Study On Mechanical Properties And Energy Evolution Of Cross-fissured Sandstone Under Cyclic Loading And Unloading

During the excavation of mineral resources,the rocks are subjected to repeated loading and unloading action,which leads to the mutual conversion of different forms of energy inside the rocks and the change of mechanical properties.Under the action of cyclic loading and unloading,micro-cracks in the rock begin to initiation,development and penetration,and eventually lead to rock instability and failure.Therefore,it is of great significance to study the mechanical properties and energy evolution characteristics of cross-fissured sandstone under cyclic loading and unloading for the construction safety of underground rock mass engineering.In this paper,the surrounding rock of coal mine roadway was taken as the research object.Through conventional triaxial compression test,triaxial cyclic loading and unloading test and acoustic emission test,the influence law of crack type,confining pressure and cyclic loading and unloading on the mechanical properties,failure mode and acoustic emission characteristics of fissured sandstone was studied.Based on the energy dissipation theory,the energy evolution law of fissured sandstone under different confining pressures was revealed.The main research results are as follows:(1)Based on the indoor test results of intact and cross-fissured sandstone specimens,the mechanical characteristics and damage modes of sandstone under triaxial compression and triaxial cyclic loading and unloading conditions were analyzed.The results show that cyclic loading and unloading has damage effect on sandstone,which makes the peak strength lower than the triaxial compression test,and the degree of breakage of the specimen increases.With the increase of the number of cycles,the elastic modulus of cross-fissured sandstone specimen decreases gradually after cyclic strengthening,and hysteretic loops and residual strains are generated in each cycle.The hysteretic loops change from thin to dense,and irreversible plastic deformation increases gradually.The peak strength,elastic modulus and peak strain of the cross-fissured sandstone specimens gradually increased with the increase of the initial surrounding pressure.With the increase of fissure dip angle,the peak strength,elastic modulus and peak strain of the specimens showed a"U"shape trend of decreasing and then increasing.The damage mode of the specimen is mainly splitting damage,with the increase of the surrounding pressure there is a tendency to change to shear damage,with the increase of the fissure inclination angle from splitting damage to mixed tensile-shear damage and then to splitting damage.(2)Based on the principle of energy calculation,the energy evolution law of cross-fissured sandstone under different conditions was explored.The results show that the total energy,elastic energy,dissipated energy and damping ratio of cross-fissured sandstone specimens increase with the increase of the number of cycles,and the proportion of elastic energy gradually increases,while the proportion of dissipated energy gradually decreases.With the increase of initial confining pressure,total energy,elastic energy and dissipated energy increase,damping ratio decreases,U_e/U gradually decreases,U_d/U gradually increases.With the increase of crack angle,the total energy,elastic energy and dissipative energy decrease first and then increase,U_e/U decreases first and then increases,and Ud/U increases first and then decreases.By introducing the energy dissipation coefficient,it is found that the brittle characteristics of fissured sandstone are more obvious under low surrounding pressure,and the brittle characteristics of intact sandstone and 0°dip and 90°dip fissured sandstone are more obvious.(3)Based on the acoustic emission test results of cross-fissured sandstone under different surrounding pressure,the characteristics of sandstone acoustic emission evolution under triaxial compression and triaxial cyclic loading and unloading conditions were analyzed.Acoustic emission information under cyclic loading and unloading has a memory function.The acoustic emission ringing count rate and amplitude increase first and then decrease in each cycle of cyclic loading and unloading.Kaiser effect appears in the early and middle stages of the cycle stage,and Felicity effect appears as the number of cycles increases.With the increase of initial confining pressure,the maximum value of acoustic emission ringing count rate and amplitude of cross-fissured sandstone specimens both increase,and a time lag effect appears.With the increase of crack angle,the acoustic emission ringing count rate and the maximum acoustic emission amplitude showed a U-shaped trend of decreasing first and then increasing.