Eddy Current Testing – Skin Depth Effect
One important point to keep in mind about eddy current testing is that this method is for surface inspections. It does not penetrate deeply in the material and it is most efficient in a thin layer of the material, about a few millimeters underneath the coil. The density of eddy currents is not constant in the material; it decreases exponentially as they spread deeper.

 

 

- High frequency - High conductivity - High permeability

- Low frequency - Low conductivity - Low permeability

 

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The standard depth of penetration (δ) is the depth where eddy current density drops to 37% of its value measured at the surface. This depth of penetration is affected by the operating frequency (ƒ), and conductivity (µ) and permeability (σ) of the material to inspect. This is what we call the skin depth effect.

 

 

 

 

This parameter is very useful to select the operating frequency based on materials characteristics. It is also used when designing probes. The coil size, the copper wire gage, and the number of turns are selected to match the impedance bridge of the ECT unit to provide more significant voltage variations at the input.