When a variable current (AC) is passed through a winding, a magnetic field is created around this winding. When this winding is brought close to the surface of an electrically conductive material, the variable magnetic field of the winding creates eddy currents on the material surface. These currents flow in a closed circuit and are called Eddy currents. Eddy currents also create their own magnetic fields. By measuring this created secondary magnetic field, surface defects can be found, and parameters such as conductivity and permeability of the material can be determined.

Scope of Application

This method is used to detect surface and subsurface discontinuities in all electrically conductive materials (copper, aluminum, etc.). With the eddy current inspection method, it is also possible to classify materials based on properties such as electrical conductivity or magnetic permeability. Apart from these, it is also possible to measure the thickness of the coating or the thickness of thin metal sheets.

Limitations

Not applicable to electrically non-conductive materials. Inspection of ferromagnetic materials requires special procedures. The depth of penetration is limited. Since the inspection result depends on the scanning direction, some errors may not be observed at the end of the scanning in unsuitable directions. It is more dependent on the user’s education and experience than other methods. Surface conditions greatly affect the inspection result. Special reference standard blocks are needed for the assembly.

Application

If crack-type symptoms are sought on the inspection surface, scanning is usually done with probes called differential probes, since they are more sensitive, but it is possible to detect crack-type errors with absolute probes. Absolut probes are mostly used for material characterization and coating thickness measurement. The “lift-off” effect is used for thickness measurement. For this, the device must first be calibrated. The thicknesses to be used for calibration should be chosen close to the thickness to be measured, with a slightly larger value and a slightly smaller value. If the magnetic permeability or electrical conductivity of the inspection piece is to be characterized by measurement, absolute probes are used again and the device values ​​should be calibrated with the help of known reference blocks before the measurement.