Detection of a Surface Flaw Using Differential Signals of MI Sensors With the Residual Magnetization Method

In the present study, the detection of surface flaws using differential signals with two MI sensors with the residual magnetization method was conducted. Vertical components of leakage magnetic fields generated from flaws were measured with the residual magnetization method by scanning with two MI sensors in the spatial domains around flaws. The measured differential signals indicate clear flaw images without signal processing because a flaw signal sharply changes from the signal generated from the weld zone although the signal amplitude becomes very small due to differential signals. However, in the case of a small flaw in the weld zone, the flaw was hard to detect because the noise due to weld metal was also measured. Therefore, wavelet analysis was applied, and the result after wavelet analysis indicated the improvement of the flaw detectability.