Extreme Sensitivity of Eddy-Currents to the Surface Conditions of Nickel

Abstract

Eddy currents are used to inspect metals for small near-surface cracks and other defects. The eddy-current signal can be calculated quantitatively, at the cost of some effort, for non-magnetic metals at room temperature. Surprisingly, the same cannot be said for ferromagnetic metals [1]. Neither a quantitative nor a qualitative understanding exists for the change in the impedance when an air-core coil is placed next to an otherwise unspecified ferromagnetic metal. Nickel and iron are the ferromagnetic metals most commonly used in commercial applications. The authors are conducting an experimental/theoretical program aimed at developing a fundamental understanding of the swept-frequency impedance of coils placed next to thick plates of these elements. We start with commercially pure nickel.