Abstract: In order to improve sensitivity through information fusion technology, a combined DC magnetic flux leakage and eddy current testing method is proposed. Experimental results indicate that the inner wall cracks in the steel pipe are detected by eddy current system. In fact, the dilemma between the experimental signal and skin effect principle is obviously. Two factors including permeability and magnetic flux leakage that have influence on eddy current signal are analyzed with finite element method and verification experiments of magnetic source. So the viewpoint, that the signal of the internal crack is caused by eddy current effect may be erroneous. Perturbed magnetic field is analyzed by using equivalent source method. Theoretical analysis shows that magnetic flux leakage produced by the crack is far stronger than perturbed magnetic field induced by eddy current effect. The induced electromotive force is created by the variation of the magnetic flux within the cross section of the pick-up coil when the probe moves along the steel pipe with a crack. The signal produced by eddy current effect is submerged. Therefore, the signal of eddy current probe is induced by magnetic flux leakage produced by the crack. Eddy current testing results of ferromagnetic materials under saturation magnetization should be reinterpreted.

Key words: Eddy current testing, Magnetic flux leakage testing, Magnetization, Perturbed magnetic field, breakout driving technology, HVD, tunnel boring machine(TBM)