The opening width of fatigue crack was very small, and conventional Bobbin
probe was very difficult to detect it in steam generator tubes. Different sizes of 8 fatigue cracks
were inspected using bobbin probe rotating probe. The analysis results showed that, bobbin
probe was not sensitive for fatigue crack even for small through wall crack mixed with denting
signal. On the other hand, the rotating probe was easily to detect all cracks. Finally, the OD
phase to depth curve for fatigue crack using rotating probe was established and the results
agreed very well with the true crack size.