Surface Roughness Influence on Eddy Current Electrical Conductivity Measurements

摘要

The measurement of eddy current conductivity, in view of its frequency dependent penetration depth, has been suggested as a possible means to allow the nondestructive testing (NDT) of subsurface residual stresses in shot peened specimens. This paper addresses the apparent reduction of the near surface electrical conductivity measured by the eddy current method in the presence of surface roughness. Experimental results are presented on shot peened pure (C11000) copper, in which the effect is particularly strong and readily measurable because of the low penetration depth caused by the very high electric conductivity of the material. Eight shot peened samples between almen intensities 2 and 16 were thermally treated and tested by X-ray diffraction measurements until the residual stress and cold work fully dissipated, leaving only the surface roughness. Eddy current electrical conductivity measurements were carried out on each fully relaxed shot peened copper specimen over a wide frequency range from 1 kHz to 10 MHz. Our results show that surface roughness, acting alone, causes a srong apparent reduction of up to 10 to 20% of the measured electrical conductivity in shot peened copper. These results can be used to estimate the much smaller effect of surface roughness on the testing of shot peened turbine engine materials, which typically have approximately 100 times lower conductivity than pure copper and therefore exhibit 10 times higher penetration depth at the same test frequency. A comparison of the predicted underestimation of the electrical conductivity in the presence of surface roughness to the expected less than 1% conductivity increase due to compressive near surface residual stresses indicates that the described artifact can significantly affect eddy current testing of shot peened turbine engine alloys, such as Ti-6A1-4V or IN100.