Conductivity Invariant Phenomenon of Eddy Current Sensor: Metal Magnetic Permeability and Probe Parameters

In eddy current non-destructive testing (EC NDT), the electrical conductivity and magnetic permeability of the sample are two fundamental factors influencing the eddy current signal. The coupling effect between the electrical conductivity and magnetic permeability of metallic materials not only reduces the accuracy of the eddy current signal but also affects the sensitivity of magnetic permeability measurements. In recent years, the conductivity invariant phenomenon (CIP) has been demonstrated to effectively eliminate the influence of electrical conductivity in magnetic permeability detection. Therefore, this study employs the finite element method (FEM) to systematically analyze the effects of magnetic permeability variations and probe parameters, including excitation signal frequency, coil turns, inner diameter, and outer diameter, on the CIP. We find that magnetic permeability variations affect the reference value of CIP but have a negligible impact on the depth of the conductivity invariant point. Additionally, optimizing probe parameters such as excitation frequency and coil turns can significantly enhance CIP performance in practical applications. The results reveal the mechanism of magnetic permeability changes that influence CIP and provide a theoretical foundation for optimizing probe design in future CIP applications. This study aims to enhance the stability and applicability of CIP, providing theoretical support for its further promotion and widespread application in engineering practices.