Experimental Study On Damage Evolution Rules Of Sandstone Subjected To Cyclic Loading

Rock is the basic material for constructing rock mass engineering,the load path and load history are very complicated during service.Cyclic loading is the most common and simple mechanical phenomenon,which is closely related to static loading.However,the damage evolution of rock under cyclic loading is significantly different from static loading.Mastering the fatigue damage evolution law of rock is conducive to maintaining the safety status of rock mass engineering,improving the service life,and ensuring the safety of people's lives and property.In this paper,red sandstone is taken as the research object.The CMT5105 microcomputer controlled electronic universal testing machine and digital image correlation(DIC)technology are used to study the deformation and damage evolution characteristics of rock under static loading and cyclic loading.The full field strain and displacement,crack opening displacement,crack initiation region main strain mean and other parameters of the cracked chevron notched brazilian disc(CCNBD)sandstone specimen under five loading angles were obtained.Through the analysis and discussion of the evolution characteristics of the above parameters,the following conclusions are obtained:(1)Under the static loading,the difference of the loading angle causes the rock to change in static load strength,stress state and crack initiation position.With the increase of the loading angle,the static load strength of the rock increases gradually,and the stress state transitions from the tension-shear composite stress state to the compression-shear composite stress state,the crack initiation position is approached from the tip of the prefabricated slitt to the center of the specimen.(2)Under the static loading,the damage evolution of rock at different loading angles has obvious stages.According to the load-displacement curve and the distribution and accumulation characteristics of rock full field strain,the damage evolution process of rock is divided into compaction phase,linear elastic phase,plastic hardening phase,plastic softening phase and failure phase.(3)Under the cyclic loading,when the rock is fractured by single-stage loading,the hysteresis curve family exhibits the evolution of "sparse-dense-sparse";the axial deformation of the rock increases rapidly in the early stage of loading,and most of the subsequent cycles it grows slowly and slowly,and accelerates when it is near to failure;the maximum principal strain(?1)is more evenly distributed in the early stage of loading,and then accumulates at a stable rate at a tip of the prefabricated slit or at a position above it,then the strain concentration,uneven deformation,crack propagation and so on gradually occurs,the accumulation of ?1 accelerates near the failure,and the crack propagates rapidly through the specimen;in the crack initiation region,the evolution of the mean value of ?1 is similar to axial deformation of the rock;in addition,the volumetric energy,the dissipative energy and the absolute values of elastic deformation energy of the rock show a U-shaped development law with the increase of the cycle period.(4)Under the cyclic loading,when the rock is fractured by multi-stage loading,the evolution of the above parameters in the final loading stage is similar to that of single-stage loading.In other loading stages,the hysteresis curve family mostly evolved from "sparse to dense";the axial deformation of rock first increased rapidly and then slowly and steadily increased;the accumulation rate of ?1 in rock was extremely low,and the mean value of ?1 in crack initiation region maintains a steady and slow growth,but it is always maintained at a low level.In addition,the volumetric energy,the dissipative energy and the absolute values of elastic deformation energy of the rock show a L-shaped development law with the increase of the cycle period.(5)Under the cyclic loading,during the whole process of fatigue loading,the value of ?1 increases first and then decreases along the direction of wing crack propagation,the value of ?1 is the largest in the region about 0.5 to 1 mm from the crack tip;Vertical the direction of the wing crack propagation,the value of ?1 is approximately parabolic,that is,the closer to the crack tip,the larger the value of ?1.The initiation and propagation of the wing cracks throughout the fatigue loading process,always controlling the fatigue damage process of the rock.