Failure wave effects in hypervelocity penetration

It has been known for several years that glass is a relatively effective armor against shaped charge jets [1] even though its performance against conventional long-rod projectiles is mediocre. Some of the authors have earlier postulated that this effect is due at least in part to an increase of the R-t value at hypervelocity. This enhancement is due to the fact that in long-rod penetration of brittle materials, a failure wave is generated in front of the penetrator which prematurely damages the material; however, if the penetrator is supersonic relative to this failure velocity, penetration is always occurring in intact material. Consequently, the true strength of a brittle material is only measured in hypervelocity experiments. In order to avoid the uncertainties of analyzing shaped charge penetration data, we have conducted experiments with L/D = 10 W alloy rods (rho = 17.2 g/cm(3)) against glass targets (rho = 2.5 g/cm(3)) to unambiguously search for this effect. In low velocity experiments, the penetration was essentially hydrodynamic, while above 3.9 km/s, the R-t-Y value was on the order of 5.7 to 7.2 GPa. This substantiates the failure wave hypothesis. (C) 1999 Elsevier Science Ltd. All rights reserved.