A Novel Measurement of Permeability, Conductivity, and Diameter for Magnetic Metallic Rod Using Multifrequency ECT

Abstract

Magnetic metallic rods are widely used in the manufacture of turbine blades and other hot end components in aircraft engines, and the material structure will be changed due to the impact of high temperature, high pressure, and corrosion environments. The structure of metallic rod can be characterized by the electromagnetic properties and size information. In this article, we have proposed a method to simultaneously reconstruct the permeability, conductivity, and diameter using multifrequency eddy current testing (ECT) system. The ECT coil encircles the metallic rod. According to the simplified classical Dodd-Deeds analytical solution, the relationship between the self-inductance variation of coil and parameters of metallic rods has been analyzed under different frequencies. Specifically, the real part of self-inductance variation is determined by both permeability and diameter at low frequency. The effect of conductivity on self-inductance variation increases as the increasing frequency. As a supplementary information, the zero-crossover frequency, a feature at midfrequency, can be used to invert conductivity. When the excitation frequency reaches high frequency (~MHz), the influence of electromagnetic information can be ignored due to skin effect. Hence, the rod diameter can be directly inverted by self-inductance variation. Based on the above information, the permeability, conductivity, and diameter can be derived, respectively. The experiments and finite element method (FEM) are also conducted to verify the proposed method. The results indicate that the relative errors of permeability, conductivity, and diameter are less than 3.0%.