eddy current

• Eddy current inspection


  One of several methods that use the principle of "electromagnetism" as the basis for conducting examinations

  
• Other methods using electromagnetism


  • Remote Field Testing (RFT)
  • Flux Leakage
  • Barkhausen Noise
• Eddy currents


  Oscillating electrical currents induced in a conductive material by an alternating magnetic field, due to electromagnetic induction

  
• Uses of Eddy Current Testing (ECT)


  • Sorting materials
  • Measurement and control of dimensions of tubes, sheets, and rods
  • Coating thickness measurement
  • Pre-service and in-service examination of heat exchanger tubes for detection of defects

  
• Eddy currents


  • Eddy current density decreases with depth exponentially (skin effect)
  • Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux
  • Eddy currents normally travel parallel to the coil's windings and the flow is limited to the area of inducing magnetic field and perpendicular to the axis of the coil's flux field

  
• Electromagnetic induction


  1. Alternating current applied to a conductor develops a magnetic field

  2. Magnetic field expands as alternating current rises to a maximum and collapses as current is reduced to zero

  3. Changing magnetic field cutting through a second conductor induces a current in the second conductor

  
• Generation of eddy currents


  1. Probe with coil of electrical conductor used

  2. Alternating current flows in the coil

  3. Dynamic expanding and collapsing magnetic field forms in and around the coil

  4. Conductive material placed in the coil's magnetic field induces eddy currents in the material

  5. Eddy currents generate their own "secondary" magnetic field opposing the coil's "primary" magnetic field

  
• Skin depth


  Region between the outer surface and the point where the eddy current drops to about 1/e or 37% of its value at the surface

  
• Principle of Eddy Current Testing


  1. AC current flows in a coil, producing an alternating magnetic field

  2. Coil brought close to conductive surface induces eddy currents in the material

  3. Presence of defects/discontinuities disturbs the eddy current flow

  
• Lift-off effect


  Distance between surface coil and test surface, affects sensitivity
• Edge effect


  Effect of component's edge or shape changes on eddy currents, can be avoided by using a balancing probe

  
• Fill factor


  Measure of how well the test piece fills the coil in external encircling probes, should be near unity
• End effect


  Disturbance of magnetic field and eddy current distribution due to proximity of coil to abrupt change in geometry

  
• Depth of penetration


  Eddy currents concentrate near the surface, their strength decreases with distance from the coil (skin effect)

  
• Applications of ECT


  • Detection of discontinuities/surface breaking cracks
  • Conductivity measurement
  • Inspection of tubes
  • Thickness measurement
• Crack detection using ECT


  1. Cracks disrupt eddy current flow and weaken their strength

  2. High frequency for surface cracks, low frequency for deeper cracks

  3. Probe placed on surface, instrument set to null point

  4. Scanning surface, monitoring for changes in impedance to detect cracks
• Conductivity measurement


  Electrical conductivity depends on factors like chemical composition and stress state, can be used for sorting metals, checking heat treatment, inspecting for heat damage
• Tube inspection using ECT


  Used to detect corrosion, erosion, cracking in tubing of heat exchangers and steam generators
• Thickness measurement using ECT


  Thicker materials support more eddy currents, strength can be measured and related to material thickness

  
• Nonconductive coating measurement using ECT


  Coating displaces eddy current probe from conductive base, reducing eddy current strength which can be measured and related to coating thickness
• Advantages of ECT


  • Sensitive to small cracks and defects
  • Detects surface and near-surface defects
  • Inspection gives immediate results
  • Equipment is portable
  • Can be used for more than just flaw detection
  • Minimum part preparation required
  • Test probe does not need to contact part
  • Inspects complex shapes and sizes of conductive materials
• Limitations of ECT


  • Only conductive materials can be inspected
  • Surface must be accessible to probe
  • Skill and training required is more extensive
  • Surface finish and roughness may interfere
  • Reference standards needed for setup
  • Depth of penetration is limited
  • Flaws parallel to probe coil winding and scan direction are undetectable