Steel and Titanium behave like you described. As long as you keep the load below tbd% of yield, there is no cycle limit. Aluminum and composites have a fatigue limit. Aluminum fatigue is well documented with Airliners. (Remember the aircraft on TV that failed because they exceeded the pressurization cycles or Landing Cycles without making mods to extend their life?) Heavy flexible composite have a very very long fatigue life. The stuff from the 90's was assumed to have an infinite cycle life. Stiff light composites with only just enough of the right material in the right place have a shorter life. Composites also have a problem with crack propagation. If you hit it in a way it was not designed for, it cracks and the crack propagates until it fails.
easy to read:
http://en.wikipedia.org/wiki/Fatigue_limithttp://www.airliners.net/aviation-forums/tech_ops/read.main/91016/Not easy to read, what the experts say to me
http://books.google.com/books?id=FW...e%20of%20carbon%20composites&f=falsehttp://scholar.lib.vt.edu/theses/av...Part_2_strength_and_life_predictions.pdfGoogle "composite fatigue life predictions" and you will find plenty of technical articles and Conference Paper Reprints. There is a lot of information in the Public Information of the FAA.
This is not my area of expertise. I can't argue with you. Everything I have seen below the PHD level is proprietary. The only article on the subject, I know of was in, I think, Mountain Bike Action in the Fall of 2011. I showed it to 2 world class experts and they both said if their assumptions about the life of the aluminum frame is correct, it looks about right. I canceled an order for a super light carbon frame and bought a heavier aluminum one.
