Rising Crack‐Growth‐Resistance ( R ‐Curve) Behavior of Toughened Alumina and Silicon Nitride

R ‐curves for a sinter/HIPed SiC(whisker)‐reinforced alumina and a sintered silicon nitride were assessed by direct measurements of lengths of cracks associated with Vickers indentation flaws. The fracture toughness measurements based on (a) initial (as‐indented) crack lengths, (b) equilibrium growth of cracks during increasing far‐field loading, and (c) crack lengths corresponding to unstable fracture showed definitive trends of R ‐curves for both materials. The fracture mechanics analyses employed an indenter‐material constant that was independently estimated using a physical model for the residual driving force and a free surface correction factor that accounted for the effects of size and shape of the cracks on stress intensity. It is shown that R ‐curve estimations based on crack length measurements have the intrinsic advantage that crack length dependence of fracture toughness is not assumed a priori as is done in conventional analysis based on strength. The measured fracture toughness of SiC(whisker)‐reinforced alumina was in agreement with the prediction of a toughening model based on crack bridging by partially debonded whiskers.