Abstract
In this study, we investigated nonlinear ultrasonic characterizations with NRUS (nonlinear resonant ultrasound spectroscopy) and three-wave mixing methods of the aluminum alloy plates subjected to zero-to-tension fatigue loading. 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. The combination with PZT and EMAT (electromagnetic acoustic transducer) were applied to for NRUS method. In three-wave mixing method, two intersecting ultrasonic waves produce a scattered wave when the resonance condition is satisfied. The wave amplitude was measured. Nonlinear ultrasonic characterizations with these two methods exhibit high sensitivity to micro-structural change of the damaged material. They rapidly increased from 60 % of fatigue life to the fracture. Nonlinearity in three-wave interaction methods sensitivity is higher than that in NRUS. These noncontact resonance-EMAT measurements can monitor the evolutions of ultrasonic characterizations 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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