抄録
To detect cracks in shape distorted cast billets, we newly propose a method that is the use of ultrasonic computerized tomography (CT) in combination with measurements of time-of-flight (TOF) of longitudinal ultrasonic waves at multiple planes as a more robust reconstruction method. The TOF is measured by applying the transmission method, and the measured TOF is used to reconstruct the apparent sound velocity profile. Cracks are identified as a decrease in the apparent sound velocity, which is caused by an increase in the TOF after the diffraction of ultrasonic waves at the cracks. The combination makes it possible to remove the effects of shape distortion to be compensated by the TOF measurements at multiple planes. Therefore, cracks are clearly detected using this method. The detection ability of this method is evaluated by numerical simulations of a shape-distorted billet. The results of this study are validated with help of wave propagation simulations using the transmission line matrix (TLM) method. More specifically, the newly proposed method allowed us to detect cracks that could hardly be distinguished with the use of our previously proposed method owing to fluctuations of the apparent sound velocity caused by shape distortions. In addition, using the new method enables the detection of cracks located near the billet’s surface whose dimensional change is as small as 5%. From the results of this study, we believe that the newly proposed method could be useful for the crack detection in shape-distorted billets.
