Applications of time-frequency distributions for ultrasonic flaw detection

In this study, we have examined time-frequency distributions Gabor transform, Wigner-Ville distribution, Choi-Williams distribution and Wavelet transform for improved flaw detection performance in ultrasonic nondestructive testing applications. A new methodology is presented with respect each T/F distribution methods, with necessary steps to achieve maximum flaw echo visibility enhancement. This methodology describes i) mapping ultrasonic signal to T/F domain, ii) projection from T/F representation back to time domain, iii) interpretation of the signal using order statistics. These steps include choosing the optimal time and frequency window sizes (based on Heisenberg principle), and the appropriate post-processing detection method to minimize the effect of null-observations. To demonstrate the validity of the methods, we discuss and draw an analogy between T/F distributions and the conventional Split-Spectrum Processing flaw detection method. The analytical and experimental studies verify the feasibility of the T/F techniques for NDE applications.