## 1 Introduction
A series of studies of different FRPs and several adhesives have been performed for structural elements including various types of cracks [17; 31; 32; 34; 35; 36; 37]. Again, the application of FRP reinforcements has proved its robustness. Not only separate and common applications of FRP reinforcements were under investigation. Here, the examples are structures or structural elements where bolted connections also appear beside the adhesively bonded overlays [14; 38]. Despite certain guidelines for the design of adhesive reinforcement for fatigue application, there is still a lack of knowledge related to the studies of the fatigue behavior of the adhesive interface [13]. In the available fatigue studies, the influence of some parameters such as the stiffness of CFRP patches and their geometry, number, position of overlays, etc. have been studied. The previously reported results of the fatigue tests [10] revealed that the adhesive has a key influence on the reinforcement effect. Recently, some guidelines for the design of adhesive joints for composite-composite structures have been proposed. It has been observed that the fatigue life of the adhesive joint can be increased by, i.e., incorporating additive particles [39], hybridization of the joint [40] or hygrothermal preconditioning [41]. However, there are no comparative studies in which different adhesives were tested under the same cyclic loading conditions. There are some studies referred to static loading conditions, but these results cannot be transferred and extended to fatigue problems. To the best knowledge of the authors, no further investigations have been made in the context of the above-mentioned issues. Based on the above insights, the novelty and the main aim of the proposed study is to formulate the guidelines for the selection of the adhesives for reinforcements of steel structures by CFRP overlays working at cyclic loading conditions. This is made by comparison of the detailed analyses and comparison of failure mechanisms (of steel core and adhesive layer) of the same structures and subjected to the same loading conditions but with different adhesives applied. In the vast majority of publications devoted to research on notched steel elements reinforced with composite overlays and subjected to periodic loads, only moderate values and variations of fatigue loadings are applied [6; 7; 8; 9; 11; 14; 15; 16; 17; 18; 20; 28; 29; 30; 31; 32; 34; 35; 36; 37; 38]. The novelty of the proposed paper relies on investigations of the failure forms of adhesive bonds subjected to extreme fatigue loadings. Here, in the performed test, the sinusoidal form of the loading cycle with stress ratio \(R\) = 0.1 and maximum tension load \(F\) equivalent to 90% of the yield limit is applied. Due to the presence of the notch, in the investigated sample zones with plastic deformations are present for such a force, which, additionally, makes the analyses difficult and complicated. These are very tough fatigue-loading conditions, which are not common and addressed in the published papers [6; 7; 8; 9; 11; 14; 15; 16; 17; 18; 20; 28; 29; 34; 35; 36; 37; 38]. Such a kind of loading is rather characteristic of highly stressed structures in mechanical appliances. In the performed failure-form analyses, particular attention is paid to the failure forms of different adhesives used and the influence of their technical data on the form of adhesive disintegration. There are many adhesives available on the market. Due to missing guidelines for the selection of adhesive for the reinforcement of steel structures by CFRP overlays, in the paper, the experimental study is performed using three chemically different kinds of adhesives. These are the two-component adhesives prepared on the chemical base of epoxy resin, polyurethane and cyanoacrylate/acrylic. Additionally, the chosen adhesives differ in the Young's modulus value from relatively hard epoxy resin to the remaining two mild adhesives.