Mesoscale analysis of volumetric and surface dissipation in granular explosive induced by uniaxial deformation waves

A Lagrangian finite and discrete element technique, combined with a finite deformation, thermo-elastic-viscoplastic, and stick-slip friction theory, is used to computationally examine volumetric and surface dissipation within the meso-structure of granular explosive (HMX, C4H8N8O8) induced by uniaxial deformation waves. Emphasis is placed on characterizing the fraction of mass heated to elevated temperature (referred to as hot-spot mass fraction) by quasi-steady waves due to plastic and friction work and its dependence on wave strength. Predictions for a large, randomly packed ensemble of HMX particles having a solid volume fraction of 0.85 and a mean diameter of 60 mu m show that plastic work principally affects the average temperature, whereas friction work affects the high frequency, high-temperature fluctuations that are likely responsible for combustion initiation. Cumulative distributions for hot-spot mass within the wave indicate that most mass (similar to 99.9%) is heated to approximately 330, 400, and 500 K by plastic work for impact speeds of 50, 250, and 500 m/s, respectively, with a small fraction (similar to 0.001%) heated to 600, 1,100, and 1,400 K by friction work. The hot-spot mass fraction induced by plastic work is well described by a Gamma distribution, though significant departures occur in the high-temperature end of the distribution due to friction work, even at higher impact speeds. Consequently, it is not possible to describe hot-spot mass fraction curves by a single classical distribution function. Implications of the predicted hot-spot mass fraction on granular HMX combustion are discussed.