Optical, video and scanning electron microscopic observations show fractographic features that are used to determine the crack propagation direction and crack origin in thermally stressed single crystal sapphire hemispherical shells. These features have been typically identified in materials with symmetrically applied stresses and simple geometries. This fractographic evaluation of sapphire is unusual because of the hemispherical geometry and complex mechanical and thermal stress conditions. Computer models predict the thermal stress response of the sapphire from fast acting asymmetrical heating patterns generated during controlled laboratory testing. These experimental tests are conducted to validate the computer model predictions. When an experimental structure fails to meet the survival limits predicted by the computer model, a fractographic evaluation is conducted to determine fracture mode and to locate the crack initiation site.

Fractured pieces are examined and photographed using a low magnification, binocular optical microscope and a high resolution video camera with digitizing capabilities.