Heterogeneity Of Granular Material And The Studies Of Its Static And Dynamic Response Based On DEM

Granular materials are widely found in nature,ranging from powder,dusts etc.to rocks and even celestial bodies.They are the most common types of substances in daily life.The applications of granular materials have a long history.Researchers have been conducting researches on the physical and mechanical properties of granular materials.Since the Discrete Element Method(DEM)was introduced,the development of particle mechanics has been advancing by leaps and bounds.A lot of research results have been gradually applied to many fields such as geological disasters,geotechnical engineering,chemical engineering,environmental science,and transportation etc.Granular matter is a macroscopic system composed of discrete solids.Owing to its discrete feature,when studying the properties of granular matter,it's natural to consider non-uniformity(heterogeneity).In addition to heterogeneity,the wave propagation behavior of granular matter is also one of the research hotspots.Granular systems present nonlinear dynamic behaviors stemming from the Hertzian contact interaction between particles and the inability to withstand tensile forces.Numerous studies have shown that granular matter is very capable of tuning waves,which makes granular matter a promising candidate for shock wave shielding,energy dissipation etc.Based on the discrete element method this thesis has studied the properties of the above two aspects of granular materials.The details are as follows:First,considering the physical property heterogeneity and geometric heterogeneity,their influence on the mechanical properties of granular materials are discussed.A series of specimens containing a certain amount of impurities(physical properties are different)are prepared.The influence of the location and distribution of impurities on the strain localization and the failure process of granular material has been analyzed.The relationships between the formation of shear bands in granular material and the initial defect distribution are confirmed.With the deepening of the loading process,the appearance of vortex structures and localized phenomena indicates the correlation of the movement between particles.Then a method for quantifying the heterogeneity of single-size particle samples based on the concepts of distance and deviation has been proposed for geometric heterogeneity to study the influence of heterogeneity on granular materials systematically.The relationships between the heterogeneity index and the particle sample are analyzed.It is proved that the existence of the heterogeneity zone in the sample is an important reason for the formation and distribution of shear bands.Two deformation modes in different areas of the granular material during the loading process have been analyzed containing the differences of particle deformation and force chain evolution.Finally,the critical dimension of the couple stress effect is suggested.Reference to the idea of coordination of strain energy with failure criteria,a failure criterion suitable for brittle transversely isotropic materials has been proposed and verified by several sets of experiment results.Then,a series of specimens containing inclusion bands that show different inclined angles are simulated.The effects of different material parameters on particle velocity amplitude and wave velocity are studied.The attenuation process of energy along the propagation direction has been studied.The relationship between the force transmission path and the wave front shape has been preliminarily explored;the relationship between wavefront shape and equivalent strain distribution is analyzed,and the related problems of wavefront shape are discussed.Then,the influence of different impact angles on the wave propagation behavior is studied,involving the acceleration,kinetic energy and wave velocity of the particles.Finally,following the previous part of the study,the evolution of microscopic structure has been introduced into the sample by using the ratio of the particle radii.The wave propagation behaviors of particle assemblies with different radius ratios under impact loading have been studied.The change of wave velocity with microstructure is analyzed,and the reasons for the change and possible explanations are given.Then,based on the assumptions about the interaction force between the particles and the energy dissipation at the contact point,the mechanism of the dissipation process is analyzed.By investigating the movement of several particles near the excitation source and tracking the distribution of impact kinetic energy,the attenuated part of energy transfer is defined by the difference between the total impact energy and the main wave energy.After explaining the relationship between wavefront shape and microstructure,the wavefront shape in the densely packed particle sample has been studied analytically,its analytical expression is obtained,and the stopping distance and propagation range are predicted respectively.