The shock-to-detonation transition in solid explosives

Abstract

The development of a shock wave and its subsequent growth-to-detonation is considered to be a necessary step in the initiation of detonation in any explosive. Using this argument as a basis, it is logical to study in detail, the development of impulse-initiated detonations, to establish the dependence of the growth-to-detonation process on physical, chemical, and geometric variables which appear to be of importance. In this paper, experiments on shock initiation of cast and pressed explosives are discussed. Plane shocks developed by explosive plane-wave generators, and degraded to desired peak amplitudes by the use of intermediate layers of inert materials, were used to initiate detonation in the test explosive sample. The velocity of the shock in each sample was measured as a function of distance into the explosive and as a function of initial shock amplitude. Initial shock pressures ranged from 28 to 140 kilobars. Explosives discussed are TNT and various cyclotols. Both cast and pressed charges were used. In cast charges other than TNT the shock velocity remained essentially constant for a period of time, the length of which depended on the initial shock amplitude, and then rapidly accelerated to normal, steady state detonation velocity. In pressed charges it was found that in the rapid rise, the shock velocity temporarily exceeded the steady detonation value, but decayed thereafter to that of the normal steady detonation. In cast TNT the velocity was found to rise to a value intermediate to that of the initial shock and the final detonation, where it persisted for a time before growth to the normal detonation value. The over-all results can be explained by a hydrodynamic model in which pressure build-up, due to chemical reaction behind the shock, reinforces the shock front as it proceeds through the charge. On the other hand, the detailed results cannot be explained by thermal reactions in homogeneous domains, but require the concept of hotspot initiation. In the discussion our findings will be compared with the work of others who have used various impact and gap-test configurations. Some of the problems and differences of opinion which have arisen in the interpretation of shock initiation will be discussed.