Diagram of eddy current inspection.
Eddy Current Inspection
Eddy current inspection uses alternating current flowing through a coil in a probe. This current has a resultant magnetic field around it that induces a magnetic field in a nearby conductive test piece. The magnetic field in the test piece in turn creates small currents ("eddies"). The nature of these eddy currents will be affected by the part conductivity and structure, and their presence is detected by the probe in a feedback loop. Variation in this electrical interaction is monitored on a test device. This method may be used to detect small surface flaws, and may be sometimes used to measure the thickness of a test sample or of a nonconductive coating on the piece. Eddy current testing is also used to sort materials based on their electrical conductivity.
Advantages of Eddy Current Inspection
- Sensitive to small cracks and other defects at the surface
- Inspection gives immediate results
- Equipment can be very portable
- Method can be used for more than flaw detection
- Minimum part preparation
Limitations of Eddy Current Inspection
- Only conductive materials can be inspected
- Ferromagnetic materials require special treatment to address permeability effects
- Skill and training required is more extensive than other techniques
- Surface finish and roughness problematic
- Reference standards needed for setup
- Depth of penetration is limited