## 3 Results and Discussion  
### Fatigue Test Results of Rim Section Specimens  
A total number of 24 specimens were tested according to the procedure presented in Section 2.2.1. All the failed specimens showed a fatigue crack development in the radius under investigation, thus confirming the appropriateness of the specimen geometry studied by FEA. As a failure criterion, the change in the slope of the strain range, \(\Delta\varepsilon\), was assumed, with reference to a graph where the strain range behavior is monitored versus the total number of load cycles, N. In the presence of a fatigue crack, in fact, the strain range value starts to increase noticeably, leading to the strain gauge failure when the crack path reaches the strain gauge grid, which is narrower than the area of the specimen subjected to the maximum stress. An example of the typical behavior indicating the nucleation and  
Figure 11: Geometry of the specimens used for uniaxial fatigue testing (**a**), with specified dimensions in unit [mm] (**b**). Figure 10: Resonant test machine (**a**) and details of the uniaxial fatigue test setup (**b**). development of a fatigue crack in the rim section specimens is shown in Figure 12, where the strain range, \(\Delta\varepsilon\), is represented in microstrain units [\(\imath\omega\)]. The number of cycles to failure, N\({}_{\text{f}}\), was assumed as the value by the slope change. When the failure criterion of the specimen was met, the test was stopped, and the actual presence of a fatigue crack was confirmed by magnetic particle inspection. An example of a detected fatigue crack is shown in Figure 13. The crack originated exactly in the desired point, confirming the development path that is usually found in the case of rim failures in service [14]. The cracked surface after the total fracture of the specimen is shown in Figure 14. The surface aspect confirmed the typical morphology of fatigue failures, with beach marks indicating the crack tip arrest and progress at each load cycle, and a small area of brittle final fracture. Due to the unidirectional bending load condition and the curved geometry of the section, the crack path is more developed along the outer side of the specimen, corresponding to the rim circumferential direction, than through the thickness of the section. This leads to a sort of elliptical shape of the fatigue region in the cross-sectional view of Figure 14, where this elliptical area is approximately 9.5 cm long and 5 mm wide. Figure 12: Example of crack start and development from the strain range behavior analysis. Figure 13: Example of a rim section specimen after test (**a**) and detail of the fatigue crack (**b**). The starting point of the crack was slightly aside the strain gauge position. This can be explained by the fact that the local stress is near the maximum value in a region of the specimen which is wider than the strain gauge grid. Therefore, any possible surface defect in that area may become the initial point of a fatigue crack. The specimens tested were obtained from two different rims of the same production batch, not manufactured consecutively but at a certain time interval. Henceforth, they will be referred to as Rim 1 and Rim 2. First, the aggregated fatigue data of both rims were considered to obtain the S-N (stress-life) diagram for the component at 50% probability of survival. The fatigue curve of the finite life region was obtained by linear regression of the fatigue data. It is represented in Figure 15, in terms of stress amplitude, \(\sigma_{a}=(\sigma_{max}-\sigma_{min})/2\), versus cycles to failure, N\({}_{\text{f}}\). The horizontal line of the graph represents the fatigue strength at runout (\(2\times 10^{6}\) cycles) for tension-tension bending tests with load ratio R = 0.1. Since it refers to the stress amplitude, it will be named as \(\sigma_{af}\) throughout this paper. From the analysis of the aggregated data of Rim 1 and Rim 2, \(\sigma_{af}\) = 148 MPa.