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In ET, electrical currents are generated in a conductive material by an induced magnetic field. Interruptions in the flow of the electrical currents (eddy currents), which are caused by imperfections or changes in a material's conductive properties, will cause changes in the induced magnetic field. These changes, when detected, indicate the presence of a change in the test object.

Eddy Current Flaw Detection.

Eddy current inspection is based on the principle of electromagnetic induction. It is used to identify or differentiate among a wide variety of physical, structural and metallurgical conditions in electrically-conductive ferromagnetic and non-ferromagnetic metals.

Eddy current inspection can be used:

  • To measure or identify conditions and properties such as electrical conductivity, magnetic permeability, grain size, heat treatment condition, hardness and physical dimensions.
  • To detect seams, laps, cracks, voids and inclusions.
  • To sort dissimilar metals and detect differences in their composition, microstructure and other properties.
  • To measure the thickness of a nonconductive coating on a conductive metal, or the thickness of a nonmagnetic metal coating on a magnetic metal.

Exchanger Tube Inspection:

  • One of the most common uses for eddy currents is in place inspection of nonmagnetic exchanger tubing. Eddy current inspection of magnetic tubing can also be conducted. However, the technique which is much more difficult than the one for nonmagnetic materials is not commonly used.
  • The magnetic tubing inspection technique includes sending an eddy current probe through the tube. As the probe moves through the tube, data relating to the condition of the tube are gathered on a gathered on a recording device. The recording device could be a strip chart recorder, an oscilloscope display or a computer. Analysis of gathered data will help determine the presence of corrosion, cracks, damaged baffles, etc. Generally, this type of inspection is conducted on a contract basis, employing inspection companies experienced in the area of eddy current techniques.

Typical Applications for Eddy Current Testing:

  • Inspection of exchanger tubes.
  • Material sorting.
  • Coating thickness.
  • Measure material properties.

One of the major applications of eddy current testing is the measurement of conductivity. This application is possible since most materials have a unique conductivity value determined by their chemical composition, and the processing and method of manufacture. Standard conductivity has been defined by the International Electrochemical Commission in terms of the amount of resistance to be found in a specific grade of high purity copper when measured at 20~C (68~F). This resistance has been arbitrarily designated as 100% International Annealed Copper Standard (IACS) conductivity. Based on this information, by knowing the IACS of a particular material or by using a calibration standard, material sorting is possible.

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