Science and technology
Babbage
Carbon-fibre composites
A high-fibre diet
Mar 7th 2011, 16:16 by The Economist online
THE new McLaren sports car is a remarkable vehicle. It is the result of a decision by what had previously been a specialist Formula 1 team to compete with the likes of Ferrari off the track as well as on it. The MP4-12C, as the new car is known, is as rapid (zero to 100kph in 3.3 seconds) as it is expensive ($275,000). But it also has extraordinary handling characteristics, especially round tight corners. These are the result of several novel features, including an active-suspension system that, instead of employing mechanical anti-roll bars, uses sensors to monitor the movement of the body and wheels, and stiffens the ride when needed using hydraulic dampers. Yet the car’s agility is also a consequence of its lightness—and that, in turn, is the result of an innovative way of using carbon-fibre composites.
Carbon fibre and McLaren go back a long way. Thirty years ago, the firm was the first to introduce a Formula 1 car that had a carbon monocoque (a body structure that also works as a chassis). John Watson used the result to win the 1981 British Grand Prix at Silverstone. Later that year he also illustrated, in dramatic fashion, its ability to withstand crashes when he walked unharmed from a spectacular pile-up at Monza. Within a few years every Formula 1 team was racing with carbon-based cars. But even today it can take 3,000 man-hours to make a carbon monocoque for an F1 car.
The reason is that it is a bespoke process. Each monocoque is made by impregnating carbon-fibre cloth with a thermosetting plastic such as expoxy resin, pressing the cloth into shape in a mould and then cooking the whole thing, mould and all, in an autoclave. The cloth, in turn, is made by weaving yarn that has itself been spun from fibres of pure carbon that are made by baking strands of a polymer such as polyacrylonitrile.
The reason for this rigmarol is that carbon-fibre body parts are stronger than steel but weigh half as much. The strength comes from the powerful chemical bonds that form between carbon atoms (think diamond). For high-performance parts needed in small numbers, such as aircraft wings and Formula 1 racing cars, the price of such lightness has been worth paying. Mass production, however, has proved elusive.
That looks as if it is changing. It takes just four hours to make the MonoCell (see right), as McLaren calls the carbon-fibre chassis it uses in the MP4-12C. That is possible because of a production process it has pioneered jointly with Carbo Tech, an Austrian firm which specialises in composites.
The new process differs from the old in two important ways. First, the carbon cloth goes into the mould dry, rather than pre-impregnated with resin. Layers of cloth are placed into a large steel tool, which provides the shape to be moulded. The tool presses the sheets together and only then injects epoxy resin into them under pressure. That is a lot less messy than handling sticky cloth. Moreover, the tool then applies the curing heat directly, rather than using an autoclave. Once hardened, the MonoCell is transferred to another device, where it is machined to provide the fixing points for the car’s other components. The process is precise, consistent and some of it is automated.
The second difference, which does not speed the process up but does make for a better product, is the use of what McLaren calls unidirectional cloth. Actually, this is not really cloth at all, since it is not woven. Instead, it is a layer of fibres all pointing in the same direction. That eliminates the strains found in woven cloth at every point where two fibres cross. It also means, according to Claudio Santoni, who is in charge of body structures at the firm, that stronger and more precise structures can be built up, layer by layer, with the exact alignment of each layer being appropriate to the strains and stresses that the body-part in question is likely to experience.
Mr Santoni reckons McLaren could make around 5,000 cars a year this way—a number that, for supercars, almost constitutes mass production. Moreover, given time, he thinks the process could be used for truly mass-produced vehicles, too. It should be possible to automate the laying of the carbon sheets. It may also be possible to use thermoplastics (which melt when heated), instead of thermosetting plastics (which do not). Not only are these easier to handle, they are also easier to recycle—which would be a consideration in a vehicle that was truly being mass produced.
McLaren is not alone in working on resin-transfer and other ways of making carbon-composite components in large quantities. BMW is planning to use carbon in a new range of electric cars for city use, and Volkswagen has just bought a stake in BMW’s partner, SGL, a German carbon-specialist. The steel age, then, may be almost over. All hail King Carbon.
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I love a carbon fibre bike.
A mass production car would both require and one hopes allow a different approach to passenger safety. We would need a protective cage more resilient than current ones. A major limiting factor now is the extra weight in an already heavy car. We might have safer conveyances even if they weigh the same by moving much of the body weight into protection.
