Eddy Current Test
Principles
Eddy Current NDT is an inductive technique which employs a probe (coil with a AC current) placed close to the inspected material. When an AC current flows in a coil in close proximity to a conducting surface the magnetic field of the coil will induce circulating (eddy) currents in that surface.
The primary flux in the Figure above induces eddy currents in the material which give rise to a secondary flux. The secondary flux is then coupled back to the coil which affects its impedance. Thus the magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance. If a discontinuity (defect) is present in the material the eddy current density will be changed which can be observed as a change in coil impedance.