适用于黏土的分数阶应力诱导剪胀方程

建立了适用于黏性土的分数阶下加载面模型,该模型所采用的分数阶塑性流动法则能够在不引入塑性势函数的情况考虑塑性流动方向与土体物理屈服面之间的非正交特性,进而统一地描述相关联和非相关联塑性流动法则。基于该流动法则可以得到一个新的应力诱导分数阶剪胀方程以考虑超固结比对黏性土剪胀特性的影响。理论分析结果表明,在相同的应力水平下,土体剪胀量会随着超固结比增大而逐渐减小。相比较修正剑桥模型,该文模型仅额外地引入一个与土体剪胀特性相关的模型参数,并且能够对超固结黏土的应变软化和剪胀特性进行合理的描述。模型计算结果与试验结果对比分析结果表明,该文模型能够准确地描述黏性土在超固结状态下的应力-应变响应和剪胀特性。     

HISS模型在ABAQUS中的三维实现、验证和应用

分级单屈服面(HISS)模型的屈服面是单一光滑的函数,解决了“帽子”模型存在的数学不完备问题。该文介绍了HISS模型中非关联流动法则各向同性硬化的模型及其主要参数对屈服面的影响,使用MATLAB对一个高斯积分点进行加载试验,观测了在加载过程中应力路径及屈服面的变化;基于ABAQUS有限元程序提供的二次开发平台,首次编写了土体HISS模型的三维用户材料子程序。使用ABAQUS外接该文开发的用户子程序模拟了Leighton Buzzard砂土的4种三轴试验算例(静水压试验、常规三轴压缩试验、三轴压缩试验和三轴拉伸试验),并将计算得到的应力-应变关系和体积变化与实验观测结果对比,模型计算的结果与实验结果符合较好,验证了HISS模型用于砂土性质描述的适用性,并将模型用于分析分层路基的平板载荷试验。HISS模型三维子程序的实现为ABAQUS分析三维土体弹塑性问题提供了一种新的本构关系。     

中应变率加载下云杉各向异性力学行为研究

采用高速加载INSTRON设备对云杉开展100 s-1~102 s-1中应变率压缩实验,研究了材料沿顺纹、横纹径向、弦向、以及径(弦)切面内与顺纹呈15°、30°、45°、60°和75°夹角方向的力学性能。实验表明随着加载方向由顺纹向横纹径(弦)向变化,材料屈服强度逐渐减小,应力-应变曲线塑性流动段由"塑性软化"向"塑性硬化"转变;试件沿不同方向压缩屈服强度表现出较强应变率敏感性。冲击压缩下云杉宏观破坏模式与加载方向相关,沿顺纹方向加载时,试件中部向外膨胀,产生褶皱、纤维屈曲折断;当载荷方向与顺纹夹角逐渐增大时,材料失效模式体现为木材纤维分层滑移、撕裂。采用简化Hill强度理论对中低应变率下云杉空间屈服行为进行了理论描述,不同应变率下云杉空间屈服面为椭圆柱曲面,屈服曲面半径长度随应变率提高而增加。实验与理论分析表明,云杉沿空间不同方向具有各向异性力学特性,屈服强度受应变率和加载方向影响较大,而破坏模式则主要依赖于载荷方向。     

内时模型“现时近似式”与塑性增量理论的关系研究

提出塑性时间的定义,Valanis K C提出的内蕴时间是其特例。分析了一类积分型塑性时间内时模型"现时近似式"的基本性质,揭示了此类"现时近似式"存在屈服面和塑性位势的必要条件。当"现时近似式"存在屈服面和塑性位势时证明了"现时近似式"的塑性因子等于塑性时间与其Pfaffy型积分分母之积。当Pfaffy型积分分母为正常数时塑性因子等于塑性时间。从经典塑性增量理论出发,采用塑性因子作为塑性时间,建立了一个小应变条件下干砂的内时模型,该模型能够考虑砂土的剪胀性,能够较好地拟合循环荷载作用下干砂的应力-应变响应。     

