Corrected Mode-Converted Wave Method for Detecting Defects in TOFD Dead Zone

The dead zone in ultrasonic time-of-flight diffraction (TOFD) restricts the quantitation of near-surface defects. Alternative TOFD techniques introduce other tip-diffracted/scattered waves with longer propagation times to implement defect detection, but the applicability is influenced due to ignoring the change of acoustic path of the diffracted shear wave in wedge. In this paper, the mode-converted wave (MCW) method is exemplified to analyze the propagation path of diffracted shear wave and the change of the received point in wedge. On this basis, the corrected MCW method is proposed by establishing the relationship between the propagation time of MCW and defect depth according to Fermat's principle. The simulation and experiments show that the depth of near-surface dead zone is reduced by 60% with the corrected MCW method. The positioning errors of the defects in the aluminum alloy sample with a thickness of 20.0 mm are no more than 0.15 mm by using the TOFD probes with 10 MHz center frequency and 40 mm probe center spacing (PCS) and the 60 & DEG; wedges. Compared to the conventional MCW method, the corrected method widens the range of applicable TOFD signals in B-scan images and reduces the measurement errors of defect depths to within 3.5% under ideal conditions.