Originally Posted by
McBTC
In other words, alloy does not have a, 'fatigue limit' although, as has been pretty well discussed, in the real world, a life expectancy of 10000000000 stress cycles is for all intents and purposes, a lifetime and then some... such that, worrying about sudden catastrophic failure due to frame fatigue is not rational.
That's right. With average loading, no corrosion, no crashing etc, an Aluminum bike can likely go 100K to 200K miles without failure. If you increase the payload...300 lb rider who rides with 60 lb panniers, the frame will likely fail sooner.
Take an aluminum beer can and cut it into a coupon. Flex it. You can flex it more aka greater displacement or less. If you displace it more aka greater loading it will fatigue in fewer cycles.
Every time a painter gets on a 30 ft aluminum ladder it flexs. When a 250lb versus 150lb painter gets on the ladder, it flexes more. A ladder will have a longer life with a 150 lb painter on it versus a 250lb painter.
Btw, I have a friend who was a good painter and he didn't like to get down off the ladder so much to relocate it. When he was young, he would pogo an aluminum ladder from one place to another. For some reason, chicken little wasn't around to tell him the ladder was going to fail with this kind of treatment.