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-te nsion (CT) specimens. BaTiO3 and PSZ exhibited pronounced R-curves that ros e 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 t o 80% larger than the initial fracture toughnesses, Ferroelastic domain swi tching was the main toughening mechanism in BaTiO3, whereas, in PSZ, transf ormation toughening was the main toughening mechanism. The crack process zo ne and crack-opening-displacement (COD) profile of each material was studie d in detail using atomic force microscopy. Cracks closure-stress distributi ons were extracted from the COD profiles, using weight-function methods. Th e 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 c rack tip. In the PSZ, crack bridging seemed to be a competing mechanism to transformation toughening.