Carbon Fibre / Carbon Fiber

Carbon fibre has many particular advantages in weight and performance but is held back by expensive fabrication, repair and recycling processes. The beauty of carbon fibre is that it can be fabricated in such a way that directional performance (in terms of response to force applied) can be manipulated to give the best possible results in virtually every circumstance. Whilst a material such as steel will have desirable performance when subjected to forces in certain ways or from certain directions, weaknesses will remain. The ability to arrange fibres to suit the particular forces affecting a component mean more areas of weakness can be eliminated.

Carbon fibre is expensive to use for several key reasons:

• The raw materials are complicated and thus costly to produce.
  
• There is a relatively high level of wastage.
  
• Automated manufacture is difficult as most of the performance benefits come from the way components are built by hand to best suit force-distribution across a component.
  
• Fabrication and curing mean product cycle times and demand on resources are high.
  
  
  

Despite the significant production costs, carbon fibre is an extremely appealing material for high-performance applications where cost restrictions are not so tight. It is commonly stated that carbon fibre can offer the same tensile strength as steel for just 25% of the weight. This is subject to careful design and fabrication to ensure the best possible performance across a component.

In order to reduce costs, increase rates of production and produce more consistent results, some new processes are being introduced. In the Mercedes-Mclaren SLR this envolves producing blanks, moulds and utilising processes from the textile industry to weave fibres to create accurate, ready made elements.

"The longitudinal members of the front body structure consist of a central cross member and the encircling moulded part or internal web. The cross member comprises several layers of carbon fibre stitched together by a machine. After the form has been cut to shape, the web blank is inserted into a braided polystyrene core. This core element is clamped into a specially developed braiding machine that produces the longitudinal member from 25,000 ultra-fine carbon filaments that are unwound simultaneously from 48 reels. This process allows the fibres to be braided around the core at a precisely defined angle to create the desired contour. Several layers are overlapped in certain areas, depending on the thickness required." – Mercedes McLaren

Images courtesy & © DaimlerChrysler

Performance in Impact

Relatively little is known about carbon fibre in crash situations. Material failure is difficult to model due to the complex nature of its composition and the variance in its construction when produced by hand. Failure is non-elastic and very different to standard automotive metals. Reinforcements are used to modify the characteristics of components to improve strength or elastic deformation characteristics. The Mercedes Mclaren SLR uses a roof made of a carbon fibre foam sandwich to improve crash-worthiness.