This NDT method consists in generating induced currents in electricity conducting materials via an alternative magnetic field (generated by a solenoid) and variable in time (low or high frequency).

The currents locally induced and generated are called Eddy currents. Their distribution and repartition depend on the magnetic excitation field , as well as on the geometry the electrical conductivity and the magnetic permeability of the controlled structure.

The presence of a defect in the testpiece generates disturbances in the circulation of Eddy currents, entailing a variation in the solenoid impedance.
The method is applicable to all electricity conducting materials.

Extract from COFREND site