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Signal processing in ultrasonic NDE using time-frequency representations

Signal processing has been used in the ultrasonic nondestructive evaluation (NDE) of materials for a number of years. The time domain analysis of NDE signals includes signal enhancement, deconvolution, matched filtering, etc. and the results can often be linked to physical parameters. Frequency (Fourier) analysis of NDE signals provides information about the major frequency components which may be closely related to the geometry of hidden defects, material properties, etc. It is clear that the use of frequency domain or time domain information alone is not adequate for effective NDE. Recent advances in digital signal processing have resulted in new and effective approaches to joint time-frequency representations that provide more information from the data which may be useful for defect characterization and classification. Among all time-frequency analysis methods, both Wigner distributions and wavelet transforms have been considered for use in ultrasonic NDE. However the full potential of time-frequency representation in ultrasonic NDE is yet to be explored. In this paper, a fundamental analysis of time frequency representations is briefly presented. Several application areas of the time-frequency representation in ultrasonic NDE including geometrical defect discrimination, NDE of bonded materials, sizing problem with geometrical defects, and waveform decomposition are examined in detail.