## 4 Discussion  
The performed experimental tests revealed the dominant influence of the adhesive type on fatigue life. The evaluation of the fracture mechanisms of the adhesive interface presented in the previous section confirms that adhesive is the weakest link and, at the same time, the decisive element for the durability of the entire structure. Based on the observation of the adhesive interface behavior and captured damage forms, the first conclusions can be drawn. However, in order to analyze and assess in more detail the influence of adhesive properties on the fatigue of the entire structure, additional measurements of fatigue fractures were made. The exemplary images of the fatigue fracture zones are presented in Figures 19-22. The images were selected in a way to present fracture zones for specimens with the smallest and highest fatigue lives for each adhesive. For each specimen, the photographs of both sides of the fracture zone are presented--on the left-hand side, the part with a larger fatigue area, and on the right-hand side, the part with a larger fast fracture zone. The mentioned above measurements were limited to the assessment of fatigue damages occurring in the steel core. First of all, the areas with slow fatigue fracture zones were designated. Here, these areas were divided into two parts--smooth (designated as 1 in Figures 19-22) and rough (designated as 2)--which differed in the smoothness of the  
Figure 17: FE results for specimen with DP 6310 NS: (**a**) distribution of equivalent plastic strains in metallic core; (**b**) distribution of tensile stresses \(\sigma_{\mathrm{x}}\) (MPa) in CFRP overlay. Figure 18: FE results for specimen with DP 6310 NS: (**a**) distribution of shear stresses \(\tau_{\mathrm{xx}}\) (MPa) in adhesive; (**b**) distribution of peel stresses \(\sigma_{\mathrm{z}}\) (MPa) in adhesive. fatigue failure zone. For better understanding, the specimens in Figures 19-22 are arranged in order of increasing fatigue zones. Additionally, the areas with beach marks (designated as 3) and fast fracture growth (designated as 4) were approximately estimated. The measurements were made for both sides of the hole of the steel core. Almost in all cases, cracks and fatigue damage were observed on both sides of the square hole. However, fatigue was always dominant on one side and on this side, a larger fatigue zone was observed (red bars in Figures 23 and 24). Here, the average values measured for each group of samples are presented. The thin grey lines in these figures mean the standard deviations of the measured quantities. Figure 23: Details of fatigue fracture surfaces of steel core and size of slow fatigue fracture zones: (**a**) smooth part; (**b**) rough part. Figure 22: Photographs of fatigue failure zone (HY 4080 GY: **(a,b)**\(N_{f}\) = 322,768 cycles and (c,d) \(N_{f}\)= 421,646 cycles) with (**a,c**) larger fatigue area and (**b,d**) smaller fatigue area. The results performed for all specimens are shown in Figures 23 and 24. The percentage contribution of smooth and rough slow fatigue fracture areas are shown in Figures 23a and 23b, respectively. It can be seen that the obtained results for reinforced specimens have a clear trend regardless of whether the part with a higher or lower fatigue contribution is assessed. By grouping the samples from the smallest to the largest slow fatigue zone, the following order of adhesives is obtained: S&P Resin 220, DP 6310 NS and HY 4080 GY. Obviously, these fatigue areas for reinforced samples are larger than the fatigue areas of bare samples. Comparing reinforced and not-reinforced samples, it can be seen that in the case of the bare specimens, there were smaller differences in the sizes of the fatigue zones (both smooth and rough) on both sides of the notch. The distance of the beach marks measured from the edge of the notch is shown in Figure 24a. The beach marks were not observed on the fracture zones of specimens in which HY 4080 GY was applied.