## 3 Results and Analysis
### Microscopic Morphology  
One of the primary reasons why samples undergo interlaminar-shear-strength failure is delamination by shear. The other kinds of failure include, but are not limited, to fiber micro buckling (i.e., fiber compressive failure) and fiber tensile breakage. In a previous study, the CNF z-threads interlocked between the carbon fibers and provided additional transversal stability support to the carbon fibers, making the ZT-CFRP composite laminates more resilient to interlaminar shear stress [25] and fiber micro buckling [20]. This may help to explain why all the ZT-CFRP laminates had better ILSS values than all the CFRP laminates, despite the different manufacturing methods tested in this study. The crack propagation in sample 1 and the delamination direction in sample 2 are shown in Figure 13 for the control CFRP/OOA-VBO laminate. Figure 14b shows an overall interlaminar delamination with a split into intralaminar delamination (i.e., crack propagation between individual carbon fibers within the same layer of the CFRP prepreg) at the end of the sample. Figure 14c shows a \(1000\times\) magnified image that highlights the absence of any CNF in the control CFRP samples, demonstrating the reason for the intralaminar delamination, interlaminar delamination, and easy crack propagation. On the other hand, ZT-CFRP samples possess more durability and resilience against interlaminar shear stress, which prevents them from completely failing through interlaminar and intralaminar delamination; rather, they hold on until failure through fiber breakage. Figures 15 and 16 display the carbon-fiber-breakage direction, the overall macroscopic-laminate-fracture direction (about 45 degrees of typical tension failure, which can also be seen in Figure 13), and the presence of pulled-off broken CNFs in the fractured region, for various ZT-CFRP/MCFA-AM samples. Figure 14: Microscopic pictures of delamination sites of control CFRP/OOA-VBO samples: (**a**) (\(100\times\)) sample 1: crack propagation. (**b**) (\(100\times\)) Sample 2: interlaminar delamination with a split into intralaminar delamination at the end of the sample. (**c**) (\(1000\times\)) Sample 2 (same spot): intralaminar delamination with no CNF. Figure 16: A microscopic picture shows (1000\(\times\)) the z-threading CNFs’ alignment in a ZT-CFRP/MCFA-AM sample with 45\({}^{\circ}\) of overall crack propagation. Figure 15: Microscopic pictures of ZT-CFRP/MCFA-AM samples: (**a**) (100\(\times\)) sample 1, fiber breakage at 45\({}^{\circ}\) angle. (**b**) (1000\(\times\)) Pulled-off broken CNFs in the fracture zone; (**c**) (100\(\times\)) Sample 2 crack propagation. (**d**) (1000\(\times\)) Sample 2 pulled-off CNFs (same spot). Figure 22 displays a comparison plot of the void contents of all the laminate types. The detailed void-area-calculation data are presented in Table A6 in Appendix A. The number of voids was highest in the no-pressure-3 DP CFRP samples, since they were printed by fusion only and without any compaction pressure; and the void content was about 23.94% with respect to the whole area in the image (this may have been different in the other spots of the no-pressure-3 DP CFRP samples, since the fusion mechanism is more susceptible to many uncontrolled factors in the materials and the processing environment). The lowest amount of voids was found in the laminates manufactured via the OOA-VBO method, since they had been cured at 0.82 bar of compaction pressure and under vacuum to remove the gas and/or volatiles inside the prepreg stacks. The void contents of the CFRP/OOA-VBO and ZT-CFRP/OOA-VBO samples were 1.37% and 1.06%, respectively. The samples printed by the novel robotic MCFA-AM printer with 0.78 bar of magnetic compaction pressure also had few void areas. The void contents were 1.36% and 3.87% for the CFRP/MCFA-AM and ZT-CFRP/MCFA-AM samples, respectively. It was also very interesting to notice that the CFRP/OOA-VBO and CFRP/MCFA-AM had the very similar void contents, of 1.37% and 1.36%, respectively.