Interpretation of Impedance in the Light of Energy Dissipation and Eddy Current Nondestructive Evaluation of Pipe Wall Thickness

This study analyzed the energy flow of eddy current testing and interpreted impedance signals in the light of energy dissipation and storage. The ac resistance, which is attributed to energy dissipation, was factorized into intrinsic ac resistance and frequency squared, whereas the reactance, as a measure of energy storage, was factorized into inductance and frequency. The intrinsic ac resistance and inductance reveal the interaction between electromagnetic field and material. It was found that when the skin depth is much larger than a specimen's thickness that the magnetic field distributes nearly uniform over the thickness, the eddy current-related quantities, i.e., intrinsic ac resistance and the reduction of inductance, barely change with magnetic property but approximately proportional to the volume of the test object, e.g., cross-sectional area of a unit length pipe, which enabled us to measure a ferromagnetic pipe and estimate wall thickness at the thick-skin regime, regardless of variation of magnetic property. The experimental study showed that, by taking difference of signals measured with two serially connected identical coils, coupled cumulatively and differentially, the coils' ohmic resistance can be eliminated. Moreover, the reduction of inductance due to the eddy current effect could be obtained by referring to signals measured at an extremely low frequency. The proposed method was numerically and experimentally validated. This study demonstrated the possibility and feasibility of measuring a ferromagnetic pipe's thickness at tens of hertz, regardless of variation of magnetic property.