T.O. 33B-1-14-124.2.2.2 EffectsOfInspectionConditionsonEddyCurrents.4.2.2.2.1 Frequency.The magnitude of the induced eddy currents in the part increases as the frequency of the inducing current increases. Inturn, the higher intensity eddy currents generate a stronger opposing magnetic field, reducing the penetration of theprimary field. Therefore, all other factors remaining constant, higher frequencies result in shallower depths ofpenetration as shown in Figure 4-9.Figure 4-9. Relative Effect of Frequency on Depth of Penetration.4.2.2.2.2 ElectromagneticCoupling.The interaction between the primary electromagnetic field, generated by the coil and the inspection article is referred toas electromagnetic coupling. Because the field decreases in strength with increasing distance from the coil, resultanteddy currents at the surface of the part will also decrease in intensity. An electrical engineering term that could also beused is inductive coupling.4.2.2.2.3 Lift-Off.As an eddy current probe is brought near a conductive part, a change in the detected signal will be noted. With theprobe near a part, a pronounced signal change will be observed in response to a small change in distance between probecoil and part. This effect is termed lift-off. The signal change occurs because the intensity of the eddy currents in thepart decreases considerably with a slight increase in coil-to-part spacing. This condition is demonstrated in Figure 4-10. Calibrated measurements of lift-off can be used to determine the thickness of nonconductive coatings on conductiveparts.
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