Crystal-scale modelling of pore collapse in cyclotetramethylene tetranitramine (HMX) under different shock strengths

A physically-based anisotropic thermal-mechanical-chemical crystal model is developed for cyclotetramethylene tetranitramine (HMX), which includes nonlinear thermoelasticity, inelasticity due to both dislocation-based plasticity and shear cracking damage, melting, and single step chemical reaction. Based on the proposed model, thermal-mechanical-chemical responses and related physics of shock induced pore collapse in HMX single crystal under low to high strengths were investigated. Under low shock strength (3.4 GPa), obvious orientation dependency of pore collapse is shown for (011) and (110) orientations, where dislocation slip and shear cracking are the dominant mechanisms. The collapsing pore morphologies of the two orientations are distinctly different. Under medium shock strength (8.8 GPa), adiabatic shear bands around the pore area become prominent and they develop along ∼45° relative to the shock direction. Under strong shock strength (23.8 GPa), significant adiabatic shear bands are formed, and the hydrodynamic jetting and impingement afterwards directly lead to the formation of hot spot and eventually the ignition. For the first time, this work clarifies the pore collapse behaviors under various loading strengths with the crystal-scale mechanisms in a unifying framework, which automatically transits from a strength-dominated regime to a hydrodynamic regime. Moreover, in this work, all of potential deformation mechanisms are decoupled and evaluated, which could provide insights into the understanding of ignition mechanism and the development of microstructure-aware predictive models at the macroscale.