Conductivity Invariance Phenomenon of Eddy Current NDT: Investigation, Verification, and Application

Both the electrical conductivity and the magnetic permeability of a measured sample have significant impact on the detected signal in eddy current nondestructive testing. Therefore, the problem of obtaining accurate measurement for the electrical conductivity or magnetic permeability has been of great interest to design and manufacturing engineers. In this paper, the conductivity invariance phenomenon (CIP) is investigated and utilized to detect the magnetic permeability of metals. First, the CIP is introduced and its sensitivity factors are investigated using a finite-element method. An experimental platform is set up based on which the CIP is verified. Finally, a magnetic permeability measurement device is proposed, which separates the coupling impact of the sample's electrical conductivity and magnetic permeability. The work reported in this paper provides an effective approach to accurately estimate the magnetic permeability of metals without the influence of its conductivity.