The video below explains the principle of Fatigue.

Summary

In material science, fatigue is the weakening of a material caused by repeatedly applied loads. Because of this cyclic loading, a failure can occur below the maximum applied load. Three stages of fatigue failure can be identified:

1. Large cycles create initial microscopic cracks, which grow into macroscopic cracks.
2. Development macroscopic crack each cycle until a critical crack length
3. Macroscopic crack development cannot sustain the peak load (critical crack length), and failure (break) occurs.