cut and fold carbon fibre advice

[Rob Davies]

I have often heard of the phrase 'cut and fold carbon fibre' but I am a little unsure about what it exactly is. It sounds like an interesting technique that could benefit our team.

Just wondering If anyone has used this technique and if so could give a little advice of what it is and how it is done and get me going in the right direction.

Thanks in advance, Robert Davies

[James Waltman]

Rob,
I meant to answer your question when you asked it another discussion (http://fsae.com/eve/forums?a=tpc&s=763607348&f=125607348&m=97510922511&r=60310174511#60310174511)in a while ago.

We (the VRI at WWU) have been doing some form of cut and fold carbon chassis for over 10 years now. Aluminum cut and fold before that. I have been involved with building two cars that use this technique and we are getting ready to make another.

The idea is pretty simple. We use prefabricated panels (usually from Hexcel). The panels have carbon fiber skins and a honeycomb core (Nomex/Aramid/Phenolic).
http://dot.etec.wwu.edu/fsae/James/Web%20Pics/NomexPanels/Panel.jpg
We use a router to cut out a section of the skin and core on the inside of a bend.
http://dot.etec.wwu.edu/fsae/James/Web%20Pics/NomexPanels/Cut.jpg
Now the panel can be bent.
http://dot.etec.wwu.edu/fsae/James/Web%20Pics/NomexPanels/Fold%20Detail.jpg
The gap gets filled in and then we do a lay-up over the joint.
http://dot.etec.wwu.edu/fsae/James/Web%20Pics/NomexPanels/Bent%20part.jpg

We use two-piece hollow aluminum bulkheads to tie everything together. For our FSAE cars we have been relying on the carbon fiber tubes for most of the chassis stiffness. We have done a few full size cars that rely solely on the cut and fold panels.

We feel that this sort of chassis is well suited for the concept of the competition because they really can be manufactured in significant volumes for relatively low cost when compared to traditional monocoque designs. The method the UWA guys have may make that a moot point but for now it is a real advantage. I don't have stiffness numbers for our latest car yet (Viking 35) but they should be very impressive (overkill?).

The process is attractive from a manufacturing point of view for several reasons. First, we don't need any molds to make these. The bulkhead design allows the whole process to be pretty much self jigging. The panels are relatively inexpensive. The cutting process can be automated very easily with a CNC router. We have one here but for the last few cars we have done all of the cuts with a handheld router or a bench mounted router. Overall the accuracy of all the chassis points can be superb. Another key advantage is that we don't have to use an autoclave for assembly. (The 6"� diameter tubes have been autoclaved pre-preg).

This is Viking 35 before assembly. All of the parts ready to go. I think that with some practice we could assemble all of the parts in less than 10 minutes.
http://dot.etec.wwu.edu/fsae/v35/bonding/images/All%20chassis%20parts%20before%20assembly%201_JPG. jpg

The Result:
http://dot.etec.wwu.edu/fsae/v35/bonding/images/Chassis%20assembled%20on%20ground%202_JPG.jpg

This is the inside of Viking 35's chassis. You can see the cut near the top and at the very bottom right.
http://dot.etec.wwu.edu/fsae/v35/bonding/images/Inside%20-%20Between%20the%20front%20bulkheads_JPG.jpg

The same section of the chassis during fitting.
http://dot.etec.wwu.edu/fsae/v35/bonding/images/Holding%20up%20the%20front%20chassis%20section_JPG .jpg

Viking 30 used this method as well.
http://dot.etec.wwu.edu/fsae/v30/image014.jpg

Some of our other guys may have stuff to add and probably stuff to correct. The website has some more pictures.

[rjwoods77]

Thank you for being cool enough to go to such detail to explain. Alot of people wouldn't go so far.

Rob,
Some of these techniques hae been used in the "the pashley project" and "project morengo" and a couple other cars in Race Tech and Race Car Engineering. Go buy the back issues that have these in them. Its a real good look into this sort of stuff.

[jack]

F1 cars in the 80's also used this technique, there is a page or so about it in "Formula 1 Technology".

[Rob Davies]

Thanks alot guys very good info from some very knowledgeable people.

I am surprised the outer sheet of carbon folds so well considering how stiff the stuff ive seen is.

We are looking into the use for a stressed skin so the core is probably too costly and not needed so much with a space frame behind it. If we can get a couple of sheets of carbon fibre that we can bend like the outer sheet of carbon fibre then bond on I think we will be happy.

Thanks,
Rob

[Denny Trimble]

Rob,
Watch out for buckling of your stressed skins on your spaceframe. We saw our stiffness fall off at a certain load point two years ago when we did a stressed-skin spaceframe, with just .050" or so (don't know off-hand) of carbon skin over a large surface area.

You might want to do a wet layup of core and additional skin to brace any large spans. But I'm the last guy you should ask for composites help...

[Kevin Hayward]

Rob,

Core material is not really expensive ... depending on which way you go. If you want stiff panels split the amount of Carbon fibre you were going to use either side of a core.

If you have seen the honeycomb panels you would not be so surprised that the carbon skins bend so easily. You are only talking about a couple of layers either side of the core.

If you want just flat skins they are pretty easy to make if you want to do wet layup and have a piece of glass hanging around.

Kev

[James Waltman]

Rob Woods,
I'm always happy to share what we are doing. I figure the more open we are the better chance I have of learning something.
I'm a little more guarded with some of the tricks and data that we have accumulated. I'm still happy to share that stuff, but prefer to trade for it.

Rob Davies,
Denny is right about the thin skins you are talking about - they can oilcan. Kevin is right about the core material - it makes a world of difference.

I think the chart from Hexcel makes it pretty clear how core material is helpful.
http://dot.etec.wwu.edu/fsae/James/Web%20Pics/NomexPanels/Core%20Chart.jpg
Dramatically better stiffness with very little weight penalty - its like magic.

This page from Hexcel (http://www.hexcelcomposites.com/Markets/Products/Panels/Sandpanel_fab/default.htm) is a very good source for more information about sandwich panel fabrication.