非饱和土等效时间流变模型

为了理论分析流变速率效应力学特性,需要建立非饱和土流变模型.首先把非饱和土屈服面分为LC和SI屈服面。其次根据LC屈服面的压缩蠕变公式,利用等效时间法建立相应的黏塑性体应变速率公式,结合巴塞罗那屈服方程和相关联流动法则,获得LC屈服面的三维流变速率方程,揭示了本文与Gennaro-Pereira提出的LC后继屈服面方程的异同。再次,根据SI屈服面的吸力压缩蠕变公式,利用等效时间法建立相应的黏塑性体应变速率方程.最后,把LC和SI屈服面的流变速率方程与弹性方程相结合,建立非饱和土等效时间三维弹黏塑性模型.当不考虑流变速率效应时,LC和SI流变屈服方程可以退化到巴塞罗那模型的塑性屈服方程.理论分析了非饱和土一维流变特性,并与土工试验数据进行对比,两者较为吻合。三轴加载条件下的流变特性数值分析结果表明,按不同方式施加到同一恒载时,随着恒载维持时间的增长,不同加载方式引起的体积流变趋向一致,但剪切流变有较明显的差别.     

非饱和土Barcelona模型修正及存在问题探讨

在分析、研究各种修正的基础上,对当前非饱和土Barcelona模型中存在的问题进行了探讨,包括对土体压缩线,LC屈服面和SI屈服面的修正,及流动法则的选用等.研究表明各种修正有很多不吻合之处,除了土体自身特性不同会引起差异外,对机理认识的不同是主要原因.除现有修正外,试验结果表明,模型中还有很多因素尚未考虑,如土体临界状态线斜率;LC和SI屈服面的选择、确定及相互间的耦合运动;滞水特性描述及合适的应力变量选用;土体凝聚力随吸力变化情况,及土体初始结构、沉积历史影响等.研究结果指出两屈服面之间的耦合作用和滞水特性尤为重要,这是实际工程中较高压力和吸力循环变化时非饱和土稳定和变形预测的理论基础.     

两类剪切屈服面的非饱和土BBM模型对比研究

针对使用椭圆形剪切屈服面的非饱和土BBM弹塑性模型计算抗剪强度偏低的问题,提出了非饱和土水滴形剪切屈服面函数,对原模型进行了改进,将水滴形屈服面扩展到非饱和土中,提出了非饱和土水滴形剪切屈服面函数,采用相关联的流动法则改进了BBM模型。通过对同一组试验的数值模拟,对比研究了改进模型的合理性以及与原模型的差别,结果表明,在控制吸力条件的三轴压缩状态下采用水滴形屈服面的改进BBM模型对非饱和土强度及变形的计算更为合理。     

常吸力下非饱和土水-力耦合循环塑性的模拟

为描述非饱和黏性土在静、动态荷载作用下的应力-应变特征及水-力耦合效应,在塑性增量流动理论的框架内建立一个描述常吸力条件下非饱和黏性土水-力耦合效应的弹塑性双面模型.基于BBM(Barcelona basic model)及Li和Meissner提出的一种塑性硬化准则,并考虑非饱和土的典型土水特征,通过边界面和加载面在...     

考虑非饱和土吸力效应的圆柱孔扩张有效应力解析

为探究吸力效应对圆柱孔周围土体应力应变的影响,推导了非饱和土中的圆柱孔扩张有效应力析解。在弹性区和塑性区分别采用Hooke定律和修正剑桥模型,将非饱和吸力作为硬化参数,建立圆柱孔周围土体的应力应变控制方程。通过引入辅助变量,将欧拉坐标系下的平衡微分方程转至拉格朗日坐标系,非饱和圆柱孔扩张问题则转换为以弹塑性边界为边值条件求解一阶常微分方程组的问题。以弹塑性边界上的应力和初始比体积作为初值,对非饱和土中圆柱孔扩张问题进行分析。结果表明,受扩孔压力作用,圆柱孔周围土体将从弹性状态进入塑性屈服状态,而土体初始吸力的变化对塑性区应力状态影响显著。随着初始吸力的增加,圆柱孔周围相同位置处的土体各应力分量增大,极限扩孔压力也随之增大。对于初始孔隙比较大的疏松土体,吸力增大使得圆孔周围塑性区范围缩小;塑性区土体比体积沿径向呈单调减小,吸力越大,比体积减小越明显。对于初始孔隙比较小的密实土体,吸力变化对塑性区范围大小影响不明显;塑性区内土体比体积沿径向先增大后减小。这是因为在弹塑性边界附近处出现了剪胀现象,且吸力越大,剪胀现象越明显。最后通过与原位旁压试验和已有数值模拟结果对比,对本文方法的可靠性进行了验证。     

