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The SLOFEC inspection technique uses the eddy current principle in combination with a magnetic field. Utilising superimposed DC-magnetisation, the depth of penetration of the eddy current field lines in the ferromagnetic material is increased. In case of a defect, the magnetic field lines have a higher density in the remaining wall thickness, which consequently changes the relative permeability in the area, which again changes the eddy current field lines, as shown in the diagram below.
The changes of the eddy current field lines are measured and analysed in comparison to its calibration in difference of the signal amplitude and the signal phase.
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