Damage in steel plates from hypervelocity impact. I. Physical changes and effects of projectile material

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Abstract
Microscopic details of physical changes occurring in steel plates impacted at hypervelocities by spherical projectiles of several materials are studied by means of metallographic examinations of polished and etched target cross sections. A hemispherical volume of material beneath the impact site undergoes the pressure‐induced α?ε polymorphic phase change. The grain structure is heavily deformed and refined, and significant hardening occurs. The boundary between transformed and untransformed material can be made visible by etching; it corresponds to about a 13‐GPa isobar. The occurrence of this phase change has a considerable effect on the stress history and on the rear‐surface fracture damage, as is shown in the following paper, Paper II. Shear banding is a dominant deformation mechanism in the crater and near‐crater regions. Furthermore, the numerous white‐etching bands of very hard untempered martensite nearly always acquire brittle cracks along their length and, hence, shear banding strongly influences cratering behavior. The formation of crater ejecta, at least in later stages, is probably controlled by shear‐band activity. Therefore, detailed predictions of near‐crater fracture patterns and ejecta size distributions require a computational model for shear bands. Projectiles having lower shock impedance than the steel targets produce quite different crater shapes and near‐crater fracture patterns than do higher‐impedance projectiles. The downward and outward patterns of shear bands and associated cracks produce by the impact of nylon and water‐filled polycarbonate, for example, are in direct contrast to the upward and outward patterns caused by steel and tungsten carbide spheres. This suggests that the maximum shear stress trajectories in the steel targets depend on the tendencies of the projectiles to penetrate or to reverse their direction upon impact. Several observed effects of projectile material on the extent of back‐surface fracture damage are related to the relative shock impedances of projectile and target.