Drilling holes in carbon fibre

[Michael Rogers]

Dear Scientists/Boffins (meaning people who Know These Things)

Will I significantly weaken a CF tube (63mm diam) by drilling 5 mm holes in it (one each side, to pass a s/s bolt through)? Do s/s and CF co-exist amicably? - I ask because I understand CF and aluminium definitely don't.

Thanks, in anticipation

Michael

[Jeremy]

CF and stainless are pretty much OK in direct contact, so no real worries there.

A 5mm hole shouldn't appreciably weaken the tube if it's in a non-critical position.  A 5mm through hole will reduce the strength of the CF tube at that point by only about 5%, but this will be enough to create a stress raiser if the hole is in a high stress region, say around the lower part of a mast where bending loads are high, or the centre of a lug yard where the halliard is attached.

You might need to take account of the bearing stress on the hole edges if the bolt is to be loaded appreciably, too.  The general rule for any fastener in relatively thin wall tube or sheet is that more often than not the fastening shear strength won't be that of the bolt or rivet itself but will be determined by the bearing stress around the hole.  This is the reason that larger diameter fasteners than seem OK for the load are often used; the larger the fastener the lower the bearing stress, because of the greater bearing area in the hole.  Unfortunately increasing the fastener size to get a greater bearing area also weakens the tube more, so selection is always something of a compromise.

[Michael Rogers]

Thanks, Jeremy. Just what I needed to know. I'm glad you're around!
Michael

[BrianP]

I was also worried about drilling holes in my carbon spars for my Scow. I also wanted to keep the tubes free from holes so that they would act as buoyancy and help support the boat on it's side if capsized.

I searched around and found various plastic fittings designed for masts and booms. I think most were from the Optimist rig components catalogues.

This shows the boom and and yard end fittings.



For rigging such as mainsheet attachment just use standard webbing.



My tubes were very thin walled so I reinforced them externally with wound kevlar strands and internally with proper structural foam discs. So it might be worth considering making a foam plug that you can slide inside the mast where you plan to drill it, then drill through mast and foam and epoxy bolt in place. If the bolt is for your main halyard the loads will be considerable at times.

Brian

[tony waller]

 

I may be late in commenting on this but my feeling is to avoid drilling holes in CF except where there is little or no loading. Certainly it can be drilled but 5mm seems to me to be quite a big hole.

By the way, wood sticks very well to CF with epoxy. Tony