Fatigue Life of Carbon Frame

harmonix1234

Eats Squid
Does anyone have a basic guide as to the general life a carbon frame should have before it's replaced / unsafe to ride?

Normally I just ride my frames (Aluminium) for two years and then replace.
They are still fine after two years, and haven't ever had problems with cracks or stress marks.

Is carbon the same in that you just ride it until it breaks, or should I replace my frame after X amount of kms / after a certain time?
On my first carbon frame now and hoping some people with more experience can offer some peace of mind.
 

MTB Wanabe

Likes Dirt
As a generalization, carbon fibre doesn't have a fatigue life if the life of the component is maintained below the yield strength of the component. You can bend a piece of CF below the yield breaking strength continuously and you will never break it, but if you exceed the yield strength of the component you will get an unwelcomed surprise with the component breaking. Aluminium on the other hand will fatigue and eventually snap if continuously cycled below the yield strength which is why aluminium eventually cracks.

Things like barely visible damage will reduce the yield strength of the CF assembly which is why after any impact the CF component receives, it should be thoroughly inspected by a qualified professional if you want to continue to safely ride the frame. No offence to the lbs mechanic but this type of inspection is well and beyond their capabilities and can be more cost effective to just replace the frame.
 

timrob

Likes Dirt
As a generalization, carbon fibre doesn't have a fatigue life if the life of the component is maintained below the yield strength of the component. You can bend a piece of CF below the yield breaking strength continuously and you will never break it, but if you exceed the yield strength of the component you will get an unwelcomed surprise with the component breaking. Aluminium on the other hand will fatigue and eventually snap if continuously cycled below the yield strength which is why aluminium eventually cracks.
Agree 100% - Carbon frame will outlive an aluminium frame any day if loaded below yeild point, aluminium with even the slightest load will begin fatiguing and start work hardening itself.

Carbon fibre generally fails in two fashions; resin failure and fibre failure.
If the resin lets go you can get delamination of the carbon matrix which is very very hard to pick up visually unless its on the surface. This mode of failure can occur mid lay between the carbon layup and is typically only spotted through ultrasound style testing - ie expert equipment. If the fibres let go then the resin wont have a chance either, resulting in catastrophic failure - ie snap!

If you are riding a ultra high end stupid light weight hard tail i would assume you have the cash to replace it. If you are on a mid range carbon frame then chances are it will be over designed and assuming there is no surface impacts, cuts or significant abrasions you should be able to ride this frame out to the end of your days. The safety factors involved in producing with any FRP is significantly higher than a much more controlled material like aluminium.


Pedal on!
 

Trickymac

Likes Dirt
i had wondered the same q myself
ive been commuting for last 2.5/3yrsyrs home on my merida scultura carbon road bike....quite often forget im on a roadie and have spent a bit of time up and down kerbs on pathways and the odd foray onto grass, plus my commute home is from hornsby to nth syd, some nice road but some shit/potholes/mahole covers etc etc
shes still going strong :)
my 2 c
 

The Duckmeister

Eats Squid
Under normal use, carbon has an infinite lifespan. However, as noted it can be weakened by impacts that exceed its yield strength.

Things like barely visible damage will reduce the yield strength of the CF assembly which is why after any impact the CF component receives, it should be thoroughly inspected by a qualified professional if you want to continue to safely ride the frame. No offence to the lbs mechanic but this type of inspection is well and beyond their capabilities and can be more cost effective to just replace the frame.
No offence taken, as it's entirely true. Even a mechanic familiar with the technical workings of composites can't pick up anything more subtle than obvious damage (I used to work in composites for several years before getting into the bike trade, and my judgement is little more than "well there's a crack there").

However, it is utterly untrue that it's more cost effective to replace the frame. Carbon is eminently repairable, with far superior material integration post-repair than aluminium, when undertaken by a skilled professional, and it's not all that expensive, often only a few hundred dollars, rather than thousands for a new frame.

Leuscher Teknik in Melbourne is a carbon bike manufacture & repair specialist, and has all the gear to thoroughly examine & properly repair even severe carbon damage. I have no affiliation, but I have referred several customers and have been impressed with the results.
 

pharmaboy

Eats Squid
Things like barely visible damage will reduce the yield strength of the CF assembly which is why after any impact the CF component receives, it should be thoroughly inspected by a qualified professional if you want to continue to safely ride the frame.
Is the implication of this, that following a crash on your carbon fibre bike, you should have it professionally inspected with testing equipment - x-ray or the like?

