含主应力轴旋转的土体平面应变问题弹塑性数值模拟

在含主应力轴旋转的土体本构关系研究的基础上,通过含主应力轴旋转的土体平面应变问题的弹塑性数值模拟结果分析,总结主应力轴旋转对土体应力分布与应形影响的规律,得出需要考虑主应力轴旋转的条件及影响的相对大小。     

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。     

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。     

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,对江西赣南崩岗土常规三轴压缩条件下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土体不同超固结比下的变形、剪胀、孔隙水压力变化特性.并利用其模拟了饱和超固结黏性土在不排水和排水条件下的真三轴和常规三轴压缩试验特性.     

一个反映土体复杂加卸荷路径的弹黏塑性模型

基于Perzyna 过应力理论,构造了一个土体各向异性弹黏塑性本构模型. 模型引入Wheeler 旋转硬化法则,能够较好的描述土体的初始各向异性和应力诱发各向异性. 借助ABAQUS 软件中UMAT 子程序接口,模型采用隐式积分算法——图形返回算法实现. 通过对一组复杂加卸荷路径的三轴不排水剪切试验(HKMD) 及一组分级加载三轴不排水蠕变试验(Sachville clay) 的模拟,表明本模型能够合理反映土体的率效应、应力松弛、蠕变及土体各向异性现象,验证了模型的有效性.     

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。     

主应力轴旋转对软土塑性变形影响分析

在姜洪伟所提出的软土各向异性弹塑性本构关系基础上，考虑类似于砂土的Ｒ－旋转试验条件，计算了软土由于主应力轴旋转所产生的塑性变形，对不同中主应力比情况下的变形规律进行分析，解释忽略主应力轴旋转影响将使设计偏于不安全的原因。     

基于进化多项式回归方法的土体本构关系

进化多项式回归方法是近几年来在国际上引起广泛关注的一种新颖的数据挖掘方法。本文在土样固结排水试验数据的基础上,首次使用该方法建立了土体本构关系。采用上海嘉定区某地软土的固结排水试验结果对模型进行了验证。结果表明,该本构关系能够更精确地描述土体的应力-应变特性,其预测精度比修正剑桥模型高19.6%~57.7%。另外,本文还用切线刚度法,对基于进化多项式回归方法的土体本构关系进行了ABAQUS二次开发,对软土固结排水试验进行了有限元数值模拟。     

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.     

基于对数应力率大应变本构关系的参数标定研究

在所有率型弹塑性本构模型中,只有对数应力率对应的本构模型能够满足自适应准则.基于对数应力率,采用实心圆轴扭转实验,对大应变弹塑性本构模型中的参数标定问题进行了讨论.推导出了考虑Swift效应时端部自由实心圆轴扭转变形的变形率、对数旋率、Kirchhoff应力及Kirchhoff应力的对数应力率.对于等向强化大应变弹塑性本构关系,给出了由实心圆轴扭转实验标定的、基于Kirchhhoff应力对数应力率的本构关系中塑性刚度函数的表达式.分析了扭转圆轴的Swift效应对塑性刚度函数的影响.结果表明,实心圆轴扭转的轴向伸长变形和径向变形对基于对数应力率大应变本构关系中的塑性刚度函数都有影响.当不考虑Swift效应时,所得塑性刚度函数表达式与不考虑Swift效应时基于Jaumann应力率的塑性刚度函数表达式相同.     

含主应力轴旋转的广义塑性位势理论

大量岩土实验与工程实践表明传统塑性位势理论无法合理反映岩土材料的基本变形机制。从塑性位势理论角度来看,当存在主应力轴旋转时,塑性应变增量与应力不共主轴,此时最一般情况下的塑性应变增量须用六个线性无关的塑性势函数来表述,从而提出含主应力轴旋转的广义塑性位势理论一般表达式。通过矩阵分析,将一般应力增量分解成共轴分量与旋转分量之和。在应力增量分解的基础上,提出含主应力轴旋转的广义塑性位势理论的分解表达式     

主应力轴旋转下黏土累积变形的分数阶元件模型

在长期交通载荷作用下土体塑性累积变形本构模型对路基沉降计算至为关键.元件组合模型可以计算岩土体循环累积应变,但现有的各类元件模型未能反映饱和软黏土的主应力轴循环旋转现象.在对饱和软黏土进行等向固结条件下的主应力轴循环旋转加载试验及非等向固结下的循环扭剪试验基础上,将Abel黏壶代替Burgers模型中的Newton黏壶,得到分数阶Burgers模型;利用遗传算法优化循环塑性累积应变的Burgers模型和分数阶Burgers模型的参数,通过对比两组模型的计算值与试验值,发现分数阶模型更适合模拟计算循环载荷下饱和软黏土的累积变形.     

