Abstract
In this study, we applied fatigue damage evaluation in pure copper and aluminum plates subjected to zero-to-tension fatigue loading through monitoring with non-contacting NRUS (Nonlinear resonant ultrasound spectroscopy), and nonlinear three-wave mixing methods, which are resonance-based techniques exploiting the significant nonlinear behavior of damaged materials. In NRUS, the resonant frequency of an object is studied as a function of the excitation level. As the excitation level increases, the elastic nonlinearity is manifest by a shift in the resonance frequency. In nonlinear three-wave mixing method, two intersecting ultrasonic waves produce a scattered wave when the resonance condition is satisfied. The wave amplitude is measured. NRUS and nonlinear three-wave interaction methods exhibit high sensitivity to micro-structural change of the damaged material. They rapidly increase from 60 % of fatigue life to the fracture. TEM (Transmission Electron Microscope) observation in copper specimens supported this view. Three-wave interaction methods sensitivity is higher than NRUS. These noncontact resonance-EMAT measurements can monitor the evolution of non-linear ultrasonic phenomena throughout the fatigue life and has a potential to assess the damage advance and to predict the fatigue life of metals.
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