Frequency selection and time shifting for maximizing the performance of low-frequency guided wave mixing

Abstract

Evaluation of fatigue damage using nonlinear guided wave mixing has been studied extensively over the past decade. It was found that the combinational harmonics as a result of wave mixing of quasi-synchronized wave modes show attractive features and are sensitive to fatigue damage. However, there were very limited studies on the frequency pair selection and time shifting of the wave mixing signals. In this study, a method is developed and theoretical equations are implemented, which provides a guide on the selection of the wave mixing frequency pair. The proposed frequency pair selection method can advance the trial-and-error method that was generally used for low-frequency wave mixing, which has a large number of possible frequency pairs, and deliberate selection is needed to avoid the overlapping of the generated harmonics. A new time shifting technique is also proposed to enhance the generation of second and combinational harmonics due to the collinear wave mixing. This technique can be very useful when there are various limitations exist on the selection of the excitation frequencies. The efficiency of the proposed method is validated by a series of numerical and experimental studies. Overall, the new findings can be utilized to further advance the development of new damage detection methods using the guided wave mixing method.