Influence of microstructure on the failure of ultra-high molecular weight polyethylene composite beams impacted by blunt projectiles

Increasing the fibre tensile strength is well-known to improve the ballistic performance of ultra-high molecular weight polyethylene (UHMWPE) fibre composite beams. Here we investigate modifying ply microstructure as an alternate way to enhance ballistic performance. Specifically, we numerically investigate the mechanisms via which failure is initiated in [0^o/90^o]_nlaminates comprising UHMWPE tape plies and compare with the commonly used UHMWPE fibre composites. Each ply of the beam is discretely modelled using a pressure-dependent crystal plasticity framework with the deformation mechanisms of the tape and fibre-based microstructures modelled by recognising differences in the available slip system in these two material systems. The numerical calculations are used to construct failure mechanism maps for a range of impact velocities, ply shear strengths and ply tensile strengths. Three dominant failure mechanisms emerged from the study: (i) failure of plies immediately under the projectile via an indirect tension mechanism in which compressive stress imposed normal to the plies by the projectile induces tensile in-plane ply stress due to the anisotropic expansion of the alternating 0^o/90^o plies; (ii) failure due to tensile straining at the rear of the impacted beam resulting from a combination of beam bending and stretching and (iii) a shear plugging like failure mode due to tensile straining at the edge of the impact site. Tensile stress generation by indirect tension is significantly reduced in the tape-based microstructures due to the unavailability of the slip systems that result in lateral plastic expansion. In fact, our calculations suggest that over the entire range of parameters modelled here the tape-based microstructures outperform the fibre-based microstructures. This suggests an alternate route to enhancing the ballistic performance compared to the traditional methods that rely mainly on increasing fibre strength.