An Eddy Current Testing Method for Thickness and Conductivity Measurement of Non-Magnetic Material

The thickness and conductivity of metal materials are important parameters because they can determine the purity, hardness, and heat treatment state of the metal. In order to better monitor the state of the metal, this paper proposes a method to simultaneously measure the thickness and conductivity of the metal sample based on a single-frequency eddy current testing scheme. First of all, it is found that the phase of the coil mutual impedance is approximately proportional to the thickness of the sample based on the Dodd and Deeds analytical solution when the conductivity is relatively low. However, as the conductivity of the sample increases, the linear fitting cannot be strictly satisfied, which means a quadratic polynomial fitting is more suitable to illustrate the relationship between the thickness and measured signal. In fact, the fitted coefficients of quadratic polynomial are approximately linearly related to conductivity, and hence a second fitting can be applied to the quadratic polynomial fitting. The correlation coefficients of the above two fitting can reach 0.998. Based on the above analysis, the relationship between the sensor phase, the thickness and conductivity of the sample can be obtained. The sensor structure consists of one excitation coil and two receiving coils(1T-2R sensor) are employed. According to the phase information of two pairs of coils, two equations related to thickness and conductivity can be obtained. Therefore, thickness and conductivity of the sample can be deduced by solving these two equations. Finally, the proposed method is verified by finite element method(FEM) numerical solutions and experiments. The mean relative error of thickness and conductivity are less than 5.4% and 5.2% for titanium alloy and stainless steel. The experimental results show the method is capable of on-line monitoring the conductivity and thickness simultaneously. More importantly, the method only needs the mode of single frequency excitation and has low requirements for the design of hardware circuit.