密实散粒体剪切破坏能量演化的离散元模拟

砂土等散粒体在剪切过程中的能量存储及耗散是其宏观力学响应的深层原因,但因量测难度较大而研究较少。将考虑抗转动的接触模型引入离散元软件PFC2D,基于热力学第一定律建立各种能量量测方法,并在平面应变双轴压缩试验中采用该方法统计密实散粒体在剪切过程中的能量演化规律。采取了4种耗散类型,即滑动-滚动（S-R）、滑动-非滚动（S-NR）、非滑动-滚动（NS-R）和非滑动-非滚动（NS-NR）。结果表明：密实散粒体加载时能量耗散以滑动摩擦为主;且小应变加载阶段,外力功主要转化为弹性应变能,但同时也存在均布于试样的耗散能;随着应变的增加,外力功的转化形式逐渐过渡为以耗散能为主,且集中分布在带状区域内;各个加载阶段的摩擦耗散均存在各向异性。     

基于离散元法的砂土破碎演化规律研究

颗粒破碎是影响砂土宏-微观力学性质的重要因素。采用改进型的可破碎颗粒生成方法,通过设置不同强度的平行黏结键模拟不同强度的可破碎颗粒,并借用基于离散元方法（DEM）的双轴压缩试验详细研究了可破碎性土在剪切过程中颗粒破碎率/平均破碎程度、微观尺度上的能量耗散分配机制、剪切破碎带形成以及断裂键各向异性的演化过程。结果表明,颗粒破碎强烈地影响砂土在宏观尺度上的力学响应、颗粒尺度上的能量分配机制以及剪切过程中的颗粒的组织结构演化。颗粒破碎主要影响小应变阶段各能量耗散元的分配机制,而在临界状态下剪切带内的颗粒摩擦以及破碎耗能是消耗外界功的主要因素。数值结果亦表明,颗粒的破碎伴随着整个剪切过程,但破碎率的增长速度却随着剪切应变的发展逐渐降低。另外,在剪切过程中,对于低破碎性土,在临界状态下剪切破碎带基本形成,带内的原有组织结构被打乱,断裂键的各向异性也随之弱化。     

Microscale Energy Dissipation Mechanisms in Cyclically-Loaded Granular Soils

This paper utilizes the Discrete Element Method to characterize energy dissipation mechanisms in cyclically loaded soils based on micromechanical considerations. Computational simulations of consolidated undrained cyclic triaxial tests were conducted at various relative densities and were subjected to cyclic loading of different frequencies and shear strain amplitudes. The different components of microscale energies were monitored during the course of the simulations and characterized into input and dissipated energies. A comparison is made between the dissipated energy computed from microscopic energy components and macroscopic energy calculated based on the area of the deviator stress-axial strain loops. These energies are then used to obtain the specific damping capacity defined as the ratio of dissipated energy during one cycle to the maximum stored elastic energy during the same cycle. The conducted simulations highlight the importance of calculating actual stored energy in the system as opposed to approximating it to be that calculated as the triangular area under the secant modulus. Finally, a series of simulations that resulted in liquefaction are discussed, and the amount of energy dissipated to liquefaction is examined based on these results.     

颗粒大小对颗粒材料力学行为影响初探

利用一种特殊颗粒材料-玻璃珠进行了一系列室内直剪试验,研究颗粒大小对颗粒材料力学行为的影响。试验一共考虑了3条近乎平行的级配曲线和4种颗粒摩擦情况：干燥状态、水浸润状态、水淹没状态和油浸润状态。试验结果表明,颗粒大小对颗粒材料的力学行为有显著影响,剪胀性随着粒径的增大而增强。为考虑颗粒大小对剪胀性的影响,提出了一种新的剪胀关系式。在该剪胀关系式中,剪胀系数为依赖于颗粒大小和颗粒摩擦等颗粒基本性质的变量。试验研究同时表明临界状态摩擦角随着颗粒大小的增加而增加。此外,从颗粒细观运动的角度提出了颗粒滑动的功能模型,推导出了功能方程,并以此揭示了颗粒大小对临界状态摩擦角影响的细观机制。     

Simple Shear Behavior of Calcareous and Quartz Sands

The monotonic and cyclic simple shear behavior of several loose skeletal calcareous sands is investigated and compared to that of common Nevada and Ottawa quartz sands in order to draw contrasts between these types of materials and to develop stiffness and modulus degradation curves for quasi-nonlinear ground response analysis. Mobilized frictional resistance and cyclic strength are generally higher for the calcareous sands, whereas shear modulus and damping are larger for quartz sand at strain levels between 0.05 and 1%. Degradation properties do not appear to be very sensitive to the initial cyclic stress ratio. Differences in behavior between the calcareous and quartz sands are presumably due to contrasts in grain geometry, hardness, gradation and the amount of intraparticle voids.     

