This paper describes the performance of the ultrasonic normal incidence method as a technique for inspection of the middle zone of wall thickness of LSAW (Longitude Submerged Arc Welding) steel pipes. Generally, the tandem method is applied to LSAW seam inspection of the middle zone of wall thickness. However, it is difficult to adjust the locations and angles of each probe.
Therefore, the normal incidence method using one probe for inspection of the middle zone of wall thickness was studied. Because the probe arrangement is simple, it is possible to reduce the time and effort required to adjust the probe location and angle. In the normal incidence method, it is possible to transmit and receive a mirror reflection because the ultrasonic wave is transmitted perpendicularly to a planar flaw under a high refraction angle. As a result of comparison experiments of the normal incidence method and tandem method, the amplitude of the echo from flat bottom drilled hole (F.B.H.) is 12dB higher in the normal incidence method than in the tandem method. The normal incidence method also has wide area sensitivity in the circumferential direction, however, the amplitude of the reflection echo changes ±25% when the incidence angle of the pipe changes ±0.3°. Thus, in practical applications, strict control of the deviation of the pipe incidence angle is necessary. This suggests that the reliability of this technique can be improved by compensating for sensitivity variations, for example, fluctuations associated with tracking backlash.
