反映粗粒料应力路径相关性的一种应变硬化模型

粗粒料已成为山区高填方工程的主要填筑材料,建立能较全面反映其力学特性的本构模型,对高填方工程的模拟分析及安全评价具有重要意义。在已有大型三轴试验研究的基础上,通过将硬化土模型与修正剑桥模型相结合同时引入对数形式的非线性强度参数关系式,建立了一种简便实用的弹塑性本构模型。该模型不仅能考虑应力水平对粗粒料应力-应变关系、体变特性、强度特性的影响,还能较好地反映粗粒料的应力路径相关性,模拟复杂应力状态下的粗粒料的变形特性,并且相关参数通过常规三轴试验即可测定。最后通过与不同应力路径试验结果进行对比,证明了模型的合理性。     

不同应力路径下石灰岩碎石力学特性的大型三轴试验研究

为了解不同应力路径下,昆明新机场石灰岩碎石填料的力学特性及该填料力学特性受水影响的程度,采用大型三轴试验机,对该填料进行不同应力路径下的三轴试验及等σ′₃压缩应力路径下的干湿对比试验。结果表明:Δσ′₁≥0 时,围压和应力比k (即小主应力增量与大主应力增量的比值)增大,试件峰值应力及初始模量亦增大,但剪胀逐渐受到抑制;石灰岩碎石的剪胀及剪缩特性与应力路径有很强的相关性;石灰岩碎石的强度非线性特征显著,围压减小的应力路径下,试件剪切强度比等σ′₃应力路径下略低;水对石灰岩碎石的剪切强度、初始模量及剪胀性有较大影响。该文的试验成果可为构建考虑应力路径影响的高填方粗颗粒填料本构模型提供试验基础。     

多围压下三轴压缩试验与不可破裂颗粒离散元法分析

采用大型三轴试验仪器对道砟散体材料进行试验,重点研究不同围压条件下三轴试验力学特性与影响规律。同时基于离散单元法(DEM),采用非破裂不规则颗粒簇,在伺服条件下建立道砟三轴试验散粒体模型,对道砟颗粒在不同围压下进行相关数值三轴模拟,探讨偏应力、体应变、围压、体积应变与轴应变之间的关系,反映道砟基本力学特性。研究结果表明:采用离散单元方法进行数值模拟与室内三轴试验得到的一系列应力-应变规律基本一致,验证了离散单元法在道砟研究中的可行性与可靠性。同时,围压对道砟散体变形、最大偏应力、偏应力比值具有影响,其中随着围压提高道砟变形降低,因此在有砟道床设计和养护维修中,增大道床围压可降低道床沉降与变形。     

堆石料真三轴条件下力学特性试验研究进展

该文综述了国内外岩土真三轴仪的发展、堆石料力学特性及机理相关的研究现状。重点介绍了该研究团队开发的大型岩土静动真三轴试验机及在堆石料力学特性试验研究方面取得的初步成果。按加载方式,土的真三轴仪可以分为3类:刚性加载真三轴仪、柔性加载真三轴仪和混合型加载真三轴仪。堆石料真三轴试验具有试样尺寸大、承压高、各方向相互干扰强、试样变形后荷载对中难、加压板与试样间摩擦效应强、试样安装和量测复杂等诸多困难。因而,目前适用于堆石料的真三轴仪和试验成果均较少。考虑堆石料真三轴及复杂应力路径条件下的颗粒破碎、各向异性等,研究其力学特性并开发相应的本构模型是该课题发展的趋势。清华大学大型岩土静动真三轴试验机中联合采用了椭圆形试样帽和异形乳胶膜的封样方式,可很好地解决真三轴试样的高压密封、拆装、量测和耐久性等一系列难题。对堆石料进行了一系列真三轴复杂应力路径试验,结果表明真三轴试验中堆石料表现出更明显的应力诱导各向异性和应力状态依赖性,特别是对小主应力-应变关系、球应力-体应变关系和广义剪应力-剪应变关系的影响更显著。球应力和广义剪应力对体积应变和广义剪应变之间存在着明显的交叉影响。     

