Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

The coil(s) in bobbin probes produce magnetic fields similar to those produced
by surface testing probes, but because the coil is concentric with material of the
tube, the eddy currents flow circumferentially around the tube wall. Since flaws
can be detected only if they distort the flow of eddy currents, this means that a
lamination or other separation of the material parallel to the surface of the tube
cannot be detected.
Fortunately, this is not a common flaw in tubes, and is usually not very harmful
to the service of the tube. However, it also means that narrow circumferential or
transverse flaws cannot be detected, as shown in FIG. 5.30. Such flaws include
fatigue cracks and stress corrosion cracks. Stress corrosion cracks are often
branched rather than a single material separation, and, if so, may be detected.
Fatigue cracks, however, are not branched, so the likelihood of detection is very
low. Flaws which progress along the tube, like seams, and flaws which are
relatively wide, like fretting grooves and corrosion pits, can be readily detected if
they are of a significant size.
Charlie Chong/ Fion Zhang