Research On Constitutive Model And Numerical Simulation Of Sand Rheology Under Low Effective Stress

Granular material,as sand,exists like solid,liquid and gas in different environments.Under 1g condition on the earth,it generally exists as solid,effective stress reduction is a key factor of solid-liquid phase transition.Shear strength significantly reduces when phase transform s,flow and large deformation occurs under shear load.Sand exhibits completely different mechanical properties from 1g to g condition.This paper proposes a constitutive model that can uniformly describe the low-speed quasi-static flow and liquid-like dense flow of sand in a low effective stress-like solid state,and it has the ability to describe unsteady flow.This paper proposes a constitutive model can uniformly descri be the solid-like quasi-static flow and liquid-like dense flow of sand under low effective stress condition,this model can also describe unsteady flow of sand.The study of sand rheological behavior under low effective stress is of great significance to seismic liquefaction engineering,space exploration engineering,granular rheology research.This thesis will serve as the preparatory work for the China Space Station scientific experiment project.It will undertake the tasks of preparatory experiments,theoretical framework construction,and numerical simulation.The main content of the thesis includes:1.Establish a low effective stress shear rheological constitutive model,called TMS.The quasi-static part is a nonlinear elastic model based on sand triaxial experiment database,the viscous part is ()rheological model based on granular rheological experiments.Model can describe the unsteady and constant rheology of granular materials,and has been verified by the triaxi al creep experiment of sand under low effective stress.Simulation of strain-ratevariation experiment reveals that sand is pressure sensitive rate-dependent material.2.Analyze high peak friction angle phenomenon in NASA's microgravity experiment.After excluding other influencing factors,it is confir med that the high peak friction angle is essential to pressure sensitive rate-dependence of sand.Based on the limit equilibrium method,inverse calculation of JAX A's shallow foundation bearing capacity experiment under variable gravity also confirmed that the friction angle increased under low gravity.The experiment was simulated by the ALE finite element model with TMS constitutive model,confirms that this phenomenon is also caused by pressure sensitive rate-dependence of sand.3.A granular materials buried rod pulling experiment under microgravity was carried out in the CAS's drop tower in Beijing,result shows that the pulling force of the buried rod changed from rate-independent under 1g condition to ratedependent under 0g condition.The experiment was simulated by CEL finite element model with TMS constitutive mode.The simulation results of the buried rod pulling force are in good agreement with the measured v alues in experiment.Through the analysis of contours,it is found that with the pressure decreases under microgravity condition,the rate sensibility of granular material increases.The viscous friction stress changes considerably with the pulling speed o f the buried rod,and this phenomenon does not occur under 1 g condition.Pressure sensitive rate-dependence of sand is essential to rate-dependency of buried rod pulling force.