Eddy current probe sensitivity as a function of coil construction parameters

We report the results of the first phase of a study designed to quantify the relationship between eddy current coil construction and the performance of these coils used in nondestructive evaluation (NDE) inspections. The ferrite core coils wound for this study are small but typical of the sizes commonly used in commercially manufactured eddy current probes. Coil diameters range from 1 mm to 7 mm with lengths from 0.5 mm to 4 mm. Seven parameters were studied and included ferrite diameter, ferrite permeability, coil aspect ratio, number of turns, distance of the windings from the inspection end of the ferrite, wire gauge, and length of the ferrite beyond the end of the windings. Additionally, the coil set was designed to provide some indication of the repeatability of identical constructions, what we have called winding inhomogeneity. The coils were incorporated into surface probes for scanning defects in flat plate specimens. The measure of sensitivity was the change of probe impedance (Z) as the probe was scanned from an unflawed area to the flawed area of the test specimen. Measurements were also made of the component of Z perpendicular to the liftoff vector.The data reported here were produced from a set of 27 probes scanned over a single defect. The defect was an electrical-discharge-machined (EDM) notch in a 19 mm thick 7075-T6 aluminum alloy specimen. The part-circular EDM notch was 9 mm long and 3 mm deep and 0.1 mm wide.Analysis of the data shows that the number of turns, the winding distance, the coil aspect ratio, and the backside ferrite length all affect the coil sensitivity. Winding inhomogeneity is significant for coils having many winding layers and can be considerably larger than the contribution made by variations in some of the construction factors. Wire gauge, ferrite diameter, and permeability showed no significant effects on our measure of sensitivity in this study.