Hypervelocity impact response of monolithic UHMWPE and HDPE plates

When developing layered or architected protective structures to mitigate hypervelocity impacts (HVIs), understanding and characterizing the ultra-high rate response and energy dissipation of each constituent is critical. Incorporation of lightweight polymeric materials as intermediate or inner-most structural layers could optimize HVI damage resistance and tolerance without compromising cost or weight. One key challenge is developing a fundamental understanding of the effects of molecular architecture on the macroscopic dynamic material response and damage formation. In this work, two common and affordable thermoplastics, namely ultra-high molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE), were assessed. Flat square targets of two distinct sizes were subjected to a series of normal HVIs with 10 mm diameter 1050 aluminum spheres at velocities in the range 2-6.5 km/s. The debris cloud velocity, mass loss, and perforation radius were found to be functions of impact velocity for both materials. High-speed images show HVIs to UHMWPE resulted in quasi-brittle responses while HVIs to HDPE resulted in apparent bulk-melting of the material and large-scale plastic deformation. When subjected to HVIs in the tested range, UHMWPE plates exhibited greater mass loss than similar HDPE plates despite the perforation radii being larger in HDPE. This suggests that the momentum and kinetic energy of the debris clouds for UHMWPE targets were greater than that for HDPE targets subjected to identical impacts. These HVI experimental results combined with polymer material characterization data indicate that differences in the polymeric molecular structure and chemistry of the polyethylenes affect their macroscopic HVI performance.