Crack growth during poling and polarisation reversal in commercial piezoceramics

The growth of Vicker's cracks during the poling of piezoelectric ceramics w as studied for two PZT based compositions with different tetragonal distort ion of the perovskite structure. Studies on the two compositions with diffe rent poling electric fields and similar initial crack lengths showed that c rack growth was not proportional to the induced stress free longitudinal st rain by 90 degrees domain reorientation. This observation is not consistent with the previously proposed mechanism of crack growth based on a strain m ismatch between the material at the crack flanks and the rest of the cerami c. This mismatch was proposed to occur because of the reduction of the elec tric field within the crack flanks. The reasons for the disappearance of th e mismatch are discussed here, and an alternative mechanism of crack growth consistent with our results is proposed, which takes into account the elec trically induced stress gradient at the crack tip, produced by the piezoele ctric effect. Cracks were also found to grow during a small number of subse quent polarisation reversals, the explanation of which must be due to some other additional effect.