The process of fatigue failure is the gradual change and damage accumulation and cracking of the weak regional organization in the material under the action of variable stress. When the crack expansion reaches a certain degree, the sudden fracture occurs, which is a process of damage accumulation starting from the local area and eventually causing the overall damage. Its characteristics are:

§ Delayed fracture, low stress fracture level (even below fatigue strength)

§ high stress, short life; Low stress, long life

§ undergo the initiation and propagation of cracks

There are various types of fatigue loads, but they all have some common characteristics:

There is no obvious macroscopic plastic deformation at the time of fracture, and there is no obvious omen before fracture, but it breaks suddenly and shows a brittle fracture. Fatigue is very sensitive to defects (notch, crack, tissue defect); The stress causing fatigue fracture is very low, often lower than the yield strength under static load, which belongs to low stress delay fracture.

Fatigue failure can clearly show three components of crack initiation, propagation and final fracture.

Its classification: high cycle fatigue (N>105), low fracture stress level, long life;

Low cycle fatigue (N=102-105), high fracture level, with plastic strain occurrence (high stress fatigue or strain fatigue)