Due to their high specific ballistic energy and high strain rate dependence, polymers are good candidates for lightweight protection against high velocity ballistic threats. Recent experiments using the Laser Induced Particle Impacts Testing (LIPIT) technique have shown enhanced abilities of polymer thin films to absorb remarkable amounts of energy during ballistic impacts.&nbsp; However, In-situ observation of the molecular-level failure and shock wave dissipation mechanisms are difficult or impossible and result in a large knowledge gap limiting the understanding and application of nanomaterials in regards to ballistic mitigation. This gap in understanding is compounded during high pressure confinement as a critically connected relationship between chain dynamics, temperature, and pressure becomes more enigmatic as shock waves travel through the material at high strain rates and pressures.&nbsp; During these shock wave events, the material state can be changed by inducing phase transitions (e.g. glass transition, melting, etc.), disassociating bonds, and catastrophically damaging the material.&nbsp;&nbsp; In this work, we use large-scale Molecular Dynamics (MD) simulations with reactive bond-breaking capabilities to gather insights into the nanoscale energy dissipation mechanisms of unconfined and confined polymers.&nbsp; Multi-layered graphene (MLG) sheets are used to create various multilayered graphene-polyethylene (PE) configurations, such as a PE with a MLG backing layer, and confined PE sandwiched between graphene.&nbsp;By varying the number and configuration of PE and MLG layers (e.g. no MLG, single backing MLG layer, and sandwich layup), the level of confinement pressure on the PE can be increased.&nbsp; The resulting PE thin films, graphene films, and confined PE composites are subjected to a range of supersonic (500 -1500 m/s) and hypersonic (2,000 – 7,500 m/s) impacts to determine the amount of energy dissipation for the given material system.&nbsp; Unconfined PE experiences large void formation and chain pullout due to a high entanglement density and low internal chain friction (small monomeric friction coefficient).&nbsp;&nbsp; With increasing levels of confinement, local chain motion is reduced increasing the frictional drag and increasing the energy absorption of the polymer.&nbsp; By increasing frictional drag, the energy dissipation mechanisms transitions from chain disentanglement to bond scission, subsequently absorbing higher amounts of energy.&nbsp; Pressure and temperature evolutions with the PE composite films are shown to reach pressures greater than 5 GPa and temperatures greater than 2,000 K for the short impact duration.&nbsp; This work provides a greater understanding of the molecular mechanisms of energy absorption for confined polymers and illustrates how such material systems have the potential to be optimized to provide enhance ballistic resistance.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;

Graphene Confined Polymer Thin Films Subjected to Supersonic Impact

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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