There are many non-destructive testing (NDT) methods but, arguably, none are as convenient as eddy current testing. With this method, eddy currents – loops of electrical current induced within conductors by a changing magnetic field in the conductor – are induced into the test object. Inspection devices can then detect surface and subsurface defects in conductive materials indicated by disturbances in the eddy currents.
Changes in the flow of the generated eddy current field caused by variations in the test object are reflected in nearby coils for subsequent analysis by an eddy current instrument. This measures variations in the electrical impedance of the coil for a chosen test frequency. Eddy current testing involves placing a probe on or near the material to be inspected. Jesse Herrin, eddy current product manager at Zetec, says: “The probe has a coil that generates an electromagnetic field that induces electrons to flow in the metal; any cracks or changes in metallurgical structure will distort the flow of electrons like eddies in a river. These distortions are captured and analysed by an instrument and displayed for the technician to review.”
Probes for eddy current testing fall into two broad categories – tubing probes for inspecting tubes in heat exchangers, and surface probes for flat and curved surfaces. Herrin again: “Within these categories, there are probes for specific applications, including welds; rivets; lap splices; bolt holes; ferromagnetic tubing; probes for tubing with bends; and more. These dedicated probes are designed so users can conduct a thorough inspection with only one pass of the area under test.”
The simplest probe design uses a single coil to generate the electromagnetic field. Multi-coil array probes have multiple coils arranged at longitudinal, transverse, or off-axis orientations that fire at coordinated times – capturing far more information in a single pass and increasing the speed, accuracy, and repeatability of an inspection. Examples of eddy current test technology include the Olympus NORTEC 600 above (www.is.gd/ufuvuw) and the MIZ-21Cfrom Zetec (www.is.gd/cepewu).
WIDE USE RANGE
Herrin points out that eddy current testing has a wide range of uses: “Some of the most common applications include tubing inspection in the power generation and oil and gas industries; materials and component testing in aerospace; and measuring thickness and conductivity to identify and sort materials in manufacturing and other industries. There are industry-wide reference standards for calibrating instruments and conducting inspections, and a growing community of inspection service providers.”
He adds that, from practical perspective, eddy current testing has several benefits: minimal site prep; the probe doesn’t have to contact the test surface; it’s effective on painted/coated surfaces; provides immediate results; and it produces a digital record.
Eddy current inspection also offers the benefit of having no impact on the material being tested. As Gary Luckett, sales and application specialist – advanced NDT at Olympus, explains: “Other forms of NDT such as penetrant testing or magnetic particle inspection may have the benefit of not destroying the part being tested, but they tend to use, for example, solvents, or they might affect the surface itself.”
However, the technique also has drawbacks, according to Herrin: “Like any NDT method, the reliability of eddy current test results depends on the skills and patience of the inspector, especially when the work environment is hazardous or uncomfortable.” And, because the testing technique is electrical in nature, it can only be used on conductive material.
As with any NDT method, the operator must meet in-depth training, experience, and examination requirements, probably in accordance with industry standards such as ISO:9712 NDT – Qualification and certification of NDT personnel. This, according to the British Institute of Non-Destructive Testing, may be an individual certified to PCN Level 3 (Personal Certification in Non-destructive testing) for the appropriate industry and product sector.
Concludes Herrin: “There are simpler and less expensive NDT methods (for example, magnetic particle testing) that don’t involve instruments and probes, but they’re more time-consuming, limited in ability to detect flaws, and don’t generate a digital record.”