Eddy current penetration depends on:
The frequency of current in the search coil ( f )
The conductivity of the specimen ( σ )
The permeability of the specimen ( µ )
As frequency increases so the depth of penetration decreases. This is known as skin effect. Furthermore, the intensity of eddy currents decreases exponentially with depth. There is a specific depth beyond which the eddy current intensity is too small to be useful for detection of defects. This depth is known as the standard depth of penetration. It is defined as the depth below the surface at which the intensity of the eddy current field has been reduced to a value of 37% of its intensity at the surface.
This depth can be calculated from a number of formulae. One of the most commonly used expressions is:-
δ - Standard depth of penetration, mm
f - frequency, Hz
σ - conductivity, m/ Ω mm 2
µ - relative permeability
A practical inspection has been known to penetrate to a depth of approximately 40mm through about 14 layers of material.
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