| Multi-layered structures are being used in a wide range of applications. The often-observed fatigue failure due to interfacial crack growth limits their performance. New accelerated fatigue techniques are needed to address this problem.; We have developed a new fatigue-testing technique for adhesive-metal interfaces. Using a piezoelectric actuator, we can apply cyclic loading of frequencies up to 20 kHz, several orders of magnitude greater than that in a conventional technique. Moreover, the technique directly addresses the debonding problem in piezoelectric multi-layered smart structures.; A general solution to an interfacial crack between a piezoelectric and an elastic material under in-plane electric loading was developed. Closed-form solutions for stresses, electric fields, and electric displacements were derived. A new stress singularity was found. We obtained special types of singular electric fields and electric displacements, which could induce 90° domain switching and affect the crack behavior.; Finite-element simulations of the crack driving force were performed. The results provided valuable information about the effect of material properties and specimen geometries on the loading mode and magnitude. Most importantly, we found that the driving force provided by the piezoelectric actuator is intrinsically three-dimensional and that the plane-strain solution underestimates the driving force.; In addition, we conducted experiments to investigate the mechanisms responsible for the nonlinearity and ferroelectric fatigue of PZT-5H, a widely used piezoelectric material. We conducted in-situ x-ray diffraction experiments on freshly poled, depolarized, and electrically fatigued PZT samples under different static electric fields. Our results provided direct evidence that non-180° domain switching and phase transition are the main mechanisms for the nonlinearity of the material. We also found that residual stress might cause the difference between piezoelectric properties for positive and negative electric fields.; Furthermore, our results showed that repeated non-180° domain switching and phase transition alone did not result in ferroelectric fatigue. Rather, this fatigue is more likely due to electric charge accumulations at grain boundaries induced by repeated 180° domain switching. |
