Eddy current density does not remain constant across the depth of a material. The density is greatest at the surface and decreases exponentially with depth (the "skin effect"). The standard depth of penetration equation (shown below) is used to explain the penetration capability of eddy current testing, which decreases with increasing frequency, conductivity, or permeability. For a material that is both thick and uniform, the standard depth of penetration is the depth at which the eddy current density is 37% of the material surface value. To detect very shallow defects in a material, and also to measure the thickness of thin sheets, very high frequencies are used. Similarly, in order to detect subsurface defects, and to test highly conductive, magnetic, or thick materials, lower frequencies must be used.

Evident’s NORTEC 600 and NORTEC 700 eddy current flaw detectors provide the frequency control needed to adapt inspections to different material depths and defect locations. The NORTEC 600 offers a frequency range of 10 Hz to 12 MHz, while the NORTEC 700 supports both conventional eddy current testing (ECT) and, on the N700i model, advanced eddy current array (ECA) inspections. By selecting an appropriate test frequency, inspectors can optimise eddy current penetration to detect surface or subsurface defects.

Eddy Current Depth of Penetration Equation

Where:
d= Standard depth of penetration (mm)
f= Test frequency (Hz)
mr= Relative magnetic permeability (dimensionless)
s= Electrical conductivity (% IACS)

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