Carbon fibres are just that – thin strands of carbon. Most carbon fibres are created from chemicals called precursors, which are turned into carbon fibres, then spun or woven into a something resembling a fabric. There are many different types of fibres and, just like normal fabrics, many different ways of weaving them into something that can be used to make a product.
A ‘composite’ is just a combination of two or more things. When we talk about composite materials, we usually mean fibres like carbon, or glass fibres, being combined with a ‘matrix’, usually some type of plastic, which acts as the ‘glue’ holding the fibres in a particular shape.
Much like glue, most matrix materials start off a liquids, but become hard and tough when cured with heat and pressure. The resulting composite is strong, tough and light.
Composite materials like Carbon Fibre Reinforced Plastics are incredibly strong, but also light. Their properties can also be ‘tuned’. Unlike metal, because they are made out of fibres, they can have different properties in different directions. For example, we can make something that bends in one direction, but is really stiff in another.
Brittle materials don’t deform much before they break. Think of glass: it can be bent a bit, and if you let go, it will return to its original shape. This is called elastic deformation as, just like an elastic band, it will snap back to shape. But bend it too far and it will break suddenly, without warning. This is because the bonds between the atoms in the glass can’t move much.
We usually illustrate this as a graph which shows how much the materials deforms before it breaks. Brittle materials deform elastically up to a certain point, then break:
In contrast to brittle materials, ductile materials deform and ‘yield’ before they ultimately break. Think of pulling apart a piece of plastic, e.g. part of a polythene bag. It gets longer, and the material gets thinner and thinner until it eventually snaps. This is because the atoms that make up the material can slide over each other.
When you pass the point where the material will not return to its original shape when you let go, this is called plastic deformation. So when we talk about ductility, we mean materials that have lots of plastic deformation before they ultimately break:
This is a good thing because:
- It can give a warning before the part fails completely
- Even if something does start to fail, the fact that it bends means it still has some strength left, making it potentially safer.
Think of something like a bicycle frame – if you crash, it would be better if the frame just bent, rather than snapped in half without warning!
What approaches are we taking?
There are lots of possible ways of making composite materials behave in a more ductile way. We are exploring:
- Different ways of making composites structures. Composites are often made of several layers of carbon fibre held together by a matrix. By changing how they are made (for example by laying the various layers of material at different angles), we can make them behave more like a ductile material – we call this ‘pseudo-ductility’.
- Using modelling like computer-based Finite Element Analysis to predict what combinations of materials we could use to achieve ductility
- Developing new types of ductile fibres, which also have high strength and stiffness
- Creating the composite constituents (fibres and a matrix material) which modelling predicts would give the best overall characteristics
- Combining the constituents and the architectures to create optimised high performance ductile composites
Want to know more?
Take a look around our site to find out more about this project and what we’ve been up to, including where we are going next and some highlights. If you’d like more general information on composites in general, check out our Composites Hub.