| As a branch of signal processing techniques, time-frequency analysis has been widely used in a variety of engineering fields. Time frequency analysis can view signal full-scale, different from traditional signal analysis methods either in time domain or in frequency domain. This paper investigates the analysis of ultrasonic Lamb wave in non-destructive testing (NDT) field and the application of time-frequency method in on-line damage monitoring of smart composite structures. It is indispensable to analysis and detecting of the lamb wave in the NDT field. Considering the multi-modes and dispersion characteristics of Lamb wave, wavelet method is applied here to obtain the dispersion curve of frequency Lamb wave in low frequency range. The dispersion curve for the tested material is acquired based on the measurement data and time-frequency analysis method. The application of the recently proposed time-frequency analysis method, the Hilbert-Huang Transform(HHT) in Lamb wave detection is also investigated. The measured Lamb wave signal is decomposed by empirical mode decomposition (EMD), a procedure of HHT. Thus, a de-noising method based on EMD is developed to extract useful signal from the experiment data. The advantages and disadvantages of HHT in signal processing of non-destructive testing by using lamb wave are discussed. For the realization of the on-line detection of Lamb wave, the smart layer technique, which combines the PZT transducers with the flexible printing circuit technology, is applied to the tested structure. By forming a network with distributed sensors, a large area on the structure can be monitored. Hardware and software are designed to perform the scan of this sensing network. Mechanic and electric property of the designed smart layers is studied through experiment and the validity of the proposed method is further evaluated. The research is supported by the National Natural Science Foundation of China under contract number of 50135030.
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