Graphene is known to be the world’s strongest material. Its first commercial uses, in sports equipment and specialty gear, rely on this mechanical resilience. Yet, it is also brittle, fracturing when the load on it passes that maximum strength level.
But what happens to the material when it faces a small mechanical load over and over again? This fatigue life of graphene and associated damage mechanism remain unknown. In a recent study published in Nature Materials (doi:10.1038/s41563-019-0586-y), researchers have found that the two-dimensional (2D) material can withstand more than a billion cycles of stress before it breaks. The research should help design next-generation carbon-reinforced composites for aircraft and cars that have high fatigue life.
“Fundamentally we answered the question, ‘can this material fail under fatigue?’” says Tobin Filleter, a mechanical and industrial engineering professor at the University of Toronto. “And the answer is yes, there’s a mechanism that will lead to failure at loads below tensile strength. This sets the stage for understanding fatigue life of this general class of 2D materials.”
The research team led by Filleter, materials science and engineering professor Chandra Veer Singh and mechanical engineering professor Yu Sun also at Toronto, conducted physical experiments as well as molecular dynamics simulations. In the experiments, they pressed an atomic force microscope (AFM) tip to the center of freestanding films of the graphene and graphene oxide material and then oscillated the tip at a frequency of 100 KHz.
AFM is a widely available technique, but the researchers had to build a special device for the experiments in order to handle these very thin 2D material samples. They etched a silicon chip with half a million holes, each just a few micrometers in diameter, and stretched a graphene sheet over the holes. “There’s no commercial approach to do this type of mechanical testing on atomically thin materials where we’re controlling static load plus cyclic load,” Filleter says.
They used a load that was 50–70% of the material’s ultimate tensile strength, the same fraction that is used in studies on metals and alloys. They found that the materials could withstand an average stress of 71 GPa for over a billion cycles before failing.
Computer simulations showed different failure mechanisms for graphene and graphene oxide. In graphene, mechanical loading and thermal fluctuation cause an irreversible bond rotation near the site of a defect, causing the material to abruptly fail without a progressive buildup of cracks or damage as would happen normally in metals.
Graphene oxide simulations, on the other hand, show “progressive damage that’s more like traditional fatigue,” Singh says. That is because epoxide functional groups present on the material undergo a structural transformation under the mechanical load into ether groups. “In these regions we see small cracks form slowly, which make the material weak.” While the billion-cycle fatigue lifetime of graphene is not surprising, the novelty of this research lies in conducting tests on graphene with the test setup used, says Ramesh Talreja, a materials science and engineering professor at Texas A&M University, who was not involved in the study. “The setup is highly specialized, very expensive, and not commonly available,” he says.