Microstructural level response of HMX-Estane polymer-bonded explosive under effects of transient stress waves

The effect of transient stress waves on the microstructure of HMX-Estane, a polymer-bonded explosive (PBX), is studied. Calculations carried out concern microstructures with HMX grain sizes on the order of 200 mm and grain volume fractions in the range of 0.50-0.82. The microstructural samples analysed have an aspect ratio of 5 : 1 (15 x 3 mm), allowing the transient wave propagation process resulting from normal impact to be resolved. Boundary loading is effected by the imposition of impact face velocities of 50200ms-1. Different levels of grain-binder interface strength are considered. The analysis uses a recently developed cohesive finite element framework that accounts for coupled thermal-mechanical processes involving deformation, heat generation and conduction, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating along crack faces. Results show that the overall wave speed through the microstructures depends on both the grain volume fraction and interface bonding strength between the constituents and that the distance traversed by the stress wave before the initiation of frictional dissipation is independent of the grain volume fraction but increases with impact velocity. Energy dissipated per unit volume owing to fracture is highest near the impact surface and deceases to zero at the stress wavefront. On the other hand, the peak temperature rises are noted to occur approximately 2-3mm from the impact surface. Scaling laws are developed for the maximum dissipation rate and the highest temperature rise as functions of impact velocity, grain volume fraction and grain-binder interfacial bonding strength.