Mode for near surface defect detection based on ultrasonic TOFD

Aims at the problem of near surface dead zone in ultrasonic TOFD (time of flight diffraction) method, a modified testing mode named as TOFDW, which employs the three-fold reflected longitudinal wave from the bottom of the piece under testing, is presented based on the conventional TOFD technique. The characteristic of sound propagation in TOFDW mode is analyzed and its detection principle is illustrated. Through artificial defect testing, the features of tested signal and image are cognized, and the testing sensitivity and accuracy of the testing mode are studied. Weld piece is tested and the result is verified by destructive testing. The experimental results show that TOFDW model can identify the near surface defect that cannot be distinguished by using conventional methods, and the artificial defect with buried depth of 1.0 mm can be effectively detected. The model has a high quantitative accuracy, and the average absolute error of artificial defects location is less than 0.3 mm.