## 3 Results and Discussion
### Comparison of the Fatigue Test Results between Rim Section Testing and Base Material Testing  
The importance of reliable material fatigue properties is discussed in Section 1. When fatigue data of actual components are not available, or cannot be obtained through standard tests, designers usually refer to fatigue data available from the technical literature or alternative sources. Many sets of data can be found, but attention must be paid to the correlation between their test conditions and the conditions of the components in service. For example, there could be substantial differences regarding the kind of load (e.g., full traction versus bending), the load or stress ratio R, and the material surface conditions, which can heavily be changed by the production process. Although empirical relationships may be used to account for some of these aspects, sometimes they are just indicative or refer to specific cases.  
To investigate further this topic, standard fatigue tests were run on the base material of the rim, with the test setup and the specimen geometry presented in Sections 2.2.3 and 2.2.4. Fourteen hourglass specimens were obtained from a strip of the material before the rim production process. They were oriented in the transverse direction of the strip (T), which is the direction of the maximum principal stress both in actual rim service condition and in rim section fatigue testing. Given the possibility of using a high-frequency test machine, special attention was paid to the fatigue strength at 2 \(\times\) 10\({}^{6}\) cycles, which was investigated by a methodology based on the staircase method (UNI 3964 and JSME standard [26]). Six specimens were available, in this preliminary analysis, for the finite life region of the S-N curve, which should only be considered as approximate. As a failure criterion, the complete break of the specimen in two parts was assumed.  
The test results are shown in Figure 17. For clarity, a detail related only to the staircase results is shown in Figure 18. The fatigue strength at 2 \(\times\) 10\({}^{6}\) cycles, for the axial testing of the base material, is \(\sigma_{af}\) = 111 MPa. Therefore, the ratio between the fatigue strength of rim sections bending tests and the one from axial tests of the base material is between 1.36 and 1.5 if we consider Rim 1 and Rim 2 separately. Being equal the cross-section and the material, the higher fatigue strength of specimens subjected to plane bending, compared to  
Figure 16: S-N curve for the bending fatigue test of rim section specimens for rim size TW27\(\times\)38\({}^{\prime\prime}\). Comparison between Rim 1 and Rim 2.  
full axial traction, is well known in technical literature. This is due to the different volume of material subjected to the maximum stress, in relation to the load condition. Although empirical coefficients are available to convert the results, in actual tests they may change, depending on the material type or the specimen geometry. For example, in [27] three different steels were tested, showing a ratio of the fatigue limit ranging from 1.11 to 1.23 between plane bending and full traction of smooth cylindrical specimens.  
It is not the aim of this study to propose a more correct correlation coefficient between bending and axial fatigue tests for steel DD11. However, the comparison made in this section highlights, once again, the importance of considering the material fatigue properties in conditions as near as possible to those of the final components in actual field use.  
Figure 17: S-N curve for the axial fatigue test of the base material DD11.  
Figure 18: S-N curve for the axial fatigue test of the base material DD11. Details of the results based on the staircase method.