## 3 Results
### Characterization of Failure Mechanisms for Particular Adhesives
#### 3.4.1 S&P Resin 220  
The samples in which S&P Resin 220 was used showed the shortest fatigue life. The exemplary photographs of the failed samples are presented in Figure 6. In all cases, S&P Resin 220 adhesive exhibited a brittle damage mechanism. The first damage forms in the adhesive layer (between the adhesive and steel core at the ends of the overlays) caused by peel stresses were initiated in the early phase of the fatigue test (Figure 5). It should be noted that such damage occurred for a smaller number of cycles than the fatigue life of core steel samples.  
Figure 5: Degradation of normalized stiffness of samples with different adhesives.  
After the final failure, on the global scale, the failure form was the same; however, some differences were observed in the local damage forms. A strong adhesion to composite was obtained and there was no visible damage of the adhesive-composite interface. The main damage form was adhesive failure between the steel core and adhesive, and three different adhesive failure mechanisms between the steel and adhesive were noticed (Figure 7). The first is complete debonding, in which there was no trace of adhesive left on the steel core (Figures 6d and 7a). In the second one, the very thin, rough and discontinuous adhesive layer (namely VTRDAL) remained on the steel (i.e., Zones A in Figures 6a and 7b). The third and the least common damage form is related to cohesive failure, and in this case, a thick adhesive layer remained on the steel (Figure 6a--in zone B, Figure 6c, and Figure 7c).  
Due to some differences in the local failure modes, the following detailed description for all tested samples is added. The description given below is related to the damage observed on the steel core:  
* Sample no.1 (\(N_{f}\) = 56,282 cycles)--on two of four surfaces of the steel core on which adhesive failure occurred, the VTRDAL layers were observed (on the whole sur  
Figure 6: Failure mode of samples with S&P Resin 220: (**a**) Sample no. 3, \(N_{f}\) = 82,169 cycles—front view; (**b**) Sample no. 3—rear view; (**c**) Sample no. 4, \(N_{f}\) = 65,576 cycles—front view; (**d**) Sample no. 4—rear view (overall length of the sample was equal to 300 mm).  
Figure 7: Adhesive failure mechanisms for joint between S&P Resin 220 and steel: (**a**) complete debonding (\(N_{f}\)= 56,282 cycles); (**b**) a very thin, rough and discontinuous adhesive layer (VTRDAL) remained on the steel (\(N_{f}\)= 94,423 cycles); (**c**) a thick adhesive layer remained on steel (\(N_{f}\)= 94,423 cycles).  
faces), including the areas around the rectangular notch and the crack edges. On the remaining two surfaces, adhesive was not observed (see Figure 7a).
* Sample no.2 (\(N_{f}\) = 94,423 cycles)--VTRDAL layers were partially observed on four surfaces (Figure 7b). On two surfaces, there was no adhesive layer around the notch and crack edges. On the remaining two surfaces, VTRDAL layers were present around the notch and crack edges. Small areas (in the form of islets) with cohesive failure were observed (Figure 7c).
* Sample no.3 (\(N_{f}\) = 82,169 cycles)--VTRDAL layers were partially observed on four surfaces, including zones around the notch and crack edges (Figure 8a).
* Sample no.4 (\(N_{f}\) = 65,576 cycles)--large areas with VTRDAL layers and without any adhesive layer. No adhesive layer was reported around the notch and cracks.  
The application of cyclic loadings led to local chippings in the adhesive layer (Figure 8b). In places where the adhesive layer was degraded, the color of the adhesive surface changed from light gray to a dark gray color.