## 3 Results
### Uncertainties of Crack Position and Progression on Total Strain Controlled Fatigue Results  
Fatigue life is defined by a damage criterion considered, which is in many traditional cases, is the complete fracture of the specimen. However, this is just one damage mechanism (and the last) that occurs during the fatigue life of a material. In many cases, the damage criterion is considered to be crack initiation only. This is to give clear recognition of the description of linear and non-linear fracture mechanics approaches being a mechanism different when compared to the degradation processes appearing before crack initiation. Care has to be taken when the crack propagation phase is included in the fatigue life evaluation, specifically when strain is the controlling mode. In that case, the number of cycles can go up to infinity when a crack opening displacement is erroneously taken as a basis to determine a specimen's strain. An alternative to crack initiation is to measure load drop when the fatigue test is performed under strain control. This is well feasible with unnotched specimens when generating materials fatigue data. However, the question arises, which crack size can be associated with a defined load drop criterion. For the description and evaluation of the incident of crack initiation, the transition between a fatigue analysis from the uncracked to the cracked structure where fracture mechanics-based evaluations in the LCF regime of the components considered are essential. Based on the results of the fatigue tests presented here, the specimens were analyzed with respect to the location of crack initiation and the assumption to be made [31]. The changes in the stress amplitudes of the total strain-controlled fatigue tests (e.g., Figure 4a) are related to crack geometries and crack sizes in order to develop an appropriate correlation. Figure 9: (**a**) Slopes of the Basquin and Manson–Coffin relationships, as well as the comparison of conventional determined fatigue data with the StrainLife curve calculated based on stress–strain data and (**b**) comparison of conventional determined fatigue data with the StrainLife curves calculated based on stress–strain, electrical resistance and magnetic field strength data for the initial condition of AISI 347 steel. For this purpose, four possible crack geometry configurations of the incipient cracks in the cross-sectional area of the specimen are considered. These possible incipient cracks are a circular crack (fisheye), a full circumferential crack, a chord crack and a lens crack, all schematically shown in Figure 10. The circular crack may initiate at an inclusion, as it could be expected for ideal geometrically unnotched specimens [32]. In comparison, the development of a chord or lens crack, e.g., due to an arbitrary unwanted condition (e.g., blade of a strain gauge transducer), may occur in non-ideal geometrically unnotched specimens. The full circumferential crack is predominantly to be expected on notched specimens. The crack area \(A\) is different for each crack initiation configuration and depends on the crack depth a. \(A\) can be calculated according to Equations (11)-(14), which is also visualized in Figure 11. For this case, the radius of the cross-sectional area of the unnotched fatigue specimens is assumed to be \(r=5\) mm. Figure 11 also contains the area fractions with associated crack depths of individual fatigue specimens with a load drop of 25%, which were tested with a strain amplitude of 0.3 \(\times\) 10\({}^{-2}\). The individual fractographs are shown in more detail in Figure 12. The fractographs show that the lens crack is a possible idealized crack shape. In addition, the location of the extensometer clip is marked with a green dot in the fractographs. The respective crack areas depend on the stress level and the present number of cycles until specimen failure has occurred.