Abstract:

When Time-of-Flight Diffraction （TOFD） technique is used to detect the bottom defects in the aluminum alloy plate， there is a near-surface dead zone under the effect of the pulse duration of lateral wave. In this paper， a TOFD half-skip mode wave method is proposed to reduce the dead zone. The characteristics of the diffracted wave of the first back-wall reflected wave at the defect tip and its symmetry in B-scan image are employed to derive the depth determination formula of bottom defect. Simulation and experiment results show that with the proposed method， the range of the near-surface dead zone is reduced by more than 64%， and the positioning errors of the defects with depth of no less than 2.0 mm are within 6.32% for the aluminum alloy plate with a thickness of 7.0 mm by using the probes with 10 MHz center frequency and 40 mm probe center spacing. Compared to the mode-converted wave and the TOFD-W wave， the half-skip mode wave is decoupled with the back-wall wave and is close to the lateral wave， presenting stronger applicability in detecting the bottom defect in aluminum alloy plate.

Key words: Time-of-Flight Diffraction (TOFD), dead zone, half-skip mode wave, aluminum alloy, quantitative detection