Study On Macro-mesoscopic Deformation Mechanism Of Binary Mixed Particle Material

The extensive application of granular materials in various engineering conditions requires a better understanding of the basic physical properties,mechanical properties and complex multiphase characteristics of granular materials.Especially for geotechnical materials,there are many complex load-bearing modes of granular materials in different engineering backgrounds.Although there are many constitutive models which can represent the stress-strain relationship of materials,it still needs to be explored deeply to fully reflect the granular mechanics and deformation mechanism at multiple scales.Therefore,the study of structural characteristics and local deformation modes at the meso-level of granular materials is necessary.It is very important to understand the loading mechanism of particle system and to establish the correlation between macro and microscopic characteristics.The contact friction characteristics,particle shape,macro loading stress path and initial consolidation characteristics of particles have different effects on the macro and micro-level behavior of particles.At the same time,the mechanical properties of discontinuous graded materials are very complicated due to their complex local structure,large particle size gap and distinct structural characteristics.The macro-strength,meso-structure,plastic instability mechanism and dynamic characteristics of particle motion within mixed granular systems need to be further studied in-depth.In this paper,the binary mixed granular system is chosen as the research object.The mechanical and geometric characteristics of continuous uniform gradation and binary mixed particle system are discussed by means of discrete element method through various loading modes,particle contact conditions and stress paths.The plastic deformation characteristics,instability mechanism and statistical characteristics of particle motion in the binary mixture are studied.On the basis of quantitative characterization of particle shape,the influence of particle shape on strength and deformation characteristics was explored.The main research contents and conclusion are summarized as follows:1.The micro-structure characterization and anti-rotation effect of granular materials within direct shear were studied.The rolling friction coefficient of granular materials significantly affects the macro-strength and micro-structure under direct shear.The deviational stress ratio at critical state is linearly correlated with the void ratio under different rolling friction conditions.The rolling friction coefficient itself is linearly correlated with the joint stress-fabric invariants.The linear relationship between the principal direction of stress and fabric under different rolling friction coefficients is very similar.The sliding friction can significantly improve the peak strength compared with rolling friction,and the anti-rotation performance of particles can also effectively improve the overall friction excitation level of particle assembly;The anti-rotation performance of particles results in the micro-structure differences in different shear zones of samples,especially for the local void ratio and the arch effect caused by rolling friction;The load-carrying capacity of contact force chains,the length of force chains and the anisotropy are all affected by the antirotational properties between particles.2.The effects of initial anisotropy and complex stress paths of granular materials are studied.The initial anisotropic consolidation degree of three-dimensional granular materials results in the difference of initial meso-structure of materials.Under the combined influence of initial stress ratio and stress path,the small strain stiffness of materials is significantly affected,and the octahedral stiffness increases with the increase of intermediate principal stress ratio.The increase of initial anisotropy corresponds to the decrease of the dilatancy magnitude and the void ratio in critical state.Under various initial anisotropic conditions,the relationship between the medium principal stress ratio and the maximum dilatancy is linear;The slope of the stress-dilatancy relationship varies greatly under different initial conditions;The quantitative relationship between macro and micro parameters is established,and the transmission characteristics of contact force and the different characteristics of the distribution of contact direction under different initial conditions and stress paths are determined.3.The mechanical and geometrical mechanism of binary mixed particle system is studied,and the correlation between peak strength and equivalent void ratio is established under different fine contents(FCs).The contact force distribution,mechanical contribution and proportional evolution of different contact types in binary mixed particle system are affected by FC;The derived anisotropic parameters calculated from the contact types with the maximum proportion are all approximate to the anisotropic parameters under global contact system.At the same time,the deviatoric stress ratio calculated by stress-fabric-force relationship under global contact is consistent with the deviatoric stress ratio calculated from the contact type with largest proportion.The stress ratio calculated by the contact type related to coarse particles is also highly similar to the global stress ratio.Contact systems are classified according to strong-weak contact and sliding-nonsliding contact.It is found that the contact normal anisotropy under strong and nonsliding contacts has a linear relationship with the macroscopic stress ratio,and the slope of the linear relationship varies with the change of the fines content.The different contributions of strong and weak contacts to the anisotropy are demonstrated.4.The local structural parameters and their probability distribution characteristics of binary granular mixtures were investigated,and the difference of packing behavior of mixtures with different fines contents was studied.The difference of packing state of mixtures at different directions was analyzed by the anisotropy of spatial distribution of radial distribution function.The distribution of local strain characteristics showed different trends of power law attenuation under different FCs.Among the particles with higher plastic strain,the variation of the relative ratios of coarse and fine grains with respect to fines content is analyzed.The plastic deformation is more obvious in the samples with lower FCs.For the binary granular mixtures in which the fines fully fill the skeleton of coarse grains,the concentration region of plastic deformation is formed near the interface between coarse and fine grains.The correlation between various local structural parameters are quantitatively studied,hence the law of interaction between different local structural characteristics are determined.5.The mesoscopic causes of the diffusive instability of binary granular mixtures are discussed.Corresponding to the macro-instability process,the evolution laws of dilatancy angle,sliding friction angle,anisotropy and the properties of sub-contact systems in the process of instability are analyzed;Based on different contact types,the evolution characteristics of different contact types and effective coordination numbers in the process of instability are compared;Based on the three-dimensional algorithm for force chain extraction,the number of coarse particles in the force chains keeps relatively stable in the process of instability.However,the fine particles disappear gradually in the force chain structure,and the average force chain length and maximum force chain length decrease gradually in the process of instability;The contact proportion and contact normal anisotropy inside and outside the force chain structure change obviously in the process of instability;The probability distribution of contact normal anisotropy and contact normal force anisotropy of single force chain shows exponential decay law.However,the distribution of contact tangential anisotropy within single chain exhibit the characteristic of back peak;The force chain disappears and reorganizes intensively in fines particles,and the contribution of coarse particles to the force chain structure becomes higher and higher during the whole instability process;During the instability process of binary mixtures,the degree of plastic deformation and the temperature of particles increase gradually,and the change of fines content and the different instability stages have significant effects on the cage effect and memory effect of particle assembly.6.Under the condition of hyperquadric particle simulation,the packing state of particle assembly corresponding to different shape characteristics is analyzed,and the influence of different shape factors on the average contact level of binary granular mixture is analyzed.The locally ordered structure formed by irregular shape in the loading process is verified by radial distribution function.The stress-fabric-force relationship based on the cross-term of different anisotropic tensors is proposed.The contribution of the anisotropy within irregularshape granulars to macroscopic strength is analyzed,and the stress-fabric-force relationship under different fines contents is validated;The evolution law of anisotropic parameters under different shape characteristics is presented;In view of the fact that the shape of coarse and fine particles varies individually in binary mixtures,it is found that the influence of the shape of coarse and fine particles on the macro-strength and micro-plastic deformation is affected by the content of coarse and fine particles.