钢筋混凝土剪力墙应变率效应试验与基于动力塑性损伤模型的模拟

应变率对混凝土材料的力学性能有着重要影响。该文在对2 片剪力墙构件进行快速加载试验的基础上,在混凝土塑性损伤模型中引入应变率效应,建立了考虑应变率效应的塑性损伤模型。应用该模型对该文所描述的快速加载下的剪力墙构件以及在拟静力作用下的另外3 个构件进行了有限元模拟,结果表明考虑应变率效应的混凝土塑性损伤模型较好地描述了钢筋混凝土剪力墙在快速加载下的非线性行为。最后,运用所建立的考虑应变率效应的混凝土塑性损伤模型对不同轴压比和加载速率下剪力墙构件的非线性性能进行了模拟与分析。     

K_0固结饱和土体柱孔扩张问题一种相似解法

采用能够考虑原状土体实际K_0固结特性弹塑性本构模型的屈服面方程为屈服准则,根据柱孔扩张基本平衡方程和相似解方法,对处于天然状态下的饱和土体柱孔扩张问题进行求解,推导出从零初始孔径柱孔扩张问题在小应变情况下塑性区的有效应力、超静孔隙水压力半解析解答。同时,通过算例对比分析表明:饱和土体在柱孔扩张后其塑性区超静孔隙水压力沿径向呈衰减趋势变化,其大小随着超固结比的增大而增大,孔壁周围土体有效应力下降明显,随着超固结比的增大,有效应力的减小趋势有所降低;考虑K_0固结影响的本文解答所得到的超静孔隙水压力大于修正剑桥模型的解答,扩孔完成后其周围土体有效应力下降大于后者。     

10Ni5CrMoV钢循环塑性变形的力学本构关系

针对屈服强度为785 MPa级别的调质态10Ni5Cr Mo V钢开展了系列应变幅的循环加载试验,并分析了循环应力应变特性,结果表明:循环塑性变形条件下表观弹性模量随应变幅的增大呈线性降低特点;循环塑性应变显著降低屈服强度,且屈服强度随应变幅的增大呈线性降低特点;塑性变形段的真应力和真应变塑性分量的对数呈良好的线性关系。基于上述结果提出了描述循环应力应变关系的单对数强化弹塑性本构关系模型σ=σp+χ·lgεp,并建立了10Ni5Cr Mo V钢循环应力应变本构方程。     

基于非共轴理论的各向异性砂土应变局部化分析

针对各向异性砂土应变局部化分析中本构模型存在的不足,采用非共轴理论进行改进。传统塑性位势理论采用各向同性假设,导致其模型不能描述非共轴特性和不能较好描述各向异性的不足,为克服传统塑性势理论的局限性,引入非共轴塑性理论建立了砂土的三维非共轴临界状态各向异性本构模型。考虑细观组构张量和应力张量的几何关系,改进模型即可描述主应力轴旋转条件下砂土材料状态的改变,材料状态变化直接导致模型的硬化规律和剪胀性发生变化,从而描述了原生各向异性的影响。非共轴修正后模型可以描述应力诱发各向异性和非共轴特性,结合分叉理论模型可以对不同沉积角度随围压变化的应变局部化特性进行分析。Toyoura砂的单剪试验和平面应变试验验证表明模型改进效果较好。     

自升式钻井平台桩靴贯入饱和砂土的机制研究

为研究桩靴贯入饱和砂土的承载机制以及桩靴周围砂土变形机理，开展桩靴的抗压承载土工模型试验以及桩靴-土体相互作用的透明土试验，测得了桩靴荷载-沉降变化规律、桩靴周围砂土的位移向量场和等值线图，初步探讨了桩靴贯入饱和砂土时的承载机制与桩靴周围砂土变形机理。基于圆孔扩张理论及分段位移迭代算法，推导出静载作用下桩靴荷载-沉降变化规律；与试验结果的对比发现：计算误差约为11.7%。通过浅应变路径法(SSPM)计算得到桩靴周围土体位移理论值；与试验结果对比发现：计算误差在16.7%～26.3%。     

饱和软粘土地基的损伤模型与震陷计算

基于各向同性弹塑性损伤和Prevost模型的基本理论,把弹塑性等向硬化、运动硬化和各向同性损伤结合起来,推导了循环荷载作用下不排水饱和软粘土弹塑性动力损伤本构模型.考虑到地震作用下土体应力的不规则性,对循环三轴试验获得的粘土残余应变经验计算公式进行了修正,最后将该残余应变引入到损伤演化方程中.通过对地基的震陷计算并与不考虑损伤的模型进行对比,结果表明,该模型能合理地考虑屈服面内应力循环对地基残余塑性应变的贡献.     

