| The macro-continuous,micro-discrete,instability and the failure of sand are progressive processes.Its complex multi-scale mechanism leads to the result that the stability and deformation analysis have been studied independently and separately for a long time.Furthermore the project practice of progressive failure is limited because of the complex traditional elastoplastic theory and its numerous model parameters.Meanwhile,the progressive failure also involves the localization deformation which forms after the shear band generates.As a result,multi-scale study is necessary and is indispensable.In this paper,the discrete element method is used to simulate the deformation and failure process of sand in real three-dimensional space.The micro-fabric is linked with the macroscopic progressive failure characteristics based on the result of quantitatively description of sand fabric proposed by Li et.al.,and a novel modeling idea,hypoplastic theory,is also used to establish the anisotropic model which could take account into fabric,the effects of principal stress rotation on the macro-mechanical properties.Finally,the progressive failure characteristics of sand are investigated from the perspective of multi-scale combination by taking the advantages of discrete element method,finite element method and micropolar theory on the description of progressive failure of sand,as well as the macroscopic phenomenon of strain localization from biaxial test.The main work and conclusions of this paper are as follows:(1)For studying the effect of the micro-fabric evolution on the macro-mechanical properties,the true triaxial numerical model and biaxial numerical model with different particle shapes and fabrics are established based on the discrete element method.Firstly,the particles with different shapes are described quantitatively.Then,the true triaxial and biaxial numerical models are established,as a consequence,we study the effects of intermediate principal stress coefficient on the strength and deformation characteristics of anisotropy sand under the condition of constant mean principal stress from the macro-scale as well as the effects of particle shape on the strength and deformation characteristics of sand from the microscopic scale.The relationship between stress and fabric is derived,and the evolution trend of the sand fabric during the whole loading process is obtained.The effects of fabric on the anisotropic degree are also described by using the concept of deviatoric fabric tensor.(2)The hypoplastic constitutive model is established to study the effects of sand fabric on the strength and deformation characteristics.The anisotropic state variable is proposed via fabric tensor and stress tensor by combination of macro and micro method,whose physical meaning is clear and the form is relatively simple.Futhermore,the anisotropic state variable is introduced into the equation of critical state of hypoplastic constitutive model.As a result,the hypoplastic constitutive model taking account into fabric is developed,which can not only consider the stress history but also could reflect the influences of pore ratio and fabric.(3)The anisotropic hypoplastic constitutive model considering principal stress rotation is proposed,by introducing the effect of the principle stress rotation and combining the fabric tensor.Consequently,the hypoplastic model is developed which could reflect the effects of principal stress rotation based on the new model.A discussion is given out,which is about the effects of principal stress rotation on anisotropic state variable,critical state,the strength and deformation characteristics under the plane strain condition.The model is also verified by the results of plane strain compression experiment.(4)The effects of sand micro-fabric on strain localization are analysized with the combination of micropolar theory and the new proposed hypoplastic constitutive model based on finite element method.By combining the micropolar theory which could consider the micro-size effects and the hypoplastic constitutive model,a series of plane strain numerical simulations are carried out with software,Abaqus.These simulations study the effects of "characteristic length" on the shear band width in micropolar theory,as well as investigate the effects of fabric on strain localization about a number of factors related to the shear band,such as the moments of formation,angle,initial shape and rotation velocity. |
