Metal Fatigue Prediction

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As the FAA wrestles with how to address the issue of keeping aging aircraft in the air (and also wrestling with the groups representing those flying old airplanes), the answer, as it so often does, may come from the military. A military-funded research project, called Prognosis, at Carnegie Mellon University is attempting to predict metal fatigue by studying how cyclical stress affects the crystalline structure of metal. "Once you've got this three-dimensional picture of a structure inside a metal along with the impurities, you can then run an engineering model where you apply a cyclic stress and figure out where in this microstructure the stress or strain will concentrate," researcher Dr. Anthony Rollett told the Pittsburgh Post-Gazette. Rollett and his team have fertile research ground to assess their theories in the Navy E-6B Prowlers that serve as a test bed. The pounding the Prowlers take in their life aboard aircraft carriers compresses the fatigue cycle and they therefore undergo constant inspection and maintenance. Program Manager Leo Christodoulou, of the Defense Advanced Research Projects Agency, told the Gazette that the goal is to equip aircraft with sensors so that the knowledge gained by Rollett's research can be used to accurately predict when fatigue problems might be imminent. He said the military now errs on the side of caution and requires periodic inspections of the airframes. However, the inspections only yield one problem for every 1,000 aircraft taken out of service for inspection. "So we've taken 999 planes out for no good reason," Christodoulou said. "Wouldn't it be best to know enough to take the bad guy out and service him and leave the good guys alone? In a sense, we want to walk up to an aircraft and say, 'How do you feel today?'