Applicability and applications of alternative TOFD techniques.

Ultrasonic time-of-flight diffraction (TOFD) technique is applied to non-destructive testing in engineering, but the dead zone influences its applicable range. Alternative TOFD techniques adopt the indirect diffracted waves having long propagation times to decouple from the lateral wave and detect near-surface defects. It should be noted that the applicability of these diffracted waves varies with parameter conditions employed for detection, e.g., sample thickness, defect depth, inspection frequency and probe center spacing (PCS). In this paper, aluminum alloy plates are selected as the research objects for comparative analysis and actual application of alternative TOFD techniques. For each alternative TOFD technique, the minimum time difference between structural waves, e.g., lateral wave and back-wall waves, and indirect diffracted wave in TOFD signals and images is defined as the evaluation index for characterizing its applicability. Three-dimensional diagrams of the applicability of mode-converted wave, LS-L wave and LL-S wave (L and S are the longitudinal wave and shear wave, respectively) are provided sequentially by theoretical calculations. On this basis, the theoretical model for selecting the optimal alternative TOFD technique under reasonable parameter conditions is established by compositing different three-dimensional diagrams. TOFD inspection was implemented on the defects with depths of 2.0-3.0 mm in the aluminum alloy plates with 7-20 mm thicknesses. The experimental results indicate that the alternative TOFD technique determined by the theoretical model is most suitable for detecting shallow subsurface defects. The range of dead zone is decreased effectively with the measurement errors of defect depths no more than 3.6 %. The related works have universality and can be extended to TOFD inspection of other materials and structures in future.