## 2 Properties of Fine Granular Area
### Characteristics of the FGA
#### 2.2.2 Characteristics Observed by Computed Tomography  
The analysis of crack initiation and early crack propagation during VHCF failure is complicated by the fact that the failure is dominated by processes inside the specimen. Because of the very localized processes and the resulting small dimensions to be investigated, there is an additional requirement for a very high resolution. because of those conditions computed tomography using synchrotron radiation (SR \(\upmu\)-CT) has gained increasing attention in recent years. [PERSON] et al. [99] used 3D digital image correlation (3D DIC) on SR \(\upmu\)-CT images of cast iron to calculate 3D strain tensors. Strains of less than 1% could be measured by this method and allowed to detect inhomogeneity due to irreversible deformation and the calculated irreversible strain fields are reported to be able to predict origin of cracks.  
[PERSON] et al. [100] investigated the crack initiation and early crack-propagation phase in Ti-6 Al-4V using SR \(\upmu\)-CT. Four cracks with a length in the grain size range of the tested material (\(<\)30 \(\upmu\)m) were detected on a specimen that had been previously stressed with \(1.1\times 10^{7}\) cycles. To evaluate the crack growth, the specimen was additionally stressed for \(1.325\times 10^{7}\) cycles and reexamined. Although no crack growth could be observed for the previously detected cracks, the initiation of four additional cracks was observed, which indicated that the failure is determined by multiple competing cracks. [PERSON] et al. [101] were able to determine the crack-propagation rate of internal cracks to be less than \(10^{-10}\) m/cycle using SR \(\upmu\)-CT, while a strong increase of the crack-propagation rate to \(10^{-8}\) m/cycle was observed once the crack reached the surface. The crack propagation rate of internal cracks corresponds to the crack-propagation rate of surface cracks in high vacuum, while it corresponds to the crack-propagation rate of surface cracks in air as soon as surface contact is reached. The slow crack-propagation rate of internal cracks was thus attributed to the vacuum-like conditions inside the specimen. Those crack-propagation rates were validated in a recent study by [PERSON] et al. [102] using SR \(\upmu\)-CT, and crack-propagation rates of \(10^{-13}\)-\(10^{-11}\) m/cycle were determined for \(\upDelta\)K-regime below 5 MPam\({}^{1/2}\). During the investigation, 28 cracks were detected within one specimen. The crack-initiation life widely varied from 20% to 70% of the average lifetime. Neither the crack-initiation time nor the size of the initiation site had an effect on the subsequent crack-propagation rate, therefore the surrounding microstructure was evaluated as the dominant influence of internal fatigue, rather than the crack origin. [PERSON] et al. [103] analyzed the structure of FGA of a martensitic-bearing steel (100 Cr6) by usage of synchrotron radiation nano-CT and 3D image processing. They found that the granular areas on the fracture surface inside the FGA observed by SEM consists of several subgrains that are tens to hundreds of nanometers in size. It was stated that the resolution of nano-CT is not sufficient, but that at present, the grain refinement was not observed and therefore the FGA-formation mechanism might be based on a rearrangement of the observed subgrains.