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R-curves, process zones, and shielding stresses of barium titanate (BaTiO 3 ) and partially stabilized zirconia (PSZ) have been studied using compact-tension (CT) specimens. BaTiO 3 and PSZ exhibited pronounced R-curves that rose over similar crack lengths and showed steady-state toughnesses of 0.7 and 6.4 MPa⅐m 1/2 , respectively. Both steady-state toughnesses were ϳ80% larger than the initial fracture toughnesses. Ferroelastic domain switching was the main toughening mechanism in BaTiO 3 , 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. Crackclosure-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 BaTiO 3 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.

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In Pb(Zr,Ti)O 3 ceramics, which are characterized by different magnitudes of coercive tensile stress, the ferroelastic domain switching induced during crack growth was measured using X-ray diffraction, and was compared with the R-curve behavior. The following correlation was established: material with smaller coercive stress has a larger amount of domain switching, and a higher toughness increment in the R-curve. The half-width of the process zone was determined for one of the four studied compositions from the spatial distribution of domain switching measured using X-ray diffraction from synchrotron source. Its value, 60 -80 m, was in good agreement with the estimate from fracture mechanics.