Fatigue: A phenomenon in which a metal material is below the ultimate strength and is subjected to an alternating load (that is, a load whose shape and direction are repeatedly changed) for a long period of time, and suddenly breaks without significant plastic deformation, and is called fatigue. The fatigue index includes fatigue limit and fatigue strength.

Fatigue limit: The maximum stress that a metal material can withstand after repeated (N) stress cycles without repeated fractures is called the fatigue limit.

Fatigue strength: The maximum stress that a metal material can withstand after repeated (N) fractures under repeated or alternating stress is called fatigue strength. At this time, N is called the fatigue life of the material. Some metal materials have no obvious fatigue limit under repeated or alternating stress, and are usually expressed by fatigue strength.