卸荷软土的应力路径及对强度影响的研究

软土基坑卸荷开挖,周围土体应力应变发生变化,导致土体结构损伤及工程性质变异,最重要是强度降低,这一影响对坑底土体尤其明显,土体扰动以后的强度才是抵抗基坑变形所能发挥出的真实强度值,而通过常规室内试验提供的基坑设计强度参数并不能很好地与这个值相吻合。这样给基坑设计与稳定性评估带来较大的误差,本文分析了这一现象的机理与规律。     

各向异性软黏土基坑抗隆起稳定极限平衡分析

天然黏土表现出明显的强度各向异性,在软黏土地区中的基坑抗隆起分析时有必要考虑土体各向异性的影响。针对桩墙式支护基坑绕下道支撑的圆弧滑动抗隆起稳定模式,采用基于空心圆柱仪（HCA）杭州原状软黏土试验结果的土体强度各向异性不排水强度公式,根据极限平衡法推导得到坑底抗隆起稳定分析公式。结合算例,讨论了土体强度各向异性、挡墙入土深度、最下道支撑和坑底间的距离等因素对坑底抗隆起稳定的影响,比较了该文方法与传统方法结果的差别,指出不考虑土体强度各向异性的软黏土基坑抗隆起分析和基于传统Casagrande各向异性公式的抗隆起分析都将高估杭州软黏土基坑的抗隆起稳定性。最后采用该方法分析杭州某失稳基坑实例,通过与已有方法结果的对比,验证了该文方法的合理性。     

结构性软黏土的修正剑桥模型

天然软黏土普遍受到土结构性的影响,如何在土体本构模型中反映这一影响显得非常重要。该文从修正剑桥模型出发,引入结构屈服应力参数表征受土结构性影响的天然土初始屈服面的形状;引入各向异性参数描述天然土体初始各向异性引起的屈服面旋转。基于土结构性突变屈服破坏机理,屈服前结构性软黏土呈现弹性的力学性质,屈服后土结构性的影响完全丧失,采用同修正剑桥模型一致的硬化规律和流动法则。根据一致性连续条件,推导增量型的应力-应变关系,建立适用于结构性软黏土的弹塑性本构模型。选取国外已有的天然沉积Bothkennar软黏土,对比典型应力路径下的试验实测结果与模型计算结果,显示了该文模型模拟结构性软黏土受力变形特性的优越性。     

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.     

循环荷载水平与加载频率耦合作用下的软粘土特性研究

通过不排水循环三轴试验,并考虑不同循环应力水平及加载频率的影响,研究了软粘土在循环荷载作用下的孔隙水压力及变形特性,分别探讨了这些特性随循环加载时间和加载次数的不同变化规律。研究结果表明,对于相同循环应力水平,相同加载次数下不同加载频率的软粘土特性有所不同,而相同加载时间下不同加载频率的软粘土特性基本相同。此外,无论加载频率为何数值,一旦循环应力水平超过临界值,软粘土破坏必将发生。为了深入研究应力水平和加载频率的耦合作用,该文从应力控制循环加载试验中的应变速率着手,对软粘土的特性进行了分析。结果表明,在应力水平相同的情况下,软粘土在不同加载频率下的应变速率是基本相同的,由此可得对于软粘土在循环荷载作用下特性的影响,应力水平比加载频率更为重要。     

层状粘性土及砂土地基中静力压桩连续贯入的数值模拟

总结分析了软硬交互层状土中静压桩沉桩阻力的变化规律。为了使ABAQUS模拟层状土地基,特别是既有粘性土层又有砂土层的层状地基中静压桩贯入,分段采用不同本构关系模型,即在粘性土层中采用修正剑桥模型,砂土层中采用扩展D-P模型。同时,采用多项措施对层状土地基土体初始应力进行平衡。结合工程实例,模拟计算了不同土层性质差异明显、软土中有硬土夹层的层状土中静压桩连续贯入过程,得出了贯入过程中沉桩阻力随深度变化曲线,反映了通过硬土层时桩周应力和沉桩阻力的突变现象,与实测资料较吻合。研究结果有助于静压桩沉桩可能性分析以及挤土效应分析。     

A continuous plasticity version of the critical state model

A modification to a conventional critical state model is described which introduces some plasticity in the stress region which is normally fully elastic. The effect is to blur the otherwise sharp distinction between elastic and yielding states. The modification is conceptually simple and does not require any additional material parameters. The model was developed for the dual purpose of providing a more robust constitutive law for finite element applications and to provide a more realistic representation of the soil. Its potential is demonstrated by simulating the response of a plane strain sample tested over a range of loading and unloading stress paths. The results are in accordance with expectations from physical tests. Further, they show that the model has some potential for at least the first few cycles of cyclic loading. A finite element boundary value analysis of an excavation is also described. This shows how the model copes with the severe test of stress paths which unload while yielding. Finally, the model is used to match stress-strain curves obtained from large-scale laboratory tests on a compacted clay. Close agreement with three of the five tests was obtained using a single set of material parameters.     

