Research On The Cyclic Constitutive Model Of Saturated Sands Based On The Phase Transformation State

Dilatancy, cyclic performance and anisotropy are the fundamental characteristics of sands, which make it different from other materials significantly. A particular point, namely the phase transformation point, can be observed on the stress-strain curves on the condition of both drainage and undrainage tests. It is one of the key features of sands, and leads the way for studying dilatancy, cyclic performance and anisotropy of saturated sands. In this paper, the behavior of saturated sands under both monotonic and cyclic loadings were studied from the three aspects mentioned above.By extending an existing constitutive model, and starting from the microscopic mechanism of sand, a saturated sand cyclic bounding surface constitutive model based on phase transformation state has been established in this paper. The model is considering the fabric changes on the influence of the accumulated plastic strain in the process of dilatancy, and combining with the cycle of saturated sand characteristics. It can describe the mechanical behavior of sand under monotonic and cyclic load under different drainage conditions.In this paper, the main research results are as follows.1、At first, based on the experimental of saturated sands under different loading conditions, macroscopic deformation mechanism of sand caused of microstructure change is analysed starting from the microcosmic of sand, and then the relationship between macroeconomic variables and the change of microscopic structures is established. A fabric-dilatancy variables is introduced, in order to reflect the effect of macro dilatancy tensor’s change caused by micro fabric on cumulative plastic deformation of the saturated sand, under cyclic load in reverse loading process. Addition, the relationship between macro and micro variables is established.2、The determination of plastic modulus in most existed cycle constitutive models is complicated, which involves many parameters. Base on the experimented results that the change of saturated dense sand decrease under cycle load in drain tests, a new changing rule for plastic modulus in the process of first loading and reloading (that is, the monotonic and cyclic loading process) is proposed. Then a modified plastic modulus expression is derived, which is easy to determine and suitable for cyclic loading is proposed.3、Ishihara proposed a e-log p’expression in 1993 years, combining state parameter proposed by Zhang Weihua, weakness is founded when it is simulated the mechanical behavior of sands under undrained condition. In this paper, the expression is improved, and a expression of ept is put forward which is suitable for calculation of phase transformation void ratio under undrained condition. And then a undrained monotonic constitutive model is build up. The calculation results which are obtained by constitutive model and phase transformation void ratio expression compered with the test data, the results show that the constitutive model can well reflect the change of effective stress, and the new expression is reasonable and effective.4、Integrated the above three work, in view of the cyclic loading (not including dynamic load), saturated sands boundary-surface constitutive model for different drainage condition is put forward. The model can describe mechanical behavior under cyclic load and the different drainage conditions, Such as deformation process of saturated sands shear shrinkage firstly and dilatancy under drainage condition, and the phenomenon which is volume change incremental be smaller and smaller, finally the change trend be stable. The model can also describe mechanics behavior of undrained condition. And it can better reflect two main characters points of sands which are peak point and phase transition.