## 4 Results and Discussion
### Results of the Impact Tension Tests  
#### 4.2.1 Dynamic Uniaxial Tension Tests  
All three types of composites were tested under impact tensile loading with a peak displacement rate of 8 m/s, corresponding to an average strain rate of 160 s-1; see dashed curves in Figure 10. The first crack stress of the composites is defined here as the first peak of the stress-strain curve, also detected with the help of DIC. The tensile strength is defined as the stress value at the peak of the ascending branch of the curve, while the ultimate global strain corresponds to the strain value at the peak stress prior to failure localization. The results of tensile strength, ultimate strain, and the work-to-fracture of the tested specimens are given in Table 7. The dynamic increase factor (DIF) of each parameter was calculated to demonstrate the rate effect on the tensile behavior of the composites. \begin{table}
\begin{tabular}{l l l l} \hline  & **Single-Yarn Pullout** & **Single-Yarn Pullout** & **Single-Yarn Pullout** \\  & **from Plain Matrix** & **from PE-SHCC** & **from PBO-SHCC** \\ \hline Peak debonding force [N] & 441 (34) & 440 (53) & 530 (94) \\ \hline \end{tabular}
\end{table} 
Table 6: Peak debonding forces in quasi-static pullout tests with three different matrices (average values; standard deviations are given in parentheses). Figure 9: Quasi-static 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 0.5 mm. It is noteworthy that all the composites exhibit multiple cracks under impact tensile loading. TRC-PBO bears the highest average maximum tensile stress of 16 MPa, followed by TRC-PE, with an average tensile strength of 13 MPa. Both the first crack stress and the tensile strength of the composites are increased pronouncedly in comparison to the corresponding values measured in the quasi-static regime. Despite the multiple cracking occurring in TRC-M, the material exhibited a strain-softening behavior with a very short plateau immediately after the initial stress peak between 0.2% and 0.3% strain. This plateau can be attributed to the pullout behavior of the textile yarns, as demonstrated in the next section. Tensile stress-strain curves and corresponding average crack widths for representative specimens of TRC-M, TRC-PE, and TRC-PBO are plotted in Figure 11. The crack formation and fracture processes were captured by high-speed cameras and then evaluated with DIC. It can be observed that micro-cracking already occurred before the formation of the first crack. The first peak of the curve corresponds to the formation of the first macro-crack, which propagates through the entire \begin{table}
\begin{tabular}{l l l l l} \hline \hline  & **First Crack Stress** & **Peak Tensile Stress** & **Ultimate Strain [\%]** & **Work-to-Fracture** \\  & **[MPa]** & **[MPa]** & **Ultimate Strain [\%]** & **[k/m3]** \\ \hline TRC-M & 8.6 (0.7) & 4.5 (0.6) & 0.3 (0.1) & 14.7 (4.2) \\ DIF & 2.5 & 1.3 & – & – \\ TRC-PE & 10.6 (1.6) & 13.0 (0.6) & 1.2 (0.2) & 122.4 (21.9) \\ DIF & 4.2 & 1.6 & 0.4 & 0.7 \\ TRC-PBO & 13.9 (0.5) & 16.0 (0.4) & 0.9 (0.1) & 122.2 (22.6) \\ DIF & 3.0 & 1.7 & 1.0 & 1.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of the uniaxial impact tension tests on the composites under investigation (average values; standard deviations are given in parentheses). Figure 10: **(a–c)** Tensile stress–strain curves of the composites under impact loading (the global strain rate curves are represented by dashed lines) and **(d)** representative stress–strain relationships for the composites under investigation. specimen's cross-section, leading to a large decrease of the composite stiffness. The momentary loss of the load-carrying capacity is revealed by the drop in the stress-strain curves after the first peak, as shown in Figure 10d.