Studies On Fracture Failure Of Piezoelectric Materials Used In Ultrasonic Motors

This project was granted by the project of the National Natural Science Foundation of China. (Contract No: 50735002).Ultrasonic motor is a high-tech product based on the piezoelectric effect and ultrasonic vibration. Due to the complex service environment, cracking or fractures may often exist in piezoelectric ceramic of ultrasonic motor so that the ultrasonic motor can not work normally. In the present work, the stator of the ultrasonic motor was modeled as an elastic-composite beam, and then studies on the fracture and failure of the piezoelectric ceramic are made theoretically and experimentally. Main content and results are as follows:The developing history of ultrasonic motor was summarized, and the fracture and failure phenomenon in the ultrasonic motor then was pointed out. According to the working principle structure of the ultrasonic motor and its working environment, the elastic-composite beam structure model was developed, and the following works were completed:The devices and the other factors which influence the working stability of ultrasonic motor were pionted out. The main mechanism of failure and two main failure types were expounds. The two main failure phenomena (mechanical fracture failure and electric breakdown failure) were compared. Critical breakdown electric field was also derived.Simulation calculation was done to the elastic-composite beam that contains a prefabricated crack. It is found, from the stress distribution at the crack tip in the samples with prefabricated cracks at different position, that the middle throughout crack is the easiest to propagate in the polarization interval. Through the simulation results for the piezoelectric ceramics under different voltage, preloading value and temperature applying at the elastic-composite beam,it is obtained that the electric field and preloading can promote the crack growth, while the temperature retard the crack growth.J integral was made on the basis of simulation calculation of different boundary conditions for the piezoelectric ceramic.It is found that the J integral value increases constantly as the electric field and preloading value increases, but it constantly decrease and finally reaches a stability value as the temperature increases.Eexperimental works about the faliue of piezoelectric materials with cracks are also conduceted, and it is found that increasing the electric field can promote the crack growth, and the decreasing of strain which causes by preloading restrain the crack gracks, while as the temperature becomes higher, the crack trends to close, which is consistent with the above theoretical predications.