超固结饱和黏性土的弹塑性本构模型及三轴试验模拟

针对饱和超固结黏性土现有的下加载面修正剑桥模型破坏应力比为定值,不能反映土体在不同应力状态下强度特性存在差异这一问题,为了更好地反映应力状态变化对破坏应力比的影响,基于三剪统一强度准则确定超固结土的破坏应力比。在此基础上得到了超固结土的弹塑性本构模型,该模型的特点是能够描述土体受力时的中间主应力效应、应力区间效应、拉压差影响。基于该本构模型作了常规三轴压缩条件下超固结饱和黏性土的模拟结果与试验结果对比,结果表明:该模型能够反映不同超固结比下土的变形、剪胀、孔隙水压力变化特性和规律。利用该本构模型模拟了排水和不排水条件下的真三轴压缩试验,得到了相应的应力-应变特性曲线。     

饱和超固结黏性土的三剪弹塑性本构模型研究

针对饱和超固结黏性土现有下加载面修正剑桥模型中破坏应力比为定值、土体黏聚力为零,以及不能准确反映不同应力状态下土的强度差异这些问题,基于三剪统一强度准则以及应力坐标平移法得到了扩展破坏应力比,其特点是能更好地反映应力状态变化以及土体黏聚力的影响。在此基础上提出了饱和超固结黏性土的三剪弹塑性本构模型,该模型的特点是能描述土体受力时的中间主应力效应,应力区间效应和拉压差影响,同时也能更好地考虑土体黏聚力的影响。基于该模型对ABAQUS软件进行了二次开发,并利用其模拟了饱和超固结黏性土在排水和不排水条件下的真三轴和常规三轴压缩试验特性。对常规三轴压缩条件下土体力学特性作了模拟和试验结果对比。结果表明所提模型能很好地反映不同超固结比下土体的变形、剪胀、孔隙水压力变化特性。     

含主应力轴旋转的土体本构关系研究进展

主应力轴旋转对岩土工程的影响日益受到人们的重视．本文从纯主应力轴旋转、纯应力洛德角变化、多种因素变化、排水与不排水等方面较全面地描述了含主应力轴旋转情况下土体的基本变形特性．对当前较有影响的含主应力轴旋转的土体本构模型（基于一般应力空间的土体本构模型、运动硬化模型、边界面模型、土体弹塑性应力应变关系的完全应力增量表述等等）进行了较为系统的评述．提出了合主应力轴旋转的实验研究中存在的核心问题与建立合主应力轴旋转的土体本构关系的根本途径．      

一种初始各向异性砂土本构模型的数值应用及试验验证

基于砂土颗粒的毛细效应特性制备砂土试样,在0°～90°范围内选取若干沉积方向,在中围压条件下进行三轴固结排水剪切试验。试验表明:沉积方向对试样强度和变形特性有显著影响。通过对试验数据进行拟合得到沉积方向和偏应力的关系,结合试验结果对Mohr-Coulomb屈服准则进行修正,建立一种密砂初始各向异性本构模型;对本构模型进行数值积分,并对密砂固结排水剪切试验进行数值模拟,分析偏应力、体积变化以及主应力方向的变化。将数值分析结果与试验结果进行对比,结果表明:二者相吻合,本构模型能够合理地预测试验结果。所建立的本构模型可用于砂土初始各向异性的研究工作中。     

恒定主应力偏转角下黏土不排水剪切特性分析

天然土的结构性、各向异性对其强度有影响.进行固结压力100kPa、主应力轴恒定偏转角为0°,15°,30°,45°,60°的恒定主应力轴偏转角的固结不排水剪切试验,分析了不同恒定主应力轴偏转角对饱和软黏土固结不排水剪切强度影响;推导了总应力控制恒定主应力轴偏转角剪切试验总应力路径,编制了总应力控制剪切三维点积分程序,对比恒定主应力轴偏转角饱和软黏土固结不排水剪切强度计算值与试验结果,表明程序能很好地反应各向异性对土剪切强度影响.     

考虑水化学损伤的岩石真三轴蠕变本构模型

为了准确描述岩石在酸性环境下真三轴蠕变行为的各阶段特征,基于水岩作用的化学动力学理论,定义了考虑PH值与时间的化学损伤因子,将弹性体,非线性Kelvin体,线性Kelvin体和黏弹塑性体进行串联,并考虑岩石在真三轴应力作用下的实际情况,建立岩石酸腐与真三轴应力耦合作用下的损伤蠕变本构模型,通过已有的蠕变试验数据对该模型进行参数辨识与验证,并通过数据拟合得到岩石在真三轴应力下的屈服面方程,探讨中间主应力对蠕变模型的影响.结果表明,推导的本构模型能很好地描述岩石在酸腐作用下真三轴蠕变行为的各阶段特性,验证了其合理性与准确性.     