正交钢丝环链网片顶压力学行为薄膜等效方法

正交钢丝环链网片广泛应用于落石防护工程,具有复杂的非线性力学行为,针对目前常用的离散接触态环梁单元模型计算效率低的问题,提出了一种薄膜等效模拟方法。开展了网片加载试验,研究了试件的P-Δ特性,分析并揭示了网片的正交化受力特征及正交拉力带的形成机制。基于最短传力路径原理构建了薄膜面外加载的正交拉力带解析模型,推导了等效薄膜的应力-应变关系,建立了薄膜顶压有限元模型。研究表明:采用指数强化弹塑性本构的薄膜能够在数值模拟中准确反演网片面外加载的宏观力学行为,模拟得到的顶破力、顶破位移和耗能能力与试验相比误差均小于10%。同时,采用该文方法建立的冲击力学模型求解得到的冲击响应与既有文献的试验结果吻合较好,加速度峰值误差4.3%。最后基于冲击模型,对比了等效模型与环梁模型的计算效率,等效方法的计算效率可提高10倍以上。     

混凝土循环加卸载动态损伤特性研究

利用大型多功能动静力三轴仪对混凝土试件进行了5种应变速率下的动态循环加卸载压缩试验。对混凝土的物理力学参数的变化规律进行了统计分析。结果表明:峰值应力和弹性模量随加载速率的提高而增大,但峰值应变随加载速率的变化表现出较大的离散性。在此基础上,进一步研究了混凝土在不同加载速率下的刚度退化规律。最后,选用基于Weibull统计理论的分段式动态损伤本构模型对试验数据进行拟合。经验证,此模型能够较好的模拟混凝土材料的本构特性。     

基于颗粒流离散元模型的弹丸侵彻细观混凝土数值模拟方法研究

利用颗粒离散单元法,研究弹丸侵彻细观混凝土模型中弹丸受到介质的阻应力与侵彻速度的关系。采用蒙特卡罗法随机生成并投放混凝土骨料且骨料的粒径分布满足级配曲线。通过对混凝土颗粒离散元细观力学模型进行单轴压缩实验、巴西劈裂实验和双轴压缩实验的参数反演,确定细观模型参数,能使细观混凝土模型具有和一般混凝土等效的力学性能。分析了骨料、过渡层和砂浆三相材料各细观参数对混凝土单轴压缩应力应变关系影响以及锥形弹和平头弹弹丸直径对侵彻阻应力的影响。将颗粒离散元细观力学模型方法计算的弹丸阻应力与空腔膨胀理论计算模型相比较,表明计算离散元方法具有良好的精度和实用性。     

基于离散元法的脆性岩石细观蠕变失稳研究

为从细观角度探究脆性岩石的蠕变失稳过程及失稳机理,该文基于三维颗粒流程序（PFC3D）考虑岩石的时效变形损伤过程,引入岩石细观单元时效损伤的应力腐蚀模型,建立了基于离散元方法的岩石时效变形损伤破裂模型,并通过单轴压缩及单轴蠕变的室内实验和数值模拟对比验证了所建立的时效变形损伤破裂模型的合理性。数值模拟再现了岩石的初始蠕变、稳态蠕变和加速蠕变三个蠕变阶段,同时模拟结果表明,在单级加载条件下,随着应力水平提高,稳态蠕变应变率显著增大,岩石蠕变失效时间逐渐缩短,初始轴向应变、初始侧向应变和初始体应变不断增大,且细观裂纹扩展形式与单轴压缩破坏形式基本相同,都是以拉伸裂纹为主,裂纹的增长速率随着时间增加而不断增大,尤其在第三蠕变阶段裂纹增长速率迅速增大;在分级加载试验过程中,模型的轴向应变、侧向应变和体应变以及裂纹最终扩展形态与单级加载基本相同;此外将三维蠕变模拟结果与二维模拟结果进行对比,结果显示三维模型拟合程度更高。     

