Strain softening of K0-consolidated Changi sand under plane-strain conditions

Experimental results are presented in this paper to study the strain softening behaviour of a marine dredged sand under plane-strain conditions. K₀ consolidated drained and undrained tests were conducted using a new plane-strain apparatus to characterize the strain softening behaviour of the sand under plane-strain conditions. For medium dense specimens, strain softening and shear bands were observed to occur under both drained and undrained conditions. For very loose specimens, no shear bands were observed and critical states were reached within the homogeneous deformation region in both drained and undrained tests. Strain softening was observed to occur at small strain for very loose specimens under undrained conditions. Two types of strain softening, the homogenous softening and banding softening, were identified and the conditions for strain softening were established. The results obtained from this study were compared with the studies by Han and Vardoulakis (Géotechnique 41(1):49–78, 1991), Finno et al. (J Geotech Eng ASCE 122(6):462–473, 1996, Géotechnique 47(1):149–165, 1997) and Mokni and Desrues (Mech Cohes-Frict Mat 4:419–441, 1998).     

Strain rate effect on the compressive strength of frozen sand

Cylindrical specimens of fine Ottawa sand (A.S.T.M. designation C-109), compacted at the optimum moisture content and saturated before unidirectional freezing, have been tested in uniaxial compression at a cold room temperature of —5.5°C and strain rates between 10⁻⁷ and 10⁻² s⁻¹. The results agree with an extrapolation of data obtained by Sayles and Epanchin [1], but are much higher than those obtained by both Goughnour and Andersland [2] and Perkins and Ruedrich [3] at strain rates below 10⁻⁵ s⁻¹. There is evidence that this may be due to variation in total moisture (ice) content, the conditions under which the specimens were frozen (closed system or an open system) and to the end effects at the platen—specimen interface.     

应变局部化弹黏塑性分析的内嵌局部软化带模型

针对岩土介质结构在破坏过程中局部化变形的问题,结合位移不连续的思想,提出内嵌局部软化带模型来捕捉结构中的局部化带。通过虚功原理建立了含局部化带影响的弹黏塑性的有限元计算模式,其中分叉理论作为局部化判断条件。模型将局部化带的形成视为一个黏塑性屈服流动过程,从而能够连续地描述局部化变形前后的力学性质。特点是计算量小、物理意义明确,可以方便地整合到传统有限元分析程序中。算例表明,计算模型是合理和有效的。     

等应力增量比路径下接触面应变软化模型

等应力增量比路径单剪试验条件下,部分土体与结构接触面表现出明显的应变软化和剪胀特性。基于广义位势理论,将土与结构的接触面问题看作应力空间上的二维问题,势函数取法向应力和切向应力,用塑性状态方程取代传统的屈服面,建立等应力增量比路径条件下的土与结构接触面应力-应变软化模型。采用指数函数,拟合单向压缩试验中法向应力与法向应变的关系,采用复合指数函数,拟合应力比与切向应变的关系,采用另一复合指数函数,拟合法向剪胀分量与切向应变的关系。通过对拟合函数进行微分,确定模型中待定系数的求解方法。结合试验结果对模型进行验证,模型拟合效果良好,具有一定的合理性。     

基于一致性理论的Drucker-Prager材料弹黏塑本构模型

系统地论述了一致性理论的主要内容,推导了相关的黏塑性切线模量,并在前人工作的基础上研究了类Hoffman材料的推广条件及其一般性的推广方法;将线性Drucker-Prager模型(简称D-P)推广到一致性的黏塑性模型;给出了推广后模型的向后欧拉算法的有限元离散列式;最后将数值算例和试验成果进行对比,验证了一致性的线性D-P模型的有效性.     

