Study On Energy Evolution And Acoustic Emission Characteristics Of Sandstone Under Uniaxial Cyclic Loading And Unloading Conditions

With the development of various underground projects and the increase of mining deep,rocks and foundations are often subjected to repeated loading and unloading during the construction of many underground projects.It is important in order to study the mechanical properties of rocks under cyclic loading and unloading.Value and engineering significance.Taking the sandstone of Chaili Coal Mine in Zaozhuang City as the research object,the uniaxial cyclic loading and unloading test of sandstone specimens was performed through different loading and unloading paths.The strength characteristics,deformation characteristics and energy of sandstone under different cyclic loading and unloading paths were,analyzed and compared.The relationship between evolution and acoustic emission characteristics is obtained.The mechanical properties of sandstone under uniaxial cyclic loading and unloading conditions are achieved.The research results obtained are as follows:(1)The sandstone is subjected to loading and unloading tests under three different paths,namely,staged cyclic loading and unloading,variable lower limit staged cyclic loading and unloading,equal amplitude cyclic loading and unloading,and the uniaxial compressive strength and elastic modulus of the sand loading and unloading to sandstone are obtained.Both Poisson's ratio have a strengthening effect.Under the grading cycle loading and unloading(Scheme B)and the lower limit grading cycle loading and unloading(Scheme C),the uniaxial compressive strength of sandstone is increased by 26% and 10%,respectively,compared with the uniaxial compression test;the elastic modulus increased by 4% and 2% respectively and the Poisson's ratio increased by 14% and 10% respectively.(2)Different cyclic loading and unloading paths will affect the mechanical properties of rocks.The uniaxial compressive strength,elastic modulus and Poisson ratio of sandstone under plan B were all improved by 14%,2% and 3% compared with plan C by comparing the two cyclic loading and unloading paths of graded cycle(plan B)and graded cycle loading and unloading with variable lower limit(plan C).(3)The stress-strain curve of the rock will form a hysteresis loop after repeated loading and unloading.The area of the hysteresis loop represents the accumulation of plastic deformation,indicating the amount of dissipated energy.Under the three cyclic loading and unloading schemes,the dissipated energy generated by the specimen is the number of cycles is positively correlated,and the average dissipation energy produced by the final circulation of the sandstone is 2-23 times that of the initial loading.(4)When the number of cyclic loading and unloading is the same,the staged cyclic loading and unloading test generates larger area of hysteresis loop and releases more dissipated energy than the graded cyclic loading and unloading test with variable lower limit.The dissipation energy of sandstone under plan B loading is about 16 times of that under plan C.The reason is that the plastic deformation is larger and the damage and damage to the rock is more serious under the loading and unloading path of the graded cycle.Therefore,the dissipation energy of the rock used to generate plastic deformation and internal damage is more.(5)The rate of change of acoustic emission events can be utilized to judge the development process of internal rock damage.Under uniaxial compression conditions,the number of acoustic emission events generated by rock during the compaction phase is about 80/s.The number of events remained relatively stable at about 20/s.The number of acoustic emission events suddenly increased by up to 25,000/s when the test piece entered the unstable phase.Under the condition of equivalent cyclic loading and unloading path,the number of acoustic emission events of the sample is significantly corelated with the stress value when the number of loading and unloading cycles is the same.When the loading value is 25 MPa,35MPa,40 MPa,the average generated during the loading and unloading process.The acoustic emission event rates were 3000/s,4500/s and 12,000/s,respectively.The acoustic emission characteristics of the rock under cyclic charging and unloading conditions exhibit a Kaiser effect.(6)The Felicity ratio can be used to determine the degree of damage to the rock.The smaller the Felicity ratio,the more damage the rock accumulates.The Felicity ratio decreases with the increase of the number of cycles in both cyclic loading and unloading conditions.The Felicity ratio of sandstone in Scheme B decreases from 1.381 to 0.409;the Felicity ratio in Scheme C decreases from 1.333 to 0.964;The Felicity ratio of the final failure of the rock sample under the path is relatively small,indicating that the sandstone is more severely damaged under the grading cycle loading and unloading path than the lower limit grading cycle loading and unloading path.