Why is graphene stronger than graphite? Why is graphene stronger than graphite when graphene is just a single layer while graphite is multiple layers of graphene?
And can't we say that diamond and coal also consist of layers of graphene as all are made of carbon or does diamond and coal have different crystalline structures?
Also can we make thin ropes out of carbon nanotubes (tube form of graphene) as it would be stronger than steel or synthetic ropes right?
 A: In real life most fractures occur at defects. Even such everyday materials as cement can have their strength increased many times by reducing the defect density within them. You'll often see claims for the incredible strength of nanostructures, but the strength is just due to the fact that these structures are free of defects. It's a lot easier to make a defect free structure when that structure is very small.
If you could make a perfect macroscopic crystal of graphite then you'd expect it to show the 
strength you'd expect by scaling up the graphene properties. But macroscopic graphite is not a perfect crystal, so it fractures far more easily than a perfect crystal would.
A: In addition to what John Rennie said about fractures, I believe that the structure of graphite itself, even if it's perfectly fracture-free, makes it "weak" in comparison. The atoms within the two-dimensional layers of carbon is covalently bonded together. But each layer is hold together "only" by weak van der Waals forces.
