High-performance nondestructive inspection method for high-attenuation billet: Ultrasonic computerized tomography using time-of-flight

A pulse echo method is usually employed to defect detection in billet. However, the pulse echo method is not suitable for high-attenuation material such as cast stainless steel because the echo wave fades away. In our previous work, it has been found that a computerized tomography (CT) using time-of-flight (TOF) of longitudinal wave with combination of pulse transmission method is effective for defect detection of high-attenuation billet. In this paper, we focused on an effect of our method to high-attenuate materials. From the numerical analysis, it was found that our proposed method is suitable for defect detection of high-attenuation materials; the defect can clearly be visualized because the effect of the wave reflected on surfaces is decreased as increase of attenuation coefficient, under high signal-to-noise (SNR) environment. However, we also found that the usable SNR condition is limited; under low SNR environment, the defect cannot be detected clearly. Therefore, we investigated the usable condition of this method from view point of the attenuation coefficient. We calculated a `required SNR map´, in which the relationship between minimum SNR required for defect detection and position of the defect is mapped. From this map, it was found that when the attenuation coefficient was high, the SNR needed to be high, i.e., adequate signal power was required. The ease of detection was varied by the location of the defect. Therefore, before inspection, this map can be used for determination of signal power or getting the scope of defect detection.