AF lab investigating microscopic crack formations, metal fatigue
By Donna Lindner, Air Force Research Laboratory
/ Published September 01, 2016
WRIGHT-PATTERSON AIR FORCE BASE, Ohio (AFNS) -- The B-52 Stratofortress is one of the oldest legacy aircraft in the Air Force. Since the 1950s, the B-52 has led the force in its dominance as the world’s best; however, just as humans begin to age, so do aircraft.
Repeated loading and unloading, changes in air pressure, exposure to altitude and more, contribute to what is referred to as “metal fatigue,” resulting in small, microscopic cracks in engine, wing or tail structures that can have detrimental effects on safety.
Research scientists from the metals branch of the Air Force Research Laboratory’s Materials and Manufacturing Directorate understand the critical importance of metal fatigue and aircraft safety. To better address this critical issue, they are studying the earliest stages of crack formation in turbine engine materials in order to alleviate cracks leading to unexpected fatigue failure. Greater understanding of crack origins leads to more effective aircraft inspection procedures, wherein maintenance teams are able to determine metal fatigue at an earlier stage and mitigate the life-limiting effects before they become a safety problem.
“The Air Force routinely inspects engine components for cracks, but it is possible to miss microscopically small cracks with current inspection methods. We want to be confident that an undetectable crack does not grow to failure before the next inspection,” said Dr. Adam Pilchak, a research scientist in the metals branch. “Because crack growth rates can vary considerably depending on how a material is processed, it is important to understand the worst-case scenario that leads to the shortest fatigue lifetime.”
Using a scanning electron microscope (SEM), Pilchak and his team are able to magnify the surfaces of fractured laboratory samples or components taken from aircraft engine surfaces up to 50,000 times to precisely determine where a crack initiated. By studying the details of the crack initiation sites and the material on which cracks form, it is possible to modify the way materials are processed, ultimately improving safety.
“Our team is using a novel methodology in SEM to quantify, non-destructively, the cracking mechanisms during early stages of fatigue. The information from these characterizations can help reduce sustainment costs and improve fleet readiness, without compromising safety,” said Vikas Sinha, a materials scientist in the metals branch.
Pilchak and his team presented their research during the International Metallographic Contest at Microscopy and Microanalysis 2016, where they were awarded first place for research in electron microscopy as well as the Jacquet-Lucas Award for excellence in metallography.
“Studying fatigue in a controlled laboratory setting can really help determine the root cause of a failure in the field,” Pilchak said. “Ultimately, understanding how cracks initiate and grow can save lives.”