R-curve behaviour of BaTiO3 due to stress-induced ferroelastic domain switching

R-curves of ferroelectric barium titanate have been measured with CT-specimens. Depending on the grain size, the R-curves exhibit an increase from an initial value of 0.5-0.7 MPa root m to a plateau value of 0.7-1.2 MPa root m after a crack length increment of approximately, 100-800 mu m. The main toughening mechanism is thought to be ferroelastic domain switching leading to the development of a process zone around the crack. The small differences in the initial values of the R-curves are attributed to grain bridging, crack deflection and crack branching association with large grains. Applying an electric field per perpendicular to the specimen plane results in an increased initial fracture toughness and a stronger R-curve behaviour. Time-dependent reorientation of domains in the crack wake causes stress relaxation combined with a strong time dependence of the R-curves. A qualitative process - one model is proposed to explain these effects. (C) 1997. Published by Elsevier Science Limited.