# Continuous Fiber-Reinforced Material Extrusion with Hybrid Composites of Carbon and Aramid Fibers
## 3 Results and Discussion
### Mechanical Properties  
The mechanical properties of the printed hybrids were determined by tensile, flexural and impact tests. The stress-strain curves of the tensile and flexural tests are shown in Figure 6.  
The comparatively low tensile strengths compared to the flexural strengths are due, on the one hand, to the lower fiber volume content and, on the other hand, to the fiber orientation in the tensile specimens. Due to the contour-based fiber orientation, the fibers at the specimen fillets are not fully loaded in the longitudinal direction, which can lead to a reduction in strength. The results show that an increasing content of aramid in the specimens leads to a significant decrease in the mechanical properties under tensile and flexural loading. Both for tensile and flexural strengths and for Young's and flexural moduli, the 100% carbon specimens show the highest measured values. This results from the higher strengths and stiffnesses of the carbon fibers compared to the aramid fibers. It can also be observed that the flexural strengths and stiffnesses decrease significantly more with increasing aramid content compared to the tensile strengths and stiffnesses. The largest  
Figure 6: Stress–strain curves of the mechanical tests: (**a**) tensile tests; (**b**) flexural tests. strain rates are observed in 100% aramid specimens with a steep decrease in strain rates when hybridized with carbon fibers. Despite hybridization, brittle fracture was observed in all tensile specimens without multiple fractures (see Figure 6a). An increasing content of aramid fibers also leads to an increase of the strain rates in the carbon fiber-reinforced specimens. This could be due to the fact that damage evolution in the carbon fiber layers is limited by the adjacent, more ductile aramid fiber layers. Similar phenomena were also described for conventional composites with interply approach in [37; 50; 62]. Multiple fractures were observed in the flexural tests of the specimens with high carbon fiber content, which can be attributed to failure of individual fiber layers at short time intervals (see Figure 6b). The averaged results of the tensile, flexural and impact tests are shown with the corresponding standard deviations in Figure 7. The experimental results show an approximately linear decrease or increase in the mechanical properties of the hybrid specimens. The 100% carbon specimens exhibit average tensile and flexural strengths of 297 MPa and 554 MPa, respectively. The aramid specimens, on the other hand, show tensile and flexural strengths of 192 MPa and 173 MPa, respectively, about 35% lower and 69% lower, respectively, compared with 100% carbon specimens. The tensile and flexural moduli of the 100% aramid composite are lower by a factor of 2.4 and 5.1, at 6.42 GPa and 7.97 GPa, respectively, compared with the pure carbon fiber-reinforced composites. In contrast, the Charpy impact strength increases significantly with increasing aramid content. The highest impact strength was measured for 100% aramid specimens, while the lowest impact strength was observed for 100% carbon specimens. The 100% aramid specimens exhibited a Charpy impact strength of 30.42 kJ/m\({}^{2}\), while the 100% carbon specimens exhibited a Charpy impact strength of only 12.60 kJ/m\({}^{2}\). This can be explained by the increased energy absorption capabilities of the aramid fibers. This observation was also made for conventionally manufactured hybrid carbon and aramid composites [42; 45]. Figure 8 compares the experimentally determined mechanical properties with the values predicted by the RoHM equations. The calculated hybrid effects as a percentage are shown in Table 3. Both positive and negative hybrid effects have been determined. From the values in Table 3, it can be seen that the prediction of the RoHM equations calculated on the basis of the average values of the non-hybrid composite systems are able to predict the mechanical properties of the hybrid composites with high accuracy.