The flexure and compressive strengths of sapphire are dependent on crystal orientation and temperature. Most notably, the c-axis compressive strength decreases below the tensile strength at temperatures >400°C and falls to 2% of the room-temperature compressive strength at 800°C. Loss of compressive strength complicates the interpretation of flexure tests. Four-point flexure specimens with no component of c-axis compression increase in strength at temperatures >500°C; however, specimens that have c-axis compression decrease in strength. It has been observed that c-axis compression causes twinning on rhombohedral crystal planes. Intersection of twins on different rhombohedral planes causes fracture that leads to mechanical failure.

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