## 3 Analytical and Experimental Results
### Mechanical Characterization  
15 specimens were tested for each fiber direction to determine the mechanical properties. The flexural tests were performed in sets. Figure 8a,d show the test specimens of a set with longitudinal and transverse fiber direction, respectively. Slight geometric irregularities of the specimens are visible, which can be attributed to the manufacturing process. Figure 8b,e show the deformation of specimens during the flexural test. A larger deformation before failure was observed for specimens with transverse fiber direction. The plastically deformed specimens (i.e., after mechanical testing) are shown in Figure 8c,f. In some of the specimens with longitudinal fiber direction, delamination of the outer layers was also observed. The stress-strain curves of 3D printed CF/PA6 specimens, as well as their variability, are of particular interest. Evaluated separately for each set, the average stress-strain curves including their standard deviation in the longitudinal fiber direction and transverse fiber direction are shown in Figures 9 and 10, respectively. The average curves and standard deviation were calculated from the measurement data. Usually, specimens fail at different  
Figure 8: Flexural test specimens before, during and after the three-point bending test. In (**a**–**c**), specimens with longitudinal fiber direction. In (**d**–**f**), specimens with transverse fiber direction. stresses. If neglected, this phenomenon can lead to calculation errors. Here, the average stress-strain curve and its standard deviation were computed between the origin and the stress level at which the first specimen failed. Additionally, the mean flexural strength was calculated for each set and marked in the figures. The shapes of the stress-strain curves differ significantly for the different fiber directions. For the longitudinal fiber direction, the stress increases almost linearly with strain, without a noticeable change of strain rate, until a sudden failure occurs. In contrast, a non-linear behavior occurs for the transverse fiber direction, starting from a strain of about 1% to 1.2%. The material follows a linear-elastic behavior in the first third of the strain range before non-linear material behavior occurs. The observed standard deviation of the stress-strain curves increases with larger strains. The magnitude of standard deviation also differs between the sets. For instance, in the longitudinal direction (see Figure 9), the standard deviation ranges from 38.3 MPa to 52.3 MPa at a strain of 1.3%. In the transverse fiber direction (see Figure 10), the standard deviation ranges from 7.5 MPa to 8.5 MPa at a strain of 2.5%. The flexural modulus was determined according to the test standard ISO 14125:1998 [52]. It is the stress-strain gradient of the strain range 0.05% to 0.25% (strain at the outer surface in the middle of the specimen, calculated from the measured deflection). Figure 10: Bending test of CF/PA6 in the transverse direction (90° fiber direction): Average stress-strain curves with standard deviation (shadow) and flexural strength (*) for each test set. Figure 9: Bending test of CF/PA6 in the longitudinal direction (0° fiber direction): Average stress-strain curves with standard deviation (shadow) and flexural strength (*) for each test set. The mechanical properties, observed in both directions, are significantly lower in the transverse direction. The flexural strength in the transverse direction is located between 29.9 MPa and 57.8 MPa. In the longitudinal direction, it is located between 470.1 MPa and 623.2 MPa. The flexural modulus was determined according to the test standard ISO 14125:1998 [52]. The observed Young's modulus values show the same kind of discrepancy between both directions. In the transverse direction, Young's modulus values between 1.3 GPa and 2.5 GPa were measured. In the longitudinal direction, Young's modulus values between 32.1 GPa and 43.1 GPa were measured. This highlights the previously described significant material anisotropy of CFC.