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1.
A new method is proposed for the development of a class of elastoplastic thermomicromechanical constitutive laws for granular
materials. The method engenders physical transparency in the constitutive formulation of multiscale phenomena from the particle
to bulk. We demonstrate this approach for dense, cohesionless granular media under quasi-static loading conditions. The resulting
constitutive law—expressed solely in terms of particle scale properties—is the first of its kind. Micromechanical relations
for the internal variables, tied to nonaffine deformation, and their evolution laws, are derived from a structural mechanical
analysis of a particular mesoscopic event: confined, elastoplastic buckling of a force chain. It is shown that the constitutive
law can reproduce the defining behavior of strain-softening under dilatation in both the mesoscopic and macroscopic scales,
and reliably predict the formation and evolution of shear bands. The thickness and angle of the shear band, the distribution
of particle rotation and the evolution of the normal contact force anisotropy inside the band, are consistent with those observed in discrete element simulations and physical experiments. 相似文献
2.
The paper provides an in-depth exploration of the role of particle crushing on particle kinematics and shear banding in sheared granular materials. As a two-dimensional approximation, a crushable granular material may be represented by an assembly of irregularly shaped polygons to include shape diversity of realistic granular materials. Particle assemblies are subjected to biaxial shearing under flexible boundary conditions. With increasing percentage of crushed particles, mesoscale deformation becomes increasingly unstable. Fragmented deformation patterns within the granular assemblies are unable to form stable and distinct shear bands. This is confirmed by the sparsity of large fluctuating velocities in highly crushable assemblies. Without generating distinct shear bands, deformation patterns and failure modes of a highly crushable assembly are similar to those of loose particle assemblies, which are regarded as diffuse deformation. High degrees of spatial association amongst the kinematical quantities confirm the key role that non-affine deformation and particle rotation play in the generation of shear bands. Therefore, particle kinematical quantities can be used to predict the onset and subsequent development of shear zones, which are generally marked by increased particle kinematic activity, such as intense particle rotation and high granular temperature. Our results indicate that shear band thickness increases, and its speed of development slows down, with increasing percentage of crushed particles. As particles crush, spatial force correlation becomes weaker, indicating a more diffuse nature of force transmission across particle contacts. 相似文献
3.
The present paper deals with the impact of shear deformation on the geometric arrangement of particles within the soil specimen,
which is termed as the microfabric of soil. A series of compression and extension lubricated end triaxial tests are performed
on cylindrical specimens of Kaolinite clay with two extreme microfabrics; dispersed and flocculated, which are obtained using
slurry consolidation technique. Flocculated microfabric has random orientation of particles within the soil mass having face-to-edge
particle contacts; however, dispersed microfabric has parallel orientation of particles containing face-to-face particle contacts.
When the specimen is subjected to large stress levels during its shear deformation, the particle orientation and the geometric
arrangement within the soil specimen gets affected due to the force acting on the clay platelets. The variation in microfabric
of soil before and after shear deformation process is evaluated by obtaining X-ray diffraction patterns of the clay specimen
at three different locations using standard X-ray diffractometer. The discussion includes an analysis of the orientation of
soil particles located at shear banding zones of the clay specimens, which may be useful for understanding the strain localization
development in clays. 相似文献
4.
5.
Undrained deformation of dilative sand generates negative excess pore pressure. It enhances the strength, which is called dilative hardening. This increased suction is not permanent. The heterogeneity at the grain scale triggers localisations causing local volume changes. The negative hydraulic gradient drives fluid into dilating shear zones. It loosens the soil and diminishes the shear strength. It is essential to understand the mechanism behind this internal drainage and to capture it numerically. The purpose of this paper is to develop a macroscopic constitutive relationship for the undrained deformation of saturated dense sand in the presence of a locally fully or partially drained shear band. Separate constitutive relations are generated for the band and intact material. Both time and scale dependence during pore fluid diffusion in saturated sand are captured, eliminating the mesh dependency for finite element implementations. The model is applied to the Gauss points that satisfy the bifurcation criterion. The proposed method is calibrated to recreate the undrained macroscopic response bestowed by an extra-small mesh. The microscopic behaviours inside and outside shear band predicted by this model are qualitatively in good agreement with individual material point behaviours inside and outside the shear band in the extra-small mesh. Depending on the loading rate and the shear band thickness, the response inside the band can be fully or partially drained, which governs the ultimate global strength. The calibrated model is exploited to simulate an upscaled biaxial compression test with semipermeable boundaries. 相似文献
6.
