Quantitative Detection of Shallow Subsurface Defects by Using Mode-Converted Waves in Time-of-Flight Diffraction Technique

An easy-to-implement method based on mode-converted waves was proposed for quantitatively detecting shallow subsurface defects using the ultrasonic time-of-flight diffraction (TOFD) technique. First, an isotropic model with a shallow subsurface defect was established to analyze the ray path of the mode-converted wave in TOFD B-scan image. The mode-converted wave with the shortest travel time and highest amplitude was identified to calculate the flaw depth via Snell's law. Subsequently, the quantitative expression of flaw depth was corrected by introducing the travel time of mode-converted wave, improving the measurement accuracy of the flaw height. Simulated results showed that the depth of the dead zone in carbon steel was reduced from 5.5 to 2.4 mm. The measurement error of shallow subsurface cracks was no more than 0.02 mm when the crack height was greater than or equal to 0.7 mm. Finally, three shallow subsurface defects were experimentally detected with the mode-converted waves in B-scan image. The quantitative errors of the measured heights were within 8.5% of the actual values, verifying the validity of the proposed method. This method is appropriate for the quantitative detection of shallow subsurface defects in thick-walled components.