Iterative Inversion Method for Eddy Current Evaluation of Near-Surface Residual Stress Profile in Surface-Treated Metals

Abstract

Because of their frequency‐dependent penetration depth, eddy current measurements are capable of mapping the near‐surface depth profile of the electric conductivity. This technique can be used to nondestructively characterize the subsurface residual stress distribution in certain types of shot‐peened metals, e.g., in nickel‐base superalloys. To predict the depth‐dependent, but frequency‐independent, intrinsic electric conductivity from the frequency‐dependent apparent eddy current conductivity (AECC), a highly convergent iterative inversion procedure is presented. The proposed technique exploits three specific features of the subsurface electric conductivity variation caused by near‐surface residual stresses in shot‐peened metals. First, compressive residual stresses are limited to a shallow surface region of depth much less than typical probe coil diameters. Second, the change in electric conductivity due to residual stresses is always very small, typically less than 1%. Third, the electric conductivity profile is fairly smooth and continuous. The accuracy of the proposed iterative inversion procedure is one order of magnitude better than that of the previously developed simpler method (J. Appl. Phys. 96, 1257 2004).