Hot spot criticality in shocked HMX over a range of pore sizes and pressures

Continuum hot spot models for explosives now incorporate information on pore size distribution. However, there is a limited understanding of the participation of different sized pores in shock initiation scenarios. The objective of this simulation- based study is to determine the criticality of single hot spots in HMX as a function of circular pore diameter and shock pressure. Simulations are performed with the multi-physics hydrocode, ALE3D. A coupled thermochemical code provides the equation-of-states and the chemical kinetics. We also employ a strain, strain-rate, and pressure-dependent strength model. We fit the results of our pore collapse simulations to an ignition criticality equation: Rig(P) = (Rmax Rmin)(P/P0)−a + Rmin. For the 2D plane strain case, Rmax = 2.5 µm, Rmin=0.25 µm, P0=5.0 GPa, and a=3.5. For the 2D axisymmetric case, Rmax=0.5 µm, Rmin=0.04 µm, P0=5.0 GPa, and a = 1. Non-reactive pore collapse simulations are also used to determine the ignition threshold but extracting definitive hot spot information from these simulations is complicated by their complex geometries and non-uniform temperature distributions. We found that using the mass of a hot spot with T > 2Tbulk appears to be a fair discriminator for ignition in complex hot spots. These studies are important because they enable upscaling grain-scale information to the continuum-scale as part of developing morphology-aware models.