The microstructure of granular explosives can affect dissipative heating within compaction shocks that can trigger combustion and initiate detonation. Because initiation occurs over distances that are much larger than the mean particle size, homogenized (macroscale) theories are often used to describe local thermodynamic states within and behind shocks that are regarded as the average manifestation of thermodynamic fields at the particle scale. In this paper, mesoscale modeling and simulation are used to examine how the initial packing density of granular HMX (C4H8N8O8) C4H8N8O8 having a narrow particle size distribution influences dissipation within resolved, planar compaction shocks. The model tracks the evolution of thermomechanical fields within large ensembles of particles due to pore collapse. Effective shock profiles, obtained by averaging mesoscale fields over space and time, are compared with those given by an independent macroscale compaction theory that predicts the variation in effective thermomechanical fields within shocks due to an imbalance between the solid pressure and a configurational stress. Reducing packing density is shown to reduce the dissipation rate within shocks but increase the integrated dissipated work over shock rise times, which is indicative of enhanced sensitivity. In all cases, dissipated work is related to shock pressure by a density-dependent power law, and shock rise time is related to pressure by a power law having an exponent of negative one.
A precursor to shock initiation of porous explosives involves the formation of reactive hot-spots at the pore scale by the lead compaction shock followed by their growth and interaction that results in vigorous burn. It remains largely unclear how the material's mesostructure (i.e., the particle packing density, and size and shape distributions) affects ignition behind compaction shocks and subsequent growth to detonation. In this study, a simple reactive burn model is formulated for sustained planar shock loading of porous HMX that incorporates an ignition criterion based on the effective reactive hot-spot formation rate established by inert mesoscale modeling and simulation of uniaxial waves. The effective rate is related to bulk wave pressure through a power law. The burn model is implemented in a two-phase (explosive and product gas) non-equilibrium continuum theory to computationally examine how ignition and initiation vary with initial packing density and shock strength. Preliminary predictions for low density HMX indicate that the burn model reasonably estimates the time and distance to detonation over a wide range of input shock pressure using a single fitting parameter that depends on initial density. Ignition and growth dominated initiation regimes are identified and discussed.
Dissipation induced by shock compaction of granular explosives (often used in initiators) can trigger combustion that results in detonation, even for relatively mild shock strengths. In this study, a computational analysis is performed to better understand how successive planar shock loading of granular explosive can affect dissipation and hotspot formation that are important in establishing the material's sensitivity. Preliminary predictions demonstrate that the trailing shock, which is overdriven with respect to the ambient material, is desensitized by the lead shock compared to a single shock of equivalent pressure. Substantial dissipation is predicted to occur as the overdriven trailing shock overtakes the lead shock that can potentially result in the ubiquitous formation of hot-spots within the material.
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