Browsing by Author "Landis, Chad M."

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    Fracture toughening of ferroelectric ceramics under electro-mechanical loading
    (2007) Wang, Jianxin; Landis, Chad M.
    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.
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    On the crack face boundary conditions in electromechanical fracture and an experimental protocol for determining energy release rates
    (2007) Li, Wenyuan; Landis, Chad M.
    An experimental protocol for measuring the energy release rate in a non-linear reversible electromechanical body is proposed and summarized. The potential results are capable of shedding light on the true physical nature of the conditions prevailing at the crack surface and in the space within the crack. The experimental procedure is simulated numerically for a linear piezoelectric specimen in a four point bending configuration subjected to electrical loading perpendicular to the crack. Two efficient finite element formulations are presented for nonlinear crack face boundary conditions. Methods for the numerical determination of the crack tip energy release rate and the simulation of the experimental method for obtaining the total energy release rate are developed. It is shown that the crack tip energy release rate calculated under energetically consistent boundary conditions is equal to the total energy release, when the exact boundary conditions are used, there is no such agreement.
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    Polarization saturation simulation of ferroelectric polycrystals
    (2004) Sheng, Jianshun; Landis, Chad M.
    A micro-electromechanical constitutive model for ferroelectric polycrystals is introduced and applied to determine possible uniaxial remanent strain and remanent polarization states in the material. Simulations based on the model are carried out to determine the polarization saturation states under different levels of prevailing remanent strain. The simulation results clearly demonstrate how the polarization develops to the saturation and how the saturation polarization depends upon the remanent strain. The obtained information of saturation states can be applied in phenomenological constitutive models.