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Eddy currents

PROFESSOR DR.-ING. DIETER STEGEMANN

Eddy current technology is becoming increasingly important in non-destructive testing. The reason for this is that it is a flexible testing technique and its probes, usually coils wound from copper wire, are very adaptable and can be manufactured without too much effort.

From everyday life we know that there are materials that conduct electricity, as is the case with cables, for example. These electrically conductive materials are mainly metals such as copper, brass, iron and steel. If such materials are brought into an alternating magnetic field, the magnetic field penetrates the material and generates eddy currents in it. They are called "eddy" currents because they normally move in the electrically conductive materials on curved paths and move back and forth in the rhythm of the exciting alternating magnetic field. They therefore indeed have something "vortex-like" about them, which is why the name vividly reflects their behaviour. Now, electric current is basically nothing more than the movement of negatively charged electrons in the conductive material. This means that our eddy currents are electrons moving back and forth in the material. In comparison with daily life, the material could be a large city in which the electrons move back and forth as people. Moving people usually have an effect. Is it the same with electrons? The answer is: yes. Through their movement, which represents a current - the eddy current - they in turn generate an alternating magnetic field, which is directed against the exciting magnetic field and weakens it.

This alternating magnetic field generated by the eddy currents is called the secondary field because it occurs as a result of the exciting magnetic field. The exciting alternating magnetic field is the primary field.

The primary alternating magnetic field is usually generated with coils through which alternating current flows. These ensure that the primary field enters the material and excites the eddy currents. As a result, "excited" electrons move back and forth along curved paths. The higher the electrical conductivity of the material, the better the eddy current generation works. The eddy currents, for their part, are not inactive and, as a result of their movement, generate a secondary alternating magnetic field which weakens the primary field and can thus be detected outside the material. Via the secondary field, a statement about the behaviour of the eddy currents inside the material can be obtained.
It is somewhat easier in the big cities, where traffic and pedestrian flows are monitored by video cameras.
can. But the secondary field can also react very sensitively to disturbances in the material and is therefore a suitable instrument for monitoring.

Why can eddy currents be used for non-destructive testing?

Eddy currents are electrons moving back and forth on curved paths in the material. These electrons can serve as "spies", so to speak, to see whether everything is running normally in the material or whether there are any disturbing influences. Such a disturbing influence could be a crack in a metal, for example. For the moving electrons, this would be a major obstacle blocking their way, so that they would have to look for a diversion. Compared to a large city, this would be a blocked road, for example. How does such an obstacle manifest itself in the material for the electrons?
No electron movement can take place in the obstacle and thus no eddy current exists at this location. This also means that no secondary magnetic field is generated at the location of the obstacle and, as a result, no weakening of the primary excitation field takes place. This means that defects in an electrically conductive material can be detected via the amplitude of the alternating magnetic field resulting from the superposition of the excitation field and the following field. If the electrons are forced to take a diversion because of the obstacle, which requires extra time, this means that a time shift occurs between the primary and the secondary magnetic field compared to the undisturbed case. We then speak of a phase shift between the two fields. The amplitudes and phases of the magnetic field changes triggered by the eddy currents are thus available for detecting defects in the material. Different defects in the material logically also cause different behaviour of the eddy currents and their signals, which makes it possible to distinguish between different types of defects. For example, material erosion or wall weakening due to corrosion provide different signal characteristics than the cracks already mentioned. For this reason, material testing with eddy currents can be used in many different ways.

Since eddy current technology usually involves measuring probes, which normally consist of a transmitter coil for generating the exciting primary field and a receiver coil for measuring the resulting magnetic field, the probe manufacturing technique is relatively simple and inexpensive. Electrical voltages and currents in the frequency range up to a few megahertz are used. The analogue signals from the probes are usually converted into digital signals so that the eddy current signals can be evaluated well and quickly.
As these are electromagnetic signals, their effect is very fast, which means that high testing speeds can be achieved. The signals and their processing and evaluation allow the creation of fully automatic eddy current testing systems for on-line or in-line testing of workpieces during manufacture.

Source:
The use of eddy currents
for non-destructive materials testing
Dieter Stegemann,
em. ord. Professor Dr.-Ing.
Faculty of Mechanical Engineering
Leibniz University Hanover

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