Mechanisms and Size Effects of Hotspot Formation due to Shock-Induced Collapse of Pores and Cracks

The shock to detonation transition in heterogeneous high energy density materials starts with the spatial localization of mechanical energy into hotspots due to the interaction of the mechanical wave with microstructural features and defects. We present large-scale molecular dynamics simulations of hotspot formation in HMX crystals following the collapse of pores of various shapes and sizes for impact velocities ranging from 0.5 to 2.5 km/s. Hotspots resulting from cracks elongated along the shock direction show significantly higher sensitivity to both shock strength and defect size. Elongated cracks 80 nm in length result in temperatures almost three times higher that voids 80 nm diameter and reach values corresponding to the ideal case of isentropic recompression of a gas. The MD trajectories reveal the atomic origin of this contrasting behavior. While circular voids undergo a transition from viscoelastic pore collapse to a hydrodynamic regime with increasing shock strength, shock focusing in elongated cracks results in jetting and vaporization, which upon recompression leads to increased heating.