Shock-driven Jetting at HMX Crystal Surface Asperities

Abstract

Shock-induced jetting and recompression are plausible mechanisms for hot spot formation in energetic materials, but remain poorly understood. To this end, we apply all-atom nonreactive molecular dynamics (MD) simulations of a shock-loaded sinusoidal surface asperity in the crystalline explosive HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) to quantify the roles of shock strength, asperity geometry, system size, and crystallographic orientation in determining the jetting response. Our simulations span a shock pressure interval of $\approx$4-47 GPa, which captures responses in both detonation initiation and steady-state detonation regimes. We find that shock strength and asperity curvature exert strong influence on jet velocity, temperature, and morphology, while feature size and crystallographic orientation play more modest roles. Jet morphology correlates strongly with the HMX ambient-pressure phase diagram and the narrow temperature range associated with its liquid phase. Coherent jets form when focusing and dissipation are sufficient to drive local temperatures above the melting point, while formation of molecular plumes is seemingly connected to jet temperatures exceeding the liquid-vapor transition temperature. Formation of plumes is unlike the jetting responses seen in metals, which typically exhibit liquid phases that are stable over wider temperature intervals. This work proposes a simple hypothesis for the connection between jet morphology and ambient-pressure phase diagrams that can be readily tested in other explosives and metals. It also provides a foundation for ongoing nonreactive and reactive MD simulations of jet impact and recompression to quantify the role played by jets in forming hot spots in HMX-based explosives.