The mechanical contact during material mechanical damage evaluation with electromechanical impedance technique was investigated, and the influencing factors, such as the times of in situ contact, contact stress, and area, were studied. The resonance frequency shift Df of electrical impedance signature was regarded as the damage identification index. Results show that the influence of mechanical contact on damage evaluation could not be neglected. With increasing times of in situ contact, the characteristic resonant peak shifted leftward gradually due to the resulted superficial indentations, and the Df value was in the order of 10 22 -10 21 kHz for each mechanical clamping. The evolution direction of the resonance peak with contact stress was reversed, and the Df value was about 10 21 -10 0 kHz. The order was equal or even more serious against that of the early-stage mechanical damage. The relationship between the Df value and the contact area, however, was elusive because of the combined effects of indentation damage and the compressive stress. Simulation experiments were conducted accordingly, and the abnormality was discussed. The quantitative results were compared between the steel and the Al alloy, and the variation with material property was demonstrated. Effective precautions to avoid the influence of mechanical contact were proposed.
The mechanical damage evaluation of Z2CN18.12N welding joint with Lamb wave was concerned in this study. The excitation parameters were optimized from the aspects of number of cycles, central excitation frequency, and the electric fields excitation profiles of the piezoelectric transducers. Moreover, the influence of the mechanical clamping was also analyzed. Results show that the enhanced anti-symmetric wave mode under the anti-symmetric electric fields excitation was better regarded as the excitation signal and the optimum excitation parameters had a strong dependence on the mechanical clamping, which should be carefully considered. The corresponding waveforms under the optimum parameters could be clearly defined and separated with each other very well. This would provide a basis for the future study of online monitoring and evaluation.
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