Eddy-Current Testing

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Eddy-current testing

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.

Introduction

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.

Basic principles

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.

Capabilities

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.

Pros:

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.

Cons:

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.

Equipment

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.

Applications

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.

Standards

Standard Description Links
British Standards (BS)
BS 3683 (part 5) 1965 (1989) Eddy current flaw detection glossary
BS 3889
Methods for non-destructive testing of pipes and tubes. Methods of automatic ultrasonic testing for the detection of imperfections in wrought steel tubes
part 2A 1986 (1991) Automatic eddy current testing of wrought steel tubes
part 213 1966 (1987) Eddy current testing of nonferrous tubes
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



External links

Eddy Current Non-Destructive Testing - B.P.C. Rao

Eddy-Current Testing Services

Introduction to Eddy-Current testing