As linear ultrasonic technique is usually limited in detection of micro-cracks in plate, a nonlinear ultrasonic technique using mixed-frequency Lamb waves was proposed. Interaction of pure S0 mode Lamb waves and micro-cracks was simulated by finite element method. Spectrum analysis was utilized to extract the amplitude of sum frequency and difference frequency in receiving signal. The impact of crack length and crack thickness on the amplitude of sum and difference frequency was also taken into account. The results showed that there is a positive relationship between the crack length and the amplitude of sum and difference frequency, and this two amplitudes would decrease while the crack thickness kept increasing. Then an experiment was conducted to prove the feasibility of micro-cracks detection using the mixed-frequency nonlinear Lamb waves technique.
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