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Composite Material

Sun Apr 17, 2005 1:36 am

Hello everybody.

During some beers yesterday I had a talk with a good friend who just started his engineering carreer with BMW. He told me a bit about the car assembly, and I was wondering why they don't use any composite material for the car fuselage.
I told him it was used in aviation because it's cheaper. But he insisted that the costs of such a composite fuselage for cars would be much higher than the costs for an aluminium or whatever they use fuselage.

So here's my question:
Why exactly is this composite material used in aviation? Because it's cheaper in the production? Or because of its less weight, which makes it cheaper in operation? Is it too expensive for the car industry because they couldn't save much weight which wouldn't lead to a noticeable fuel burn decrease?

Any help is appreciated  Smile
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RE: Composite Material

Sun Apr 17, 2005 2:15 am

Composites are lighter but expensive so you'll mostly find composites on cars where weight saving is critical, such as racing where parts like the hood, wings etc are composite. Steel on the other hand has better energy absorbing properties than composites, that's why it's difficult to build a car chassis from it. Formula one cars though have a composite monocoque, but formula one cars aren't known to be cheap either..  Smile

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RE: Composite Material

Sun Apr 17, 2005 2:31 am

While composites are usually pitched as being lighter, in my experience, they quite often come out the same weight or heavier than the comparable metal part. This is real world aerospace experience, not theoretical.

The advantages of composites for aircraft construction are that they are much easier to form into shapes with complex curvature and they are much stronger than comparable metal parts.

In car production, they want to quickly turn out a lot of one shape, so steel has an advantage in that parts can be stamp formed. In addition, steel can be easily welded, these days by robots. Composites have a tiny elastic deformation range, something necessary for absorbing crash loads, and steel has a large elastic deformation range.
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RE: Composite Material

Sun Apr 17, 2005 8:26 pm

Composites can be moulded to include the strengthening substructure. Metal structures, unless milled from solid, require a lot of additional strengthening to be fastened on. Thus composites reduce the number of fasteners and improve integrity.

If composites really didn't save weight no one would use them. You have to look at the overall picture, not just the weight of individual components.

Composites won't corrode, reducing the need for inspection. Crack resistance is improved and so fatigue life. On the other hand, composites require new inspection techniques to detect hidden failures.

By adjusting the alignment of fibres in the lay-up process, unique directional strength qualities can be built in which are not possible with conventional metal structures.

Cars are mass produced cheaply, so exotic composites aren't used. GRP is used a lot for body panels and trim though. On a car these aren't load bearing, so don't require complex lay-up and moulding procedures. Gets expensive after a collision though, GRP panels don't dent, they crack and shatter, so the entire panel must be replaced.
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RE: Composite Material

Mon Apr 18, 2005 8:26 am

Composites are many different things. "Composite" just means that the final product has been made up (composed) from two or more different things.

When talking about aircrafts, then it is mostly glass fiber, carbon fiber or aramide fiber. Aramide fiber is often called Kevlar, which is a Du Pont trade name for the product. Boron fiber is also used to a minor extent, especially for helicopter rotor blades.

When talking about composites on airliners these days, then it is mostly carbon fibers, even if glass fibers have been used for radomes and many other substructures on airliners for well over fifty years.

Carbon fibers have the properties mentioned in earlier posts: Lightweight, strong, expensive, easy to mould in strange shapes, and very good fatigue properties. It is also very demanding to produce large structures since it has to be cured at a high and very exact temperature, all of it at the same time.

Carbon fiber is a very bad product for car body production. First of all it is prohibitively expensive. But even more important, it is very stiff, and when it breaks it becomes a thousand knifes, not exactly what you need in a traffic accident. Steel is almost unbeatable for a good and safe car.

In a F1 race car the driver is protected from the carbon fiber by sitting in a "bathtub" made of aramide fiber, which is also used to produce bulletproof vests and such. It has entirely different properties. A bulletproof vest made of carbon fiber would be totally useless since it would explode into small scrapnels when hit. Even if it is much stronger than aramide fiber.

For consumer products we see carbon fibers used for the most expensive fishing rods, and also some other special sports equipment where price is not important. Rackets, certain bicycle components stc. These days it is also finding its way to the most expensive amateur astronomy telescopes where ultimate stiffness and portability is more important than a few thousand dollars wallet drain.
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RE: Composite Material

Mon Apr 18, 2005 12:41 pm

Another major reason why we don't see carbon fiber or other composite based cars is that until very recently, carbon fiber parts had to be laid up by hand...an extremely time consuming process.

With a mass-produced car, two of the most important criteria in materials are the cost of of the materials, tools to form it, etc., and the amount of time and labour expended to make the part. In almost every case, steel is the fastest and cheapest material to make a car out of.

Also, the ability to repair parts easily and quickly is a minor but still important goal of a car manufacturer. Having the car written off because high repair costs associated with a parking lot bumper-bashing is never a good way to patronize consumers.

Having said that, there are an increasing number of uses of carbon fiber composites in automobiles. The roof panel of the new BMW M5 is made of carbon fiber (if I'm not mistaken), and a good number of the new Corvette Z06 body panels are made from carbon fiber.
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RE: Composite Material

Tue Jun 14, 2005 1:36 pm

There was something else I have always wondered about composites: Stress related to thermal expansion. From sealevel to FL360, there is a change of 70 K. While for most materials the expansion is not much more than a few centimeters or less, what about higher speeds beyond Mach 1.0? The JSF competition had 3 then 2 contenders (Boeing bought MD), all aircraft were made from composites as well as being designed for 9-g manuvering and Mach 1.8+.

I'm not too knowledgable in material science, I hope someone with composite experience can help me out.

Aren't composites are the plastic-metal equivalents of metal-metal alloys? So not all of them are the same, can I have instead of an Aluminum base, maybe a Titanium or even Tungsten based composite? Could that work? I suppose little has been done beyond 'practical or proven uses', so it would be difficult to asertain.
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RE: Composite Material

Wed Jun 15, 2005 2:23 am

Concorde's fuselage stretched considerably when cruising at Mach 2. However, thermal expansion does not cause stress unless it is constrained in some way, for example if dissimilar metals are bonded together.

When composites are mentioned these days it is usually carbon fibre or something similar, which are basically plastics with a reinforcing matrix. The idea is to use the high tensile strength of a material like carbon or glass but avoid it's brittle nature be embedding it in a plastic material. Alternatively you could see it as the carbon fibre matrix providing strength to the plastic.

I should imagine such composites are less affected by thermal expansion than metals.

Glare (as used on the A380 for example) does involve the use of metal. It's a laminate material made up of thin aluminium and fibre glass layers. It might have limitations regarding thermal stresses due to the differential expansion of the various layers.
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