Recent development of nanoscale ballistic testing has shown the applicability of nanomaterials for ballistic protection but have raised questions regarding the structure-property relationships leading to enhanced ballistic responses at the nanoscale. In this work, large-scale (multi-million atom) reactive Molecular Dynamics (MD) simulations are used to simulate the impact and ballistic performance of Polyethylene (PE) and Polystyrene (PS) thin films.  The large-scale nature of the MD simulations (multi-million atoms) in conjunction with the 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 that has yet to be observed directly without extrapolation or time-temperature superposition.  Micro-particle ballistic experiments have shed light on the dominant mechanisms of energy absorption during impacts, as  the work of fracture, yielding, bending of the film, and adiabatic heating have been identified to occur in different polymers to varying degrees. However, the molecular mechanisms leading to the dominance of one energy absorption mechanism over the other are unknown. The reported simulated specific penetration energy (E~p~) versus impact velocity in this work is shown to predict the experimentally determined E~p~ of PS to within 15% for a range of supersonic impact velocities.  At low impact velocity (\< 1,000 m/s), crazing/tearing failure dominates due to entanglement pullout.  At high impact velocities (> 1,000 m/s), the failure mode is governed by fragmentation associated with an increasing number of ruptured chains.  Interestingly, PE systems exhibit a higher E~p~ at low velocities due to a higher entanglement density than PS, whereas PS exhibits a higher E~p~ at elevated velocities due to preferential chain scission over chain pullout.  For supersonic velocities, the E~p~ 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.  The temperature evolution during impact is also studied. Polystyrene is shown to undergo a larger temperature increase during impact compared to polyethylene.  Even at low impact velocities (\< 1,000 m/s) temperatures in polystyrene can rise above the glass transition temperature causing a more ductile-like failure. At hypersonic velocities, temperatures can spike into the thousands of degrees leading to the dissociation of bonds and enhances the fragmentation of chains.  In combination with experiments, these simulations provide insights into the underlying molecular failure mechanisms associated with nanoscale ballistic impacts and provides a useful tool for the design and development of impact resistance nanomaterials.

"Permission to publish was granted by Director, Geotechnical & Structures Laboratory."


Supersonic Impact Response of Polymers via Large-Scale Molecular Dynamics Simulations

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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