In fact, the fatigue failure of materials is mainly due to the irreversible change of the material structure. When the stress borne by the material itself is higher than the material bearing limit, the material structure will be damaged, and the damage will accumulate continuously. When the accumulation reaches a certain value, the material will fail suddenly. The fatigue failure process includes three stages: crack nucleation and initiation, crack propagation and final instantaneous fracture. The fatigue life NF is composed of crack initiation life no and crack propagation life NP, namely:
Macro fatigue cracks are formed by nucleation, initiation, propagation and interconnection of a large number of micro cracks. Fatigue cracks originate from uneven parts of material surface or internal structure (prone to stress concentration). Generally, there are inclusion particles, internal defects of structure, etc., and grain boundary cracking or local slip occurs under the action of stress. The generation of fatigue cracks is also related to the material microstructure, such as dislocation, interstitial atom, vacancy and other point defects in the material also provide conditions for the nucleation of microcracks, and finally evolve into the propagation of macro cracks. Dislocations widely exist in materials and belong to a kind of lattice defects. Dislocations in metal materials include edge dislocations, screw dislocations and mixed dislocations. Dislocations are in an unstable state in the crystal structure. The more incomplete the crystal is, the easier the dislocation will move [16]. Different sign ductile dislocations can move in reverse and opposite directions, so when two different sign edge dislocations meet, they will cancel each other and disappear. The edge dislocation can also climb in the vertical direction of the slip surface, while the screw dislocation can move arbitrarily in the parallel direction of its Berger vector. During the movement, the slip trace is left on the crystal surface in the direction perpendicular to the dislocation line to form a slip step. The screw dislocation can also transition between the two slip surfaces to form cross slip; The motion of mixed dislocations has many characteristics of dislocation motion. When dislocations cross the whole slip plane, they will leave deformation traces on both sides of the crystal. The interaction between many dislocations can be divided into two types: the interaction between parallel dislocations and the interaction between dislocation. There is a Bailey Hirsh relationship between the macroscopic stress of the crystal and the dislocation density:
Where, σ Is macro stress, χ Is the lattice parameter, μ Is the shear modulus of the material, B is the Berger vector, P is the dislocation density, σ I is the macroscopic stress of the crystal without dislocation. In the initial stage, the number of dislocations in the material structure is small and relatively stable. When bearing the stress, some dislocations begin to slip. On the one hand, the slip of dislocations causes the increment of dislocations and the increase of dislocation density P. on the other hand, it continuously reduces the distance between adjacent dislocations and increases the interaction force between parallel dislocations, which is reflected in the increase of macroscopic stress of crystals, The material has hardened. Therefore, various mechanical phenomena (such as cyclic hardening, cyclic softening, Bauschinger effect, etc.) in the process of fatigue nucleation and initiation to final fracture are closely related to the number, structure and movement of dislocations in the material.
In some cases, the fatigue crack propagation does not follow the maximum stress direction, but extends in another direction, which is another fatigue crack propagation mode caused by the energy principle.
The lower the energy required, the easier the fatigue crack will expand. The energy principle means that the crack propagation is always carried out in the direction with the lowest energy. Generally, the route with the lowest energy required is the route with the lowest crack propagation resistance. The place with the lowest energy is generally the defective part of the material surface or the place where stress concentration is easy to occur inside the material.