非饱和土的黏弹塑性本构模型研究

非饱和土变形是随时间发展的,为了描述非饱和土随时间变化的长期变形行为,本文建立了非饱和土黏弹塑性模型.在已有的非饱和土率无关模型的基础上,考虑应变率对变形和强度影响,建立了一个非饱和土率相关的黏弹塑性模型.根据试验结果,应变率越大,屈服应力则越大,将屈服应力表示为应变率的函数,给出率相关影响的加载湿陷屈服面.利用一致性条件,给出了一维条件下非饱和土的增量方程.对模型进行数值分析,对模型基本行为进行预测.将模型退化饱和条件和率无关2种情形,利用已有的试验结果对模型进行了验证,结果表明模型可以用于预测非饱和土的蠕变行为.     

非饱和土强度统一解及地基承载力的拓展研究

为改善现有非饱和土抗剪强度统一解与地基极限承载力的应用灵活性,分别对二者进行了拓展研究.研究包括非饱和土抗剪强度统一解的适用范围,非饱和土地基极限承载力的模型试验结果验证、线性吸力和高吸力对地基极限承载力的影响.结果表明:拓展后的非饱和土通用抗剪强度统一解可适用于双应力状态变量理论、有效应力理论及其他理论方法;扩展后的地基极限承载力公式得到模型试验结果的验证,且统一强度理论参数为0或1都不能反映试验地基土的真实承载特性;高吸力具有双重影响,线性吸力分布下的地基极限承载力明显小于均布吸力下的地基极限承载力.应实测地基土中基质吸力的大小及其分布,并慎重选择高吸力双曲线模型的截距.     

正常固结饱和黏性土的三剪次加载面模型

以三剪统一强度准则为破坏准则,采用坐标平移法推导出能够反映中间主应力效应、土体拉压差效应和区间效应的三剪破坏应力比,来替换修正剑桥模型的原本破坏应力比从而得到三剪统一屈服函数。基于该屈服函数,并结合次加载面理论,提出适用于循环荷载条件下的正常固结饱和黏性土的三剪次加载面本构模型。对模型参数u,C和χ做了敏感性分析,发现参数u,C和χ大小都会影响对循环荷载条件下饱和黏性土的棘轮效应和曼辛效应;为验证所提模型的正确性,将该模型的计算结果、文献中两种饱和黏土动三轴试验结果以及饱和Kaolin黏土的双面模型预测结果做了比较,结果表明所提模型能够较好地反映正常固结饱和黏土的应力应变关系,随着循环次数的不断增加,所建模型的模拟结果能不断产生累计塑性应变和累积孔隙水压力,且能够形成滞回圈。通过真三轴数值模拟分析可知,饱和黏性土的塑性偏应变在加载前期发展较快,之后相对减小;单一变量下,分别增大最小主应力和中间主应力,均会增大土体的抗剪强度。     

