Ultra-High Strain Rate Impact Response of Polyethylene at Critical Transition Temperatures

All spacecraft are increasingly at risk of hypervelocity impact (HVI) from micrometeoroids and orbital debris (MMOD). Meanwhile, new hypersonic weapons pose significant ballistic threats to military and civilian assets. In both scenarios, the declining effectiveness of traditional armor necessitates the creation of specialized, layered HVI protective structures. Ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) are promising intermediate layers due to their high mass-specific energy absorption and customizability. However, their behavior at HVI-induced strain rates (>10⁶ s⁻¹) is not well understood, particularly near their glass transition (−116°C) and melt (130°C) temperatures. A recent HVI study showed that UHMWPE targets impacted at room temperature experienced bulk fragmentation, while HDPE samples exhibited extensive melting and visco-plastic flow. This follow-up study examines the effects of initial target temperature (₀), impact velocity (₀), and average entanglements per chain () on polyethylene’s (PE’s) HVI response. UHMWPE and HDPE plates, 12.7 mm thick, at ₀ ≈ −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. The HVI response of PE was found to be primarily influenced by the competition between strain rates and polymer chain relaxation. Lowering ₀ for a fixed limited chain motion similarly to increasing  at a fixed ₀, causing HDPE’s HVI response to resemble UHMWPE’s at similar ₀. Conversely, increasing ₀ made both materials more susceptible to widespread fracture by increasing strain rates beyond the rates of chain disentanglement and reorientation. The material that exhibited the most visco-plastic flow without subsequent bulk fragmentation lost less mass, had smaller perforations, and better absorbed energy, suggesting an optimal  value that maximizes PE energy absorption for a given ₀ and ₀. Increasing ₀ or decreasing ₀ requires reducing  to maintain the molecular mobility necessary for maximum energy absorption. These findings support the development of a protective structure composed of PE layers, each optimized for an anticipated average strain rate.