一种预测材料单轴饱和棘轮应变的本构模型

通过大量单轴棘轮实验 ,研究了均值、幅值、峰值和谷值应力对 30 4不锈钢的饱和棘轮应变的影响规律以及棘轮历史对材料棘轮饱和变形状态的影响。结果表明 ,均值、幅值和谷值 3种应力参量两两之间构成制约棘轮变形的二元参量 ,峰值应力与饱和棘轮应变之间存在不受均值、幅值和谷值应力影响的单调函数关系 ,因而峰值应力是导致材料正向棘轮变形的根本原因。根据这一现象 ,提出了棘轮门槛应力值σrth和棘轮应力σr的概念 ,建立了基于单参数控制的、用于饱和棘轮应变预测的饱和棘轮本构模型SRM。实验发现 ,先前低循环应力水平下材料棘轮行为对后继高循环应力水平下的饱和棘轮变形无影响 ,进而提出了单试样法 ,利用该方法来确定SRM本构模型材料参数只需 1～ 3个试样。基于单试样法建立的SRM模型用来预测在独立加载工况下 30 4不锈钢试样的饱和棘轮应变 ,其安全因子介于 1～ 1.3之间。

异戊橡胶在低温压缩应变下的结晶性能研究

研究了温度、压缩应变和时间3个主要因素对异戊橡胶结晶的影响。结果表明,在0℃及-25℃下,胶料低温结晶时的饱和压缩应变为40%;在-40℃下,胶料低温结晶时的临界压缩应变为35%左右。在0℃下,胶料低温结晶发生较为明显的时间区间为2-4 h;在-25℃下,胶料低温结晶发生较为明显的时间区间为4-8 h。在-40℃下,压缩变形量在30%以下将会延迟胶料低温结晶现象的发生,压缩变形量在40%以上时将会诱导低温结晶现象的发生。

用Kaiser效应测得的地应力是实时应力的证明

本文采用对试件施加持续荷载的方法证明：用Ｋａｉｓｅｒ效应测得的地应力，不是岩石在历史上曾经受到过的最大应力，而是实时应力。

Effect of Initial Stress Conditions on the Threshold Shear Strain of Nanjing's Saturated Fine Sand

By using GDS dynamic hollow cylinder torsional apparatus, a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction αo, the initial ratio of deviatoric stress η0, the initial average effective principal stress Po and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively, while the other three parameters remain constant. ao has a great influence on γt, which is reduced when ao increases from 0° to 45°and increased when α0 increases from 45° to 90°. The effect of α0 on γt, plays a leading role and the effect of η0 will weaken when ao is approximately 45°.

Constitutive equations and finite element implementation of strain localization in sand deformation

A recently proposed model coupling with the solid-fluid of the saturated sand was utilized to study the deformation band. Based on the critical state plasticity model by Borja and Andrade, the hydraulic conductivity tensor was naturally treated as a function of the spatial discretization matrix about the displacement and the stress field, allowing a more realistic representation of the physical phenomenon. The fully Lagrangian form of the Darcy law was resolved by Piola algorithm, and then the flow law was gained, leading to the implementation of a modified model of the saturated sand. Then the criterion for the onset of localization was derived and utilized to detect instability. The constitutive model was implemented in a finite element program coded by FORTRAN, which was used to predict the formation and development of shear bands in plane strain compression of saturated sand. At last, the formation mechanism of the shear band was discussed. It is shown that the model works well, and the simulation sample bifurcates at 1.18% axial strain, which is in a good qualitative agreement with the experiment. The pore pressure greatly affects the onset and development of the deformation band, and it obviously increases around the localization-prone regions with the direction toward the outer side of the normal of the shear band, while the pore stress flows nearly horizontally and is distributed equally far away the shear band region.

Behaviour of large post-liquefaction deformation in saturated sand-gravel composites

The laboratory tests on the post-liquefaction deformation of saturated sand-gravel composites were performed to investigate the characteristics of stress-strain relation and the dissipation of pore water pressure by the hollow cylinder apparatus. It is found that the stress-strain response and the dissipation process of pore water pressure are composed of three stages, including the low intensive strength stage, the superlinear strength recovery stage and the sublinear strength recovery stage, and the demarcation points of the curve of pore water pressure are lag behind those of the stress-strain response. The comparison results of the behaviour of large post-liquefaction deformation between saturated sand-gravel composites and Nanjing fine sand show that the low intensive strength stage and the superlinear strength recovery stage of saturated sand-gravel composites are shorter while the sublinear strength recovery stage is longer. A stress-strain model and a dissipation model of excess pore water pressure of liquefied sand-gravel composites are established, in which the initial confining pressure and the relative density can be considered synthetically. And it is found that the predicted results by the two models are in good agreement with experimental data.

An improved Mesri creep model for unsaturated weak intercalated soils

The weak intercalated soils in redbed soft rocks of Badong formation have obvious creep characters. In order to predict the unsaturated creep behaviors of weak intercalated soils, an unsaturated creep model was established based on the unsaturated creep tests of weak intercalated soils by using GDS triaxial apparatus. The results show that the creep behaviors of intercalated soils are apparent and significantly affected by matric suction. Based on this, an empirical Mesri creep model for intercalated soils under varying matric suctions was built. The fitting results show that the parameters Ed and m of this model are in good power relations with matric suction s and stress level Dr, respectively. An improved Mesri creep model was established involving stress-matric suction-strain-time, which is more precise than the Mesri creep model in predicting the unsaturated creep behaviors of weak intercalated soils.

Stress-strain relationship of unsaturated cohesive soil

A moisture-content based constitutive model was proposed based on the hyperbolic model as an attempt to move towards the implementation of unsaturated soil mechanics into routine geotechnical engineering practice. The stress-strain behavior of in-situ soil at a depth of 5 m was investigated by conducting undrained triaxial compression tests using the remolded soil samples. The test results show that the stress-strain relationship of unsaturated cohesive soil is still hyperbolic. The values of parameters a and b given in the model decrease with increasing the confining pressure for soil samples with the same moisture content and increase with increasing the moisture content for soil samples under the same confining pressure. The relationships between parameters a, b and moisture content were studied for confining pressures of 100, 150, 200 and 250 kPa. The comparison between the measured and predicted stress-strain curves for an additional group of soil samples, having a moisture content of 25.4%, shows that the proposed moisture content-dependent hyperbolic model provides a good prediction of stress-strain behavior of unsaturated cohesive soil.