FEM simulation of viscous properties for granular materials considering the loading rate effect

The deformation and strength characteristics of sandy soils as a kind of granular materials are very complex. The experimental results show that when the strain rate suddenly changes in monotonic loading (ML) case, the stress–strain curve of sandy soils changes sharply and then gradually converges into the original inferred one that would be obtained by continuous ML at constant strain rate after having exhibited clear yielding. Similar behaviors are also observed when ML is restarted at a constant strain rate following a creep loading or stress relaxation stage. An elasto-viscoplastic constitutive model for granular materials is developed, which consists of three components. One of the most important features of the model is that it can take into account the effects of loading rate due to viscous properties on the stress–strain behavior. The stress ratio-axial strain–time relations from four drained plain strain compression (PSC) tests on the saturated Toyoura sand are successfully simulated by the finite element method (FEM) code incorporating the proposed constitutive model. It is shown that the FEM code can simulate the viscous behaviors of sand accurately under arbitrary loading history.     

Strain localisation modelling and pore pressure in saturated sand samples

Dynamic strain localisation theory together with a multiphase material model is used to simulate shear band dominated processes in fully saturated sand samples. The fluid-saturated medium is viewed as multi-phase continuum consisting of a solid skeleton and pores filled by fluids. The governing equations are based on the general framework of averaging theories. A generalised plasticity constitutive model for fully saturated soils is adopted in the computational process. Both samples of medium-loose and dense sands are studied. Negative water pressures, which are important in localisation phenomena of fully saturated dilatant geomaterials, are obtained for dense sands, while positive water pressures result for medium-loose sands.     

Construction of a constitutive model in calculations of pressure-dependent material

To make constitutive modeling of materials more approaching reality, a new theory is proposed, in which a corresponding constitutive model can be constructed and characterized experimentally via two steps, one relates to the characterization of yielding behavior of material, and the second relates to the characterization of plastic flow of material deformation. The constitutive model involves two functions, yield function and plastic potential. A relationship between two functions is suggested, therefore, a corresponding plastic potential can be easily created after we have an appropriate yield function. To consider the non-isotropic hardening feature of strength differential in the constitutive model, the concept of equivalent hardening state is introduced, and then, multi-experimental flow stresses can be addressed in the model. When pressure sensitive materials are taken as an example in discussions, the Drucker–Prager yield function is employed to express the yielding behavior of material and a differently experimental characterization of the model is created as the corresponding plastic potential to describe the feature of plastic flow of material. This simple constitutive model can reproduce three sets of experimental results; including two flow-stresses and the volumetric plastic strain. The constitutive model can also well predict stress–strain relations with different pressures loaded on the material. Study shows that the feature of plastic flow is not that sensitive to the pressure loaded on the material when the yielding stress is.     

A mixture model for the compaction of saturated sand

Free draining water saturated sands and dry sands compact progressively under cyclic shear loading, and the rate of compaction increases as the shear strain amplitude increases, independent of the confining stress magnitude. As the sand grains are relatively incompressible, this compaction is essentially an irreversible porosity decrease induced by rearrangement of the granular structure. We present a constitutive model of differential type for porosity variation which has the minimal ingredients necessary to reflect the observed cyclic loading phenomena, and determine the associated material functions of a particular form by correlation with cyclic loading data. A hypoelastic shear response is also correlated to cyclic loading data to complete an isotropic constitutive model. Pore pressure generation under cyclic loading in undrained conditions is evaluated to illustrate the predictions of the model.     

