## 4 Experimental Program
### Monotonic Loading  
Nine (VFS-A) and six (VFS-B) specimens were subjected to monotonically increasing tensile loading. The loading rate was 0.01 mm/s for each test. The measured loads of the testing machine were converted into the tensile stress of the fibre by \(\sigma_{\text{st}}^{\text{f}}=F/A\). The diameter of the fibres was assumed to be in good order to 0.19 and 0.13 mm. Figure 3 shows the tensile stress-strain curves for VFS-A and VFS-B (black lines) and the mean curves (red lines) of the test series. The curves of VFS-A (\(\varnothing_{\text{f}}=0.19\) mm) in Figure 2(a) show an approximately linear elastic behaviour up to 90% of the failure load. After that, the curves are slightly curved until fibre rupture occurs abruptly. The failure occurs at an elongation of about 22 to 27%, and an average ultimate tensile strength of approximately 3576 N/mm\({}^{2}\). The curves of VFS-B (\(\varnothing_{\text{f}}=0.13\) mm) show slightly uneven characteristics. The curves increase nonlinearly, and the curve's curvature does not change upon failure load. The failure also occurs without preannouncement. The strain at the failure point of about 22 to 27% is nearly equal to the micro steel fibres of test series VFS-A. However, the ultimate tensile strength of VFS-B is approximately 15% higher. Table 1 lists the detailed results of the tensile tests of the high-strength micro steel fibres under monotonic loading. The value's exponent is its standard deviation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Number** & **Diameter \(\mathcal{O}_{\text{f}}\)** & **Ultimate Tensile Strength \(f_{\text{st}}^{\text{f}}\)** & **Ultimate Strain \(\epsilon_{\text{st}}^{\text{f}}\)** \\ \hline
[-] & [Num.] & [mm] & [N/mm\({}^{2}\)] & [\%] \\ VFS-A & 9 & 0.19 & 3575.8\({}^{\text{+}\text{0.2}}\) & 22.8\({}^{\text{+}\text{0.7}}\) \\ VFS-B & 6 & 0.13 & 4250.4\({}^{\text{+}\text{95.2}}\) & 24.6\({}^{\text{+}\text{2.3}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of steel fibres under monotonic tensile loading. Figure 3: Tensile stress–strain curves of high-strength micro steel fibres of test series VFS-A (**a**) and VFS-B (**b**). Figure 2: “Truss” of the ordered high-strength micro steel fibre (**a**) and test specimen (**b**). Due to the unexpected high deviation of the tensile strengths between the manufacturer's specification and the test results in Table 1, scanning electron microscopy (SEM) images were obtained in order to confirm the \"true\" diameter of the fibre. In this connection, removal of the brass coating from the steel surface of the fibre was inevitable. No chemical removal was possible because it might have affected the steel of the fibre. Removing the brass coating with sandpaper was also unrewarding because it also damaged the steel's material. The third possibility was an embedment of the steel fibre in UHPC and a careful pullout with a precision testing machine. The embedment in concrete seemed to be a good solution, which is why the pulled out fibre was prepared for SEM images. Figure 4 shows the steel fibre's surface with brass coating (a) and without brass coating (b) in SEM. The steel fibre's surface (VFS-A) with brass coating in Figure 4a is quite smooth. Only some faults are noticeable on the surface with a very slight depth. In contrast to that, the fibre's surface without brass coating in Figure 4b shows many gouges located mostly in the longitudinal direction of the fibre. The production process might have caused these gouges and the brass coating hides them underneath. As the cross-sectional area is very important for the calculation of the tensile strength of the micro steel fibre, accurate determination of the gouge's depth was attempted in additional SEM investigations. Instead of the micro steel fibre's surface, the cross-sectional area should reveal the depth of the gouges. The fibre had to be located parallel to the electron beam for visualization. Nevertheless, a steel fibre located in the electron beam's direction works like an antenna.