Modelling shear band is an important problem in analysing failure of earth structures in soil mechanics. Shear banding is the result of localization of deformation in soil masses. Most finite element schemes are unable to model discrete shear band formation and propagation due to the difficulties in modelling strain and displacement discontinuities. In this paper, a framework to generate shear band elements automatically and continuously is developed. The propagating shear band is modelled using discrete shear band elements by splitting the original finite element mesh. The location or orientation of the shear band is not predetermined in the original finite element mesh. Based on the elasto‐perfect plasticity with an associated flow rule, empirical bifurcation and location criteria are proposed which make band propagation as realistic as possible. Using the Mohr–Coulomb material model, various results from numerical simulations of biaxial tests and passive earth pressure problems have shown that the proposed framework is able to display actual patterns of shear banding in geomaterials. In the numerical examples, the occurrence of multiple shear bands in biaxial test and in the passive earth pressure problem is confirmed by field and laboratory observations. The effects of mesh density and mesh alignment on the shear band patterns and limit loads are also investigated. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
7.
为合理反映颗粒破碎对堆石料力学特性的影响,基于试验结果分析,得出了堆石料在压缩和剪切作用下的颗粒破碎特性规律。通过引入压缩破碎和剪切破碎的相关参数,借鉴已有本构模型的合理定义,吸收临界状态理论和边界面理论的优点,发展了考虑颗粒破碎和状态相关的堆石料静动力统一弹塑性本构模型,并阐述了模型参数的确定方法。该模型不仅能够反映堆石料在静力荷载作用下的低压剪胀、高压剪缩、应变软化和硬化等特性,还能够反映在循环荷载作用下应力-应变的滞回特性和残余变形的累积效应。最后为验证模型的合理性,分别对堆石料的静力三轴和循环三轴试验进行了数值模拟预测,结果表明：模型预测与试验数据吻合良好,所提出的本构模型能够合理地描述颗粒破碎对堆石料静动力变形特性的影响。 相似文献
8.
This article presents a fundamental study on the role of particle breakage on the shear behavior of granular soils using the three‐dimensional (3‐D) discrete element method. The effects of particle breakage on the stress ratio, volumetric strain, plastic deformation, and shear failure behavior of dense crushable specimens undergoing plane strain shearing conditions are thoroughly investigated through a variety of micromechanical analyses and mechanism demonstrations. The simulation of a granular specimen is based on the effective modeling of realistic fracture behavior of single soil particles, which is demonstrated by the qualitative agreement between the results from platen compression simulations and those from physical laboratory tests. The simulation results show that the major effects of particle breakage include the reduction of volumetric dilation and peak stress ratio and more importantly the plastic deformation mechanisms and the shear failure modes vary as a function of soil crushability. Consistent macro‐ and micromechanical evidence demonstrates that shear banding and massive volumetric contraction depict the two end failure modes of a dense specimen, which is dominated by particle rearrangement–induced dilation and particle crushing–induced compression, respectively, with a more general case being the combination and competition of the two failure modes in the medium range of soil crushability and confining stress. However, it is further shown that a highly crushable specimen will eventually develop a shear band at a large strain because of the continuous decay of particle breakage. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
9.
Conditions for instabilities in collapsible solids including volume implosion and compaction banding
Ronaldo I. Borja 《Acta Geotechnica》2006,1(2):107-122
We review conditions for material instabilities in porous solids induced by a bifurcation of solution into non-unique strain rate fields. Bifurcation modes considered include jumps in the strain rate tensor of ranks one and higher representing deformation band and diffuse instability modes, respectively. Eigenmodes (e-modes) are extracted for each type of instability to fully characterize various frameworks of deformation in collapsible solids. For diffuse instability these e-modes are determined from a homogeneous system of linear equations emanating from the condition of zero jump in the stress rate tensor, which in turn demands that the tangent constitutive tensor be singular for the existence of nontrivial solutions. For isotropic materials we describe two types of singularity of the constitutive tensor: (a) singularity of the constitutive matrix in principal axes, and (b) singularity of spin. Accordingly, we derive the e-modes for each type of singularity. We utilize the singularity of the constitutive matrix in principal axes as a precursor to volume implosion in collapsible solids such as loose sands undergoing liquefaction instability and high-porosity rocks undergoing cataclastic flow. Finally, we compare conditions and e-modes for volume implosion and compaction banding, two similar failure modes ubiquitous in granular soils and rocks.Supported by U.S. Department of Energy, Grant DE-FG02-03ER15454, and U.S. National Science Foundation, Grants CMS-0201317 and CMS-0324674. 相似文献
10.
