Eddy Current Probe Impedance Response to Residual Stress and Small-Scale Surface Roughness Variations

Surface roughness can create noise and errors in eddy current measurements. This study investigates the effect of small-scale roughness variations on the eddy current probe response, which are known to influence the current flow when the skin depth is close to the roughness height. Small roughness variations were made using silica carbide paper on two test pieces made from steel. Treated areas were measured using eddy current probe with frequencies ranging from 0.125 to 8 MHz. High frequencies and ferromagnetic test pieces lowered the skin depth close to the roughness height. Significant eddy current probe responses were observed in the treated areas. Changes in surface roughness were measured using the white light interferometer. The treatments also induce residual stresses and work hardening, which were estimated using X-ray diffraction measurements. Test pieces were annealed after the measurements to remove the residual stress and work hardening. Consequently, the response magnitudes were significantly reduced. The results implied that the residual stresses had the most significant influence, and the roughness changes themselves had a weak frequency-dependent effect on the probe response, which was observed after annealing. It was concluded that the influence of small-scale roughness variations itself is weak enough to be relevant only in specific high-accuracy applications.