冻结砂土力学性质的离散元模拟

基于离散单元法颗粒流理论,土体颗粒单元间采用接触黏结模型中来考虑冻土中冰的胶结作用,建立了冻结砂土的颗粒流模型。通过改变计算模型中颗粒单元的参数,模拟了在不同冻结温度以及不同围压下冻结砂土的宏观力学性质,并与冻结砂土的室内试验结果进行了比较,结果表明：颗粒流方法可以较好地模拟冻结砂土的应力-应变关系以及剪切带的发展变化过程,颗粒流细观参数对温度具有显著的依赖性。研究结果对离散单元法在特殊土中的应用具有一定的理论和应用价值。     

一个描述软黏土时效特性的一维弹黏塑性模型

为了描述软黏土一维应力-应变关系的时效特性,基于Bjerrum的等时间线体系,提出等黏塑性应变率线概念,推导了黏塑性应变率与黏塑性应变增量的关系,建立了软黏土的一维弹黏塑性模型;从理论上分析了新建模型与3种典型的一维弹黏塑性时效本构模型的内在联系,表明新建模型与其他3种模型在本质上是等效的,且形式更简洁,物理意义更明确;利用新建模型对软黏土的固结-蠕变耦合效应、应变率效应、应力松弛效应等时效特性进行了理论分析,并得到了相应的解析解;结合宁波软黏土的一维固结试验,阐述了模型参数的确定方法,并用新建模型对宁波软黏土的固结-蠕变试验、温州软黏土的一维多级等应变率试验、香港海相软黏土的一维应力松弛试验进行模拟,验证了新建模型的有效性。研究结果表明,新建模型能很好地模拟软黏土的一维时效特性。     

基于应变软化的多阶边坡稳定分析

多阶边坡除了最危险滑面需要关注,每级台阶的潜在滑面及其他不满足规范要求的滑面亦需要得到重视。以双台阶边坡和复杂多台阶边坡为例,基于应变软化理论,利用FLAC3D软件分析边坡的剪应变发展规律,根据剪应变的集中带获取边坡的多个潜在滑面。分析滑面的剪入、剪出和滑面中部点的强度参数变化规律,并结合矢量和法分析滑面在应变软化过程中安全系数的演化规律。研究结果表明：采用该方法可获取边坡的最危险滑面及次级滑动面;随着软化的发展,滑面上点的强度参数从峰值强度逐渐降低至残余强度,然而演化的速度不同;随着应变软化过程的发展,所有滑面的安全系数均逐渐降低,最后趋于一个定值。     

中主应力系数影响下的冻结砂土损伤本构模型

人工冻结法是饱水砂层开挖过程中常用的止水和临时支护方法,通过冻土损伤特性研究为冻土力学特性和冻结体稳定性分析奠定基础。为研究冻结砂土的损伤力学特性,在-5℃下进行了不同中主应力系数的冻结砂土三维室内试验。从冻土微元破坏服从Weibull随机分布的特点出发,将Drucker-Prager强度准则作为冻土微元统计分布变量,利用应变等价性假说,建立了三维应力状态下冻结砂土损伤本构模型;在此基础上,讨论模型参数F₀与m和中主应力系数的关系,对模型参数进行合理修正,建立中主应力系数影响下的冻结砂土损伤本构模型,并与试验结果进行对比。分析结果表明：参数F₀和m随着中主应力系数的增大呈现先减小后增大的趋势;参数F₀反映了冻结砂土的强度特性,参数m代表了冻结砂土的延性及脆性特征,考虑中主应力系数影响的冻结砂土损伤本构模型能很好地模拟冻结砂土应力-应变全过程曲线。研究成果为人工冻结法工程设计提供一定的理论依据。移动阅读     

Strain rate, temperature, and sample size effects on compression and tensile properties of frozen sand

Selection of material properties for use in design of frozen earth structures has been a limiting factor for some field applications. In particular, the mechanical properties governing the behavior of a frozen soil structure subjected to bending stresses are of interest. The effects of strain rate, temperature, and sample size on the compressive and tensile properties of frozen silica sand have been determined experimentally using uniaxial compression and split cylinder tests. Data included on the initial tangent modulus, compressive strength, failure strains, and tensile strength help delineate some limitations of available test procedures. Failure modes for various test conditions are described.

