The onset of damage in shocked alumina

Shock wave profiles have been measured simultaneously using the velocity interferometry system for any reflector (VISAR) and embedded foil stress gauges. The gauge is backed using a transparent rear window made of polymethylmethacrylate, through which the laser beam entering the VISAR cavity is also shone. The technique is applied to grades of alumina of differing purity shocked just above the threshold normally taken to be the Hugoniot elastic limit (HEL). In the case of one of the aluminas, the result is analogous to metals with a small spall (dynamic tensile strength) signal recorded. In the case of the other materials, the signals show marked differences, indicating both the need for simultaneous measurement using a variety of sensors and the limitations of the present definitions for the mechanical thresholds in dynamic compression (HEL) and tension (spall strength).