## 4 Discussion
This can be explained by the fact that the fatigue zones constituted most of the fracture zone (see Figure 23b, >90% and Figure 22a-c) or the entire part was subjected to a fast fracture (i.e., Figure 22d). In the remaining cases, the above-described trend was maintained (the range of beach marks is as follows, from the smallest to the largest: bare sample, S&P Resin 220 and DP 6310 NS). The narrowing of the samples is shown in Figure 24b. Obviously, the largest narrowing was obtained for bare samples with similar values on both sides of the hole. The use of the reinforcement significantly reduces narrowing on one side (on which a larger fatigue zone occurred); however, on the second side, the observed narrowing was close (DP 6310 NS and HY 4080 GY) or larger (S&P Resin 220) than the values obtained for bare samples. This can be explained by the fact that the reinforcement relieved the steel core during damage growth (mainly on one side with a larger fatigue zone) and when the damage in the steel core reached a critical level, the adhesive joints are suddenly broken. This usually resulted  
Figure 24: Details of fatigue fracture surfaces of steel core: (**a**) size of zones with beach marks; (**b**) Z-reduction of the area of the samples. in the fatigue damage on the second side (with a smaller fatigue zone) not having enough time to fully develop or not being initiated at all. This is also in agreement with the results presented in Figures 23 and 24 for smaller fatigue zones in which the beach marks and the size of the slow fatigue fracture zones on bare samples are sometimes larger or comparable with those on the reinforced samples. Different damage mechanisms were observed (Section 3.4) for the tested adhesive joints. The main failure form of degradation of the adhesive joints, which was observed in all cases, was adhesive failure between the steel and adhesive. The scale of the particular failure modes was specified for each sample. The obtained results are presented in Figure 25. The presented bars show the percentage participation of particular parameters on the measured damaged surfaces of the adhesive joint. The strongest adhesion of the adhesive to the steel core (in the range of 22-33%) was obtained for HY4080 GY adhesive. In the case of S&P Resin 220, three adhesion failure forms were observed: 1--a thick adhesive layer remained on the steel core (0.5-4%), 2--a very thin adhesive layer remained on the steel core (24-78%), and 3--the complete lack of adhesion to steel (18-74%). The DP6310 NS adhesive also revealed an unsatisfactory adhesion to the steel core (2.7-20%). The weakest adhesion to the composite was observed for HY4080 GY adhesive (65-78%). The remaining adhesive (S&P Resin and DP6310 NS) showed a high adhesion to the composite surface (99-100%). Slight composite damages were observed only in three cases and can be neglected in further analyses. A slight cohesion failure (below 3%) was observed in almost all cases, except for one specimen (DP 6310 NS--1) in which it achieved a value of 20.6%. In this case, it was caused by higher adhesion to the steel core. Finally, it can be observed that the fatigue life of samples in which S&P Resin 220 and DP 6310 NS were applied is correlated with adhesion to steel. On the other hand, in the case of the HY 4080 GY adhesive, the fatigue life seems to be correlated with adhesion to composite overlays. In Section 3.5, it was shown that the stresses in an adhesive interface depend on its tensile modulus. This applies to normal and shear stresses at the ends of the overlays as well as in the surroundings of the notch in the steel core. All stresses increased (with a nonlinear parabolic trend) with increasing adhesive tensile modulus. So, the application of the stiff and strong adhesive may lead to failure due to the pe  
Figure 25: Failure modes observed in tested adhesive joints. increase at the end of the joint with an increase in the tensile modulus.