## 3 Results
### Failure Analyzation  
In the following section, the observed failure behavior of the samples is described and first explanations for the underlying failure mechanisms are given. In the course of this study, two distinct failure modes occurred depending on the fiber orientation in the CFRT component and the type and orientation of the pin geometry in relation to the load direction. The study shows that with a fiber orientation of 0\({}^{\circ}\), the samples tend to fail due to failure of the CFRT component while with fibers oriented in 90\({}^{\circ}\), the samples more often fail due to an extraction of the pin structure at higher loads, as was also found in [13]. Figure 10 shows images of samples created after manually stopping the single lap shear test after the initial drop of force was noticed. On the left, an example of a sample with 90\({}^{\circ}\) fiber orientation and a cylindrical pin is shown with a clearly visible separation between the CFRT and metal component which is a sign for beginning pin extraction and  
Figure 9: Force–displacement curves for cylindrical (**top**), elliptical (**bottom left**) and polygonal pins (**bottom right**). also a significant bending of the CFRT sample, which is a result of the samples' low bending stiffness perpendicular to the fiber orientation and the resulting rotary moment induced by the testing forces. In the middle, a microscopic image of an exemplary CFRT sample with a 90\({}^{\circ}\) fiber orientation after the extraction of the steel component is shown, which shows no significant deformation of the pin hole. Despite the lack of hole-deformation, dark areas can be seen below the pin hole that is the area where the testing forces are introduced into the laminate, which can be interpreted as matrix cracks due to the introduced compressive stress perpendicular to the fiber orientation. These cracks are a result of the unidirectional composite's low strength under compressive loads perpendicular to the load direction. However, despite the low compressive strength perpendicular to the load direction, samples loaded in 90\({}^{\circ}\) to the fiber orientation still show higher maximum forces before failure of the joint, which needs further explanation. This can be explained by the more efficient introductions of the testing load into the fibers in comparison to 0\({}^{\circ}\) fiber orientation. In 90\({}^{\circ}\), the fibers take up the testing load similar to an arresting cable used at aircraft carriers to rapidly decelerate landing airplanes and translate the testing load into a tensile load in the fibers. On the right, a sample with fibers in 0\({}^{\circ}\) orientation is shown. This sample clearly shows an ovalization of the pin hole as a result of the introduced compressive stresses. An explanation for this deformation can be found in the distinct fiber morphology, which occurs, in the direct pin pressing process (compare [8]) which leads to practically fiber free, matrix rich zones, which are located next to the pin hole in the direction of fibers. When samples are loaded in 0\({}^{\circ}\) to the fiber orientation, the testing force is directly introduced into these matrix rich zones, which have no reinforcing fibers and consequently are less stiff leading to the shown ovalization of the pin-hole and to a reduction in force in the mechanical tests. Table 8 summarizes the observed failure modes. A tendency for failure of the CFRT component with 0\({}^{\circ}\) fiber orientation and pin extraction failure with 90\({}^{\circ}\) fiber orientation can be seen, which also corresponds with the findings in the previous section. Exceptions to this are elliptical 0\({}^{\circ}\)/90\({}^{\circ}\) and polynomial blunt/90\({}^{\circ}\) samples, which also showed divergent force-displacement curves in Figure 9. This can be explained by the comparably sharp edge of the pin structure at the point of load transmission in combination with a pin geometry, which has an increased resistance against pin bending and consequently less likely pin extraction.