胶结砂砾石层面直剪试验及破坏行为细观研究

本文采用宏观试验和细观模拟相结合的方法研究胶结砂砾石层面在剪切过程中的破坏行为．首先进行了不同法向应力作用下的胶结砂砾石层面直剪试验,获得了不同的剪切面破坏特征：随着法向应力的不断增加,剪切破坏面凹凸起伏程度、骨料脱落现象越发明显．其次为了深入探究层面破坏现象,按照室内试验采用的骨料级配粒径建立细观颗粒数值模型,结合物理试验对模型进行参数标定,并进行数值模型的层面直剪模拟和细观分析．结果表明,数值模型可以再现宏观直剪试验层面破坏特征;法向应力越大,层面区域颗粒发生错动和翻转的数量越多;层面破坏方式为颗粒间的张拉和剪切混合破坏,裂隙均集中在层面位置,随着法向应力的增加,裂隙的集中区域逐渐由“面”向“带”转变．     

振动效应对铁路道砟剪切性能影响的离散元数值模拟

铁路道砟在传递移动列车载荷时会产生明显振动及接触迁徙行为,从而诱发道床的破碎劣化及非均匀沉降变形,道砟材料的抗剪性能研究对于碎石道床动力学行为及维养评估非常重要。为进一步探究铁路道砟的剪切力学特性,基于Minkowski Sum理论构造了扩展多面体单元,采用三维Voronoi切割算法生成了非规则扩展多面体道砟颗粒。通过最小投影方法对单元进行粒径识别和筛选,建立了符合铁路道砟级配要求的扩展多面体道砟模型数据库。对比了不同法向应力下铁路道砟准静态直剪试验及离散元模拟结果,验证了扩展多面体道砟接触参数的合理性。在此基础上,开展了振动效应下的道砟动态直剪离散元模拟研究,分析了振动角度、幅值及频率对道砟材料抗剪性能的影响。结果表明：构造的扩展多面体道砟离散元模型能够较好地反映非规则道砟间的强咬合互锁接触作用,并有效模拟直剪过程中的剪切应力–应变的变化及剪缩–剪胀行为。正则化下不同法向应力对应的铁道道砟剪切强度更适合采用幂函数拟合表示。振动场的存在会明显降低道砟颗粒的抗剪性能。当振动方向与剪切方向一致时（振动角度为0°）,道砟的剪切强度被明显削弱;随着振幅及频率的增大,道砟颗粒的剪切强度也会显著降...     

碎石料直剪实验的组合颗粒单元数值模拟

通过构造三维组合颗粒单元来描述颗粒间的互锁效应,对非规则颗粒材料的力学行为进行了离散元数值模拟,并通过碎石料的直剪实验进行了验证.该组合颗粒的质量与碎石块具有相同的概率分布特性,其几何形态则由不同数目、镶嵌尺寸、组合方位和粒径的球形颗粒进行随机构造.组合颗粒单元在局部与整体坐标之间的转动、力矩和方位关系通过四元素方法进行确定;颗粒之间的作用力采用具有Mohr-Coulomb摩擦定侓的Hertz-Mindlin 非线性接触模型,并考虑了非线性法向粘滞力的影响.在不同的法向应力下,对碎石料在直剪实验中的剪切应力和剪胀现象进行了离散元模拟,计算结果与实测结果相吻合;此外,在不同的法向应力和接触摩擦系数下,对碎石料的有效摩擦系数进行了计算和讨论.本文工作验证了组合颗粒单元在非规则颗粒材料的离散元模拟中的可行性.     

非充填岩石节理的动态剪切力学行为实验研究进展

岩石节理的动态剪切力学特性是岩体力学的基本问题之一,在地震工程、采矿工程、隧道工程等诸多领域有广泛应用。获取准确的岩石节理动态剪切力学参数是认识节理剪切力学行为的基础,国内外学者经过持续探索,研发了一系列动态剪切实验设备,形成了以传统定速直剪和循环剪切为主的实验技术体系。近几年发展的冲击直剪实验技术使这一体系更加完善。本文针对非充填岩石节理的动态剪切力学特性室内实验研究,首先系统阐述了相关实验技术的特点及其发展,然后总结了岩石节理动态剪切强度和变形特征的研究成果,最后提出值得进一步探索的问题。     

