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Eddy current test (ET)

Eddy current test (ET)

The Induced Currents method allows all kinds of applications that can be related to changes in the chemical and physical characteristics of any conductor

Eddy current test – Non-Destructive Testing Laboratory

Principle of the method

During a Nondestructive Inspection using Induced Currents, an alternating current-fed coil generates a magnetic field that induces currents in the part being examined.
These currents affect the impedance of the coil that generated them.
Any discontinuity in the part changes the intensity and path of the currents, thus altering the impedance of the circuit.
This variation is an indication of possible defectiveness.

Examination techniques

Examination techniques differ in two respects:

  • The manual or automated application of the method;
  • the type of signal produced by the defect, in relation to the different geometric and electrical characteristics of the probes used.

Applicability

The applicability of the Induced Currents method is closely related to the characteristics of the materials under test. The method is applicable on conducting materials and for locating surface or subsurface discontinuities at depths not exceeding 10-15 mm, for materials of medium to low conductivity. This depth decreases dramatically for higher conductivity values and for ferromagnetic materials.

Information

To enable FBR Control to prepare an offer suited to the customer’s needs, it is advisable to verify some useful information, such as:

  • Type of product to be checked and material
  • Part geometries and if possible construction drawing
  • surface condition
  • quantity to be checked
  • inspection to be performed at the site or at the customer’s site
  • control methodology if defined
  • control specification if defined
  • acceptability if defined

Applications

The eddy current method is highly versatile in that it can be applied to any type of conducting material and detect even the smallest inhomogeneity, whether geometric, electrical or magnetic, through the test coil.

As a result, the method can be adapted to the specific needs of each control, which may include the detection of inhomogeneities related to material geometry, such as cracks, deformations, inclusions, thickness variations, oxidation, and so on, the measurement of thicknesses of nonconductive layers on conductive substrates or conductive layers on substrates with different conductivities.

The detection of changes in material conductivity, inhomogeneities in alloys, localized overheating, heat treatment errors, and finally the measurement of magnetic field strength to detect changes in material permeability.

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