Fatigue is a cumulative process, which may or may not result in a failure. Fatigue progresses through 3 stages. The first is called the crack nucleation stage. Every product made from virtually every material has tiny defects that are virtually undetectable by inspection. (After all, no part is ever completely perfect.) These defects could be voids or inclusions in the base metal, tiny scratches, cracks, or burrs from machining or forming processes, etc. After a series of cycles of stress application, the defect may develop into a tiny crack. Sometimes, even a sharp notch or corner in a highly stressed area on a part can concentrate enough stress to generate a minute crack at the tip.