In this work the crack propagation behaviour of a commercially obtained lead zirconate titanate (PZT) piezoceramic is studied under monotonic and cyclic loading. Piezoelectric ceramics contain an electrical dipole in their crystal structure such that when a mechanical load is applied an electrical potential is formed, and vice versa. Permanent electrical domain orientation may be induced by the application of a large electrical potential in a process known as poling. Piezoelectric ceramics are used for applications such as actuators, sonar transducers and microphones. These and most other application involve high levels of cyclic loading and fatigue degradation is will known. However, cyclic fatigue to date has only been considered under electrical loading despite the fact that most components are subjected to high levels of mechanical load. Crack growth studies are rare. In the vicinity of a crack tip, local stress causes switching of the crystal domains leading to dilation perpendicular to the crack propagation direction. This explains the observed crack growth resistance (R-curve) toughening which is characterised in this work. Furthermore, it is shown that the extent of toughening is dependent upon the crack propagation direction relative to the poling direction. When a cyclic mechanical load is applied, subcritical crack growth is shown to occur below the plateau value of the R-curve and at rates which are independent of poling direction. A measurement of static fatigue rates show that a true cyclic fatigue degradation process exists. Another experiment which measured the R-curve behaviour as a function of intermittent loading time showed that the observed fatigue behaviour could not be explained by reverse switching of the crystal domains. The question arises therefore as to the mechanism of cyclic fatigue degradation in PZT and whether it is the same under both cyclic electrical and mechanical loading. In both cases it is currently unknown however a parallel study has shown that plastic strain accumulates in bulk PZT under mechanical loading in a strain-softening type process. The link between this and crack propagation behaviour will be discussed.
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