Eddy Current Testing
Eddy Current Testing is one of the NDT tools that uses the principal of electromagnetism to conduct inspection. Alternating current is energized to a probe at a chosen frequency depending on the type of inspection and enhances the production of magnetic field around the coil in the direction ascertained by the right-hand-rule. As another conductive material is brought inside the magnetic field, electromagnetic induction occurs and an eddy current is induced in the material. The eddy current induced flows in a circular path which causes its own magnetic field to occur, which is opposite to the probe’s “primary” magnetic field. Amplitude and pattern of the eddy current and the resulting magnetic field will be interrupted and altered when there are changes in metal thickness or defects like near-surface cracking.
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Eddy Current Testing is used in a wide range of industries. However, since it is electrical in nature, it is limited to conductive material such as metal. Eddy Current Testing can be used not only to detect surface and near-surface discontinuities, but is also applicable to measure thickness of thin sheets of metal, thickness of nonconductive coatings such as paint, and sorting material based on electrical conductivity and magnetic permeability.
Tubing inspection is one of the major applications of Eddy Current Testing,which is used to detect cracking, erosion, corrosion and other changes in tubing. Thousands of tubes can be found in heat exchangers and steam generators, which are widely used in power plants, where just small leaks can contribute to a huge loss not only to the industries but also to the humankind. Therefore, it is very important for the power plant to periodically shutdown to enable tubes and other equipment to be inspected and repaired. Where a high-speed inspection technique is required, Eddy Current Testing seems to fit the criteria the most.
Among the advantages of Eddy Current testing is the ability to detect surface and near surface flaws, is sensitive to small cracks and other flaws, can be used for much more than flaw detection, able to inspect complex shapes and sizes of conductive materials, minimum preparation is required for the parts being inspected, equipment is very portable.