Fatigue Damage Mechanism and Deformation Behaviour of Granite Under Ultrahigh-Frequency Cyclic Loading Conditions

Ultrasonic vibration-assisted rock breaking is a potentially effective technique to accelerate hard rock drilling processes. Fatigue damage is a primary factor that governs rock fragmentation subject to ultrasonic vibration, and when such damage accumulates to a critical level via crack initiation and propagation, macro-damage (e.g., macro-cracks) will occur. To date, however, the specific fatigue damage mechanism of hard rock materials under high-frequency and low-amplitude cyclic loading conditions is still unclear. In the present study, we applied a 2D digital image correlation (2D-DIC) method to measure the full-field strain in granite samples with different loading amplitudes. From these deformation data, the threshold value for rock fragmentation under ultrasonic vibration was obtained, and it was also found that the logarithm of the time required to meet this value decreases linearly with an increasing amplitude coefficient. Then, we conducted numerical simulation based on a 2D particle flow code (PFC2D) to reproduce the crack initiation and propagation processes and explore their mechanisms. The results from the simulation show that due to irreversible sliding under ultrasonic vibration, the difference in the displacement between particles on either side of a crack tip will increase, which leads to an increase in the concentrated lateral tensile stress. When the tensile stress exceeds the strength limit, the crack will initiate and propagate, resulting in fragmentation of rocks.