Abstract: In time of flight diffraction(TOFD) technology，the D-scan image has poor lateral resolution, large quantitative error of crack length and arcs at the crack tip due to the wide beam of transducers. In order to improve the sizing accuracy of crack length, the synthetic aperture focusing technology (SAFT) is introduced and a mathematical model of SAFT processing on the D-scan image is built. Based on CIVA software, nine rectangular cracks is established and the ultrasonic TOFD D-scan is simulated. The range of crack length is from 5.0 mm to 45.0 mm. The effective diffraction signals of cracks are enhanced and the arcs at the crack tip are weakened in the simulated D-scan image which is processed and reconstructed by SAFT. For the rectangle crack with a length of 5.0 mm, the measurement error is only 0.4 mm. This method is verified in the experiments of two surface breaking notches, of which the length is 5.0 mm and 40.0 mm, the quantification errors are 0.4 mm and 0.8 mm respectively. It shows that D-SAFT technique can be utilized to obtain high resolution D-scan image and accurately measure the crack length.

Key words: arcs, flaw length sizing, synthetic aperture focusing technology, time of flight diffraction