基于室内试验与数值模拟的膨胀土裂隙对强度影响规律研究

针对室内试验测试多裂隙发育膨胀土抗剪强度指标的困难,提出以室内试验与数值模拟相结合的研究方法;以不同产状单裂隙面膨胀土三轴试样在200kPa围压下的CD试验结果为样本,基于颗粒流原理,采用PFC3D软件进行仿真试验,获得了与三轴试验结果相一致的PFC3D球颗粒参数和裂隙参数;以此参数为基础,进行不同裂隙产状膨胀土的仿真试验,初步探寻了裂隙发育对广西宁明膨胀土的强度影响规律。研究发现:裂隙发育是膨胀土强度衰减的主要原因;裂隙的空间角度、不同裂隙的组合、裂隙发育程度均对膨胀土抗剪强度指标产生影响,且对黏聚力的影响较大;与无裂隙情况相比,裂隙率达14.06%时,C值降低71.2%。     

基于离散元模型的土石混合体直剪试验分析

土石混合体是由高强度块石和低强度土体组成的一类特殊工程地质材料,其力学性质可通过直剪试验进行确定。本文针对土石混合体的细观材料特性,分别采用球形颗粒单元和非规则组合颗粒单元模拟土体和块石材料,对其在不同含石量和颗粒粘结强度下的直剪试验过程进行离散元分析。计算结果表明,土石混合体的抗剪强度随着含石量和粘结强度的增加而增加;通过不同法向应力下直剪试验的离散元分析,确定了不同含石量下土石混合体材料的内摩擦角和粘聚力。本文研究结果有助于进一步揭示土石混合体的抗剪强度特性。     

红砂岩单轴压缩宏细观参数映射关系研究

颗粒流离散元软件PFC2D在岩土工程中的应用十分广泛,存在的主要问题是如何标定其细观参数,目前大多使用"试凑法",此方法的缺点在于工作量大、效率低等,本文提出以反向传播算法(back propagation,BP)神经网络的方式代替此方法,利用PFC2D内置的FISH以及Python语言对其进行二次开发,使之自动运行并获取40组宏细观参数样本。结果表明:BP神经网络可以快速准确地建立宏细观参数映射关系,误差均保持在0.01以内,模拟得到的应力–应变曲线与室内试验曲线高度吻合,且无需大量的数据样本便可创建网络,效率较高;另外,经验证本文选用的平直节理模型,可以有效地解决平行粘结模型UCS/TS值偏小问题,确定平直节理模型可以更好地模拟岩石。     

MACRO AND MICROSCOPIC PARAMETER MAPPING RELATIONSHIP FOR RED SANDSTONE 1)

颗粒流离散元软件PFC2D在岩土工程中的应用十分广泛,存在的主要问题是如何标定其细观参数,目前大多使用"试凑法",此方法的缺点在于工作量大、效率低等,本文提出以反向传播算法(back propagation,BP)神经网络的方式代替此方法,利用PFC2D 内置的FISH 以及Python 语言对其进行二次开发,使之自动运行并获取40组宏细观参数样本。结果表明：BP神经网络可以快速准确地建立宏细观参数映射关系,误差均保持在0.01 以内,模拟得到的应力--应变曲线与室内试验曲线高度吻合,且无需大量的数据样本便可创建网络,效率较高;另外,经验证本文选用的平直节理模型,可以有效地解决平行粘结模型UCS/TS值偏小问题,确定平直节理模型可以更好地模拟岩石。     

基于PFC~(3D)的砂土直剪模拟及宏细观分析

为能更好地分析砂土在剪切过程中的体积变化及力学行为,通过PFC~(3D)颗粒流程序,按照室内实际级配建立数值试样,进行了不同垂直压力下的砂土直剪试验模拟,并将模拟结果与室内试验结果进行了对比;研究了砂土在剪切过程中的体积变化,并用两种方法分析了砂土剪切带的演变过程;从细观角度对试样颗粒的速度场及力链网络的发展变化进行了分析研究。结果表明,数值模拟结果与室内试验结果基本一致,砂土的体积变化表现为先剪缩后剪胀,剪胀量与垂直压力成反比;剪切带厚度约为11～12倍d_(50),在剪切带内颗粒的位移和欧拉角变化较大;试样内部强力链的演变较为明显,力链网络对外荷载变化的敏感性很高;剪切过程中上盒颗粒速度场方向的变化能够较好地解释砂土产生的剪胀现象。     

岩体结构面非线性弹性-塑性软化本构模型研究

在总结评述现有岩体结构面本构模型的基础上,将非线性弹性模型和弹塑性模型结合起来,并采用起伏角磨损演化方程来定量描述结构面的磨损软化,建立了岩体结构面非线性弹性-塑性软化本构模型.利用新建立的模型对岩体结构面直剪试验进行了预测,模型预测结果与试验结果吻合良好,验证了模型的有效性和模拟能力.该模型概念清晰,参数易于确定,能够合理描述岩体结构面的非线性变形、塑性软化、弹塑性耦合、剪胀和磨损等主要力学特性.     

