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Frictional Clamping on a Composite Structure

Frictional Clamping on a Composite Structure

(OP)
Hi,

I am currently designing a composite structure that requires aluminium brackets to be attached to this structure.

I am currently working with frictional clamping between the aluminium bracket and the composite structure, but my coefficient of friction (approx 0.25) is too low.

Does anyone have any experience in this are or can suggest a way of increasing my coefficient of friction?

Thankyou

RE: Frictional Clamping on a Composite Structure

You will crush the composite tube, or else break the fibres near the edge of the clamp. You could try gluing it, but will still have the same problem. If you fit an internal reinforcing tube you might get it to work, but if you've done that you might as well go the whole hog and create a proper hardpoint insert.

Cheers

Greg Locock


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RE: Frictional Clamping on a Composite Structure

Serrations, cast into the composite and stamped or cut into the clamp material. Or like Greg says.

RE: Frictional Clamping on a Composite Structure

I'd prep the surfaces and glue it with epoxy.


Factory Hood support on C3 Corvette.
I believe there is a steel gizmo embedded in there.

http://image.corvettefever.com/f/9795430/ccrp_0805...

If I could not torque the bolts enough to provide sufficient clamping I'd expect the faying surface to collapse and the bracket to embed and the joint to become loose without the fasteners backing off one bit



RE: Frictional Clamping on a Composite Structure

This is a common situation with carbon bicycle seat tubes I believe, and the industry has come up with various solutions to help deal with it. For example a 'carbon paste' that is used to increase friction - although I don't know how effective it is. I have heard it works by having a small grit in it, small enough to not strongly damage the carbon, but just large enough to indent into the surface of the carbon and aly slightly and hence resist relative motion (kind of like thousands of tiny ball detents).

Having said that, the way I see it, your coefficient of friction is generally not 'too low' - it's your overall pullout force that is too low - and that is effected by other factors as well - your clamping force. Get more clamping force on there. Yes, there are limitations in what pressures carbon can take without failure - so you need more surface area as well. If you have used up all your possible surface area and applied as much pressure to it as you can in the situation, then yes you have to start getting creative.

RE: Frictional Clamping on a Composite Structure

Do note also that if you are using carbon, not glass fibre - it can cause aluminium clamps to corrode I believe. One solution is a glass fibre composite sleeve between the aly and carbon.

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