Carbon fibre is used in a range of applications where lightweight materials are favoured but strength still important. For example, in motorsport. How does its composition make it fulfill these requirements?
What is Carbon Fibre?
Carbon fibre is a composite material made up of filaments of long chains of carbon atoms. These chains are woven together to make sheets of carbon fibre. Its structure means it is very strong but also very lightweight.
Carbon fibre has a density of around 1800kgm-3. This makes it more than four times lighter than steel, which has a density of 8000kgm-3. It is also very stiff, with a Young modulus of around 230GPa. It therefore holds its shape well and shows very little deformation.
Its high ultimate strength also makes it highly desirable. It has an ultimate strength over ten times greater than steel (5GPa compared to 0.4GPa).
Carbon fibre is made from the polymer polyacrylonitrile. This polymer is first drawn into long strands, which helps to align the carbon atoms. Its polymeric properties allow it to stretch in this way.
The strands are then oxidised, improving their thermal stability so they can be heated to high temperatures of 3000oC. This second heat is done in the absence of oxygen so that it doesn’t burn. The non-carbon atoms are lost and remaining carbon atoms then make a tightly bonded carbon crystal structure. They are aligned parallel to the fibre length.
Epoxy resin is added and strands woven to further improve strength. In the end, the carbon fibre will be about 92% carbon, although this figure depends on the type of carbon fibre.
The structure of carbon fibre is actually very similar to that of graphite. In a similar way, it is made up of sheets of carbon atoms in a regular hexagonal pattern. Each of these layers is very strong due to the covalent bonds between carbon atoms.
But graphite has weak intermolecular forces between layers so they slide over one another. Due to this, graphite is soft and lacks strength. Carbon fibre is layered in a different way, with a turbostratic stucture, meaning that the sheets are layered irregularly and crumpled together. For this reason, they will not slide over one another.
Carbon fibre has a range of applications for example: aeroplanes, sporting equipment and high performance cars.
It was first used in Formula One in 1981, revolutionising the sport with a multitude of advantages over previously used steel and aluminium. One of the key factors was its weight, the cars became much lighter which immediately gave huge performance advantages.
Carbon fibre is also stiff so the load will be transmitted into the structure as a whole, allowing higher loads to be absorbed. It shows little deformation under high aerodynamic load, improving stability of the car.