VMIB (Virtual Multi-dimensional Internal Bonds)模型是在VIB(Virtual Internal Bond) 模型基础上发展起来的一种多尺度力学模型。VIB认为,固体材料在微观上是由随机分布的质量微粒（Material particle）构成,微粒与微粒之间由一虚内键（virtual internal bond）连结,材料的宏观本构方程则直接由微粒之间的连结法则（Cohesive law）导出。而在VMIB中,微粒之间引入了切向效应,材料的宏观本构方程由虚内键刚度系数导出。由于考虑了微粒之间的切向效应,VMIB能够再现材料不同泊松比。依据VMIB,材料的宏观力学行为决定于微观虚内键的演化。岩石材料在围压条件下强度显著增强,破坏模式一般表现为剪切破坏,其微观机制在于虚内键的演化决定于自身的法向和切向变形,并且演化速度决定于连续层次上材料微元的应力状态。为描述这种微观虚内键的演化机制,提出了一种虚内键演化方程。通过该演化方程,可以宏观再现岩石材料在围压条件下的断裂破坏行为。作为初步应用,没有考虑材料的塑性变形机制,因此,还有待于进一步的理论完善。 相似文献
11.
The shear behavior at the interface between the soil and a structure is investigated at the macroscale and particle‐scale levels using a 3‐dimensional discrete element method (DEM). The macroscopic mechanical properties and microscopic quantities affected by the normalized interface roughness and the loading parameters are analyzed. The macro‐response shows that the shear strength of the interface increases as the normalized roughness of the interface increases, and stress softening and dilatancy of the soil material are observed in the tests that feature rough interfaces. The particle‐scale analysis illustrates that a localized band characterized by intense shear deformation emerges from the contact plane and gradually expands as shearing progresses before stabilizing at the residual stress state. The thickness of the localized band is affected by the normalized roughness of the interface and the normal stress, which ranges between 4 and 5 times that of the median grain diameter. A thicker localized band is formed when the soil has a rough shearing interface. After the localized band appears, the granular material structuralizes into 2 regions: the interface zone and the upper zone. The mechanical behavior in the interface zone is representative of the interface according to the local average stress analysis. Certain microscopic quantities in the interface zone are analyzed, including the coordination number and the material fabric. Shear at the interface creates an anisotropic material fabric and leads to the rotation of the major principal stress. 相似文献
12.
黏土的变形主要受土颗粒之间的联结方式控制,将土颗粒之间的联结方式分为完善联结和滑动联结。在弹性变形阶段,颗粒之间的联结为完善联结,随着剪应力的增大,骨架中一部分完善联结逐渐变成滑动联结,这种转变即为损伤的演化。骨架的损伤和破坏遵循Mohr-Coulomb准则,在p-q平面中以应力点到初始损伤线和破坏线的相对距离表示损伤比,给出了一种描述骨架损伤和计算损伤演化的方法,进而提出了一种描述黏土在不排水条件下剪切变形的损伤本构模型。模型中的参数可根据常规三轴压缩试验确定,模型的形式简单,可适用于复杂的应力路径情况。对试验结果的拟合表明,该模型能较好地反映黏土在不排水条件下的剪切变形特征。 相似文献
13.
城市固体废弃物的剪切强度机理及本构关系 总被引：2，自引：0，他引：2
根据城市固体废弃物（简称MSW）的大三轴固结排水剪试验结果,延伸岩土工程中研究土体的概念和方法,研究MSW的剪切强度机制。MSW与大变形相关的强度特性主要是由于含有大量的纤维状成分所致,在剪切过程中试样总的抗剪能力由剪切面上的摩擦力和纤维状加筋相的拉力所产生。MSW的摩擦强度主要由3部分构成：纤维状成分与土颗粒之间的摩擦,纤维状成分之间的摩擦,和土颗粒之间的摩擦。基于MSW的剪切强度特性,采用邓肯-张本构模型来描述MSW的应力-应变关系。 相似文献
14.
A computational framework is presented for dynamic strain localization and deformation analyses of water‐saturated clay by using a cyclic elasto‐viscoplastic constitutive model. In the model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil particles are considered. In order to appropriately simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a two‐phase mixture is derived in the updated Lagrangian framework. The shear band development is shown through the distributions of viscoplastic shear strain, the axial strain, the mean effective stress, and the pore water pressure in a normally consolidated clay specimen. From the local stress–strain relations, more brittleness is found inside the shear bands than outside of them. The effects of partially drained conditions and mesh‐size dependency on the shear banding are also investigated. The effect of a partially drained boundary is found to be insignificant on the dynamic shear band propagation because of the rapid rate of applied loading and low permeability of the clay. Using the finer mesh results in slightly narrower shear bands; nonetheless, the results manifest convergency through the mesh refinement in terms of the overall shape of shear banding and stress–strain relations. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
15.
