| Summary: | Abstract Assisted ultrasonic vibration technique can significantly improve the efficiency of hard rock drilling in petroleum and mineral engineering. In this study, to determine the fragmentation mechanism of rocks under ultrasonic vibration, numerical simulations using the discrete element method (DEM) were performed. A novel flat‐joint model (FJM), combined with an ultra‐high‐frequency loading boundary condition, was used to model the damage process of the hard rock under ultrasonic vibration loading. The numerical results demonstrated that the evolution of local strain and fragmentation were in good agreement with the experimental results. Based on the established model, the influence of loading parameters was investigated. Furthermore, by analyzing the development of the full strain field, crack orientations, and crack distribution, the fragmentation mechanism was revealed for the rock under ultrasonic vibration. Under ultra‐high‐frequency loading, the rock deformed in a heterogeneous manner and produced both compressive and tensile strain zones. The compressive zones were mainly distributed in the fringe and tensile zones in the top center. The generated tensile cracks caused by compression and tension in these two strain zones led to the rock failure.
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