饱和砂土基于相变状态的不排水本构模型

饱和砂土基于相变状态的改进本构模型能较好地描述砂土不排水情况下的应力-应变关系。通过改进模型中相变状态参量的计算方法,将e-lgp°表达式加以改进,通过详细的阐述,提出e_pt-lgp的关系式,并通过三轴压缩试验的数据对提出的关系式加以验证。应用新的关系式计算不排水条件下试样的相变孔隙比e_pt,进而计算出模型中的相变状态参量。改进后的不排水本构模型的计算结果与不排水三轴压缩的试验结果能较好地吻合,即验证了本构模型的有效性,也验证了e_pt-lgp关系式的合理性,使原有的边界面本构模型更好地描述不排水的情况下、不同密度和围压状态下饱和砂土的力学行为。     

A constitutive model for unsaturated soils: thermomechanical and computational aspects

This paper first presents a complete formulation of a constitutive model that deals with the irreversible behaviour of unsaturated soils under various loading and drying/wetting conditions. A standard form of incremental stress-strain relations is derived. The constitutive model is then cast into the thermodynamical theories and verified using the thermomechanical principles. It is shown that hydraulic hysteresis does not contribute to the plastic dissipation, though it contributes to the plastic work. All plastic work associated with a plastic increment of the degree of saturation is stored and can be recovered in a reversed plastic increment of saturation. The incremental constitutive equations are also reformulated for implementation in finite element codes where displacements and pore pressures are primary unknowns. Qualitative predictions of the constitutive model show that incorporating two suction related yield surfaces and non-associated flow rules into the Barcelona Basic Model opens a full range of possibilities in modelling unsaturated soil behaviour.     

A direct approach toward simulating cyclic and non-cyclic fatigue failure of metals

A new finite \(J_2-\)flow rate-dependent elastoplastic model is proposed toward directly simulating cyclic and non-cyclic fatigue failure behavior of metals. Novel results in four respects are presented: (1) the concept of yielding is rendered irrelevant with smooth transitions to both the plastic state and the rate-independent case; (2) asymptotic loss of the strength is incorporated as inherent constitutive feature; (3) the failure behavior may accordingly be derived as a direct consequence of the proposed model, without involving any additional failure criteria and any additional variables; and (4) direct and explicit procedures are established for identifying each rate-dependent parameter based on suitable test data. Numerical examples are provided for various uniaxial loading cases up to failure, including monotone and cyclic strain cases from low to high strain rates and cyclic loading cases as well as non-cyclic loading cases with either variable stress amplitudes or variable strain rates. Model predictions compare well with test data.     

基于黏塑性理论的碾压混凝土动态剪切本构模型

由于碾压混凝土大坝是逐层碾压而成,坝体层面处的静动抗剪强度均低于其本体强度,在地震、振动或冲击作用下,坝体层面(包括坝基界面)有可能发生沿层面的动态滑移失稳破坏。基于Perzyna黏塑性连续理论,提出了一个用于描述碾压混凝土层面动态剪切断裂行为的本构模型。该模型的特点有:混凝土材料的软化塑性和扩容特性直接与界面处断裂失效过程相联系;使用Carol率相关界面方程作为屈服判据来描述碾压混凝土材料的率相关性;使用经典塑性断裂理论来描述剪切面上的断裂失效和摩擦滑动过程,并且只需要较少的模型参数。利用该模型与含层面碾压混凝土的动态强度试验结果进行了对比分析,包括在不同的应力路径如单轴拉、压和压剪状态下和不同加载速率下的试验结果。结果表明模型与试验得出的结论吻合较好,这种弹-黏塑性动态剪切本构模型对预测包含薄弱层面的碾压混凝土动力破坏性能是有效的,这为相关工程问题的研究提供有益的思路和有效的工具。     

A unified theory of elasticity and plasticity

It is shown that a certain set of rate theory type constitutive equations can give rise to both elastic behavior and plastic yield in infinitesmal strain both in loading and unloading. Elastic behavior corresponds to uniqueness of solutions of initial value problems of differential equations and holds when the equations satisfy a Lipschitz condition. This Lipschitz condition fails when a von Mises yield condition holds, and the corresponding nonuniqueness gives rise to plastic yield during loading. During plastic yield, the basic constitutive relations automatically turn into the Prandtl-Reuss equations. Upon unloading from yield, the stable solution of the equations dictates reentry into an elastic regime. The transition to and from plastic yield is exact and sudden, not asymptotic, but the transition is smooth. It is shown that linear behavior in the elastic regimes can be approximated arbitrarily closely. The theory can be extended to include strain hardening.