When metallic components in airplanes, bridges, and other structures crack, the results are often catastrophic. Now, researchers have developed a new method for testing metals at a microscopic scale that allows them to rapidly inflict repetitive loads on materials while recording how ensuing damage evolves into cracks. The process has proven a connection between early, micron-scale damage to the eventual location of the fracture, predicting locations of cracks from early features.
Whether it is the pounding of vehicles on bridges or shifts in air pressure on airplanes, such continuous change, called cyclic loading, gradually induces slips in the internal molecular structure of the most durable metals until cracks occur that could have been anticipated long before their perilous appearance. Fatigue failure plagues all metals and 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. But the life of those parts could be more accurately determined by better understanding the origins of crack initiation. 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.
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. 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 loads that lead to localized damage that could become cracks.