Experimental study on dynamic characteristics of granular materials under axial high-frequency vibration

The fundamental understanding of the behavior of granular materials by the effect of vibration is necessary to properly address a number of engineering issues, such as long-term settlement of high-speed railway, vibratory pile driving in sandy stratum, and earthquake-induced geotechnical disaster. Triaxial compression tests of dry Pingtan sand were carried out by a modified triaxial apparatus, where axial high-frequency vibration was super-imposed on the specimen at pre-peak, peak, and post-peak stress states during monotonic shearing. The influences of vibration conditions, confining pressure, and the initial relative density on the vibration-induced responses of Pingtan sand are mainly considered. It is shown that the super-imposed vibration leads to significant deviatoric stress reduction and vibro-induced additional axial strain. This owes to the fact that the static inter-particle friction turns to dynamic friction, and consequently, the frictional resistance has a considerable reduction when vibration is applied to the sand specimen. The vibration-induced stress–strain behavior of sand specimen is characterized into three states by two thresholds concerning vibration intensity and confining pressure: (1) stable state, (2) vibro-compression state and (3) vibro-instability state. For the vibro-compression state, the deviatoric stress reduction has a positive linear correlation with the increase in vibration intensity, while the vibro-induced additional axial strain follows a power-law increase with vibration intensity. Given a vibration condition, the deviatoric stress reductions and the vibro-induced additional axial strains at pre-peak, peak, and post-peak stress state follow a descending order. Besides, the influences of vibration on shear strength and critical state were also discussed.

     

法向刚度对土与结构接触面三维特性影响的试验研究

运用80 t大型三维接触面试验机,对不同法向刚度下粗粒土与结构接触面三维静动力学特性进行了试验研究,重点分析了法向刚度对接触面力学特性的影响规律。法向常刚度条件下,接触面在单调剪切时均先剪缩再剪胀、法向应力先减小后增大;循环剪切时接触面不可逆性剪切体变呈单调增长、可逆性剪切体变峰值逐渐减小后趋于稳定,切向应力峰值不断减小,抗剪强度逐渐减小,主应力比峰值则基本保持不变。法向刚度主要影响剪切体变、可逆性剪切体变、切向应力、主剪应力等接触面力学性能参数的大小;法向刚度越大,单调剪切时法向应力变化越大、切向应力峰值越大、剪缩和剪胀量越小;循环剪切时不可逆性剪切体变增长越慢、最终值越小,抗剪强度减小越快、达到0时对应的循环周次越少。法向刚度对剪切体变、可逆性剪切体变、主剪应力、主应力比等性能参数与切向位移的关系形式及接触面摩擦角基本没有影响。     

A simplified analysis of interface failure under compressive normal stress and monotonic or cyclic shear loading

Interface damage and delamination is usually accompanied by frictional slip at contacting interfaces under compressive normal stress. The present work is concerned with an analysis of progressive interface failure using the cohesive crack model with the critical stress softening and frictional traction present at the contact. Both monotonic and cyclic loadings are considered for anti‐plane shear of an elastic plate bonded to a rigid substrate by means of cohesive interface. An analytical solution can be obtained by neglecting the effect of minor shear stress component. The analysis of progressive delamination process revealed three solution types, namely: short, medium and long plate solutions. The long plate solution was obtained under an assumption of quasistatic progressive growth of the delamination zone. In view of snap back response, the quasistatic deformation process cannot be executed by either traction or displacement control. The states of frictional slip accompanied by shake down or incremental failure are distinguished in the case of cyclic loading, related to load amplitude and structural dimensions. The analysis provides a reference solution for numerical treatment of more complex cases. Copyright © 2005 John Wiley & Sons, Ltd.     

