Crack deflection in piezoelectric ceramics

Crack deflection can occur in a specimen subject to a stress gradient of high tensile stresses near the surface which decreases with increasing depth. Such a stress gradient can be induced by strain incompatibilities. These can for example arise under electric fields between the electroded and external regions of a piezoelectric material. Such incompatibilities have been realized in thin rectangular model specimens from PZT-piezoelectric ceramics with top and bottom partial electrodes. Under an electric field, controlled crack propagation has been observed in-situ in an optical microscope. The crack paths are reproducible with very high accuracy. Small electrode widths lead to straight cracks with two transitions between stable and unstable crack growth, while large electrode widths result in curved cracks with four transitions. Poling the specimen prior to the experiment alters the crack path and introduces an anisotropy in the R-curve behavior as well as in the achievable strain mismatch. The crack path selection and crack length can be explained by means of a qualitative fracture mechanics analysis.