土的工程力学性质的颗粒流模拟

基于颗粒流理论,引入不同的颗粒接触连接本构模型,分别建立了砂土和粘性土的颗粒流模型．通过颗粒流数值模型试验,对砂土和粘性土的室内平面应变试验及其剪切带形成和发展进行了数值模拟,分别对比了不同围压下颗粒流试样与室内试验的应力应变关系曲线,基本再现了砂土和粘性土试样应力．应变关系．通过砂土和粘性土PFC试样剪切带模拟表明,当围压较小时试样内部颗粒位移量小而且分布范围较广,当围压增大时,试样内部颗粒位移量也增大,而且发生较大位移颗粒的分布范围趋于集中,同时随着围压的增大试样内部形成明显的剪切带．无论砂土还是粘性土的PFC试样,随着围压的增加剪切带的形状趋于集中,而且剪切带宽度在减小．在围压很小时,试样内形成大的破坏区域,在围压较大时出现明显的线破坏区．这些规律基本与室内试验结果相似。     

DEM analysis of the effect of joint geometry on the shear behavior of rocks

In order to comprehensively investigate the effect of different joint geometries on the shear behavior of rocks, the Distinct Element Method (DEM) was utilized with a new bond contact model. A series of direct shear tests on coplanar and non-coplanar jointed rocks was simulated using the PFC2D software, which incorporates our bond contact model. Both coplanar jointed rocks with different joint persistence and non-coplanar ones with different joint inclinations were simulated and investigated numerically. The numerical results were compared and discussed with relevant laboratory tests as well as some reported numerical works. The results show that for coplanar jointed rocks, the peak shear stress decreases nonlinearly with the joint persistence, and the failure process can be divided into four stages: elastic shearing phase, crack propagation, failure of rock bridges, and residual phase. For non-coplanar jointed rocks, as the absolute value of the inclination angle of the rock joints increases, its shear strength increases, changing the failure patterns and the length of new fractures between existing cracks. When the absolute value increases from 15° to 30°, the average shear capacity increases the most as 39%, while the shear capacity increases the least as 2.9% when the absolute value changes from 45° to 60°. There is a good consistency of the failure patterns obtained from experiments and numerical tests. All these demonstrate that the DEM can be further applied to rock mechanics and practical rock engineering with confidence in the future.     

Critical state shear behavior of the soil-structure interface determined by discrete element modeling

The interface between soil and structure can be referred to as a soil-structure system, and its behavior plays an important role in many geotechnical engineering practices. In this study, results are presented from a series of monotonic direct shear tests performed on a sand-structure interface under constant normal stiffness using the discrete element method (DEM). Strain localization and dilatancy behavior of the interface is carefully examined at both macroscopic and microscopic scales. The effects of soil initial relative density and normal stress on the interface shear behavior are also investigated. The results show that a shear band progressively develops along the structural surface as shear displacement increases. At large shear displacement a unique relationship between stress ratio and void ratio is reached in the shear band for a certain normal stress, indicating that a critical state exists in the shear band. It is also found that the thickness and void ratio of the shear band at the critical state decreases with increasing normal stress. Comparison of the DEM simulation results with experimental results provides insight into the shear behavior of a sand-structure interface and offers a means for quantitative modeling of such interfaces based on the critical state soil mechanics.     

Scaling granular material with polygonal particles in discrete element modeling

Despite advancements in computational resources, the discrete element method (DEM) still requires considerable computational time to solve detailed problems, especially when it comes to the large-scale models. In addition to the geometry scale of the problem, the particle shape has a dramatic effect on the computational cost of DEM. Therefore, many studies have been performed with simplified spherical particles or clumps. Particle scaling is an approach to increase the particle size to reduce the number of particles in the DEM. Although several particle scaling methods have been introduced, there are still some disagreements regarding their applicability to certain aspects of problems. In this study, the effect of particle scalping on the shear behavior of granular material is explored. Real granular particles were scanned and imported as polygonal particles in the direct shear test. The effect of particle size distribution, particle angularity, and the amount of scalping were investigated. The results show that particle scalping can simulate the correct shear behavior of the model with significant improvement in computational time. Also, the accuracy of the scalping method depends on the particle angularity and particle size range.     

