Eddy current testing is an NDT method used in conductive materials such as metallic plates, sheets, tubes, rods and bars for detection and sizing of material discontinuities during manufacturing or in-service. It is applied across a variety of sectors such as nuclear, aerospace, power generation, petrochemical or manufacturing industries.
Eddy current testing uses the principal of electromagnetism to induce swirling or closed loops of currents, so called eddy currents in conductive materials. By monitoring the voltage across the probe, surface characteristics can be examined.
By generating an alternating current in a coil, a magnetic field is developed. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. When another conductive material is brought into the close proximity to this changing magnetic field, current will be induced in this second conductive material.
Eddy currents are induced electrical currents that flow in a circular path. They get their name from eddies that are formed when a liquid or gas flows in a circular path around obstacles.
Variations in the phase and magnitude of these eddy currents can be monitored using a second coil, or by measuring changes to the current flowing in the primary 'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current flow and a corresponding change in the phase and amplitude of the measured current.
Monitoring the changes of the measured current allows detecting flaws, defects or material property variations. The locus of impedance change is called eddy current signal. Defect location can be determined with the help of the phase angle of the eddy current signal. And the amplitude of the eddy current signal provides information about the defect severity.
Eddy-current testing can detect and locate very small cracks in or near the surface of the material, the surfaces need minimal preparation, and physically complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements.
Some of the advantages of eddy current technique include:
- Nearly all metallic materials can be tested.
- Detects surface and near surface defects.
- Inspections can be proceeded quickly, results will be provided immediately.
- Equipment could be very portable.
- Method can be used for much more than flaw detection.
- Minimum part preparation is required.
- Testing machine can be calibrated to accept parts with a certain range of signal interpretation by utilizing a series of known good samples.
- Test probe does not need to contact the part.
- Inspects complex shapes and sizes of conductive materials.
Some of the limitations of eddy current technique include:
- Only conductive materials can be inspected.
- Surface must be accessible to the probe.
- Skill and training required is more extensive than other techniques.
- Testing equipment is relatively expensive and complex in nature.
- Surface finish and roughness may interfere.
- Reference standards needed for setup.
- Depth of penetration is limited.
- Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable.
Both analog and digital instruments are available. They monitor the probe output and display information for analysis. Measurements, adjustments, controls, data storage, management and analysis are performed by computer software.
Eddy current is best suited for:
- Quality assurance during manufacturing and in-service inspections.
- Sorting of materials with different heat treatment.
- Detection of flaws in metallic plates, tubes, rods and bars.
- Detection and characterization of intergranular corrosion in stainless steel.
- Measurement of conductive and non-conductive coating thickness.
- Measurement of electrical conductivity and magnetic permeability.
|British Standards (BS)|
|BS 3683 (part 5)||1965 (1989) Eddy current flaw detection glossary|
|BS 5411 (part 3)||1984 Eddy current methods for measurement of coating thickness of nonconductive coatings on nonmagnetic base material. Withdrawn: now known as BS EN 2360 (1995).|
|American Society for Testing and Materials (ASTM)|
|ASTM A 450/A450M||General requirements for carbon, ferritic alloys and austenitic alloy steel tubes|
|ASTM B 244||Method for measurement of thickness of anodic coatings of aluminum and other nonconductive coatings on nonmagnetic base materials with eddy current instruments|
|ASTM B 659||Recommended practice for measurement of thickness of metallic coatings on nonmetallic substrates|
|ASTM E 215||Standardizing equipment for electromagnetic testing of seamless aluminum alloy tube|
|ASTM E 243||Electromagnetic (eddy current) testing of seamless copper and copper alloy tubes|
|ASTM E 309||Eddy current examination of steel tubular products using magnetic saturation|
|ASTM E 376||Measuring coating thickness by magnetic field or eddy current (electromagnetic) test methods|
|ASTM E 426||Electromagnetic (eddy current) testing of seamless and welded tubular products austenitic stainless steel and similar alloys|
|ASTM E 566||Electromagnetic (eddy current) sorting of ferrous metals|
|ASTM E 571||A Electromagnetic (eddy current) examination of nickel and nickel alloy tubular products|
|ASTM E 690||In-situ electromagnetic (eddy current) examination of nonmagnetic heat-exchanger tubes|
|ASTM E 703||Electromagnetic (eddy current) sorting of nonferrous metals|
|ASTM E 1004||Electromagnetic (eddy current) measurements of electrical conductivity|
|ASTM E 1033||Electromagnetic (eddy current) examination of type F continuously welded (CW) ferromagnetic pipe and tubing above the Curie temperature|
|ASTM E 1316||Definition of terms relating to electromagnetic testing|
|ASTM G 46||Recommended practice for examination and evaluation of pitting corrosion|