Near Surface Flaws Detection Based on Modified Time-of-Flight-Diffraction Approach

摘要

TOFD (Time-of-Flight-Diffraction) approach is an accurate approach of sizing and locating embedded vertical (or almost vertical) flaws. Its major limitation is insensitive to near surface flaws (can not detect flaws 5mm or smaller under the surface). A modified TOFD approach (TOFDR) is proposed. It detects flaw tip diffraction signals after they reflected at the back wall first so as to avoid the overlapping of lateral signal and tip signal. The new theory is presented in detail, the expressions denoting the size and location of a flaw are deduced, and the influence of the distance and frequency of the probe pairs is discussed. Simulation and experiment in lab shows that this approach can detect 94% standard flaws correctly located from 2mm to 60mm under the surface with an average accuracy of 0.4mm.