Discrete Element Simulation Of Complex Stress Paths And Fabric Evolution Of Granular Materials

The mechanical properties of sand-like granular materials,such as strength and deformation,are greatly affected by stress path and microstructure evolution.The discrete element method(DEM)can be used to simulate the interaction between particles and their microstructure evolution,and to realize various complicated stress paths.Therefore,the discrete element method(DEM)has unique advantages in mechanical response simulation and mechanism description of granular materials.In order to find out the mechanical response and fabric evolution of air-solid two-phase granular materials,the conventional triaxial tests of sand and German glass beads were carried out under anhydrous condition,the onedimensional and two-dimensional mechanical responses of air-solid two-phase system were obtained.Then,based on the test results,the discrete element method is used to simulate one-dimensional consolidation,conventional triaxial and hollow cylinder torsional shear tests,the stress-strain and fabric evolution of granular materials under one-dimensional,two-dimensional and complex stress paths are discussed.The main achievements and innovations are as follows:(1)The consolidation experiment of glass beads was carried out,and the discrete element method was used to simulate the experiment.Based on the experimental results of glass beads under onedimensional compression,a discrete element model is established,the effects of particle shape,gradation and deposition angle on the compressibility,stress path and fabric anisotropy of granular materials were studied.The results showed that particle shape had a significant impact on the compressibility of the samples under one-dimensional compression.The sphericity of particles showed a "W"-shaped relationship with the static earth pressure coefficient,while the normal contact microstructure showed an"M"-shaped relationship.Under the action of the deposition angle,the initial anisotropy of the microstructure was evident.With the loading of axial force,the anisotropy of the contact fabric gradually increased,while the anisotropy of the long axis of the particles gradually decreased.With increasing deposition angle,the compressibility of the sample showed a characteristic of initially increasing and then decreasing,and the static earth pressure coefficient increased approximately linearly.(2)A series of conventional triaxial tests of dry aeolian sands were carried out under different relative initial compactness and confining pressure conditions,including exhaust,no exhaust and low confining pressure conditions.The effects of relative initial compactness,confining pressure and pore air on deformation and failure characteristics of aeolian sand are discussed.The results show that the stressstrain relationship of aeolian sand can be divided into two types:strong softening type and weak softening type.Under the condition of no exhaust,the compressibility of pore air decreases with the increase of relative density and confining pressure.and the pore air pressure produced significantly increased the shear strength of aeolian sand.Based on the experimental results,linear relationships between peak friction angle,deformation modulus,and shear strength of Tengger Desert sand and relative initial density were established.(3)The triaxial exhaust tests on glass beads under different confining pressures and particle sizes in dry state were carried out,and the general triaxial test model of glass beads with unified macro and micro parameters was established based on the test results.The macroscopic and microscopic mechanical characteristics of granular materials under two stress paths were analyzed.The results showed that largescale glass beads had higher strength,critical stress ratio,internal friction angle,and more obvious stickyslip phenomenon.Under the triaxial compression and triaxial tension conditions,the contact normal fabric of the original isotropic specimen has evolved,and under the triaxial compression conditions,the anisotropy of contact normal fabric is higher than that of triaxial tension.With increasing confining pressure,the number of contact force chains increased,the proportion of strong contact forces became larger,and at the same time,the development of anisotropy of the contact structure was limited.(4)Based on the numerical calibration parameters of the glass bead triaxial test,a numerical model for the hollow torsion test was established,and the influence of the principal stress rotation angle on the strength,deformation,and anisotropy of the sample was studied.The results showed that the peak stress ratio of the sample decreased as the principal stress rotation angle increased,and there was a significant anisotropy in deformation.Under complex stress state,when the principal stress direction rotates 0～90°,the anisotropy of contact fabric decreases with the increase of the principal stress direction angle.When the principal stress direction is rotated by 90～180°,the anisotropy of the contact fabric increases with the increase of the principal stress direction angle.