Study On Mechanical-Acoustic Emission Characteristics Test And Discrete Element Simulation Of Sandstone Under Tensile-shear State

Tension-shear composite stress state is common in valley undercutting and overhanging dangerous rock conditions.Tensile-shear failure is a failure mode produced under combined tension-shear stress.The fracture surface is parallel to the shear stress and also perpendicular to the normal stress,so the rock is susceptible to disturbance and instability.Therefore,it is particularly important to study the strength and failure evolution law of rock under the combined tension and shear stress,which can provide reference for the protection and treatment of tunnels or dangerous rock projects.The research in this article is completed on the basis of the National Natural Science Foundation of China(52004072),the Guizhou University Introduced Talents Research Project(201901),and the Guizhou University's Natural Science Special Research Fund Project(201903).The current tensile-shear test methods are mostly based on specific shape specimens and complex instruments,based on conventional test instruments,we cannot obtain rock tensile-shear mechanical properties efficiently and quickly.Based on this,we independently designed a set of tensile-shear devices that can be directly used on conventional uniaxial compression testing machines,and used sandstone as the research object,combined with acoustic emission detection technology,and carried out tensile shearing of sandstone under different conditions.Experiments and numerical simulation studies have been carried out as well.The main research work and conclusions are as follows:(1)Tensile-shear tests of sandstone under different height-to-diameter ratios,different sizes,different indenter shapes and different base sizes were carried out.The strength characteristics and crack development laws of sandstone specimens under different variables were studied and the sandstone failure mode under combined tension-shear stress.The results show that the greater the height-to-diameter ratio,the greater the peak load of the sandstone specimen;the smaller the size of the base,the greater the peak load of the sandstone specimen;under the same conditions,the peak load of the square indenter is greater than that of the circular indenter;when the specimen is doubled in the same proportion,the fluctuation of the peak load is small.The overall failure mode of the sandstone specimen is mainly tensile-shear failure.the sandstone in contact with the tensile-shear device first undergoes shear failure,then the entirety undergoes tensile-shear failure.Most of the specimens show the effect of truncated cones or ridges after failure:the smaller the ratio of height to diameter and the larger the size of the base,the better the potential tensile-shear failure surface of the specimen;the fracture mode is similar when the specimen size is enlarged;the apex of the head is more prone to stress concentration.(2)During the test,combined with acoustic emission detection technology,the data of ringing count,event number,event location,rise time and average frequency of sandstone during the whole test process were monitored and analyzed.The results show that the development of acoustic emission ringing count rate and event count rate are divided into four stages: quiet stage,initial stage,intense stage and residual stage,which are consistent with the rising and falling trend of load-displacement curve;when the specimen ruptures,the initial stage cracks appear at the bottom and top and subsequent cracks develop mainly in the middle of the specimen;the larger the height-to-diameter ratio,the greater the proportion of overall shear cracks;the square indenter produces more tensile cracks than the circular indenter;the size of the base The larger the value,the greater the number of tensile cracks when the sandstone fails in tension and shear.(3)Used the PFC3D particle flow program,established numerical models of sandstone under different conditions,combined the uniaxial compression stress-strain curves of sandstone specimens,calibrated the meso-mechanical parameters of sandstone materials and simulated the tensile and shear tests of sandstone under different conditions.Analyze the meso-evolution characteristics of tensile-shear cracks in sandstone.The results show that the numerical simulation curve is in good agreement with the actual test curve.The crack initiation position first appears at the contact position with the tensileshear device,and then the cracks mainly propagate in the middle of the specimen.The fracture surface cracks are tensile cracks and shear cracks.Mainly,the changes in the position and number of cracks during the propagation process are consistent with the load-displacement curve.As the height-to-diameter ratio increases,the simulated tensile cracks and shear cracks increase;the number of cracks under different indenters is not much different;the size of the base increases,and the number of tensile cracks and shear cracks decreases.Using displacement field and contact force chain analysis methods,the evolution and development of meso-cracks can be observed intuitively.