In production, every aircraft structure component will undergo inspection by one of the primary NDT methods. In-service aircraft will experience subsequent nondestructive testing, eddy current inspection being one the primary methods. This article serves to highlight the more common forms of eddy current applications on in-service aircraft.
Nondestructive testing methods range from simple manual and visual techniques to sophisticated eddy current and ultrasound technologies that can be used for spot-checks or incorporated right into the production line.
Since the development of eddy current testing in the early to mid-1900s this method has been used to detect defects and properties of many types of metals. The most common applications are testing tubular products for transverse defects, testing bar or wire products for longitudinal surface defects, and testing parts for defects and properties such as hardness.
Aero-engine discs are very critical parts since they are exposed to high stress and temperature during flights. Engine disc components such as bearing rings, fan and compressor discs are subjected to extensive inspections during the different manufacturing stages, including dimensional controls up to the detection of the tiniest surface flaws on rolling surfaces (bearing raceways).
First, a little history. The alternating current field measurement (ACFM) nondestructive testing technique was developed in the ‘80s to detect and estimate penetration depth of fatigue cracks in underwater welded tubular intersections of offshore oil platforms.
In 1968, Zetec won a government contract to inspect steam generator tubing and developed an analog, single-frequency eddy current tester, a state-of-the-art instrument at the time. Over the next 20 years, Zetec’s eddy current testing equipment, training, and methodology became industry standards for nuclear power NDT, then in balance-of-plant and process industry applications.
Eddy current displacement sensors combine several advantages. They provide high precision and resolution on a wear-free, noncontact basis. They are resistant to external influences such as dirt, pressure and fluctuating temperatures and therefore are ideally suited to harsh industrial environments.