Data analysis shows that the initial yield stress, the compressive peak strength, and the initial tangent modulus increase with decreasing temperatures and increasing strain rates. Tensile strengths from split cylinder tests appear to be independent of deformation rates. Uniaxial compressive strengths decreased slightly and the initial tangent modulus increased with increasing sample diameter (constant length to diameter ratio). Deformation and failure modes changed from a plastic to a brittle behavior when strain rates were increased from low to high values. Larger failure strains at slower strain rates (more time available) appear to be a result of pressure melting, water migration and refreezing, permitting more particle readjustments before development of the peak strength. Observations on failure strains suggest limiting values for design situations.     

主动围压状态人工冻结砂土SHPB试验与分析

为研究主动围压状态人工冻结砂土的动态力学性能,利用直径50 mm变截面分离式霍普金森压杆试验装置,以取自山东济宁某矿-94.52 m处冻结砂土为研究对象,对长径比为0.5的冻结砂土试件进行主动围压状态下的冲击压缩试验,分析了主动围压和应变率对冻结砂土动态力学性能的影响。研究结果表明,无围压状态下,冻结砂土动态应力-应变曲线可分为弹性阶段、塑性阶段、黏性阶段和破坏阶段,冻结砂土呈脆性破坏;主动围压状态下,应力-应变曲线塑性阶段明显增长,无黏性阶段,冻结砂土呈微裂破坏。相同条件下,主动围压状态下冻结砂土的动态抗压强度均高于无围压状态,且随着围压的增大,动态抗压强度逐渐增大;当应变率为220 s~(-1),温度为-15℃时,主动围压为0.5、1.0、1.5、2.0 MPa的动态抗压强度为无围压状态下的119%、140%、158%和181%。不同主动围压下的应力-应变曲线表现出汇聚现象,汇聚点趋向于无围压状态。冻结砂土的动态抗压强度随应变率的提高而增加。     

Strain localization in sand: an overview of the experimental results obtained in Grenoble using stereophotogrammetry

Experimental results are presented from the extensive program of drained plane strain compression tests on sand carried out in Grenoble over the last two decades. Systematic analysis of photographs of the deforming specimen allowed for measuring deformations and determining strain fields throughout the test, that is: prior to, at, and after the onset of strain localization. The principles, details and accuracy of the procedure are described, as well as its suitability to properly depict the patterns of deformation. Findings concerning the occurrence and progression of strain localization are discussed. The issues of shear band orientation and thickness are addressed, as well as temporary and persistent complex localization patterns, and the volumetric behaviour inside a band after its formation. The influence of such variables as initial state of the sand (effective stress and relative density), specimen size and slenderness, as well as grain size, is discussed. Copyright © 2004 John Wiley & Sons, Ltd     

考虑体积变化影响的岩石应变软硬化损伤本构模型及参数确定方法

针对现有岩石损伤模型的局限性与不足,将应力作用下的岩石抽象为空隙、损伤与未损伤材料三部分,以空隙率反映岩石体积或空隙的变化,以损伤变量或损伤因子反映岩石力学性态的改变程度,通过对各部分材料的微观受力及其与宏观应力和应变的关系进行分析,建立了可反映岩石变形过程中体积或空隙变化特性的新型岩石损伤模型;从岩石微元强度合理度量方法研究入手,引进统计损伤理论,建立了可考虑损伤阀值影响的岩石统计损伤演化模型,进而建立了能模拟应变软硬化全过程的岩石统计损伤本构模型,并提出了参数确定方法。该模型不仅能反映岩石应变软硬化特性,而且能反映岩石变形的初期压密和后期扩容的特性。通过实例分析与讨论,表明模型合理与可行。     

