Eddy Current Testing (ET) is an NDT method based on electromagnetic induction. A coil carrying AC is brought close to a metal, and small circulating currents, called eddy currents, flow in the metal. Anything that disturbs them, such as a crack, a change of alloy or a thinner wall, changes the coil’s impedance and shows on the instrument. The key condition: ET only works on electrically conductive materials.
How it works: induction and the impedance plane
The coil’s primary field induces eddy currents in the part. These create a secondary field that opposes the primary one (Lenz’s law), so the coil’s impedance changes. The impedance plane display shows that change: distance from the balance point is amplitude, the angle is phase. The inspector rotates the phase so the lift-off signal runs horizontally, and a crack then shows as a signal at a different angle.
Eddy currents are densest at the surface and weaken with depth (skin effect). The standard depth of penetration δ is where the current density has fallen to 37% of its surface value. Rule of thumb: high frequency for surface cracks, low frequency for subsurface flaws.
Probes and equipment
- Impedance-plane flaw detector with frequency, gain, balance and phase rotation controls.
- Surface (pancake/pencil) probes for surface cracks, conductivity and coating thickness.
- Bobbin probes pushed through heat-exchanger and condenser tubes.
- Encircling coils for bar, tube and wire on production lines.
- Absolute, differential and reflection probes, each with its own strengths.
- Reference standards of the same material with EDM notches or drilled holes; ET is a comparison method.
- Conductivity meter, usually working at 60 kHz and reading directly in % IACS.
Basic surface ET set-up
- Connect the probe, switch on and let the instrument warm up for at least 5 minutes.
- Make sure the test area is free of loose paint and dirt.
- Choose the frequency, place the probe on the reference block away from the notches and balance.
- Lift the probe slightly and rotate the phase until the lift-off signal moves horizontally.
- Scan the reference notch and set the gain as the procedure requires.
- On the part, re-balance without touching the gain, then scan with an index no wider than the coil.
- Re-check sensitivity regularly and at the end of the job. If it has drifted, re-inspect everything since the last good check.
ET applications
- Fatigue and corrosion cracks in aircraft, including fastener holes and multi-layer structures.
- Fatigue cracks at in-service steel welds, even through paint up to about 2 mm (MT needs paint under about 0.5 mm).
- Heat-exchanger and condenser tubes: corrosion, pitting, wall thinning and cracking from the inside or outside.
- Sorting alloys, checking heat treatment and measuring conductivity.
- Measuring the thickness of paint, anodising or plastic on metal.
Advantages and limitations
| Advantages | Limitations |
|---|---|
| No radiation and no chemicals | Electrically conductive materials only |
| Works through paint and coatings | Limited penetration, about 5 mm (sometimes 10 mm) |
| Measures conductivity, coating and wall thickness, not just cracks | Ferromagnetic steel is difficult because its permeability varies |
| Well suited to automated production lines | Many variables (lift-off, edges, temperature) can give false signals |
Learn ET from scratch
In the Eddy Current Testing (ET) Level II course you watch the impedance plane move live, practise on an eddy current flaw detector simulator with absolute, differential and reflection probes, and prepare for the Level II exam with a workbook and question bank. Start with free Lesson 1: Electricity & AC circuits.
Fact source: ET Level II workbook, pp. 7–13, 23–65, 69–83, 91–92.
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