## 6 Conclusions and Outlook  
The experimental investigations presented in this paper were carried out within the framework of the DFG-SPP 2020. They included uniaxial tensile tests on high-strength micro steel fibres as well as pullout tests of embedded fibre groups, each under monotonic and cyclic loading. In addition, extensive efforts were made to further localise the damage mechanisms on the fibre surface and in the fibre cross-section with the help of SEM images.  
The experimental investigations and the evaluations of the imaging methods provided the following findings:  
* The stress-strain relationship of the high-strength micro steel fibres showed no considerable ductility like conventional steel. The failure is brittle.
* The brass coating of the high-strength micro steel fibre hides gouges located in the longitudinal direction of the fibre underneath. The gouges reduce the cross-sectional insignificantly and do not affect the tensile strength in a considerable manner.
* The mean S/N curve of the high-strength micro steel fibre showed in the first instance no good agreement with prestressing steel. After converting the mean S/N curve into characteristic values, a comparison with prestressing steel was possible and provided relatively good agreement for the high-cycle fatigue. Equation (4) provides a recommendation for a mathematical formulation of the S/N curve.
* The used high-strength steel fibres always pulled out of the concrete matrix under monotonic loading, i.e., no fibre rupture occurred. The maximal pullout resistance in the test program was approximately one-third of the fibre's tensile strength. After the peak load, the pullout curves show an almost linear decreasing course of curves.
* Under cyclic loading, fibre rupture occurred partly at orientations of 60\({}^{\circ}\) but not for 30\({}^{\circ}\) and 90\({}^{\circ}\). The fatigue curves of the un-cracked micro steel fibres showed the typical course of curves like fatigue of concrete: strain stabilisation, linear strain increase, and exponential strain increase.  
Since the experimental investigations showed that fibre breakage might occur for some embedded lengths and orientations, the current design (slanderness) of the fibres \(l_{t}\varnothing_{t}=13.00.19\approx 68\) might not be suited for cyclic loading. Damaging bending effects might have large impacts in thin fibres so that the slenderness should be considered to be below 68 [-] for fibres subjected to cyclic loading. However, an accurate determination of the fibre design (geometry and tensile strength) that is suitable for cyclic loading requires further fundamental investigation in terms of pullout tests with varying fibre slenderness.  
Figure 12: Test specimens VM after cyclic loading with a fibre orientation of (**a**) 90\({}^{\circ}\), (**b**) 60\({}^{\circ}\), and (**c**) 30\({}^{\circ}\).  
**Author Contributions:** Conceptualization, methodology, formal analysis, investigation, resources, visualization, data curation, J.-P.L. and M.E.; writing--original draft preparation, J.-P.L.; writing--review and editing, supervision, project administration, funding acquisition, M.E. All authors have read and agreed to the published version of the manuscript.  
**Funding:** German Research Foundation: Priority Program 2020 (DFG number: 353961703).  
**Acknowledgments:** This cooperative research project conducted at iBMB, Division of Concrete Construction, and Institute of Structural Analysis, both at TU Braunschweig, is part of the Priority Program 2020, which is funded by the German Research Foundation (DFG). The authors acknowledge the financial support.  
**Conflicts of Interest:** The authors declare no conflict of interest.