EDDY CURRENTS (ET)
This NDT method creates currents in electricity-conducting materials that are induced by a variable magnetic field using a probe.
These induced currents, known as eddy currents, circulate locally in the material. Their distribution and division depend on the magnetic excitation field, the geometry and characteristics of electrical conductivity and magnetic permeability of the part tested.
An anomaly found in the part tested can disturb the movements of currents and thus cause a variation in the loaded coil impedance of the probe that depends on the type of anomaly and its volume.
An analysis of this variation in impedance provides the indications that can be used to conduct the test. The signals collected are interpreted by comparing signals recorded in the tested material with signals in a reference part containing anomalies representative of the phenomena sought.
This variation is converted into amplitude and phase on a screen as curves known as “Lissajous”.
2. Test method
Absolute measurement, for detecting long defects (corrosion, wear, erosion, etc.)
Highly sensitive to slow variations in electrical conductivity, magnetic permeability and material thickness, it is used in static or dynamic characterisation mode to:
- measure conductivities;
- sort materials;
- measure quench depths;
- measure the thickness of coatings (paint, electrolytic deposits, plastics, anodising, etc.) or thermochemical treatments (carburizing, nitriding, etc.).
Differential measurement, for detecting short defects (cracks, blowholes, inclusions, corrosion spots, etc.)
As this requires a relative movement between the part and the probe (dynamic measurement), this measurement is used to test health as it is insensitive to the gradual variations in quantities that influence the path of the eddy currents.
This NDT method is often used to detect surface defects as the eddy currents tend to gather on the surface of conducting bodies (skin effect); it is therefore applied to the testing of metal sheets. But it can prove highly efficient in testing cylindrical structures (bars, tubes).
Eddy currents can also detect the variations in composition of an alloy and even measure coating thicknesses. It is therefore a method found in all industries, especially in tube manufacture and maintenance.
4. Advantage of the method
- Finds breaking (even obstructed) or shallow (zero to a few mm) defects;
- dimensional measurements (measurement of the thickness of insulating coatings or with a conductivity very different from that of the substrate);
- sorting of parts with different electrical conductivity (same alloy but different heat treatment, grades of alloys mixed accidentally, etc.);
- high flaw sensitivity, with possibility of assessing the depth;
- discrimination between different types of anomaly (dimensional, structural, etc.) ;
- possibility of automatic, continuous testing of long and profiled parts (tubes, bars, ribbons, strips, etc.); ;
- fast throughput speeds, the probe is not necessarily in contact with the part;
- measurement possible under water;
- measurements possible at very high temperatures (900°);
- perfectly adapted to maintenance tests (steam generator tubes, ski lifts, suspended bridges, aircraft engines, etc.); ;
- no harmful environmental impacts;
- compatible with health and safety requirements.
Eddy current testing is highly appreciated given the miscellaneous possibilities offered by the detection sensitivity and the easy automation of the method. The lack of contact between the probe and the part to be tested, the possibility of high speed throughput and the ease in integrating the process in production lines are the main advantages of eddy currents.
In addition, the reproducibility of measurements, despite the complexity of electromagnetic phenomena and the many settings used, make it a widely-used NDT method for maintenance operations or equipment calibration.
5. Related standards
General standards currently in force
Non-destructive testing - Eddy current testing - Vocabulary
Non-destructive testing - Eddy current testing - General principles
Non-destructive testing - Equipment for eddy current examination - Part 1
Instrument characteristics and verification
Non-destructive testing - Equipment for eddy current examination - Part 2
Probe characteristics and verification
Non-destructive testing - Equipment for eddy current examination - Part 3
System characteristics and verification
Non-destructive testing - Equipment for eddy current examination - Array probe characteristics and verification
Text prepared by COFREND in conjunction with Robert Lévy.