Explanation
In the eddy current test an eddy current is induced in the examined material by means of an alternating magnetic field. The generated magnetic field is detected using a receiver coil that forms an eddy current sensor together with the excitation coil.
Conclusions about the examined test piece can be drawn in an analysis. Depending on the application, cracks can be detected here or statements can be made about the hardness of the material.
The working principle of the eddy current test
Generation of the alternating magnetic field Hp by means of an alternating electrical field.
This alternating magnetic field Hp creates eddy currents gw in an electrically conductive material.
The magnetic field generated from the eddy currents is overlaid with the excitation magnetic field and is converted to a measurable electrical alternating voltage that can then be analysed in the eddy current test unit.
The two most important pieces of information that serve analysis in the eddy current test are:
a.) the amplitude of the receiving alternating signal – i.e. the intensity of the generated eddy currents
b.) the phase displacement – i.e. the temporal offset between the sent sine signal and received sine signal
Usually, in most cases, the complex impedance level is selected to represent the signals of the eddy current test. This allows both the sum (=amplitude) and phase to be shown in one diagram.
In this example, the resulting eddy current signals from various conductive materials are entered into the complex impedance level.
The use of the electrical and magnetic fields for the non-destructive material test
The eddy currents, more specifically the eddy current density, is influenced by the electrical conductivity and permeability (=penetrability for magnetic fields) of the tested material. The eddy current test makes use of this dependency so that the material can be examined for cracks, hardness or also grinding burn.
