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Demonstrating Eddy 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.


NDT
Teaching Tips

Ultrasonic
Reflectors
Signal Interpretation
DAC Curve

Eddy Current
Depth of Penetration
Lenz Law

X-ray
Backscatter Radiation
Film Handling Artifacts
Focal Spot Size
Lead Screens
Source/Object Orientation