Abstract: Research on the damage mechanism of rock under the action of water jet has always been a difficult problem due to high turbulence of water jet and complicacy of rock material. According to fully-decoupled fluid-structure interaction (FSI) theory, the standard k-ε two equations and control volume method for water jet, and the elastic orthotropic continuum and finite element method for rocks, are employed respectively to establish a numerical analyzing model of ultra-high pressure water jet impinging on rock. In terms of the stress distribution of rock interior, which is computed under submerged water jet with different velocities, and based on the theory that the rock micro-unit strength obeys Weibull probability distribution, a theoretical equation of damage variables evolution and a rock statistical constitutive model are developed, with Mohr-Coulomb damage criterion as distribution variable. A damage criterion, with non-dimensional coefficient to characterize rock damage, is also set up for analyzing rock failure mechanism with water jet. The process of jet impact on the rock is simulated, by using the FSI model, the statistical constitutive damage model and the damage criterion. Micro failure mechanism test and analysis with scanning electron microscope (SEM) for rock failure surface by jets cutting were performed, whose results show that the micro-mechanism of rock failure due to water jet impingement is a brittle fracture in the condition of tensile and shearing stress. The test results also agree well with the numerical simulation analysis, which constructs a bridge between the micro-failure mechanism and macro-breaking analysis of rock with water jets impact. The investigation affords a method for studying the mechanism of rock failure under ultra-high pressure water jet impingement.

Key words: Fluid-structure interaction, Rock damage, Scanning electron microscope, Ultra-high pressure, Water jet