Carbon fiber crack repair

[pd9]

I have a spare seat post for a bicycle that I am working on as my project involving carbon fiber. I’m doing this primarily to get some experience with carbon fiber. I plan on sanding down the area then reinforcing by wrapping it with carbon fiber fabric. I am unclear about a few things though:

How far down should I sand the area around the crack? If I don’t sand it down far and wrap it, there may be too much of a bulge in the repaired area.

It seems that I have a few options for what kind of fabric to use. I’m thinking maybe 2x ud (0,90deg) folded over the end of the post (the other side also has a small crack in it) then wrapping the entire post in the affected area with a twill fabric. The dunstone heat wrapping looks like it will work very nicely :-).

One concern that I have is that when the heat wrap compresses the seat post, the portion around the crack will cave in a little. Maybe I can use an expanding foam that I can dissolve with acetone. Is this necessary?

The wide selection of epoxy resins available also have me a little confused. Can someone recommend a strong epoxy resin system? I This will be my first time working with carbon fiber so I will most likely go with a hardener that gives me ~20min in the cup and around an hour when spread out. I read that the type of epoxy resin used has a significant impact on the final product’s strength. The final product needs to be strong enough so I can clamp it at or around the previously cracked area. Here is an image of how it is clamped on.

Here is a picture of the crack (it runs vertically). I can easily push one side down.

This can give you an idea of the material’s thickness. Notice the film the manufacturer forgot to remove!

[zhefei]

The picture showing the crack is out of focus. We can’t see the crack. Try macro mode on your camera, or get farther away (camera have a minimum focusing distance).

You’ll need to sand all the way down and completely remove the crack. It sounds like the crank goes through the whole thickness, since you can push it down. Maybe you can reinforce it from the inside, and then sand and repair the crack.

You mention a heat shrink wrap tape. This will not compress the whole area evenly, as the flatter sections will get very little compression compared the the high curvature areas. Because of this, I’m not sure how well that will work.

I’ve not heard of an expanding foam that dissolves with acetone. Most of the commonly found 2-part polyurethane foams are very, very hard to dissolve once cured.

Epoxy systems all have easily available spec sheets for download, where you can compare curing requirements and post-cured strengths. Also, to a hobbyist like you and me, a lot of the selection will depend on who will sell you a small quantity. There are several out there, and it’s easy enough to compare them to narrow down the initial selection.

How did this crack happen?

Good luck with the repair.

[pd9]

Thanks for the response, I will try to get better pictures. I guess you can call it a fracture, not a crack? You can barely see that it is cracked because both sides are level and there is no material missing.

I think a better plan is to vacuum bag it for compression.

As far as epoxy resin goes, I think I have narrowed it down to aeropoxy.

The crack happened when someone over torqued the two fixing bolts at the rear. With hollow pieces like this the torque specifications are pretty important! Luckily I had a spare so its no big deal if I mess this one up

[datas_br]

I’ve done some work in this area but I’m not the most experienced her. So keep that in mind, but:

I would have my doubts about that part being repairable. The problem is that post needs to withstand compression from 2 directions. Vertically from the seat and around the circumference from the post clamp. you also have horizontal forces trying to make it bend at the clamp from the weight of you body plus road shocks. The layup would be rather complex to deal with that but you will need to find out and replicate it.

Since you have to keep the outside dimensions you will need to apply the patch from the inside. This means sanding down a wedge along the crack and using a bladder to apply pressure to the patch. You also need to place it in a makeshift mold to keep the crack from bulging out under pressure.

It’s not completely impossible but I think it would be easier to just replace it.

Cheers
DB