Abstract: Vibration contributes to crushing of rocks. Research on the mechanism of vibrating rock is usually carried out in the low frequency range. In order to fill in the gap of the mechanism of rock crushing by vibration of ultra-high frequency section, an experimental study on the effect of ultrasonic vibration on rock strength under different stress conditions was conducted based on static and dynamic loading models, under the condition that the ultrasonic vibration frequency was 20 kHz, and the preloading range was 100-500 N. It is demonstrated that the rock strength remained unchanged basically when the preloading was less than 200 N. The reason is that the internal stress state of rock did not meet the strength criterion. When the preloading was more than 200 N, the rock strength increased with vibration duration, and there is an optimal preloading value (400 N) to minimize the rock strength. The efficiency of ultrasonic vibration rock crushing can be improved by narrowing the difference between the vibration frequency and the natural frequency of the rock.

Key words: ultrasonic vibration, preloading, numerical simulation, compressive strength