Lift-off effect in high-frequency eddy current conductivity spectroscopy

Precision eddy current measurements have been shown to be capable of characterizing the near-surface residual stress and cold work profiles in surface-treated components. To capture the peak compressive residual stress in moderately shot-peened (Almen 4–8 A) nickel-base superalloys, the eddy current inspection frequency has to be as high as 50–80 MHz. Unfortunately, spurious self- and stray-capacitance effects render the complex eddy current coil impedance variation with lift-off, the so-called lift-off curve, highly nonlinear, which makes it difficult to achieve accurate eddy current conductivity measurements beyond 25 MHz in the presence of even the slightest lift-off uncertainties. As opposed to the well-known inductive lift-off effect that decreases with increasing probe size, the capacitive lift-off effect increases with probe size. Both effects increase with frequency with the inductive effect being initially stronger, but then taken over at high frequencies by the faster growing capacitive effect. Since the two effects produce opposite curvature in the lift-off curve, in the frequency range where they are approximately equal to the lift-off curve becomes essentially linear and fairly accurate, conductivity measurements can be conducted even in the presence of lift-off variations.