Found cracks inside of seat post

Question

Fairly new to carbon bikes. Found these cracks while adjusting saddle. Were using torque wrench up to 8Nm. Anything serious to worry about? No surface traces so far :/

Answer 1

I'm not a carbon fiber expert. Nevertheless, here's my take. I am assuming that the photo shows the inside of the seat post, although I'm not sure exactly what the item is.

Carbon fiber derives its strength from the fibers being in their intended orientations. The graphic below is from a 2017 paper by Bowkett and Thanapalan on failure detection methods for carbon. This shows some of the potential ways carbon can fail.

Now, my understanding is that, as with all manufacturing process, there are always a few minor flaws in any carbon item. I think that absolutely zero voids of any size throughout the whole structure may be impossible or at least not economically practical, for example. Engineers will build in a safety margin to accommodate a reasonable amount of flaws.

Inside the post, I think I see some wrinkles at the surface. While these are at the surface, I suspect that the item is not at 100% of the design strength because of this. There's an internal bladder or mold of some sort in there while the structure is getting baked, otherwise you couldn't make hollow parts. In some cases, perhaps the mold doesn't cleanly separate from the part or it got wrinkled when it was inserted. This is what I think happened. I don't know how strongly surface wrinkles are correlated with further defects under the surface.

Nevertheless, I would approach the manufacturer. It's possible that they will say it's just a surface issue (i.e. there's no relationship between this and underlying defects) and that there's no problem. This post by a carbon bike manufacturer claims that "at least one major manufacturer" does this, but describes the technique as inferior. If the manufacturer refuses a warranty, then you should use watchful waiting - that is, inspect the seatpost regularly for signs of failure, especially around where the seat binder holds it in the frame.

If this is a D-shaped seatpost, then there is no standard shape that manufacturers use, i.e. you can't really go off to, say, Thomson or some other 3rd party manufacturer and buy a replacement D-shaped post. Manufacturers won't always stock replacement posts forever - 5-7 years after the frame was last manufactured might be what you can reasonably hope for. It's possible that you might consider getting a replacement post just in case, although I realize this is expensive.

Last, the manufacturer would have specified a maximum torque. 8 Nm sounds pretty plausible for a seatpost, but it will be stated in the manual or online. Furthermore, you don't need to use the maximum torque, you could in principle tighten the binder just enough to keep the post in place, using carbon grip paste to help (actually, the manufacturer should have used that paste from the start, although not everyone may do so). In fact, due to safety margins, the seatpost won't crumble to dust because you tightened it beyond 8 Nm mid-ride - assuming the limits of what you can generate with a small multi-tool, of course. As an aside, if you need to adjust something mid-ride, you should redo the job with a torque wrench when you're home. And if this is an ultralight part, that advice is invalid.

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In technical terms, delamination is the CF word for a fracture, meaning that the layers have broken apart. Generally that means an impact did that, or else a void formed a stress concentration that caused the layers to fracture. I am not 100% sure what the technical term is for what I think happened to the OP's seatpost. Hence, I'm going with "wrinkle" for now, with the understanding that we are not seeing an internal wrinkle. In the image, porosity means an area with many micro voids. A foreign object lodged inside the matrix is usually called an inclusion. Resin rich areas mean areas where there was too much resin, which is basically the glue that holds the structure in place. It's also possible to have resin dry areas. Both are potential problems.

Answer 2

If this is a seatpost or saddle, the safest option is to just replace it.

If this is your frame, that's a large expense so let's get some more info.

(more answer waiting on more photos)