Abstract: Combining the autoregressive spectral extrapolation method with the ultrasonic time of flight diffraction method, the near surface detection blind area caused by the pulse width of lateral wave is suppressed. A carbon steel test block model having bottom notches with depth of 2.5 mm, 3 mm, 5 mm and 7 mm is set up. In the simulation, the model is detected using TOFD probes with 5 MHz center frequency and 34.0 mm probe center spacing (PCS). Autoregressive spectral extrapolation method extends the frequency band, compresses time-domain pulse width and separates the overlapping signal. The depth of dead zone is reduced from 8.3 mm to 2.5 mm and four bottom notchesare detected and positioned. Bottom notch having 3.0 mm depth is calculated with the same testing parameters in experiment, and the detected result has an absolute error of 0.30 mm. Compared with conventional TOFD and spectrum analysis methods, autoregressive spectral extrapolation method based on high SNR frequency bands and extrapolate the high frequency and low frequency range data outside the effective band, achieves the goal of improving time resolution and provides a new way for TOFD to detect defects in blind areas.

Key words: autoregressive spectral extrapolation, dead zone, defect positioning, time of flight diffraction