ADVANCES IN SWEPT HIGH FREQUENCY EDDY CURRENT RESIDUAL STRESS CHARACTERIZATION

We report on recent progress of a swept high frequency eddy current (SHFEC) technique for nondestructive characterization of residual stresses in engine materials with specific applications to shot‐peened superalloy components. The technique determines conductivity depth profiles by model‐based inversion of SHFEC data, and converts them into residual stress profiles using a material‐based model which generally includes the piezoresistivity (PR) and possibly other non‐PR effects. Improvement in detection coil and upgrades to the measurement system have extended the effective frequency upper bound to 70 MHz. A modified piezoresistivity theory, including the effect of texture on stress‐induced conductivity changes, was developed in light of the experimental evidences of shot‐induced changes of texture in peened Inconel 718 samples. The theory was applied to convert the conductivity profile to a residual stress depth profile, which agrees with the data taken by the standard layer removal XRD method.