Intergranular Hotspots: A Molecular Dynamics Study on the Influence of Compressive and Shear Work

TL;DR

This study uses molecular dynamics simulations to distinguish the effects of compression and shear on hotspot formation at crystal interfaces in high explosives, revealing compression's dominant role and shear's conditional influence.

Contribution

It provides a novel simulation approach to isolate and compare the effects of compressive and shear work on hotspot localization in explosive materials.

Findings

Compression significantly localizes temperature and strain energy.

Shear increases energy localization only at low shock pressures (~10 GPa).

Hotspots are highly localized within 5-20 nm of interfaces.

Abstract

Numerous crystal- and microstructural-level mechanisms are at play in the formation of hotspots, which are known to govern high explosive initiation behavior. Most of these mechanisms, including pore collapse, interfacial friction, and shear banding, involve both compressive and shear work done within the material and have thus far remained difficult to separate. We assess hotspots formed at shocked crystal-crystal interfaces using quasi-1D molecular dynamics simulations that isolate effects due to compression and shear. Two high explosive materials are considered (TATB and PETN) that exhibit distinctly different levels of molecular conformational flexibility and crystal packing anisotropy. Temperature and intra-molecular strain energy localization in the hotspot is assessed through parametric variation of the crystal orientation and two velocity components that respectively modulate compression and shear work. The resulting hotspots are found to be highly localized to a region within 5-20 nm of the crystal-crystal interface. Compressive work plays a considerably larger role in localizing temperature and intra-molecular strain energy for both materials and all crystal orientations considered. Shear induces a moderate increase in energy localization relative to unsheared cases only for relatively weak compressive shock pressures of approximately 10 GPa. These results help isolate and rank the relative importance of hotspot generation mechanisms and are anticipated to guide the treatment of crystal-crystal interfaces in coarse-grained models of polycrystalline high explosive materials.