Planar rectangular spiral coils in eddy-current non-destructive inspection

The results of a theoretical and experimental study of planar rectangular coils in eddy-current NDE are presented. Three fundamental cases are examined: a planar coil (i) in free space, (ii) above a defect-free conductive half-space and (iii) centered above a long, uniform crack. Closed-form expressions for the coil impedance are presented for these three cases and the theoretical predictions are validated against experiment using a series of coils with different aspect ratios and both non-magnetic and magnetic plates. While the change in normalized coil impedance due to the half-space depends only weakly on the coil aspect ratio, the change in normalized impedance due to the crack depends strongly on coil aspect ratio with the maximum normalized response observed for an aspect ratio of 1:1, i.e. for square coils. In contrast to planar circular coils, the response of planar rectangular coils depends on their orientation with respect to the crack. This orientation dependence is investigated in detail.