## 3 Results and Discussion
### Material Behavior under Quasistatic and Cyclic Loads
In contrast, differences become obvious with respect to the location of the crack initiation sites depending on the stress level during cyclic  
Figure 4: Monotonic stress–strain response of the specimen deep rolled at 550 \({}^{\circ}\)C applying a deep rolling force of 855 N. Data of the initial state condition and of the condition deep rolled at RT using a deep rolling force of 855 N were recompiled from [39]. Figure 5: S-N curve of the specimen deep rolled at 550 \({}^{\circ}\)C applying a deep rolling force of 855 N. Data of the initial state condition and of the condition deep rolled at RT using a deep rolling force of 855 N were (partly) recompiled from [39]. loading. At lower stress amplitudes of 450 and 460 MPa respectively, typical characteristics related to deep rolling known from the literature, i.e., a shift of the crack initiation points away from surface areas towards internal defects as a result of the near-surface compressive residual stresses, become obvious. Thus, crack initiation can clearly be traced back to the marked sites in the interior of the specimens, as further highlighted by the magnified details below the overview images. As no inclusions or pores responsible for fracture initiation can be located at the crack initiation sites, it is assumed that localized embrittlement is a major contributing factor eventually leading to crack initiation in the present study. According to a review by [PERSON] et al. [68], hydrogen in high-manganese steels is segregated on the phase, grain and twin boundaries, eventually affecting the local properties. In the present study, however, hydrogen-induced crack initiation cannot be clearly proven, and more research is necessary in order to fully understand this topic. At higher stress amplitudes, demonstrated by the fracture surfaces of specimens fatigued at 520 and 540 MPa, respectively, crack initiation is shifted to the surface again, pointing to sufficient plastic strain for the formation of persistent slip bands in the strengthened surface layer. Therefore, it can be assumed that effects shifting the location of crack initiation to the interior of the specimen, such as compressive residual stresses in the surface layer, lose their effect at those increased stress amplitudes. In order to further elucidate these observations, analysis of the residual stress stability was conducted for both the dr_855_RT and the dr_855_550 \({}^{\circ}\)C condition. Figure 7 details pre-fatigue and half-life surface residual stresses in dependence of the applied stress amplitude for specimens deep rolled at RT and 550 \({}^{\circ}\)C, respectively. Generally, irrespective of the deep rolling temperature, a decreasing stability of the surface residual stresses can be seen with increasing stress amplitudes (with one exception in Figure 7b). Taking a closer look at the surface residual stress stability of the dr_855_550 \({}^{\circ}\)C condition, a significant decrease of 62.1% of the half-life surface residual stress value was only found for the specimen fatigued at the highest stress amplitude of 540 MPa. Specimens tested at lower stress amplitudes showed less degradation in a range between 8.6% and 26.4%, while the lowest relative loss was found for the specimen tested at 460 MPa. Depth profiles in Figure 7d indicate that the trend of very stable residual stresses at low stress amplitudes seems to apply to the whole surface layer, as the specimen tested at 425 MPa  
Figure 6: Fracture surfaces of the dr_855N_550 \({}^{\circ}\)C condition after cyclic loading at loading amplitudes of 450, 460, 520 and 540 MPa. The pictures below the overview images of the fracture surfaces show magnified details of crack initiation sites marked with white arrows in the overview. For fracture surfaces characterized by multiple crack initiation, only the major site is highlighted and shown in the magnified view. only shows minor deviation compared to the pre-fatigue measurement.