This work stems from a novel, high frequency fatigue testing technique for adhesive-metal interface crack growth using a piezoelectric actuator. It consists of several parts. (i) We obtained a general solution to an interfacial crack between a piezoelectric and an elastic material under in-plane electric loading which predicted a special type of singular electric field and electric displacement that could induce 90° domain switching and affect the crack behavior. (ii) We performed experiments to investigate the mechanisms responsible for the nonlinearity and ferroelectric fatigue of PZT-5H. Our in-situ x-ray diffraction experiments on freshly poled, depolarized, and electrically fatigued PZT samples provided direct evidence that non-180° domain switching and phase transition are the main mechanisms responsible for the nonlinearity of the material.