Numerical and experimental study of circular disc electromechanical impedance spectroscopy signature changes due to structural damage and sensor degradation

Abstract

The article describes a study on the use of the electromechanical impedance spectroscopy method for crack-damage detection with piezoelectric wafer active sensors bonded to thin-walled structures. The study combines analytical, finite element method, and experimental methods. The specimens under consideration consisted of circular plates with circular piezoelectric wafer active sensors bonded at the center. Some of the plates were pristine, and others were damaged. The damage consists of simulated cracks (thin laser-machined slits) placed at various locations away from the center. For pristine plates, the electromechanical impedance spectroscopy spectrum was predicted with two numerical methods: (a) analytically using closed-form solutions and (b) Comsol 4.3 Multi-physics software using two-dimensional axisymmetric elements. The results were compared with experimental measurements performed with an HP 4194 impedance analyzer. This comparison allowed us to tune the model parameters. For plates with simulated cracks, the electromechanical impedance spectroscopy spectrum was predicted with three-dimensional finite element method models and then compared with experiments. Overall, the effect of crack damage on electromechanical impedance spectroscopy signature was found to consist of frequency shifts, peak splitting, and appearance of new peaks. These effects were predicted by the numerical models and confirmed by experiments. Modeshapes were also measured using a scanning laser Doppler vibrometer and compared with finite element method predictions. Interesting new modeshape features (local high-frequency modes) were observed near the crack. These features tend to explain some of the new peaks appearing in the electromechanical impedance spectroscopy spectrum. Further refining of the finite element method model was performed to capture variability effects associated with plate thickness, piezoelectric wafer active sensors thickness, adhesive layer thickness, and imperfect adhesive layer. The effect on the piezoelectric wafer active sensors asymmetrically placed electrodes and soldering points was also studied. It was found that these variations modify the electromechanical impedance spectroscopy signatures, but much less than the changes induced by the presence of the crack. Hence, it was concluded that the capability of the electromechanical impedance spectroscopy method to detect actual damage was not affected by these variations. A proper physics-based analysis and interpretation of the electromechanical impedance spectroscopy spectral changes are performed.