Optical techniques for the investigation of the ballistic impact of thin plates

The normal impact of chrome steel spheres upon mild steel and copper plates of various thicknesses was investigated. The bulging of their rear surfaces was monitored during the first 30 μs after impact using stereo digital speckle photography. The experimental data for copper was compared with two path-dependent models, one optimised for annealed and the other for pre-stressed (work-hardened) copper. It was not possible to differentiate between the two copper models using single-point displacement data. However, full-field data revealed clear differences in the shapes of the bulges, with greater global deformation and closer agreement with experiment for the model that included pre-stress. For both mild steel and copper, thin plates resulted in localised bulging whilst thicker plates produced global dishing. The bulge peak also grew earlier and more quickly in thinner specimens. The greater ductility of copper resulted in narrower, faster-growing bulges compared with mild steel of comparable thickness.