Numerical simulation analyses of phase transition for a finite-sized HMX single crystal subjected to thermal loading

Phase transition of HMX single crystals is the very first process prior to chemical reaction and ignition of polymer-bonded explosives (PBX). A mesoscale phase transition model is developed to investigate the role of solid phase transformation on the thermo-mechanical behavior of HMX single crystals. The model captures nonlinear elasticity, dislocation-based crystalline plasticity and temperature-dependent phase transition. Phase transition evolutions of HMX subjected to different heating rates with a certain hydrostatic pressure were investigated based on the finite element software ABAQUS. The simulated results showed that with the thermal heating and cooling boundary conditions, the phase transition state is reversible, but its path is irreversible. The path-dependence of the phase transition is reflected by the residual strain and stress that comes into being in the absence of mechanical constraints for 1 mm size HMX single crystals during a temperature cycle. Moreover, the phase transition is inhibited by higher temperature gradients and hydrostatic pressure. As the -phase of HMX crystal converts to the -phase, the crystal volume expands due to the larger expansion coefficients of -HMX so that the stress concentration can be found at the sample center.