## 3 Results and Discussion
### Fibre/Matrix Interfacial Assessment by Single Yarn Fragmentation Test
This is especially true for the flax yarns enzymatically treated using 10 wt % Peclyve for 6 h. A poor interfacial adhesion between the treated flax yarns and both the epoxy and vinylester matrices could be inferred from these results. However, as highlighted previously for the critical yarn length values, it is important to emphasize that the IFSS results are affected by the change in mechanical properties of flax yarns caused by the biochemical treatments. After the 10 wt % Peclyve 6 h treatment, the tensile strength at a length equal to the critical yarn length was reduced by 50%, resulting in a strong decrease in the interfacial shear strength value with the epoxy and vinylester resin. The main problem with this kind of test for yarns is related to the correct measurement of the cross-sectional area of the yarn. In fact, a yarn includes voids among the technical fibres that are replaced by the matrix during the impregnation stage. This suggests that the IFSS values in the present work can only be used for a qualitative screening of the different flax yarns. Nevertheless, once again the better compatibility of the flax yarns with the epoxy matrix can be confirmed. A more reliable assessment of the level of interfacial adhesion can be based on another parameter, i.e., the debonding length [51]. An extended debonding zone is typical for weak interfacial bonds. In addition, this parameter does not depend on the constraints that are required to be satisfied for a sound single fibre fragmentation test (SFFT). Values included in Table 6 show reduced debonding lengths for the enzymatically treated yarns compared to the untreated ones, thus fostering a better interfacial adhesion of flax yarns with the epoxy and the vinylester matrices. The decrease in debonding length found for the enzymatically treated flax yarn with both epoxy and vinylester matrices can be linked to the increase in hydrophobic behaviour produced by the Peclyve enzymatic solution and a better fibre individualization that could promote the wettability of the resin. Similar debonding length values were measured for both the 5 wt %/4.5 h and 10 wt %/6 h treatment conditions. A decrease in the debonding length of about 18% was found for both enzymatically treated flax yarns in the epoxy-based composites. This result indicates that the different solution concentrations and treatment durations used affect the adhesion quality between the flax yarn and the epoxy matrix in the same way. The results found for the flax yarn/vinylester system are different. In this case, the enzymatic treatment performed with 10 wt % Peclyve for 6 h produced better adhesion results than the treatment with 5 wt % Peclyve for 4.5 h. If compared with the as-received flax yarn/vinylester system, a decrease in the debonding length of about 36% was found for the Peclyve 10 wt %/6 h flax yarn/vinylester composite. The improved interfacial adhesion can be also inferred from micro-CT micrographs. This analysis was carried out only for the best performing treatment, i.e., the Peclyve 10 wt %/6 h (Figure 9). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Matrix** & **Flex Yarn** & **\(\sigma_{\text{f}}\)(L\({}_{\text{c}}\)) [MPa]** & **IFSS [MPa]** \\ \hline \multirow{3}{*}{Epoxy} & Neat & 406 \(\pm\) 18 & 19.3 \(\pm\) 3.7 \\  & Peclyve 5 wt \%, 4.5 h & 283 \(\pm\) 10 & 18.1 \(\pm\) 2.7 \\  & Peclyve 10 wt \%, 6 h & 203 \(\pm\) 3 & 12.2 \(\pm\) 1.1 \\ \multirow{3}{*}{Vinylester} & Neat & 376 \(\pm\) 15 & 13.9 \(\pm\) 2.8 \\  & Peclyve 5 wt \%, 4.5 h & 262 \(\pm\) 2 & 10. 5 \(\pm\) 1.4 \\ \multirow{3}{*}{Vinylester} & Peclyve 10 wt \%, 6 h & 188 \(\pm\) 9 & 7.8 \(\pm\) 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Strength values at critical fragment length and interfacial shear strength (IFSS) values for the neat and enzymatically modified flax/epoxy and flax/vinylester systems.