考虑应力路径和加载速率效应砂土的弹黏塑性本构模型

 利用室内多应力路径平面应变压缩试验结果,分析和研究密实砂土变形和强度特征的应力路径和加载速率效应。试验结果表明：一方面,不可恢复体积应变和剪切应变都具有明显的应力路径相关性,因而在传统塑性理论中将其作为硬化参量存在不合理性;另一方面,砂土的应力–应变特性与加载速率的变化存在着显著的关系。加载速率效应与蠕变和应力松弛一样均是砂土黏性的外在反映,其最重要的特征之一是加载速率发生突变时,应力也发生相应的突变,并呈现出刚性很大、近似弹性的特性。对试验结果的进一步分析发现,一种修正的不可恢复应变能W ir*及相关的函数与应力路径不相关。将W ir*作为硬化参量,并在非线性三要素模型的理论框架下,提出一种基于能量的砂土弹黏塑性本构模型。该模型可以考虑应力路径、压力水平、固有各向异性、孔隙比等因素对砂土变形和强度特征的影响,以及应变局部化和加载速率变化所产生的黏性特性。将上述模型嵌入到有限元程序中,并对平面应变压缩试验进行模拟计算,验证模型的精确性。研究结果表明,与现有的砂土本构模型相比,所提出的模型能更好地模拟应力路径及加载速率变化对砂土变形和强度特征的影响。     

平面应变条件下砂土局部化剪切带的有限元模拟

 根据饱和砂土的排水平面应变压缩试验的应变场,分析研究砂土的应变局部化现象以及剪切带的形成。在平面应变压缩条件下,砂土在峰值应力状态附近出现应变局部化现象,在残余状态最终形成一条V型剪切带。剪切带的形成是一个渐进过程,砂土呈现出渐进性破坏特性。这种由软化特性引起的应变局部化剪切带是砂土材料非常重要的变形和强度特性之一。基于砂土的三要素弹黏塑性本构模型和动态松弛有限元求解技术,对砂土的应变局部化现象和剪切带的形成进行有限元模拟。其中,砂土本构模型中引入应变局部化参数S来表示砂土峰值以后的软化和剪切带效果,剪切带在单元内未考虑其方向性,而是假设剪切带方向与最大剪切应变的方向一致。分析结果表明,动态松弛有限元法及砂土三要素模型能合理地模拟砂土的应变局部化现象,剪切带附近的最大剪应变值也非常接近,从而实现对砂土材料从硬化→峰值→软化→残余的全过程模拟以及对砂土应变局部化剪切带的定量化分析。     

黏土和砂土简单的三维本构模型(英文)

把修正剑桥模型推广为适合于黏土和砂土的简单、统一三维本构模型是基于以下两点提出的 :第一点 ,把由笔者已提出的基于SMP准则的一个变换应力张量用于修正剑桥模型使其简单实现了三维化 ,改进的模型对于黏土实现了从剪切屈服到剪切破坏的统一及临界状态理论同SMP准则的结合 ;第二点 ,为了建立对于黏土和砂土简单、统一的本构模型 ,推导出了一个新的硬化参数 ,新硬化参数不仅能描述砂土不同程度的剪胀性 ,对于正常固结黏土又能退化成塑性体积应变 ,所提硬化参数的合理性也被各种路径下分别在三轴压缩和三轴伸长下的试验结果所证实。所提模型的计算参数仅为 5个常规试验参数 ,易于确定     

一个能够模拟软土时效特性的简单弹黏塑性模型(英文)

基于Perzyna超应力理论与修正剑桥模型,建立了一个能够模拟软土时效特性的简单的弹黏塑性本构模型,提出了参数的实验室确定方法。以室内试验为基础,模拟了不同试验条件下软土的时效特性:应变速率对先期固结压力和不排水抗剪强度的影响;一维固结与次固结特征及竖向应力对次固结系数的影响;不同应力水平下的不排水蠕变特征;不同应变水平下的应力松弛特征。通过实验数据与数值模拟的比较,对模型进行了验证,发现上述本构模型能够较好地描述不同加载路径下的正常固结与微超固结土的时效特征。同时,通过对同一试样的多阶段加卸载三轴实验、现场压力仪实验及实验室压力仪实验的模拟,发现此模型可以较好地拟合实验过程中复杂应力路径下软土的时效特征。     

Fem Simulation of the Viscous Effects on the Stress-Strain Behaviour of Sand in Plane Strain Compression

