Review of Progress in Quantitative Nondestructive Evaluation
This paper reports on a self‐consistent, swept frequency eddy current (SFEC) technique for characterizing surface and sub‐surface conditions of materials, with specific applications to detecting residual stresses in shot‐peened Ni‐base superalloys and surface oxidation in engineering components. The technique involves measuring lift‐off normalized vertical component signal to suppress lift off noise and instrumentation effect. Theoretical study shows that the vertical component signals are insensitive to coil dimensions, thus enabling EC measurements in separate frequency bands using multiple coils, while yielding continuous broad‐band data so that both the bulk conductivity and near‐surface conductivity profile can be determined by model‐based inversion. We demonstrate the technique on two surface‐modified materials, namely Inconel 718 samples shot peened at 4 A to 8 A, and an Ag‐1.5at%Al alloy which was used as a model material for a fundamental study of internal oxidation. For each sample set, the vertical component signals measured using two dissimilar sets of coils and instruments were found to overlap, confirming that the signals are insensitive to coil dimensions and instrumentation. The bulk conductivities of the samples were determined by inverting the low frequency data. The results were then used to constrain model‐based inversion of the high frequency data to obtain near‐surface conductivity profiles, from which the residual stress profile of the shot‐peened Inconel 718 and the oxidization depth of the Ag‐Al alloy can be inferred.
American Institute of Physics
Lo, Chester C.H.; Frishman, Anatoli M.; Shen, Y.; and Nakagawa, Norio, "Self-consistent swept frequency eddy current measurements for characterization of near surface material conditions" (2008). Center for Nondestructive Evaluation Conference Papers, Posters and Presentations. 93.