## 4 Results and Discussion  
### Results of Quasi-Static Tests  
#### 4.1.1 Uniaxial Tension Tests on Plate-Like Composite Specimens  
The small specimen length and the insufficient yarn anchorage resulted in undesirable failure modes of the plate-like composite specimens. As mentioned in the previous sections, the carbon yarns were longer than the specimens and the protruding ends were bent over the specimens' edges and glued between the specimens and the adapters. Due to the relatively weak bond strength between the carbon yarns and the surrounding matrix, and because of the small specimen length, the failure of the yarns occurred in the bent-over segments in the adapters. Such a failure was mostly facilitated by the poor transversal properties of the carbon fibers and the resulting damage induced during bending. Thus, in this configuration, the strength of the carbon textile reinforcement cannot be fully exploited and the specimens fail under considerably lower loads compared to long specimens with a proper textile anchorage [2; 8; 11]. Nevertheless, despite these limitations, the comparative study offers an insight into the influence of the short fiber reinforcement on the composite behavior and addresses further improvements in terms of composite design and testing configuration. The uniaxial quasi-static tensile stress-strain curves are plotted in Figure 7. The comparison of the representative curves indicates the contributions of SHCCs to the total response of the composite in the yarn pullout stage. Figure 7: (a–c) Tensile stress–strain curves of the composites under quasi-static loading and (d) representative stress–strain relationships for the composites under investigation. It should be noted that the specimens had already exhibited a few minor cracks prior to loading due to the forced mechanical clamping, which makes it challenging to define the accurate value of the first crack stress. However, the results of different materials are still comparable under the same testing conditions. Hence the results of the first crack stress, peak tensile stress, ultimate global strain, and the work-to-fracture of the tested specimens under uniaxial quasi-static tension tests are given in Table 4. The tensile strength of the composites was obtained by dividing the peak load by the composite cross-section. The global strains were calculated as specimen deformation over their gauge length; the ultimate value (strain capacity) corresponds to the peak load. Note that the global strain is a measure of the material deformability, and it is not associated with a uniform strain field in the specimens. It is rather representative of the extent of multiple cracking in the samples before peak loading. The post-peak behavior is associated with crack localization and widening accompanied by yarn pullout. In the case of TRC-M without any discrete fiber reinforcement, the yarn failure and subsequent pullout are accompanied by the widening of the localization crack with no multiple cracking, yielding an average composite tensile strength of only 3.4 MPa, which is also the first crack stress; see Figure 7a. On the contrary, the materials reinforced additionally with short micro-fibers, i.e., the ones with SHCC matrix TRC-PE or TRC-PBO, exhibit strain-hardening behavior. The tensile stress increases after the first crack accompanied by the formation of multiple cracks; see Figure 7b,c. The relatively low first crack stress of TRC-PE (2.5 MPa) can be traced back to the relatively high porosity of the matrix, as well as to the highly hydrophobic nature of short PE fibers. Due to the purely frictional bond, the fibers are only activated after crack formation, while prior to that they act as micro-defects [23], leading to even lower first crack stress than that measured for TRC-M (3.4 MPa). The short PBO-AS fibers, in contrast, possess a weak hydrophilic character. The smaller diameter results in higher aspect ratio and larger amount of fibers in the case of the same volume fraction of 2%.