## 1 Introduction  
Ultra-high-performance concrete (UHPC) offers enormous possibilities in relation to thin-walled, light, and filigree concrete constructions. Thus, UHPFRC concrete constructions have substantial advantages over conventional reinforced concrete constructions when considering important ecological aspects like saving of material, resource efficiency, and sustainability, as well as the load bearing capacities.  
Due to the very brittle failure of plain UHPC under tensile and compressive loading, recent research projects (e.g., [1, 2]) as well as several European standards (Swiss SIA [3], French national additions to Eurocode 2 [4]) recommend a fibre reinforcement in UHPC-components, leading to ultra-high-performance fibre-reinforced concrete (UHPFRC). UHPFRC usually requires high steel fibre contents (up to 250 kg/m\({}^{3}\)) to provide a ductile failure mechanism. Besides the pure addition of steel fibres and in regard to the aforementioned ecological aspects, the design and the performance of micro steel fibres in UHPFRC under monotonic and cyclic tensile loading is of great importance.  
The fatigue behaviour of a material is quite complex. High-strength micro steel fibres embedded in UHPC provide a ductile failure mode under monotonic loading. However, the effect of cyclic loading might be rupture, which leads to sudden failure of the components.  
[PERSON] [5] ran tests on UHPFRC beams under bending with an almost fibre-free tension zone. He found that the fatigue strength under cyclic loading decreases considerably after only a few load cycles and is clearly below the \"static\" strength.  
[PERSON] [6] investigated the flexural fatigue behaviour of un-cracked UHPFRC-specimens under cyclic loading and ascertained that the fatigue strength of the concrete matrix is in the lower scatter range of the \"static\" strength. In contrast, [PERSON] [7] observed a positive fatigue behaviour of cracked UHPFRC influenced by steel fibres.  
[PERSON] [6], [PERSON] [8], [PERSON] [9], [PERSON] [10], and [PERSON] [11] investigated the pullout behaviour of embedded fibres in UHPC more fundamentally. The test programmes consisted of fibre pullout tests of embedded UHPC under monotonic loading with varying steel fibre geometries, UHPC-properties, and number of fibres. All tests showed a continuous fibre pullout. [PERSON] [12], [PERSON] [6], [PERSON] [13], and [PERSON] [14] also observed continuous fibre pullout but under cyclic tensile loading. However, these observations rest upon UHPFRC-specimens with a 3-D fibre orientation and not upon pullout tests.  
Beyond this, [PERSON] [6] also recognised fibre rupture under cyclic loading during the test's execution. Actually, the geometric design of steel fibres and the high tensile strength always provide continuous fibre pullout. The bond stress between the fibre and UHPC must have been so high that fatigue failure of the steel fibre material occurred un-der the cyclic loading.  
As part of a tandem research project funded by the German Research Foundation under the Priority Program 2020 (Ger. SPP 2020), various damage processes in UHPFRC under monotonic and cyclic tensile loading were investigated at the iBMB, Division of Concrete Construction of TU Braunschweig. The following article reports the experimental and theoretical investigations.