Eddy Current Testing

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This method is widely used to detect surface flaws, to measure thin walls from one surface only, to measure thin coatings and in some applications to measure depth. This method is applicable to electrically conductive materials only. In this method eddy currents are produced in the product by bringing it close to an alternating current carrying coil. The main applications of the eddy current technique are for the detection of surface or subsurface flaws, conductivity measurement and coating thickness measurement.

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Application of ECT

Crack DetectionCorrosion MonitoringMaterial Thickness MeasurementsCoating Thickness MeasurementsConductivity Measurements

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Electromagnetic Induction

•Eddy currents are created through a process called

electromagnetic induction.

•When alternating current is applied to the conductor, such as

copper wire, a magnetic field develops in and around the

conductor.

•This magnetic field expands as the alternating current rises to

maximum and collapses as the current is reduced to zero.

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Current Flow

Electromagnetic Induction

If another electrical conductor is brought into the proximity of this changing magnetic field, the reverse effect will occur. Magnetic field cutting through the second conductor will cause an “induced” current to flow in this second conductor. Eddy currents are a form of induced currents!

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Crack Detection

Crack detection is one of the primary uses of eddy current inspection. Cracks cause a disruption in the circular flow patterns of the eddy currents and weaken their strength. This change in strength at the crack location can be detected.

Magnetic FieldFrom Test Coil

Magnetic Field From

Eddy Currents

Eddy Currents

Crack

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Eddy Current Testing Equipments

Probes - Mode of Operation

Eddy current probes are available in a large variety of shapes and sizes. In fact, one of the major advantages of eddy current inspection is that probes can be custom designed for a wide variety of applications. Eddy current probes are classified by the configuration and mode of operation of the test coils.

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Different Types of Probes

Absolute Probes Surface Probes Bobbin Probes

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Advantages of Eddy Current Testing

Sensitive to small cracks and other defectsDetect surface and near surface defectsInspection gives immediate resultsEquipment is very portableMethod can be used for much more than flaw detectionInspects complex shapes and sizes of conductive materials

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Limitations of Eddy Current Testing

Only conductive materials can be inspected. Surface must be accessible to the probe.Skill and training required is more extensive than other techniques.Surface finish and roughness may interfere.Depth of penetration is limited.