Constitutive Model For Soils Based On Thermomechanics In Three Dimensions

There are many problems when using classical plasticity theory originated from the research of metals to geomaterials.In recent years,the plasticity theory(or “hyperplasticity)based on thermomechanics was proposed,which is more general and applicable compared with classical plasticity theory.The elementary theory and the general procedure of constituting models based on thermomechanics are introduced.1)The yield condition in dissipation stress space can be deduced by choosing a proper dissipation function.And the flow rule in dissipation stress space is determined by Ziegler orthogonality principle.2)The shift stresses and elasticity law are deduced from free energy function.3)The yield condition and the flow rule in true stress space can be deduced by adding the shift stresses to dissipation stresses.4)A complete constitutive model can be established by adding the hardening law.Some well-known existing critical-state models are re-examined in the framework of thermomechanics.The isotropic model(?-? model)proposed by Collins is analyzed and the validity of this model is confirmed by test data.The direction of plastic strain increment for geomaterials is determined by analogy between geomaterials and metals based on the concept of characteristic plane.The plastic strain increment for metals is determined by the stress on its characteristic plane-octahedral plane.Based on the analogy between geomaterials and metals,the direction of plastic strain increment for geomaterials can be determined by the direction of the stress on the characteristic plane(SMP plane).The equations for the calculation of the direction of plastic strain increment are proposed.Furthermore,it can be applied to the cohesive materials by adding a material parameter.According to the proposed method,the characteristic plane for geomaterials—SMP plane reduces to octahedral plane which is the characteristic plane for metals.The plastic strain behavior of metals and geomaterials can be unified formally.Furthermore these equations are confirmed by test data.In order to describe the strength and deformation in 3D,the ?-? Model should be revised by transformed stress,since it is a 2D model.In transformed stress space,the direction of plastic strain increment obtained here is used to define the direction of shear stress.The ?-? Model revised by transformed stress can simulate the test data well.