Fracture toughening of ferroelectric ceramics under electro-mechanical loading
Landis, Chad M.
Doctor of Philosophy
In this dissertation, the fracture toughening behavior of ferroelectric materials under different electromechanical loading conditions is predicted and compared to available experimental observations. A multi-axial, electromechanically coupled, incremental phenomenological constitutive model for ferroelectric ceramics is developed first. The constitutive model is then implemented within the finite element method to study the effects of electric field on the Mode I steady crack growth under plane stress and plane strain conditions. Toughening behaviors of electrically permeable cracks are simulated on both initially unpoled and poled materials with electric field applied in-plane or parallel to the crack front. The finite element results give detailed electromechanical fields, remanent strain and remanent polarization distributions, domain switching zone shapes and sizes, and the crack tip energy release rate. It is shown that the toughening is related to the size of the concentrated switching zone that is confined to a small region around the crack tip. The model predicts a range of phenomena that indicate that the toughening is dependent on both the level of electric filed applied and on the polarization state. In addition to the effects of electric field, the effects of the plane-stress constraint and transverse stress are also established in the out-of-plane poled cases. In a similar manner to the electrically permeable cracks, the crack growth simulations for electrically conducting crack face boundary conditions are also performed. The results predict the toughening variations under combined electromechanical loadings for poled or unpoled materials. The electromechanical fields from the finite element results are used to determine the stress and electric field intensity factors around the crack tip. The favorable comparison of the present model to the experimental observations suggest that ferroelectric switching behavior is more accurately modeled with an incremental plasticity formulation, rather than as an unstable phase transformation. The nonlinear studies of this dissertation not only explain most available experimental phenomena but also enhance the understanding of the nature of fracture in ferroelectric ceramics.
Mechanical engineering; Engineering; Materials science