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Intended as a canonical answer to the many questions we have about damage to carbon parts.

It is well known that carbon fiber parts can be damaged and fail, and carbon parts can be fragile when stressed in certain ways, but we get a large number of questions on this site asking if a carbon part (Frame, rim, stem) with damage is a concern or not. Many of these parts are obviously suffering no more than a minor cosmetic blemish, and very few are suffering from clearly significant damage. Most fall into the gap between where physical inspection is required to make an assessment.

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Carbon fiber composites are made up of sheets of carbon fiber filaments. These are usually impregnated with resin. To make CF structures, manufacturers will cut up the sheets, then lay them up onto a mandrel or other structure. They will then bake them at high pressure and temperature to cure the resin. The image below is from a Youtube video showing how Mad Fiber wheels are made - that company is now defunct, this is just for illustrative purposes. The metal item is a mandrel, which is a bit like a mold except that you put sheets of carbon on to a mandrel, versus you would put things in a mold.

enter image description here

If you have a CF frame or component and it's been damaged such that you can see torn fibers, it needs repair or replacement. This is obvious but probably worth stating. The image below is from Appleman Bicycles, who is a carbon framebuilder and repairer in my city. Disclosure, he's local to me and we are acquaintances, but I'm not a customer. On the left side of the frame, the fibers are clearly broken. It may not be as obvious on the right side, but I'm pretty sure that if you saw that frame in person, you'd see broken fibers also.

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With that trivial question out of the way, it is frequently hard to know what to do if you crash on a carbon frame or component and there's no obvious damage. Damage to carbon items is frequently not visible at the surface. However, an impact, even a fairly soft one, might lead to a delamination below the surface that later grows into a larger failure. Delamination is the CF term of art for a fracture.

Ultrasound or other imaging is used to inspect CF parts in the aerospace industry. Ideally, we would have this capability widely available, but we do not. This is the only reliable way to detect a delamination under the surface. There may be a carbon repair shop close to you with this capability, but your local bike store cannot do this. Many people will propose a tap test, where you tap a coin lightly around the suspected fracture and listen for a change in pitch. The problem is that this test is most reliable on flat surfaces, and bikes don't really have a lot of those.

What to do: a proposed set of steps for consumers

This section concerns what to do if you sustain an impact that doesn't leave visible damage. For major impacts like a car vs. bike accident or a crash at racing speeds, more caution is recommended. It also depends on what part is involved; if your handlebars are damaged and they break, that will almost surely cause a crash, although if you are extremely skilled you may be able to recover (but you shouldn't count on being able to do this). I think the same would hold for stems and forks. I would lean towards replacing these parts immediately if you take a major impact. Minor impacts can be handled through watchful waiting, but some level of caution and regular inspection is still recommended.

I'm less certain about frames. In many cases, they are shielded from direct hits by the other parts of the bicycle and by your body. Again, the ideal case would involve an ultrasound or equivalent inspection after a crash. In the absence of that, I would propose watchful waiting. Be familiar with how your bike sounds and how it flexes under your pedaling. If you notice a sudden change, stop riding and try to see if there are any cracks. The picture below is from a video by Raoul Luescher, a carbon bike repairer based in Australia. Here, a metal part associated with the steering system hit the bike's head tube from inside due to arguably poor design. There was a small visible crack from the outside.

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I believe that paint chips around frequently handled parts can be ignored. These might include areas around the dropouts. Gravel bikes may frequently take paint chips from flying bits of gravel. Some examples of what I believe to be paint chips are below, taken from recent posts.

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Last, consumers should be aware that damaged carbon frames can be repaired. These repairs are not that expensive compared to repairing metal frames. On metal frames, damage will frequently involve replacing the entire affected tube. In contrast, with carbon, the second photo from Appleman bicycles shows that repairs can be localized to just the damaged area. I believe that forks, wheels, stems, and handlebars are not cost effective to repair; you should just replace those if damaged.

