BACK TO BASICS: The eddy current inspection method, Part 4. Applications, practical testing and advanced concepts

Abstract

Eddy current applications encompass a wide range of capabilities. Here is given a summary of some of these and a discussion about their practical requirements. If we refer to part one of this series we will find the standard equation for calculating the effective depth of penetration. This equation has parameters for resistivity, test frequency, relative magnetic permeability and depth of penetration. The change in any one of these parameters gives the basis for an eddy current. Figure 24 illustrates some of the applications possible. Defect detection Surface crack detection This is normally carried out with pencil probes or 'pancake' type probes on ferrous or non-ferrous metals. Frequencies from 100 kHz to a few MHz are commonly used. Depending on surface condition it is usually possible to find cracks 0.1 mm or less in depth. Shielded probes, with their focused field, add the ability to test very close to edges or dissimilar materials such as ferrous fasteners in an aluminium structure. Differential probes are sometimes used, particularly in automated applications, but care must be taken to ensure that the orientation of flaws is correct for detection.