Crack-Resistance Curves of Surface Cracks in Alumina

The resistance of coarse-grained alumina to stable crack growth, initiated from "naturally'occurring surface flaws, was studied in strength tests with a bend-bar technique. The surface cracks experienced R-curve behavior over the first few hundred micrometers; i.e., the applied crack driving force had to be increased with crack extension. Compared to R curves generated from long cracks in standard single-edge-notched-bend specimens, the surface-crack curves have lower initial values. The possible influence of localized residual stresses, counteracting the applied crack driving force, is discussed.