Eddy Current Inspection
Eddy current inspection is one of the NDT methods that use the principal of “electromagnetism” as the basis for conducting examinations.
Eddy currents are created in the test piece through a process called electromagnetic induction. When alternating current (AC) is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. The magnetic filed expands as the alternating current rises collapses as the current is reduced to zero. If another electrical conductor (i.e. Test Piece) is brought into the close proximity to this changing magnetic field, eddy currents will then be induced into this second conductor.
Eddy currents are therefore induced electrical currents that flow in a circular path. Below is a diagram to illustrate the principle.
The major advantage of eddy current inspection is the variety of inspections and measurements that can be performed. Eddy current inspection can be used for:
- Crack detection (i.e. Welding)
- Material thickness measurement (with limitations)
- Coating thickness measurements (i.e. Paint Thickness)
- Conductivity measurements for:
- Material identification
- Heat damage detection
- Heat treatment monitoring
Some of the major advantages of eddy current inspection include:
- Sensitive to small cracks and other defects, this is a awesome tool when it comes to in service components or high stress components
- Detects surface and near surface defects
- Electronic equipment give immediate feedback.
- Equipment is very portable, ideal for Rope Access use
- The item being tested need minimal surface preparation, i.e. crane boom can be inspected without removing coating system. Saving time and money…………!
- Ability to inspect complex shapes and sizes of different conductive materials
Some of the limitations of eddy current inspection include:
- Only materials that are conductive can be inspected
- High skill and extensive operator training is required
- Surface finish, roughness or even rust may interfere with inspection
- A individual reference standards is needed for each different conductive test piece.
- Depth of penetration is limited
- Some flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are likely not to be detected.