# Continuous Fiber-Reinforced Material Extrusion with Hybrid Composites of Carbon and Aramid Fibers
## 3 Results and Discussion
### Damage and Failure Analysis  
In order to draw conclusions on the main failure mechanisms and possible differences between the fiber types, the microstructure of the fracture surfaces was investigated. For this purpose, SEM images were taken of specimens with 50% relative fiber content of carbon and aramid, respectively. The fracture surfaces of specimens from each test method were examined. The images of the fracture surfaces are shown in Figure 9.  
The analyzed fracture surfaces of the different specimens show similar failure mechanisms. Differences with regards to the different test methods cannot be identified. However, the different fiber types can be clearly identified. The aramid fibers can be identified by the ductile fracture typical of aramid fibers with necked fibers (see, e.g., Figure 9a,d). The carbon fibers, on the other hand, exhibit brittle fracture surfaces. This is also consistent with the strongly different strain rates of the carbon and aramid fiber-reinforced specimens in the mechanical tests. Furthermore, the higher energy absorption and the resulting impact strength can be justified with increasing aramid content. In addition, the aramid fibers show more debonding, which was also observed by [PERSON] et al. [46]. This may indicate poorer impregnation of the aramid fibers. The ductile fractures and the significant difference in length compared to the carbon fibers may reinforce this impression.  
Nevertheless, similar failure mechanisms can be identified for both the carbon and aramid fibers. For example, strong debonding occurs in all specimens. This can be seen from the numerous exposed fibers, which are surrounded by little or no matrix. This is more evident in the aramid fibers due to the large difference in length. In addition to exposed fibers, the carbon fiber layers also show vacancies caused by pulled-out fibers (see Figure 9d,e). In addition, delamination can be seen on all fracture surfaces. Delamination indicates a low adhesion strength between the layers or between the fiber strands. This could be further promoted by the different elongation rates of the fibers in the layers. Debonding and delamination are among the main failure mechanisms in printed composites and have been widely reported in the literature [21; 22; 23; 24].  
Another aspect that stands out in the analysis of the fracture surfaces is the partially irregular distribution of the fibers in the surrounding matrix (see e.g., Figure 9a,d). This phenomenon occurs commonly in printed composites and is also described in [63]. This could be due to the comparatively low fiber volume fraction of the fiber filaments (approx. 34%).  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Specimen** & **Tensile Strength** & **Young’s Modulus** & **Flexural Strength** & **Flexural Modulus** & **Charpy Impact Strength** \\ \hline
70 CF/30 AF & +3.1\% & +3.3\% & \(-6.0\%\) & \(-10.5\%\) & \(-5.2\%\) \\
50 CF/50 AF & \(-1.8\%\) & +8.8\% & +1.5\% & \(-9.5\%\) & +4.0\% \\
30 CF/70 AF & \(-1.9\%\) & +4.7\% & \(-9.4\%\) & \(-5.6\%\) & +9.0\% \\ \hline \end{tabular}
\end{table}
Table 3: Experimentally determined hybrid effects as a function of mixing ratio.