Semi-analytical method for characterization slit defects in conducting metal by Eddy current nondestructive technique

Abstract

Defects in the metal components is a typical cause of component failure in engineering, with a lack of timely detection and elimination of these defects usually causing serious accidents. A nondestructive testing method using eddy current, however, exhibits remarkable capabilities in quantitatively monitoring the formation of these artificial defects in metal materials. Owing that the commonly appeared defect species in metal structures is slit defect instead of cylindrical defect, a deeper understanding of the magnetic testing signal correlated to slit defects in eddy current nondestructive method is required. In this paper, based on pervious investigation on cylindrical defect, a semi-analytical method for the magnetic field of a slit defect in eddy current testing system is proposed. Firstly, an analytical expression to the magnetic field for a cylindrical defect in metal is briefly introduced. Then, a model with an elliptical cylindrical defect is studied, and a semi-analytical method is deduced in details. Next, a semi-analytical formula is proposed to calculate the magnetic signals generated by slit defect. Finally, with a case study, the proposed semi-analytical method is applied to calculate the slit defect magnetic testing signal for slit defect in eddy current technique. The results indicated that the maximal relative errors between the semi-analytical solution and FEM are less than 2 % and the computation efficiency is higher than 10 times. With the support of these results, not only our proposed semi-analytical method can be readily used to investigate the forward problem due to the slit defects in eddy current technique, but also it can be further extended to seek the theoretical expression for defects with complex shapes, including those natural defects in conductive metal components (e.g. defects in oil pipe, rail track, iron bridge, oil tank, ocean ship).