Explanation of terms of metal materials (II): Fatigue

Many mechanical parts and engineering components work under alternating load. Under the action of alternating load, although the stress level is lower than the yield limit of the material, after a long time of repeated stress cycle, sudden brittle fracture will also occur. This phenomenon is called fatigue of metal materials.

The characteristics of fatigue fracture of metal materials are:

 (1) The load stress is alternating;

 (2) The action time of load is long;

 (3) The fracture occurs instantaneously;

 (4) Both plastic and brittle materials are brittle in the fatigue fracture zone.

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

The fatigue phenomena of metal materials can be divided into the following types according to different conditions:

 (1) High cycle fatigue: refers to the fatigue in which the number of stress cycles is more than 100000 under the condition of low stress (the working stress is lower than the yield limit of the material, or even lower than the elastic limit). It is the most common kind of fatigue failure. High cycle fatigue is generally referred to as fatigue.

 (2) Low cycle fatigue: refers to the fatigue with the number of stress cycles below 10000 ~ 100000 under the condition of high stress (the working stress is close to the yield limit of the material) or high strain. Because the alternating plastic strain plays a major role in this kind of fatigue failure, it is also called plastic fatigue or strain fatigue.

 (3) Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress caused by temperature change.

 (4) Corrosion fatigue: refers to the fatigue damage of machine components under the joint action of alternating load and corrosive media (such as acid, alkali, seawater, active gas, etc.).

 (5) Contact fatigue: it refers to the pitting peeling or surface crushing peeling of the contact surface of machine parts under the repeated action of contact stress, resulting in the failure and damage of machine parts.