It was a result so unexpected that MIT researchers initially thought it must be a mistake: Under certain conditions, putting a cracked piece of metal under tension—that is, exerting a force that would be expected to pull it apart—has the reverse effect, causing the crack to close and its edges to fuse together.

The surprising finding could lead to self-healing materials that repair incipient damage before it has a chance to spread. The results were published in the journal Physical Review Letters in a paper by graduate student Guoqiang Xu and professor of materials science and engineering Michael Demkowicz.

“We had to go back and check,” Demkowicz says, when “instead of extending, [the crack] was closing up. First, we figured out that, indeed, nothing was wrong. The next question was: ‘Why is this happening?’”

The answer turned out to lie in how grain boundaries interact with cracks in the crystalline microstructure of a metal—in this case nickel, which is the basis for “superalloys” used in extreme environments, such as in deep-sea oil wells.

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