Olympic composites

Badminton rackets used to be made from wood. Now they will sometimes contain buckminsterfullerene or bucky balls. Canoes were also made of wood. Now a variety of materials are used, including carbon fibre. These, and many other materials used in sport, are composites. A composite material is made by combining two or more materials – often ones that have very different properties.

The two materials work together to give the composite unique properties. However, within the composite you can easily tell the different materials apart as they do not dissolve or blend into each other. 

People have been making composites for many thousands of years. One early example is mud bricks. Mud can be dried out into a brick shape to give a building material. It is strong if you try to squash it (it has good compressive strength), but it breaks quite easily if you try to bend it (it has poor tensile strength). Straw seems very strong if you try to stretch it, but you can crumple it up easily. By mixing mud and straw together, it is possible to make bricks that are resistant to both squeezing and tearing thus making excellent building blocks.

Another ancient composite is concrete. Concrete is a mix of aggregate (small stones or gravel), cement and sand. It has good compressive strength (it resists squashing). In more recent times it has been found that adding metal rods or wires to the concrete can increase its tensile (bending) strength. Concrete containing such rods or wires is called reinforced concrete. Many tonnes of reinforced concrete have been used to build the Olympic venues. 

Most composites are made of just two materials. One is the matrix or binder. It surrounds and binds together fibres or fragments of the other material, which is called the reinforcement.

The first modern composite material was fibreglass. It is still widely used today for boat hulls and sports equipment. The matrix is a plastic and the reinforcement is glass that has been made into fine threads and often woven into a sort of cloth. By itself the glass is very strong, but brittle and it will break if bent sharply. The plastic matrix holds the glass fibres together and also protects them from damage by sharing out the forces acting on them. Some advanced composites are now made using carbon fibres instead of glass. These materials are lighter and stronger than fibreglass, but more expensive to produce. They are used in expensive sports equipment such tennis rackets and racing bikes. Carbon nanotubes and buckminsterfullerene (or bucky balls) have also been used successfully to make new composites. These are even lighter and stronger than composites made with ordinary carbon fibres but they are currently extremely expensive.

Composites also exist in nature. Bone is a composite. Without bone we could not stand up or walk, throw or use a racket. Sport could not exist (slug racing is not very exciting)! Bone is made from a hard but brittle material called hydroxyapatite (which is mainly calcium phosphate) and a soft and flexible material called collagen (which is a protein). Collagen is also found in hair and finger nails. On its own it would not be much use in the skeleton but it can combine with hydroxyapatite to give bone the properties that are needed to support the body.

Natural composites are also found in plants. Wood is a composite – it is made from long cellulose fibres (a polymer) held together by a much weaker substance called lignin. Cellulose is also found in cotton, but without the lignin to bind it together, it is much weaker. The two weak substances – lignin and cellulose – together form a much stronger one. Wood was used in the ancient Olympics for a variety of purposes including the javelin, so using composites for sport is nothing new. What has changed is the huge variety of different composites which can now be made.