Attaching Carbon Fiber Tubes

[neeba]

Hi everyone,

I am from Lafayette Motorsports and I am trying to implement carbon fiber A-arms. We manufactured aluminum plugs that inserted into the carbon fiber tubes with a bond gap of 0.011 in. The epoxy that I used is Loctite 9340 Hysol with glass beads with a diameter of 0.007 in. Each of the surfaces was roughened and cleaned prior to application of the epoxy. I let the epoxy cure for 4 days and then tested it in the Tensile tester, and the epoxy began to pull apart at 500 lbs. Is there a better process or epoxy that I should use in order for the epoxy not to slip at such a low force? Any other suggestions would be greatly appreciated.

Thanks for your help.
Brendan

[Loz]

The product is a good adhesive, it sounds like your process could do with better technical execution.

I've tested the 9340 (pull out shear tests) achieving close 15 MPa shear strength (~0.5" diam. aluminium inserts in CF tube failing at upwards of 15 kN).

It could be inadequate cleanliness, poor mixing of the epoxy and hardener, over use of filler, inconsistent bondline, expired product, poor test methods, the list goes on...

If it is "slipping" (and that means a bondline failure) then it is most likely a surface preparation problem (which is almost always associated with the metallic part - especially oxidising aluminium). It doesn't hurt to use an adhesive surface primer/cleaner as part of the prep process.

[MCoach]

Sanding parts to get a high surface roughness for bonding is a way of life in composites.
Live it. Breathe it.

[Swiftus]

Breathe it.

Don't actually breath it. :) That would give you cancer ;)

[MCoach]

Bah, that's just what they said about that silly asbestos stuff too. ;)

[Freddie]

If I remember correctly from our tests:
Remember to get rid of all the release agent from the tubing, it's easy to have release agent and/or dust left which destroys the bond
Prepare the inserts by abrasive blasting, that seems to be the most effective way of roughing up the surface
Push the insert in, don't twist it into the tubing

[Rory Hourihan]

Pretty good tutorial on it here. http://www.carbonfibertubeshop.com/cut%20&%20bond.html. Also a good place to source tubes. They're very nice and will give discounts to FSAE teams. Execution in surface prep and manufacturing is very important though. If you get it wrong you could very well lose the whole corner at once, think of a house of cards. Do some research on surface prep and consider proof testing your tubes before putting them on the car. For comparison we achieved around 4000psi with a similar bonding method.

[neeba]

Thanks for the advice guys! Hopefully it'll work when I make them again.