The objective of the contribution is to understand the fatigue bond behaviour of brass-coated high-strength micro steel fibres embedded in ultra-high performance concrete (UHPC). The study contains experimental pullout tests with variating parameters like load amplitude, fibre orientation, and fibre-embedded length. The test results show that fibres are generally pulled out of the concrete under monotonic loading and rupture partly under cyclic tensile loading. The maximum tensile stress per fibre is approximately 1176 N/mm2, which is approximately one third of the fibre tensile strength (3576 N/mm2). The load-displacement curves under monotonic loading were transformed into a bond stress-slip relationship, which includes the effect of fibre orientation. The highest bond stress occurs for an orientation of 30° by approximately 10 N/mm2. Under cyclic loading, no rupture occurs for fibres with an orientation of 90° within 100,000 load changes. Established S/N-curves of 30°- and 45°-inclined fibres do not show fatigue resistance of more than 1,000,000 load cycles for each tested load amplitude. For the simulation of fibre pullout tests with three-dimensional FEM, a model was developed that describes the local debonding between micro steel fibre and the UHPC-matrix and captures the elastic and inelastic stress-deformation behaviour of the interface using plasticity theory and a damage formulation. The model for the bond zone includes transverse pressure-independent composite mechanisms, such as adhesion and micro-interlocking and transverse pressure-induced static and sliding friction. This allows one to represent the interaction of the coupled structures with the bond zone. The progressive cracking in the contact zone and associated effects on the fibre load-bearing capacity are the decisive factors concerning the failure of the bond zone. With the developed model, it is possible to make detailed statements regarding the stress-deformation state along the fibre length. The fatigue process of the fibre-matrix bond with respect to cyclic loading is presented and analysed in the paper.

In ultra‐high performance concrete (UHPC) embedded micro steel fibres increase the ductility and improve the post cracking load‐bearing behaviour under tensile loading. For an efficent and economical dimensioning of components, especially for fatigue susceptible structures, it is necessary to characterise the crack‐bridging load‐bearing effect of the micro steel fibres and the associated processes taking place in the intersection between both material components. Thus, the development of model equations to predict the deformation and damage behaviour of UHPFRC with numerical simulations on the macro‐level is very useful. In the contribution, a material model, which describes the phenomenological damage processes of the composite material, is presented. The numerical realisation with the finite element method applies a hybrid‐mixed element formulation, where a direct coupling with the material model is performed. Numerical studies of the material behaviour of UHPFRC are presented by means of an illustrative example.