Cincinnati -- University of Cincinnati aerospace engineer Peter Nagy is testing two different techniques to spot early signs of fatigue in aging aircraft. Both of them rely on heat.
He explained one during a summer workshop at the Fraunhofer Institute for Nondestructive Testing, in Saarbruchen, Germany and presented results on the second during the Twenty-Fourth Annual International Conference entitled Review of Progress in Quantitative Nondestructive Evaluation in San Diego July 27 to Aug. 1.
Nagy says it's more important to spot problems early in older aircraft, because in many ways, they're like older people. "When you're getting older, you lose your ability to resist sickness. Then you have to detect sickness much earlier because it will progress much faster," said Nagy.
In the same way, a microscopic crack in an older aircraft will grow and spread much more quickly than defects in newer planes. "The material is degrading on the microscopic level," said Nagy. "You really don't see cracks yet, but if there's a crack, instead of taking two years to propagate through the material, it will take two months."
Ironically, some techniques are "too good" at finding these microscopic defects. "You zoom in and you see everything. You start to see scratches on the surface, corrosion pits. Things really become disgusting when you use high magnification."
To pinpoint damage specific to fatigue, Nagy is using a special technique called laser irradiation. He heats the surface of the metal for a few microseconds with a laser. The quick blast of heat only affects the surface, not the cooler metal underneath. "We use very little heat, but it's concentrated on one area," explained Nagy. "Just like when you fry a chicken too fast, you burn the surface but it's cool inside."
An airplane has an outer skin too. That skin expands because of the heat, but the cool material inside doesn't change. That creates a pressure, pulling the skin back to its original size. The pressure actually squeezes any microcracks together. That quick change from open to closed crack can be detected fairly easily with conventional techniques such as ultrasonic or eddy current flaw detectors, so Nagy believes the system has promise in fatigue detection.
He was invited to Germany to explain the new technique during a workshop on industrial application of new scientific methods this June.
At the meeting in San Diego, which was co-sponsored by the Federal Aviation Administration and the Air Force, Nagy talked about a novel application for thermocouples. This research project is part of an interdisciplinary effort including the University of Dayton, Ohio State University and the Wright Laboratories at the Wright-Patterson Air Force Base.
The thermo-electric fatigue test relies on the fact that metals respond differently when heated. For example, if you use thermocouples to heat aluminum and another kind of conducting metal, an electrical signal is produced which can be detected.
To study fatigue, Nagy wanted to find out if there is also a measurable difference between fatigued metal and non-fatigued metal. "It turns out fatigued aluminum is different from virgin aluminum." The difference is small, and the behavior is non-linear, but Nagy reported that the differences can be detected and measured.
The metallic composition of individual aircraft can vary, however. So Nagy says it is critical to find points of comparison on the same plane. He compares parts of the plane that aren't under heavy stress and parts where fatigue normally shows up first.
Nagy believes the thermo-electric measurement will provide a good measure of the plane's general health. "The problem with most early detection systems is they're more sensitive to cracks than anything else. If there are cracks, you don't measure the inherent degradation of the material, the aging, the underlying weakening of the material."
Nagy's research is funded through a special $5 million federal program to improve the safety of aging aircraft.