Measuring Coaxial Hole Size of Finite-Size Metallic Disk Based on a Dual-Constraint Integration Feature Using Multifrequency Eddy Current Testing

This article presents a new approach of eddy current methods for determining the size of the coaxial hole in the metallic circular disk. In recent decades, for the air-cored sensor probe, the impedance change due to the presence of an infinite metal plate can be calculated by the Dodd-Deeds model. However, in practical measurements, the sample cannot match with the condition required - 'infinite'; thus, the Dodd-Deeds model could not be applied to the disk with finite size and certainly not a coaxial hole in the center. In this article, a dual-constraint analytical method is proposed. That is, the upper and lower limits of the integration are substituted with specific values instead of the original 0 and ∞. Besides, it is found that, once the outer radius of the disk is fixed (i.e., the lower limit of integration is fixed), the upper limit reduces linearly as the size of the coaxial hole increases. Both the FEM simulation and experiments have been carried out to validate this method. The radius of the hole can be estimated based on the dual-constraint integration feature.