Fatigue fracture is a kind of fracture that occurs after a long period of work under the action of alternating stress. Fatigue fracture is one of the common failure forms of automobile parts, and it is also one of the most harmful failure forms.
According to the characteristics of parts and the total number of stress cycles, fatigue failure can be divided into: crack-free parts of the fatigue fracture failure and crack parts of the fatigue fracture failure.
For different types of fatigue failure, the analysis methods are different: when high cycle fatigue occurs, the yield strength should be below, and the life of the parts is mainly controlled by the crack nucleation life. The stress of low cycle fatigue is higher than that of yield strength, and its life is greatly influenced by the crack propagation life. Generally, automobile parts are low stress and high cycle fatigue fracture.
Fatigue fracture failure mechanism: Fatigue fracture of metal parts is essentially a cumulative damage process, which can be divided into slip, crack nucleation, emblem crack propagation, macroscopic crack propagation and final fracture.
Methods to improve fatigue fracture resistance of automotive parts:
1) Delay the initiation time of fatigue crack: strengthen the surface of metal alloy, such as surface rolling, shot peening and surface heat treatment. Grain refinement can improve the fatigue strength limit; heat treatment can prevent grain boundaries from becoming fatigue crack propagation channels; improve the purity of metal materials, reduce the size of inclusions; improve the surface integrity of parts design level, as far as possible to avoid the phenomenon of stress concentration.
2) Reduce the rate of fatigue crack growth: the main methods are: crack arrest method: that is,drilling holes in the front of crack growth to prevent crack growth; reaming method: that is, without affecting the strength of the premise, using reaming method to increase the diameter of fatigue crack, fatigue crack will be removed; scraping repair method: that is, using scraping method: Remove the cracks on the part surface.
3) Increasing the threshold length of fatigue crack: The threshold value of crack growth of metal parts is the maximum stress intensity factor amplitude of non-propagation (stability) of fatigue crack. Its value is usually determined directly by experiments.