The Generalized Plasticity Cosserat Continuum Model And Its Application

The geotechnical materials are generally heterogeneity in natural conditions,which has a significant impact on the stress-strain relationship,bearing capacity,deformation characteristics of materials,development of shear band zone.In current theories,it is still assumed that the soil mass is uniform in space and few constitutive theories can consider the static and dynamic response at the same time.In addition,particle breakage is also a common phenomenon in the process of loading and its influence on the stress-strain relationship can not be ignored.In addition,in some actual projects such as earth and rockfill dam,particle materials such as rockfill and sand often are changed in the aspect of the stress and strain relationship due to particle breakage under high confining pressure.Moreover,it may affect the overall settlement and deformation of the dam.At the same time,some studies show that particle breakage is one of the most important factors to affect the deformation of dam body under earthquake load.In order to develop a constitutive theory which can carry out both static and dynamic analysis at the same time and take the effect of particle breakage into account,the following work is done in this paper:(1)The generalized plasticity theory proposed by Pastor,Zienkiewicz et al.,including the improvement of this theory by subsequent scholars,is develpoed as a constitutive program in the framework of ABAQUS secondary development.It is used in conventional triaxial compression experiment analysis,dynamic triaxial experiment analysis,filling phase and seismic analysis of dam.The numerical simulation results are compared with the experimental results and the simulation results,and the correctness of the generalized plasticity model is verified.It takes the particle breakage characteristics of materials into account and proves that the particle breakage has an important influence on the stress-strain relationship and the relationship between bearing capacity and displacement.It also proves that the generalized plasticity theory can keep the consistency of model parameters in the condition of considering the static and dynamic loads at the same time.The analysis of rockfill dam under static and dynamic load conditions reveals the influence of particle breakage on settlement deformation.At the same time,from the perspective of the number of fill layers,the inverse correlation between the settlement deformation and the number of layers is explained.(2)By simulating the plane strain experiment of which Borja,a Stanford University professor,has done and considering the spatial non-uniform distribution of soil,the generalized plasticity model is used to simulate the plane strain experiment in heterogeneity soil and the conventional homogeneous soil at the same time,and the correctness of the generalized plasticity model is furtherly verified.Taking the void ratio as a special case of non-uniformity,the influence of soil non-uniformity on the development of shear zone is analyzed in this paper.In addition,the influence of heterogeneity on the development of shear band and the softening of bearing capacity is revealed by comparison.It is pointed out that the X-type shear band in common cognition will show a certain tendency under the action of non-uniformity.By using different types of elements,the generalized plastic constitutive subroutine developed in this paper is used to simulate the plane strain experiment and the influence of element type on the development of shear band zone in nonhomonity soil is obtained.With a series of simulation tests by using a few kinds of elements,it shows that C3D20 R element has an excellent advantage in the aspect of simulating shear band with heterogeneity soil.The influence of particle breakage on the number of shear bands is revealed by correcting the model parameters.The controlling effect of some key parameters on shear band is also revealed.(3)The generalized plasticity theory is deduced to the three dimensional Cosserat continuum,and the generalized plasticity theory program is developed under the classical continuum and Cosserat continuum.The generalized plasticity classical continuum element shows the influence of soil heterogeneity on shear band zone more clearly and the conclusion that the strain localization in classical theory is mesh-dependent is obtained by dividing grids in different densities.In addition,through the analysis of the void ratio and generalized shear stress at some points in the sample,the reason of the softening bearing capacity in the classical theory is obtained.Finally,through the plane strain simulation of generalized plastic Cosserat continuum,it is proved that the Cosserat theory has the characteristics of overcoming the griddependence phenomenon in the phase of strain localization caused by the classical theory,which have reasonable numerical simulation results.At the same time,the curves of bearing capacity and displacement are highly consistent without serious bifurcation.Besides,a relationship between inner length parameter and median particle diameter is proposed.In this paper,a relationship between relative breakage rate and plasticity work has been described and another relationship between average particle size and relative breakage rate is proposed.The model can get the change of average particle size through particle breakage,and then it can deduce the change of internal length parameters.So in this way,the model proposed in this paper can get a function which is the dynamic simulation of Cosserat internal length parameters according to the particle size in the phase of particle breakage.The furthur meaning of this improvement measure is to give a real physical significance to Cosserat internal length parameter which is not meaningful in general cognizance.From the perspective of shear band width simulation results,it is in harmony with the internal length parameters meaning proposed by the Cosserat brothers to represent the microdimensions of micro-structural in the materials.What is more,the relationship between internal parameter and relative breakage rate is proved to be an important factor in controling the width of shear band in plain strain experiment.