An Eddy Current Testing Primer

How to find the flaws that lead to disaster


“In the aircraft industry, there are two areas where eddy current testing is commonly used; the airframe and the engines,” says Dr. Norio Nakagawa, adjunct professor of aerospace engineering at Iowa State University and a guiding light for the NDT Resource Center educational website (www.ndt-ed.org). “The testing works best in near-surface applications. It is simple and easy to use, which makes eddy current testing a valuable tool for aircraft maintenance technicians.”

Applications
Eddy current testing has many uses in the maintenance hangar. On airframes, for instance, “it is primarily deployed for detecting cracks and faults in riveted areas,” says Dr. Nakagawa. “Since these rivets literally hold the airframe together, the integrity of these joints and their holes is paramount to the aircraft’s overall integrity.”

Areas that take a lot of stress, such as undercarriages and engine casings, are also prime targets for eddy current testing. “The amount of stress undercarriages experience during repeated takeoffs and landings makes them vulnerable to cracks and other forms of fatigue,” says Jankowski. “Meanwhile, if the engines are kept running too long after landing, materials in those areas can be inadvertently backtempered; making them brittle and more prone to failure. The symptoms of these problems can all be detected using eddy current testing.”

Limits
Because it is built upon electromagnetic induction, eddy current testing is only useful for detecting flaws on or near the surface of the conductive material being checked. The PEC technique can achieve deeper penetrations, but will eventually encounter a limit in the penetration depth it can measure.

As well, only a skilled technician can interpret the wealth of information offered by an eddy current test. A novice can see that there’s a problem being detected, but not much else. This is why effective eddy current testing requires the tester to be properly trained. The NDT Resource Center web site offers a number of training recommendations at www.ndt-ed.org. The site also has a detailed rundown on the science of eddy current testing for interested readers.

One last hint: Eddy current testing equipment needs to be well-built and properly calibrated to ensure accurate, meaningful readings. Careful and consistent probe positioning during testing is also a must.

The bottom line
Eddy current testing is a profoundly powerful, accurate, yet easy-to-use NDT approach to finding cracks, flaws, and other forms of surface and near-surface damage in airframes and engines. It is an essential tool for any aircraft technician, whether working on Cessnas or 777s.

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