The velocity of expansion of a pellet (diameter d = 10-15 mm, thickness h = 0.1-0.6 mm) of a solid organic substance under impact between two flat polished steel surfaces has been determined by means of a rotating-mirror camera. Several nitrocompounds (melting points from about 50 to 300ยฐC) and benzoic acid have been studied. It has been estimated that the first stage of the process is a slow deformation of the pellet accompanied by a rise in pressure and density. The low deformation velocity is of the order of several m/sec, and the duration varies for different substances from 0.04 to 0.2 msec.
When the pressure reaches some critical value, depending on the layer dimensions (Pl '~d/h), the pellet ruptures and the fragments escape from between the impacting surfaces. The velocity of escaping also depends on the dimensions of the pellet (u I ~,d/h) and correspondingly on the pressure P l" The mean velocity u I for tetryl and PETN is u I = 32 (Pl -0.6), m/sec, Pl in kilobars.
The first destruction is followed usually by a second and sometimes by a third one. The velocity can reach 0.4-0.5 km/sec. If it is sufficiently high, the temperature rise due to friction can lead to decomposition and to ignition of the nitrocompouuds. The critical velocity required to bring about ignition does not depend on the energy of impact or on the pellet dimensions. For different substances, it is in the range of 0.25-0.55 km/sec. It has been shown that melting of the substance usually precedes the ignition and the temperature of the melting film may be well above the melting point of the substance.