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Eddy current testing (ECT) is known as an effective technology for inspecting surface and near surface defects in metallic components. It is well known that the amplitude of eddy current (EC) density decreases with increasing depth, which is referred to as skin effect. Skin depth is an important parameter that quantifies the speed of attenuation of EC in the depth direction and is closely related to the capability of ECT for detecting deeply hidden defects. It is found that the traditional formula for calculating skin depth derived under the assumption of uniform plane field excitation is not applicable to the cases of ECT with coils. The skin effect in component with flat surface excited by pancake coil has been investigated by the authors. The skin effect in conductive tube tested by bobbin coil and that in conductive bar tested by encircling coil are more complex. The paper studies the skin effect in these two cases. Finite element analysis shows that the attenuation of EC is not only due to the ohmic loss, but also influenced by the diffusion effects, the aggregation effect, and the combined cancellation/diffusion effect of EC. The skin depth of EC associated with bobbin coil is always smaller than that associated with uniform plane field excitation, whereas the skin depth of EC associated with encircling coil can be greater than that associated with uniform plane field excitation under certain conditions.

SKIN EFFECT IN EDDY CURRENT TESTING WITH BOBBIN COIL AND ENCIRCLING COIL