基于柔性边界的非饱和接触模型参数标定方法研究

离散元法(DEM)是研究非饱和土力学性质的重要方法，其模拟的可靠性取决于非饱和接触模型细观参数的准确标定。目前，对非饱和接触模型细观参数的标定方法缺乏系统研究。本文提出了一种基于柔性边界的标定DEM中非饱和接触模型细观参数的方法，对非饱和接触模型细观参数进行了系统标定，并通过参数敏感性分析揭示了非饱和接触模型中细观参数与宏观力学性质的相关性。首先，通过正交试验设计对细观参数的取值范围进行优化，分析细观参数对宏观响应的敏感性，敏感性分析结果表明：非饱和土的割线模量E50主要受摩擦系数μ、吸力?、杨氏模量E影响，影响程度大小为μ>? > E；非饱和土的峰值强度Sp主要受吸力?、摩擦系数μ影响，其影响程度大小为??>μ>E。其次，根据敏感性分析结果确定相关宏观响应的显著影响参数，利用基于中心复合试验设计(CCD)的响应面分析方法建立非饱和土宏观力学参数与显著影响参数之间的回归方程。再次，根据回归方程建立非饱和土细观参数的最优化模型，使用MATLAB中的FMINCON函数对其进行求解，获得细观参数的最优解。最后，利用获得的最优参数，模拟不同围压下非饱和土的三轴试验并与试验结果进行对比，验证所标定参数的有效性。该方法可为进一步分析非饱和土的细观力学性质提供基础。     

非饱和土本构模型研究进展

总结了非饱和土本构模型的研究历史和现状,介绍了非饱和土的各类模型,包括弹塑性模型、广义吸力模型、损伤模型、基于热力学的混合物理论模型.把弹塑性模型分成3个部分进行了介绍:未考虑饱和度影响,考虑饱和度影响但未考虑滞后现象和考虑滞后现象.并介绍了利用热力学和混合物理论建立非饱和土本构模型的新方法,指出了目前非饱和土本构模型研究中存在的问题,给出了今后可能的发展趋势.     

Coupled hydro-mechanical analysis of slope under rainfall using modified elasto-plastic model for unsaturated soils

Two modifications for the basic Barcelona model(BBM) are present. One is the replacement of the net stress by the average skeleton stress in unsaturated soil modeling, and the other is the adoption of an expression for the load-collapse(LC) yield surface that can match flexibly the normal compression lines at different suctions. The predictions of the modified BBM for the controlled-suction triaxial test on the unsaturated compacted clay are presented and compared with the experimental results. A good agreement between the predicted and experimental results demonstrates the reasonability of the modified BBM. On this basis, the coupled processes of groundwater flow and soil deformation in a homogeneous soil slope under a long heavy rainfall are simulated with the proposed elasto-plastic model. The numerical results reveal that the failure of a slope under rainfall infiltration is due to both the reduction of soil suction and the significant rise in groundwater table. The evolution of the displacements is greatly related to the change of suction. The maximum collapse deformation happens near the surface of slope where infiltrated rainwater can quickly reach. The results may provide a helpful reference for hazard assessment and control of rainfall-induced landslides.     

Modeling of strength and deformation of overconsolidated clays based on bounding surface plasticity

An elastoplastic constitutive model for overconsolidated clays is established in the framework of the critical state theory and bounding surface plasticity theory. The bounding surface is defined as the maximum yield surface in the loading history. A yielding ratio, i.e., an internal variant, is defined as the size ratio of the current yield surface to the corresponding bounding surface. The yielding ratio instead of the overconsolidation ratio (OCR) is used to evaluate the strength and stress-strain behaviors of overconsolidated clays in the shearing process. The bounding stress ratio incorporating the effect of the yielding ratio is used to characterize the potential failure strength of the overconsolidated clays. The dilation stress ratio taking into account the effect of the yielding ratio is applied to describe the dilatancy behaviors of the overconsolidated clays. Comparisons between model predictions and test data show that the proposed model could well capture the strength and stress-strain behaviors of normally consolidated and overconsolidated clays.     

A unified criterion for yielding behavior of metallic glasses

The yield behavior of metallic glasses was studied. Three yield criteria, including von Mises yield criterion, Mohr-Coulomb yield criterion and the unified yield criterion were used to describe the yield phenomena of the metallic glasses. Two classes of the experimental data were chosen to draw the yield loci using the unified yield criterion. It is shown that the unified yield criterion can be used to describe the yield behavior of the metallic glasses no matter whether the metallic glasses show strength-different effect or non-strength-different effect. Almost all the widely accepted yield criteria are the subsets of the unified yield criterion if the intermediate principle stress and/or the intermediate principle shear stress are not considered at all.     

