Discrete Simulation On The Mechanical Behavior Of Granular Media

Granular materials are commonly encountered in nature and in various industrial processes.Different from the cases of fluids and solids,the stress distributions in compressed dense granular media demonstrate the characteristics of notable anisotropy and heterogeneity,leading to the formation of force chain meso-scale structure.From the industrial application point of view,the heterogeneous stress distributions could lead to the un-uniform or unstable flows of granular materials,which might subsequently result into the abnormal transfer and reaction performances.From the theoretical point of view,the anisotropic and heterogeneous stress distributions make the dynamic response of granular particle present strong spatio-temporal correlations,which challenges the applications of continuous models to simulate the flow and mechanical behavior of dense granular materials.Thus,thoroughly understanding the complex dynamic response of dense granular materials is not only valuable for developing the theory of granular rheology but also useful for guiding various granular handlings.By performing the Discrete Element Method(DEM)simulations,in this work the influence of material properties including the frictional coefficients and granular composition on the probability distributions of contact forces,stress distribution and also the macroscopical strain-stress behavior was thoroughly investigated.The main findings of this work are as follow:(1)In the range of parameters investigated,the influences of frictional coefficient and the volume fraction of granular components on the probability distributions of dimensionless normal and tangential contact forces were ignorable.The probability distributions of the normalized contact forces only depend on the stress condition.By comparing the simulation data and those predicted from the five typical force distribution functions available in literature,it has been found that the Weibull model and Nagan model give the best fitting degrees.And most importantly,the variations of the empirical parameters appearing in these two models show well fitted functional dependence on the maximal principal stress,which indicate that these two models have better generality.(2)For dense granular media made of single component,the simulation results show that the value of the efficient frictional coefficient increases with the increase of particle-particle frictional coefficient.At the particle scale,increasing particle-particle frictional coefficient leads to the increases of the anisotropy of particle contact direction,the spatial heterogeneity of stress distribution and the average contact forces,and the decrease of particle coordination number.(3)For the binary mixture made of soft and hard particles,the contribution of each type of particles to the macroscopical stress changes when the volume fraction of soft particle is around 60%.Increasing the volume fraction of soft particles leads to the decrease of the anisotropy of particle contact direction,and the increase of the anisotropies of normal and tangential contact forces.Such two opponent tendencies result into the fact that the value of the efficient frictional coefficient keeps constant.(4)For the binary mixture made of different size particles,the distribution of macroscopical load among each type of particles changes when the volume of small size particles is around 30%,below which the macroscopical stress in mainly contributed by the large size particles.increasing the volume fraction of small size particles increases the anisotropy of contact length but decreases the anisotropy of contact direction,leading to fact that the value of the efficient frictional coefficient is nearly independent from the volume variation of small size particle.