Supersonic Impact Response of Polymer Thin Films via Large-Scale Atomistic Simulations
Andrew L. Bowman, Edwin P. Chan, William B. Lawrimore, John K. Newman
Recent nanoscale ballistic testing studies have shown the applicability of nanomaterials for ballistic protection but have raised new questions regarding the nanoscale structure-property relationships that contribute to the enhanced ballistic response of thin films. Here we report on multi-million atom reactive Molecular Dynamics (MD) simulations of the supersonic impact, penetration, and failure response of polymer thin films of polyethylene (PE) and polystyrene (PS). The reported simulated specific penetration energy (Ep*) versus impact velocity predicts to within 15% the experimentally determined Ep* for polystyrene. For low impact velocities (<1 km s-1), the polymers undergo a crazing/petalling failure mode due to chain disentanglement. For high impact velocities (>1.5 km s-1), the failure mode transitions to fragmentation dominant due to increased propensity for chain scission. The temperature evolution around the impact zone is characterized and shows that impacts in PS produce an overall higher temperature, which can be above the glass transition temperature even for low velocity impacts. Interestingly, our results reveal that the two materials systems display unique energy dissipation mechanisms. The PE systems exhibit a higher Ep* at low velocities due to a higher entanglement density, whereas PS exhibits a higher Ep* at elevated velocities due to an adiabatic heating induced glass transition occurring at impact velocities greater than 1 km s-1. Finally, our simulations also have capabilities to simulate hypervelocity impacts to show that polymers can rival the ballistic response of pristine graphite due to their high strain rate sensitivity.
Molecular Dynamics, Energy Absorption, LIPIT, Ballistic Impact, Adiabatic Heating, Thin Film