高应力高水压下砂岩三轴蠕变特性试验研究

对砂岩进行高围压高水压条件下的三轴压缩蠕变试验。试验表明,在整个加载过程中,孔隙水压力主要起到增强轴向变形和横向变形的作用;但在加载的初始阶段,孔隙水压力在一定程度上抑制了轴向变形。当应力水平高于屈服应力时,横向蠕变速率明显大于轴向蠕变速率,且横向蠕变率先进入加速蠕变阶段。本文提出一个新的非线性黏性元件,并引入一个能判断是否进入加速蠕变阶段的计时器元件,组建一个非线性黏塑性加速蠕变启动模型,将该黏塑性模型与Burgers模型串联,构建一个新的非线性黏弹塑性蠕变模型,推导了该模型在常规三轴应力状态下的本构方程。基于试验结果,通过对非线性优化算法(BFGS)的Matlab编程,实现对本文提出蠕变模型的参数识别,识别效果比较理想。对比试验曲线与拟合曲线,二者相当吻合,验证了新提出的非线性黏弹塑性蠕变模型的正确性。     

K0超固结土的不排水抗剪强度

结合K0超固结上模型、旋转角w公式、临界状态不排水条件以及基于SMP的变换应力张量建立了不排水抗剪强度的统一表达式;采用三轴压缩、三轴拉伸的应力洛德角θ、旋转角w建立了三轴压缩、三轴拉伸条件下的不排水抗剪强度公式;基于临界状态士力学,推导出了平面应变条件下的应力洛德角θ、旋转角w的表达式,进而得到平面应变条件下的不排水抗剪强度公式.分别采用三轴压缩、三轴拉伸和平面应变条件下试验数据对所提出的不排水抗剪强度公式进行验证,预测结果和试验数据的基本吻合表明了不排水抗剪强度公式的合理性.     

正常固结原状饱和黏性土的本构模型研究

基于扰动状态概念研究了正常固结饱和原状黏性土的本构模型。其中,相对完整状态采用基于原状黏性土的理想非线性弹性本构模型来表征;完全调整状态则是基于临界状态土力学框架,将修正剑桥模型与三剪强度准则相结合而建立的弹塑性本构模型来表征。为了更好地反映黏性土的黏聚力影响,提出了等量代换法和坐标平移法两种处理方法,并作了对比研究。针对基于临界状态土力学框架和扰动状态概念建立的原状土本构模型为平均应力、广义剪应力与体应变、广义剪应变间的关系且无屈服函数显式表达式,提出了一个将其三维化的方法。三维化后的原状黏性土本构模型为空间内6个应力分量和6个应变分量之间的关系,更便于复杂应力状态下的土体变形分析及有限元编程。为验证模型的正确性,对正常固结江西原状饱和红黏土土样作了固结排水和固结不排水条件下的常规三轴压缩试验和K0固结试验,并与模型计算结果作了对比,结果表明:所提模型能够较好地反映江西原状饱和红黏土的力学和变形特性。另外,用所提模型对江西原状饱和红黏土进行了真三轴压缩数值模拟实验,探讨了中间主应力对其力学和变形特性的影响,结果表明:中间主应力及其影响系数对土的变形、孔隙水压力、强度均有影响;固结不排水条件下,在相同剪应力的作用下,土体剪应变随中间主应力影响系数的增大而减小,在相同轴向应变条件下,孔隙水压力随中间主应力影响系数的增大而增大;固结排水条件下,在相同平均正应力作用下,土体体应变随中间主应力影响系数的增大而减小,在相同轴向应变条件下,土体体应变随中主应力影响系数的增大而增大。     

基于西原模型的非线性黏弹塑性流变模型

在西原模型的基础上,引入能反映加速蠕变起始点的非线性黏塑性元件和瞬时塑性变形的塑性元件,建立了一个能完整反映蠕变全过程的非线性黏弹塑性流变模型. 在其一维流变模型结构的基础上建立三维的流变模型,推导三维流变方程,接着探讨了在三轴压缩试验中确定三维蠕变参数的方法,并对参数进行了敏感性分析. 用三轴压缩试验结果验证模型的有效性,结果表明模型能很好地模拟蠕变的三阶段,并且在多级偏应力水平下可以采用同一组蠕变参数.     

MODELLING STRAIN-RATE-DEPENDENCY OF NATURAL SOFT CLAYS COMBINED WITH ANISOTROPY AND DESTRUCTURATION

The paper aims to investigate modelling the strain-rate-dependency of natural soft clays combined with anisotropy and destructuration using an elasto-viscoplastic model.The model is based on Perzyna’s overstress theory and the elastoplastic model S-CLAY1S.Tests at constant strain-rate and creep tests under both one-dimensional and triaxial conditions on several clays are simulated.Simulations highlight the loading scenarios in which it is necessary to account for anisotropy and/or destructuration in order to get accurate predictions.Comparisons between the predicted and measured results demonstrate that the proposed model can successfully reproduce the time-dependent behaviour of natural soft clays under different loading conditions.     

