carbon fiber mounting points

[jonnycowboy]

hey all,
I had a question nagging me for a while about the use of carbon fiber. When you design the CF part, lets say in this case a whole tub, how do you attach metallic parts to it? As in suspension mounting points, engine mounting points, etc. Is there a layer of metal in between the layers of CF? or just bonded to the outside with loctite? I'd be worried with just CF, you can't tap it since it isn't uniform enough, so you wouldn't be able to bolt anything on? Also, bearing mounts, etc that i'd worry about. Maybe CF wheels too, just bonded to the alum. part or bolted or what?

Anybody care to enlighten me? I feel like a nooB!!

[jonnycowboy]

hey all,
I had a question nagging me for a while about the use of carbon fiber. When you design the CF part, lets say in this case a whole tub, how do you attach metallic parts to it? As in suspension mounting points, engine mounting points, etc. Is there a layer of metal in between the layers of CF? or just bonded to the outside with loctite? I'd be worried with just CF, you can't tap it since it isn't uniform enough, so you wouldn't be able to bolt anything on? Also, bearing mounts, etc that i'd worry about. Maybe CF wheels too, just bonded to the alum. part or bolted or what?

Anybody care to enlighten me? I feel like a nooB!!

[gug]

im sure someone will describe some better methods later, but for the moment ill have a shot.

one method in to make small inserts (we made ours out of aluminium). the inserts were a cylinder about 10mm diameter (for about M4 screws), and a bit longer than the thread lengths. they were tapped and the outside was roughened (gnurled? knurled? spelling?). these were then included in the layup by cutting a chunk of foam out of the forming material (ie., the foam centre in our wings) about 30mm diameter at the surface and tapering to a point like a cone about the length of the insert in from the skin. then the insert is put in and bonded to the foam with a combination of microspheres and epoxy (kinda like body filler, only harder). the microsphere and epoxy base distributes the load from the mounting point over a larger area of the skin, stops the insert from ripping straight out.

sanding back the microspheres and epoxy is important too. i dont fully understand the explanation of why though, its got to do with the curing reaction and something like the surface mollecules are underbonded...

then the cf is laid up over the insert. sometimes you can leave a screw in there and lay up around the hole (difficult to vac. bag), or you can lay up over the insert and cut the cf later to get to it. im sure there is a better method of doing this step, just cant think of it/havent heard of it yet.

there are heaps of websites about all this kind of stuff. but dont take anyone (especially me ) on their word. you'll find heaps of so-called professionals who will tell you completly contradictory things. like "use calibrated pumps, its what all the professionals use", then you go talk to the suppliers themselves and they say "do not use calibrated pumps. we refuse to sell them because they are not accurate enough"...

[TomF]

Some adhesives to glue your (Alu) brackets to the CF would work quite well. Just look into some aerospace adhesives, that are strong enough. One disadvantsge could be the generation of an electrical potential over the layer of adhesive, but that should not be too large. Otherwise some inserts might suit you well. There are tons of ways to fasten them to the CF, just let your creative mind work for a while. One other oppurtunity is to drill a hole into your tub (small off course) and put an alu bus in there, which you load with some bolted connection. This way you do not introduce normal loads to the CF, but you can attach something there.

[Tony K]

Loctite Hysol is perfect for bonding just about anything to CF. We use it bond CF to hard points and our nomex honeycomb core. Good stuff.