Numerical Simulation Of Static Mechanical Behavior Of Ordered And Disordered Particles

As one of the most existing substances on the earth,granular material is widespread in nature and widely used in production practice in daily life.Granular matter is composed of a large number of solids with similar properties,which are closely related to each other.It is a typical disordered system of multi-body interaction.Therefore,it shows many unique mechanical properties,such as Brazilian fruit effect,jamming,fluid-solid transformation,etc.Although human research on granular materials can be traced back to thousands of years ago,the cognitive level of granular materials is still limited,which is only equivalent to the cognitive level of solid physics in the 1930 s.The physical nature and motion law of granular materials still plague the scientific community and limit the development of industrial production.Therefore,the study of this complex special system and the understanding of its solid state in motion law not only have important application background,but also can promote the development of physics.In this paper,two typical loading cases,namely,hexagonal close-packed particle material and randomly distributed particle system under triaxial loading,are taken as the research objects,and the mechanical properties of the two particle systems are studied by the discrete element(DEM)method of numerical simulation.Compared with the traditional particle material experiment methods(photoelasticity method,CT imaging technology,etc),DEM can conveniently obtain all kinds of data in the particle system at various stages of the loading process,such as the size and direction of the contact force,the accurate position of the particles,and the evolution of various energies,and its cost is lower.The main research contents of this paper are as follows:(1)The basic principle and idea of discrete element method are introduced,and the hard ball model is briefly introduced.Two soft ball models used in this paper are mainly introduced:linear contact model and parallel bond model.(2)The equilibrium state of hexagonal close packed particle system under point load is simulated by discrete element method.The characteristics of the mechanical response changing from single peak to double peak under different loads are analyzed.The result shows that the mechanical response under different depths is consistent with the depth of the broken contact network.The force chain network in the system is divided into three categories,and the role of each force chain network is analyzed.A method to control the mechanical response of particle system by bonding is proposed,and the influence of bonding strength on mechanical response is given.At the same time,the influence of bonding on three force chain networks is pointed out.Finally,the phase diagram of the effect of bonding on the mechanical response of hexagonal close-packed particle system is given.(3)The triaxial loading process of randomly distributed particle samples was simulated by discrete element method.The results show that the dense particle samples and loose particle samples show different mechanical properties in the loading process.According to the variation characteristics of macro stress and volume strain,the loading process of the two samples is divided into different stages,and the characteristics of each stage are given.Through the comparison with the macro stress,it is found that the number of force chains depends on the transmission direction of force rather than the change of system geometry.At the same time,the size of force chains in each direction reflects the change characteristics of macro stress.The evolution law of various energies in the loading process is analyzed from the perspective of energy,and the contribution of friction coefficient in stabilizing the structure of force chain is pointed out.The increase of friction coefficient effectively increases the stress peak of dense particle samples.(4)Through the simulation results,some assumptions are made on the displacement field in the process of triaxial loading,and a function characterizing the displacement field of particles is proposed.Using the method of subsection fitting of characterization function,the particle displacement along the diagonal direction is given,and has a certain accuracy.The characterization function is extended to the whole loading process by the error of boundary condition,and the width of shear band in dense samples is studied by this function.