Study On Dynamic And Static Mechanical Properties And Fracture Mechanism Of Hard Rock Under Microwave Irradiation

The gradual depletion of shallow resources and the increasing demand for resources have prompted the development of mining activities to deeper levels.Hard rock fragmentation is a prerequisite for tunnel construction,oil and gas drilling,deep geothermal development,and deep mining,and it is indispensable for safe and efficient mining of deep resources.Microwave-assisted mechanical rock fragmentation is considered to be a promising technical solution that can replace the traditional drilling and blasting method to achieve high-efficiency deep hard rock fragmentation.This paper focuses on the dynamic and static mechanics of hard rock under microwave radiation through theoretical analysis,experimental exploration and numerical simulation.The main research contents and conclusions are as follows:(1)The thermal response law of typical hard rock was obtained based on the microwave radiation test system.The temperature of the granite is positively correlated with the microwave radiation time,and the heating rate was positively correlated with the microwave power.SEM results show that intergranular cracks of minerals contribute the most to granite damage under microwave radiation.The pore size distribution and pore structure characteristics of granite were obtained by NMR,and microwave radiation can promote the expansion of granite micro-cracks,which is manifested in the increase of porosity,the increase of pore connectivity and the decrease of fractal dimension.Relationships between thermophysical properties,compressive and tensile strength and ultrasonic properties of microwave-damaged granite and microwave operation parameters were obtained.(2)The fracture properties and brittle-ductility transformation characteristics of microwave-damaged granite were obtained based on the basic static tests,and the weakening law of dynamic tensile strength was obtained according to the dynamic Brazilian splitting test;microwave radiation induced faster extension of tensile fracture and wider tensile fracture area of granite,while microwave radiation-induced microcracks promoted the dissipation of impact energy,and microwave treated could significantly reduce the loading rate dependent.The established dynamic tensile intrinsic model based on the maximum tensile strength theory,statistical damage model and tensile deformation volume damage model can well describe the dynamic tensile response of microwave-damaged granite.(3)The fracture properties of microwave-damaged granite were obtained,and the fracture toughness decayed exponentially with increasing heating time;the weakening of fracture brittleness of granite under microwave radiation was manifested by the prolongation of the active period of acoustic emission and post-peak stage and the increase of fracture process zone size,and the fractal dimension of microwave radiation damaged granite was exponentially related to the fracture toughness;Microwave radiation can significantly weaken the peak indentation load and reduce the specific energy,and the stripping of indentation fragments is mainly induced by tensile damage;the indentation fragment size is positively correlated with temperature and indentation depth;the specific energy of microwave radiation-damaged granite can be predicted by the specific energy model based on fracture mechanics.(4)Through the dynamic indentation test,it is found that the increase of temperature of granite under microwave radiation leads to the decrease of maximum intrusion force and the increase of maximum penetration depth,and the energy transfer efficiency and specific energy are negatively correlated with temperature;the increase of confining stress hinders the indentation fragmentation of granite under microwave radiation,which leads to the increase of energy transfer efficiency and specific energy,and the crushing volume can be described by the negative exponential function.The increase of impact velocity increases the crushing volume and energy transfer efficiency of microwave irradiated granite,and significantly reduces the energy consumption.(5)A discrete element method(DEM)based numerical model of microwave damaged hard rock was established.The parameters of the bonding model were calibrated by the temperature response and static and dynamic mechanical experiments,and the applicability of the model was verified.Numerical simulations of indentation breakage shows that microwave damage to hard rock can weaken the inhibitory effect of high confining stress on the expansion of damage zone.The simulation results of cutting and impact fragmentation of microwave damaged granite show that the increase of temperature can significantly reduce the average cutting force and promote the interactions between multiple impact teeth,which can improve fragmentation and significantly reduce specific energy.In addition,a drilling tool based on microwave radiation assistance was designed,and the feasibility of engineering applications of microwaveassisted mechanical hard rock fragmentation was analyzed.