# Toughening Behavior of Carbon/Epoxy Laminates Interleaved by PSF/PVDF Composite Nanofibers
## 3 Results  
### Test Results  
Figure 5 shows the force-displacement curves for the reference and modified samples under DCB loadings, and Table 3 presents the results. As seen, the PSF and PVDF did not affect the slope of the linear loading phase before crack propagation. An interesting phenomenon is observed while the crack propagates. In the reference laminate, a high number of short rises and falls of the force is registered, unlike the modified laminates, especially the PSF- and PVDF/PSF-modified ones, where a lower number of variations is registered (see the orange ovals in the figures). In nanomodified samples, the force rises after a drop up to about 6N, which is 15% of the maximum load.  
The maximum load (Fmax) was 32.7 N for the reference laminate, and it increased 11% and 21% by applying PSF and PVDF nanofibers, respectively; G\({}_{\text{IC}}\) for the non-modified sample is 255 N/m, whereas, for the PVDF, PSF, and PVDF/PSF samples, it is 423, 384 and 454 N/m, respectively. By comparison with the reference, the energy-release rate of PSF- and PVDF-modified laminates was enhanced by 51% and 66%, respectively, and a higher enhancement of 78% was obtained by using the mixed nanofibrous mat.  
Figure 4: End-notched flexure (ENF) sample under Mode-II test.  
ENF test curves are shown in Figure 6, and the results are presented in Table 4. The behavior of the two types of samples differs at the fracture initiation stage. The crack started to propagate 60-80 N below the Fmax in both control the PSF-modified samples. In this critical point, the slope of force-displacement curve decreases, flagging a crack propagation. In the PVDF-modified laminate, the crack initiation was followed by a force drop, about 40-60 N, in various samples. Then, the force increased again up to the maximum load. The force-displacement curve of the laminates interleaved by PVDF/PSF has some similarities with both of the two other modified samples. In the stage of crack initiation, a very small force drop was observed, and then the load increased about 10-20 N, up to the F\({}_{\max}\) with a lower slope.  
Figure 5: Mode-I fracture test (DCB) outcome for reference (**a**) PSF-only, (**b**) PVDF-only, (**c**) PVDF/PSF (**d**) nanomodified samples.  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Reference** & ** PVDF** & **PSF** & **PVDF/PSF** \\ \hline Maximum load (N) & 32.7 \(\pm\) 2 & 39.6 \(\pm\) 0.5 & 36.3 \(\pm\) 1.5 & 39.6 \(\pm\) 0.5 \\ Variation (\%) & & +21 & +11 & +21 \\ G\({}_{\text{IC}}\) (N/m) & 255 \(\pm\) 7 & 423 \(\pm\) 53 & 384 \(\pm\) 9 & 454 \(\pm\) 26 \\ Variation (\%) & & +66 & +51 & +78 \\ \hline \end{tabular}
\end{table}
Table 3: DCB test results.  
According to Table 4, reference and PSF-modified laminates have similar values of maximum load and GIIC. Therefore, the PSF nanofibers do not show significant effect on toughening the virgin laminate, while its influence in Mode-I loading was positive. On the other hand, PVDF and PVDF/PSF nanofibers increased the GIIC of the laminate by 57% and 75%, respectively. It is interesting to note that, although the influence of PSF nanofibers on GIIC was negligible, its mixture with PVDF had a synergistic effect. A similar phenomenon was observed by [PERSON] et al. [39]. They used PCL film, nylon nanofibers, and their mixture for toughening carbon/epoxy laminates: According to their results, presented in Table 5, each interlayer individually increased GIIC by 20%, while their mixture almost doubled it.  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Reference** & **PVDF** & **PSF** & **PVDF/PSF** \\ \hline Maximum load (N) & 477 \(\pm\) 13 & 523 \(\pm\) 15 & 478 \(\pm\) 12 & 558 \(\pm\) 16 \\ Variation (\%) & & +10 & / & +17 \\ GIIC (N/m) & 182 \(\pm\) 3 & 285 \(\pm\) 2 & 199 \(\pm\) 9 & 318 \(\pm\) 29 \\ Variation (\%) & & +57 & +9 & +75 \\ \hline \end{tabular}
\end{table}
Table 4: ENF test results.  
Figure 6: Mode-II fracture test (ENF) outcomes for reference (**a**), PSF-only (**b**), PVDF-only (**c**) and PVDF/PSF (**d**) nanomodified samples.