The fracture toughness KIC of a brittle material is defined as
KIC = YSVc (7)
in which S is the applied stress required to propagate a crack of depth c, and Y is a geometrical parameter. Values of KIC are often measured for ceramics from the lengths of cracks around a hardness indent. KIC is not material parameter; it depends on sample history and many uncontrolled factors. It is based on the Griffith equation, which gives a necessary but not sufficient criterion for crack propagation . Thus KIC is not a very useful quantity for defining mechanical properties of brittle material. A value of about 3.0 MPam1/2 is often found for alumina .