Originally Posted by
Eric F
Your materials knowledge depth is clearly way beyond mine, but I am a bit puzzled that multiple CF bike manufacturers have stated that the fatigue life of CF is infinite. This seems to conflict with what you're saying. Or, maybe I'm just not understanding the different terms clearly, which is certainly possible. Are you referring to the fibers themselves, or to the composite, which includes both fibers and resin? If it's the composite, I would expect that to vary a lot with the properties of the resin material.
"Fatigue life" and "fatigue limit" do not refer to the same concept.
Fatigue life is the number of stress cycles using a specified amount of stress at which a material can be expected to fail due to material fatigue.
Fatigue limit (some sources instead use the term an
endurance limit) refers to the amount of stress
below which a material will never fail due to material fatigue,
regardless of how many times that stress is applied and removed.
Every material has a fatigue life for large stresses. Not every material has a fatigue limit.
Steel and titanium are generally believed to have a fatigue limit. AL and CF composites are not.
CF fatigues. Due to its nature, it behaves and fatigues differently than metals - but it does fatigue. IMO anyone claiming otherwise is either (1) incorrect or (2) is deliberately choosing misleading wording that implies that without actually stating it outright (e.g., "Will last a lifetime!"). I suppose I could be wrong, but I don't believe I am.
Lennard Zinn wrote an article for
VeloNews that gave a pretty decent, nontechnical explanation of the issue some time back. If you're interested, here's the link.
https://www.velonews.com/gear/techni...fiber-fatigue/
As I've said before: CF technology is fairly mature now; ditto AL. Frames and forks can be made from either to last as long as desired using proper design, and doubtless today are designed to last a long time - probably longer than you or I will be around. Absent damage, I wouldn't sweat their durability.
But I would check either periodically for signs of damage. And if you notice anything suspicious, I'd have it checked out pronto by a competent pro.