In order to describe the principle of ultrasonic TOFD

finite element method is used for analysis on ultrasonic time of flight diffraction（ TOFD） detecting technology. Firstly

use COMSOL for modeling; then process of generation

transmission and reception of ultrasonic are analyzed with the help of finite element model of TOFD;finally

effects of defect sizes on ultrasonic TOFD detecting signals is studied by changing model parameters.Research results show that the phase of direct wave

upper diffraction wave

lower diffraction wave

and bottom reflection wave in TOFD signals change in turn 180°; probe can only receive part of the four basic ultrasonic signals if the defect lies on the sample surface.