水波与淤泥质底床相互作用数值模拟——基于滞后回路特征的淤泥体流变模型

应用一个滞后回路特征的半经验的淤泥体流变模型描述表面水波作用下底床软泥层的振荡运动，建立了表面水波与淤泥质底床相互作用的垂向二维耦合模型。在数值处理上，为提高数值格式对水－泥界面附近流动的分辨力，较好地再现界面附近速度梯度大的流动特点，采用垂向对数网格处理技巧。模型验证是通过计算水波波高衰减率和底床软泥体输移速度，并与有关的实验测量数据进行比较。     

A nonlinear strain gradient beam formulation

In this paper, a nonlinear size-dependent Euler–Bernoulli beam model is developed based on a strain gradient theory, capable of capturing the size effect. Considering the mid-plane stretching as the source of the nonlinearity in the beam behavior, the governing nonlinear partial differential equation of motion and the corresponding classical and non-classical boundary conditions are determined using the variational method. As an example, the free-vibration response of hinged-hinged microbeams is derived analytically using the Method of Multiple Scales. Also, the nonlinear size-dependent static bending of hinged-hinged beams is evaluated numerically. The results of the new model are compared with the results based on the linear strain gradient theory, linear and nonlinear modified couple stress theory, and also the linear and non-linear classical models, noting that the couple stress and the classical theories are indeed special cases of the strain gradient theory.     

Incremental constitutive equation for large strain elasto plasticity

Starting from the standard theory with intermediate configuration for finite deformations of an isotropic elasto-plastic material with isotropic hardening, rate type constitutive equations are obtained. The small elastic strain approximation is then discussed and it is shown that, in this approximation, these equations reduce to Hill's formalism of large strain elasto-plasticity obtained from the classical Prandtl-Reuss relations of infinitesimal plasticity by substituting for the infinitesimal strain rate, stress and stress rate respectively the rate of deformation tensor, the Cauchy stress tensor and the Jaumann stress rate tensor. The limiting case of perfect plasticity is also investigated.     

Cement stabilised rammed earth. Part B: compressive strength and stress–strain characteristics

Strength and behaviour of cement stabilised rammed earth (CSRE) is a scantily explored area. The present study is focused on the strength and elastic properties of CSRE. Characteristics of CSRE are influenced by soil composition, density of rammed earth, cement and moisture content. The study is focused on examining (a) role of clay content of the soil on strength of CSRE and arriving at optimum clay fraction of the soil mix, (b) influence of moisture content, cement content and density on strength and (c) stress–strain relationships and elastic properties for CSRE. Major conclusions are (a) there is considerable difference between dry and wet compressive strength of CSRE and the wet to dry strength ratio depends upon the clay fraction of soil mix and cement content, (b) optimum clay fraction yielding maximum compressive strength for CSRE is about 16%, (c) strength of CSRE is highly sensitive to density and for a 20% increase in density the strength increases by 300–500% and (d) in dry state the ultimate strain at failure for CSRE is as high as 1.5%, which is unusual for brittle materials.     

Incremental forms of Schapery’s model: convergence and inversion to simulate strain controlled ramps

Schapery’s nonlinear viscoelastic model is written in incremental form, and three different approximations of nonlinearity functions in the time increment are systematically analysed with respect to the convergence rate. It is shown that secant slope is the best approximation of the time shift factor, leading to significantly higher convergence rate. This incremental form of the viscoelastic model, Zapas’ model for viscoplasticity, supplemented with terms accounting for damage effect is used to predict inelastic behaviour of material in stress controlled tests. Then the incremental formulation is inverted to simulate stress development in ramps where strain is the input parameter. A comparison with tests shows good ability of the model in inverted form to predict stress–strain response as long as the applied strain is increasing. However, in strain controlled ramps with unloading, the inverted model shows unrealistic hysteresis loops. This is believed to be a proof of the theoretically known incompatibility of the stress and strain controlled formulations for nonlinear materials. It also shows limitations of material models identified in stress controlled tests for use in strain controlled tests.     

