Carbon Steel Frame (CSF)

[bahous]

Hi people,

This year, my university is going one step closer to building a carbon monocoque. We have chosen to go with a carbon steel frame. For those that wonder what is that, here's the idea:

Replace some key tubes by carbon panels to obtain a lighter chassis with equal or better stiffness.
This would be applied to the side impact zone and the side of the nose section. as for the rest of the nose, we decided to go with a one piece removable fiber glass section.


So, I seeking your help for ideas on how to attach the carbon panels to the steel frame based on knowledge, experience, articles, anything.

That would be really appreciated.

Thanks

[bahous]

Hi people,

This year, my university is going one step closer to building a carbon monocoque. We have chosen to go with a carbon steel frame. For those that wonder what is that, here's the idea:

Replace some key tubes by carbon panels to obtain a lighter chassis with equal or better stiffness.
This would be applied to the side impact zone and the side of the nose section. as for the rest of the nose, we decided to go with a one piece removable fiber glass section.


So, I seeking your help for ideas on how to attach the carbon panels to the steel frame based on knowledge, experience, articles, anything.

That would be really appreciated.

Thanks

[exFSAE]

All those tubes in the side impact structure should be pretty much set and non-replaceable, no? Been a while since I looked at rules.

What I always thought would be cool would be to replace sections of a steel spaceframe with carbon tubes. Mainly cross-bracing in non-safety critical areas that is there to make a decently rigid frame.

Would be a pain to work out, but man it'd be tight.

[Go Hawks]

I think exFSAE's idea is much more doable. I've never thought about doing it the way you're suggesting but I have thought about what exFSAE suggested. The biggest problem I always come back to is that you'll end up with a lot of joints.
This is the biggest problem area really for any work with carbon you will do. There definitely are ways you could make ex's idea work. You could bond the carbon tube to a steel tube then weld that to a steel node. You'd have to make sure and test your bond joints though. The manufacturer supplied shear strength of the epoxy would only be a starting point for your calculations of how much bonding area you would need. There are too many variables in the bonding process, the metal etc. to accurately predict your bond strength.

Also I would imagine analyzing the idea you through out would be an absolute nightmare. You'd have some areas of your FEA that would be beams and some that would be composite plates. With the carbon tube idea you can just give a different E value and strength to the tubes depending on which is carbon and which is steel.

Good luck with whichever route you decide.

[NRBaer]

When building a carbon monocoque one of the largest areas of concern for our team always seems to be the side impact zone. Under that assumption I would advise you to look at replacing non-critical areas with thinner wall and/or smaller dia tubing and then bond panels onto those tubes. Bonding area is the key component and if you don't get it right they will not hold, but of course you don't know that until that specific bond is tested.

As far as replacing with cf tubes, remember:
they tend to transfer load in one direction
epoxy can be heat sensitive and welding once the carbon panels/tubes are on can lead to more problems

[RacingManiac]

uh...
(big pic)
http://www.fsae.utoronto.ca/2007/pictures/2007%20Constr...n/PanelBending24.JPG

We've done that since 2002, the pic shows the 2007 car where the front section was replaced with carbon penal as well. The earlier car mainly have panels around the cockpit. In 2008 they have moved on to full monocoque.

And no I don't do the chassis, so don't direct question to me...

more pic of the past years' cars here:
http://www.fsae.utoronto.ca/2007/gallery.html

[JVC]

Ok, I guess before your question can be answered properly, first of all some things need to be put straight.

First of all ask yourself what your goal for stiffness in torsion is. You want to get your stiffness-to-weight ratio as high as possible, not your torsion stiffness per se.
Having sorted that out, we come to the next point, how will these panels be loaded? In shear most likely, so you want a continuous connection between your steel tubing and your CFRP panels. You could go for adhesive bonding, but you might also consider, after your frame has been welded together (welding resin will kill your welder), to establish a connection using a hand lay-up. Effectively what you do is taking another piece of carbon fibre cloth (of appropriate size), which is draped over your panel on one side, folds around the tube, and meets the panel again at the other side. Beware, there are no holes in your frame, otherwise you might end up filling it with resin, making things a whole lot heavier!
Then considering the SEF. It is just some simple mechanics to show your composite side impact structure complies with the rules. As for FEA, if you use decent FE software this should be not to difficult either. Combining shell and beam elements is not a nightmare, probably you will even be able to find benchmark examples in your FE tools library showing you how, and otherwise use your engineering skills to figure it out. Any team driving round with a carbon fibre monocoque can tell you how.

Speaking of which, instead of the hybrid form you propose you might consider this: make part of the car a full monocoque and the rest a space frame. There have been and are quite some cars out there with such a design. To my mind this is a more sensible solution. I am not saying that a carbon fibre space frame would not work, but in that case you are not making full use of the materials properties.

Anyway, this is what I had to say. Good luck with designing.
I am looking forward to see your final solution.

Cheers,
Jules

[bahous]

we have thought of making a semi-monocoque (ie a section of the frame is carbon monocoque and the rest is a steel space-frame). however, our university does not have the facilities and the equipment to accommodate such pieces.

therefore, in order to "introduce" the team to a carbon fiber monocoque and still develop a car for 2009, we have decided to go with a "simpler" design (it's relative, right).

i know that U of Toronto implemented this concept on their previous cars. any tips from members from UofT on the lay-up of carbon fiber cloths to the space frame? how many layers did you use?

[RacingManiac]

As I am no chassis person, the detail I can tell you is based purely on observation. We made our own panel in 2007, with exactly what layup I have no clue, its certain layers of bi-directional and uni-directional on the surface, and a nomex core. We used to purchase penals before that with similar layup, but because they weren't made for this purpose they were less than optimum. In 2007 we also used penals for the front bulkhead and since we are feeding suspension loads into those areas we used denser foam core and balsa core in those places. The penals are made in big sheets then were cut and folded with router to fit to the spaceframe. The bonding between the 2 were done with some commercially available epoxy(something aerospace, again I am not the person to ask about the exact stuff). Then the mating area were wrapped and bonded with carbon cloth on the outside to protect the mating area, as suggested by Formula Student officials in 2006(previous years these places were left exposed, not that it actually presented an issue, but we did it to satisfy the organizers)

[bahous]

do you have the contact information of the person (persons) who were responsible for that on your previous cars? if so, can you please send me their info as a PM?

thanks