Joining Carbon Fiber

[jonnycowboy]

Hey guys,
I have a question re: carbon fiber usage... I've already searched and read a few books (competition car composites probably the 1st coming to mind) and I still don't know/understand how they join two CF parts together..

Usually when you have a part - lets use a wing in this example - you make a female mold of the top and a female mold of the bottom... so you make the top and bottom and have two smooth surfaces.. now how do you join the two to create smooth leading and trailing edges? I know there are adhesives that will join composites but how do wings always look so smooth?

Also, same question for body tubs... they're always made in 2 parts right, top and bottom, how do they stay together?
thanks for your help!!

[James Waltman]

Man, 'how do you join carbon fiber' is about as open ended as 'how do you design a suspension system'.

I'll try to cover a few basics and then you'll have to ask more specific questions. I'm going to have to make some generalizations. This is going to be difficult without drawing some sketches.

You need to have a joint. This can be designed into the part (and tooling) or it can be secondary.

Loctite has some good diagrams in their user's guide: Available here (http://www.loctite.com/int_henkel/loctite_us/binarydata/pdf/lt1475b_DoItRight.pdf)
http://dot.etec.wwu.edu/fsae/HostedPics/Adhesives_and_Bonding/Loctite_Joint_Guide.JPG

A plain butt joint is going to have relatively poor performance. The good joints all have some sort of doubler. This can be a preformed piece (molded), built into the part, standard forms (think sheet stock), or a layup of some kind. So a butt joint with a layup down the seam should work pretty well.

A joggle lap joint needs to be designed into the tooling. This is easy to do – you just need to make sure that the joggle in the tool won't trap the part when you are trying to pull it out. A joggle lap joint with an additional doubler is pretty close to the double strap joint. I think that a joggle lap joint with a doubler is probably the best design option (for something like a chassis). The joggle can also serve as a locating feature.

For your example of a wing...
There are a lot of options – you can even make them as one piece. You could have an upper skin, lower skin, and leading edge as three separate parts. If you're going to join multiple parts you just need a joint. A butt joint is probably a poor choice for a wing. A butt joint with an external doubler is probably worse. You could use a joggle lap joint pretty easily if you are going to make your wing from two pieces (upper and lower). You just need to add the joggle detail to one tool (and extend it appropriately). I would shift the joint away from the center of the leading edge. I would put the joint near the forward section of the lower skin. The upper skin would include the leading edge that would wrap under the wing.

The goal of all of these joint designs is to minimize peel. Adhesives are pretty worthless for any peel loads. The same principles still apply even if you are going to co-cure the parts or join them with a layup or some other doubler.
There are other joint designs that aren't shown in the Loctite guide. Mostly they are just variations on the same themes (often with different names).

Do a Google search for "Adhesive Joint Design"�. You'll find quite a few good resources.

You can also use traditional fasteners to join composite parts – but it's a shame to do so. I don't think that is what you were asking about though.

Does any of that help or even make sense? Is that the kind of answer you were looking for?

[jonnycowboy]

Wow what a detailed answer, thanks James!!
The thing I was worried about was indeed peel, since theres so much wind on the wing, especially the bottom half which has a low pressure area underneath i'd be worried about peel.
I guess I've never looked at a wing close enough to notice any joints. I'm guessing that theres some kind of sandable black pigment that goes on top of the joint to make it 'dissapear'

[conekilr]

A two piece mold does not necessarily mean that the product must be two pieces. We most definitely do not bond two halves to make our chassis. For example, we make a two piece symmetrical CF mold, bolt the two halves together and layup inside. This allows us to produce a seamless layup one piece chassis. Many complex parts, such as a bike frame, can be produced by laying up on top of a removable tool. I surely don't know half of the processes but I'm assuming a wing could be made similarly.

[NetKev92]

A lot of composite aircraft right now do mold a lower surface, an upper surface, and bond the parts together. The standard right now seems to be to recess or joggle both pieces and use a third piece, a nose cap for the leading edge. Once you get it all together, bondo or higher grade fillers are your friends. There's a lot of bodywork involved in building a kitplane.

As suggested, there are some other options for an automotive wing. Some of the Porsche racers use hollow wings that are molded as a single piece. I don't know the details but fairly cool. If you make the part foam filled, molding a single piece part gets easier. The difficulty is in making the female molds because you never get a part better than your molds.

Another approach is to use male tooling or mold-less construction. To start, hot-wire a good foam likeness of the part and sand it as required for smoothness so you can lay-up on the outside. To get a good surface finish, wrap around the wet layup with something stiff and smooth like mylar, bag the thing, and don't use any breather cloth over the part. Breather cloth that has give in it will leave wrinkles in the part. If you smooth the bag so there are no wrinkles, you can get a smooth finish without any molds. Some of the AIAA Design/Build/Fly teams have really worked out the process so they can make single-piece wings in one shot without any extra bodywork and smoothing.

Remember to scuff and clean your parts if you bond anything. Poor prep will ruin the bond.

[Andy]

The best way to make a wing as efficiently as possible without a lot of hassle is to use the two piece, top and bottom approach. Lay up the bottom skin with a good joggle on the front edge so you'll have something to bond the top surface too. Then you can make the upper surface and bond the two together using some Hysol® EA 9686 structural adhesive. You can also use this adhesive to bond in any endfish for endplates or bonding wing spars. In order to have the best profile, it is recommended that a glue jig is made either from the molds or seperately. Having really smooth trailing and leading edges is always very difficult and in most cases a highly skilled composites trimmer will have to do some sanding. If you've got any more questions, drop me an email a.j.miller@cranfield.ac.uk. I've got some good pictures you might be interested in.

[Erich Ohlde]

a good way for making wing molds is to make a "Taco" style mold. This allows you a seamless leading edge. after you have cured the carbon you can pinch the trailing edges together (after spars have been introduced) and bond them

[Erich Ohlde]

Originally posted by conekilr:
A two piece mold does not necessarily mean that the product must be two pieces. We most definitely do not bond two halves to make our chassis. For example, we make a two piece symmetrical CF mold, bolt the two halves together and layup inside. This allows us to produce a seamless layup one piece chassis. Many complex parts, such as a bike frame, can be produced by laying up on top of a removable tool. I surely don't know half of the processes but I'm assuming a wing could be made similarly.

well said jared.

[conekilr]

Originally posted by jayhawk_electrical:
a good way for making wing molds is to make a "Taco" style mold. This allows you a seamless leading edge. after you have cured the carbon you can pinch the trailing edges together (after spars have been introduced) and bond them

Hmm, I thought about that after posting but wasn't sure if it had been done. Are you getting better yet? You are supposed to be helping us put the car together lol! By the way, the intake plenum came out nicely and we bonded the runners tonight so come see it.