A Thickness Measurement Method Immune to Lift-Off Fluctuation Using Sweep-Frequency Eddy Current Testing

Eddy current testing (ECT) is a highly effective technique for measuring the thickness of metal samples. However, the fluctuation of lift-off distance easily affects the accuracy of measurement. In this paper, a thickness measurement immune to lift-off strategy based on the sweep frequency eddy current testing is investigated. First of all, we conducted an analysis on the relationship between the peak frequency of mutual inductance variation and the thickness of metal plates in line with Dodd-Deeds analytical solution. Moreover, we have demonstrated that the real part of mutual inductance variation at high frequencies (~ MHz) can be directly employed to invert and estimate lift-off, which is immune to the thickness and electromagnetic properties of metal samples. According to the estimated lift-off, the instrument factor for thickness measurement can be compensated to improve accuracy of thickness measurement. Both experiment and numerical solution have been applied to verify the proposed method, and the results indicate the relative error is only within 2.4%, which provides an approach to actual online measurement in the future.