## 3 Results and Discussion
### Material Behavior under Quasistatic and Cyclic Loads
Only specimens fatigued at >500 MPa experienced substantial residual stress reduction through plastic deformation, eventually rationalizing the observations revealed by fracture surface analysis. Thus, a degradation in the residual stress stability at stress amplitudes > 500 MPa leads to crack initiation in the direct vicinity of the surface, whereas stable residual stresses result in a shift of the point of crack initiation to the specimen interior. When directly compared to the RT deep rolled specimen, it becomes obvious that microstructure stability is superior for the dr 855_550 \({}^{\circ}\)C condition, as the reduction in the surface residual stress for dr_855_RT reaches 50.3% already for the smallest stress level considered, whereas at the highest stress amplitude of 540 MPa, finally, only one quarter of the initial surface residual stress value remains. Nevertheless, it has to be noted that despite a decrease by a factor of approximately 2 in terms of the residual stress value directly at the surface after half-life fatigue (for the lowest stress amplitude of 425 MPa) almost the same maximum value remains, as deduced from the residual stress depth profile depicted in Figure 7c. For the highest stress level, both conditions, i.e., dr_855_RT and dr_855_550 \({}^{\circ}\)C, are characterized by similar half-life residual stress depth profiles. In the literature, dynamic strain aging during deep rolling at elevated temperatures is considered to be the major mechanism to increase the microstructural stability of different kinds of steel grades, in turn reducing dislocation movement and annihilation [69; 70]. Thus, the aged surface layer microstructure stabilizes the inherent residual stresses to such an extent that fatigue life is measurably increased for the TWIP steel under investigation when compared to non-aged, RT deep rolled specimens. Although the near-surface layer of the dr_855_550 \({}^{\circ}\)C condition is characterized by significantly lower hardness and maximum compressive residual stress values, the stability of residual stresses induced by dynamic strain aging effects eventually seems to dominate the fatigue life in the HCF regime (increase up to a factor of 2), as  
Figure 7: Residual stress stability: Comparison of pre-fatigue and half-life surface residual stress values (N\({}_{\text{B}}\)/2) for varying stress amplitudes and comparison of pre-fatigue and half-life residual stress depth profiles (fatigued at lowest and highest stress amplitude) for ((**a**) + (**c**)) dr_855N_RT and ((**b**) + (**d**)) dr_855N_550 \({}^{\circ}\)C condition. compared to the RT deep rolled counterparts. However, discussion at that point mainly focused on the direct surface of the specimens, obviously revealing positive effects of dynamic strain aging after deep rolling at 550 \({}^{\circ}\)C. In order to further analyze these effects in the bulk of the specimen, evaluation of the plastic strain amplitude, being a measure well-known for having a significant influence on the fatigue behavior, is part of ongoing studies, eventually allowing for additional interpretation of the results shown.