The Effect Of Particle Size Ratio And Particle Shape On The Mechanical Behavior Of Granular Materials Based On DEM

Granular materials are macro systems composed of a large number of discrete particles,and widely exist in many fields,such as geotechnical engineering and transportation.The complex macroscopic mechanical properties of granular materials are the macroscopic manifestation of the interaction of various microscopic factors,such as the particle size,shape,arrangement,contact,and material properties etc.For binary granular materials,the particle size ratio,as an important factor affecting the packing structure,has a great influence on the static mechanical properties of binary granular materials.In addition,the particle shape,as a basic geometric feature of the particles,also has a great influence on the packing structure of the granular materials.At present,most studies are conducted based on round or spherical particles,and most of the actual particle shapes are non-spherical or noncircular.Therefore,it is of great importance to consider the influence of particle shape on the macroscopic mechanical properties of granular materials.In this thesis,the effects of particle size and shape on the macro and micro mechanical properties of granular materials are studied based on the discrete element method.The research contents are as follows:Firstly,the influence of particle size ratio on the static properties of binary granular materials with different packing structures is discussed.The effects of particle size ratio on the shear modulus,strength and shear band of binary granular materials in biaxial compression tests are analyzed by considering three types of packing structures,namely,regular packing,layered packing and random packing.For the regular packing granular samples,the particle size ratio has a significant effect on the strength and deformation behavior.The larger the particle size ratio,the greater the shear strength and the greater the shear dilation occurring during loading.The relationship between the peak stress ratio and the particle size ratio is obtained based on the structural stress analysis of the regular packing granular samples and verified with the numerical experimental results.For the layered packing granular samples,the similar microstructure makes the effect of the particle size ratio on their static properties smaller,and the different contact models used in the numerical calculation will lead to the difference of the shear modulus during the biaxial compression test.For random packing granular samples,in the larger particle size ratio range,the smaller the particle size ratio is,the larger the shear modulus is.The strong force chains are mainly distributed in the large particle-large particle contact,and the weak force chains are distributed in a large number of small particle-small particle contacts.Secondly,the effect of aspect ratio on the wave propagation behavior of granular samples composed of elliptical particles is investigated.The energy decay,wave velocity,wavefront shape and dispersion behavior during wave propagation are mainly focused.The energy decays sharply along the wave propagation direction and is influenced by the aspect ratio and the confining stress.The wave velocity is related to the microstructure of the granular sample.It is positively correlated with the aspect ratio and has a power-law relationship with the confining stress.The relationship between the wave velocity and the aspect ratio and the confining stress is obtained by analyzing the branch fabric tensor.The wavefront shape has the same elliptical shape as the composed particle of granular sample.The relationship between the wavefront shape and the aspect ratio is analyzed by the velocity propagation path,and the analytical expression is obtained.The dispersion curves of the granular samples are fitted sinusoidally,and it is found that the greater the aspect ratio,the stronger the dispersion of the granular samples.The conclusions can provide a reference for wave-guided granular crystals design.Finally,rate process theory is introduced to investigate the creep and stress relaxation behaviors of granular materials with different particle shapes.The changes of particle shapes is characterized by the aspect ratio and the blockiness.The macroscopic behaviors of creep strain during creep and deviatoric stress decay during stress relaxation of the granular samples are investigated.It is found that for the aspect ratio and blockiness,when one is small,the increase of the other one will lead to the greater creep strain and creep strain rate during creep,and the greater creep strain when the granular samples enter the secondary creep stage and enter the tertiary creep stage,and the greater deviatoric stress decay and the lower the volumetric dilation during stress relaxation.However,the aspect ratio and blockiness will hinder each other’s influence on the creep and stress relaxation of the granular sample when they are large,and the variation pattern of the granular sample with aspect ratio or blockiness will no longer follow the variation pattern when the aspect ratio is small or the blockiness is small.By investigating the evolution of the coordination number,contact force and contact force anisotropy of the granular samples,it is found that the influence of the aspect ratio and blockiness on the creep of the granular sample mainly comes from the change of the ratio of shear contact force to normal contact force,and the influence of the aspect ratio and blockiness on the stress relaxation process mainly comes from the change of the anisotropy of the normal contact force of the granular sample.