If so, that is completely insane, and you should immediately join the tiddly winks club locally, and if thats too dangerous, perhaps lawn bowls on astroturf is more appropriate! ;D

A mountain bike is made for crashing - its an absolutely guarantee for a properly used product, that it and its rider will be propelled into the weeds and trees at speed enough to take rider for enforced vacation from riding for some time. I thought it was only roadies that had these sorts of panic attacks after riding furiously into the back of small parked sedan.
 

carpetrunner

Likes Dirt
Carbon road frame experience

I've been riding a Look KG361 since around 2003. Carbon tubes with magnesium lugs,
The early years were mostly Sunday rides, but I was pushing 93kg so not a good life for a bike,
about 3 years back I had a mid life crisis, lost 13kg and joined some group rides,
(and bought a Spesh Enduro - that's why I'm here).

The Look now gets a caning, 3 hard training rides per week with a good sprinkling of out of the seat sprinting. It still feels fantastic - probably not as stiff as modern carbon - but it was originally designed to feel like steel 531 tube.

In the meantime I've broken a couple of steel frames commuting, both were cracks in lugs.

A mate at work has been commuting on Sydney roads, 4 days a week for more than 10 years on a Cadex, it looks like crap now, missing lots of paint - but it still rides OK and he has strava evidence of it still being very quick.

so maybe they last forever?

- carpetrunner
 

MTB Wanabe

Likes Dirt
Is the implication of this, that following a crash on your carbon fibre bike, you should have it professionally inspected with testing equipment - x-ray or the like?

If so, that is completely insane, and you should immediately join the tiddly winks club locally, and if thats too dangerous, perhaps lawn bowls on astroturf is more appropriate! ;D

A mountain bike is made for crashing - its an absolutely guarantee for a properly used product, that it and its rider will be propelled into the weeds and trees at speed enough to take rider for enforced vacation from riding for some time. I thought it was only roadies that had these sorts of panic attacks after riding furiously into the back of small parked sedan.
Yes. It is part of my trade to inspect cf panels on aircraft, and this is what aircraft maintenance manuals stipulate to be carried out when a tool is dropped on a cf panel. The same potential damage could occur as a result of a stack, so I recommend getting the frame inspected by someone that is familiar in inspecting cf frames.

At the end of the day, if you think my recommenation is overly conservative, then you are free to ignore it and continue to ride, but I'm not prepared to take that chance.
 

pharmaboy

Eats Squid
I'm very glad that aircraft are held to far higher standards when I get on one - I suspect I'd be bankrupt in a year or 2, if my cars had the same maintenance schedule of a commercial aircraft, and I sure wouldnt be riding a carbon bike if i had to spend $1000 a year on inspections.

maybe i crash too much..... ;(
 

cramhobart

Likes Dirt
Firstly let me say I know next to nothing about carbon bikes, never owned one hardy ridden one.
However I work with some guys that sail high performance carbon sailing boats(foiling moths) and their experience is
that after a couple of seasons the boats start to get "soft" and lose a bit of boat speed.
Anyone noticed a carbon frame getting more "compliant" with age?
 

brendonj

Likes Dirt
To say that carbon fibre has an infinite fatigue life is somewhat misleading. Carbon fibre is also affected by cyclic loads below its "yield" strength. There are a variety of reasons for this. This usually results in softening of the structure. Sometimes cracks. In most instances, visual detection of the cracks on something like a bike is easily achieved long before catastrophic failures occurs. Because CF is a composite, the crack most grow through each fibre one at a time so to speak. So a large crack will tend to still grow slowly. Whereas with steel or aluminium this might not be the case. A crack through 5% of a section which is not easily detectable could go straight through the rest of the structure in one hit due to the stress concentration at the crack tip. This is why people say carbon fibre is better in fatigue.

So bottom line is that CF frames also need replacing. The issue is that you could be prepared go closer towards the end of their life as you should see a failure before it breaks and hurts - as the risk is lower probably lower.
 

pharmaboy

Eats Squid
Interesting stuff^^^

Just a real world observable - in an xc riding group, we have had maybe a dozen frame failures over the last couple of years . Some have been catastrophic in that the bike could no longer be ridden, but none had the frame snap in 2 resulting in a crash. Most were cracks that are found while looking for a creak or washing the bike, and the big failures were either seat tubes or chainstays that parted company, so in those cases the downtube holds the bike together and the other side of the chain stay keeps the rear together.

In terms of the front triangle, It would seem to me that the nature of a triangle is the flex causes one angle to break, but by definition, the triangle tends to stay together
 
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