Eddy Current Testing Basic Knowledge

This method is widely used to detect surface flaws, to sort materials, to measure thin walls from one surface only, to measure thin coatings and in some applications to measure case depth. This method is applicable to electrically conductive materials only. In the method eddy currents are produced in the product by bringing it close to an alternating current carrying coil. The alternating magnetic field of the coil is modified by the magnetic fields of the eddy currents. This modification, which depends on the condition of the part near to the coil, is then shown as a meter reading or cathode ray tube presentation. The Figure gives the basic principles of eddy current testing.

There are three types of probes Figure used in eddy current testing. Internal probes are usually used for the in-service testing of heat exchanger tubes. Encircling probes are commonly used for the testing of rods and tubes during manufacturing. The uses of surface probes include the location of cracks, sorting of materials, measurement of wall and coating thickness, and case depth measurement.

This method may be used for:
(1) For the detection of defects in tubings.
(2) For sorting materials.
(3) For measurement of thin wall thickness’ from one surface only.
(4) For measuring thin coatings.
(5) For measuring case depth.

Some of the advantages of eddy current testing include:
(1) Does not require couplant.
(2) It gives instantaneous response.
(3) Has uncomplicated steps during set-up.
(4) Is extremely sensitive to flaws.
(5) Is very repeatable.
(6) High scanning speeds can be used.
(7) Is very accurate for dimensional analysis of flaws or coating thickness.

Some of the limitations of eddy current testing include the following:
(1) The theory requires a good academic background in electrical principles and in mathematics.
(2) Extremely sensitive to surface variations and therefore requires a good surface.
(3) It is applicable to conductor materials only.
(4) Can be used on non-magnetic and magnetic material but is not reliable on carbon steel for the detection of subsurface flaws.
(5) Its depth of penetration is limited.
(6) Crack tightness and orientation of eddy current flow to a crack or linear discontinuity will affect detectability.

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