Acoustic Emission Source Characteristics Of Granite Creep Damage And Its Evolutionary Pattern

Rock creep is one of the main causes of ground pressure activity in metal mines,and it is of great engineering significance to explore the deformation and damage mechanisms and prediction methods in the process of rock creep under different stress conditions.In this paper,a graded loading creep acoustic emission source localization test is conducted on granite,and the relationship between creep characteristics,acoustic emission characteristic parameters and loading stress is investigated,and an improved steady-state creep rate method is proposed as a long-term strength estimation method for rocks.Based on the simplex algorithm,the distribution characteristics of the damage creep acoustic emission sources are analyzed,and the evolution of the sources along the loading direction and the vertical loading direction,as well as their spatial fractal dimensional characteristics,are studied.On this basis,the moment tensor analysis method is used as the main research tool to analyze the magnitude,absolute energy and principal frequency characteristics of the shear,tensile and hybrid acoustic emission sources during creep damage.The following main conclusions were obtained.The following main conclusions were obtained.(1)Compared with the uniaxial compression specimens,the angle between the inclination of the rupture surface and the maximum principal stress decreases in the creep specimens,with the mean value being about 76°,and the creep specimens are mainly shear-type damage;the long-term strength of granite is determined to be about 61.1 MPa using the improved steady-state creep rate method,which accounts for about 81.5%of the mean uniaxial compressive strength.With the increasing creep stress,the instantaneous strain showed a trend of decreasing first and then stabilizing;the creep rate curve in the deceleration,isokinetic and accelerated creep stages during creep damage was"U"shaped,showing a trend of decreasing first,then basically constant and then increasing suddenly.(2)Relatively speaking,when the creep stress is less than the long-term strength,the average number of acoustic emission events and the average energy does not change significantly as the creep stress increases,but the average amplitude of acoustic emission tends to decrease,and when the creep stress is greater than the long-term strength,the average number of acoustic emission events,the average energy and the amplitude increase significantly.To a certain extent,the sudden increase in cumulative energy can be taken as a precursor to the rock entering the accelerated creep stage.(3)When the creep stress is close to the long-term strength,the seismic sources are mainly distributed at the upper and lower ends of the specimen,and the range of seismic source z_ivalues along the loading direction is less in the range of 42mm～58mm;the seismic sources in the vertical loading direction are gradually approaching the axis from the side walls,and the seismic source _ir values are almost non-existent in the range of 0mm～5mm.When the creep stress is greater than the long-term strength,in the deceleration,equal creep phase,along the loading direction seismic source z_i values are mainly in the range of 0mm～42mm and58mm～100mm;vertical loading direction seismic source _ir values are mainly located in the range of 6mm～25mm in the outer core circle.During the accelerated creep phase,the seismic sources converge from the end to the middle of the specimen in the loading direction;perpendicular to the loading direction,the seismic sources converge rapidly from the outer wall of the specimen to the axis and penetrate the specimen.When the creep stress is close to the long-term strength,the shear sources converge in the middle and lower part of the specimen;the tension sources converge towards the middle of the specimen,and the mixed sources become more and more numerous at the ends.When the creep stress is greater than the long-term strength,the shear,tension and hybrid sources are densely distributed in the interior of the specimen.(4)During the creep damage process,the number of shear-type sources was the largest,accounting for 67.3%～75.2%of the total number of sources;the number of tension-type sources was the second largest,accounting for 15.5%～23.1%of the total number of sources;the number of hybrid-type sources was the smallest,accounting for 9.2%～17.8%of the total number of sources.For the magnitude,as the creep stress increases,the magnitude of the hybrid,shear and tensile sources tends to increase gradually.When the creep stress is greater than the long-term strength,the mean value of the shear source magnitude is the largest and the mean value of the hybrid source magnitude is the smallest.(5)When the creep stress is less than the long-term strength,the main frequency range is mainly located at 230k Hz～340k Hz at first,and as the creep stress increases,two main frequency bands are added,ranging from 30k Hz～35k Hz and 100k Hz～130k Hz,respectively.When the creep stress is greater than the long-term strength,the main frequency bands of the deceleration phase and the equal creep phase increase and are located from 25k Hz～45k Hz,90k Hz～145k Hz,215k Hz～350k Hz.In the accelerated creep stage,the main band is significantly broadened,and the main band can start to be significantly broadened as an identification method for stepping into the accelerated creep stage.In creep damage,the main band ranges of shear and tension-type sources are similar in the deceleration and isokinetic creep stages,and their main band ranges are larger than the main band ranges of hybrid sources.(6)During creep,the elastic strain energy,dissipation energy and creep stress are exponentially related.When the creep stress is greater than the long-term strength,the elastic strain energy and dissipation energy gradually increase in the deceleration creep stage and the equal creep stage,and in the acceleration creep stage,the dissipation energy suddenly increases,and the elastic strain energy first drops briefly and then suddenly increases.In the accelerated creep stage,the dissipation energy suddenly increases,and the elastic strain energy drops briefly and then suddenly increases.