A stress-strain model called TESRA (Temporary Effects of Strain Rate and Acceleration), described in a non-linear three-component framework, has been proposed to simulate the effects of viscous property on the stress-strain behaviour observed in drained plane strain compression (PSC) tests on clean sands. According to the TESRA model, the current viscous stress component is obtained by integrating for a given history of irreversible strain increments of viscous stress component that developed by respective instantaneous irrecoverable strain increment and its rate and have decayed with an increase in the irreversible strain until the present. The TESRA model was implemented into a generalized elasto-plastic isotropic strain-hardening non-linear FE code. The integration scheme to obtain the viscous and inviscid stress components according to the TESRA model in FEM analysis needs some specific considerations including the relevant choice of the suitable rate parameter. The shear stress—shear (or axial) strain—time relations from five drained PSC tests on saturated Toyoura sand and air-dried Hostun sand were successfully simulated by the FE code embedded with the TESRA model. It is shown that the FE code can simulate the time-dependent stress-strain behaviour of sand accurately without spending any significant extra computational time or storage. The results of simulation using one element and multi-element are essentially the same.     

An Elasto-viscoplastic Constitutive Model with Strain-softening for Soft Sedimentary Rocks

In general, soft sedimentary rocks exhibit strain-softening and time-dependent behavior. In the present paper, an elasto-viscoplastic constitutive model for soft rocks is proposed, that can universally describe time-dependent behavior such as creep, stress relaxation and strain rate sensitivity by extending Adachi and Oka's elasto-viscoplastic model for frozen sand. It is found that the proposed model can be applied to the time-dependent behavior of a soft tuffaceous rock called Tomuro Stone under the strain rate constant triaxial compression, drained triaxial creep and undrained triaxial stress relaxation.     

加载速率变化条件下砂土的黏塑特性及本构模型

 分析并研究饱和的、风干的砂土在平面应变压缩试验条件下的弹黏塑性特征,尤其是黏性特性。加载速率效应、蠕变以及应力松弛都是砂土材料本身黏性的反映,而与超孔隙水压力的消散无关。试验不仅实现了应变率逐步变化加载过程,同时也实现了蠕变加载和应力松弛过程。试验结果表明,在加载应变率发生突变时,对砂土应力–应变关系有明显的影响,呈现出刚性很高、近似于弹性的特性,而后随着应变的进一步增加,在明显屈服之后黏性应力逐渐衰减至基本惟一的应力–应变曲线。类似现象同样也发生在蠕变加载或应力松弛后以一恒定应变率突然重新加载的情况。基于非线性三要素模型框架,针对所观察到的砂土黏性特性,提出一种弹黏塑性本构模型。该模型可以描述砂土在任意加载历史下的黏性效应,包括加载应变率发生任意变化时的应力–应变响应、蠕变以及应力松弛。最后,利用该模型对上述砂土平面应变压缩试验结果进行模拟,所提案的三要素弹黏塑性本构模型能够很好地模拟砂土的黏性特性。     

剪切双曲线型等效时间流变模型

为了预测变剪应力荷载下的流变变形,运用热力学内变量连续介质力学和Yin-Graham等效时间法来推导剪切流变模型。首先把Mesri双曲线型蠕变公式推广到耗散空间,在耗散空间绘制黏塑性剪应变与剪应力等效时间线。其次利用Yin-Graham等效时间法建立了黏塑性剪应变速率与等效时间关系方程,获得黏塑性剪应变速率与应力水平和黏塑性剪应变之间的函数表达式。最后利用耗散剪应力等于真实剪应力这个力学关系式,把黏塑性剪应变速率表达式和线弹性本构方程相结合,建立了一维弹黏塑性剪切流变方程,研究了流变方程分别采用绝对等效时间和相对等效时间表示的表达式。基于这个流变方程,获得了三轴固结排水剪切蠕变试验中单级加载和多级加载的剪切流变解。实例表明,剪切流变模型能够较好地模拟软土三轴固结排水剪切蠕变试验成果。     

砂土的双硬化特性与弹塑性本构模型

 基于高精度的丰浦砂三轴等向加卸载试验与多应力路径平面应变压缩试验结果,提出平面应变条件下的修正塑性功体积硬化函数,得到双硬化框架下的修正塑性功剪切硬化函数,建立应力路径不相关的丰浦砂修正塑性功剪切–体积双硬化函数。在该双硬化函数基础上,推导基于修正塑性功的增量型剪切–体积双硬化弹塑性刚度矩阵,构建可以考虑多种变形强度影响因素的丰浦砂本构模型。数值计算与室内试验结果的比较表明,该砂土双硬化弹塑性本构模型可以合理地模拟砂土材料的变形强度特性。     

