Experimental And Numerical Simulation On Mechanical Properties Of Red Sandstone Under Cyclic Loading

To investigate the fatigue mechanical properties of red sandstone under cyclic loading,this paper based on the project of National Natural Science Foundation of China（NO.51979293:Subcritical Age-effect Propagation of Rock Fatigue Cracks under Cyclic Loading and Multi-scale Analysis of Fracture Morphology）.Taking red sandstone as the research object,the macroscopic mechanical properties of red sandstone under cyclic loading were analyzed by laboratory tests.The particle flow software PFC^2D was used to carry out the numerical simulation test of red sandstone under cyclic loading,and the rock failure process was studied from the mesoscopic perspective.The accumulation degree of damage in rock is quantitatively determined by theoretical analysis.The main research results are as follows:（1）The fatigue failure of rock is controlled by the static loading curve,and the failure strain under cyclic loading is the strain in the post-peak stage of the static whole process curve.The threshold value of fatigue failure of red sandstone is between（75%-85%σ_c）.The overall hysteresis loop shows the characteristics of"sparse-dense-sparse",and moves towards the direction of strain increase.The evolution of the loading and unloading elastic modulus is generally divided into three stages.The accumulation rate of residual strain and maximum strain increases with the increase of stress ratio.（2）The initial damage of sandstone under uniaxial compression is caused by the cracking of tensile cracks.The rock failure belongs to tensile failure.The tensile crack angle is mainly concentrated in the axial loading direction[80°,110°],and the particle displacement near the macro fracture zone reaches the peak.Under cyclic loading,the number of microcracks increases step by steps with the increase of the axial strain,and tensile cracks dominate the whole process.The micro-cracks in the rock sample were initially distributed in the void area with weak bond and structure,and then continuously gather in a certain area until the macroscopic fracture zone is formed.Near the later stage of cyclic failure,the expansion of micro-cracks is mainly controlled by the direction of principal stress loading.（3）The dissipation energy measures the degree of rock lithology deterioration and strength loss.Under high stress level,most of the external input energy is converted into elastic energy and stored in the rock,and the energy loss accounts for only a small part.The damage variable defined by the maximum axial strain method and the energy dissipation method is only applicable to describe the degree of damage deterioration in rock under low cycle fatigue load.The overall evolution curve of the damage variable is divided into three stages.Based on the continuous damage theory,the fatigue damage evolution model of rock was established to quantitatively measure the damage accumulation degree in rock.