FLAT R-CURVE FROM STABLE PROPAGATION OF INDENTATION CRACKS IN COARSE-GRAINED ALUMINA

The fracture toughness of coarse-grained Al2O3, known for pronounced ''long''-crack R-curve behavior, was studied in the ''short'-crack regime utilizing the stable propagation of indentation cracks in bending. A combination of in situ microscopic crack growth observations and mechanical testing enabled measurement of crack extension curves. They reflect the contributions of residual indentation stress intensity and applied bending stress intensity on the total crack driving stress intensity and allow determination of the residual stress factor X and the toughness K,. The results indicate that X depends on indentation load and K, is surprisingly constant rather than increasing. To resolve the latter contradiction with long-crack R-curve behavior, combined short/long-crack fracture tests were performed with the same specimens. Starting with stable indentation crack growth and continuing with stable long-crack extension, the previous toughness results were confirmed, i.e., constant toughness from indentation cracks and increasing toughness from long cracks. The influence of crack-opening behavior on bridging-controlled R-curve toughening can qualitatively explain the observed discrepancies.