Study On Mechanical Properties And Energy Evolution Of Sandstone Under C Yclic Loading And Unloading

Since the industrial revolution,people's demand for resources has increased geometrically.Today,the shallow underground resources are not enough to meet the development of society.Therefore,the excavation and research of deep resources is imperative and of great significance.During the excavation of deep resources,rocks are often subjected to complex stress of repeated loading and unloading,which leads to the mutual transformation of different forms of internal energy,usually accompanied by the input,accumulation,dissipation and release of energy.Under the action of cyclic loading and unloading,the microcracks and fissures in the rock begin to sprout and develop continuously.When the cyclic stress continues to increase,the new microcracks and fissures further expand and intersect,and the internal damage gradually accumulates,resulting in macro cracks and rock mass instability,which poses an unexpected threat to the construction safety.Therefore,it is of great significance to study the mechanical properties and energy evolution characteristics of rock under cyclic loading and unloading for the construction safety of underground rock engineering.In this paper,the mechanical properties and energy evolution law of sandstone under cyclic loading and unloading conditions are studied.Uniaxial compression test,conventional triaxial test and uniaxial and Triaxial Cyclic Loading and unloading test are carried out by combining indoor test and energy theory.The stress-strain curve characteristics,mechanical properties and failure modes of sandstone are mainly studied,The energy evolution and distribution law of sandstone before peak under different confining pressures are studied.The relationship between hysteretic effect energy,damping ratio and cycle times is analyzed,and the change law of them under various confining pressures is discussed.The results show that the area of hysteresis loop and residual strain of sandstone increase with the increase of cycle times,and the higher the confining pressure is,the greater the value is;In different loading and unloading stages,the secant modulus of elasticity shows different trends with the increase of the number of cycles,and the Poisson's ratio is basically the same;In the case of no confining pressure,the failure mode of sandstone is mainly columnar splitting failure,and in the case of confining pressure,it is shear failure;The energy evolution behavior of sandstone in pre peak stage is energy accumulation.The ratio of total energy,elastic energy,dissipated energy and elastic energy increases with the increase of cyclic stress,and the higher the confining pressure is,the larger the corresponding value is;The hysteresis effect of sandstone increases with the increase of confining pressure,showing the same trend;The variation of damping ratio of sandstone has little relationship with confining pressure and tends to a stable value.