As just about any video game, movie, book, etc. will teach you, no matter how strong something is, it has a weakness that can lead to its failure. Graphene, that amazing atom-thick sheet of carbon is no exception to this, as researchers at Rice University and Tsinghua University have found.
If you were to zoom in far enough, graphene would look like a collection of carbon atoms arranged in a hexagonal pattern, like chicken wire. At least if you were looking at a perfect piece of graphene, which is very hard to create. The easier form of graphene to make is polycrystalline, so it has many domains of hexagonal patterns, and where they meet, the patterns may not line up, causing other shapes to form. What the researchers found is that the seven-atom rings at these domain borders are greater weaknesses to the material than previously thought. As stress is applied to graphene, the force will collect at these defects and will be amplified by the length of the domain border. With enough stress, the ring will fail and potentially cause the entire graphene sheet to break apart.
This has not been found before because previous studies analyzed the defect less accurately than in this study, because it was easier both on the computer and the researchers to do so. Fortunately this weakness can be dealt with by using defect-free monocrystalline graphene, or polycrystalline graphene with smaller domains, so the stress is not amplified as much.
Source: Rice University