When a crack forms in a material, typically it is downhill from there onwards. Any further tension makes the crack spread and only increases the damage. At least, that is what intuition and conventional fracture mechanics indicate. Now, the opposite effect has been observed. Under certain circumstances, applying tension or other loading to cracks can actually trigger these fissures to close.
“This really turns our understanding of what is possible in fracture mechanics on its head,” said Michael Demkowicz, an assistant professor of Materials Science and Engineering at the Massachusetts Institute of Technology and co-author with graduate student Guoqiang Xu, of an article published in the October 4 issue of Physical Review Letters (DOI: 10.1103/PhysRevLett.111.145501; 145501). “It’s not surprising if a crack closes under compression, but if you pull on the crack and see it close, that’s very unexpected.”
Demkowicz and Xu stumbled across this discovery by accident. While studying hydrogen embrittlement in nickel-based superalloys as part of a project on deep-sea oil well applications, they noticed that one of their simulations was behaving in a counterintuitive way. Rather than spreading, the nanocracks they observed seemed to be healing. The researchers assumed there was a mistake with the program or in their parameters, so they combed over the setup for any possible glitches. “We went back and eliminated all of those options,” Demkowicz said. “Eventually, we convinced ourselves that it was really happening.”
The challenge, then, was to figure out why this was happening. Xu and Demkowicz created detailed computer models simulating how the microstructure of nickel behaved under a number of conditions. Disclinations—a somewhat exotic class of string-like, one-dimensional defects that form in metal but have a much stronger internal stress field than the more common dislocations—turned out to be responsible, based upon quantitative measurements of the strength of the stress field. The stress field intensity causes the material to pull together rather than separate apart under an applied force. Depending on the kind of microstructure evolution occurring, external loads ranging from hundreds of MPa to GPa triggered the crack closure.
Molecular dynamics simulation of nanocrystalline Pd, where a grain boundary migrates under shear stress and heals a crack. Pure shear loading would neither close nor open the crack in the absence of grain-boundary migration. Credit: Guoqiang Xu.
“A lot of work has already been done on self-healing of soft matter such as polymers and biomaterials, which are generally weak in comparison to metals or ceramics,” Demkowicz said. “Our finding is interesting because it’s a mechanism that allows for self-healing in much stronger materials.”
Demkowicz and Xu are only in the very early stages of investigating this phenomenon, but they can already imagine several possible applications the finding may open in the future. One is to try to design materials with microstructures that use the mechanism to heal internal damage. If typical wear-and-tear could be prevented or arrested, fatigue—one of the most common forms of failure in metal components—may be reduced. The mechanism may also help prevent surface cracks from forming in harsh environments, such as in the deep-sea oil wells that Xu and Demkowicz were originally investigating. Thinking even further down the line, the energy stored within disclinations may even be harnessed to help modify other material properties, such as strain hardening, they said.
Demkowicz looks forward to further exploring the newly discovered mechanism. These efforts will include in situ experiments, developing design tools to create microstructures best suited to self-healing and, eventually, figuring out how to undertake cheap and efficient large-scale processing. “Here we have a truly new mechanism, something previously not known, that goes against conventional wisdom of fracture mechanics,” he said. “To me it’s extremely exciting because it opens up opportunities that previously did not exist.”
