Characterizing grain-size effects in the shock heating of idealized PBXs

Heterogeneous polycrystalline microstructure in plastic bonded explosives (PBX) leads to thermal localization under dynamic impact. We describe a method to characterize the heterogeneity of these temperature fields as a function of the grain size. We performed a systematic study of how grain size affects temperature in model PBXs using finite element simulations and correlation techniques. The modeling framework combines a dislocation-based, anisotropic, single crystal plasticity model for RDX with a visco-elastic constitutive model for the estane binder. Our simulations used constant binder volume while the grain size was varied in the range of 25 to 200 microns. Smaller grains gave rise to lower average temperatures, with lower spatial correlation amplitudes. The microstructure of the preceding grains hit by the shock wave are observed to affect the later shock wave evolution itself and thereby the temperature of the ensuing grains. Fast Fourier Transform analysis of the spatial correlation revealed this effect in each of the grain sizes tested. High thermal localization in all grain sizes revealed a grain size length-scale effect. The spatial correlation between temperatures in the shock direction decreased, while the temperatures in the transverse direction retained their strong correlations.