Influence of Shock Pressure on Hot Spot Formation in a Model Plastic-Bonded Explosive

Hot spot behavior influences the sensitivity and shock-to-detonation transition of plastic-bonded explosives (PBX). Yet, experimental observation of hot spots has been precluded by detection techniques which can resolve the multiple time and length scales at which they exist (fs-µs, nm-µm). We developed a model PBX consisting of a single crystal of the high explosive HMX (cyclotetramethylene-tetranitramine) embedded in a transparent polyurethane binder which allowed us to directly observe hot spots at the time and length scales they exist. In this work, we investigated the influence of shock pressures on hot spot behavior by 1) visually tracking hot spots with micron-resolved high-speed gated imaging, and 2) tracking hot spot temperatures with nanosecond-resolved optical pyrometry. We shocked ~100 HMX single crystals at pressures ranging from 12-26 GPa. Two distinct shock pressure thresholds were observed. At 15 GPa some crystals began producing slow growing, discrete hot spots, while over 23 GPa the hot spot density was sufficient to result in massive deflagration. Additionally, initial estimates for the velocity of the flame front across an HMX crystal were calculated from the high speed images taken at median pressures.