## 4 Discussion
This negative effect of peel stresses growth can be mitigated by using different adhesives (stiffer and stronger close to the notch and softer at the ends of the overlays) or by grading of the properties of the adhesive along the joint [6; 47]. Based on the concluding remarks from the experiments that adhesions to steel and CFRP are most important for ensuring a high fatigue life, the obtained numerical results for different adhesives from Table 2 were compared with admissible tensile and shear stresses. The summary of this comparison is shown in Figure 26.  
Here, it can be seen that adhesives with a low tensile modulus have also smaller admissible stress limits (both shear and tensile) than adhesives with a larger tensile modulus. However, the increase in tensile modulus results in increasing stresses in the adhesive interface, which, in many cases, exceed admissible limits. The admissible tensile stresses \(\sigma_{z}\) are exceeded in S&P Resin 220 (no. 11--in the surrounding of the hole and at the ends of overlays) and Sikadur 30 (no. 12--at the ends of overlays) adhesives. The admissible shear stresses at the adhesive/steel interface are exceeded for Araldite 2014-2 and S&P Resin 220 (in both cases, at the ends of overlays) and Sikadur 30 (at the ends of overlays and in the notch area). The admissible shear stresses at the adhesive/CFRP interface are exceeded for Araldite 2012, Araldite 2014-2 and S&P Resin 220 (at the ends of the overlays) and Sikadur 30 (at the ends of overlays and in the notch area). These observations explain the fatigue behavior of S&P Resin 220 (detachment of overlays and low fatigue life) revealed in the experimental tests and discussed in the previous section. On the other hand, the performed FEM analyses and summary shown in Figure 26 do not explain the difference in fatigue life of samples with DP 6310 NS and HY 4080 GY. It is most likely that the fatigue life, in these two cases, is additionally dependent on the adhesion forces of adhesive to the steel core. Adhesion to steel seems to be crucial because the first fatigue phenomena (plasticity, crack initiation and growth, etc.) occur in the steel core around the notch. Worse adhesion to steel was observed for DP 6310 NS. This led to detachments of the ends of overlays during the tests, which, consequently, also resulted in a change in stress redistribution in the adhesive interface and, finally, in accelerated fatigue damage growth. Another factor that should increase the fatigue life of samples with  
Figure 26: Maximal stresses in different adhesives and comparison with admissible stresses. HY 4080 GY is that this adhesive seems to be the most resistant to the influence of peel stresses at the ends of the overlays. During tests with this glue, no symptoms of premature detachment of the overlays were observed. Moreover, HY 4080 GY revealed the highest adhesion to the steel core in the vicinity of the notch. Finally, the selected properties of different adhesives are compared together in Figure 27 (2-C epoxy adhesives) and Figure 28 (2-C acrylic, polyurethane and methacrylate adhesives). Based on the performed study, the comparison concerned the following material properties: tensile modulus, shear strength on steel, shear strength on CFRP, tensile strength and elongation at failure. The comparison is presented in dimensionless form maintaining the scale of particular parameters. The minimal properties are close to the center of the wheel, while the largest ones are on the circumference of the circle. In the attached comparison, three adhesives are not shown--S&P Resin 220, Sikadur 30 and SikaFast 5221 NT. They are omitted to ensure the readability of the drawings because they have significantly different properties from other adhesives--a tensile modulus a few times larger (S&P Resin 220 and Sikadur 30) and an elongation at failure at least twice as large (SikaFast 5221 NT). Generally, epoxy adhesives (Figure 27) are characterized by a high tensile modulus, low elongation at failure and higher shear and tensile strengths.