Study On The Fracture And Fatigue Of Ferroelectric Materials

Ferroelectric and piezoelectric ceramics have been widely used due to the coupling between mechanical and electrical behavior. However, their inherent brittleness limits their application. Hence, the study on fracture and fatigue of ferroelectrics becomes more and more important.In this dissertation, theoretical, computational and experimental methods have been adopted to investigate fracture and fatigue of ferroelectrics. The dissertation contains the following work:In order to judge whether a crack will grow, the surface energy release rate with consideration of electrical saturation is introduced by analyzing the energy exchange in ferroelectrics with a main crack. For the field including electrical yielding, the criterion is consistent with the experimental results, and has a clear physical sense in comparison with other criteria.The effect of remanet polarization on electro-mechanical fields in ferroelectric ceramics with a flaw has been investigated. Two types of flaw, i.e. an elliptical cavity and a line crack, have been studied in terms of the exact boundary conditions. The results show that the effect of remanent polarization is similar to that of electric field and should not be ignored. Moreover, the effect of remanent polarization can be used to explain some experimental phenomena and make the solution with a linear constitutive equation more complete.The tests of the electric fatigue on ferroelectric specimens with through pre-cracks were conducted. As far as we know, it is the first time that the electric fatigue test with a through pre-crack is performed. The experimental results show that there were two distinct fatigue patterns: under a low electric load, the emergence and growth of microcracks was a major fatigue mechanism, while under a high electric field, the propagation of macroscopic crack is a dominant fatigue mechanism. The crack growth was in company with the electric breakdown occurring inside the crack.Two theoretical models are developed to explain the fracture induced by alternate electric fields and predict the crack growth rates measured in the tests. Another model is proposed to analyze the mechanism of the emergence of microcracks and also can be used to explain some experimental results.A finite element model based on the simulation of domain switching is developed. The solving process is nonlinear and electromechanically coupling. And the variation of remanent strain is considered. In order to obtain a stable and accurate solution, a new Double-Gibbs energy criterion for domain switching is proposed and an iterative procedure is designed. The numerical examples such as cavity problem, central crack problem as well as the multilayer electrode-piezoelectric structure display that the proposed FEM can simulate the domain switching and the computational results are consistent with experimental data. In addition, a full energy criterion of domain switching is also proposed to overcome the shortage of other existing criteria in FEM. It is found that the simulations with this full energy criterion can characterize the experimental constitutive curves well.