Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.1.4 Detection of subsurface flaws
5.1.4.1 Introduction
The depth of penetration of the magnetic field produced by eddy current test
coils, and therefore of the eddy currents produced by this field is highly
dependent of the coil diameter, the larger the coil diameter the greater the
depth of penetration. This is the predominant effect in limiting eddy current
penetration, and explains why the penetration is often far less than that
calculated using the effective depth of penetration formula. Typically, the
magnetic field in the axial direction is relatively strong only for a distance of
approximately one tenth of the coil diameter, and drops rapidly to only
approximately one tenth of the field strength near the coil at a distance of one
coil diameter. To penetrate deeply, therefore, large coil diameters are
required. However as the coil diameter increases, the sensitivity to small
flaws, whether surface or subsurface, decreases. For this reason, eddy
current flaw detection is generally limited to depths most commonly of up to
approximately 5 mm only, occasionally up to 10 mm.
Charlie Chong/ Fion Zhang