Measurement of energy release rates for cracks in PZT under electromechanical loads

An experimental method is presented, which allows measuring all components of the total energy release rate under conditions of controlled crack growth in a piezoelectric material. V-notched specimens of PZT poled parallel to the long axis are fractured in 4-point-bending. Additionally electric fields are applied parallel and anti-parallel to the poling direction, i.e. perpendicular to the crack surface. To determine the total energy release rate the mechanical and the piezoelectric compliance as well as the electrical capacitance of the sample are acquired continuously, using a small-signal-compliance method. The derivation of the data with respect to the crack surface area allows calculating the mechanical, piezoelectric, and electrical contributions to the total energy release rate. For an electric field of 500 V/mm the electrical contribution is negative and its absolute value is as high as the mechanical one whereas the piezoelectric value is comparatively small. Hence, the total energy release rate, based on the used method, almost vanishes under these conditions and cannot be taken as a valid fracture criterion. The expected effect in retarding crack growth has not been observed.