The quantitative relations between true and standard depth of penetration for air-cored probe coils in eddy current testing

Abstract

The true depth of penetration of eddy currents generated in a conducting sample by an air-cored probe coil, besides depending on the electromagnetic wave frequency and the magnetic permeability and electrical conductivity of the sample, also depends strongly on the coil dimensions and the sample thickness. The standard depth of penetration widely used as a guide for eddy current inspection purposes is calculated for a plane electromagnetic wave incident perpendicularly on a conducting half-space and is thus a material/test parameter rather than a true measure of penetration. In this paper the quantitative relations between true depth of penetration and standard depth of penetration are presented for three configurations of eddy current probe and test material. First an air-cored coil above a conducting half-space is considered, then the same coil above a conducting sheet, and finally the true depth of penetration is calculated in a conducting half-space covered with cladding for different ratios of condictivity between cladding and base material.