Accurate Defect Sizing using Phased Array and Signal Processing

Accurately sizing cracks has been a main objective of ultrasonic nondestructive evaluation for decades. Early sizing techniques based on echo amplitude have repeatedly been shown to be deficient. Diffraction techniques offer a significant improvement, but are also limited by pulse length and reliance on operator interpretation. In particular, Time-Of-Flight Diffraction is limited by pulse length in terms of the smallest midwall defect it can size. Tip back diffraction works well with phased array S-scans, but is also limited by similar factors to those limiting TOFD. Overall, diffraction techniques (TOFD and back diffraction) currently have a typical accuracy of + 1 mm, with best practice pushing the accuracy down to ~0.5 mm. The University of Toronto and Olympus NDT Canada have been collaborating on the development of digital signal processing techniques to improve the time resolution of ultrasonic signals, based on Weiner filtering combined with autoregressive spectral extrapolation. Early results on synthetic signals and simple laboratory test specimens showed promise, and the technique has now been incorporated into S-scans on single fatigue cracks. The results showed a significant improvement over current sizing accuracies using traditional S-scans.