Visualization of wave propagation and impact damage in a polycrystalline transparent ceramic-AlON

The interaction of high velocity projectiles with brittle ceramics results in the formation and propagation of severe fragmentation in the ceramic, which precedes the penetration of the projectile. An Edge-on Impact (EOI) technique, developed at the Ernst-Mach-Institute (EMI), coupled with a Cranz-Schardin high-speed camera, has been successfully utilized to visualize dynamic fracture in many brittle materials during the first 20 μs after impact. For the first time this technique has been used to visualize internal damage evolution. Transparent polycrystalline AlON specimens measuring 100 à 100 à 10 mm were impacted using solid cylinder steel projectiles with velocities ranging from 270 to 925 m/s. Pairs of impact tests at approximately equivalent velocities were carried out in transmitted plain (shadowgraphs) and crossed polarized light. Stress wave and damage velocities were determined as well as the nucleation of crack centers ahead of the apparent fracture front, growing from the impacted edge of the specimens.