T.O. 33B-1-1
4-11
4.2.2.1.5
Edges (Including Corners and Radii).
For most eddy current techniques, the flow is circular parallel to the surface of the part. If the flow of eddy currents
intercepts an edge, corner, or radius of the part, the circular pattern is disrupted and the eddy currents are confined to a
smaller volume. This action changes the magnitude and distribution of the eddy currents and is known as edge effect
(see Figure 4-7). As illustrated, the current density will be slightly greater at the edge of the part than at the interior.
This will result in a slight increase in sensitivity to discontinuities located at the edge.
Figure 4-7. Distortion of Eddy Current Flow at the Edge of a Part.
4.2.2.1.6
Discontinuities.
Discontinuities in an electrically conductive material can also change the circular eddy current flow pattern as shown in
Figure 4-8. Discontinuities include cracks, inclusions, voids, seams, pits, laps, and numerous other inhomogeneities
related to the production, fabrication and use of metallic parts. The change in the magnitude and distribution of the
eddy currents is roughly proportional to the size of the discontinuity intercepted by the eddy currents. Because of the
weaker eddy currents at increasing depths beneath the surface, the eddy current response to flaws at or near the surface
is greater than the reaction from same size flaws at greater depths.
Figure 4-8. Effect of Discontinuities on Distribution of Eddy Currents.