An anisotropic elastic–viscoplastic model for soft clays

Experimental evidences have shown deficiencies of the existing overstress and creep models for viscous behaviour of natural soft clay. The purpose of this paper is to develop a modelling method for viscous behaviour of soft clays without these deficiencies. A new anisotropic elastic–viscoplastic model is extended from overstress theory of Perzyna. A scaling function based on the experimental results of constant strain-rate oedometer tests is adopted, which allows viscoplastic strain-rate occurring whether the stress state is inside or outside of the yielding surface. The inherent and induced anisotropy is modelled using the formulations of yield surface with kinematic hardening and rotation (S-CLAY1). The parameter determination is straightforward and no additional experimental test is needed, compared to the Modified Cam Clay model. Parameters determined from two types of tests (i.e., the constant strain-rate oedometer test and the 24 h standard oedometer test) are examined. Experimental verifications are carried out using the constant strain-rate and creep tests on St. Herblain clay. All comparisons between predicted and measured results demonstrate that the proposed model can successfully reproduce the anisotropic and viscous behaviours of natural soft clays under different loading conditions.     

An anisotropic fibre-network model for mechano-sorptive creep in paper

In this paper a simplified network model for mechano-sorptive creep is presented, which is a further development of an earlier paper [Strömbro, J., Gudmundson, P., 2008. Mechano-sorptive creep under compressive loading – a micromechanical model. International Journal of Solids and Structures 45 (9), 2420–2450.]. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive effects in compression than in tension. Moisture dependent material parameters and anisotropy in the fibre distribution have been introduced. Theoretical predictions based on the model are compared to experimental results for an anisotropic paper both under tensile and compressive loading at varying moisture content and it is found that the important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined.     

动力UH模型及其有限元应用

饱和砂土在循环载荷下具有复杂的应力应变关系, 通常表现出液化过程中的大变形以及往返活动性现象. 为简单有效地模拟上述特性, 在超固结UH模型的基础上, 将其扩展为可考虑砂土动力加载下的本构模型. 具体做法有3点: (1)改变屈服面椭圆长短轴之比, 将比值定义为反映应力诱导各向异性转轴斜率的函数; (2)引入旋转硬化规则, 用来反映应力诱导各向异性; (3)建立一个与旋转硬化规则以及临界状态特性相协调的统一硬化参数. 模型预测结果表明, 所提动力模型可简单、有效地用于砂土在动力载荷下应力应变关系的模拟. 最后将该动力UH模型嵌入到有限元软件中, 三维地基的动力加载模拟结果表明, 动力UH模型可方便地应用于岩土工程实践中.      

Applied creep theory for bodies with anisotropy due to plastic prestrain

Plastic strains in structures at the stages of manufacturing, testing, and approaching the operation regime cause anisotropic variations in the mechanical properties of materials, including creep strength. We consider the following special but practically important class of loading processes for originally isotropic materials: a simple active plastic strain is followed by a long-term steady-state loading within the elastic limits. To describe the second stage, we present the creep strain deviator in the form of an additive orthogonal decomposition in the directions of the repeated loading and the vector anisotropy. The coefficients in the decomposition are material functions of time, of the intensities of the preliminary and repeated loadings, and of the angle between the directions of these loadings. We obtain conditions on the material functions under which, at any given time instant, there is a one-to-one continuous correspondence between the stress and strain tensors for the model proposed and the boundary-value problem in the generalized statement has a unique solution; we also prove the convergence of the iteration method of elastic solutions used to find this unique solution. The model is identified according to the creep diagrams (under steady-state stresses of different values) determined for the material in the original state and after the plastic prestrain at an angle (zero, extended, and intermediate) to the direction of the repeated loading. We show that our results are in good agreement with the results available in the literature concerning experiments in this class of processes for stainless steel at high temperature. We propose an engineering version of the theory in which only the experimental data for uniaxial tension are used. We discuss the versions of the model for the cases in which the plastic preloading is cyclic (one-dimensional or circular) and the repeated loading is unsteady.     

基于三维细观模型的全级配混凝土静态力学性能的数值模拟

根据混凝土材料的细观组成和力学特性,研究了骨料几何形状和空间分布规律,建立全级配混凝土三维凸多面体随机细观模型,引入了混凝土细观组份材料的本构模型,分别模拟了单轴、双轴和三轴状态下混凝土的静态力学性能,并建立混凝土梁的三维宏细观分析模型,研究了三点弯曲梁的变形及裂缝扩展情况。结果表明,本文建立的细观力学模型的计算结果与实验数据吻合较好,可以较好地模拟各种复杂应力条件下混凝土的静态力学性能和损伤破坏机理。     

岩石三轴蠕变试验黏弹性解析及参数识别

基于岩石三轴蠕变试验,推导了相应的黏弹性解析解,并对三参量H-K流变模型给出 了黏弹性解析解的显式以及流变参数的识别方法. 在此基础上,针对花岗岩及片岩两种岩石 的室内蠕变分级加载试验曲线进行了流变参数的识别与反演. 分析表明,同一岩样在不同加 载级别下的弹性及黏弹性参数在量值上均有所不同,其中黏弹性模量和黏滞系数的量值随 载荷的增加出现损伤衰减,基于试验反演所获参数的解析蠕变曲线与试验蠕变曲线吻合较好.