Carbon Fibre Tub Design

[adriantaylor09]

Hey Eng-tippers.

This is a question that I have out of pure interest.

I am planning for my second kit car build, and while doing a bit of research, I couldn't help but entertain the notion of a carbon fibre tub style chassis.

The benefits of carbon fibre chassis are well known: Weight reduction and excellent chassis stiffness. The downsides are also well known: Carbon is very expensive, carbon fibre is an an-isotropic material meaning that it is strong in one direction and weak in another which makes the layup design interesting, but the biggest downside of all is the prohibitively expensive manufacturing processes involved i.e. the use of an autoclave.

I was ready to squash the idea completely when I came across the following article:

http://www.compositesworld.com/articles/innovative-composite-design-may-replace-aluminum-chassis

The article describes a method of building automotive carbon tub's using mostly out of autoclave processes. Essientially the chassis/tub is made in several sections using RTM methods and then glued together at the end and cured in an autoclave.

My questions are:

Does anybody have any experience with this type of construction for composites in general? Is this process used in other industries like aerospace?
What type of resin/adhesive would be used to join the pieces together?
Since the individual pieces have already been molded and cured before being assembled, would they receive any benefit from the final autoclave process?
I'm assuming that the reason for using an autoclave to join everything together is to achieve a bond strength necessary for the high loading conditions that the chassis will see, with this in mind, is it possible to achieve this bond strength without the use of an autoclave?

Any discussion is welcomed!!

 

[MikeHalloran]

Of course you'd just eyeball-engineer the whole thing instead of bothering with expensive FEA and such.

Okay, suppose you go ahead and make your own tub.

In the unlikely event that you survive the inevitable crash,

who you gonna call?



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Autoclaves large enough for a complete tub are few and far between. We built a hot room instead, I'm not sure about the second one but the first got to 80 deg C, which is worth doing, but not really ideal.

If you want a room temperature cure it's really a case of sitting down with the adhesives catalogue and finding a cold cure epoxy that is strong enough. If you think carbon fibre is expensive be prepared for sticker shock with specialised epoxies. A cynic might suggest that normal boring polyester resin is good enough.

Plenty of people have built their own composite tubs, although I'm not sure eng-tips is a great place to start. In my opinion for a one off it is likely to be a bit of a pain, but it is possible. One approach we used is to build the shape from polystyrene foam, install buttons at each hardpoint, then layup prepreg and uniaxial over the top, vac bag it, and cure in the hot room.

The devil is in the details, our lead constructor on that makes motorbikes the same way so he knew what worked.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Screwman1]

Are you going to use prepreg or dry fabric. If prepreg, then the issue becomes cure temps. If using dry fabric, then you have to think of how to set up a vacuum bagging system and the right bleeder plies for a room temp cure. I know it can be done, but if you look at all of the Formula car chassis except pure pro racing, they still use tube frames because it is simple to fabricate, the strength is known and repairs are a cinch. On a tube frame, if a pick-up point is mislocated, you just cut it off and weld on a new one. On a fiber tub, you ponder how you are going to save all your work.