Investigation on conductivity invariance in eddy current NDT and its application on magnetic permeability measurement

Eddy current (EC) non-destructive testing (NDT) is one of the most widely used NDT techniques. Electrical conductivity and magnetic permeability of the sample have serious impacts on the detected signal-magnetic flux density (Bz) in the EC NDT. Therefore, how to separate one´s impact on the magnetic flux density from another is an important issue, which has abstracted more and more researchers´ interests. In this paper, the conductivity invariance (CI) phenomenon was found when we investigated the impact that the conductivity random distribution in vertical direction on the detected signal using finite element method. The CI phenomenon also could be found when the excitation frequency changes. According to the conductivity invariance (CI) phenomenon, when the liftoff is appropriate, the conductivity of the sample has little influence on the magnetic flux density. Therefore, the CI phenomenon is proposed to the measurement of magnetic permeability in this paper.