Detonation waves in PBX 9501

Abstract

For a planar detonation wave propagating in the plastic-bonded explosive PBX 9501, measurements of the reaction zone display a classical ZND profile. Moreover, the reaction-zone width is substantially less than the average size of an explosive grain. We show that the reaction zone is compatible with realistic constitutive properties and an Arrhenius reaction rate based on the bulk temperature. Thus, contrary to conventional wisdom, hot spots are not needed to propagate a detonation wave. Conventional wisdom is based, in part, on shock desensitization experiments; the observation that precompressing a PBX with a weak shock – which eliminates voids as nucleation sites for hot spots – can quench a propagating detonation wave. By analysing the temperature behind two shocks compared to a single shock and the corresponding change of the induction time, we show that a detonation wave sustained by the bulk reaction rate from shock heating is compatible with shock desensitization. Shock desensitization depends on having a temperature sensitive rate, which usually is associated with detonation wave instability. However, for PBX 9501 the temperature variation in the reaction zone is small, and one-dimensional simulations show that this results in a stable detonation wave. Furthermore, we show that two additional phenomena are compatible with the perspective that bulk burn can sustain a planar detonation wave: failure diameter, which does depend on the heterogeneous structure of a PBX; and PBXs with a lower HE content which display an irregular detonation front.