XVI - Void Precursors

Abstract

Voids are only collapsed by pressure below a definite limiting pressure. This is a very general mechanism, difficult to establish in inert materials but easy in reactive materials because such a mechanism leads to the onset of chemical reaction or a high-velocity detonation (HVD). It is conceivable that collapsing bubbles relate to low-velocity detonations (LVD), while expanding bubbles ahead of the shock front to HVD. The precursors from the high pressure side are messengers between the reacted and the unreacted explosive, passing information to the unreacted region that something is about to happen. A messenger of this kind is not predicted by the classical detonation model, but such information is transmitted by fluctuations of the sound velocity in the reacted medium in more modern modifications. The activation mechanism of precursors works for any shock-to-detonation transition (SDT) when the entering shock is sufficiently powerful. Because of the parametric amplification of bubble migration by their size variation, LVD to HVD and HVD to LVD transitions also become possible. Statistical scatter in HVD experiments is to be expected because of microscopic mechanisms.