Can I repair carbon fibre, using epoxy?

[Greenheart]

Dumb question I know, I should just look it up online.

I saw separate kits at Force 4 for repairing carbon fibre, so I daresay it responds best to similar chemicals. But I've got some epoxy resin and hardener left over from whatever my last sticky-business was, so I'm hoping it will lend itself to fixing a small split in a lightweight carbon spar.

It's not catastrophic damage, just a two-inch crack in the inner section where the upper sleeve of a windsurfer mast overlaps the lower.

Same old project - making a bowsprit from bits of random stuff, and persuading an asymmetric to fit where it wasn't designed to.

Thanks, in advance.

 

[TQA]

Yes

You can use CF cloth or glass cloth as reinforcement. .

 

[johnwest]

Cf cloth or glass cloth but not chopped strand mat.

j

 

[Yngmar]

Yes, that's how they're made (not always epoxy, can be polyester or other resins): https://en.wikipedia.org/wiki/Carbon_fiber_reinforced_polymer

Do remember that epoxy is not UV stable though, so you must then protect it further or it'll crumble away again. Paint or tape depending on use may do the job.

 

[William_H]

Yes epoxy will bond to CF just make sure you clean and abrade the CF well. You may find that the glass does not have the same strength as CF and may fail. If you add too much glass to get the strength you may end up with a stiff section of the CF pole. ie when the pole bends it bends in a lesser radius where you have reinforced and a tighter radius where you have not reinforced. At least taper the glass reinforcement. I might suggest you look for a CF wind surfer mast that is all in one piece. That will be able to bow a lot without problem. As I have found with a CF windsurfer mast as a spin pole. olewill

 

[Greenheart]

Thanks for the advice gents. So...epoxy resin is okay, but the cloth needs to be the real stuff like a carbon fibre repair kit contains?

I'm thinking that as the damage is at the end of an inner section of tube, I could make a rigid 'plug' to fill 90% of the end of the tube...

...then pour epoxy into the gap between the plug and the damaged outer...so the tube wall will effectively be much thicker there...

...or does epoxy need a fibrous armature to achieve its full strength? I thought solid resin would be stronger.

 

[TQA]

The strength is in the carbon fiber or glass fiber. Use the minimum amount of resin to fully wet out the cloth. Carbon fiber cloth is stronger but small quantities can be hard to find except on the internet. For what you are doing glass cloth is fine.

 

[Greenheart]

Thanks again TQA. I just realised I hadn't actually understood the earlier replies. :hopeless:

It really shouldn't be hard to fix at all.

Funny thing is, I only need about 30 inches of the narrower, top section of the mast, to make the bowsprit. I guess I'll want to abrade the outer section of both tubes where they meet, and glass-over the join with more well-wetted cloth?

 

[SvenH]

Cf cloth or glass cloth but not chopped strand mat.

j

That used to be a hard fact but nowadays there is at least one chopped strand mat out there that can be used with epoxy.
At least, that is what a sales rep told me.

He got scared stiff when he saw wind turbines being made with the stuff so went to the supplier to check. Sure enough, it was ok for use with epoxy.

 

[Quandary]

With epoxy, you use powder bound mat, any reputable GRP supplier will stock it. Epoxy resins are not compatible with the binder in the mat used with polyester resins.
There are a lot of jobs where using woven fabric gives less than tidy results, the guy who fitted the battens for hanging ropes in my cockpit locker can attest to that.

 

[Greenheart]

Hmm. Thinking back, I'm not sure any of my epoxy repairs ever used a cloth sold specifically for use with epoxy resin.

But I guess there's no reason really why I shouldn't use layers of polyester resin (and the cloth sold with it) to fix my carbon pole.

 

[lw395]

Cloth and tape are generally OK with any resin.
Mat is mostly made for polyester only, the strands are held together with a 'binder' which dissolves in polyester resin but not epoxy.
Epoxy will make a better repair of carbon, it will stick to the existing stuff better, and flex a bit more before unsticking.
My local chandler sells various widths of glass tape by the metre which is handy.
Several online places sell carbon tape the same way.
When working on the outsides of tubes, a useful tip is to lay up the repair, cover with polythene, then bind it with insulting tape to keep it all in place and under pressure.

 

[Greenheart]

Great advice, thank you. I'll hope to find time this weekend.

An unrelated question is how much of the undamaged upper-mast to cut off and add, to lengthen the lower. The 8ft lower is definitely too short for dinghy-bowsprit purposes.

It occurred to me that as the upper tapers, I could trim as much of its bottom end as necessary to make it narrow enough to slide it telescopically up inside the lower half...

...thus maintaining a a good long overlap to lessen the stress where the two pieces meet...then cut off as much of the very narrow top end as I think will be superfluous...

...and bond them together with epoxy. Sound plan?

 

[lw395]

Great advice, thank you. I'll hope to find time this weekend.

An unrelated question is how much of the undamaged upper-mast to cut off and add, to lengthen the lower. The 8ft lower is definitely too short for dinghy-bowsprit purposes.

It occurred to me that as the upper tapers, I could trim as much of its bottom end as necessary to make it narrow enough to slide it telescopically up inside the lower half...

...thus maintaining a a good long overlap to lessen the stress where the two pieces meet...then cut off as much of the very narrow top end as I think will be superfluous...

...and bond them together with epoxy. Sound plan?

Seems fairly reasonable.
8ft is quite a lot of bowsprit. I don't think a B14 sprit is much longer than that. But they are all about sailing in 20knots. And they have a long foredeck I suppose.

 

[Greenheart]

My foredeck's fairly big too, for a dinghy - the forestay is nearly six foot in front of the mast.

The reason I need greater length of bowsprit, is that I won't have any engineered, moulded housing for the sprit, such as established classes use...

...so if I secure it on deck at a point beside the forestay, I'll need plenty more length behind that point, to maintain the stable pointing of the bowsprit, and secure its mounting.

I've thought for years, inconclusively, about the upsides and downsides of making the sprit's bow-connection rotate a few degrees, to enable the asymm to fill, deep downwind...

...at the moment, I think I'll be happy to make it remotely controllable on any point of sailing, but I welcome all views on what's the best way, in practice.

Worth remembering that I single-hand, so I won't have much time for systems which need continuous diligent attendance.

Sorry for drifting my own thread.

 

[Greenheart]

I won't apologise for waking up my own old thread, because you gentlemen's advice answered my initial question perfectly.

I epoxied then sanded the repair which seems very solid, and the sleeve still fits well over the narrower lower tube.

I plan to fit them together and make the join permanent...what I'm wondering is whether a few layers of epoxied glass cloth, over the join between the upper and lower sections, is likely to be enough to hold them together when the tack of the spinnaker is pulling forward on the end of the bowsprit?

I could easily drill a 4mm hole through the inner section and outer sleeve, and put a bolt through...then build up a smooth 'rise' of epoxied cloth so the bolt doesn't obstruct the rapid launching and retraction of the bowsprit in use...

...but perhaps drilling holes in carbon fibre can seriously compromise its strength? I doubt it - I don't see why it should be any worse than drilling a metal spar - but I've no idea.

Thanks.