Fracture behavior of ultra-high performance concrete under complex stress states

Ultra-high performance concrete （UHPC） holds broad application prospects in hydraulic engineering， yet its fracture behavior under complex stress states remains unclear. This paper investigates the fracture properties of UHPC under combined tensile-shear stress through an integrated experimental and numerical simulation approach. Experiments were conducted on slant-notched short beam specimens， systematically varying the notch inclination angle and steel fiber volume content to analyze the effects of fracture mode mixity and fiber content on crack initiation， propagation， and load-bearing capacity. A three-dimensional meso‑scale numerical model was developed to analyze the fracture failure of UHPC under complex stress states by explicitly modeling fiber degrees of freedom and adopting appropriate constitutive relations for the matrix and matrix-interface bond-slip behavior. The research findings indicate that the critical state for mixed-mode fracture of the UHPC matrix conforms to the maximum energy release rate criterion. An increase in fiber content and a higher shear component in the combined stress state enlarges the fracture process zone and intensify matrix spalling. The developed 3D meso‑scale numerical model can successfully replicate the experimental results and effectively predict the fracture failure characteristics of UHPC under combined stress states. This study enhances the understanding of UHPC's fracture behavior under complex stress and provides a reference for its safe design and application in hydraulic engineering.