The Sensing Area and Effective Diameter of Surface Impedance Coil Eddy Current Probes over Flat Nonmagnetic Metal Surfaces

To detect localized flaws in nonmagnetic metal components with eddy current testing (ET) probes, the flaws need to be in the sensing areas of the probe coils. Flaw signals are generated when the flaws redirect the normal flow of the induced eddy current from the probe coils. Over the last several decades, analytic and numerical mathematical models of eddy current probe coils have been developed with capabilities of accurately calculating induced eddy current distributions in metal components. Results of calculations using methods similar to those developed by Dodd et al [1][2] are documented in this report that show the sensing areas of typical surface impedance eddy current probe coils in nonmagnetic metal plates. These results indicate that when the coil diameter is less and the defined skin depth of penetration the extent of the maximum amplitude of the current density distribution is significantly beyond the coil. However, when the coil diameter is larger than the skin depth, the location of the maximum amplitude of the current density is located under the coil windings. These results indicate that the previously defined "effective diameter" [3] of surface impedance coil probes should be reconsidered. References: [1] Dodd C.V. and Deeds W.E., "Analytical Solutions to Eddy-Current Probe Coil Problems", Oak Ridge National Laboratory Report ORNL-TM-1987, November 1967. [2] Dodd C.V., Cox C.D. and Deeds W.E., "Experimental Verification Of Eddy-Current Flaw Theory", Review of Progress in Quantitative Nondestructive Evaluation, 1985, pp. 359-364. [3] V.S. Cecco, G. Van Drunen, G., Sharp, F.L., Eddy Current Testing Manual on Eddy Current Method, Vol. 1, Atomic Energy of Canada Report AECL-7523, November 1981, pp. 66-67.