Use of inspection technology provides many safety benefits through the removal of defective components form commercial service. A variety of defect types can result from routine operation caused by inherent material flaws, manufacturing induced anomalies, aging phenomena such as fatigue cracking and corrosion, and/or inadvertent damage that comes from use such as impact from foreign objects or weather (hail damage, lightning strikes). For today’s fleet, the detection of cracks and corrosion remains a high priority. A variety of methods are utilized for defect detection with the selection of the technique dependent on the component geometry, material, expected defect location, defect type/size, and inspection cost. An often used method for detection of cracks and corrosion in aluminum structures is eddy current inspection (ECI). The advantages of this method include sensitivity to small defects with detection of cracks that are 0.1” and smaller being quite typical for surface cracks. The inspection is used on a routine basis and has proven quite cost effective. However, as the fleet ages and as designs become more complex, detection at multiple layers and greater depths becomes more important. Detection and quantification of corrosion has also risen in priority for the commercial and military sectors.

Given the need for improved crack and corrosion detection, the Federal Aviation Administration William J. Hughes Technical Center funded the development of a new method that addresses limitations of conventional eddy current. Investigators at Iowa State University have developed a new pulsed eddy current system as part of the Center for Aviation Systems Reliability (CASR). In contrast to the conventional ECI method, pulsed eddy current (PEC) induces a range of frequencies in a single measurement. This has the advantage of inspecting multiple depths in a single pass, a more cost effective and thorough approach. CASR staff has worked with several industry partners in the development of the technology including testing of specific applications. This has included measurements of bond thickness variations in specimens supplied by Cessna and participation in a round-robin blind corrosion experiment arranged by SAIC Ultra Image Inc.
Both methods utilize a probe which produces an electrical field in the part and measures changes in the signal which indicate the presence of flaws. The CASR program also includes a project to develop new probes based on Hall sensor technology, a sensor originally developed for the electronics industry. Because the Hall sensors measure magnetic fields, this avoids some of the typical disadvantages with eddy current sensors such as reduced sensitivity at lower frequencies and low spatial resolution due to physical size.

The advantages of the pulsed eddy current approach were well recognized in a recent demonstration of the system to industry at the Air Transport Association Nondestructive Testing Forum, held in Houston, October 1 – 3, 2002. Discussion is underway with several commercial equipment vendors regarding potential licensing agreements. For more details about the project, please contact Lisa Brasche, CASR Program Manager at lbrasche@cnde.iastate.edu or the developer, Marcus Johnson at marcus@cnde.iastate.edu.