Hypoplastic Model And Discrete Element Simulation Of Anisotropic Sand

Sand is a typical discontinuous scattered material.During the natural deposition process,particles are influenced by gravity.The spatial arrangement shows obvious orientation,showing anisotropy.anisotropy and scatter are naturally the fundamental characteristics of sand.Most of the existing constitutive models are based on the elastoplastic theory of continuity and isotropy assumptions.The model describes the effect of the shape,size,and orientation of the meso-scale soil particles on macroscopic mechanical properties,as well as complex loading methods,stress paths,etc.the influence of anisotropy on mechanical response under conditions is very difficult,and modeling is relatively complicated.The hypoplastic theory directly establishes the incremental relationship between stress and strain,avoids the solution of a large number of partial differential equations,and is simple in modeling.For this reason,this paper introduces a new anisotropy based on the hypoplastic constitutive theory and the macro-mechanism combination method.The anisotropic model was established with state variables.in combination with PFC2D/3D discrete element software,the plane strain test and the true triaxial test were simulated from the aspects of particle shape,force chain contact,failure laws,etc.The comparison and analysis of the model was conducted.The relationship between the anisotropy of sand and its discrete properties,and its main conclusions are as follows:(1)The basic idea of the anisotropic constitutive model of sand based on the hypoplastic theory of Wu is summarized.Based on the basic rules of tensor calculations,the process of deducing the isotropy to anisotropy of the hypoplastic constitutive theory is described.Through the description of the characteristics of sand failure,the meanings of the failure surface and the flow rule are analyzed,and the porosity is taken into account.The influence mechanism of sedimentation angle and the model on the model drainage test analyzes the validity and rationality of the theory.(2)The anisotropic constitutive model for sands was established and the simulation and verification of the biaxial and true triaxial drainage tests were carried out.Using the macro mechanism method,Xue Feng Li etc.Introduced the newly defined anisotropic state variables into the critical state equation of sand,described the influence of the meso structure on its critical state,and discussed the anisotropic parameters,rode angle,and pores.The law of the ratio of anisotropic state variables and critical state lines.By introducing concepts such as anisotropic state variables,critical state lines and fabrics,Wu’s quantitative description of the anisotropy of sand is optimized.the parameters of the hypoplastic model under different loading conditions were analyzed.Through the biaxial drainage and the true triaxial drainage test simulation,the influence of the orthotropic parameters on the stress strain relationship was analyzed,and the rationality of the model was initially verified.(3)Plane strain tests under different particle shapes were simulated using PFC2D.In order to reflect the anisotropy of sand particles,a plane strain program was developed based on the discrete particle flow in the fish language.Based on the circular particles,the elliptical,trigonometric,rectangular,and square-like structures were successively constructed using clump.The influence of particle shape on the macroscopic mechanical properties of the five particle shaped specimens was investigated under biaxial test conditions.The evolution of contact force and velocity fields and the stress and strain laws were analyzed.The particle flow simulations show that the more irregular the shape of the particles,the more obvious the anisotropic characteristics.Through the simulation comparison with the constitutive model established in this paper,the rationality and effectiveness of the sand flow anisotropy described by particle flow and hypoplastic models are further verified.(4)Based on the PFC3D,a true three axis discrete element program with different particle shape samples was programmed and simulated.a variety of geometrically shaped solid particles were constructed and simulated by the discrete element program of the triaxial test to compare the effect of intermediate principal stress coefficients on the dilatancy of different loading paths.The different medium principal stress coefficients were used to describe contact force chains and displacements.Field evolution process.using the established anisotropic plasticity model,the results of the discrete element program are simulated.The results show that the larger the intermediate principal stress coefficient is,the more obvious the dilatancy of sand particles is,and the greater the confining pressure is,the more the contact force chains between the particles are.Intensive,model building and discrete element programs can better describe the mechanical response of anisotropic sands.