I don't remember how much the body of a car weighs versus the chassis, engine, etc.
The new Boeing Dreamliner is supposed to use the most composite materials than any other aircraft in production, making it 20% lighter. It will be really interesting when carbon-fiber starts becoming the material of choice for large commerical aircraft on a grand scale, or if it starts getting used extensively in shipping.
Wet lay-ups and unidirectional fibers aren't terribly new innovations; the way they bypassed the need to use an autoclave is a lot more impressive.
Good stuff, and adroitly explained. But to address a criticism of carbon fiber / plastic composites that keeps cropping up about recycling:
True, thermosetting plastics are harder to recycle than thermoplastics. But we burn old thermoset plastics, like used tires, as fuel all the time.
All the plastics are just long hydrocarbons (carbon backboned chains with hydrogens hanging off) synthesized from short hydrocarbons (crude oil and NGLs). Burning or oxidizing any of these fuels results in CO2 and H2O, regardless.
We can burn the old plastic, and use the crude oil and NGLs saved from the fire to make virgin plastics. Whether we conserve our petroleum by combusting or recycling plastics makes no great difference.
Of course in the US we are crowding out coal by burning tires for electricity, but most would agree that is even better. Either way we don't need to toss old thermosets into the landfill.
What happens when the matrix surrounding the fibre catches fire? Toxic fumes? What are the standards applicable? Or will it be a fudge as on carbonfibre planes?
If that is the case, shouldn't our 'carbon footprint' be increased so that there will be sufficient carbon to meet the demand? Just curious.
If carbon-fibre is used to produce mass market, fuel efficient cars that are stronger and safer then it would seem that the benefits derived are greater than those of their steel counterparts, even factoring in the recycling costs. However, to produce fuel-guzzling super cars like the McLaren for a chosen few seems a needless waste of technology and resources. When can you realistically do 0 to 100kph in 3.3 seconds, and where can you drive at >200kph?
Acuara1, are you responding to my comment?
No, our carbon footprint doesn't go up when we burn plastics (hydrocarbons) for electricity.
In the US that displaces coal. Coal is dirtier than plastics with (1) a higher carbon to hydrogen ratio (so more C02 to harmless H20 exhaust) and (2) more impurities, like mercury.
How does one recycle the carbon fibre body parts at end of life? More and more composites in car bodies reduce recyclability and will pose serious problems in the fuure. Let us analyse sustainability and environmental aspects carefully and not use superficially attractive materials, which are practically indestructible. We should look at biodegradable plastics as an option.
It would be nice if journalists who write about science and technology were to learn the distinction between "strain" and "stress".
And, whilst it is not an issue in this item, it would also be a good thing if they were to learn to distinguish between "energy" and "power", rather than treating them as synonyms, which they are not.
Fair article. The tooling for this must be expensive. It's also worth noting that strength isn't necessarily the most important property here. Toughness, or the ability to absorb energy during impact is critical to automotive safety, while stiffness (elastic modulus) helps this McLaren turn corners, and minimizes the vibration in aircraft wings. Metals typically have to compromise between strength and toughness, where carbon fiber composites do both well for the weight. Carbon fiber composites truly are supreme in stiffness per weight, but this is much less important in a Buick than (say) a McLaren...
@ZeFox
Pretty sure it won't catch fire until your interior is either burning or melted.
Hail King Carbon! But with gas prices going thro' the roof, McLaren may be hard pressed to sell 5,000 MP4-12Cs annually, anywhere but in the ME & possibly China.
Goodlookin' hot rod, needless to say.
I likeit that the Economist includes articles on a broad range of subjects, but I always feel the science/technology pieces are written(or edited?) by people who patently don't understand the subject.
"These are the result of several novel features, including an active-suspension system that, instead of employing mechanical anti-roll bars, uses sensors to monitor the movement of the body and wheels, and stiffens the ride when needed using hydraulic dampers."
To my knowledge, anti roll bars play no role (no pun intended) in a suspension system. They are a safety feature designed to preserve the integrity of the passenger compartment duringa crash (hence anti-roll). They can be used to stiffen a chassis, but again, this has no direct affect on suspension.
The traditional passive, as opposed to active, suspension system is mechanical shock absorbers (springs) and dampers (pistons moving in oil).