## 2 Properties of Fine Granular Area
### Influences Regarding the Formation Process of FGA
#### Influence of the Environment  
VMCF tests in corrosive media lead to a drastic reduction of lifetime or fatigue strength [73; 74; 75; 76]. The corrosive medium results in multiple crack originations, surface cracking coalesced with subsurface growing micro-voids in early crack growth period and to an intergranular crack propagation with a widespread formation of secondary cracks during stable crack propagation [75]. [PERSON] and [PERSON] [77] attributed the characteristic S-N curve to the rapid crack initiation and propagation of surface cracks due to the influence of the environment (atmospheric water vapor) and slow crack initiation and propagation of internal defects due to the lack of the environmental influence. Early studies on the effect of vacuum environment on metals showed an increase of the plastic zone size [78; 79; 80] and an easier motion and interaction of dislocations as well as the activation of more sliding systems and less favorably oriented grain participation in the deformation [80]. It was shown that crack-propagation rates become lower and threshold SIF ranges increase with decreasing vacuum pressure [81; 82].  
Compact tension specimens formed FGA for tests in vacuum, while no FGA was observed for identical tests in air [81]. While the FGA-formation is usually only observed for internal fractures in the VMCF-regime while testing in air conditions it has been shown to also occur for surface defects in vacuum environment. [PERSON] et al. [61] report FGA-like structures on the fracture surfaces of a previously introduced crack for a low-temperature-tempered Ni-Cr-Mo steel (corresponding to AISI 4340 steel) after repeated cyclic loading for \(5\times 10^{7}\) cycles with stresses resulting in a \(\mathrm{K_{th}}\) with a stress ratio of \(\mathrm{R}=-1\) in vacuum environment, while the FGA-like morphology was not found for the same test in air environment. [PERSON] et al. [83, 84] observed FGA-formation on artificial surface defects under vacuum environment for specimens failed in the VHCF-regime, while no FGA-formation was observed when tested in air. The microstructural investigations that were performed in the FGA formed at the surface in vacuum environment showed that the VHCF failure mechanism for artificial surface defects tested in vacuum are comparable to those of subsurface inclusions. They therefore concluded that the vacuum conditions seem to be crucial for the late crack initiation. However, according to [PERSON] et al. [26, 85], cold-rolled stainless steels also show FGA-formation for surface crack initiation even while tested in air environments. It should be noted that this result is unique and was obtained only for very thin specimens and therefore needs further validation. Given that FGA-formation for high-strength-bearing steel is only observed under vacuum-like conditions and not for air environment, the vacuum-like conditions seem to have a very favorable influence on the mechanism of FGA-formation and failure in VHCF-regime, if FGA-formation on surface defects of cold rolled stainless steels would be validated even for tests performed in air environment, vacuum-like conditions should no longer be understood as a mandatory requirement. However, the dependency of different materials on vacuum-like conditions for FGA-formation is not yet fully understood.