Eddy Current Testing (ET): Principles and Industrial Applications is a nondestructive method using electromagnetic induction to detect surface and near-surface flaws, measure coating thickness, and sort materials by conductivity, largely without surface prep. It is a mainstay for heat-exchanger tube inspection, surface crack detection, and material sorting on production lines.
A probe with an AC-driven coil is placed near or inside a conductive part. Its magnetic field induces eddy currents beneath the surface, generating an opposing field that changes the coil's electrical impedance. Any discontinuity, a crack, a wall-thickness change, an alloy variation, or a conductivity shift, alters this signature, usually read on an impedance plane display, letting the operator identify and characterize flaws.
ET only works on electrically conductive materials, metals and some conductive composites; it cannot inspect plastics, ceramics, or most composites unless they contain conductive fibers. This separates it from liquid penetrant testing, usable on any nonporous surface, or magnetic particle testing, restricted to ferromagnetic materials but sharing ET's speed and minimal prep.
Eddy currents concentrate near the surface and decay exponentially with depth, the skin effect. The depth at which current density falls to about 37 percent (1/e) of its surface value is the standard depth of penetration, inversely proportional to the square root of frequency, conductivity and permeability.
Frequency choice trades depth against resolution; technicians often mix frequencies to suppress signals such as support-plate indications while keeping flaw sensitivity.
Lift-off is the distance between the probe coil and the part surface. Small variations, from surface roughness, coating thickness, or inconsistent handling, produce large impedance changes that can mask or mimic real flaws. It is usually the dominant noise source in surface-probe ET, though it is exploited in one application: lift-off signal strength correlates predictably with probe-to-substrate gap, making it the working principle behind non-contact coating-thickness measurement. In flaw detection, lift-off must instead be controlled or nulled through probe design, spring-loaded contact, or signal processing that separates lift-off from flaw response.
The largest industrial application of ET is inspecting thousands of thin-wall tubes inside shell-and-tube heat exchangers, condensers and boilers. A bobbin-style probe is pulled through each tube, scanning the full length in a single pass.
| Probe type | Primary use | Typical detects |
|---|---|---|
| Bobbin probe | Full-length scan, non-ferromagnetic tubes | Wall loss, pitting, corrosion, baffle wear |
| Rotating probe (RPC/MRPC) | Tube-sheet and U-bend regions | Circumferential/axial cracking, stress corrosion cracking |
| Remote field testing (RFT) | Ferromagnetic tubing (carbon steel) | Wall thinning, general corrosion |
| Array/surface probe | Flat and curved surfaces | Surface-breaking, fatigue cracking |
Common reference standards include ASTM E243 for copper and copper-alloy tubes, ASTM E426 for stainless steel, titanium and similar alloys, and ASME Section V Article 8 for general eddy current examination requirements. Programs typically flag tubes exceeding a wall-loss threshold, often cited around 20 to 40 percent, for plugging or repair, the exact figure set by the owner's fitness-for-service criteria.
Surface probes scan welds, machined components, fastener holes and forgings for fatigue and stress-corrosion cracks under cyclic loading, including spots liquid penetrant testing cannot reach once coated. A low-frequency variant, pulsed eddy current testing, can screen carbon steel piping for wall loss through insulation and cladding without stripping it first, a useful screening step ahead of targeted corrosion under insulation inspection. Because conductivity tracks alloy composition and heat treatment, ET is also used for material sorting, verifying alloy grade and catching mix-ups between similar parts without cutting a sample.
ET's advantages are speed, repeatability and minimal prep: no couplant, automatable, works through thin coatings and paint, and yields a quantitative, storable signal for trending. Its limits: it only works on conductive materials, depth is capped by the skin effect, it is sensitive to lift-off and handling noise, and complex signals need trained, often certified, personnel. For deep subsurface flaws in thick sections, ultrasonic methods usually fit better.
Because tube bundles are inspected on a recurring cycle, the practical challenge is rarely the physics, it is tracking which tubes were scanned, each one's wall-loss trend, and when a bundle crosses its plugging threshold. Maintenance teams increasingly log eddy current results, defect maps and plugging decisions inside a CMMS such as Fabrico, so tube history carries into the next outage plan instead of living in a standalone report. Book a Fabrico demo to see how inspection data connects to work orders and asset history.
Not directly. ET needs a conductive material to induce eddy currents, so it cannot inspect plastics, ceramics or non-conductive composites unless they contain conductive fibers such as carbon fiber, and even then sensitivity is reduced.
Effective depth is capped by the skin effect, typically a few tenths of a millimeter to a few millimeters, depending on frequency, conductivity and permeability. Lower frequencies reach deeper but lose resolution for small flaws.
Coil-to-surface distance strongly affects impedance, so small variations from surface roughness or coating can produce signals as large as real flaws. Separating lift-off from flaw response is central to reliable interpretation.
Both are fast, couplant-free surface methods. Magnetic particle testing works only on ferromagnetic materials and needs magnetization plus indicating media, while ET works on any conductive material and gives a quantitative signal that is easier to automate and record.
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