Originally Posted by
Koyote
AL doesn't rust, but it can corrode. It also has the shortest fatigue life of any commonly-used bike frame material. CF, on the other hand, effectively has NO fatigue life.
There's nothing wrong with AL, but let's be realistic.
Not sure what you are trying to say here, but the fatigue limit of steel and Ti is what makes stress cycle failure less of an issue while the lack of a fatigue limit for Al means that stress cycle failure is more of an issue. As a reminder, the fatigue limit, if it exists, is a level of stress below which an infinite number of stress events will not lead to stress failure.
Fiber-reinforced resins do not exhibit a fatigue limit, which makes stress failure harder to design for. Presumably, bike frame engineers are being very cautious and building in big strength safety margins to be on the safe side.
Also, current fiber-reinforced resins typically are using a resin that is relatively brittle which is an issue because initial fractures may be hard to detect and tend to fail catastrophically. There are promising new matrix options that will significantly reduce brittleness but are currently too expensive.
Otto