For the first time, a team of researchers from Sandia National Laboratories and Texas A&M University has observed metal cracks melting back together without human intervention.
"We've shown that metals have their own natural ability to heal themselves, at least in the case of nanoscale fatigue damage," explains Brad Boyce, a materials scientist at Sandia.
The discovery overturns basic scientific theories of how engineers design and evaluate the fatigue life of structural materials. If the phenomenon could be harnessed, it could usher in an engineering revolution in which self-healing engines, bridges, and aircraft reverse wear-related damage, making them safer and more durable.
Astonishing discovery
The process of fatigue in metals means gradual failure. It is caused by the gradual propagation of cracks under repeated mechanical loading and can eventually lead to fracture. In structural applications, fatigue is responsible for up to 90% of in-service failures. Fatigue damage leads to wear of solder joints in electronic equipment, motor failure, or bridge failure. The crack Boyce and his team saw disappear was one of these tiny but momentous fractures, measured in nanometers, that cause hundreds of billions of dollars in economic damage in the U.S. each year.
"It was assumed that cracks in metals would only grow larger, not smaller. Even some of the basic equations describing crack growth exclude the possibility of such healing processes," Boyce explains.
Discovery with the help of chance
Back in 2013, Michael Demkowicz, a full professor at Texas A&M, published a theory that metal may be able to 'heal' wear-related cracks under certain conditions.
Now, confirmation of his theory came purely by accident, when scientists at the Center for Integrated Nanotechnologies wanted to study how cracks form and propagate in a nanoscale piece of platinum. They applied a special electron microscopy technique that allowed them to pull on the ends of the metal 200 times per second. As expected, the nanoscale cracks advanced, deflected, and stopped at local microstructural barriers. Surprisingly, however, cracks were also observed to heal by a process that can be described as cold welding of the crack flanks, triggered by a combination of local stress state and grain boundary migration.
After about 40 minutes of the experiment, the damage reversed. The crack melted back together at one end, leaving no trace of the earlier injury.
"We weren't looking for that," Boyce said.
The experiment was replicated by Demkowicz on a computer model, and he confirmed that the phenomenon observed at Sandia was the same one he had theorized years earlier. The question of whether the self-healing process can be used in practice or is also possible with conventional metals in air remains unanswered at this time.
"The extent to which these findings are generalizable will likely be the subject of extensive research. I hope that this finding will encourage materials scientists to consider that, under the right circumstances, materials can do things we never expected," Boyce explains.
Despite all the unknowns, this discovery is a major advance at the frontier of materials science. The team of researchers from Sandia National Laboratories and Texas A&M University published their findings in the journal Nature.
