Another variation is to use the shock wave for spallation testing of materials. This application is based on the behavior of shockwaves to reflect from the rear free surface of a work piece as a tensile wave. Depending on the material properties and the shock wave characteristics, the reflected tensile wave may be strong enough to form microcracks or voids near the back surface, or actually "blow-off" or spall material from the back surface. This approach has some value for testing ballistic materials.