TOFD detection of shallow subsurface defects in aluminum alloy thin plates by half-skip mode-converted wave

Aluminum alloy is widely applied to the aerospace field. However, the inspection of thin plates using Time-of-Flight Diffraction (TOFD) technique is restricted by the near-surface dead zone because of the coupling between diffracted longitudinal wave and lateral wave. The half-skip mode-converted wave is introduced to decrease dead zone and detect defects in aluminum alloy thin plates by increasing ray path and propagation time. The quantitative correlation for the diffracted shear wave from longitudinal back-wall wave is deduced in combination with the acoustic path, realizing the accurate location of shallow subsurface defects. Simulated and experimental results indicate that the dead zone is decreased by 38% by the half-skip mode-converted wave, and the location errors are within 5% for the aluminum alloy plate with a thickness of 7.0 mm. Compared to other alternative TOFD techniques, half-skip mode-converted wave has better response amplitude and positioning accuracy, demonstrating strong applicability in TOFD inspection of thin plates.