How F1's vital aero parts are made strong

To many people the words 'carbon fibre' describe a single material, but it is as generic a term as the word 'metal'. In the same way metal covers a range of elements from aluminium to zinc, so too does carbon fibre describe a plethora of different combinations of fibre and resin, each with unique and exploitable properties.

In general engineering carbon fibre is still thought of as a relatively new material, but the fibres as we know them today were first made by Swan for use in lightbulbs in 1860. In the mid twentieth century small samples of high-performance fibres were being made in laboratories, but it wasn't until 1963, when researchers at the Royal Aircraft Establishment in Farnborough developed a method many companies went on to commercialise, that the material became viable.

I remember as a child my father bringing home a sample from Farnborough and telling me it was the material of the future. How right he was, but neither of us could have guessed how much it would play a part in my professional life.

It's difficult to pin down carbon fibre's first use in motorsport, but it could have been in the Ford GT40 that won Le Mans in 1968. It had bunches of fibres wet-laminated to the inner surfaces of the bodywork to provide additional stiffness.

The vast majority of fibres used in F1 and elsewhere are called PAN fibres. They take this name from the material the fibres are made from, which is polyacrylonitrile or PAN for short. It's an organic polymer resin which, although a thermoplastic, doesn't melt under normal heating.

To make the fibres PAN is first heated in air at 230°C to form an oxidised PAN fibre, and then carbonised above 1000°C in an inert atmosphere to make carbon fibres. The single continuous fibres, which have a diameter around one tenth of that of a human hair, are wound on to a reel and the many thousands of them form a bunch known as a tow.

The strength of these fibres is largely a function of the size of the defects in their crystalline structure. The smaller the defects the higher the strength, and improved manufacturing processes are continually providing us with stronger materials. The latest fibre from Toray, T1100, is twice the strength of an early fibre such as T300, which was used in the first composite monocoque I was involved with at Toleman.

Sometimes it is stiffness that is more at a premium than strength and again there are many different types of fibre to satisfy this requirement.

Stiffness is a function of the alignment of the crystal structure of the fibre. The closer the alignment is to the long axis of the fibre, the stiffer the finished fibre will be. Again, modern fibres are available that have twice the stiffness of early fibres although this comes at the expense of some strength and some additional brittleness.

The difference in composite materials doesn't stop at the fibre properties, though.

Each bundle or tow of fibres can be woven in many different ways, each having different properties. The simplest of these is unidirectional fibre which, as its name suggests, consists of the bundle being aligned lengthwise.

This gives immensely good properties in one direction but no strength at ninety degrees to the bundle. If, however, the load path in a component is known and is simple this can be the most efficient use of carbon fibre.

More generally a component, such as a monocoque, needs to handle multiple load paths in various directions and it's here that woven mat comes into its own.

The weave is defined by the arrangement of the warp (0 degree) fibres and the weft (90 degree) fibres. In a plain weave the warp fibres pass alternately under and over the weft fibres.

A twill weave is one where one or more warp fibres alternately weave over and under two or more weft fibres but there are many more varieties, each with different integrity and different abilities to be draped in a complex three-dimensional mould shape.

The final component that makes carbon fibre such a versatile engineering material is the resin that encases the carbon and gives rise to the true name of the material, carbon fibre reinforced plastic or CFRP.

Just as in old-fashioned fibreglass laminating, it's the resin that bonds the fibres together and, once cured by heating, gives rise to the solid nature of the CFRP.

In F1 manufacturing the resins are impregnated into the woven carbon fibre mat as part of the manufacturing stage, giving rise to the term pre-preg or pre-impregnated fibre.

Once again there are a variety of resins available, each with different properties that allow tuning of the finished material.

By a large margin the majority of resins are of the epoxy family but even within this group different types of resins are used for different applications. The uncured resin in the pre-preg holds the fibres together in a tacky state, allowing the laminate to be draped into the moulds. But once heated and cured it contributes some mechanical properties to the finished product, particularly in the plane at ninety degrees to the woven laminate.

Finally a carbon fibre component will often be stabilised by sandwiching a core material between two sheets of carbon fibre to further enhance the out-of-plane properties of the finished article.

As you may by now have guessed, the core materials are also numerous. The first core material I used in a design was end-grain balsa wood in the late 1970s.

Today cores are generally a honeycomb material made of either very thin aluminium foil or a paper-like honeycomb of Nomex. For complex shapes a machined syntactic core is often used. This is a foam-like material that combines very light weight with easy machining to allow shaping to complex curvatures. The purpose of the core isn't to add strength as such but to stabilise the carbon skins and provide the separation needed for enhanced structural properties.

That just about sums up the basic materials and terminology of modern carbon composites - next month we'll look at the production process that allows these materials to be so well exploited in F1.