Study On The Damage Evolution Law Of Red Sandstone Under Different Cyclic Loading And Unloading Paths

Many engineering practices have shown that rock masses are often subjected to repeated loading and unloading during the construction and operation stages of geotechnical engineering.Therefore,it is necessary to study the law of rock damage evolution under the action of cyclic loading and unloading,which has certain positive significance for disaster prevention and safe construction of related geotechnical engineering.In this paper,red sandstone was selected as the research object,uniaxial compression test and cyclic loading and unloading tests of different paths were carried out.Since acoustic emission and digital image correlation technology can help to understand the law of rock damage evolution,the monitoring of the acoustic emission and digital image correlation technology was carried out simultaneously.And the mechanical properties,acoustic emission characteristics,surface deformation field evolution information,and energy evolution laws of the rock under different cyclic loading and unloading paths were analyzed.At the same time,numerical simulation software RFPA^2Dwas used to obtain the spatial distribution of acoustic emission and the development of micro-fracture of the rock.Based on the above perspectives,the damage evolution law of rock under different cyclic loading and unloading paths was studied.The main research results obtained are as follows:(1)The average peak stress of rock measured under constant lower limit cyclic loading and unloading and variable lower limit cyclic loading and unloading reached80.1 MPa and 76.2 MPa,respectively.The plastic deformation of the rock in the constant lower limit cyclic loading and unloading process increases with the increase of the number of cycles,while the elastic deformation is mainly manifested in the variable lower limit cyclic loading and unloading.(2)The acoustic emission of the two loading methods shows the Kaiser effect in the early stage,and the Felicity effect gradually appears in the later stage of the loading.The average value of the Felicity ratio in the constant lower limit cyclic loading and unloading continues to decrease with the increase of the stress,and the Felicity ratio in the later stage of the variable lower limit cyclic loading and unloading is basically remained between 0.89 and 0.96.The average value of the Felicity ratio when the rock sample fails is respectively 0.49 and 0.93,reflecting that the rock damage degree is more serious under constant lower limit cyclic loading and unloading.(3)The displacement field and strain field cloud images obtained based on the digital image correlation technology(DIC)can reflect the evolution of the deformation and fracture process of the rock.Among them,the horizontal displacement cloud map gradually shows a vertical strip-like distribution,and the vertical displacement shows an increasing trend in value.Through the damage analysis of the apparent strain,it is found that the damage gradually accumulates before the rock fails under the constant lower limit cyclic loading and unloading,the cracks develop steadily.However,when loaded to 66%of the compressive strength under variable lower limit cyclic loading and unloading,the large apparent strain points of the rock are rapidly concentrated from dispersion,and the local area deformation increases sharply,and finally severe brittle failure occurs.(4)The change trend of elastic strain energy and dissipated energy of rock under constant lower limit cyclic loading and unloading is very close.When the stress level is approximately 50 MPa,that is,62%of the compressive strength,the growth rate of elastic strain energy tends to be stable.The average dissipated energy produced when the rock finally fails is 0.0495 MJ/mm³.In the process of variable lower limit cyclic loading and unloading tests,the elastic strain energy of the rock is higher than the dissipated energy at the early loading stage.As the number of cycles increases,the fracture inside the rock expands rapidly,and the damage degree increases continuously.The dissipated energy increases faster,and the dissipated energy gradually exceeds the elastic strain energy.Correspondingly,The average dissipative energy generated in the final failure is 0.0391 MJ/mm³.This indicates that more dissipated energy is used to produce internal damage and plastic deformation of rock under constant lower limit cyclic loading and unloading,and the damage degree of rock under constant lower limit cyclic loading and unloading is more serious than that under variable lower limit cyclic loading and unloading.(5)Regarding the red sandstone sample used in this test,it is found that when the homogeneity of 3 is used in the numerical simulation test,the simulation result is closest to the actual result.Through the two-dimensional images of the acoustic emission activity diagram and the maximum principal stress failure process diagram obtained by numerical simulation,it is found that the failure of the rock model at the meso level in the constant lower limit cyclic loading and unloading and the variable lower limit cyclic loading and unloading is a combination of tensile failure and shear failure,the main failure is tensile failure,supplemented by shear failure.