Skin Depth Considerations in Eddy Current NDT

Abstract

Eddy current nondestructive testing depends upon the interaction of time-varying electromagnetic fields with the material under test. The electromagnetic fields are applied to the material under test via some finite sized transducer, usually an inductive coil or set of coils. If the material is conducting the fields will penetrate the conductor, but will attenuate to negligible levels after some distance. The classical skin depth, ${\delta _s} = \sqrt {2/\omega \mu \sigma } = \sqrt {1/\pi f\mu \sigma }$ is the standard assumption for the characteristic distance of field penetration in conductors. This paper reports the results of a study which examined the decay of sinusoidal steady-state (AC) fields in conductors induced by finite sized coils. Comparisons are made among the classic Dodd and Deeds formulations [1], the 3D-axisymmetric finite element method (FEM) [2], and published results [3]. Where possible experimental observations were compared to the computed and published results.