Originally Posted by Engineer1911

Ferrous alloys when subjected to loading conditions that are less than yield strength of the base material, have an infinite fatigue life. You can run the machine forever at that loading condition without the parts and pieces breaking.

Aluminum does not have an infinite fatigue life. Aluminum alloys work hardens at the points of highest and will eventually crack and fail.

An aluminum alloy part may have a service load of 500 pounds, a working stress of 8,000 psi, and a yield strength of 25,000 psi. Even though the working stress is 1/3 of the yield strength of the material, the part will eventually crack and fail. That's what aluminum alloys do.