MIT researchers have unexpectedly discovered that 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.
“We had to go back and check,” says professor of materials science and engineering Michael Demkowicz, 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.
By creating a computer model of that microstructure and studying its response to various conditions, the researchers found that there is a mechanism that can, in principle, close cracks under any applied stress.
This computer simulation of the molecular structure of a metal alloy – displaying the boundaries between microcrystalline grains (white lines forming hexagons) – shows a small crack (dark horizontal bar just right of bottom centre) that mends itself as the metal is put under stress. This simulation was one of several the MIT researchers used to uncover this new self-healing phenomenon. (Simulation courtesy of Guoqiang Xu and Michael Demkowicz.)
– David L Chandler, MIT News Office