Study On Propagation Characteristics Of Rock Internal Crack Based On Acoustic Emission

With the rapid development of urban road construction in central and Western China,a series of geotechnical engineering problems caused by slope rock mass can not be ignored,such as road bedrock rock slope instability,dangerous rock fall,tunnel excavation rock collapse and so on,which are all engineering problems caused by rock internal crack propagation.In practical engineering,it is found that the rock mass is often not completely dense,and there are different forms of cracks in its interior But there are cracks.In this paper,aiming at the granite samples in Chongqing area,the cracks with different inclination angles are prefabricated in the middle of cylinder and Brazilian disc specimens.By means of indoor test,theoretical analysis and numerical simulation,the acoustic emission tests of the whole failure process of two models with different inclination angles of prefabricated cracks under uniaxial compression are compared and analyzed,and the geometric characteristics of prefabricated cracks and the relationship between their inclination angles are found Different types of rock will affect the failure mode and mechanical properties of rock samples,so as to explore the variation law of rock deformation and strength,and reveal the propagation mechanism of internal cracks in rock.The main contents and achievements are as follows:(1)For the stress-strain curve of cylinder specimen,the compression process of rock can be divided into four stages: compaction stage,elastic stage,plastic stage and residual deformation stage.With the increase of the inclination angle of the precast crack,the compressive strength and the initiation stress of the rock specimen decrease,and the initiation angle increases;the initiation position is always at the tip of the precast crack;the elastic modulus first increases and then decreases,and reaches the maximum when the inclination angle of the precast crack is 30 degrees;the fracture mode of the rock gradually changes from splitting tensile failure to splitting shear failure.(2)For the disk specimen,the crack initiation angle increases with the increase of the inclination angle of the prefabricated crack;the crack initiation position tends to the middle of the crack from the tip of the prefabricated crack;the bearing capacity and the crack initiation load of the rock decrease first and then increase,reaching the minimum value at 30° and 60°respectively;the elastic modulus decreases first and then increases,reaching the minimum value at 30 ° inclination angle of the prefabricated crack.The fracture mode is mainly tensile splitting failure,while the disc specimen with prefabricated crack angle between 30° and 60° shows tension shear composite failure.(3)The acoustic emission energy shows a sudden increase type when the stress and load peak value drop suddenly,which can better reflect the brittle fracture instability of rock;the acoustic emission ringing count divides the rock compression fracture process into the initial stage,the acoustic emission gradual growth stage and the acoustic emission sudden increase stage,and the amplitude change rule is not obvious compared with that,the ringing count can better reflect the rock compression fracture process Changes in the process.The change trend of AE cumulative energy,amplitude and ringing count is consistent,which is positively correlated with the bearing capacity of the specimen.The difference is that the AE cumulative parameters of the disc specimen are also affected by the fracture failure mode of the specimen after the peak load.(4)The R value of AE reflects the concentration degree of AE energy,which first increases and then decreases in the loading process.Because the lowest r value appears in the middle of loading,it indicates that a small number of high-energy AE events occur,which means the beginning of fracture.The lowest r value can be regarded as the starting point of main fracture.By normalizing the ra-af value of AE and studying its distribution,the fracture mode of rock can be inferred,which is in good agreement with the experimental observation.The crack initiation stress of rock internal crack can be obtained by acoustic emission caeh method.The calculated values of cylinder specimen are larger than the actual results,while the error of disk specimen is relatively small because there is no sudden stress drop in the early stage.(5)Based on the maximum circumferential stress criterion,a compression shear tensile fracture criterion considering the geometric characteristics of unclosed cracks and t stress is established,which can be used to accurately calculate the crack initiation angles of cylinder and disk specimens with different precast crack angles.With the increase of the width length ratio of cracks,the maximum circumferential stress increases and the crack initiation angle decreases;with the increase of the inclination angle of prefabricated cracks,the crack initiation angle gradually transits from normal to reverse.T-stress can prevent or delay crack initiation and propagation for cylinder specimen and disk specimen with prefabricated crack inclination less than 30° and promote crack initiation and propagation for disk specimen with prefabricated crack inclination greater than 30°.The uniaxial compression test and Brazilian disc test are simulated by XFEM combined with the maximum principal stress criterion.The simulation results are compared with the results of laboratory test and theoretical calculation.It is found that the simulated crack initiation angle is in good agreement with the measured crack initiation angle,and the deviation from the theoretical solution is large.