基于抗转模型的颗粒材料宏−细观关系研究

接触模型的宏?细观参数标定是成功使用离散元方法的关键。在离散元的接触模型中线性接触模型与抗转线性接触模型均可用于模拟砂性土的力学行为,其中抗转线性接触模型在模拟密砂的剪胀性方面具备优势。采用抗转线性接触模型对室内密实砂土三轴试验进行了离散元模拟,验证了抗转线性接触模型的可靠性;进而系统分析了颗粒间摩擦系数、刚度比和抗转动系数等细观参数与砂土峰值内摩擦角、残余内摩擦角、峰值剪胀角等宏观参数的相关关系并进行了验证;揭示了偏应力作用下,细观参数对密实砂土试样内部剪切带宽度与倾角变化的影响规律,提出了考虑剪胀角的剪切带倾角经验公式。通过研究建立了抗转线性接触模型宏?细观参数的量化关系并给出了标定参数的具体流程图,提出了快速标定宏观参数的方法并应用实例进行了验证,为采用抗转线性接触模型精准模拟密实砂土的力学特性提供依据。     

不同级配颗粒材料的循环硬化行为与能量耗散

颗粒材料在循环荷载作用下呈现循环硬化行为,然而关于循环硬化行为的宏-微观相对应的机理解释的研究却不多见。本文通过3D-DEM实施了循环三轴数值模拟试验,研究了不同级配下颗粒材料在循环荷载下的循环硬化行为,并分析了(轴向)平均模量和由滑动接触造成的能量耗散的演化趋势。结果表明随着循环荷载的进行,平均模量逐渐增加并趋于渐进值。每个循环内的能量耗散随着循环数的增加不会一直衰减,存在一个阈值。不同颗粒级配下平均模量的增加指数和能量耗散衰减指数的大小相近(约为0.6),表明在描述循环荷载硬化行为时,无论是使用变形模量还是归一化能量耗散在结果上是一致的,即微观尺度上滑动接触引起的能量耗散映射出宏观尺度上平均模量的变化。颗粒平均粒径越大的颗粒体系,其平均模量越大,而且越早完成能量耗散过程,但不同的颗粒级配下对应的能量耗散阈值差异较小,基本上维持在一个常数(约为0.15)。     

高应力下颗粒材料一维力学特性研究(I):压缩性质

高水平应力作用下,砂土等颗粒材料中的颗粒将发生破碎。一方面,颗粒破碎导致材料的颗粒分布曲线发生变化：材料中的粗颗粒含量减少,细颗粒含量增加;另一方面,颗粒的破碎引起了能量的转化。由能量守恒定律,作用过程中外力所做的功一部分由粒间摩擦力转化成热能,而另一部分则消耗到颗粒破碎过程中。利用表面物理学理论,颗粒破碎能可以表达为颗粒表面张力在颗粒破碎中所作的功。由此得到了一维压缩条件下颗粒破碎量与宏观压缩量之间的关系表达式。为了验证得到的关系式,开展了砂土的一维压缩试验,并进行了试验数据的整理分析。研究结果表明,所得关系表达式能较好地反映高水平应力作用下颗粒破碎对颗粒材料压缩性的影响。     

Constitutive description of interface behavior including cyclic loading and particle breakage within the framework of critical state soil mechanics

The cyclic behavior of soil–structure interface can be very important in dynamic problems. The cyclic behavior of soil–structure interface may be nonlinear, which includes hysteresis, hardening, degradation, and particle breakage. The breakage of granular soil particles during shearing of granular soil–structure interface is associated with cyclic degradation and can be critical to the dynamic behavior of soil–structure system. The critical state soil mechanics concept formerly used to simulate the nonlinear monotonic behavior of granular soil–structure interface was modified and extended to describe the cyclic behavior, especially soil‐particle breakage and degradation during cyclic shearing. Soil‐particle breakage was assumed to relate to the energy consumption during cyclic shearing and the critical state line of the soil–structure interface was assumed to translate with the consumption of shearing energy during cyclic shearing as the threshold value is attained. The model was formulated in the framework of generalized plasticity and is capable of describing the salient features of granular soil–structure interface under cyclic loading. Most of the model parameters have straightforward physical meanings and are calibrated using monotonic or cyclic interface test results. The proposed model was calibrated and validated against published test results. The dependency of interface behavior on stress path and cyclic degradation can be successfully described by the proposed model. Copyright © 2008 John Wiley & Sons, Ltd.     

