Study On Meso-scale Characteristics Of Strain Localization Of Sand Based On PFC Discrete Element Simulation

The failure process of geotechnical materials is usually accompanied by the phenomenon of strain localization.Studying the strain localization of granular materials from the meso-scale is helpful to understand its physical and mechanical nature.When the soil reaches and exceeds the peak strength,the deformation of the soil gradually concentrates in a narrow band-shaped area from a uniform distribution.Such banded regions are shear bands.The appearance and evolution of shear bands are typical features of strain localization.Geotechnical materials,especially soil particles,are discrete.Therefore,the use of continuum mechanics to analyze geotechnical problems often has limitations.For example,it is difficult to simulate the shear failure problem in the large deformation stage,and it is difficult to simulate the particle behavior at the meso-scale.The discrete element method is suitable for specific and precise analysis of the particle behavior at the meso-scale.At present,in the discrete element method,the observation methods of the localization of the strain(such as the width of the shear band,the inclination angle,the type of distribution,etc.)are mainly within the scope of manual,visual inspection,and estimation.Intelligent and automated observation methods have yet to be studied.In the study of macroscopic rock and soil strain localization,the Cosserat continuum theory has advantages over discrete element and other numerical methods based on particle scale.By introducing the internal characteristic length,not only can the mesh sensitivity problem in the simulation of the shear band be solved,but also the width of the shear band in the finite element simulation can be controlled.In this study,numerical simulation and analysis of the strain localization process in dense sand are carried out,based on the discrete element method.The main research contents and results are as follows:(1)The numerical simulation of the biaxial compression experiment of dense sand was carried out in the discrete element software PFC.Developed the code for generating and compacting particles in eight layers.The development process of stress,strain,velocity field,displacement field,rotation angle field,force chain network,porosity field,coordination number field,etc.in the experiment is analyzed.(2)The distribution types of shear bands observed in the samples are classified.There are three types: "Single Shear Band","Dual-X Shear Band",and "Other Types".(3)A shear band observation method is defined in the PFC software,based on the "accumulated rotation of particles".A program for measuring the width of the shear band was developed,based on the principle of "α-shape".The program can automatically measure the width,inclination,and distribution type of the shear band obtained in the biaxial compression experiment of PFC.(4)When using the developed identification program,a number of experiments were carried out in order to study the influencing factors of the width of the shear band.A variety of influencing factors(including random seeds,confining pressure,porosity,particle gradation,etc.)and their influence mechanisms are analyzed.The essence of the impact on the shear zone is that these factors affect the rotation characteristics of soil particles.The development of the shear band is related to the rotation effect of the larger particles in the sample.A brief comparative analysis was made between the influencing factors of the shear band studied in the discrete element method and the influencing factors contained in the evolution formula of the Internal characteristic length in the Cosserat finite element theory.