基于离散元法的脆性岩石细观蠕变失稳研究EI北大核心CSCD

为从细观角度探究脆性岩石的蠕变失稳过程及失稳机理,该文基于三维颗粒流程序(PFC3D)考虑岩石的时效变形损伤过程,引入岩石细观单元时效损伤的应力腐蚀模型,建立了基于离散元方法的岩石时效变形损伤破裂模型,并通过单轴压缩及单轴蠕变的室内实验和数值模拟对比验证了所建立的时效变形损伤破裂模型的合理性。数值模拟再现了岩石的初始蠕变、稳态蠕变和加速蠕变三个蠕变阶段,同时模拟结果表明,在单级加载条件下,随着应力水平提高,稳态蠕变应变率显著增大,岩石蠕变失效时间逐渐缩短,初始轴向应变、初始侧向应变和初始体应变不断增大,且细观裂纹扩展形式与单轴压缩破坏形式基本相同,都是以拉伸裂纹为主,裂纹的增长速率随着时间增加而不断增大,尤其在第三蠕变阶段裂纹增长速率迅速增大;在分级加载试验过程中,模型的轴向应变、侧向应变和体应变以及裂纹最终扩展形态与单级加载基本相同;此外将三维蠕变模拟结果与二维模拟结果进行对比,结果显示三维模型拟合程度更高。     

2D-C/SiC复合材料偏轴拉伸力学行为研究

通过对2D-C/SiC复合材料试件进行不同偏轴角度的拉伸实验,研究了偏轴角度对材料拉伸力学特性的影响。通过应变片分别测得了材料加载方向和纤维束方向上的应力-应变行为,对比分析了偏轴角度对上述应力-应变行为的影响;并结合试件断口扫描电镜照片,阐释了纤维束方向上拉伸和剪切损伤间的相互耦合效应。实验结果表明,材料的拉伸模量和强度随偏轴角度的增大出现明显下降;材料纤维束方向上的拉伸损伤和剪切损伤具有显著的相互促进作用。最后,以材料0°拉伸和45°拉伸实验数据为基础,建立了材料的偏轴拉伸应力-应变行为预测模型,模型预测结果与实验结果吻合较好。     

A hybrid approach for modeling of breakable granular materials using combined finite-discrete element method

It is well known that particle breakage plays a critical role in the mechanical behavior of granular materials and has been a topic subject to intensive studies. This paper presents a three dimensional fracture model in the context of combined finite-discrete element method (FDEM) to simulate the breakage of irregular shaped granular materials, e.g., sands, gravels, and rockfills. In this method, each particle is discretized into a finite element mesh. The potential fracture paths are represented by pre-inserted non-thickness cohesive interface elements with a progressive damage model. The Mohr–Coulomb model with tension cut-off is employed as the damage initiation criterion to rupture the predominant failure mode at the particle scale. The particle breakage modeling using combined FDEM is validated by the qualitative agreement between the results of simulated single particle crushing tests and those obtained from laboratory tests and prior DEM simulations. A comprehensive numerical triaxial tests are carried out on both the unbreakable and breakable particle assemblies with varied confining pressure and particle crushability. The simulated stress–strain–dilation responses of breakable granular assembly are qualitatively in good agreement with the experimental observations. The effects of particle breakage on the compressibility, shear strength, volumetric response of the fairly dense breakable granular assembly are thoroughly investigated through a variety of mechanism demonstrations and micromechanical analysis. This paper also reports the energy input and dissipation behavior and its relation to the mechanical response.     

Micromechanical behaviour of Si-based particulate assemblies: A comparative study using DEM and atomistic simulations