As a side note, carbon fiber does not have inherently poor potential lifespan. It has a very long fatigue life, although @NathanKnutson correctly notes that the engineering concept of fatigue in metals does not apply to carbon fiber composites.

A note on handlebars and forks

If you crash or drop a bike, by virtue of their position, the handlebars are likely to take an impact. In contrast, it is unlikely that you will hit a carbon crank but not the handlebars. In the case of head-on collisions, you can assume that the fork has taken a hard impact as well.

If a handlebar or fork fails under load, you are extremely likely to crash. It may be worth being more conservative with these parts if they're made of carbon, and leaning towards replacing them if damage is suspected. In fact, it may be good advice to replace these parts regularly. I realize that this is easier said than done, and I am not aware of empirically-backed replacement intervals for these parts.

For many frame tubes, if one tube fails under load, a skilled rider can often come to a controlled stop, and there is some degree of redundancy if the tube is a chainstay or seatstay.

Some notes on prevention

My understanding is that CF does especially poorly under point loads - i.e. getting hit by a small hard object. While this is easier to say than to do, users can consider all the scenarios where hard objects might hit the frame and try to mitigate them. In this question, the poster packed their bike in a bag, but the fork rotated and the rim brake caliper hit the frame. While the solution would again be watchful waiting, it's worth trying to avoid such things happening. For example, if packing a bike, one could try padding the rim brake calipers or otherwise not pack hard objects in the same case.

A side note: minor surface damage inside the frame

As I discuss on another answer, frame manufacturers designing for high-level competition obviously push frame design. There may be cases when a carbon tube is flexing against a metal item. For example, in late 2021, Specialized recalled their Tarmac SL7 frames and forks. The headset had a compression ring on the outside of the fork steerer. As the bike hit bumps, the compression ring had an relatively sharp edge that dug into the steerer tube's surface. This would eventually cause a failure. However, Raoul Luescher raised an issue with the fix, namely that now the compression plug, which was cut to length, could have a sharp edge as well. He showed a borescope shot of the inside of the fork where the edge of a compression plug marked the surface of the fork, and he argued that this is likely to fail eventually. For performance road bikes with hydraulic hoses routed inside the steerer, an unexplored issue is that over time, the hoses might abrade through the steerer tube's fibers as well.

The watchful waiting advice applies to damage from one-time events. The issue of damage from repeated events is a more serious one. However, this sort of damage is likely to happen outside your line of sight, and it might require specialized tools to even see.

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    @paul23 Bars don’t break spontaneously. It’s always when you hit a bump or hole, or you’re sprinting. Good luck steering with just your butt at that point. Don’t even think about staying upright while riding MTB.
    – MaplePanda
    Apr 6 '21 at 17:27
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    @paul23 Wow, that's the best armchair theorist comment I've read on any subject a long time. Chapeau. Apr 6 '21 at 17:46
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    @paul23 "I've head my handlebars get off multiple times. Same for whole fork or sadle pin." It seems like a good moment to start properly assembling your bike, or at least changing your bike mechanic!
    – EarlGrey
    Apr 9 '21 at 10:44
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    @paul23 that assertion is demonstrably counterproductive. We cyclists are safer for the road when there are more of us. We are more politically powerful when there are more of us. Not all of us have the skill or the time to repair all aspects of the bike, and anyway it would put a lot of shops out of business. Also, if those are your friends, I'd hate to meet your enemies - sawing through your steerer could injure you, possibly severely or even fatally.
    – Weiwen Ng
    Apr 9 '21 at 15:42
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    @WeiwenNg what do you mean "more of us"? Everyone bikes already, just like everyone needs to learn how to repair a tire and screw some things. Or swim or walk for that matter. But those are just parnks we did to each other when 15, it's a good laugh after you've been at the pub and getting home. Don't be so sore, do you feel like that when watching (say) top gear? It's the silly flavours that make life fun, just like trying to impress by standing on the saddle (and then failing big time).
    – paul23
    Apr 9 '21 at 15:55

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