The role of localized plastic flow in the impact initiation of explosives

Abstract

Individual crystals of silver and lead azide have been impacted by spherical particles. Im pact was at normal incidence on flat faces of the explosives. A novel feature of the experiment was that the particles were very small compared with the millimetre dimension crystals. This meant that the impact damage was independent of the shape of the explosive. Initiation took place when critical conditions of particle size and velocity were exceeded. It was possible to examine the deformation produced when the impact conditions were just subcritical and show that it was primarily plastic. If the deformation energy were uniformly distributed around the indentation, only a small temperature rise would be produced. However, it is suggested that, because of thermal softening, the deformation is concentrated in narrow bands of material by adiabatic shear. The temperature rise in these regions could become high enough for initiation. The discussion is extended to compacted explosives and conditions are considered under which local plastic flow can take place. Finally, the advantages of microparticle impact for testing the sensitiveness of explosives, and situations where particle impact has practical interest are described.