Discrete Element Numerical Simulation On Mechanical Response Characteristic Of Dense Granular Materials

Dense granular materials are widely encountered in nature and various industrial processes.Due to its discrete nature,the stress distribution in dense granular material demonstrates clearly anisotropic and heterogeneous characteristics,which lead to the intense spatial correlations of the mechanical response of granular particles.From the industrial application point of view,the heterogeneous stress distribution and the spatial correlations of particle mechanical response can lead to the un-uniform or unstable flows of granular materials,which subsequently result into the abnormal transfer and reaction performances.From the theoretical point of view,the strong spatial correlations of particle mechanical response challenges the development of general rheological constitutive laws based on spatial-averaging methods.Thus,thoroughly understanding the complex mechanical response of dense granular materials is not only useful for guiding various granular handlings but also valuable for developing the general theory of granular rheology.By performing two-dimensional Discrete Element Method(DEM)simulations,in this work the mechanical responses of dense granular materials under the action of dragged intruder and/or horizontal shear are investigated.The main findings of this work are as follows:(1)For dragged intruder simulations,the horizontal force experienced by the intruder is closely related to intruder size and the friction coefficients of particles.The intruder size decides the amount of surrounded particles that are dragged by the intruder.And friction coefficients affect the average normal contact force between particles.(2)The analysis of the stick-slip phenomena shows that every slip event is accompanied by the sharp change of the horizontal position of intruder.The movement of intruder leads to the formations of two vortices in the velocity/displacement field.And the arrangement of the two vortexes is nearly symmetric according to the drag direction.By analyzing the particle contact information,it is found that during the stick stage the difference between the average angle of strong contacts and that of weak contacts slowly increase,indicating that the geometrical supports from weak contacts to strong contacts are strengthened.However,the number of strong contacts decreases,and the number of weak contacts in friction sliding state increases.Such results suggests that the formation of slip event is due to the accumulated particle sliding during the stick stage.(3)As to horizontal shear,in the range of parameters investigated the effects of confining pressure and shear velocity on macroscopic friction coefficient are nearly ignorable.The macroscopic friction coefficient is closely related to particle friction coefficient.Based on the analyses of particle contact information,it is found that such relativity is due to the fact that particle friction coefficient changes the anisotropies of contact direction and normal contact force.(4)The horizontal movement of bottom wall significantly decrease the traction force experienced by the dragged intruder.The traction force decreases with the increase of shear velocity.Macroscopically,this is due to the fact that horizontal shear of the bottom wall decreases the voidage of granular materials outside the shear band.Microscopically,the increase of shear velocity decreases the anisotropy of normal force at the contact located outside the shear band.The decrease of the anisotropy of normal contact force results into the decrease of macroscopic friction coefficient,which subsequently leads to the decrease of traction force.