What it finds

  • Surface and near-surface cracks, including fatigue cracks around aircraft fastener holes.
  • Corrosion and wall loss in thin structures and in tubing.
  • Material differences, such as conductivity changes from heat treatment, which helps with sorting alloys.
  • Coating thickness of non-conductive coatings on metal.

How it works

Alternating current in a probe coil creates a changing magnetic field. When the coil is near a conductive material, that field induces circular electrical currents in it, called eddy currents. They make their own field, which pushes back on the coil. A crack forces the currents to go around it, and that changes the coil's response. The instrument shows the change, often as a moving trace on a screen.

Frequency is the main control. Higher frequencies stay close to the surface and are sensitive to small surface cracks. Lower frequencies reach a little deeper but with less sensitivity. Probes are compared against reference standards with known flaws. Bobbin probes, rotating probes and array probes (ECA) extend the method to tubing and larger areas.

Eddy current is the best known part of electromagnetic testing, a wider family that SNT-TC-1A also treats as including remote field testing.

Where it is used

ET is widely used in aircraft maintenance and aerospace manufacturing, on heat exchanger and condenser tubing, on wheels and engine parts, and in production lines for bar and tube. See our guides to aircraft maintenance and tube inspection.

Limits

  • Conductive materials only. It does not work on plastics or ceramics, and most composites are not good candidates.
  • Shallow reach. The currents concentrate near the surface, so it is no replacement for UT or RT on deep flaws.
  • Magnetic steels are harder. Their magnetic properties can swamp the signal, which is why they often need special probes or other methods.
  • Lift-off and geometry. Changes in the gap between probe and part, edges and fasteners all produce signals of their own.
  • Interpretation. Setup and signal reading take training, and results depend on good reference standards.

Who is allowed to do this work is set by a certification scheme. See How to Become an NDT Level I, II or III.

Frequently asked questions

Can eddy current see through paint?

Often, yes. It does not need bare metal, so thin non-conductive coatings can usually stay on. The coating adds lift-off, and the procedure has to allow for it.

Does eddy current work on aluminum?

Yes. Aluminum is a good conductor and is not magnetic, which is one reason the method is so common in aircraft work.

Eddy current or penetrant for surface cracks?

Eddy current needs no chemicals and can work through thin coatings, but it needs a suitable probe and standards. Penetrant works on many materials and shows the whole surface at once, but the surface must be clean and bare.

How NDT LLC can help

Our team of Level IIIs helps with ET procedures, technique selection and program reviews, and we take on field NDT and inspection work. Contact us.

This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.

NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by ASNT, ASME, ASTM International, or any other organization named in this article; names are used only to identify the organizations and documents referred to.