For a member with an initial crack a0, when bearing static load, as long as the working stress σ is less than the critical stress σc, the member will work safely and reliably under the static stress level, and brittle failure will occur only when σ=σc or K1=K1c.
However, if the component bears an alternating stress of σ<σc, the initial crack a0 will slowly expand under the action of the alternating stress, and when it expands to a=ac, the component will be unstable and damaged.
The growth process from the initial value a0 to the critical value ac is called the subcritical growth of fatigue crack, also called the residual life stage of macro crack a0, as shown in Fig. 1.
The total fatigue life N of the material consists of two parts, namely the formation life Ni (initiation) and the life Np (propagation) from crack growth to fracture:
The fatigue fracture process is complicated due to many factors, but it can be roughly divided into four stages according to the crack development process:
N = Ni + Np
1. Crack nucleation stage
Under the action of alternating load, if the component has no cracks or is a smooth component without defects, although the nominal stress is less than the yield limit of the material, the local area of the component surface can still slip because of the uneven material.
It is explained by the mechanical principle that the surface of the component is in a plane stress state, so it is easy to slip, but there is no plastic deformation feature.
As a result of repeated cyclic slip processes, metal extrusion and extrusion slip bands will be generated, thus forming the nucleus of microcracks.
2. Microcrack propagation stage
After the crack pole is formed, the microcrack propagates along the 45 °slip surface with the main stress bearing.
At this stage, the extension depth into the surface is very shallow, about ten microns, and there are many cracks along the slip band, as shown in Fig. 2.
This process is the first stage of crack growth.
3. Macro crack growth stage
This stage is a macro stage that gradually transits from the micro cracks.
The crack growth rate increases, the growth direction is perpendicular to the tensile stress, and it is a single crack growth.
It is generally considered that the crack length within the range of 0.01mm~ac is the macro crack growth stage, also known as the second stage of crack growth.
4. Final fracture stage
When the crack is large enough to reach the critical size ac, instability propagation will occur and fracture will occur quickly.
The above are typical fatigue fracture processes of smooth surface components without initial cracks.
For high-strength materials, because of the high yield strength, large notch sensitivity, and many internal inclusions and hard particles, cracks often form nuclei directly at the macro stress concentration, and crack first along the inclusion and the basic interface, thus starting the macro crack stable growth stage, instead of the inclined micro crack growth stage.
The macro crack growth stage is the most important in analyzing fatigue from the viewpoint of fracture mechanics.