Energy equation and stress–dilatancy relationship for sand

The energy equation is an expression of the first law of thermodynamics or the law of conservation of energy. According to the first law of thermodynamics, the externally applied work to a system is equal to the sum of dissipation energy and Helmholtz free energy of the system. However, most of the currently available stress–dilatancy relationships are based on the energy equation of Taylor-Cam Clay type, which hypothesizes that the applied plastic work is equal solely to the frictional dissipation energy. The Helmholtz free energy has been completely neglected. Recently, observed from acoustic experiments, it has been recognized that Helmholtz free energy can be caused by deformation mechanisms other than friction between particles. Thus, it is necessary to include additional terms in the energy equation in order to correctly model the stress-dilatancy behavior. This paper addresses the issue regarding the balance of this energy equation. Analyses of experimental results are presented. Specific forms of the frictional energy and Helmholtz free energy are proposed. The proposed energy equation is verified with the experimental data obtained from Silica sand, Ottawa sand, and Nevada sand.

     

Micromechanical investigation of the particle size effect on the shear strength of uncrushable granular materials

Particle size strongly influences the shear strength of granular materials. However, previous studies of the particle size effect have focused mainly on the macroscopic behavior of granular materials, neglecting the associated micro-mechanism. In this study, the effect of particle size on the shear strength of uncrushable granular materials in biaxial testing is investigated using the discrete element method (DEM). First, a comprehensive calibration against experimental results is conducted to obtain the DEM parameters for two types of quartz sand. Then, a series of biaxial tests are simulated on sands with parallel particle size distributions to investigate the effect of particle size on macro- and microscopic behaviors. Finally, by adopting the rolling resistance method and the clump method, irregular-shaped particles are simulated to investigate how the particle size effect will be influenced by the particle shape. Simulation results demonstrate that (1) the peak shear strength increases with particle size, whereas the residual shear strength is independent of particle size; (2) the thickness of the shear band increases with the particle size, but its ratio decreases with particle size; (3) the particle size effect can be explained by the increase of friction utilization ratio with particle size; and (4) the particle size effect is more significant in granular materials that consist of particles with higher angularity.

     

Induced fabric under cyclic and rotational loads in a strain space multiple mechanism model for granular materials

The strain space multiple mechanism model idealizes the behavior of granular materials on the basis of a multitude of virtual simple shear mechanisms oriented in arbitrary directions. Within this modeling framework, the virtual simple shear stress is defined as a quantity dependent on the contact distribution function as well as the normal and tangential components of interparticle contact forces, which evolve independently during the loading process. In other terms, the virtual simple shear stress is an intermediate quantity in the upscaling process from the microscopic level (characterized by contact distribution and interparticle contact forces) to the macroscopic stress. The stress space fabric produces macroscopic stress through the tensorial average. Thus, the stress space fabric characterizes the fundamental and higher modes of anisotropy induced in granular materials. Herein, the induced fabric is associated with monotonic and cyclic loadings, loading with the rotation of the principal stress, and general loading. Upon loading with the rotation of the principal stress axis, some of the virtual simple shear mechanisms undergo loading whereas others undergo unloading. This process of fabric evolution is the primary cause of noncoaxiality between the axes of principal stresses and strains. Although cyclic behavior and behavior under the rotation of the principal stress axis seem to originate from two distinct mechanisms, the strain space multiple mechanism model demonstrates that these behaviors are closely related through the hysteretic damping factor. Copyright © 2011 John Wiley & Sons, Ltd.     

分级循环荷载下岩石动力学行为试验研究

利用WDT-1500多功能材料试验机,对砂岩、砾岩和砂砾岩进行了分级循环荷载试验,研究了岩石的动弹性模量对应力幅值和应力水平的响应特性,得到了动弹性模量和耗散能随应力幅值、应力水平及含水率的变化规律。试验结果表明,分级循环荷载下岩石的能量耗散越多,动弹性模量越小;应力水平越高,动弹性模量和耗散能越大;含水率和应力幅值越大,动弹性模量越小,耗散能越大。讨论了邓肯-张模型能够描述分级循环荷载作用下岩石的应力-应变关系,构建了动弹性模量随应力水平、应力幅值及含水率变化的演化模型,探讨了模型参数的确定方法。根据能量耗散的经验法则,建立了耗散能演化模型,结果表明,该模型能够描述分级循环荷载过程中能量耗散行为。     

