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What is Eddy Current Testing (ET)? An NDT Method Overview
Eddy current testing (ET) is an electromagnetic NDT method based on electromagnetic induction. It is used to detect surface and near-surface flaws in conductive materials, measure wall thickness changes, and determine coating thickness. Its portability and speed make it particularly popular in aerospace, nuclear, and piping industries.
How It Works
When a coil (probe) carrying alternating current is brought close to a conductive test surface, circular induced currents — eddy currents — form in the material. A flaw or property change in the material disrupts the path of those currents; the disruption shows up as a change in the probe's impedance, which is measured and displayed.
The method only works on electrically conductive materials: steel, aluminum, copper, titanium, and nickel alloys.
Applications
Heat Exchanger Tubing
One of the most common ET applications. Corrosion, cracks, and wall thinning on the inner surface of thin-walled heat exchanger tubes are scanned without removing the tube bundle. Purpose-built multi-frequency probe arrays are used for this application.
Aerospace Inspection
Surface cracks on aircraft skin panels, fatigue cracks around fastener holes, and surface irregularities on engine components are scanned quickly with ET. No radiation is used, which is a field advantage.
Bar, Rod, and Profile Material
Automated ET systems are used for continuous surface inspection of bar stock, coil, and profiles on production lines.
Coating Thickness
The thickness of non-conductive coatings (paint, anodizing) over a conductive base can be measured with ET.
Material Sorting
Conductivity differences linked to alloy composition or heat-treatment state allow similar-looking materials to be distinguished.
Strengths
- No contact required — inspection through paint or coating
- High scanning speed — especially with automated systems
- No hazardous materials (no radiation, no fluids)
- Portable equipment suited to field work
- Coating thickness, conductivity, and flaw detection in one instrument
Limitations
- Only works on conductive materials
- Depth limit: typically a few millimetres from the surface; shallower in ferromagnetic steels
- Probe geometry changes and surface irregularities create noise
- Signal interpretation requires experience — analysis is done on the impedance plane
- Not suitable for deep internal flaws in thick-walled components (UT is preferred there)
Relevant Standards
| Standard | Scope |
|---|---|
| EN ISO 15549 | Eddy current examination — general principles |
| EN ISO 17643 | ET of welds |
| ASME Sec. V Article 8 | Eddy current examination |
| EN ISO 9712 | ET personnel certification |
| ASTM E243 | ET of heat exchanger tubing |
Personnel Qualification
ET requires personnel certified under EN ISO 9712 or ASNT SNT-TC-1A. The method code is ET; the certificate scope (welds, tubes, products) must be specified. Heat exchanger tube applications in particular require specific technical proficiency, and some clients request additional supporting documentation.
ET Work Orders and Reporting in NDT Software
ET work orders should capture:
- Probe type, frequency setting, and scan speed
- Reference standard and acceptance criteria
- Calibration block and reference sample details
- Scan results mapped onto a surface or tube layout
For heat exchanger work, recording results tube-by-tube and comparing with the previous inspection is critical for corrosion trend analysis.
ceres-ndt handles work orders, reports, and equipment management for ET and all other NDT methods on one platform. Get in touch to book a demo.
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