An electric conductor traversed by a current is surrounded by a circular magnetic field (eddy field). If the straight conductor wire is now wound into a circular conductor loop, the eddy field lines overlap in such a way that they form a magnetic dipole (with north pole/south pole structure).
The strength of the magnetic field generated can be increased by winding the coils, just as they are used in eddy current testing as probe elements, with a larger number of coil turns. With increasing coil length their magnetic field becomes more and more like that of rod-shaped permanent magnets.
The magnetic field outside of the coil penetrates the electrically conductive product to be tested. Since the coil is traversed by an alternating current, circular currents are induced close to the surface of the product to be tested, which are called eddy currents.
These eddy currents run counter to the direction of the coil current, and can in a sense be understood as mirror image of the coil current. The flowing eddy currents in turn are surrounded by a magnetic eddy field.
In a “defect-free” test specimen (homogenous material) the eddy currents can propagate unimpeded.
The magnetic field generated by the eddy current, too, is characterized by a dipole structure. This secondary magnetic field is in opposition to the primary magnetic field of the coil. The superimposition of both magnetic fields leads to a resulting magnetic field, which, compared to the primary magnetic field of the coil, has a smaller field strength.
Should the product to be tested have any local defects (e.g. cracks, corrosion pits, pores, non-metallic inclusions or similar), the eddy currents won’t be able flow unimpeded anymore. In a sense, such inhomogeneities represent an insurmountable obstacle. The eddy currents must go around to the side and/or in direction of depth, and therefore experience a weakening. Consequently, the magnetic eddy field around them is also weakened. The reduced magnetic counter-effects on the primary field of the coil lead to changes in the resulting magnetic field so that it differs from that of the defect-free test specimen.
The strength of the magnetic field under the influence of an electrically conductive product can be registered by suitable probes (receiving coils), then analysed, and displayed in a suitable way. This way, conclusions about the properties of the test specimen can be drawn, for example regarding geometry, dimensions, material parameters and the presence of local defects.
