## 3 Results and Discussion
### Surface Hardness Measurement  
Surface hardness measurements were made on the rim section specimens of Section 3.1 to assess if the plastic deformation induced by the rolling process contributed to the improvement of the material fatigue properties due to strain-hardening effects. It must be noted that the fatigue results of the base material, discussed in Section 3.2, are mainly related to its elastic properties, since the stress levels used were below the yield strength of Table 2. Also, the fatigue results of rim section testing are all located in the high-cycle fatigue region, which is usually considered for life longer than \(10^{5}\) cycles. Here, the material elastic properties prevail. Therefore, the linear elastic model was used to convert the initial strain values recorded by the strain gauge to the corresponding stress levels, as a valid approximation solution. The results indicated an improvement of the mechanical characteristics of the material after the production process, as already discussed by other authors. For example, in [8; 23], a similar low-carbon steel was proven to show better fatigue properties after plastic deformation, comparable to the ones of 30% cold work sheet metals subjected to conventional fully reversed bending. For those 30% cold work specimens, both the tensile strength and the yield strength were highly increased if compared to the base material. In that case, the total deformation of the zone tested was higher than the case study presented in this paper due to a different geometry of the rim.  
If noticeable plasticity phenomena were experienced using the fatigue testing approach proposed in this study, more extensive constitutive models of the material should be considered and analyzed [37].  
The surface hardness values of the rim section specimens are shown in Table 5. Hardness was measured according to Rockwell B scale (HRB) on four different specimens: two specimens from Rim 1 and two specimens from Rim 2. They are referred to as A, B (Rim 1) and C, D (Rim 2). For each specimen, two different zones were measured:  
* Zone 1: at minimum 3 cm-distance from the radius, along the axial direction. Here the deformation is negligible. This zone is named as \"flat\", henceforth;
* Zone 2: the radius under investigation, where plastic deformation is present.  
Values of Table 5 are averaged from three different measurements for each zone, with negligible scattering.  
In Table 6, the hardness values are converted to tensile strength (R\({}_{\text{m}}\)), according to the conversion table from ASTM A370 [38] for non-austenitic steels. These values must be assumed just as an approximation, because, currently, there is no general method of accurately converting the hardness numbers determined on one scale to tensile strength values.  
Considering that approximation, the values measured in the flat area of the specimens are in line with the tensile strength of the base material of Table 2, which is 391 MPa. In the radius zone the hardness tends to increase, with slightly higher converted values of tensile strength. That is an indication of improved local material characteristics and confirms the importance of testing structural components as near as possible to their final condition. This is one of the main purposes for which the section test approach was developed.  
Another interesting aspect that can be noticed from Table 6 is that, on average, the converted values of tensile strength in the radius of specimens from Rim 1 are higher than those from Rim 2. That could also explain the fatigue test results of Figure 16, where Rim 1 showed slightly better results.