Originally Posted by
tdracer
... Metals such as steel have a nearly infinite fatigue life, so long as you stay away from the plastic deformation area - aluminum is somewhat unique it that it does eventually fatigue even if you are operating well away from the plastic deformation area.
Steel (and many other materials) may fail by
low cycle fatigue, where repeated plastic deformation such as bending a wire coathanger rapidly causes failure.
But if you think simply avoiding plastic deformation will protect your steel structure from fatigue failure, then
high cycle fatigue will come as a
nasty surprise.
Aluminium is not at all unique; it is steel which is unusual in exhibiting a
fatigue limit, a stress level about half that required for plastic deformation, below which it does not accumulate fatigue damage.
Typically if a steel structure will withstand over about 10 million cycles, it is not loaded past its fatigue limit and should last indefinitely:
When you consider that an engine running at, say, 1700 RPM, will accumulate about 100,000 cycles per hour, 10,000,000 cycles in 100 hours (roughly 2000 miles for a car in city traffic), you begin to appreciate why we don't make crankshafts in aluminium.