Acoustic Emission Characteristics During Red Sandstone Creep Failure Under Different Water Condition

Rock creep damage is the main factor to induce metal ore pillar damage and roof caving,and the presence of water aggravates the complexity of rock creep damage.Therefore,it is of great engineering significance to explore the response characteristics and prediction methods of the rock creep damage process under different water-bearing conditions.In this paper,the creep emission test is carried out on red sandstone under dry,natural and saturated conditions with graded loading,and the influence of creep stress and water content on the acoustic emission activity is investigated by using source localization,moment tensor inversion,and fractal theory as the main research methods to analyze the creep characteristics,macro damage forms and their relationship with water content under different creep stress conditions.On this basis,the spatial and temporal evolution of the source values along the loading direction and the vertical loading direction of the specimen and their relationship with creep stress and water content state are analyzed.On this basis,the spatial and temporal evolution of the sourceZ_i-value along the specimen loading direction and the sourceR_i-value in the vertical loading direction are analyzed,as well as their relationship with creep stress and water content state;the identification method of microcrack nucleation,extension and penetration based on the fractal dimensional characteristics of the sourceZ_i-value and source R_i-value is proposed;the long-term strength estimation method based on the energy characteristics of the source is discussed,and the following conclusions are mainly obtained.(1)With the increase of water content,the axial strain and transverse strain at creep damage tended to decrease,and the long-term strengths of dry,natural and water-filled specimens were 44.08MPa,40.41MPa and 31.8MPa,respectively.(2)When the creep stress is less than the long-term strength,the acoustic emission activity is weak,and the main frequency range of acoustic emission is small,and the number of frequency bands is small.When the creep stress is greater than the long-term strength,the acoustic emission activity increases with the creep stress,and the main frequency range increases,and the number of frequency bands becomes larger.In the accelerated creep stage,high-frequency sources start to appear,and the main frequency range of the sources continues to increase,and the number of frequency bands increases significantly.With the increase of the specimen’s water content,the main frequency range and the number of rings in the accelerated creep stage are larger,but the energy rate is relatively low.(3)When the creep stress is less than the long-term strength,the number of sources decreases with increasing creep stress,and the spatial and temporal evolution characteristics of the sources reflect to some extent the process of the compression of the primary microcracks.When the creep stress is greater than the long-term strength,the number of sources increases with the creep stress,and the number of sources decreases with the increase of water content.The spatial and temporal evolution of the seismic sources is,to some extent,a reflection of the process of nascent crack emergence,nucleation,expansion and penetration.Microcrack emergence is mainly generated in the deceleration creep stage,nucleation is mainly generated in the isokinetic creep stage,and microcrack expansion and penetration are formed in the accelerated creep stage.For the creep damage of each specimen,the shear type seismic source accounted for about 70%,the tension type seismic source accounted for about20%,and the mixed type seismic source accounted for about 10%.(4)The spatial and temporal evolution trends of the seismic sources under the last level of creep stress are different for specimens with different water content.In the loading direction of the specimen,the seismic sources of the dry group gather from the two ends to the middle,the seismic sources of the natural group gradually expand from one end to the other,and the seismic sources of the water-saturated group expand from the top to the bottom of the specimen.In the direction perpendicular to the specimen loading,the seismic sources of each group of specimens are gathered from the outer or middle ring of the specimen to the core area.(5)The fractal dimension of the source coordinateZ_i-value along the loading direction and the source radiusR_i-value perpendicular to the loading direction gradually decreases during the deceleration creep phase and stabilizes during the isokinetic creep phase.To a certain extent,the stabilization of the fractal dimension curves of the sourceZ_i andR_i can be taken as a sign that the microcracks have converged into a nucleus.In the accelerated creep stage,the fractal dimension ofZ_i-value andR_i-value of the seismic source coordinates continue to increase.To a certain extent,the beginning of the increase in the source space’s fractal dimension can be taken as a sign of microcrack expansion and penetration,and this sign can be used as a precursor feature of creep damage in red sandstone.(6)As the water content of the specimen increases,the acoustic emission source energy at each creep stress becomes smaller.To some extent,the creep stress corresponding to the lowest average source energy or average shear energy can be taken as the long-term strength value of the specimen.