:Experimental Study On Damage Characteristics Of Granite Under Microwave Cyclic Irradiation

In the process of tunnel excavation,microwave irradiation technology can effectively cause thermal damage inside the rock,induce the initiation and propagation of micro cracks,so as to reduce the difficulty of construction and improve the efficiency of rock breaking.The environmental temperature of the working face is too high after high power microwave irradiation,which is not conducive to the safety of machinery and workers.Low power cyclic irradiation can not only meet the requirements of cracking,but also reduce the construction temperature of the working face.Therefore,it is of great theoretical significance and application value to study the effect of low power cyclic irradiation on rock damage.Firstly,the fracture mechanism and damage characteristics of granite under microwave are analyzed;Secondly,the variation laws of temperature rise characteristics,longitudinal wave velocity and macroscopic mechanical properties of granite under different microwave power,cycle times and cooling methods were studied,and the damage mechanism and failure mechanism of microwave irradiation on granite were revealed from the macro and meso levels.Finally,based on the damage mechanics theory and rock constitutive theory,the damage constitutive model of granite considering multi-factor coupling is established.The main research results and contents are as follows:(1)With the increase of microwave power,the final surface temperature of granite sample increases continuously,and the decrease rate of longitudinal wave velocity is faster.When the microwave power is the same,the longitudinal wave velocity of the sample decreases with the increase of heating times.Under the same microwave effect,the decrease of the longitudinal wave velocity of the specimen under the water cooling mode is higher than that under the natural cooling mode,which is the result of the combined effect of microwave irradiation and water cooling.(2)The fracture morphology of granite was microscopically tested,and the effects of microwave power and irradiation times on the crack morphology and development characteristics of granite fracture were analyzed.It was found that the crack forms of the fracture morphology of the specimen were intergranular crack,transgranular crack and intragranular crack.The opening,number and density of cracks are related to microwave power and heating times,and the degree of crack development increases with the increase of microwave power and heating times.(3)Quasi-static uniaxial compression tests were carried out on granite irradiated by microwave.With the increase of microwave power and irradiation times,the stress-strain curve gradually moves to the lower right;after microwave irradiation,the mechanical properties of granite decreased to varying degrees,in which the peak stress and elastic modulus decreased,the peak strain increased,the ejection ability of fragments decreased and the fragmentation degree increased.The way of water cooling has a certain effect on the damage degree of the sample,but the effect is far less than the number of irradiation.(4)The damage characteristics of granite irradiated by microwave were analyzed by different methods.The damage factor was positively correlated with the increase of microwave parameters and heating times.Based on the strain equivalence principle,the granite damage variable considering the influence of multiple factors was deduced,and the pore closure characteristics of the specimen at the initial loading stage were characterized.The granite damage constitutive model considering the microwave effect was established,and the model parameters were determined.The reliability of the constitutive model was verified by comparing the theoretical curve with the experimental curve.In this paper,the mechanical properties and damage characteristics of rock caused by low power microwave cyclic irradiation are tentatively explored.The regular changes between rock damage and microwave power and cyclic irradiation are summarized,which promotes the development of microwave assisted rock breaking technology.At the same time,it also provides theoretical basis and reference for future engineering development.