Fracturing Mechanism And Application Of Hard Rock Under Microwave Treatment

Microwave-assisted rock-breaking has the advantages of green,high efficiency and low energy consumption,which is a promising method to achieve the efficient excavation of hard rock strata in deep coal resources development.Therefore,the mechanism of Microwave-assisted rock-breaking has attracted the attention of engineering and academic fields.In this study,based on the difference of microwave absorption capacity of granite minerals,by using the continuum analysis of microwave electromagnetic and discrete element mechanics model,the micro mineral temperature discrete assignment was implemented.And a method for calculating and analyzing the mechanical properties of granite under microwave treatment is proposed and verified as a reliable approach in term of temperature field,mechanical properties and failure pattern.Then,the fracture evolution,mechanical behavior and fracture mechanism under the uniaxial/triaxial loading conditions of the microwave treated granite specimen,were revealed by numerical simulation.Whereafter the simulation of granite rock breaking by cutter after microwave treatment is carried out to analyze the influence of different waveguide positions and microwave power on the effect of microwaveassisted rock breaking.The main work and conclusion of this thesis is as follows:(1)The heterogeneous granite grain-based model has been established and verified.The particle flow model of granite was established,then the influence of contact microscopic parameters on macro-mechanical parameters such as uniaxial compressive strength,elastic modulus and Poisson’s ratio of granite was analyzed,finally the relationship between macro-mechanical parameters and micro-mechanical parameters was obtained.It is found that the uniaxial compressive strength is greatly affected by the microscopic bond strength,the macroscopic elastic modulus is related to the microscopic contact modulus,and Poisson’s ratio is mainly affected by the stiffness ratio.At last,the heterogeneous granite GBM model is constructed and its reliability is verified.(2)The continuous-discrete calculation method of granite fracture process under microwave is established and the heating rule and fracture mechanism of granite under microwave are investigated.By means of microwave electromagnetic analysis of continuum basis,the macroscopic characterization of granite temperature under microwave treatment is described.Based on the heating characteristics of mineral absorption to microwave and GBM model,the discrete temperature assignment of microscopic minerals is realized.A discrete element method for the mechanical property analysis of granite under microwave treatment is proposed,and the reliability of this method is verified from the temperature field distribution and mechanical properties.It is found that two relatively low temperature regions and one hot spot appear in the sample after microwave treatment,and these two regions become more obvious with the increase of microwave power.Under the condition of low microwave power(≤ 2k W),the number of microcracks in the sample is very small.At high microwave power(≥ 3k W),the microcracks of the sample developed gradually dominated by the grain boundary tensile failure,and the microcrack net was formed with the hot spot area as the center,and finally the macroscopic cracks appeared.(3)The uniaxial compression simulation,mechanical behavior and fracture mechanism of the damaged granite model after microwave treatment are studied.The uniaxial compression simulation experiment was carried out on the standard sample model after microwave treatment.It was found that the peak stress,elastic modulus and damage threshold of the sample all decreased with the increase of microwave power,and these mechanical parameters dropped more obviously under the treatment of high microwave power(≥3k W).Combined with the fracture phenomenon,it can be seen that there is no significant difference in the crack evolution and failure pattern of the specimens under load when the microwave power is less than 2k W.When the microwave power was higher(≥3k W),the microcrack after microwave irradiation dominated the crack evolution in the loading process,and the specimen showed a tendency to propagate along the initial crack and appeared significant local failure,resulting in the loss of bearing capacity of the sample.(4)The mechanical behavior and fracture characteristics of granite under triaxial compression under different confining pressures and irradiation conditions are simulated and analyzed.Combined with the uniaxial compression results,it was found that for triaxial compression,with the increase of microwave power,the initial confining pressure increased the strength of specimen.At low microwave power(≤2k W),the increase effect of the confining pressure on the elastic modulus is not obvious.At high microwave power(≥3k W),the increase effect of the initial confining pressure on the elastic modulus is particularly significant.According to the failure pattern,the above result can be explained as follows: with the increase of microwave power,the confining pressure promotes the continued bearing and stress transfer of broken particles,making the failure mode of samples change from intergranular failure leading to intragranular failure leading.Furthermore,the crack network can be fully developed and the concentrated failure of local structure caused by high power(≥3k W)irradiation can be inhibited.(5)A model of two-dimensional constant section cutter intruded into microwave treated rock block is established.The influence of waveguide position and microwave power on microwave-assisted rock breaking is studied.The results show that when the waveguide is directly facing the center line of the single cutter,with the increase of microwave power,the maximum invasion force during the whole process is unchanged first and then decreases,and the degree of breakage increases with the increase of the microwave power.When the waveguide is facing the middle slot of the double cutters,the decreasing trend of the maximum invasion force is similar to that under the condition of single cutter,but the decreasing degree of breakage is bad.Finally,it is considered that better microwave-assisted rock breaking can be achieved under the condition that the waveguide is directly facing the center line of the single cutter,and the microwave power selection of 30-60 k W is appropriate.