A frequency-mixing nonlinear ultrasonic technique for micro-crack detection

Toward invalidity of the linear ultrasonic technique for nondestructive detection of micro-crack in structures, a frequency-mixing nonlinear ultrasonic technique was developed for micro-crack detecting. Firstly, the mechanism of frequency-mixing and signal processing method for feature extraction were investigated; secondly, By experimental measurement of frequency response of mixing frequencies, the optimum parameters for frequency-fixing detection frequencymixing were determined; experiments for micro-crack detection were conducted in mode of heterolateral excitation, and research focused on the influence of frequencies of excitations on the detection results. It was shown that in mode of heterolateral excitation, not only fatigue micro-cracks in the structure can be detected, but also the position of the defect can be located. Meanwhile, the frequencies of excitations have great influence on signal-to-noise ratio. The performance of damage detection experiments was best when the amplitudes of mixing frequencies are maximum. Therefore, frequency-mixing nonlinear ultrasonic technique is a valid approach for crack detection and location.