Ph.D. Dissertation Defense by William S. Oates
Friday, July 9, 2004
(Dr. Christopher S. Lynch, Chair)
"Fracture of Ferroelectric Materials"
Ferroelectric materials continue to find increasing use in actuator, sensor and transducer design. Questions regarding lifetime and reliability remain a concern due to the inherent low fracture toughness and complex material behavior. The poling procedure required for use in actuator and sensing devices introduces anisotropy in elastic and dielectric coefficients as well as piezoelectric coupling between the mechanical and electrical fields. This introduces complex fracture behavior which necessitates advanced analytical techniques and fracture characterization.
In this dissertation, fracture mechanics of ferroelectric materials is evaluated by employing different analytical techniques and experimental methodology. The theoretical work has focused on linear piezoelectric coupling that accounts for the influence of anisotropy and heterogeneity on fracture. A new orthotropic rescaling technique is presented that explicitly solves the anisotropic linear elastic piezoelectric crack problem in terms of material coefficients. The effects of heterogeneities on electric field induced microfracture are analyzed by implementing a crack at the edge of a heterogeneous piezoelectric inclusion. A positive, flaw-localized driving force is realized when permeable crack face boundary conditions are considered.
The experimental portion of the work evaluates fracture behavior in the ferroelectric
ceramic, lead zirconate titanate (PZT), and the ferroelectric relaxor single
crystal PZN-4.5%PT. Relative humidity and electric boundary conditions are shown
to have significant effects on crack kinetics in PZT. Fracture anisotropy in
single crystal PZN-4.5%PT is characterized using the Single-Edge V-notch Beam
(SEVNB) method and Vickers indentations. Scanning electron micrographs are used
to determine the crack profile near the crack tip which leads to a prediction
of crack tip toughness and local energy release rate. A weak cleavage plane
is identified in the single crystal relaxor which contains a significantly lower
toughness relative to ferroelectric ceramic PZT.