Assisted ultrasonic vibration technology has received great interest in the past few years for petroleum and mining engineering related to hard rock breaking. Understanding the fatigue behaviour and damage characteristics of rock subject to ultra-high-frequency loading is vital for its application. In this research, we conducted ultrasonic vibration breaking rock experiments combined with an ultra-dynamic data receiver and strain gauges to monitor the development of strain in real time. The experimental results show that the strain curve is U-shaped, and it can be divided into three stages: the strain first decreases, then remains steady (with light fluctuations) and finally increases. The sample first underwent compressive deformation, and no rupture occurred. As the vibration continued, the compressive deformation decreased with the initiation and propagation of cracks, and fragmentation occurred. To elucidate the crack evolution mechanism of the granite specimens, numerical simulations were performed using particle flow code in two dimensions (PFC2D), and an improved fatigue damage model based on the flat-joint contact model was proposed. The numerical results indicate that this model can effectively reproduce the fatigue characteristics of hard brittle rocks under ultrasonic vibration. By analysing the stress and strain fields and cracking process, the crack evolution mechanism in the brittle hard rock under ultra-high-frequency loading is revealed. These experimental and numerical results are expected to improve the understanding of the fragmentation mechanism of rock under assisted ultrasonic vibration.

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.