高含蜡胶凝油的屈服特性研究

提出了一种高含蜡胶凝油屈服测试方法,量程范围较常规方法宽。基于此研究了低温下测试油蜡混合物胶凝油的屈服特性,详细阐明了熔蜡温度、熔蜡温度下的静置时间和屈服应力测试温度下的静置时间对特高含蜡胶凝油屈服特性的影响。研究发现,熔蜡温度越高,胶凝油的屈服应力越小;熔蜡温度下静置时间越长,油蜡混合物的低温屈服应力越大;胶凝油受屈服应力测试温度下静置时间影响较小。静置时间超过30min,其屈服值基本一致。研究结果对于科学制备高含蜡胶凝油,进而探索清管过程中蜡层剥离机制具有重要意义。     

Zr63Al10Ni10Cu14Nb3大块非晶合金的力学行为研究

通过电弧熔炼并利用吸铸的方法制备了长70 mm、直径3 mm的Zr63Al10Ni10Cu14Nb3大块非晶合金.热稳定性分析表明:该合金具有较大的过冷液相区.室温压缩实验显示合金具有很高的屈服强度,在合金发生屈服之后应力-应变曲线上有明显的锯齿现象和一个明显的应变软化阶段.结合断口形貌特征,分析并讨论了非晶合金的剪切断裂机理.     

Unified characteristics line theory of spacial axisymmetric plastic problem

The unified strength theory proposed by Yu in 1991 is extended to special axisymmetric problem. A unified special axismymmetric characteristics line theory based on the unified strength theory is proposed. This theory takes account of different effects of intermediate principal stress on yielding or failure and the SD effect (tensile-compression strength difference) of materials. Various conventional axisymmetric characteristics line theories, whihc are based on the Haar-von Karman plastic condition, Szczepinski hypothesis, Tresca criterion, von Mises criterion and Mohr-Coulomb theory, are special cases of the new theory. Besides, a series of new spacial axisymmetric characteristics fields for different materials can be introduced. It forms a unified spacial axisymmetric characteristics theory. Two examples are calculated with the new theory, the results are compared with those obtained by the finite element program UEPP and those based on the Mohr-Coulomb strength theory. It is shown that the new theory     

Constitutive model for overconsolidated clays

Based on the relationships between the Hvorslev envelope, the current yield surface and the reference yield surface, a new constitutive model for overconsolidated clays is proposed. It adopts the unified hardening parameter, to which the potential failure stress ratio and the characteristic state stress ratio are introduced. The model can describe many characteristics of overconsolidated clays, including stress-strain relationships, strain hardening and softening, stress dilatancy, and stress path dependency. Compared with the Cam-clay model, the model only requires one additional soil parameter which is the slope of the Hvorslev envelope. Comparisons with data from triaxial drained compression tests for Fujinomori clay show that the proposed model can rationally describe overconsolidated properties. In addition, the model is also used to predict the stress-strain relationship in the isotropic consolidation condition and the stress paths in the undrained triaxial compression tests. Supported by the National Natural Science Foundation of China (Grant Nos. 50479001 and 10672010), the National Science and Technology Supporting Item (Grant No. 2006BAK12B12), and the National Basic Research Program of China (Grant No. 2007CB714203)     

Analysis of complete plasticity assumption for solid circular shaft under pure torsion and calculation of shear stress

The distribution of shear stress on the cross-section of plastic metal solid circular shaft under pure torsion yielding,the applicability of complete plastic model assumption and the shear stress formula were researched.Based on the shear stress formula of circular shaft under pure torsion in elastic stage,the formula of torque in elastic stage and the definition of yield,it is obtained that the yielding stage of plastic metal shaft under pure torsion is only a surface phenomenon of torque-torsion angle rel...     

剪应力屈服准则的统一形式

本文建立了剪应力屈服准则的统一形式。这是一个以等腰八面体切向剪应力的两个分量为函数的屈服准则。说明了各种剪应力屈服准则是有联系的,屈服准则与所研究的材料相关。这个一般的屈服准则,可以还原为各种现已知道的剪应力屈服准则。