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

Abstract

All spacecraft continue to face a growing risk of hypervelocity impact (HVI) by micrometeoroids and orbital debris (MMOD). Concurrently, 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 mass-specific energy absorption and tailorability. Yet, their behaviors at HVI-induced strain rates (>10⁶ s^-1) remain understudied and poorly understood, particularly near their glass transition (-116°C) and melt (130°C) temperatures. A recent HVI study revealed that, when impacted at room temperature, UHMWPE targets exhibited bulk fragmentation while similar HDPE samples showed extensive melting and visco-plastic flow, differences attributed to molecular mobility. This follow-on study probes the interplay of initial target temperature (T₀), impact velocity (v₀), and average entanglements per chain (N_e) on polyethylene’s (PE’s) HVI response. 12.7 mm thick UHMWPE and HDPE plates at T₀ = -120°C, 20°C, and 140°C were subjected to 2.5 km/s and 6.0 km/s HVIs by 6.35 mm diameter aluminum spheres. PE’s HVI response was found to be largely governed by a competition between rates of strain and polymer chain relaxation. Lowering T₀ for a fixed N_e constrained chain motion analogously to increasing N_e 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, suggesting that, for a given T₀ and v₀, there is an optimal N_e value that maximizes PE energy absorption. Increasing v₀ or decreasing T₀ necessitates a reduction in N_e 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.