I’ve heard carbon fiber “has no fatigue life”, meaning it doesn’t weaken from (intended) use, as long as it is free of defects and damage.
But there are a lot of different types of carbon fiber…
I was told this years ago by a renowned expert, but never been able to verify the tidbit conclusively.
I would venture to say that it couldn’t be true for carbon intended to flex, but it seems reasonably plausible for carbon that is not flexed during use.
Fatigue is a concept in metallurgy with a very specific meaning. It is degradation of the crystalline structure of a metal. A metal object or material's fatigue limit is the amount of load it can sustain before any of this effect occurs. Some metals like aluminum technically have no fatigue limit, so the loaded areas have to be designed to give good lifespan regardless. The structural metal objects all around us (bikes, buildings, planes, cars) are designed, repaired, maintained, and/or retired and replaced to work around the fatigue properties of the material.
Carbon fiber isn't a metal and doesn't have a metal-like crystalline structure, so the metallurgical concept of fatigue doesn't apply. When its internal structure is degraded from load in any way, it's broken. It simply relies on its extremely high strength to keep that from happening. When it's momentarily very heavily loaded to the cusp of what it can sustain structurally, but doesn't break, it's fine and can do that forever. (Someone that designs or inspects carbon wings or whatever might have some more to say about that, but it's the basic principle).
Note that there's kind of a linguistic tug-of-war involved with the term fatigue as it applies to carbon fiber bikes. Fatigue by the metallurgical definition is an important consideration in the care and feeding of metal bikes and parts, which is to say most of them in the world, and so cyclists and mechanics might be grounded in the metallurgical definition and bristle at drawing an analog with the durability considerations involved with carbon fiber, because it's important to understand about carbon fiber that it plays by a different set of rules. So in that sense, using the term there is or can be argued as wrong. You can use it to try to suggest an analogous relationship with metal fatigue, but in carbon fiber's case of living in a strong broken/not-broken binary, there really isn't much of an analog. But with some other materials there might be more of an analog and suddenly it doesn't look as wrong or useless to borrow the term, and some people might be in the habit of doing that, or otherwise not care about what metallurgists have to say about the word.
Fatigue occurs in ALL materials [1]. It is most famous in metals due to the behaviour of "dislocations": in malleable crystalline materials under slip. Here is an article on fatigue in carbon fibre, including S-N plots, where fatigue limits are obtained from [2].
So yes. Repeated loading causes fatigue in all real solid materials.
[2] https://onlinelibrary.wiley.com/doi/abs/10.1002/pc.750020311
I'll answer more generally on the subject of lifespan rather than fatigue life, which Nathan has argued is a concept derived from metallurgy that doesn't operate the same way in carbon fiber.
First, remember that carbon fiber composites are made from cut-up bits of carbon fiber sheet (usually pre-impregnated with resin, aka pre-preg carbon). Those are laid up onto a mandrel. The number, shape, orientation, and position of the sheets can be altered to alter the properties of the tube. When people talk about the carbon fiber "layup", that concept parallels tube selection in metal frames, where framebuilders might choose a tube with a certain thickness (or thicknesses, if butted tubes) and shapes (e.g. ovalized at the end, or they might bend their own tubes if a custom builder or do something like hydroforming if a mass manufacturer). However, there are more dimensions of freedom to vary carbon fiber construction in.
If a carbon frame is perfectly constructed and is perfectly handled, then per this Cyclingtips article, bicycle industry insiders think it would outperform metal frames in laboratory fatigue tests. That involves putting frames on a jig and running them through load cycles until they fail. One quote is:
“Composites do not behave like metals,” explained Chuck Texiera. “In fact, they don’t actually fatigue like metals in the same classic sense of the word. The fatigue life of the fibre itself is just about infinite.” ... In the absence of any impact, the matrix can deteriorate with use, but it takes an extremely long time. “The epoxy matrix will at some point start to form little cracks,” explained Chuck Texiera, “and then over time it will just have the connectivity of the fibre. So really what’s happening, over really extended periods of time, you can expect the stiffness of the frame to change ever so slightly but it’s such a small number. We can measure it but I really wouldn’t think it would be perceivable. But it takes hundreds of thousands of cycles to even get to that. Two years would be far too short for that to occur with any kind of typical age group racer.”
The issue is that all manufacturing processes have variation. At present, carbon fiber manufacturing involves a lot of hand labor, and hand methods inherently have more variation than machine-based methods. Moreover, the layup process is more complex than with metal tubes - a metal frame has 8 metal tubes, a bottom bracket shell, and other items, but in CF, each tube is composed of many segments of carbon fiber. The more steps there are in the process, the more opportunities there are for some error.
One manufacturing process error in carbon is that you can get voids in the cured structure - those are literally air bubbles, and more layers of fabric or a more complex structure probably make them more likely. Voids weaken the structure. A void in the wrong spot makes the structure more likely to fail over time.
(NB: in metal frames, there's no exact parallel. However, improper welds, especially with earlier titanium frames, have some similarities. Titanium must be welded under inert gas, and a contaminated weld is particularly prone to eventual failure.)
This is more of a design rather than a process issue, but when a when a carbon structure flexes against a metal part, that can cause failures as well. If the metal part has an edge, even if it isn't necessarily a 90 degree one, that edge can cut into the fibers over time. Raoul Luescher has been documenting some issues on a recent recall on the Specialized Tarmac SL7 fork, and the link goes to his most recent YouTube video on the problem. I'd characterize this as design-related, but it still does affect potential lifespan. Again, metal frames aren't inherently immune from design-related failures, e.g. ultralight metal bikes have really thin tubing. However, designers aren't trying to push the design limits of metal bikes.
Last, I mentioned handling. Imperfect handling can cause invisible delaminations (a technical term, in this context like a fracture) inside the carbon. Over time and repeated loading, the delaminations can propagate and eventually cause the whole structure to fail. In principle, a pretty light impact can cause a delamination, particularly if the carbon struck something hard.
What should riders take away from this? Carbon fiber's lifespan is potentially very long. It's not infinite, but arguably no frame's lifespan can be expected to be infinite under regular conditions (i.e. you're riding the frame and it takes a few knocks here and there and your maintenance isn't perfect). Carbon fiber is a newer material, and the designs are more complex than with metal tubes, so the manufacturers are still perfecting their manufacturing processes. Additionally, more cutting edge frames are inherently more likely to have manufacturing or design flaws because they are a) pushing the limits and b) because the layup is more complex, there are more steps in manufacturing, and thus there are more chances for an error to occur. Additionally, the lighter the carbon (or any other material) frame, the more of a risk handling mishaps pose.
NB: I have no engineering or cycling industry background. The statements above are drawn from industry interviews that I have listened to, plus my own inferences.
I think the resin portion of carbon frames degrades from UV exposure much as plastics do, and develop micro cracks from repeated bottom bracket pedal stresses. Were talking fractions of a mm in movement. It will be most evident around the bottom bracket, which will gradually flex more after a few years, when compared to new. The frame won't fail, but gradually become less rigid and less efficient. It's a subtle but measurable change. Racing teams get new carbon frames yearly, to keep up with evolving tech, so they are immune to this issue. Other parts of a carbon bike frame would be less likely to develop resin stress cracks. Similarly, demanding tennis players replace carbon fiber racquets frequently because the frames subtlely begin to flex more where stressed most, the throat area.
Recently Titan deep water submarine collapsed, and there are claims that due fatigue of its carbon composite hull. As a result I would careful with assumptions that carbon composites are fundamentally not prone to fatigue, even if there may be some differences on they loads between a bicycle and submarine.
