A cracked metal disk from an airline engine. The crack was caused by the type of metal fatigue that the new method would be able to predict long before a crack ever appears. Photo: NTSB.When metallic components in airplanes, bridges and other structures crack, the results are often catastrophic. But researchers at Johns Hopkins University (JHU) have now found a reliable way to predict vulnerabilities in these components earlier than possible with current tests.
In a paper in Science, the researchers detail a new method for testing metals at a microscopic scale that allows them to rapidly inflict repetitive loads on materials while recording how the ensuing damage evolves into cracks.
This method has already proven a connection between early, micron-scale damage and the eventual location of the crack – “suggesting you can predict locations of cracks from such early features,” said author Jaafar El-Awady, a mechanical engineering professor at JHU's Whiting School of Engineering.
“We’re able now to have a more fundamental understanding about what leads up to cracks,” El-Awady said. “The practical implication is that it will allow us to understand and predict when or how the material is going to fail.”
Whether it is the pounding of vehicles on bridges or shifts in air pressure on airplanes, such continuous change, known as 'cyclic loading', gradually induces slips in the internal molecular structure of even the most durable metals. Eventually, cracks occur that could have been anticipated long before their perilous appearance.
“Fatigue failure plagues all metals and mitigating it is of great importance,” El-Awady said. “It is the leading cause of cracks in metallic components of aircraft.”
That is why it is common practice in the airline industry to adhere to regular – and expensive – replacement schedules for many parts. French investigators recently called for design reviews of the Airbus A380 to determine if they guard against metal fatigue risks. A better understanding of the origins of crack initiation could, however, allow the lifespan of airplane parts to be determined more accurately.
“With the lack of understanding of the mechanisms that lead to crack initiation, it has been difficult to predict with any reasonable accuracy the remaining life of a cyclically loaded material,” El-Awady explained. “The component could actually be fine and never fail but they throw it away anyway solely on the bases of statistical arguments. That’s a huge waste of money.”
Most current tests to understand the origins of crack initiation have focused on the moments just prior to or after cracking to assess what happened in the makeup of the metal. And many of those tests use far larger samples that preclude tracking the initiation of damage, which is a sub-micrometer scale feature. The new method narrows the lens as small as feasible, and begins when metals are first exposed to the loads that lead to localized damage that could become cracks.
This story is adapted from material from Johns Hopkins University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.