Researchers at Cornell University (Ithaca, New York, USA) used atomic modeling to explore the ways environment can influence the growth of cracks in alloys such as aluminum and steel. This knowledge could help engineers better predict, and possibly postpone, the failure of structures.
Their findings were recently published in Physical Review Letters in a paper entitled “Dissolution at a Ductile Crack Tip,” co-written by Cornell Ph.D. student Wenjia Gu and Cornell professor Derek Warner. Among the paper’s key findings: by removing atoms from the tip of a crack in its model, the researchers could prevent a crack from propagating, thereby improving the material’s mechanical performance.
“People have been modeling crack growth and fracture for a long time, but the actual process by which it occurs has not really been clear, at least for structural alloys in complicated environments,” says Warner, an associate professor of civil and environmental engineering at the university and the paper’s senior author. “Tt can be a very large-scale phenomenon—big structures can fracture—but it can be controlled at the atomic scale, particularly when you look at environmental effects.”
Warner is the current director of the Cornell Fracture Group, which conducts both scientific and engineering research to understand and predict the deformation and failure of structures. This research group focused on dissolution as a mechanism that adversely impacts materials ranging from corroded metal surfaces to eroded human bone.
The Cornell Fracture Group created a series of atomic 2D models of a structural alloy, similar to aluminum and steel, that was also pliable enough not to shatter when deformed. After running numerous simulations to determine the different ways atoms interacted, the research team found that removing surface material inhibited cracks from growing.
“The proclivity of a crack to grow depends on how sharp it is,” says Warner. “If you have a big round notch, it’s unlikely to propagate like a crack. But if you have some sharp feature, like a slit cut with a knife, it is more likely to grow. So in this way, material removal, akin to what occurs during corrosion, can actually improve mechanical performance.”
As Warner notes, there is a biological corollary to this mechanical phenomenon—for instance, osteoclasts, a type of bone cell, dissolves bone tissue in order to promote bone growth and resist fracturing. Such an approach could have plenty of practical applications, he suggests.
“There are some situations where you would have an engineered structure, a structural alloy, and you could say well, it might actually be beneficial to let it corrode a little bit because it can blunt the cracks that are there already,” Warner says.
The Cornell study was funded by the Office of Naval Research, whose investment could pay off in the long run if it enables them to keep expensive aircraft in safe working condition amid the extreme ocean environment.
Source: Cornell Chronicle, https://news.cornell.edu.