Research On Damage Mechanism And Acoustic Emission Characteristics Of Sandstone With Different Granularity Under Loading

In the process of geotechnical engineering,rock with different grain sizes is often encountered,and the peak strength,compressive strength and deformation resistance of rock with different grain sizes are very different.Therefore,it is very important to study the influence of grain size on rock mechanics and acoustic emission characteristics.In this paper,by collecting the mechanics and acoustic emission data of different size sandstone In the process of uniaxial compression failure,and taking the time as reference variable,the coupling relationship between mechanical and acoustic emission was obtained to study the influence of grain size on the process of uniaxial compressive failure of sandstone and further reveal the microscopic mechanism of rock failure.In order to find a reasonable criterion of rock failure,based on the fractal theory,the variation of fractal dimension in the process of deformation was studied,and the damage evolution process during the deformation and failure of sandstone was analyzed.The main research contents and conclusions are as follows:(1)According to the stress-strain curve of sandstone,the failure process of sandstone can be divided into four stages: compaction stage,elastic stage,yield stage and failure stage.With the increase of grain size,the time of elastic stage of sandstone is longer,and more elastic potential energy is gathered in rock,leading to the bigger possibility to take place rock burst.Grain size has little effect on the macroscopic failure mode of sandstone,and most of failures are single shear failures.However,with the increase of grain size,the slip marks on the fracture surface show obvious slip direction,which indicates that the contact force between the fracture surfaces is bigger and plastic slip characteristics is more obvious with grain size increasing.The effect of grain size on the mechanical properties of sandstone is also obvious.Increasing grain size causes the greater the compressive strength and elastic modulus of sandstone and the smaller elastic strain.This indicates that under the same conditions,the compressive strength and deformation resistance of sandstone increase with the increase in grain size.(2)Under uniaxial compression,acoustic emission of sandstone can be divided into three stages: initial compaction stage,quiet stage and outbreak stage.Grain size has little effect on the initial compaction period and quiet period,but the impact of the grain size on outbreak stage is very large.The larger the grain size leads to more active acoustic emission of sandstone in the outbreak period and the greater damage.Through the curve of sandstone,it can find that the stress threshold value of sandstone(?cd)equals to the 87% of peak strength.The failure process of sandstone under uniaxial compression is a mixture of tensile failure and shear crack,but it is mainly shear failure.And only a small amount of tensile failure occurred in the failure stage.It can be found that with the increase of particle size,the RA value of the acoustic emission of sandstone in outbreak stage is obviously increased,and the peak value is bigger,which show that the tensile failure is easier to occur in the failure process of sandstone with increasing grain size.Taking three sandstones of different grain sizes as experiment materials,RA values of acoustic emission in the rock before the main fracture all have a sudden increase and reach to the peak,and then rock macro fracture can be seen.It can be seen that the sudden increase of RA value indicates that the damage occurs.Therefore,it can take the sharp increase of RA value as the precursory information of sandstone failure.(3)In the time domain,the acoustic emission ringing count of sandstone has self-similarity and fractal characteristics.The evolution pattern of fractal characteristics is rising-sudden drop.In the initial stage of loading,a large number of small shear cracks are formed in rock.These cracks develop in the early stage,and the fractal dimension increases gradually and reaches to the peak value.With the expansion of the crack,the crack penetrates into each other and the fracture surface is formed,bringing about the sudden drop of fractal dimension.This reveals that the cracks in the rock begin to develop along the fracture surface until the macro crack appears before rock failure.Therefore,the first drop of fractal dimension can be used as the precursor of rock mass failure.The effect of grain size on the overall variation of acoustic emission fractal characteristics of sandstone is not very large.However,in the quiet period of acoustic emission,the curve of the correlation dimension is more gradual with grain size increase.These results indicate that using the rock with bigger grain size as material,in the elastic stage,the change of the rock interior is more gentle,and the degree of internal crack growth is relatively low.Moreover,the work done by external loads is mainly converted into elastic potential energy stored in rocks.In addition,the peak value of the correlation dimension of the ring number of sandstone acoustic emission is also slightly increased with increasing grain size,which shows that the failure of sandstone is more severe,and the release of energy is greater with grain size increases.The sandstone with bigger grain size has higher possibility to produce rock burst.