Atomistic Simulations of Pore Collapse Initiation and Deflagration in HMX

The initiation processes associated with the collapse of pores in crystalline HMX have been studied with Reactive Molecular Dynamics (RMD). Very large-scale simulations (up to 400 million atoms) were performed on systems with 25, 50, 100 and 200 nm pores, and with shock impact velocities ranging from 1.0 to 4.0 km/s. The simulations were performed primarily on two-dimensional (2D) slabs with cylindrical pores, with a few 3D simulations with spherical pores also being performed. The LAMMPS code was used for the simulations with the ReaxFF-lg RMD potential function. The reaction progress is projected onto a previously developed 3-stage reaction model which contains separate branches depending on the pressure being greater or less than ~5 GPa. The ignition process is sensitive to the size of the void and the shock strength, consistent with expectations. For the lowest impact velocities (≤1.0 km/s), the reaction only proceeds to the first stage of reaction, which is only a mildly exothermic process, and this could thermally quench. For moderate impact velocities (1.5-2.0 km/sec), self-sustaining hot spots with deflagration waves are formed. These have a resolved 3-stage reaction structure with an overall reaction zone width of 10-30 nm. The velocities of these waves range from 50-2000 m/s, depending upon the upstream (with respect to direction of the deflagration propagation) temperature and pressure of the material. In particular, there is a very rapid (~2000 m/s) burning back into the former void space, which contains material that is at an elevated temperature and pressure because it has now been doubly shocked. For the highest impact velocities (2.5-4.0 km/sec), Mach stems are formed which channel a rapid deflagration wave (≥2000 m/s) in the forward shock direction. Under these strong shock conditions, homogeneous initiation is also seen to be occurring away from the void locus.