Eddy-current testing

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Eddy-current testing uses electromagnetic induction to detect flaws in conductive materials. There are several limitations, among them: only conductive materials can be tested, the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of penetration into the material is limited by the materials' conductivity, and flaws that lie parallel to the probe may be undetectable.

In a standard eddy current testing a circular coil carrying current is placed in proximity to the test specimen (which must be electrically conductive).The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy current. Variations in the phase and magnitude of these eddy currents can be monitored using a second 'receiver' coil, or by measuring changes to the current flowing in the primary 'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique.

However, eddy-current testing can detect very small cracks in or near the surface of the material, the surfaces need minimal preparation, and physically complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements.

The testing devices are portable, provide immediate feedback, and do not need to contact the item in question. Recently tomographic notion of ECT has been explored see for example:

Another eddy-current testing technique is pulsed eddy-current testing. A major advantage of this type of testing is that there is no need for direct contact with the tested object. The measurement can be performed through coatings, weather sheetings, corrosion products and insulation materials.[1] This way even high temperature inspections are possible. Compared to the conventional eddy-current testing, pulsed eddy-current testing allows multi-frequency operation.[2]

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