Atomistics-consistent continuum models and pore-collapse-generated hotspot temperatures in energetic crystals I: beta-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (beta-HMX)

Hotspot formation due to pore collapse is a key mechanism for initiating detonation of shocked energetic materials. Energy localization at and around the pore collapse site leads to high-temperature hotspots, initiating chemical reactions. Because chemical reaction rates depend sensitively on temperature, predictive continuum models need to get the pore-collapse dynamics and resulting hotspot temperatures right; this imposes stringent demands on the fidelity of thermophysical model forms and parameters, and on the numerical methods employed to perform high-resolution meso-scale calculations. Here, continuum material models for beta-HMX are examined in the context of nanoscale shock-induced pore collapse, treating predictions from molecular dynamics (MD) simulations as ground truth. Using MD-consistent material properties, we show that the currently available strength models for HMX fail to correctly capture pore collapse and hotspot temperatures. Insights from MD are then employed to advance a Modified Johnson-Cook (M-JC) strength model form that captures aspects of shear strain and strain-rate dependency not represented by the standard JC form, but which are shown to be critical for accurately describing the nanoscale physics of shock-induced localization in HMX. The study culminates in a fully MD-determined strength model for beta-HMX that produces continuum pore-collapse results well aligned in all aspects with those predicted by MD, including pore-collapse mechanism and rate, shear-band formation in the collapse zone, and temperature, strain, and stress fields in the hotspot zone and surrounding material. The resulting MD-informed/MD-determined M-JC model should improve the fidelity of simulations to predict the detonation initiation of HMX-based energetic materials in microstructure-aware multi-scale frameworks.