不同法向边界条件接触面三维力学特性的试验研究

运用最新研制的80 t三维多功能土工试验机对粗粒土与结构接触面在3种典型法向边界条件下的三维力学特性进行了试验研究。结果表明,接触面法向边界条件对其力学特性有重要影响,不同法向边界条件下接触面表现出了一些相似和不同的力学响应：(1)接触面总体上剪缩或有剪缩趋势,但数值不同;(2)除常应力条件外,接触面切向应力-切向位移关系曲线同一循环内一般不闭合,不同循环也不重合;(3)3种法向边界条件下接触面应力比-切向位移关系曲线同一循环内基本闭合,不同循环也基本重合,且形状较为相似,均呈椭圆形;(4)接触面静、动抗剪强度指标随法向边界条件变化不大,但动抗剪强度则差别很大。     

土工膜-土工布界面动力剪切特性试验研究

糙面土工膜（GMX）和无纺土工布（GT）是垃圾填埋场中周边衬垫系统的重要组成部分,其界面特性对于整体填埋场的稳定性尤为重要。但是,目前对GMX-GT界面的动力剪切特性研究较少。为此,利用大型直剪仪,开展了一系列全饱和条件及干燥条件下GMX-GT界面的循环剪切试验,主要研究了竖向应力、位移幅值和循环次数等对GMX-GT界面动力剪切特性的影响,并对比分析了全饱和及干燥两种条件下GMX-GT界面动力剪切特性的差异。研究结果表明,随着位移幅值的增加,GMX-GT界面呈现出由剪切硬化向剪切软化转变的特性。循环剪切作用使得界面的内摩擦角随着位移幅值的增加而增大。GMX-GT界面主要表现为剪缩特性,且总剪缩量随竖向应力、位移幅值和循环次数的增加而增加。剪切刚度随竖向应力和循环次数的增大而增大,随位移幅值的增大而减小。阻尼比随位移幅值的增大而增大,随循环次数的增加而减小,说明位移幅值会增加GMX-GT界面的能量的耗散。GMX-GT界面在干燥条件下的破坏模式与饱和条件下的存在明显差异,干燥条件下GT内部的破坏更加显著,全饱和条件下GMX表面的破坏更加明显。     

土工袋减振隔振机制分析及试验研究

设置于地基中的土工袋不仅可以提高地基承载力,而且具有减振隔振效果。通过水平循环剪切试验研究了土工袋的动力特性,验证了土工袋具有可变的水平刚度和较大的阻尼比,表明土工袋是一种良好的基础减振隔振材料。土工袋的等效阻尼比随着竖向压力的增大而减小,随着最大剪应变的增大而增大。同时,采用离散单元法进行了土工袋在动力荷载作用下的数值模拟计算,土颗粒间采用弹簧-阻尼器接触模型;而土工袋被看成具有张力的小颗粒薄层,这些小颗粒间的接触与袋内土颗粒间的接触模型相同,但仅设置法向的接触而无切向接触,且只受拉不受压。结果表明,土工袋的减振消能效果主要来自于袋内材料的摩擦耗能、黏滞耗能以及袋子张力引起的耗能。另外,还进行了土工袋沟槽的原位振动测试,验证了土工袋的减振隔振效果。     

Numerical evaluation of three non‐coaxial kinematic models using the distinct element method for elliptical granular materials

This paper presents a numerical evaluation of three non‐coaxial kinematic models by performing Distinct Element Method (DEM) simple shear tests on specimens composed of elliptical particles with different aspect ratios of 1.4 and 1.7. The models evaluated are the double‐shearing model, the double‐sliding free‐rotating model and the double slip and rotation rate model (DSR² model). Two modes of monotonic and cyclic simple shear tests were simulated to evaluate the role played by the inherent anisotropy of the specimens. The main findings are supported by all the DEM simple shear tests, irrespective of particle shape, specimen density or shear mode. The evaluation demonstrates that the assumption in the double‐shearing model is inconsistent with the DEM results and that the energy dissipation requirements in the double‐sliding free‐rotating model appear to be too restrictive to describe the kinematic flow of elliptical particle systems. In contrast, the predictions made by the DSR² model agree reasonably well with the DEM data, which demonstrates that the DSR² model can effectively predict the non‐coaxial kinematic behavior of elliptical particle systems. Copyright © 2016 John Wiley & Sons, Ltd.