Eddy current test (ET)



The inspection by eddy current is one of several methods of non-destructive testing using the principle of electromagnetism.
The eddy currents are created through a process called induced electromagnetism. When an alternating current is applied to a conductor, such as a copper wire, a magnetic field develops around it and the conductor. This magnetic field expands and collapses when the current increases when the current is reduced to zero. If another electrical conductor is brought close to the magnetic field determines a variation with a resulting induction of current in the second conductor.
The principle is used in non-destructive testing: the magnetic field generated by a coil fed with alternating current produces, in the piece to be tested, the induced currents. These currents affect the impedance value of the coil that generated them. The presence of any discontinuities in the specimen examined, changes the intensity and the path of the eddy currents and thus the impedance of the circuit, whose variation, detected by the device, is indicative of possible defects.

APPLICATIONS
One of the biggest advantages in using eddy current inspection is the wide variety of applications.
In appropriate circumstances the induced currents can be used to
  • Detection of cracks
  • Detection of corrosion damage
  • Measurement of thickness of materials
  • Measurement of thickness of coatings
  • Measurement of conductivity (for identification of materials, determination of the depth of the treatment, detection of damage from overheating, monitoring of heat treatments).

ADVANTAGES
Some of the advantages of inspection with eddy currents are represented by:
  • sensitivity to small cracks and / or other defects
  • detection of surface defects and below the first surface layer
  • immediacy of the results of the inspection
  • portability of the equipment and therefore a great versatility on site
  • applications not only in detecting defects
  • it is requested a minimum preparation of the article to be tested
  • it is possible to inspect complex parts, of different shapes and sizes


LIMITS
Some of the limitations of the inspection with eddy current:
  • Only conductive materials can be inspected
  • The surface must be accessible to the probe
  • It is required an excellent training and experience of the operator
  • The superficial finish, such as roughness, can interfere with test
  • It is required a standard of comparison on which perform the resetting
  • the depth of penetration is limited
  • the method is not suitable to monitor large areas, as this method allows only a very localized control
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