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Current Depth of Penetration
The depth of penetration of eddy currents decreases with increasing frequency, increasing electrical conductivity and increasing magnetic permeability. These three variables are used to calculate the standard depth of penetration of eddy currents. The dependence of depth of penetration on these variables can be demonstrated to students using a set of step or tapered wedges.
The dependence of eddy current depth of penetration on test frequency can be easily demonstrated with a single nonmagnetic (aluminum) step or tapered wedge. The magnetic permeability is 1.0 for nonmagnetic materials and the conductivity of a particular material is generally known and relatively constant. Therefore, the depth of penetration will be affected only by operating frequency.
Start the experiment with a high frequency setup and scan the wedge down the flat side looking for an indication of a change in the metal thickness. With a high frequency setup, only changes in the thinner regions of the wedge will be detected. Then, repeat the scan at a lower frequency. Thickness changes deeper in the material should also become detectable. Calculated values for depth of penetration can be compared to experimentally measured values.
The effect of electrical conductivity and magnetic permeability on
depth of penetration can be demonstrated in similar way but multiple
test wedges will be required. To evaluate conductivity effects, use
two nonconductive wedges with significantly different conductivity values.
Select a test frequency that will detect only thickness changes in the
thin region of the high conductivity material but will detect changes
in the thicker region of the low conductivity material. To evaluate
magnetic permeability affects repeat this experiment using a magnetic
wedge and a nonmagnetic wedge with similar electrical conductivity.