Damage law and acoustic emission characteristics of three types of hard rocks under low power microwave irradiation

Microwave-assisted rock breaking is a new method to achieve efficient and low-damage excavation of hard rocks. There are some differences in the wave absorption capacity, water loss, warming and damage laws of different hard rocks, and the comparative study has important theoretical and engineering significance for accurate and efficient rock breaking. By comparing the mass changes of the specimens before and after pretreatment and microwave radiation, the water loss law of the three types hard rocks were obtained. The whole process of surface temperature rise and fracture of hard rock in microwave field is monitored by infrared thermal imager, and the temperature rise and macro crack propagation characteristics of three types of hard rock under different water content were obtained. The mechanism of hard rock damage effect with increasing radiation energy was elaborated by the changes of longitudinal wave velocity, acoustic emission and SEM microstructure of three types hard rocks under different time of radiation in dry condition. The results show that the effect of water in the lower power microwave radiation within 300 s is as follows: the evaporation of water is the dominant factor in the mass loss of hard rock, water has a significant role in promoting the warming of hard rock, and the degree of influence of water content on the average temperature of the surface area is divided into three stages according to the radiation time “small(0-50 s), large(50-200 s), stable(200-300 s)”, in which the saturated water absorption has the most significant effect on the temperature rise and the sandstone and limestone with high saturated water absorption burst in the cavity at 55 s and 240 s respectively. The damage law of different hard rocks with temperature rise in dry state is as follows: the crack propagation of limestone is more obvious than that of sandstone and granite due to large surface temperature difference. COMSOL numerical simulation of continuous radiation for 420, 600 and 720 s has a good correlation with the change trend of the damage factor obtained from the experiment. After the three types of hard rock damage, the acoustic emission of the specimens presents three stages of stability, growth and failure, and the uniaxial compressive strength loss is from fast to stable, and the ring count and energy are different in order of magnitude; At the micro scale, the fracture surface of granite and sandstone after 720 s of microwave radiation appears the bonding of molten minerals, which leads to the slow reduction of the longitudinal wave velocity, which explains the internal cause of the damage stabilization; With the increase of the radiation time, the mass loss, the damage factor characterized by the wave velocity, the heating rate and the strength loss obtained by the numerical simulation all show a consistent change law from fast to slow and a “inflection point” from fast to stable. The research results can provide some reference for the differentiation, low energy consumption and high efficiency damage of different hard rocks. © 2023 Academia Sinica. All rights reserved.