Nonlinear behavior of piezoelectric ceramic under combined electro-mechanical loads.

摘要

To generate large forces and displacements piezoelectric ceramics are operated in the nonlinear regime at high electric and mechanical fields. Current understanding of the nonlinear piezoelectric behavior is limited. This dissertation contributes to an understanding of physical mechanisms of nonlinear piezoelectric behavior by investigating, first, the effect of constant prestress on the dielectric (polarization) and piezoelectric (strain) response and, second, the effect of bias electric field on the stress-strain response of a lead zirconate-lead titanate PZT-5H piezoelectric ceramic. It is found that a properly selected mechanical preload will substantially increase the actuation characteristics of the ceramic. That is, the displacement output can be more than doubled under optimum preload. It is found that a cyclic mechanical loading of piezoceramics under constant electric field results in relatively large closed stress-strain hysteresis loops representing energy absorbed during the work cycle. The energy absorbed of up to 40% may be achieved under optimum bias electric field. A detailed physical explanation of the observed nonlinearities is provided by considering a representative material volume divided by one non-180° domain wall. To complement the description, an analytical model is developed based on the rule of mixtures formulation and a domain wall pressure concept. The principal conclusion is that the piezoelectric, dielectric, or elastic response of the ceramic under combined electro-mechanical loading is proportional to the volume fraction of the domains available for switching and the balance of domain wall pressures from electrical and mechanical loads. While the domain wall motion increases the response of the material, it does so at the cost of reducing fatigue life. An investigation into fatigue behavior of piezoelectric ceramics is conducted with a novel specimen configuration with a notch between macroscopically engineered dissimilar domain structures. The notch is cut perpendicular to the electrodes as opposed to traditional studies where the notch is introduced parallel to the electrodes. It is found that fatigue degradation is larger for larger magnitudes of negative electric field because negative electric field may result in microscopic domain reversal which gives rise to high stress concentration.