冻融循环下青藏粉砂土双屈服面本构模型研究

 基于青藏粉砂土在冻融循环下的常规三轴固结剪切试验,通过引入模量残余比和冻融循环次数,建立考虑冻融循环影响的双屈服面本构模型。试验结果表明,粉砂土的应力–应变关系呈应变硬化型,而在整个剪切过程中,体变呈现剪缩的特性。粉砂土的剪切模量随着围压的增大而增大,随着冻融循环的发展而出现先减小后增大的趋势。与未冻融粉砂相比,冻融以后的弹性剪切模量可降低约36%左右。其应力平面p-q上的剪切屈服面和体积屈服面可分别用过原点的线性函数和椭圆型曲线进行描述。对于剪切和体积硬化特性,建立与塑性应变及冻融循环次数相关的硬化参数,且均采用非相关联的流动法则。所提出的双屈服面本构模型能够很好地反映土体的应力–应变特性,模型计算值与试验值较为吻合,该模型能较为准确地预测不同围压及不同冻融循环次数下的应力–应变关系曲线,较好地反映冻融循环对粉砂力学性质的影响。     

考虑时间效应的K_0各向异性UH模型

将所建立的考虑时间效应的超固结土统一硬化模型(时间UH模型)推广为能够考虑K0各向异性的弹黏塑性模型。基于广义非线性强度准则的变换应力三维化方法,将新模型扩展到三维应力状态。使用新模型预测K0固结土的一维和三轴等应变率加载试验,预测结果表明新模型能够合理反映应变率对K0固结土前期固结压力和应力应变曲线的影响。根据模型的基本方程,推导三轴压缩条件下不排水抗剪强度的理论公式,并将该公式的计算结果与试验结果进行对比,发现该公式能够反映不排水抗剪强度随超固结度和应变率的增大而提高的性质。     

砂土剪胀剪缩特性及其对抗剪强度的影响

为了研究饱和砂土的剪胀剪缩特性及其对抗剪强度的影响,选取滹沱河细砂,利用空心圆柱扭剪仪较系统地开展了一系列不同初始密度、不同固结压力条件下的排水与不排水纯扭剪试验研究,在总应力保持不变的情况下研究了砂土的剪胀剪缩特性,着重探讨了在排水与不排水试验中,不同密度和不同有效围压的砂土在单调剪切荷载作用下的应力-应变关系、硬化与软化、土体的剪胀剪缩以及强度等特性。结果表明:砂土密度和固结压力对砂土剪胀剪缩特性具有显著的影响;砂土的剪胀剪缩特性对砂土的排水、不排水强度以及应力-应变关系产生显著的影响;由于剪胀剪缩特性的影响,砂土的不排水抗剪强度甚至可能高于排水抗剪强度;研究成果可为今后砂土的本构模型和数值模拟提供试验资料。     

Modelling and Simulation of Rate-Dependent Stress-Strain Behaviour of Granular Materials in Shear

A constitutive modelling of the elasto-viscoplastic stress-strain behaviour of geomaterials in shear that has been developed within a non-linear three-component model framework is validated by simulating a comprehensive series of drained triaxial compression (TC) and direct shear (DS) tests on a wide variety of granular materials. Illustrative simulations of rate-dependent stress-strain behaviour of geomaterial under typical laboratory test conditions were performed to analyse the structure of the model. The versatility of the proposed model and its applicability to a wide variety of shear loading histories is examined and demonstrated by these simulations. The following results are shown. Commonly with different basic viscosity types, Isotach, TESRA and P&N, the viscous stress component has a positive component that increases with an increase in the irreversible strain rate, which makes feasible stable and realistic simulations of rate-dependent stress-strain behaviour, including creep deformation, based on the proposed model. With different unbound granular material types having similar relative densities, the creep strain in TC tests and creep shear displacement in DS tests that develop by sustained loading at a given shear stress level for a given period tends to decrease with an increase in the particle roundness. This trend of behaviours is explained by a decrease in the viscosity type parameter, θ, associated with an increase in the particle roundness based on the simulations of these tests.     

Cyclic behavior of Q345GJ steel used in energy dissipation shear links

The cyclic behavior of Q345GJ steel was explored by strain cycle tests, stress cycle tests and uniaxial tension tests. The results of strain cycle tests confirmed that Q345GJ steel has no cyclic hardening, and it had rare strain amplitude effect or strain history effect. The stress cycle tests confirmed that the cumulative plastic strain goes up stably under eccentric stress with increasing loading loops. Thus, it can be concluded that the hardening characteristics of Q345GJ coincide with kinematic hardening, having almost no isotropic hardening. The superposition of four nonlinear kinematic hardening model based on Chaboche’s constitutive model was used to predict the hardening characteristics of Q345GJ steel, and the parameters of the constitutive model were calibrated based on the test results. The numerical validation of the constitutive model confirmed that the proposed parameters can give good simulation for strain cycle behavior and stress cycle behavior of Q345GJ steel.     

