Anisotropic Damage Constitutive Model Based On The Mechanism Of Cracking In Rock-Like Material Under True Triaxial Compression

With the continuous growth of energy demand and the rapid development of the underground engineering,some activities such as mining,transportation,water conservancy,and even scientific laboratories are gradually expanded into underground space,even deep underground space.A large number of in-situ tests show that true triaxial stress is the most general stress state in underground engineering.Rock deformation and fracture have always been the focus of geotechnical engineering research.Therefore,it is of great significance to investigate the mechanical behavior of rock under true triaxial stress state.The mechanical behavior of rock is closely related to its internal microstructure,such as pores and cracks.The initiation and propagation of cracks will affect the macroscopic mechanical response of rocks.The study of crack evolution mode under true triaxial stress state will be helpful to understand various mechanical behaviors of rock true triaxial test and reveal the mechanism of influence of intermediate principal stress on rock failure.Therefore,this paper systematically studies the crack propagation and evolution process under the true triaxial stress state,and analyzes the influence process and mechanism of the true triaxial stress from the crack initiation stress,failure stress and propagation failure mode of the crack.Based on the results of the mesoscopic study,a damage model coupled with mesoscopic fracture mechanics is established.This model can reflect the influence of true triaxial stress on deformation of rocks.The contents of the research and the main conclusions of this dissertation are as follows:(1)By utilizing a true triaxial loading system,the evolution process of closed cracks in different directions is studied.By studying the propagation of closed and penetrated cracks in different directions under different true triaxial stress states,it is found that the closed crack parallel to the intermediate principal stress direction is mainly constrained by the minimum principal stress,and its crack initiation stress and failure stress(characteristic stresses)is less affected by the intermediate principal stress.The cracks parallel to the minimum principal stress is strongly constrained by the intermediate principal stress,which significantly increases the characteristic stresses of the cracks,and when the intermediate principal stress is large,the prefabricated cracks will no longer propagate,but will start from the direction parallel to the intermediate principal stress,and a new crack is generated.When the cracks parallel to the direction of the maximum principal stress,they are restricted first with the increase of the intermediate principal stress.When the intermediate principal stress exceeds the threshold,it will promote the propagation of the cracks in this direction.By studying the law of crack evolution in different directions,the mechanism of intermediate principal stress effect on failure of specimen is effectively revealed.(2)A comparative study is carried out on the difference between the propagation of the open cracks and the closed cracks.Through the true triaxial experiments of two kinds of cracks parallel to the intermediate principal stress direction,the differences in failure strength and brittleness-ductility transformation of two kinds of pre-cracked specimens under the same stress state are clarified.The influence of the intermediate principal stress and the minimum principal stress on characteristic stresses of the two types of cracks is explored.The results show that under the same stress state,the brittle failure of the specimen with open cracks is more obvious.The propagation of open cracks is dominated by wing cracks,and anti-wing cracks are also generated.The influence of the minimum principal stress on the propagation mode of the open crack is not obvious.The propagation of closed cracks gradually changes from wing cracks to shear cracks with the increase of the minimum principal stress.Therefore,the effect of friction between crack surfaces and normal stress on the crack propagation mode cannot be ignored.In addition,the increase of the intermediate principal stress can reduce the characteristic stresses of the opening crack,indicating that the effect of the intermediate principal stress on the penetrated crack cannot be ignored,and it is proved that the twodimensional crack propagation model analyzing the propagation process of the through crack under the true triaxial stress state will make certain errors.(3)The evolution features of 3D cracks under true triaxial compression test are studied.The influence of the intermediate principal stress on the characteristic stresses of the surface crack is analyzed,and the results show that with the increase of the intermediate principal stress,the characteristic stresses first increase and then decrease.The propagation of three-dimensional cracks was observed by surface images and CT,and it was found that the propagation paths of cracks on the surface were dominated by wing cracks and anti-wing cracks,and the internal cracks would be along the direction perpendicular to the intermediate principal stress.As the intermediate principal stress increases,the expansion in this direction is restrained,but at the same time,the internal crack is stimulated to expand along the direction approximately parallel to the intermediate principal stress,which causes the surface crack to propagate and coalescence,which decreasing the characteristic stresses of the crack.Through these comparative studies of the above three kinds of cracks under the true triaxial stress state,this thesis reveals the mechanism the effect of the intermediate principal stress on strength based on the fracture mechanics.(4)Based on the microscopic fracture mechanics test,a constitutive model coupling fracture mechanics with damage mechanics is derived.The model based on the dilute homogenization method,and uses the crack density as the damage variable,closely combines the damage process with the crack development,then one can derive the damage driving force and strain energy release rate based on the laws of thermodynamics.The model then calculates the inelastic strain caused by crack growth,by introducing a novel damage potential function.All eight parameters in the model have sound physical meanings.Numerical simulation using various true triaxial stress paths were carried out by this model,and the simulation results were in good agreement with the experimental results.Thus it is verifying the rationality and effectiveness of the model,which can characterize the effect of intermediate principal stress on rock deformation.(5)Based on the parameters sensitivity analysis,an intelligent model calibration program is proposed.By machine learning method and the parallel computing toolbox built-in Matlab,the program can intelligently and quickly calibrate the accurate model parameters according to the true triaxial stress-strain curve.It can effectively reduce the difficulty in model calibration,and lays a foundation for subsequent engineering applications.