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.