Straining every fibre
Carbon composites can significantly reduce vehicle mass but optimising body design for light weight and passive safety is one of the biggest challenges ahead
- Published in Focus.
Crash course: Carbon composites can be used for crumple zones and the passenger cell
Despite the improvements still being made with steel, and the increasing use of aluminium, the industry is looking at carbonfibre composites as a body material for high-volume applications.
If successful, the materials will deliver significant weight reductions, enabling further powertrain downsizing and compensating for the extra bulk of electric and fuel-cell vehicles’ batteries and hydrogen tanks.
The OEMs need maximum weight savings to justify the investment. Partnering with the materials suppliers will help – BMW has a joint venture with SGL Carbon, Audi with Voith and Daimler with Toray. But much of what has been learned so far about body-in-white engineering in terms of crash performance has to be rethought.
“If you design a carbon-fibre bodyshell it makes no sense to have the same concept and shape you would for a metal structure – it has to fit to the material,” says Karl-Heinz Baumann, Daimler’s head of passive safety concepts and strategy. “So we can forget what we do for steel or aluminium: we need to find new ways to design the passenger compartment and the energy-absorbing structures.”
Load-deflection characteristics are totally different – unlike metals, carbon composites do not progressively deform as they absorb energy, so crumple zone and passenger cell design have very different challenges if made from composites.
Composites’ specific stiffness is excellent so the material is ideally suited to making a light, rigid occupant compartment – any deformation or intrusion is highly undesirable. For crash cans and crumple zones, however, you need as much crushed length as possible to absorb as much impact energy as possible.
BMW has chosen aluminium for the front and rear crash structures of its Megacity electric vehicle, due in 2013. In the past, Daimler used tapering composite structures for the all-carbon SLR supercar it developed with race firm McLaren.
Baumann believes composites could still be relevant in future designs too: “Per kilogram of material, you have higher energy absorption compared to metal but you have to ensure the structure stays together during and after the impact.”
Side impacts are perhaps the hardest to mitigate against simply because the B-pillar has to cope with such high stresses – it’s become one of the most highly evolved parts of the body structure.
Ultra-high strength boron steels are becoming the standard, with reinforcement pressings behind. Enhancements such as tailored tempering and tailor-rolled thicknesses are also used now. Replicating this level of performance using composites will be hard.
