Shallow Buried Defect Testing Method Based on Ultrasonic TOFD

Ultrasonic time of flight diffraction (TOFD) suffers from the shortcoming of lack of near surface resolution, primarily owning to the superposition of the lateral wave and the shallow buried defect (SBD) wave, and the insensitive region beneath the inspection surface because of the restricted beam width of the probes. This paper presents a method for the detection of SBD based on conventional TOFD. The method is named as TOFDW because it employs a three-fold reflected longitudinal wave whose propagation path in the testing piece looks like the letter “W”. Based on Pythagoras’s theory, mathematical models are developed to locate SBD. In TOFDW mode, the time difference between the lateral wave and the SBD wave can be enlarged greatly so that they would never be superposed. In addition, through reflecting the incident longitudinal wave by bottom surface, the near surface region of the testing piece is covered with an intense acoustic field, making TOFDW free of the restriction of the probes’ beam width. These contribute to the improvement of the near surface resolution. The experiments show that with the proposed TOFDW and mathematical models, the artificial defect tip with the buried depth of 1 mm can be detected, and the tips can be measured within an error of 0.3 mm; the weld defect with the buried depth of 2 mm can be identified effectively, and the location can be measured within an error of 0.5 mm.