peak strength as well as the compressive response of the composite at moderate levels of applied strain. The calculations also show that misalignment of the specimen between the compression platens strongly influences the compression response and especially the initial stiffness. Importantly, the FE calculations reveal that over the range of strain rates investigated here, inertial stabilisation has a negligible contribution to the strong rate sensitivity observed for strain rates above 100 s^(-1) and that this sensitivity is primarily associated with the strain rate sensitivity of the polymeric matrix.