Geometrically nonlinear stress–strain behavior of hyperelastic solid foams

The present study is concerned with an effective stress analysis of cellular solids in the finite strain regime. The homogenization of the microstructure is performed by means of a strain energy based RVE-procedure which assumes macroscopic equivalence of a representative volume element for the given microstructure and a similar volume element consisting of the effective medium, if the average strain energy density in both volume elements is equal provided that the deformation gradient with respect to both elements is equal in a volume average sense. Disordered microstructures are considered by means of a randomized periodic model in conjunction with a stochastic approach. The model is applied to an analysis of the effective stress–strain behavior of two-dimensional model foams with periodic and disordered microstructure. Special interest is directed to effects of the geometric nonlinearity.     

Residual stress analysis in the oxide scale/metal substrate system due to oxidation growth strain and creep deformation

High-temperature oxidation of metals induces residual stresses both in the metals and in the growing oxide scales. In this work, considering the case of asymmetric oxidation, a new analysis model to elucidate the residual stress evolutions in an oxide scale/metal substrate system during an isothermal oxidation process is developed. Elastic and creep deformations in both oxide and metal phases are considered in this work. The oxidation growth strain generated in oxide scale is also taken into account and is described by the Clarke model, that is, the growth strain rate increases linearly with the oxide thickening rate during isothermal oxidation. A comprehensive numerical study is carried out by the present approach. Results reveal that the proposed model can lead to an excellent agreement with the published experimental results and thus well validate the present model. Effects of the growth parameter, creep constants, Young’s modulus, and substrate thickness on the residual stress evolutions in the oxide scale/metal substrate system have also been discussed.     

考虑刚度非线性软化的修正硬化土模型

基于硬化土模型(HS模型)只能描述土体应变硬化行为和刚度线性软化现象,提出了考虑刚度非线性软化的修正硬化土模型。修正模型采用与分段幂函数关系描述初始骨干曲线,并考虑了初始静偏应力和加载速率对应力-应变关系的影响。修正HS模型能较好地描述不同初始静偏应力和加载速率下土体的应变硬化、应变软化和刚度非线性软化等特征。最后,基于原状杭州软粘土的三轴试验,验证了修正模型的合理性。     

A modelling technique for calculating stress intensity factors for structures reinforced by bonded straps. Part II: Validation

In this second part of the two-part paper validation of the 2D FE modelling technique described in the first part is presented for a range of test configurations. Each mechanism that influences crack growth behaviour of strap reinforced structures is modelled for different substrate geometries, strap materials and dimensions in order to test the accuracy and robustness of the methodology. First, calculated through-thickness strain energy release rate distribution is compared with the result of a 3D FE model to validate this 2D model. Second, calculated disbond areas, thermal residual stresses and their redistribution with crack propagation are validated against experimental measurements. Third, influence of geometric nonlinearity and the need to use the alternate analysis method described in part I are demonstrated by examples, and errors generated by not following this analysis rule are given. Finally, using the 2D model calculated stress intensity factors, fatigue crack growth rates and lives are predicted for different specimens, strap materials and applied stress levels and are compared with the experimental tests. Good or acceptable agreement has been achieved for each case.     

中低应变率加载下弱胶结软岩动力学特性

我国西部侏罗系煤层上覆巨厚白垩系富水软岩,为了解此类软岩在冲击荷载作用下的力学本构关系及损伤演化规律,利用Hopkinson压杆装置对干燥、饱和红砂软岩进行中低应变率下的冲击试验,结果表明:红砂软岩峰值应力、峰值应变均表现出明显的应变率效应,其中峰值应力与应变率呈指数关系;相同应变率下,干燥红砂软岩的强度大于饱和状态,对冲击荷载表现出更强的抵抗能力,但饱和红砂软岩的宏观破坏强度大于干燥状态;低应变率加载下,干燥红砂软岩出现负损伤;结合微观机理分析,低应变率下,水对红砂软岩的弱化作用占据主导地位,随着应变率的增大,在惯性效应和水的Stefan效应共同作用下,饱和红砂软岩的动态强度得到强化;基于Z-W-T模型和应变等效原理,建立了服从Weibull分布的损伤本构方程,经验证能很好的反映红砂软岩的动态本构关系,具有一定的工程实际意义。     

Mechanical response of a soft rubber compound compressed at various strain rates

Rate-dependent material constitutive behavior models are needed in numerical simulations of shock-mitigation structures. In this research, compressive stress–strain response of a soft rubber compound is obtained experimentally at quasi-static, intermediate and high strain rates under uniaxial-stress and uniaxial-strain loading states. Kolsky bars with modifications for characterizing soft materials and a long Kolsky bar are used to conduct the dynamic experiments, while an MTS load frame is used for conducting experiments at quasi-static rates. Compression experiments are conducted at each decade in the strain-rate scale without any gap typically seen in the intermediate range. The experimental results show significant strain-rate effects on the mechanical behavior of this soft material, which are summarized via a rate-dependent constitutive model.