I’ve always been interested in finding out how carbon fibre fatigues, with a thought to whether it would make for a suitable commuter bike (cost aside). My understanding is:

Aluminium: will always fatigue, even at fairly low stress cycles.

Steel: does not fatigue, just as long as it is kept within it’s (very generous!) stress range

Carbon fibre: no idea!

My own anecdotal experience of this has been how my steel frame (Surly Troll) commuter has been comfortably outlasting a lot of the aluminium parts that I originally built it up with- namely cranks, pedals and rims (x2!).

My only carbon fibre frame was a mountain bike, which lasted fine, but even using it every weekend it is never going to get the same high mileage a commuter gets..

Anecdotally, rocket jr has a racing drone/quadcopter that has a carbon frame

He’s crashed it many times – bounced it off trees and concrete – and I’ve helped repair it. As you might expect the carbon shows no signs of fatigue until it fails catastrophically.

The only issue I’ve read is that tensile fatigue makes the carbon slightly less stiff before it breaks

A mate of mine once spent the best part of a mutual friend’s wedding explaining to me that whereas aluminium, steel and ti would also eventually crack due to repeated flex beyond a certain number of cycles / range, a properly made and designed carbon fibre bike would not.

He was interested because he was about to ride around the world on a carbon fibre bike and was also an engineer. His experience suggests he was correct.

Coincidentally, I own the same cross frame and after about 10,000 km of sometimes quite brutal use mostly in the Peak, it developed a small crack in the headtube and another in one of the chainstays. To be fair, it has been hammered across stuff like the Roych and various other mildly rocky tracks. It was fixed by Rob Hayles at Recarb a few months back and is still going strong.

I guess the crucial factor is whether the frame has been properly designed and manufactured. I have no way of knowing why my frame cracked and the round-the-world one didn’t afaik. Moreover, I’m not an engineer or any sort of expert, so this is anecdotal. Though my mate was an engineer and knew a fair bit about bikes.

So, yeah, I don’t see why not.

Doesn’t fatigue in the same way as metal. But it can in theory degrade via UV or other chemicals. But manufacturers know this and take steps to avoid it. Consensus seems to be UV degradation is not an issue unless you live in New Mexico and store your bike outside in the sun.

My 1999 Scott Genius was replaced in 2000 when the carbon bit stuck on to hold the front mech snapped. It was a little stick of carbon with bugger all bracing getting pushed sideways every time you used the front mech.

The replacement had the same part as an ali piece bonded into the carbon. That frame got battered and abused (check my user name) bouncing off trees and rocks until 2014/15(ish) when the bonded ali main pivot worked loose from the carbon. I had replaced the ali rear triangle a couple of years earlier as cable rub had worn a hole right through it.

The carbon doesn’t fail when properly designed, the stuff it is stuck too and the sticking point is what failed.

i’ve had a carbon frame since 1994. Did 5000 miles a year on it for a few years, then it had a few years off, and since 2009 it’s been my commuter doing about 3000 miles a year. No issues yet 🙂

Yes it does. Well not quite in the same way, but it still fatigues.

I’d not commented before, because I’m far from an expert and was likely to post incorrect information, but there’s plenty of proper science stuff out there, eg http://www.cirmib.ing.unitn.it/Compositi/textbookCOMP/18.pdf

Here’s the google I used to find that https://www.google.co.uk/search?q=carbon+composite+fatigue

Good work aracer! I’ll be honest tho and say I’m struggling to completely understand all of that. Does anybody know how carbon fibre fatigues compared to aluminium and steel?

Not really. The SN curve for composites is pretty flat when compared to metals.

Link to research paper

They do degrade in manner that most metals do not. They also continue to cure when exposed to sunlight so become stiffer/more brittle as they get older unless they’re protected.

What is an SN curve?

You can also get bimetallic corrosion on naked carbon which will corrode things like seatposts pretty quickly. Ask me how I know this.

You see that tool I used earlier to find information? 😉

SN is basically load applied (stress) over the number of times that load is applied. Usually 10^7 cycles is considered to be endurance or safe-life.

Not really. The SN curve for composites is pretty flat when compared to metals.

So you can expect a composite frame to pretty much last forever then? That sounds good!

It is the resin that fatigues. The polyester resin they use in boats does eventually, but epoxy is much better (and I think epoxy is what they use in all bikes).

Last year I part-ex’d a few windsurf masts 25%, 50% and 60% carbon content. The oldest was about 25 years old. They take a beating, get scratched to buggery, and the rule is to tighten the boom clamp until you can hear a sort of cracking sound.

I don’t know what that proves, but I think carbon is pretty tough.

I swapped them for 2 new carbon masts. A 100% and a 75%.

Epoxy degrades. Seen it happen with vibration, flex, thermal cycling