Due to external loads, damage extension forces will drive mechanical and electrical damages to evolve in piezoelectric materials from the viewpoint of continuum damage mechanics, so it is important for understanding piezoelectric fracture mechanism and estimating the utilizing life and safety of a piezoelectric device to study the damage extension forces. Based on the static piezoelectric damage constitutive model established in our previously published papers, general expressions of the damage extension forces are given. The finite element method and the iterative procedure are utilized to calculate the extension forces at crack-tips in the samples for the experiments of Park and Sun's, and then the linear and nonlinear fracture criteria are presented. It is revealed from the list of the normalized mean square deviations that the nonlinear criterion is better.
