Transient Evolution And Microscopic Mechanism Of Shear Rheological Properties Of Dense Granular System

Granular matter is a complex multi-body system formed by the interaction of a large number of discrete solid particles,and it can be divided into granular gas,granular liquid and granular solid according to its motion state.Among them,dense granular system can exhibit dual mechanical properties similar to solids and fluids,and undergo solid-liquid phase transitions under certain conditions.The solid-liquid phase transition of dense granular system is the key to develop granular lubrication as a new lubrication method.At the same time,it is also crucial to optimize industrial processes such as powder metallurgy and grain transportation,and to improve the understanding of the formation mechanism of natural disasters such as desertification,avalanches,and landslides.Due to the discontinuous,non-uniform,and energy dissipation characteristics of granular matter,the solid-liquid phase transition process of dense granular system strongly depends on multiple physical scale levels such as particle properties,contact characteristics,microstructure and external boundary conditions.At present,the necessary conditions and internal mechanism for the transformation of the flow state are still unclear.Therefore,my thesis starts with the contact geometry and mechanical characteristics and evolution law of the internal microstructure of the granular system,establishes the internal correlation of the mechanical characteristics at different physical scale levels,and explores the influence law and mechanism of flow state changes on the shear rheological characteristics of the granular system.Firstly,taking the two-dimensional granular lubrication system with periodic boundary conditions as the object,the evolution process from initial static to steady flow state under the action of plane shear is simulated by discrete element method,and the motion characteristics such as particle motion velocity,shear strain rate and shear expansion and the transient evolution law of macroscopic mechanical response are analyzed in detail,and the influence of flow rate on shear rheological characteristics is studied.The simulation results show that the granular system undergoes three stages of transfer,adjustment and stabilization during the evolution of the flow state,and the increase of shear expansion degree and effective friction coefficient occurs in the transfer stage.With the increase of flow rate,the macroscopic stress-strain behavior of the granular system changes from strain softening to strain hardening,and the effective coefficient of friction increases significantly when the flow state is stabilized.Subsequently,the stress-force-fabric relationship is used to describe the transient evolution of the anisotropy characteristics of the microstructure of the granular system in three aspects: contact direction,normal contact force and tangential contact force distribution,and the correlation between shear rheological characteristics and microstructure anisotropy characteristics was established.The results show that the anisotropy degree of contact direction and tangential contact force gradually increases during the evolution of flow state,and the increase of anisotropy degree in contact direction is at the cost of the decrease of the anisotropy degree of normal contact force.The increased flow rate leads to an increased degree of anisotropy of the internal microstructure,thereby increasing the shear strength of the granular system.In addition,according to the particle contact force threshold,the whole set of the granular system is divided into strong and weak contact subsets,and the effect and contribution weight of the strong and weak contact subsets on the shear strength of the granular system are analyzed by using the unified characterization method of anisotropy of the partition contact subset.The results show that the strong contact subset plays a leading role in determining the bearing and shear strength of the granular system,while the weak contact subset plays a stabilizing role.In addition to the normal contact force anisotropy of the weak contact subset,the other anisotropies of the contact subset contribute positively to the shear strength of the granular system.Moreover,the normal contact force anisotropy of the strong contact subset is the largest contributor to shear strength.With the increase of flow rate,the carrying capacity of strong contact subsets is further enhanced,but its contribution weight to shear strength decreases,mainly due to the anisotropy of weak contact subsets being more sensitive to changes in flow rate.Furthermore,the micromechanical properties of sliding and non-sliding contact subsets are quantitatively analyzed by the key contact feature partitioning method,and the mechanism of the transition of contact sliding state on the flow state evolution of the granular system is explored.From the topological network structure characteristics of contact force,it can be found that the non-sliding contact subset contains a large number of longer-range force chains,and its contact subnetwork is relatively dense,reflecting solid-like characteristics,which is the bearing skeleton of the granular system.The sliding contact subset consists of a discrete short-range force chain structure that exhibits a fluidlike mechanical behavior.The segmentation of sliding contact subsets over non-sliding contact subsets disrupts the stability of the microstructure of the granular system.The sliding and non-sliding contact subsets exhibit differences and compensation in maintaining the stability of the granular system,which is the fundamental reason why the granular system microstructure maintains a dynamic equilibrium between collapse and reconstruction.Finally,a linear reciprocating granular friction test device is designed and built,and the load-bearing characteristics of the granular system and the evolution of the force chain structure are explored by using the change results of the normal pressure of the shear plane,and the dependencies of the granular friction coefficient on the applied load and the sliding speed are determined dependencies.The results show that in the process of reciprocating sliding,the change of the normal pressure on the shear plane corresponds to the evolution process of the force chain structure relaxation,collapse,reconstruction and stability,and the change of the normal pressure on the left and right sides is closely related to the sliding directions,which can reflect the dynamic change process of the spatial distribution of the force chain network.There exists a significant powerexponential decreasing law between the experimentally measured granular friction coefficient and the applied load,and a weak linear increasing relationship with the sliding velocity.In this thesis,the influence of flow state changes on the shear rheological properties of dense granular system and the microscopic mechanisms are studied from macroscopic,microscopic,and mesoscopic scales.The research results strengthen the internal connection between different scales of the granular system,highlight the importance of key feature contacts in exploring the granular rheological mechanism,and expand the theoretical thinking and experimental methods of granular mechanics research,and have important guiding significance for the practical application of granular lubrication.