Micro-Gap Experiments and Insensitive Explosives

Early research on shock desensitized plastic-bonded explosives (circa 1970) also studied large single crystals of explosive. High quality crystals --- free from voids that serve as nucleation sites for hot spots --- have been found to be very insensitive to shock initiation. In fact, experiments were not able to initiate a large single crystal of HMX ($\sim$ 10\,mm) with a detonation wave in PBX~9404, which is 94 weight\,\% HMX and has a Chapman-Jouget pressure of 35\,GPa. Yet a single crystal of HMX can be initiated by a flyer plate that drives a shock at a similar pressure. This is especially puzzling since the detonation wave in PBX~9404 has a peak pressure at the von~Neumann spike of nearly 60\,GPa. An important difference between the two drive systems is a small gap at the PBX~9404/HMX interface due to surface roughness of the PBX; estimated to be 30 to 50 microns. Conceptually, the experiment is equivalent to the gap test used to compare the sensitivity of different explosives; albeit with a micro-gap and a very insensitive explosive. The inability of a PBX~9404 detonation wave to initiate a single crystal of~HMX is due to the reaction zone in the PBX~9404 being of comparable length to the gap in the experiment and the rarefaction or Taylor wave behind the detonation wave.