The Full Face Rock Tunnel Boring Machine (TBM) is increasingly prevalent in China's tunnel construction. However, the significant wear of the disk cutter in extremely hard rock layers has resulted in low efficiency in rock-breaking, leading to project cost escalation and other contradictions. These challenges are increasingly prominent and hinder the TBM's normal usage. Microwave-assisted TBM disk cutter rock-breaking, a novel technology, is gaining attention from engineering researchers due to its advanced technology, safe and controllable nature, rapid heating, and environmental friendliness. In light of this, this study explores the damage mechanism of microwaves on rocks under various factors. Basalt from Chifeng, Inner Mongolia, was chosen as the study object. X-ray diffraction results revealed that the rock primarily consists of 65% plagioclase, 23.9% pyroxene, and 11.1% olivine. The Particle Discrete Element PFC2D simulation software was employed to model the three-phase mineral rocks by randomly distributing them based on mineral content ratios. To eliminate boundary effects and enhance simulation model efficiency, a mineral particle size of 1.7 mm to 2.0 mm and a rock model size of 400 mm × 160 mm were selected. The rock model’s left and right sides act as confining pressure boundaries, the bottom is a fixed boundary, and the top (Tunnel face) is a free boundary, with the initial rock temperature set at 25 ℃. Simplifying the heating effect of microwave radiation on the rock, the heating area of microwave radiation on the rock, and the attenuation of microwave with depth, enables a more efficient and intuitive study of the microwave radiation’s influence mechanism. To validate the numerical model calculation’s rationality, a 90 mm × 180 mm three-phase mineral rock model is established to analyze the microwave radiation rock cracks’ change rule and expansion characteristics. The study utilizes a dual waveguide with a microwave power density of 1 × 1010 W/m3 to radiate the rock. The effects of waveguide spacing (60 mm, 70 mm, 80 mm, 90 mm, 100 mm), radiation time (0 s, 0.04 s, 0.06 s, 0.08 s, 0.1 s), and confining pressure (5 MPa, 10 MPa, 15 MPa, 20 MPa) on rock damage, crack sprouting, and expansion are investigated. The results show that the change characteristics of rock cracks follow a consistent pattern during microwave irradiation. Cracks begin to appear when the microwave radiation time is 0.04 s, with no cracks sprouting before that time. Subsequently, as the microwave radiation time increases, the thermal expansion and thermal stress inside the rock gradually intensify, resulting in more bonding fractures between mineral particles. The number of cracks rises, leading to crack penetration between the dual waveguide radiation regions. Upon analyzing the location of damage cracks in relation to the distribution of three-phase minerals within the microwave radiation area, it becomes evident that these cracks predominantly occur between plagioclase feldspar and pyroxene. Additionally, a majority of the cracks terminate at olivine minerals. The impact of different waveguide spacings on rock cracks follows a consistent pattern: the variation in waveguide spacings exhibits a similar influence on the expansion of rock cracks. As radiation time increases, there is a growing occurrence of cementation breaks at the pyroxene-plagioclase junction, leading to the formation of extension cracks. For instance, considering the rock model with a confining pressure of 5 MPa, when the waveguide spacing is 60 mm, 70 mm, 80 mm, 90 mm, and 100 mm, the radiation time required for crack penetration between microwave radiation areas is 0.08 s, 0.10 s, 0.12 s, 0.145 s, and 0.15 s, respectively. The corresponding number of cracks generated inside the rock is 91, 163, 280, 495, and 462. The effects of different confining pressures on rock cracking exhibit consistency within the 20 MPa range. Comparative analysis, using confining pressures of 5 MPa and 10 MPa as representatives, indicates that, when exposed to microwave radiation for 0.08 s, the rock sample with a confining pressure of 10 MPa produces five fewer cracks than the sample with a confining pressure of 5 MPa. However, the remaining cracks are identical. Continuous radiation of rock samples at a confining pressure of 10 MPa for 0.10 s reveals the regeneration of the same cracks at the locations where the initial five cracks were absent. In summary, when utilizing microwave power density of 1 × 1010 W/m3 for basalt heating, a radiation time of no less than 0.04 s is necessary to achieve effective rock-breaking assistance. Microwave radiation induces large temperature gradients and thermal stresses between strong wave-absorbing minerals (pyroxene) and weak wave-absorbing minerals (plagioclase), resulting in damage cracks. The location of these cracks is closely related to the distribution of rock minerals. As waveguide spacing increases, the radiation time needed for crack penetration between microwave radiation regions gradually extends. Confining pressure exerts inhibiting and restraining effects on crack generation and expansion. Changes in confining pressure do not significantly impact the location and expansion path of damage cracks in rock samples. Compensating for the reduction of cracks due to increased confining pressure can be achieved by prolonging the microwave radiation time. © 2024 Sichuan University. All rights reserved.
