Mitigation of the effect of variations in the electrical conductivity of the material via two-frequency eddy current testing of the thickness of the electrically conductive wall under significantly varying influence parameters

The paper analyzes feasibility of the two-frequency eddy current method for measuring the thickness of an electrically conductive wall under significantly varying test and influence parameters of the test object — the lift-off between the eddy current probe and the test object surface, and the electrical conductivity of the material. An analytical solution was used to determine the dependence of the two-frequency signal of the surface eddy current probe on the influence parameters of the test object. The informative parameters used to simultaneously mitigate the effect of the two influence parameters were the amplitude of the added high-frequency voltage to determine the lift-off, the phase of the added low-frequency voltage to determine the wall thickness, and the phase of the added high-frequency voltage to suppress variations in the electrical conductivity of the material. The calculated dependences of the informative parameters on the test and influence parameters were analyzed. The use of nonlinear functions of the inverse transformation of the informative parameter into the test parameter was shown to efficiently mitigate the effect of variations in the lift-off on measurement results. A method to suppress variations in the electrical conductivity of the test object material is proposed. It implies the correction of the phase of the added low-frequency voltage by the correction value calculated from the parameters of the lift-off and wall thickness, and high-frequency phase variation caused by varying the electrical conductivity of the material.