How much damage is too much damage?

[fyrstormer]

So I have another carbon swingarm sitting in my garage now, and it has a scratch and a chip on it; the scratch doesn't go through the epoxy coating, but the chip does; I can't tell how many carbon fibers were cut, but at least a few were. (I'm talking about individual fibers, not whole strands in the weave.) I've heard CF can let go pretty much without warning once the fibers are compromised, but how big does the damage area need to be before it becomes hazardous?

[Colin]

send it to me and I'll test it to destruction for you and let you know the result.

Please be aware that this may take 2 to 3 years..........or maybe even forever.........<GRIN>

Go on! get out and ride it! <SMILE>

I've only ever seen or heard about one carbon swing arm failing and that was someone on an 857 dirtjumping and slammed into the front face of a jump......ouch!

I should think Aly would break without much warning as well, as it is by nature a pretty brittle metal?

I'd think that if you were using it for XC then no worries, use it as an excuse to wash and inspect it regularly!

Col. (always willing to help in breaking other people's stuff!)

[fyrstormer]

The problem with CF in regards to its failure mode is, because it's a composite that relies on epoxy for rigidity and fibers for strength, when one of the two is damaged it either becomes extremely flexible or extremely brittle, which usually causes the failure of the other component in a short period of time. Anyone who's ever handled bare carbon fiber knows that while it has a high tensile strength, it's actually quite easy to cut, and the edges of cracked epoxy can act like a serrated knife to cut the fibers once they're allowed to wiggle around a bit; conversely, once the fibers are compromised, there is nothing to prevent the epoxy from being stressed past the point of shattering.

I tested this myself a few years ago with a rock hammer and a spare CF steer-tube spacer. I stood it on its side and went tap tap tap, Tap Tap Tap, TAP*CRUNCH*. There was no deformation before the spacer cracked, like there had been with any number of aluminum and steel rings that I destroyed over the years because I had nothing better to do. I just tapped it hard enough and it broke into four pieces, severing the fibers almost all the way through; the remaining fibers posed no challenge to tear apart by hand.

But I don't know how much strength is lost if only a few fibers are cut, and if the damage is caused by surface abrasion instead of by impact. My instinct says "not nearly enough to cause problems", but I don't know for sure, so I thought I'd ask in case anyone else had ever experienced a failure before.

[orange]

I've not heard of any failures.

[fyrstormer]

That's good to know. I cleaned and filled the chips with cyanoacrylate epoxy (i.e. superglue) last night, and wet-sanded the damaged area, so now it pretty much looks like nothing ever happened. I used superglue because I wanted something thin enough to soak into the frayed carbon fibers, and it worked just as I had expected. The better appearance is nice, but mainly I wanted to hold the ends of the fibers in place so they couldn't continue to fray anymore.

On a note that is related, but not in any obvious way, how much are non-carbon Girvin Vector forks worth these days, if they are undamaged and use a coil shock?

[orange]

Not sure where you are, but in the UK around £65-£85

[fyrstormer]

Ashburn, VA USA. Given current exchange rates, that works out to $100-125. Not too shabby.

Anyway, I ask because I may have one to sell in the near future.

[orange]

you might find in the States that despite exchange rates, that's $65-$85 unfortunately as everything is cheaper there

[spikebender]

Would anyone know if Carbon fiber can be aplyed over something to streghthin it something like a cast? 

[fyrstormer]

In theory, yes it can be applied over top of another material to strengthen it, but carbon fiber by itself is only really useful for its tensile strength, not its abrasion resistance. In fact, carbon fiber is easier to cut than a similar thickness of aluminum, so it would be much worse as an outer skin than most other materials you could choose.

If I were going to improve on carbon fiber, I would use it as a substrate and I could coat it with a layer of rubber or thermoplastic to absorb impacts and allow for easy surface repairs.