Experimental Study On Damage Rupture Characteristics Of Sandstone Affected By Different Hydrostatic Stress And Lode Angles

In recent years,with the development of a large number of the deep rock excavation projects,high ground stress problems are increasingly outstanding.Many experts and scholars in related fields pay great attention to the stability of the deep surrounding rock during the excavation process.Therefore,sandstone is taken as the research object in this paper,and the independently developed test equipment named‘Multi-functional ture triaxial fluid-solid coupling experiment system' is used to carry out the loading and unloading test of sandstones under true triaxial stress conditions.This paper discusses mainly the effects of different initial hydrostatic stress and different Lode angles on the mechanical properties,acoustic emission characteristics,crack propagation and energy evolution law of sandstones.It is critical to further study the damage rupture mechanism of rocks.The main research results are as follows:(1)Performing the mechanical property experiments of sandstones under the conditions of different initial hydrostatic stress and different Lode angles.In the failure process of sandstones,the amount of strain compression or expansion of three principal stress directions increases,and with increase of the deviatoric stress,the stress-strain curves of samples gradually deviates from the linear segment,some samples appear volume-expanding phenomena in final failure stage.The increase of initial hydrostatic stress can increase the deformation of each principal stress direction,and the yield point gradually lags,the plasticity of samples decreases continuously,and the peak strength and minimum principal stress of sandstones in final failure stage increase significantly.At the same time,deviator stress carrying coefficient ? also increases,it can explain that the debilitation of the deviator stress on the rock strength is gradually inhibited.The reduction of Lode angle causes the maximum principal strain to increase,the minimum principal strain to reduce,the dilatancy of sandstones gradually weaken or even disappear,the anti deformation capacity of samples is enhanced,and the peak strength is remarkably improved.(2)The effects of different initial hydrostatic stress and Lode angles on the damage characteristics of sandstones are analyzed combined with acoustic emission technology.In prophase loading stage,acoustic emission signal is weak,AF and RA values are mainly concentrated in a local range.After entering the yield stage,the signal begins to be active,and data points spread toward the directions that AF values reduce and RA values increase,moreover,the shearing effect on rocks becomes more obvious.When samples are close to unstable failure stage,energy rate,amplitude,AF and RA successively reach the peak point.And a lot of location points at high amplitudes generated in later phase,in the meanwhile,they are concentrated in the internal local area of samples,it eventually causes many deterioration zones to be interconnected,then through cracks are formed inside the rock samples and induce structural unstable failure.Besides,with the increase of initial hydrostatic stress,the signal points in distribution figure of AF and RA are gradually close to the AF axis,RA axis and coordinate origin.Moreover,RA is significantly reduced in this process,the carrying capacity of samples is enhanced,and the debilitation of the shearing effect on the rock strength is inhibited.The distribution figure of AF and RA have similar variation characteristics when initial hydrostatic stress increases or Lode angle reduces,it indicates that these two methods have overlapping effects on the action effect of sandstones.(3)The effects of different initial hydrostatic stress and different Lode angles on the rupture mechanism of sandstones are analyzed combined with CT reconstruction technology.When the sandstones are under true triaxial stress conditions,the penetrating fracture planes formed in unstable failure stage are mainly caused by shear-tensile combined effect,and the damage areas formed by these two action modes have great differences on macro-scale.As the hydrostatic stress increases,the plasticity of sandstones is weaken,the degree of damage is further reduced.As the Lode angle reduces,the brittle failure characteristics of samples gradually disappear,and the quantity of the internal principal fracture planes and non-penetrating secondary cracks is reduced.(4)Study on the energy evolution law of sandstones under different initial hydrostatic stress and Lode angles.At compression stage and elastic stage,most absorbed energy is converted to elastic strain energy.Enter the yield stage,dissipated energy increases rapidly and reaches its maximum eventually.The damage limit of sandstone sample leads to instability failure.As the Lode angle reduces,dissipated energy in yield stage increases,the growth rate of energy consumption ratio constantly decreases.In addition,the increments of energy consumption ratio under different stress conditions are almost same from yield stage to peak unstable failure stage,which keep in the range of 0.4?0.6.It is not affected by hydrostatic stress and Lode Angle.that means the dissipated energy is positively correlated with the total energy absorbed by the samples.The range can be regarded as the precursor feature of cataclysmic rocks.