双剪统一弹塑性应变软化本构模型研究

 根据统一强度理论应力不变量的形式,确定所要建立模型的屈服函数以及塑性势函数,考虑土体强度随加载过程的逐渐发挥,确定土体硬化函数。采用非相关联流动法则建立复杂应力状态下的土体弹塑性本构方程,并且对模型中所产生的奇异性进行分析和处理,指出模型中的奇异性是屈服函数、塑性势函数的流动矢量、以及硬化模量所引起的。给出模型中所含参数的确定方法,采用膨胀性泥岩的常规三轴试验模型进行验证,验证结果表明,所建立弹塑性本构模型能够模拟泥岩的应力–应变关系,较好地反映土体的应变软化特性。     

Dynamic behavior of reinforced clayey sand under cyclic loading

Experimental investigations and modeling of linear elasticity of fiber-reinforced clayey sand under cyclic loading unloading are conducted in this paper. Experimental studies are focused on four aspects. First, a series of cyclic triaxial tests, with different confining pressures and deviator stress ratios up to 150 cycles, are performed. Impacts of fiber content, cell pressure, deviator stress ratio and loading unloading repetition that affect dynamic behavior of the composite material are discussed. It is shown that shear modulus decreases with increasing deviator stress ratio at high confining pressure and the rate of loss of shear modulus found to be much lower for fiber reinforced specimens. Other results show that increase of shear modulus with loading repetition is more pronounced at higher deviator stress ratios. Second, the optimum fiber content is experimented under cyclic loading unloading and is expressed as a power function of deviatoric stress ratio. It is shown that optimum fiber content is not constant and it is affected by deviator stress ratio. Third, a function is introduced to describe the linear stress–strain curve under cyclic loading unloading using equivalent linear analysis. The shear modulus G is expressed as a function of fiber content, confining pressure, deviatoric stress ratio and loading repetition. Finally constitutive coefficients of the model parameters are calibrated by the results of cyclic triaxial shear tests and using the linear regression.     

K0固结软黏土的弹黏塑性本构模型

在修正剑桥模型基础上综合考虑了软黏土的各向异性及率相关性,建立了适用于K0固结软黏土的弹黏塑性本构模型。模型借鉴过应力理论的基本思想,定义了与动态加载面相对应的参考屈服面,应用径向映射准则将两者联系起来,流动函数通过分析一维情况下土体的体积蠕变速率得到。以黏塑性体积应变为硬化参数,将一维情况扩展到三维应力状态,直接用次固结系数描述土体黏性强弱,所有参数可通过压缩试验及三轴不排水剪切试验得到。分别计算了代表性等向和K0固结黏土的三轴不排水等应变率加载、不排水剪切蠕变及蠕变破坏过程,与试验结果进行对比,验证本文模型的有效可行性。     

砂土非共轴特性的本构模拟

传统的本构理论隐含了应力和塑性应变率的共轴条件,无法客观描述主应力轴旋转试验中的非共轴现象。因此,在已有的本构模型中引入非共轴塑性流动理论来描述这种非共轴现象,并对形状函数进行了修正。通过对真三轴试验、单剪试验和空心圆柱试验进行数值模拟,表明修正后的形状函数能够更好地描述砂土在真三维状态下的变形特性,更重要的是非共轴理论的引入使得模型能够合理预测主应力轴旋转过程中主应力和主应变率的方向变化规律。     

土的清华弹塑性模型及其发展

 土的清华弹塑性模型具有独特的建模方式。在Drucker假说的基础上,无需其他的附加假设,通过试验资料直接确定塑性势面和塑性势函数,选择适当的硬化参数使屈服函数与塑性势函数一致。通过真三轴试验及平面应变试验,利用该模型的建模方法,在π平面上确定了一种双圆弧的屈服轨迹,从而建立了该模型的三维形式,提出了相应的流动规则表达式;在小浪底堆石料的三轴湿化试验基础上,发现湿化应变作为一种塑性应变与通过该点的屈服面正交,因而只需分别确定干土与饱和土的清华弹塑性模型的参数,同时进行各向等压条件下的湿化试验,测量其湿化体应变,就可以计算出在任意应力状态下浸水湿化的应力应变全过程;与湿化的清华模型相似,进行了不同含水率的土的三轴试验,并进行在干土试样中预加冰屑然后在指定应力状态下使其融化均匀增湿的三轴试验,发现屈服函数不变,硬化参数可以表示为塑性应变和含水率的函数,从而绕过了基质吸力这一变量,建立了非饱和土的清华模型,试验表明它可以合理地预测从干试样增湿到其他含水率的应力应变全过程;密实的永定河砂的试验表明,在相同应力状态下,应变硬化段与应变软化段的塑性应变增量的方向是一致的,将硬化参数表示为塑性功的函数,则可以描述土的应变软化,模型试验结果表明了建立的反映应变软化的清华模型可以合理地计算浅基础的荷载沉降关系;将等向硬化改为旋转硬化,就可以计算砂土在减载和循环加载下的应力应变关系。清华弹塑性模型是一个具有很大发展空间的模型。