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Time reversal technique is investigated for health monitoring. The propagation characteristics of Lamb waves in plate-like structure, with emphasis on group velocity and characteristic wave curves are investigated theoretically. The measured signal were decomposed by matching pursuit to enhance the time reversibility of Lamb wave in plate-like structure, the result shows that the defect signals is easy to recognize, then a damage imaging technique is developed by using the time reversal method.

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Ultrasonic guided wave have the multi-modes and dispersive characteristics, and its modes are easy to be converted at boundary or when running into defects in pipes, which makes the discrimination of different guided waves modes of the reflection signals in pipes NDT very hard. In this work, firstly, the experiments are carried out to test two kinds of stainless steel pipes by applying guided waves NDT, one is integrated pipe and another is non-integrated pipe with a small hole defect, and the detected guided waves echo signals are respectively obtained. Secondly, the measured signals are processed by matching pursuit method and the Chirplet matching atom parameters are calculated. By calculating the time-frequency distributions spectrum of detected guided waves echo signals, torsional, flexural and longitudinal guided waves modes are identified from the intact pipe, and the two wave-packets with torsional and flexural guided waves modes are also identified from the pipe with hole defect. The results showed that the matching pursuit method has a tremendous advantage to identify different guided waves modes in pipes nondestructive testing.

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