A peridynamics model for strain localization analysis of geomaterials

Geomaterials such as sand and clay are highly heterogeneous multiphase materials. Nonlocality (or a characteristic length scale) in modeling geomaterials based on the continuum theory can be associated with several factors, for instance, the physical interactions of material points within finite distance, the homogenization or smoothing process of material heterogeneity, and the particle or problem size-dependent mechanical behavior (eg, the thickness of shear bands) of geomaterials. In this article, we formulate a nonlocal elastoplastic constitutive model for geomaterials by adapting a local elastoplastic model for geomaterials at a constant suction through the constitutive correspondence principle of the state-based peridynamics theory. We numerically implement this nonlocal constitutive model via the classical return-mapping algorithm of computational plasticity. We first conduct a one-dimensional compression test of a soil sample at a constant suction through the numerical model with three different values of the nonlocal variable (horizon) δ. We then present a strain localization analysis of a soil sample under the constant suction and plane strain conditions with different nonlocal variables. The numerical results show that the proposed nonlocal model can be used to simulate the inception and propagation of shear banding as well as to capture the thickness of shear bands in geomaterials at a constant suction.     

Aging and strain softening model for episodic faulting

Episodic slip on shallow crustal faults can be qualitatively explained by postulating a fault constitutive law that is the superposition of two limiting material responses: (1) strain softening after peak stress during large strain rates, and (2) strength (peak stress) recovery during aging at small strain rates. A single law permits a variety of seismic and aseismic phenomena to occur over a range of space and time scales. Specific cases are determined by the spatial variation of material constants, recent deformation history, crustal rigidity, and remote forcing.     

适用于砂土循环加载分析的边界面塑性模型

基于临界状态土力学框架,建立了一个适用于砂土排水循环加载的边界面塑性模型。采用了考虑虚拟峰值应力比的偏应变硬化准则,初始加载阶段应力点位于边界面上,反向加载阶段以历史最大屈服面作为边界面,同时实现了对密砂软化现象的模拟和对历史所受最大应力的记忆。边界面采用修正的椭圆形,引入考虑密度与应力水平的状态相关剪胀函数,采用非相关联流动法则和以应力反向点作为映射中心的径向映射准则。模型仅有10个参数,通过常规三轴试验即可确定,并且使用一套参数可以模拟不同围压、密度的单调和循环加载情况。分别对饱和砂土的单调、循环排水三轴试验进行模拟,结果表明,该模型能够合理地反映饱和砂土排水条件下的应力-应变特性。     

基于判别分析法的地震砂土液化预测研究

将距离判别分析方法应用于砂土液化的预测问题中,建立了砂土液化预测的距离判别模型。选用震级、研究深度、震中距、标贯击数、地下水位及地震持续时间等6项指标作为判别因子,以大量的工程实例数据作为学习样本进行训练,建立了线性判别函数对待评样本进行了评价。研究结果表明,距离判别分析模型判别砂土液化效果良好,预测准确度高,回判估计误判率低,可望成为砂土液化预测的有效手段。     

预压作用下软土微观结构试验及弹黏塑性模型研究

为了揭示软土固结蠕变机制,计算软基“工后沉降”,开展了室内模拟真空、堆载以及真空-堆载联合预压加固软土的固结蠕变三轴试验及三轴试验前、后的微观结构试验。建立了能考虑微观结构改变的弹黏塑性(EVP)软土蠕变模型。研究结果表明：原状软土样在真空预压、堆载预压、联合预压3种预压荷载作用下,固结过程和蠕变过程具有耦合效应;土体微观结构参数的改变与宏观工程性质的改变具有相辅相成的关系;EVP模型弱化了具体应力路径的变化,其反映弹性的参数和反映黏塑性的参数可由微观结构参数确定;改进的EVP模型既能计算土体的弹黏塑性变形,也考虑了土体微观结构的变化,能更全面地反映土变形的实质。