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*Unit –V*EDDY CURRENT TETING

*By:Zoha NasirAssistant professorJIT, Barabanki

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*Eddy Current Testing

Electromagnetic analysis is a term which describes the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is eddy current method.

Eddy currents are composed of free electrons under the influence of induced electromagnetic field which are made to “drift” through metal.

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*Basic Principles

When an alternating current is passed through a coil, it develops a magnetic field around the coil, which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current.

If this coil is placed in such a way that the magnetic field passes through an electrically conducting specimen, eddy currents will be induced into the specimen.

The eddy currents create their own field which varies the original field’s opposition to the flow of original current.

The specimen’s susceptibility to eddy currents determines the current flow through the coil.

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Eddy current inspection can frequently be performed without removing the surface coatings such as primer, paint, and anodized films.

It can be effective in detecting surface and subsurface corrosion, pots and heat treat condition.

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*Methods of ECI

Pulsed Eddy Current TechniqueTransmission MethodReflection Method

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* Types Of Eddy Current Inspection Coils

Concentric CoilProbe Type CoilBobbin CoilEncircling CoilAbsolute And Differential Coil

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*Advantages

Low cost.No final cleanup required. ECI is very sensitive to small cracks and defects.Equipment are portable.Minimum part preparation required.Applicable to both ferrous and non ferrous metals.

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* Limitations

Trained operators are generally required.Surface finish and roughness may interfere.Depth of penetration is limited.Only applicable to conductive materials.Interpretation may be difficult.

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* Applications

In field of welding, casting, forging.For crack detection.Material thickness measurement.online testing of wires and tubes.Coating thickness measurement.

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*

Porosity.Undercutting. Rollover or “Cold Lap”.Slag inclusion. Poor penetration. Voids.

Defects