Failure mechanical behavior and energy evolution of coal and rock under cyclic loading and unloading
In the process of close-distance seam group mining,multiple mining will cause significant cyclic loading and unloading effects.In order to study the stability of coal pillar and surrounding rock under cyclic stress,uniaxial cyclic loading and unloading tests of coal or rock single body and coal-rock combination combined with acoustic emission signal monitoring and PFC3D numerical simulation calculation were carried out.The results show that,under cyclic loading and unloading,the stress-strain curve of the specimen presents the characteristics of"thinning-dense-thinning".Combined with the acoustic emission count and the number of cracks derived,the deformation process can be divided into the pore and fracture compaction stage,the elastic deformation and crack stable development stage,and the crack unstable development stage.Cyclic load can strengthen the strength of coal,but weaken the strength of rock.In the test,the average elastic modulus of different compositions increases by 7.03％to 18.95％compared with that of single coal,and decreases by 0.48％to 6.76％compared with that of single rock.The strengthening effect for coal or weakening effect for rock is positively correlated with the strength difference between the single specimens that forming the combination.In the process of cyclic loading and unloading,the crack initiation rate and crack number of specimens are related to their strength and deformation characteristics.The crack development of brittle rock specimens is mainly concentrated in the stage of crack unstable development,and the higher the strength of specimens,the more concentrated the stage of crack unstable development,while the crack development of plastic rock specimens is in a relatively active state during the whole process of cyclic loading and unloading.The fracture pattern of coal and rock single body specimens under cyclic loading and unloading test is mainly compression-shear failure,showing a single inclined plane shear failure crack.When the coal-rock combination fails,the end of the coal body first forms columnar splitting or inclined plane shear failure,the fracture surface is deflected when extends to the coal-rock interface,and extending to the combination boundary.The deflection angle increases with the increase of rock strength in the combination.In the cyclic loading and unloading test,both the elastic energy and dissipative energy of the specimen showed an increasing trend,and the elastic energy and dissipative energy generated in a single cycle were negatively correlated with their own strength characteristics,but the elastic energy and dissipative energy generated in the cyclic accumulation were positively correlated with the specimen's own strength.According to the research results,it is beneficial to improve the supporting capacity of coal pillar under the disturbance of cyclic mining stress to change the strength of surrounding rock and the cementation between coal pillar and surrounding rock.
rock mechanicscyclic loading and unloadingcoal-rock combinationacoustic emissioncrack propagationenergy evolution
NETL
NSTL
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