利用自主开发的基于无网格法的岩石力学计算分析程序,采用Mohr-Coulomb破坏准则,考虑材料塑性屈服后的软化特性,研究单轴压缩条件下含一个缺陷的岩石试样的变形特性,探讨含缺陷材料的岩石变形局部化的演化规律。计算结果表明,岩石试件的破坏是一个渐进破坏的过程;加载过程中试件中的缺陷弱粒子首先出现粒子破坏并产生声发射,随加载进行试件产生微裂隙并扩张形成一个变形局部化带,局部化带扩张、发展,最终形成一个明显的剪切破坏带,缺陷粒子对试件的破坏起到了控制作用。岩石的声发射记录了岩石材料的塑性破坏过程,岩石试件的剪切破坏带与塑性区的发展具有非常类似的规律,但塑性区面积要大于剪切破坏带,从所处位置来看剪切破坏带位于塑性区之内,通过研究声发射特征信息发现变形局部化发生在应力-加载步（应变）达到峰值强度前。 相似文献
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17.
This paper aims at extending the well‐known critical state concept, associated with quasi‐static conditions, by accounting for the role played by the strain rate when focusing on the steady, simple shear flow of a dry assembly of identical, inelastic, soft spheres. An additional state variable for the system, the granular temperature, is accounted for. The granular temperature is related to the particle velocity fluctuations and measures the agitation of the system. This state variable, as is in the context of kinetic theories of granular gases, is assumed to govern the response of the material at large strain rates and low concentrations. The stresses of the system are associated with enduring, frictional contacts among particles involved in force chains and nearly instantaneous collisions. When the first mechanism prevails, the material behaves like a solid, and constitutive models of soil mechanics hold, whereas when inelastic collisions dominate, the material flows like a granular gas, and kinetic theories apply. Considering a pressure‐imposed flow, at large values of the normal stress and small values of the shear rate, the theory predicts a nonmonotonic shear rate dependence of the stress ratio at the steady state, which is likely to govern the evolution of landslides. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
18.
Geotechnical experiments show that Lode angle‐dependent constitutive formulations are appropriate to describe the failure of geomaterials. In the present study, we have adopted one such class of failure criteria along with a versatile constitutive relationship to theoretically analyze the effects of Lode angle on localized shear deformation or shear band formation in loose sand for both drained and undrained conditions. We determine the variation in the possible stress states for shear localization due to the introduction of Lode angle by considering the localized deformation as a bifurcation problem. Further, similar bifurcation analysis is performed for the stress states along a specific loading path, namely, plane strain compression at the constitutive level. In addition, the plane strain compression tests have been simulated as a boundary value finite element problem to see how Lode angle affects the post‐localization response. Results show that the inclusion of a Lode angle parameter within the failure criterion has considerable effects on the onset, plastic strain, and propagation of shear localization in loose sand specimens. For drained condition, we notice early inception of shear localization and multiple band formation when the Lode angle‐dependent failure criterion is used. Undrained localization characteristics, however, found to be independent of Lode angle consideration. 相似文献
19.
Two types of stress path-controlled plane strain compression tests were performed on both loose and dense specimens of angular and sub-angular sands and two rounded glass beads with different particle sizes. Digital image correlation method was used to analyze local deformation developments, especially shear band patterns. The material behavior in response to shearing has been found to be dependent on the relative density, particle shape, and stress path. The results of analysis on local deformation developments showed that the onset of shear bands occurred prior to their peak strengths in both dense and loose specimens. The growth rates of local maximum shear strain along a shear band were approximately consistent with an increasing global axial strain after the onset of shear band. The shear band width was influenced by both the mean particle size and the particle shape. The measured shear band inclination angles were in between those estimated by Coulomb’s and Roscoe’s formulas. 相似文献
20.
针对平均粒径对砂土剪切特性的影响作用,结合室内试验和离散元模拟方法对不同平均粒径砂土进行了细观研究。基于3种不同平均粒径砂土的直剪试验结果,通过建立反映砂土剪切试验特征的PFC（particle flow code）颗粒流模型,详细研究了不同粒径砂土在剪切过程中土样体积变化、力链网络、孔隙率和配位数等细观结构参数的变化特征和规律,并从细观角度分析了颗粒粒径对土样宏观剪切特性的影响机制。结果表明：具有不同平均粒径砂土的细观结构参数在剪切过程中存在显著差异,并且其细观参数差异主要集中体现在剪切带处;剪切力学特性的影响主要体现在抗剪强度和剪胀效应方面,砂土平均粒径越大,抗剪强度越高,剪胀效应越明显;具有不同平均粒径的砂土在剪切过程中土颗粒运动规律及剪切带形态变化特征存在一定的差异,平均粒径越大,剪切带内上跨式颗粒占比越大,剪切带厚度越大。 相似文献