metal fatigue is not about over-stressing. It is accumulated stress changes, ie the changes that happen to metal when moved a little, often. The metal crystal structure can change, hence properties.
This is why airframes have a design life, say 2000 hours. The structure may never have been over-stressed or even heavily stressed, but has work hardened to the point the metal is brittle. It may then fail suddenly despite being under design stress.
Here lies the question that simulations and sensibly, ground aging tests check, how long will it really last ?
With minor modifications mechanical components can have their life doubled, cutting hourly cost significantly. Conversly, design faults, like the Comet 1 windows can show up before someone dies.
and its Friday; where is a Paris snippet or the BOFH ?