Fatigue crack evaluation using the non-collinear wave mixing technique

The non-collinear wave mixing technique was used for the quantitative evaluation of fatigue cracks. Two shear waves intersect and interact with fatigue cracks, which lead to the generation of a longitudinal wave. Non-collinear wave mixing experiments were conducted on different specimens, whereby the position at which the two incident waves intersected was scanned vertically below the notch and the measured signals were processed by filtering, time–frequency analysis and bispectral analysis. The presence of a wave packet in waveform, or wave-mixing component at the sum frequency in time–frequency domain, or bispectral peak at the sum frequency was used to characterize the fatigue cracks. The length of the fatigue cracks in two samples was estimated according to the spatial distribution of the ultrasonic nonlinear coefficient. The measured results corresponded closely with the actual lengths; therefore, the proposed method is effective for the quantitative evaluation of fatigue cracks.