SURFACE DAMAGE OF POLYMETHYLMETHACRYLATE PLATES BY ICE AND NYLON BALL IMPACTS

A study is described of the impact process and the corresponding surface damage to PMMA (polymethylmethacrylate) targets impacted by ice and nylon spheres. A gas gun system was used to project the spheres and the impacts were recorded using a high-speed image converter camera (Imacon 792). Special attention was paid to the conditions causing projectile failure and the surface damage to the target. The experimental results show that although the deformation and failure modes for ice and nylon are different, the surface damage to the PMMA target has a similar failure pattern, i.e., a central, circular undamaged region surrounded by an annular damaged region containing many short circumferential cracks. Analysis shows that the diameter d of the central undamaged region and the annular damaged area A(m) can be used to characterize the surface damage which depends on the projectile material, sphere radius R, as well as the impact velocity V-0. For a given projectile, two critical impact velocities exist: One is the threshold velocity (V-0)(th), below which no visible surface damage is observed, and another is the fracture velocity (V-0)(f), above which a plate target is broken. Damage in the annular region is caused by the Rayleigh surface wave in the present experiments. The conditions for the Rayleigh surface wave to pass ahead of the expanding edge contact velocity are analyzed. Comparisons between the theoretical predictions and experimental results, including the present study and earlier data, are made for PMMA plates impacted by projectiles of different materials (ice, nylon spheres, and water drops) and at different impact angles. (C) 1995 American Institute of Physics.