the techniques and test geometries that have been use to measure subcritical crack growth in ceramic are several, but they share a common principle, namely, the subjection of a well-defined crack to a well-defined stress intensity KI,and measurement of its velocity v, the technique considered here, the advantages of which are elaborated upon below, is the double torsion geometry shown in Fig.12.8 for the double torsion specimen,KI is given by