For a component with an initial crack of size a0, when subjected to static loads, as long as the working stress (σ) is less than the critical stress (σc), the component will function safely and reliably under the static stress level. Brittle failure will only occur when σ=σc or K1=K1c.
However, if the component undergoes alternating stress with a value of σ<σc, the initial crack a0 will gradually increase in size under the influence of the alternating stress. When it reaches the critical size of a=ac, the component will become unstable and damaged.
The process of initial crack size a0 growing to critical size ac is referred to as subcritical fatigue crack growth or macrocrack residual life stage a0, as depicted in Figure 1.
Figure 1
The total fatigue life (N) of a material consists of two stages: the initiation life (Ni) and the propagation life (Np) from crack growth to fracture.
The fatigue fracture process is complex and influenced by many factors, but can generally be divided into four stages based on crack development:
N = N i +N p
1. Crack nucleation stage
When a component is subjected to alternating loads and does not present cracks or defects, even if the nominal stress is below the material's yield limit, the surface of the component may still suffer slippage in localized areas due to the uneven material.
This occurs because the surface of the component is in a state of plane stress, making it susceptible to sliding without any plastic deformation. Over time, repeated cyclic slip processes result in the formation of metal extrusion and slip bands, creating the nucleus for microcracks.
2. Microcrack propagation stage
Once the crack core is formed, the microcrack propagates along the 45° sliding surface under the influence of the principal stress.
At this stage, the crack depth on the surface is very shallow, only about ten microns, and there are many cracks along the slip band, as illustrated in Figure 2.
This is the initial stage of crack growth.
3. Macro stage of crack growth
This phase marks the transition from microcracks to macrocracks.
The crack growth rate increases and the growth direction is perpendicular to the tensile stress, with single crack growth.
It is generally accepted that crack lengths in the range of 0.01 mm ac represent the macrocrack growth stage, also known as the second crack growth stage.
4. Final stage of the fracture
Once the crack size reaches the critical size ac, instability propagation will occur and fracture will occur rapidly.
This is a typical fatigue fracture process for components with smooth surfaces and no initial cracks.
For high-strength materials, due to their high yield strength, high notch sensitivity, and the presence of internal inclusions and hard particles, cracks generally form directly at the macrostress concentration points and the first crack along the inclusions and the matrix interface, starting stability. macrocrack growth stage instead of the inclined microcrack growth stage.
The macrocrack growth phase is the most significant phase for fatigue analysis from the point of view of fracture mechanics.
