## 3 Results and Discussion  
As previously mentioned, all samples maintained a constant overlap length of 30 mm, joint width of 25 mm, and a distance between loading points (S) equal to 120 mm. The only variation lay in the type of adherent materials used. Specifically, the single-lap adhesive joints were constructed using combinations of aluminum/aluminum, CFRP/CFRP, and CFRP/aluminum materials. The typical load-displacement plots for the tested single-lap adhesive joints under three-point bending loading conditions are depicted in Figure 7. These experimental results and pictures of the fractured surfaces are shown in Table 4  
Figure 5: The three-point bending test and the respective bending moment diagram. The blue regions represent the moment outside the overlap length. Figure 6: The three-point test of single-lap adhesive joint (Al-Al). and Figure 8, respectively. As can be seen from Figure 8, the fracture surface patterns are combinations of cohesive (a type of failure so that a layer of adhesive remains on both surfaces of the adherends) and adhesive (interfacial bond failure between the adhesive layer and the surfaces of the substrate) failures. These types of failures of the single-lap adhesive joints are also reported in many papers authored by different researchers [2; 10; 11]. The deformed shape of the single-lap adhesive joint under three-point bending is depicted in Figure 9, which was described according to the principles of beam mechanics theory in bending loading conditions. In this illustration, it becomes evident that the bottom region of the neutral axis experiences tensile stress, while the upper regions, located above the neutral axis, undergo compression stress. As a result, any failure would originate from the side experiencing tensile stress, specifically the lower part of the neutral axis. Conversely, no failures occurred in the adhesive above the neutral axis. Figure 10 further confirms this phenomenon, as observed in the tested single-lap joint specimens. In this figure, it is evident that crack initiation and propagation have occurred from the lower section of the joint, precisely at the interface between the adhesive layer and the lower adherend (here, the aluminum plate), where tensile stresses dominate. In other words, the crack near the interface is expanding, and it might experience kinking as it progresses toward the upper interface. However, the failure is cohesive in nature because a thin layer persists on the lower adhesive surface (Figure 8). As shown in Table 4, notably, there is a clear correlation between the displacement at peak loads and the joint's strength. Specifically, the CFRP/CFRP configuration showed the highest displacement, surpassing that of the aluminum/aluminum single-lap adhesive joints by 167%. This behavior can be attributed to the varying stiffnesses of the aluminum and CFRP adherends. Notably, the CFRP adherend offers greater flexibility compared to the aluminum adherend, as indicated by their respective Young's modulus values, as shown in Tables 2 and 3. Figure 10: The crack initiation and propagation of the single-lap adhesive joints from the interface between the upper adherend (here the CFRP plate) and the lower adherend (here the aluminum plate). Figure 9: The deformation of the single-lap adhesive joints under three-point bending loading conditions. As previously illustrated in Figure 5, the bending moment in the three-point bending test follows a linear variation. Its highest point is achieved at the midpoint of the overlap and subsequently diminishes in a linear manner until it reaches a value of zero at the outer supports.