Fracture Mechanical Behaviors And Acoustic Emission Characteristics Of Beishan Granite Under CCNBD Test

With the utilization of nuclear energy in the world,the disposal of nuclear waste has become more and more important."Deep geological disposal method"is the most suitable disposal method for high-level radioactive waste.There are many structural defects in the natural rock mass,which will have a great impact on the bearing capacity and failure process of the surrounding rock,thus threatening the stability of the underground laboratory.The research shows that tensile failure is the most common failure form in the rock.In this paper,the fracture mechanical behavior and acoustic emission characteristics of Beishan granite with prefabricated cracks under tension are analyzed based on the Cracked Chevron Notched Brazilian Disk(CCNBD)test.The main research contents and achievements are as follows:(1)Based on the research results of some scholars at home and abroad on the fracture characteristics of Beishan granite in recent years,because of only three-point bending test has been carried out in this research,cracked chevron notched Brazilian disk(CCNBD)test with different loading model is designed.The fracture toughness of Beishan granite under different test conditions is obtained:Under static loading,the fracture toughness of Beishan granite is about 1.180MPa·m^1/2.Under cyclic loading,the fracture toughness of Beishan granite is 1.153MPa·m^1/2,cyclic loading will reduce the bearing capacity of the rock with prefabricated cracks under tension pressure.(2)The deformation characteristics of CCNBD specimen of Beishan granite under different loading modes are studied.CCNBD test shows that Beishan granite has a significant brittleness,and cyclic loading would not change the failure mode of the sample.Beishan granite shows a significant memory function in cyclic loading.(3)Based on the acoustic emission(AE)characteristics of CCNBD under different loading modes,this paper determines the characteristic strength value and studies the damage evolution law.It is turn out that FPZ generally exists in CCNBD test,which is related to the anisotropy of materials.The macro cracks in rock are the result of the development and connection of micro cracks,not the growth of a single crack.Under static loading,the specimen is basically cracked along the prefabricated crack surface.However,under cyclic loading,the development of micro cracks in the FPZ at the end of the specimen will produce macro cracks,and change the direction of crack propagation.Under static loading,the CCNBD specimen of Beishan granite enters the stable crack growth stage at 42%fracture toughness and enters rapid crack growth stage at 78%fracture toughness;the damage of specimen increases with the increase of load,and the AE characteristics of rapid crack growth stage is the most significant,which is the main stage of specimen crack development.The damage evolution of the specimen under cyclic loading is different from that under static loading and triaxial loading.The damage of the specimen decreases first(before the rapid crack growth stage)and then increases with the increase of load.(4)Based on the particle flow program PFC^3D,a three-dimensional numerical model of CCNBD sample is established,the meso-parameters of Beishan granite are calibrated,the fracture process of the specimen under static loading is simulated,and the crack propagation law and stress distribution characteristics are analyzed.It is concluded that crack initialized at the tip of the prefabricated crack and spreads to both ends of the notch,the crack propagates along the prefabricated crack to the end of the specimen in the shape of"U",and the curvature decreases with the crack growth.Stress distribution of the specimen is obviously controlled by the shape of the crack,and the contour lines of tensile stress and axial stress are U-shaped.The direction of the minimum principal stress changes from tension to compression at the front of the crack.Fracture mechanism of CCNBD is not pure mode?crack growth type,but a mode of synthetic stress of the dominant tensile stress and the in-ligament shear stress.