The Effect Of Internal Structure And Stress History On Sand Response To Small Loading Increment

Sand as a typical granular material,its mechanical response is determined by the microscopic characteristics.In order to better study the small-strain elastic modulus of sand particles,it is necessary to conduct macro-micro relationship on multiple scale.This article mainly studies the internal structure and stress history of sand granular materials at small strain.The discrete element method is used as a research tool to obtain the internal structure information of granular material,the uniform field assumption and the contact stiffness matrix are used to construct the elastic modulus of the granular material system using the internal structure information.To study the elastic modulus of sand granular materials from macro-micro scales,the analysic is compared with the DEM output.The main research results are shown as follows:(1)The granular material exhibits elastic property under the action of small strain,and its stress shows a linear relationship with strain.If the shear amplitude is further increased,the shear modulus will be decresed The small strain shear modulus increases with the increase of confining pressure,and decreases with the increase of the initial friction coefficient of the granular material.It shows that as the internal particles are more in close contact,the elastic modulus of the system will increase.(2)Through internal structure and contact information of particles,the elastic modulus of granular materials can be predicted by using the uniform field theory.The predicted value of the elastic modulus is greater than the true value of the granular materials.The elastic modulus of the system constructed by the particle contact element stiffness matrix can approximate the true value of the elastic modulus of the granular material.For smooth granular materials,the elastic modulus obtained under the assumption of uniform stress has difference with the actual situation.The shear modulus obtained by the truss analysis method has a better approach to the true value.(3)The predicted value of the elastic modulus through the assumption of the uniform field and the construction of the structure matrix increases with the increase of the effective confining pressure,the trend of decreasing with the increase of the friction coefficient is the same as DEM silumation.It shows that the change of internal structure will affect the change of elastic modulus of granular material.It is a feasible method to predict the elastic modulus of granular materials through contact information and particle position information.The real elastic modulus of sand granular materials is smaller than the homogeneous field assumption.(4)Macroscopic properties can be explained through meso analysis.The shear modulus obtained from the biaxial shear test on the sample is smaller than the bulk modulus obtained from the biaxial compression test,which can be interpreted from the microscopic view as the uneven distribution of the internal contact force under the biaxial shear path.The added value is concentrated in the contact of a few particles.The difference between the elastic modulus of the granular material and the elastic modulus assumed by the uniform field is mainly due to the difference in the contact force of the particles and the difference in the position of the particles under external action.(5)In the history of isotropic stress,with the increase of confining pressure,the particles are more dense,and the elastic modulus of the granular material increases with the increase of confining pressure.Under the effect of the history of shear stress,even if the structural information of the particles does not change,due to the increase in the anisotropy of the particulate material,the small strain shear modulus of the particulate material will produce a large difference.