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Basic Principles of Eddy Current Inspection

 

Eddy current inspections use the principals of electromagnetism as the basis for conducting examinations.

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Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to a conductor, such as copper wire, a magnetic field develops in and around the conductor.  If another material which is electrically conductive is brought into  close proximity to this primary magnetic field, current will be induced in the secondary conductive material that will flow in a circular path perpendicular to the primary field.  Any defects in the secondary material will affect the overall inductive reactance which can be detected and displayed on specialized instrumentation. 

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Eddy Currents can be used for:

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• Crack detection

• Bolt Hole Inspection

• Material thickness measurements

• Conductivity measurements for Material identification

 

Some of the advantages of eddy current inspection include:

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• Sensitive to small cracks and other defects

• Detects surface and near surface defects

• Inspection gives immediate results

• Equipment is very portable

• Minimum part preparation is required

• Inspects complex shapes and sizes of conductive materials

 

Some of the limitations of eddy current inspection include:

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• Only conductive materials can be inspected

• Surface must be accessible to the probe

• Surface finish and roughness may interfere with measurement

• Reference standards of similar material and geometry are needed for setup calibration.

• Depth of penetration is limited to surface or near surface

• Skill and training required is more extensive than other techniques

• Flaws that lie parallel to the probe coil winding and probe scan direction are undetectable

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