Temperature Dependence of the Hypervelocity Impact Response of Polyethylene Plates from Tg to Tm

All spacecraft continue to face a growing risk of hypervelocity impacts (HVIs) by micrometeoroids and orbital debris (MMOD). Similarly, emerging hypersonic weapons pose acute ballistic threats to military and civilian assets. In both cases, the diminishing effectiveness of legacy armor demands the development of specialized, layered HVI protective structures. Ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) stand out as promising intermediate layers due to their high specific energy absorption and tailorability. Yet, polyethylene's (PE's) behavior at HVI-induced strain rates (>106 s-1) remains understudied and poorly understood, particularly near its glass transition (-116°C) and melt (130°C) temperatures. A recent HVI study revealed that UHMWPE targets impacted at room temperature exhibited bulk fragmentation while similar HDPE samples showed extensive melting and visco-plastic flow, as a consequence of differences in the two polymers' molecular mobility. In this current study, the interplay of target temperature (T0), impact velocity (v0), and average entanglements per chain (Ne) on PE's HVI response is investigated. 12.7 mm thick UHMWPE and HDPE plates at T0 = -120°C, 23°C, and 140°C were subjected to 2.5 km/s and 6.0 km/s HVIs by 6.35 mm diameter aluminum spheres. The PEs' HVI responses were found to be largely governed by a competition between rates of strain and polymer chain relaxation. Lowering T0 for a fixed Ne constrained chain motion analogous to increasing Ne at a fixed T0. This caused HDPE's HVI response to increasingly align with UHMWPE's at similar v0. The opposite was also observed. Increasing v0 alone made both materials more prone to widespread fracture by raising strain rates beyond rates of chain disentanglement and reorientation. The material exhibiting the most visco-plastic flow without subsequent bulk fragmentation lost less mass, had smaller perforations, and better absorbed energy. This suggested that for fixed T0 and v0, there is an optimal Ne value that maximizes a given PE's energy absorption. Increasing v0 or decreasing T0 requires a lower degree of entanglement to sustain the degree of molecular mobility that gives maximum energy absorption. These findings motivate the development of a protective structure composed of PE layers, each optimized for an anticipated average strain rate.