Failure and penetration response of borosilicate glass during short-rod impact

A series of reverse ballistic experiments reveal further properties of the failure front (FF) associated with the penetration of borosilicate glass by a gold rod. Importantly, the FF ceases to propagate a short time after the rod is fully eroded. The rods, 1 mm in diameter, were short (5-11 mm). The glass targets were 20-mm diameter cylinders, 60-mm or 100-mm long. Impact velocities varied between 1 and 2 km/s. The impact and penetration process was observed with five flash X-rays and a 16-frame high-speed optical camera. A FF propagates from the impact region. The velocity of FF propagation is an increasing function of the impact velocity. The termination of the FF can reasonably be predicted in most cases with a simple model that assumes a rarefaction wave, originating at the time of complete rod erosion, propagates from the bottom of the penetration channel to the FF at a speed equal to the bulk wave speed of undamaged glass