Conductivity Measurement of Non-magnetic Material Using the Phase Feature of Eddy Current Testing

Eddy current testing (ECT) have been widely applied for electromagnetic parameters measurement and structural health monitoring due to the advantages of non-contact and high sensitivity. However, lift-off variation affects the measurement accuracy of ECT. In this paper, a novel strategy to handle this issue has been proposed for conductivity measurement of non-magnetic materials. First of all, a simplified analytical solution is derived from classical Dodd-Deeds analytical solution. According to the simplified analytical solution, the conductivity of sample is proved to be proportional to reciprocal of the crossover frequency with phase equaling to -3π/4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{ - 3\pi } \mathord{\left/ {\vphantom {{ - 3\pi } 4}} \right. \kern-0pt} 4}$$\end{document}. Hence, the conductivity of samples can be estimated based on the characteristics. Furthermore, compared with magnitude signal of ECT, the phase feature is demonstrated minimally affected by lift-off variation from aspects of theoretical derivation. Meanwhile, we have also analyzed the relationship between size of coil and lift-off suppression capability of phase measurement through simplified analytical solution. Specifically, as coil size increases, the influence of lift-off fluctuations can be ignored on phase measurement in ECT. In order to verify the prosed strategy, the experiments involving samples with different conductivities and coils with different sizes have been carried out. The results indicate the proposed method can achieve high precise conductivity measurement and suppress the interference caused by lift-off to some extent.