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For a metallic component, prediction of a remaining useful life (RUL) is of extreme importance to prevent a sudden failure of the structure. If a fatigue crack exists, the nonlinear interaction of a low frequency pumping wave (LF) and a high frequency probing wave (HF) generates additional responses at the frequencies equal to the sum and difference of the input frequencies. In this study, the nonlinear ultrasonic modulation is used for fatigue crack prognosis of the lifting-lug that connects a tension cable. Nonlinear ultrasonic modulation components are used to define a fatigue index. The functional relationship between fatigue index and the number of loading cycles is established based on Paris–Erdogan theories. Then, by curve fitting of the established function to the fatigue index values obtained at various loading cycles, the RUL corresponding to a fatigue index is estimated from the fitted function. The proposed RUL estimation method is associated with the following advantages: (1) the fatigue index is defined as a function of nonlinear ultrasonic modulation components, (2) the functional relationship between fatigue index and loading cycles is established for lifting-lug structure, and (3) online remaining useful life is predicted and updated using sequentially measured ultrasonic signals. To validate the proposed method, the lifting-lug specimen is used and subjected to cyclic loading. The experiment is conducted using surface-mounted piezoelectric sensors for ultrasonic signal excitation and measurement. The experimental results confirm that the proposed method can estimate the RUL successfully.
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