A study of energy localization mechanisms in PBXs under shock loading through interface-resolved reactive simulations*

The shock-induced initiation of plastic bonded explosives (PBXs) is a complex phenomenon wherein various mechanisms are at play at the grain-scale. For example, the interaction of binder-crystal and crystal-crystal interfaces, the shock focusing due to tortuosity of the embedded microstructures, and the collapse of pores may lead to energy localization followed by ignition of HMX crystals. However, the relative importance of these mechanisms is not well understood. In this work, we take a computational approach to investigate the mechanistic details of shock response of HMX crystals embedded in polyurethane binder, for a set of actual images of PBX samples. We have employed interface-resolved reactive simulations using a sharp-interface Eulerian framework. The interfaces are represented using levelsets, with appropriate boundary conditions being enforced using the ghost fluid method. High-resolution calculations are performed on crystals of different shapes, sizes, and crystals with/without pores. The results show that energy localization due to collapse of pores in HMX crystals is the dominant mechanism of hotspot formation at the short timescales relevant to shock-to-detonation transition in PBXs.