Carbon Fibre and Lightweight Materials

Engineers have always wanted materials that are both strong and light. A material that can bear heavy loads without adding much weight allows aircraft to fly further on less fuel, cars to move more efficiently and sports equipment to perform better. For most of history the choice lay between metals, which are strong but heavy, and lighter substances such as wood, which are weaker. In recent decades a new class of materials has begun to change this balance, and among the most important of them is carbon fibre. Its combination of lightness and strength has made it one of the defining materials of modern engineering.

Carbon fibre is made, as its name suggests, from extremely thin threads of carbon. Each individual fibre is far thinner than a human hair, yet within it the carbon atoms are locked into long, ordered chains that run along the length of the thread. Because of this arrangement, a fibre resists being pulled apart along its length with astonishing strength for its weight. Thousands of these fibres are bundled together to form a yarn, which can then be woven into cloth much as ordinary thread is woven into fabric. On its own, however, this cloth is flexible and would offer little resistance to being bent or crushed.

The real material used in engineering is not the fibre alone but a combination of two substances working together. The woven carbon cloth is soaked in a liquid plastic, usually a type of resin, which is then hardened so that it sets solid around the fibres. The result is a composite: a material made of two distinct parts that together perform far better than either could alone. In this partnership the fibres provide the strength, bearing the loads placed on the material, while the surrounding plastic holds the fibres in place, keeps them properly aligned and protects them. Neither part could do the job by itself, but combined they form a material that is remarkably stiff and strong.

The chief advantage of carbon fibre composites is their exceptional strength for their weight. A component made of carbon fibre can be far lighter than the same component made of steel while still carrying similar loads. This is why the material is prized in aircraft and racing cars, where every reduction in weight brings a direct saving in fuel or a gain in speed. It has spread from these demanding fields into a growing range of everyday products, from bicycle frames to tennis rackets, wherever lightness and stiffness are worth paying for.

The material has other useful qualities besides its strength. Unlike metals, carbon fibre does not rust, and it holds its shape well as temperatures change. It can also be moulded into smooth, complex curves that would be difficult or costly to achieve in metal, which gives designers a good deal of freedom. Because the fibres can be laid in chosen directions, engineers are able to make a part stronger along the lines where the greatest forces will act, tailoring the material to the job in a way that a uniform block of metal does not allow.

Carbon fibre is not without drawbacks, however, and these explain why it has not replaced metals everywhere. The greatest obstacle is cost. Producing the fibres and forming them into finished parts is slow and expensive, far more so than working with ordinary steel or aluminium, so the material is reserved for uses where its advantages justify the price. Its behaviour when it fails is also different from that of metal. A metal part often bends and gives warning before it breaks, whereas a carbon fibre part tends to remain rigid and then fail suddenly, which engineers must take carefully into account.

A further difficulty concerns what happens at the end of a product's life. Metals can be melted down and recycled again and again, but the composite structure that makes carbon fibre so useful also makes it hard to recycle, because the fibres and the hardened plastic are bonded tightly together and are not easily separated. As the material becomes more widespread, finding good ways to recover and reuse it has become an active area of research, so that its benefits during use are not offset by waste afterwards.

Carbon fibre is thus a clear example of how modern engineering has moved beyond simply choosing among natural materials towards designing materials to order. By combining strong fibres with a supporting plastic, engineers have created something lighter than metal yet strong enough to trust in an aircraft wing. Its high cost and the challenge of recycling it remain real limitations, but its unusual blend of properties ensures that carbon fibre, and composites like it, will remain central to the search for structures that are at once strong and light.