The whole fatigue process includes three stages: crack initiation, crack propagation and fracture failure caused by final instability propagation. The fatigue fracture can be divided into three corresponding areas: fatigue source (initiation area), radiation area (expansion area) and rapid fracture area (instantaneous fracture area). There are two sources of fatigue:

1) Because the material itself inevitably contains micro defects, such as inclusions, voids, etc., or there are cuts, steps, etc. in the structure. These micro defects will cause discontinuities in the material, which will produce high stress locally and become the “crack source”.

2) In the process of production, processing, transportation and use, mechanical structures often cause scratches or corrosion on the surface of the structure, become high stress areas and produce microcracks. Metallic materials are polycrystalline. Under the action of high stress, slip occurs in the grain and forms slip bands. Under the repeated action of load, the slip band passes through the grain and becomes a “persistent slip band”. The persistent slip band further develops and forms microcracks, and several microcracks develop into a main crack. At this point, the crack initiation process is over.