The Micro-damage Constitutive Model For Soil

Because the damage constitutive model based on continuum damage mechanics does not concern the physical mechanism of microscopic damage, it seems to only provide an estimate of the macroscopic deformation of soil and its applicability is limited to the particular stress conditions or stress paths. In the present studies for microscopic damage of soil, only the descriptions about the phenomena and the result obtained from the CT test, rather than the constitutive models that can reflect the influence of microscopic damage, are given.This paper explores two methods to establish the microscopic damage constitutive model of soil:(1) Assuming the skeleton of soil is composed of perfect skeleton and damage skeleton in the damage deformation phase, and the conversion from perfect skeleton to damage skeleton obeys Mohr-Coulomb law, a calculation of the volume fraction of damage skeleton is conducted according to the relative distance from the stress circle to the initial damage line. Regarding the perfect skeleton as perfectly bonded impurity and the damage skeleton as imperfectly bonded impurity, the nonlinear functions for calculating the composing modulus are obtained by using micromechanics method. To evaluate the performance of the proposed constitutive model, comparisons between simulated deformation and the actual deformation were conducted. The consequence indicates that the proposed micromechanics model was capable of simulating the nonlinear shear deformation behavior for the soil which has not significant amount of shear dilation.(2) A hypothesis that the skeleton deformation is only the deformation of the interfaces of grains, and the deformation characteristics including nonlinearity, plastic deformation and shear dilation are controlled by the sliding of interfaces of grains within soil skeleton, is presented in this paper. According to the relative distance from the stress circle to the initial sliding envelope line, direction-range value of the sliding interfaces can be determined. The damage ratio is defined as the ratio of the previous direction-range values to its maximum value (determined by failure stress circle). The overall shear modulus is the weighting average of the perfect contact shear modulus and sliding contact shear modulus. The volumetric strain is made up of two parts: the one induced by hydrostatic stress, the other one induced by plastic shear strain. Test on the versatility of the proposed damage model based on the micro-sliding mechanism, including varying hydrostatic stresses, consolidation methods and stress paths, indicate that the proposed micro-sliding damage model is capable of describing deformation characteristics for various conditions. In addition, the relations between damage ration and shear stress, stress ratio and generalized stress ratio proposed in this paper are illustrated and compared. The result reveals that the damage ration has a one-to-one relation with the generalized stress ratio, namely, the generalized stress ratio proposed in this paper can be also used to describe the damage state.