Discrete Element Investigation Of Physical And Mechanical Properties Of Granular Materials

Granular materials widely exist in nature,industry and engineering.It is of importance for engineering practice to investigate physical and mechanical properties of granular materials.There is a series of key factors affecting physical and mechanical properties of granular materials,e.g.,material property,particle shape,particle size distribution,internal and external perturbations,and so forth.The discrete element method(DEM)becomes one of the most effective numerical tools used to investigate granular mechanics in the recent three decades.This thesis presents a comprehensive study on the theory of DEM with creative theoretical and technical contributions to DEM modeling.A series of DEM simulations from granular random packing to mechanical behaviors under perturbations were carried out.The physical and mechanical properties of granular materials were examined with considering particle shape effect.Major work and findings from this thesis are summarized below:(1)Versatile polyhedral and super-ellipsoidal DEM models were established.The collision detection algorithm among non-spherical particles was studies first,then a robust in-house DEM code for non-spherical particles,Sudo DEM,was developed based on a general framework provided by an open-source code YADE.Moreover,specific codes for random packing,direct shear and triaxial compression tests of non-spherical particles were prepared to investigate the effect of particle shape on packing structure and mechanical responses in the coming work.(2)Effect of particle properties on fabric of granular random packing was investigated.A series of random packing of non-spherical particles was simulated.Effects of friction coefficient,contact stiffness,container size and particle resolution on packing density and coordination number of regular tetrahedral particles were examined.Analysis of effect of particle shape(aspect ratio and eccentricity)on packing structure of tetrahedral particles was conducted.Moreover,another comprehensive meso-scale analysis of packing structure of super-ellipsoidal particles was conducted with considering effect of particle shape(aspect ratio and blockiness)as well,where the effect of particle shape on fabric of granular random packing was further revealed.The whole study also validated the established non-spherical DEM models in the application for random packing,which was a basis of conducting direct and triaxial compression tests of non-spherical particles.Furthermore,benefits and drawbacks of multi-spherical,polyhedral and super-ellipsoidal models were discussed.(3)Sheared mechanical behaviors of non-spherical granular materials were investigated.The Hertz-Mindlin(HM)contact model for super-ellipsoidal particles was studied and implemented in the present code.With a comparison of macro-and meso-mechanical behaviors of granular materials during triaxial shear tests between the linear spring(LS)and HM models,it was found that the LS model can be applied to investigating meso-mechanics of granular materials with complex particle shapes,thereby speeding up simulations.Next,two particle shape descriptors,i.e.,angularity and asphericity,was proposed.Quasi-spherical polyhedron and superball models were established for investigating effects of particle angularity and asphericity on the mechanical behaviors of granular materials during direct shear and triaxial shear simulations.The meso-mechanism that particle angularity and asphericity enhance macroscopic strength and stiffness was analysed.Finally,the algorithm of 3D Set Voronoi tessellation for super-ellipsoidal particles was studied and implemented,by which the internal structure of sheared granular materials was analysed.Relationship between average Voronoi cell-related quantities and global quantities(global porosity and mean coordination number)was found.Further,effect of particle shape on characteristics of Voronoi cells was examined,and a preliminary understanding of the relationship between particle and void networks was developed.(4)Thermally-induced shakedown of granular materials under internal perturbation(long-term thermal cycling)was investigated.A thermal-cycling DEM model was established,and fabric evolution was examined during thermal cycling,as well as effects of the heating rate,the initial consolidation stress and the magnitude of temperature change.A significantly accumulative effect on shakedown of granular materials(with significant stress relaxation like granular rheology)was observed,and the corresponding meso-mechanism was preliminarily analysed.