## 3 Results and Discussion
### Fractographic Study  
A fractographic study was also performed on the fracture surfaces of selected SENB specimens to give valuable insights into the failure mechanisms acting on the different systems. Specifically, the binary CNTs0.5-III and CB2-II and the ternary CNTs0.5_CB2-IV specimens with the maximum observed K\({}_{\rm ic}\) and G\({}_{\rm ic}\) values were selected for observation and the SEM micrographs are presented in Figures 6-8. A good distribution of the filler particles was observed all over the surfaces of the different systems. However, the morphology of the fractured surfaces differed greatly for each of the binary and the ternary systems. In more detail, the binary CNTs0.5-III specimen exhibited a rather smooth fracture surface, where only some micro-cracks were evident, indicated by the green circle in Figure 6a. A closer examination revealed the homogenous distribution of the MWCNTs into the resin matrix, as well as the presence of some agglomerated particles (indicated by red circles in Figure 6b). By examining even further the area indicated by the yellow rectangle in Figure 6b, the agglomerates were found to cause crack pinning phenomena (indicated by yellow arrows in Figure 6c) to the secondary generated nano cracks, which are known as tail cracks [40]. Moreover, specific spots were also observed on the fracture surface of the CNTs0.5-III specimen where the network of disentangled CNTs was visible and was accompanied by the presence of agglomerated particles (indicated by the red ellipses in Figure 6d). A closer examination of the CNT network (Figure 6e,f) revealed interconnected long CNT bundles with dimensions with a rough approximation in the range of 50-150 nm. As reported by the manufacturer, the MWCNTs diameter is 10 to 15 nm, and their length is 1 \(\upmu\)m to 10 \(\upmu\)m and their initial form is that of agglomerated bundles with an average diameter of 400 \(\upmu\)m (50 \(\upmu\)m to 900 \(\upmu\)m). Therefore, the dimensions observed in Figure 6f indicated that these CNT networks were most probably comprised of disentangled bundles of a few CNTs with mostly unaltered length in comparison to their initial state prior to the dispersion process. A different morphology was observed at the fracture surface of the binary CB2-II specimen, where more and bigger matrix cracks were observed in comparison to the CNTs0.5-III binary specimen. In this case, the crack pinning was the dominant failure mechanism (as indicated by the green circles in Figure 6a,b). Furthermore, particle pull-out and breakage along with void creation, were observed for the CNTs0.5-III binary specimen (as indicated by the blue circles in Figure 6a,b). Finally, a combination of all the fracture mechanisms which were observed for the binary CNTs0.5-III and the CB2-II specimens were exhibited by the ternary CNTs0.5_CB2-IV specimen. In this case, the simultaneous use of the different scale fillers resulted in the presence of micro-scaled CB particles, micro- and nano-scaled MWCNTs agglomerates, as well as fully disentangled MWCNTs, which were interconnected in a 3D hybrid micro/nano network. This 3D network was capable of impeding crack initiation and propagation more effectively in comparison to the binary systems as indicated by the fracture toughness results, presented above, as well as to cause a more pronounced decrease of the electrical resistivity of the hybrid composite. Figure 6: Scanning electron micrographs recorded from the fracture surface of a CNTs0.5-III SENB specimen at (**a**) \(\times\)100, (**b**) \(\times\)1000, (**c**) and (**d**) \(\times\)5000, (**e**) \(\times\)20,000, and (**f**) \(\times\)37,000 magnification. Figure 7: Scanning electron micrographs recorded from the fracture surface of a CB2-IV SENB specimen at (**a**) \(\times\)100 and (**b**) \(\times\)250 magnifications. In more detail, both disentangled MWCNTs and CB particles were present in combination with chopped Carbon Fibers (CF) and agglomerated MWCNTs and CB particulates on the fracture surface of the CNTs0.5_CB2-IV specimen.