Wave velocity is the most critical parameter used by the synthetic aperture focusing technique for the ultrasonic imaging of concrete structures. However, it has been challenging to estimate wave velocity with the traditional time-domain method that requires the picking of direct waves. The dispersion trend of direct waves is theoretically a horizontal line with frequency as the x-axis and phase velocity as the y-axis, and the horizontal line is due to the nondispersive phenomenon of direct waves in an elastic, homogeneous material. To overcome the challenge of picking direct waves in the time domain, this study proposes a frequency-domain method to first obtain the dispersion trend of direct waves based on the nondispersive phenomenon and then estimate the shear-wave velocity from the average phase velocity of the nondispersive trend in the frequency domain. This frequency-domain method was verified with the numerical and experimental ultrasonic data from concrete specimens with embedded delamination. A comparison of the results obtained using the two methods indicates that the frequency-domain method can significantly improve the accuracy of estimating the shear-wave velocity and eventually improve the accuracy of ultrasonic imaging.
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