Detection of a Crack and Determination of Its Position in a Plate by the Nonlinear Modulation Method Using Lamb Waves

Features of detection of a single crack in a plate are experimentally studied by the nonlinear acoustic modulation method provided that single defects (cracks and cavities) are located between a low-frequency (tens of kilohertz) ultrasonic transducer of Lamb waves and a high-frequency (units of megahertz) ultrasonic transducer. It is shown that using modulation of the defect characteristics by a sequence of phase-inverted Lamb waves, by the level of modulation of the high-frequency wave reflected from the defect one can determine its type (a crack or a cavity). Changing step by step the interaction region of low-frequency and high-frequency waves by varying a delay between their radiation, it is possible to study selectively the nonlinear acoustic characteristics of the defects arranged in series on the location path. Based on the obtained results, we created a combined transducer which scans a defect and modulates its characteristics simultaneously in the preset area and which is suitable for moving it by hand on the surface of a plate for seeking hidden cracks.