Birth and growth of explosion in liquids and solids initiated by impact and friction

Abstract

The birth and growth of explosions initiated by mechanical and thermal means have been studied. Liquid and solid explosives show a striking similarity. The point of initiation is always located at a source of local high temperature, for example, a hot wire, an electric spark, an impacted grit particle, or at a gas pocket suddenly compressed during impact. There is an appreciable time lag between the first moment of impact and the first appearance of light from the explosion. With secondary explosives the time lag depends on the conditions of impact (it could be varied from 60 to 150 μsec.), but for all the explosives studied the delays under similar conditions are approximately the same. For the primary explosives the time lags are usually much shorter, indicating that a different mechanism of initiation may be operative. The first stage of explosion in liquids is a burning which begins slowly and accelerates to speeds of 500 or even 1000 m./sec. This speed may represent, in the main, a mass movement of the gas products away from the centre of explosion. In most solid explosives (both primary and secondary) the first stage is again a slow burning which accelerates to speeds of several hundred metres per sec. A second stage of constant velocity detonation then sets in. The detonation velocity (which varies from 1100 to 2300 m./sec. according to the explosive and the physical conditions of the layer) may be identified with the low-velocity detonation in large charges, and the correct order of velocity has been explained on hydrodynamic grounds. It is suggested that the continued propagation of the low-velocity detonation stage in a liquid is made possible by the rapid breaking up of the explosive by the detonation shock front, particularly if the liquid has a low viscosity. In more viscous liquids and solids propagation is possible only if hot-spot sources are present in the explosive. The hot spots may be developed by rapid compression of gas pockets, or, if the solid has a high enough meltingpoint, by intercrystalline friction.