## 4 Results and Discussion
### Results of the Impact Tension Tests
#### 4.2.2 Dynamic Single-Yarn Pullout Tests  
The dynamic pullout curves are plotted in Figure 13. They show the same pattern as those obtained from the quasi-static tests. Note that the curves exhibit oscillations in the pullout stage, which are due to the intrinsic digital noise of the strain gauges, significant for such low pullout forces. Similar to the quasi-static tests, the peak bond strength between carbon yarn and the matrix with the addition of short PBO fibers, is the highest among the three composites, with an average of 1455 N; see Table 9. Note that the SHTB could ensure only a limited displacement during one wave passage, which was 2.5 mm.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline  & **TRC-M** & **TRC-PE** & **TRC-PBO** \\ \hline Average crack width at 0.1\% strain level [\(\mathrm{\SIUnitSymbolMicro m}\)] & 17 & 16 & 29 \\ Average crack spacing at 0.1\% strain level [mm] & 25 & 17 & 25 \\ Applied stress at 0.1\% strain level [MPa] & 5.8 & 8.2 & 14.1 \\ \hline Average crack width at 0.3\% strain level [\(\mathrm{\SIUnitSymbolMicro m}\)] & 63 & 25 & 40 \\ Average crack spacing at 0.3\% strain level [mm] & 17 & 10 & 13 \\ Applied stress at 0.3\% strain level [MPa] & 3.8 & 8.2 & 12.6 \\ \hline Average crack width at 0.4\% strain level [\(\mathrm{\SIUnitSymbolMicro m}\)] & 83 & 43 & 58 \\ Average crack spacing at 0.4\% strain level [mm] & 13 & 10 & 13 \\ Applied stress at 0.4\% strain level [MPa] & 3.4 & 9.2 & 14.4 \\ \hline Average crack width at the ultimate strain level [\(\mathrm{\SIUnitSymbolMicro m}\)] & 138 & 72 & 107 \\ Average crack spacing at the ultimate strain level [mm] & 13 & 7 & 13 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Average crack width and crack spacing at the strain levels of 0.1%, 0.3%, and 0.4% for the representative specimens subjected to uniaxial impact tension tests (localization crack excluded).  
Figure 12: Average crack width and applied stress for representative specimens under impact tensile loading.  
Figure 13: Dynamic force-slip relationships of single carbon yarns pulled out from: (**a**) plain matrix; (**b**) PE-SHCC; (**c**) PBO-SHCC; and (**d**) representative force-slip relationships for different composites up to a slip extent of 2.5 mm (the global strain rate curves are represented by dashed lines).  
It is obvious that the increasing displacement rate led to an increase in the peak pullout force with a dynamic increase factor (DIF) of around three for all three parameter combinations. The increased bonding between carbon yarn and matrix contributes to a higher tensile strength of the composites. Moreover, taking into consideration the increased tensile strength of the uncracked regions under a higher strain rate, higher stresses are needed to generate new cracks. In the previous section, it was shown that the number of cracks in TRC-M increased in impact tests, but it decreased in TRC-PE and TRC-PBO. In a future study, tension tests of single carbon yarn and short PE and PBO-AS fibers, and the pullout tests of the above-mentioned short fibers, need to be performed in order to attain a comprehensive understanding of the strain rate sensitivities of tensile properties and the bonding properties of the reinforcements to various matrices. What is more, as presented in [34], the bond strength could be enhanced by decreasing the spacing between weft yarns, upon the premise of a sufficient mesh spacing for short fibers to penetrate, which indicates the approach to further improve the bond properties of textiles by optimizing the mesh to an extent.