New near surface detection method of ultrasonic TOFD based on transformed wave

LIN Nai-chang
（Xiamen Inspection Institute of Special Equipments， Xiamen 361004， China）

Abstract： Aiming at the problem of near surface dead zone in ultrasonic time of flight diffraction（TOFD） such as the near surface defect that cannot be distinguished by using conventional methods and low quantitative accuracy and the low resolving power of detection，a new near surface detection method was proposed. The characteristic of distortion wave whose velocity is slow has been used，the condition that transformed wave appears in the windows of lengthways wave detection was obtained by means of calculating transformed wave，through wave and reflected wave of undersurface. To solve the problem of near surface dead zone in ultrasonic TOFD，the defect was detected by transformed wave to reduce the measurement error and to enhance the resolving power of detection. The result of experiment has been compared and analyzed by the measurement error and the resolving power of detection has been calculated by the method based on transformed wave and conventional methods respectively in the ten artificial defects with different buried depth. The results indicate that the method can identify the near surface defect that cannot be distinguished by using conventional methods，and the artificial defect with buried depth of 2.0 mm can be defected effectively. The method has a high quantitative accuracy and a high resolving power.
Key words： ultrasonic time of flight diffraction technique（TOFD）； dead zone； transformed wave