R-curve behavior and crack-closure stresses in barium titanate and (Mg,Y)-PSZ ceramics

R-curves, process zones, and shielding stresses of barium titanate (BaTiO3) and partially stabilized zirconia (PSZ) have been studied using compact-tension (CT) specimens. BaTiO3 and PSZ exhibited pronounced R-curves that rose over similar crack lengths and showed steady-state toughnesses of 0.7 and 6.4 MPa.m(1/2), respectively. Both steady-state toughnesses were similar to 80% larger than the initial fracture toughnesses, Ferroelastic domain switching was the main toughening mechanism in BaTiO3, whereas, in PSZ, transformation toughening was the main toughening mechanism. The crack process zone and crack-opening-displacement (COD) profile of each material was studied in detail using atomic force microscopy. Cracks closure-stress distributions were extracted from the COD profiles, using weight-function methods. The resulting stress profiles indicated that compressive residual stresses of 40 MPa in BaTiO3 and 400 MPa in PSZ acted in a limited region behind the crack tip. In the PSZ, crack bridging seemed to be a competing mechanism to transformation toughening.