Due to their high specific ballistic energy and high strain rate dependence, polymers have been identified as candidates for lightweight protection against ballistic impacts. Recent development of nanoscale ballistic testing has displayed the remarkable impact resistance properties of nanomaterials but have raised questions regarding the structure-property relationships leading to enhanced ballistic responses at the nanoscale. Nanoscale materials exhibit size-dependent behaviors that vary greatly from their macroscale counterparts; both favorably and unfavorably. However, nanoscale experiments are difficult to perform, and in-situ observation of nanoscale energy dissipation mechanisms are difficult or impossible, resulting in a large knowledge gap. In this work, multimillion atom reactive Molecular Dynamics (MD) simulations are used to predict the ballistic impact performance of polyethylene (PE) and polystyrene (PS) thin films and to observe the failure mechanisms under impact. The large-scale nature of the presented MD simulations in conjunction with small scale experimental ballistic testing of nanoscale thin films (~60 nm film thicknesses) minimizes the size discrepancy typically found between MD simulations and experiments and provides a comparison not typically observed without extrapolation. The reported simulated specific penetration energy (Ep*) versus impact velocity in this work is shown to predict the experimentally determined Ep* of PS to within 15% for a range of supersonic impact velocities.  At low impact velocity (&lt; 1,000 m/s), crazing/tearing failure dominates due to entanglement pullout. At high impact velocities (&gt; 1,000 m/s), the failure mode transitions to fragmentation dominant due to increased propensity for chain scission. Interestingly, due to a very high entanglement density, PE systems exhibit a higher Ep* at low velocities than PS, whereas PS exhibits a higher Ep at elevated velocities due to an adiabatic heating induced glass transition and increased chain mobility. For supersonic velocities, the Ep* for each material is shown to scale with the entanglement density and closely follows trends observed through micro-particle ballistic testing within the same velocity range. Finally, we simulate hypervelocity impacts to show that polymers can rival the ballistic response of pristine graphite due to their high strain rate sensitivity. Permission to publish was granted by Director, Geotechnical & Structures Laboratory.

Supersonic impact response of polymers via multimillion atom molecular dynamics simulations

ACS Spring Meeting 2021

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