Simulating shear behavior of a sandy soil under different soil conditions

Understanding of soil shear behavior is very important in the field of agricultural machinery and soil dynamics. In this study, a discrete element model was developed using a simulation tool, Particle Flow Code in Three Dimensions (PFC^3D). The model simulates direct shear tests of soil and predicts soil shear behavior, in terms of shear forces and displacements. To determine and calibrate model parameters (stiffness of particles, strength and stiffness of bond between particles), laboratory direct shear tests were conducted to examine effects of soil moisture content and bulk density on shear behaviors of a sandy soil. Three soil moisture levels (0.02%, 13.0%, and 21.5%) and four bulk density levels (0.99, 1.28, 1.36, and 1.50 Mg/m³) were used in the tests. The test results showed that in general drier and denser soil conditions produced higher shear forces. Based on the test results, the bond strengths of the model particles were determined from soil cohesion and internal friction angle. The model particle stiffness was calibrated based on the yield forces from the tests. The calibrated particle stiffness varied from 1.0 × 10³ to 8.2 × 10³ N/m, depending on soil moisture and density levels. The bond stiffness calibrated was 1.0 × 10⁷ Pa/m for all soil conditions.     

建筑结构倒塌破坏的数值模拟方法综述

对建筑结构倒塌破坏的数值模拟方法进行了综述,可归纳为预设损伤与破坏准则、隐式有限元、显式有限元、颗粒离散单元法等4类,分别就其发展历史和实际应用过程中存在的若干缺陷和问题进行探讨。根据颗粒离散元法的基本原理与特点,以某3跨10层的高层框架结构为算例,建立颗粒流模型,基于PFC2D软件编写程序,模拟该结构在突遭水平强击作用下从构件裂纹扩展、断裂、失效、退出工作、结构局部性坍塌一直到整体倒塌的全部动态过程,表明颗粒离散元法可对二维、三维倒塌过程中存在的大量非线性问题进行处理,与其他方法相比较具有较大优势。     

Shear of rubber tube springs

Rubber tube springs consist basically of cylindrical rubber tubes bonded on their inner and outer curved surfaces to rigid cylindrical tubes. They are widely used as flexible linkages, for example in vehicle suspensions. Rotation of one rigid tube with respect to the other about their common axis subjects the rubber tube to azimuthal shear. Displacement of one rigid tube with respect to the other along their common axis puts the rubber tube into axial shear. Using FEA, we have calculated the stresses set up in both cases, for a long rubber tube of a non-linearly elastic (neo-Hookean) material. The results are compared for the two modes of deformation, and with analytical predictions where available. For a long tube the shear stresses are substantially independent of the end conditions, but the normal stresses are strongly affected, as found previously for sheared rectangular blocks [A.N. Gent, J.B. Suh, S.G. Kelly III, Mechanics of rubber shear springs, Int. J. Nonlinear Mech. 42 (2007) 241-249]. If the end surfaces are stress-free, unexpectedly large normal stresses are generated, even in azimuthal shear. These high tensile stresses are attributed to restraints at the inner and outer cylindrical boundaries that compensate for the absence of stresses on the end surfaces that would be needed to maintain a simple shear deformation. Thus, the boundary conditions affect the stresses everywhere (in contrast to an “end effect” that would diminish away from the ends). Small departures from complete incompressibility are found to lower the internal stresses markedly, and even cause the sign of the stresses to be reversed.     

砂土地基锚板基础抗拔承载力PFC数值分析

在模型试验的基础上,采用PFC离散单元法对条形锚板基础在中密砂土地基中的抗拔性能进行了数值分析。数值模拟采用簇颗粒单元来模拟砂粒的不规则形状,颗粒级配根据模型试验福建标准砂的级配按照相似级配法生成,细观参数根据数值双轴试验确定。水平锚板数值模拟结果与模型试验结果基本一致。与模型试验结果相比,颗粒流数值模拟能得到颗粒间接触力链的分布及其演化规律,能从细观角度来探明宏观抗拔承载力特性的演化机理。在此基础上对倾斜锚板上拔过程进行了模拟,分析了锚板前后砂粒的运动趋势以及接触力链的演化规律,并与已有承载力结果进行了对比分析。     

Influence of particle shape on the microstructure evolution and the mechanical properties of granular materials

In order to study the influence of particle shape on the microstructure evolution and the mechanical properties of granular materials, a two-dimensional DEM analysis of samples with three particle shapes, including circular particles, triangular particles, and elongated particles, is proposed here to simulate the direct shear tests of coarse-grained soils. For the numerical test results, analyses are conducted in terms of particle rotations, fabric evolution, and average path length evolution. A modified Rowe's stress–dilatancy equation is also proposed and successfully fitted onto simulation data.