The low-velocity-to-hypervelocity penetration transition for impact craters in metal targets

Projectile/target behavior for 1100 Al/Cu, soda-lime glass/Cu, soda-lime glass/1100 Al, ferritic stainless steel/Cu, and ferritic stainless steel/1100 Al for spherical (3.18 mm diameter) projectiles at impact velocities ranging from 0.8 to ∼6 km s−1 has been examined by light metallography, SEM, and TEM. At a reference velocity of 1 km s−1, the crater depth/crater diameter ratio (p/Dc) is observed to be linearly related to bulk density ratios (ρp/ρt)1/2 and elastic modulus ratios (Ep/Et)(ρp/ρt)1/2, and to vary from about 0.2 to 2.95. The hypervelocity (uo>5 km s−1) threshold value for p/Dc is also shown to be linearly related to these functionalities and ranges from p/Dc=0.4 for the 1100 Al/Cu system and 0.85 for the ferritic stainless steel/1100 Al system. The residual crater microstructures are all characterized by a zone of dynamic recrystallization at the crater wall (which thickens with impact velocity), and decreasing dislocation density beyond this zone; consistent with residual hardness profiles whose amplitudes decrease with distance from the crater wall. Computer simulations and validation of these simulations utilizing the ranges of experimentally measured crater geometries with impact velocity were developed which fairly accurately represented residual crater shapes and related features. These results also demonstrate the importance of appropriate projectile/target strength ratios in computer simulations; and illustrate the potential for extrapolations to new systems, and for impact velocities well beyond those achievable in the laboratory.