When testing with an encircling coil, where are eddy currents densest?

In eddy current testing, the distribution of eddy currents is influenced by both the electromagnetic properties of the material being tested and the geometry of the coil used. When employing an encircling coil, the eddy currents that are induced by the alternating magnetic field are indeed densest at the surface of the material. This phenomenon occurs due to the skin effect, which describes how alternating currents tend to flow near the surface of conductive materials.

As the frequency of the applied AC signal increases, the penetration of the electromagnetic field into the material decreases, resulting in a concentration of eddy currents just below the surface. This means that most of the current loops created by the induction process are found in the shallowest layers of the material, diminishing in intensity with depth. Therefore, when testing with an encircling coil, the correct assessment of where eddy currents are densest confirms that they are primarily concentrated at the surface.