RECENT IMPROVEMENTS IN HIGH-FREQUENCY EDDY CURRENT CONDUCTIVITY SPECTROSCOPY

Abstract

Due to its frequency‐dependent penetration depth, eddy current measurements are capable of mapping near‐surface residual stress profiles based on the so‐called piezoresistivity effect, i.e., the stress‐dependence of electric conductivity. To capture the peak compressive residual stress in moderately shot‐peened (Almen 4–8A) nickel‐base superalloys, the eddy current inspection frequency has to go as high as 50–80 MHz. Recently, we have reported the development of a new high‐frequency eddy current conductivity measuring system that offers an extended inspection frequency range up to 80 MHz. Unfortunately, spurious self‐ and stray‐capacitance effects render the complex coil impedance variation with lift‐off more nonlinear as the frequency increases, which makes it difficult to achieve accurate apparent eddy current conductivity (AECC) measurements with the standard four‐point linear interpolation method beyond 25 MHz. In this paper, we will demonstrate that reducing the coil size reduces its sensitivity to capacitive lift‐off variations, which is just the opposite of the better known inductive lift‐off effect. Although reducing the coil size also reduces its absolute electric impedance and relative sensitivity to conductivity variations, a smaller coil still yields better overall performance for residual stress assessment. In addition, we will demonstrate the benefits of a semi‐quadratic interpolation scheme that, together with the reduced lift‐off sensitivity of the smaller probe coil, minimizes and in some cases completely eliminates the sensitivity of AECC measurements to lift‐off uncertainties. These modifications allow us to do much more robust measurements up to as high as 80–100 MHz with the required high relative accuracy of +/−0.1%.