Many mechanical parts and engineering components are subjected to alternating loads. Under the action of alternating load, although the stress level is lower than the yield limit of the material, but after a long time of repeated stress cycle, sudden brittle fracture will occur, this phenomenon is called fatigue of metal materials.

The characteristics of fatigue fracture of metal materials:

①The load stress is alternating;

② the operation time of the load is longer;

③ The fracture is instantaneous;

④Whether it is plastic material or brittle material, it is brittle in the fatigue fracture zone.

Therefore, fatigue fracture is a common and dangerous fracture form in engineering.