Microstructure in shaped charge cones and its relationship to plastic instability and jet performance

Abstract

When a plane wedge of relatively thin plates of metal or a cone of sheet metal is backed up on the outside with explosive and detonated, the walls of these wedge-type geometries are caused to collapse so that the metal is forced to concentrate on the corresponding axis. In the case of a plane wedge a planar jet of metal is formed which conceptually approximates the blade of a sword moving at speeds up to 30,000 ft./sec. (∼ 9 km/s). When strategically placed on structural girders of a building, demolition can be achieved in a remarkable collapse process which can be carried out in the midst of a city. As a metal cone, the ensuing axial jet can punch a hole the size of a U.S. quarter in more than a meter of steel armor. These shaped charge jets, especially those originating from explosively collapsed metal cones, also exhibit remarkable mechanical behavior: metals such as copper are stretched nearly 900 percent (in the solid state) before necking into small particulates like a non-viscous fluid breaking up into small droplets. As it turns out this jet breakup, and its corresponding armor penetrating ability, seems to be dependent upon the initial metallurgical processing of the cone, especially the initial grain size in the case of copper. However since the jet break up for copper seems to occur over a very narrow range of grain sizes (between about 35 and 45 μm), more subtle features of the cone microstructure could be controlling the jet performance.