THE GENERAL STRESS STRAIN RELATION OF SOILS INVOLVING THE ROTATION OF PRINCIPAL STRESS AXES

1.IntroductionThestudyofsoils'constitutiverelationismostlylimitedinsuchStressstatethattherotationofprincipalstressaxesisabletobeignored.Soonlythevalueofprincipalstressneedtobeconsideredandthedirectionofprincipal'stressisregardedtobeunchangeable.Onthiscondition,theprincipalaxesof'stressincrement,strainincrementjstressandstrainarethesame.Therelevantexperimentsandengineeringsshowthattherotationofprincipalstressaxeswillgeneratesignificantplasticdeformationandthenoncoaxalityofstressandstrain.First…     

A STRESS VECTOR-BASED CONSTITUTIVE MODEL FOR COHESIONLESS SOIL (Ⅱ)-APPLICATION

ＩｎｔｒｏｄｕｃｔｉｏｎＡｃｏｍｐａｎｉｏｎｐａｐｅｒ [1 ]ｄｅｓｃｒｉｂｅｄａｓｔｒｅｓｓｖｅｃｔｏｒ＿ｂａｓｅｄｃｏｎｓｔｉｔｕｔｉｖｅｍｏｄｅｌｆｏｒｃｏｈｅｓｉｏｎｌｅｓｓｓｏｉｌ.Ｔｈｅｍｏｄｅｌｉｓｂａｓｅｄｏｎｔｈｅｅｑｕｉｖａｌｅｎｔｄｅｃｏｍｐｏｓｉｔｉｏｎｏｆｔｈｅａｃｔｉｏｎｅｆｆｅｃｔｏｆｓｔｒｅｓｓｖｅｃｔｏｒｆｏｒｔｈｅｍａｉｎ .Ｔｈａｔｉｓ ,ｕｎｄｅｒｐｌａｎｅｓｔｒａｉｎｃｏｎｄｉｔｉｏｎ ,ｔｈｅａｃｔｉｏｎｅｆｆｅｃｔｏｆｓｔｒｅｓｓｖｅｃｔｏｒｃａｎｂｅｅｑｕｉ…     

A STRESS VECTOR-BASED CONSTITUTIVE MODEL FOR COHESIONLESS SOIL (Ⅱ)-APPLICATION

The stress vector-based constitutive model for cohesionless soil, proposed by SHI Hong-yan et al., was applied to analyze the deformation behaviors of materials subjected to various stress paths. The result of analysis shows that the constitutive model can capture well the main deformation behavior of cohesionless soil, such as stress-strain nonlinearity, hardening property, dilatancy, stress path dependency, non-coaxiality between the principal stress and the principal strain increment directions, and the coupling of mean effective and deviatoric stress with deformation. In addition, the model can also take into account the rotation of principal stress axes and the influence of intermediate principal stress on deformation and strength of soil simultaneously. The excellent agreement between the predicted and measured behavior indicates the comprehensive applicability of the model. Biography: SHI Hong-yan (1962-), Associate Professor, Doctor     

Dynamic damage constitutive relation of mesoscopic heterogenous brittle rock under rotation of principal stress axes

A micro-mechanics-based model is developed to investigate microcrack damage mechanism of four stages of brittle rock under rotation of the principal stress axes. They consist of linear elastic, non-linear hardening, rapid stress drop and strain softening. The frictional sliding crack model is applied to analyze microcracks nucleation, propagation and coalescence. The strain energy density factor approach is applied to determine the critical condition of microcrack nucleation, propagation and coalescence. The inelastic strain increments are formulated within the framework of thermodynamics with internal variables. Rotation of principal stress axes affect the dynamic damage constitutive relationship and the failure strength of brittle rock.     

NONLINEAR MICRO－MECHANICAL MODEL FOR PLAIN WOVEN FABRIC

The warp yarns and weft yarns of plain woven fabric which, being the principal axes of material of fabric, are orthogonal in the original configuration, but are obliquely crossed in the deformed configuration in general. The orthotropic constitutive model is unsuitable for fabric. In the oblique principal axes system the relations between loaded stress vectors and stress tensor are investigated, the stress fields of micro-weaving structures of fabric due to pure shear are carefully studied and, finally, a nonlinear micro-mechanical model for plain woven fabric is proposed. This model can accurately describe the nonlinear mechanical behavior of fabric observed in experiments. Under the assumption of small deformation and linearity of mechanical properties of fabric the model will degenerate into the existing linear model.     

Constitutive response of idealized granular media under the principal stress axes rotation

This paper is dedicated to the understanding of the phenomena, which give rise to anisotropy and non-coaxiality in granular materials. In achieving three-dimensional numerical simulation under static condition of granular media, granular element method (GEM) is adopted in this study. The method has been incorporated into the so-called mathematical homogenization theory for quasi-static equilibrium problems, which enables us to obtain the macroscopic/phenomenological inelastic deformation response of a representative volume element (RVE). To examine the anisotropic macroscopic deformation properties of the assumed RVE, which is solved by granular element method (GEM), a series of numerical experiments involving the pure rotation of the principal stress axes are carried out, and its results are discussed in relation to induced anisotropy and non-coaxiality.