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Crack Propagation and Detection in PZT-Multilayer Ceramics
Keywords: Impedance Spectroscopy, Fracture Mechanics, PZT
Piezo multilayer ceramics are increasingly used under extreme condition such as high pressures in engine injection systems. The mechanical stability and reliability of the ceramic device is of major importance for proper operation. Critical functional defects are caused by material fracture and crack propagation in the device. The flaw propagation in PZT multilayer ceramics under mechanical load was examined using impedance spectroscopy and three-point-bending studies. Initial flaws were generated by applying a sinus ac-field on the specimen. The crack was successively promoted and after the release of the external mechanical load the impedance spectroscopy was conducted. As a measure for flaw extension, the shift in the resonance frequencies and the sub-resonance height of the impedance spectroscopy was used. A functional dependence of the resonance frequency and the phase shift on the crack length was found. The crack propagation was studied on flaws starting at the positive and negative electrode, respectively. Therewith it was shown, that the maximum fracture strength as well as the crack propagation depends on the electrode potentials. The variation in the fracture strength is caused by the different observed fracture mode: interface cracking, matrix cracking and a mixed-mode. The morphology of the examined fracture faces were ascribed to an anisotropic behaviour, which is created by the sample processing, e.g. the poling process. A modified poling procedure with a lower poling temperature was analysed, which yielded a reduction of the anisotropy of the electrode strength. The result revealed an influence of the lattice defects in the ceramic on the mechanical strength anisotropy, which was prevented by lower defect mobility at a lower poling temperature.
Tobias Kuehnlein, Development Engineer / Ph.D. Student
Robert Bosch GmbH
Bamberg, Bavaria 96050,
Germany


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