Ability of Detecting Defect inside Billet by Ultrasonic CT Method Using Time-of-Flight of Longitudinal Wave

Abstract

For detecting defects inside a square billet, pulse echo method is generally used. This method has a problem that it is difficult to detect a tiny defect because the received signal power obtained by the reflected wave is too small. Then, we has proposed a detection method by ultrasonic CT method using time-of-flight of ultrasonic signal instead of the pulse echo method. However, the ability for detecting a defect by this method is unrevealed. In this paper, to clarify the ability for detecting a defect by this method, we numerically analyzed this ability by varying scan pitch and signal frequency. The result by changing the scan pitch showed that the necessary scan pitch depends on the signal wavelength. The result by changing the signal frequency showed that it was efficient to use the lower frequency signal for detecting the tiny defect. This was caused by that the measured receiving wave contained some effects of phase delay which was generated by the interference among the direct wave, the diffracted wave by the defect and the scattered wave from the defect. In this result, it was found that high frequency was not necessarily useful for the practical purpose of detecting defects. This method is more efficient for detecting the tiny defect than pulse echo method, by which it cannot be detected. In addition, it was found that the scan pitch can be set larger in this method.