Why CFRP makes Formula One cars safer
McLaren and Lotus debuted cars in the 1981 Formula One season featuring CFRP (carbon fibre reinforced polymer) chassis, and since then the sport has been dominated by the material. Its combination of high strength and light weight makes it the perfect material for car construction. And in the intervening 30 years, engineers’ understanding of the material’s properties and capabilities have improved markedly.
Some of the biggest advances have been in the area of design and simulation. The behaviour of CFRP is considerably harder to predict than uniform materials: with finite element methods having been developed originally to model solid homogeneous and isotropic materials, simulations are naturally more complex for composites. The past decade though has seen the rapid development of new modelling techniques that can predict the behaviour of CFRP, providing engineers with a much deeper understanding of what is and isn’t possible.
The focus of these simulation and design packages is on the interaction between the separate plies of a composite structure, enabling components to be tailored to the load cases they receive, by optimising the composite lay-up (as in, for example, the number of ‘plies’ or carbon cloth layers, direction of fibre weaves and local addition of material in high-stress areas).
Not only did the introduction of CFRP construction into Formula One provide engineers with the ability to build cars that are lighter and stiffer than was possible using other construction techniques, it also brought great benefits to driver safety. These days, one of the most important milestones in a new Formula One car’s development is passing the mandatory FIA crash tests. While it is still a struggle for some teams to pass these tests – usually those lower down the grid with less powerful development facilities – they do not prove problematic for most.
It is the failure mode of composites that makes them hard to model. Traditional metallic structures absorb impact energy by deforming and folding; CFRP is very different, as the energy is absorbed by fracturing. The static simulation of CFRP is now well-developed, but accurate prediction of crash performance is somewhat harder, with FEA (finite element analysis) requiring significant real-world testing, reducing the benefits of FEA as a tool here.
New processes, however, allow accurate predictive FEA simulations based on more economical testing of small material samples, or ‘coupons’. The coupons are placed in a fixture in a compression test machine, then crushed at a variety of speeds, and the results of the tests inform the basis of the material properties and behaviour modes for the analysis. Overall though, composites are largely unrivalled as a material for impact absorption, with a specific energy absorption (SEA, measured in kJ per kg of material used) far higher than their metallic counterparts – providing sufficient optimisation has taken place of course.
The decision to use CFRP for impact absorption is a fairly easy one. Comparing the SEAs of various materials, steels achieve about 12 kJ/kg while aluminium reaches around 20k J/kg. However, a well-optimised carbon fibre structure – that is, one with an optimised lay-up/fibre orientation and component geometry – can absorb anything from 40 kJ/kg up to 70 kJ/kg in a highly refined and tested design.
Suffice to say, from a safety perspective, CFRP does not look likely to be superseded as Formula One’s material of choice any time soon.
Written by Lawrence Butcher