... under cycled loading: stress on... stress off.

The lower the stress the more cycles aluminum can withstand before failure. But, aluminum will eventually fail under cycled loading. Steel, on the other hand, can last for ever under cycled loading if the stress is low (~ under half of the yield stress).

Remember that 737 in Hawaii that had the skin of the fuselage peel off in flight. That was an old plane that had a lot of flights, but not necessarily a lot of flight hours. Every time it took off and landed the cabin was pressurized and depressurized. That cycle-loaded the fuselage, and eventually the aluminum skin let go.