Predictions for multi-scale shock heating of a granular energetic material

This thesis addresses the multi scale heating of a granular energetic solid due to shock loading. To this end, an existing mathematical model that has been used to predict low pressure bulk and localized heating of the granular high-explosive HMX ([CH2NNO2]4) is extended to account for compressibility and melting of the pure phase solid. Dense granular HMX has a heterogeneous structure composed of randomly packed small grains (average size ~ 100 µm) having a free-pour density that is approximately 65% of the pure phase solid density. The shock loading response of this material is complex and consists of both bulk heating due to compression and compaction, and grain scale heating due to stress localization and plastic deformation in the vicinity of intergranular contact surfaces. Such dissipative processes at the grain scale induce high frequency temperature fluctuations (referred to as “hot-spots”) that can trigger combustion initiation even though the bulk temperature remains quite low. The work presented here is an attempt to characterize hot-spot evolution within the framework of a thermodynamically compatible bulk compaction model that can be used for engineering calculations. The model is shown to admit both steady subsonic and supersonic compaction wave structures that result in significant localized heating at the grain scale based on grain contact theory. Peak hot spot temperatures in the range of 1000 K are estimated for subsonic compaction waves that could induce combustion initiation and influence ignition sensitivity of the material. Thermal conduction and phase change are shown to be significant at low impact speeds, but become less important at higher speeds. Compressive grain heating had little effect on hot spot temperatures for the range of impact conditions considered in our study (up = 100-1000 m/s). A parametric sensitivity analysis was performed to characterize the effect piston impact speed, initial solid volume fraction, and other key model parameters on both compaction wave structure and localized heating. At higher initial volume fractions (\u3e 0.90), it was found that viscoelastic heating dominates over the viscoplastic heating. Also, predictions for the variation of bulk plastic strain, pressure, and porosity through the compaction zone are shown to qualitatively agree with results obtained by detailed micromechanical models