In this paper, we investigate the micromechanical behaviour of Si-based particulate systems subjected to tri-axial compression loading. The investigations are based on three-dimensional discrete element modelling (DEM) and simulations. At first, we compare the variation of mean compressive stress for a silicon assembly subjected to tri-axial compression, predicted at two different scales: at the particulate scale, using the DEM simulation (mono-dispersed particulates) and at the atomistic scale using the molecular dynamics (MD) simulation results for silicon mono-crystal reported by Mylvaganam and Zhang (2003) [K. Mylvaganam, L. Zhang, Key Eng. Mater. 233–236 (2003) 615–620]. Both the simulation methods considered the silicon assembly subjected to an identical (tri-axial) loading condition. We observed a good qualitative agreement between the DEM and MD simulation results for the mean compressive stress when the assembly was subjected to small volumetric strain. However, at large volumetric strain, the mean stress of the silicon assembly predicted from MD simulation did not scale-up with the DEM results. This discrepancy could be due to that MD simulation is only valid for particle contacts, which are independent of one another and does not consider the inherent ‘discrete’ nature of particulates and the induced anisotropy prevailing at particulate scale. The micromechanical behaviour of particulate assemblies strongly depends on the inherent discrete nature of the particles, their single-particle properties and the induced anisotropy during mechanical loading. At the second stage, using DEM, we present the evolution of macroscopic compressive stress and several micromechanical features for four cases of the commonly used Si based poly-dispersed particulate assemblies (Si, SiC,Si₃N₄ and SiO₂) under tri-axial compression loading. We also present the evolution of several other phenomena occurring at particulate scale, such as the energy dissipation characteristics due to sliding contacts and the features of fabric structures developed during mechanical loading. The study shows that the single-particle properties of the Si based assemblies considered here significantly affect the micromechanical behaviour of the assemblies and DEM is a powerful tool to get insights on the internal behaviour of discrete particulates under mechanical loading.     

A calibration framework for the microparameters of the DEM model using the improved PSO algorithm

The discrete element method (DEM) is commonly used for simulating the mechanical characteristics of rock materials; however, constructing a DEM model requires the specification of a number of microparameters. In this paper, to obtain the microparameters of the DEM model, the improved particle swarm optimization (PSO) calibration method was presented. Based on numerical simulation examples, the new approach is considered valid for calibrating the microparameters of the DEM model. Moreover, it is concluded that different sets of microparameters can be determined when few macroparameters are used, which indicates that the empirical formula between microparameters and macroparameters is not reliable. From the analysis of the numerical simulation results, it is suggested that more macroparameters should be used to calibrate the microparameters of the DEM model, and the corresponding numerical simulation results could be more reliable; otherwise, the generated numerical model may not accurately simulate the mechanical characteristics of rock materials.     

团块缺陷结构对岩石静动力学特性影响的数值模拟分析

基于颗粒流离散元法和PFC2D程序构建含团块缺陷结构的岩石数值模型试样,指定试样中部1 cm半径范围为团块缺陷,通过0.1倍、1倍和10倍关系改变团块细观力学参数,构建A、B和C三种数值试样类别,分别进行三轴压缩和三轴疲劳试验。通过试样破坏特征、强度特征、团块及周边应力、应变率特征分析缺陷岩石静、动力学特性。试验结果表明,压缩和疲劳荷载下A类试样在团块左右方向应力集中,而上下方向（包括团块内部）为低应力区;C类试样与之相反,B类试样无明显应力集中现象。无论压缩或疲劳荷载,A类试样团块应变率高于四周,C类试样与之相反,B类试样无明显应变率差异化。研究成果揭示:1）岩石内部团块状缺陷结构无论其材料强度高低均劣化岩石抗压强度;2）静动荷载加载过程中含团块缺陷岩石内部表现出应力场不均匀性和应变差异性。     

Micromechanical modelling of oval particulates subjected to bi-axial compression

One of the questions that still remain unanswered among researchers dealing with granular materials is how far the particle shape affects the micro-macroscopic features of granular assemblies under mechanical loading. The latest advances made with particle instrumentation allow us to capture realistic particle shapes and size distribution of powders to a fair degree of accuracy at different length scales. Industrial applications often require information on the micromechanical behaviour of granular assemblies having different particle shapes and varying surface characteristics, which still remains largely unanswered. Traditionally, simulations based on discrete element method (DEM) idealise the shape of individual particles as either circular or spherical. In the present investigation, we analyse the influence of particle shape on the shear deformation characteristics of two dimensional granular assemblies using DEM. We prepared the assemblies having nearly an identical initial packing fraction (dense), but with different basic shapes of the individual particles: (a) oval and (b) circular for comparison purposes. The granular assemblies were subjected to bi-axial compression test. We present the evolution of macroscopic strength parameters and microscopic structural/topological parameters during mechanical loading. We show that the micromechanical properties of granular systems are significantly influenced by the shape of the individual particles constituting the granular assemblies.     

Validation of microparameters in discrete element modeling of sea ice failure process

A GPU-based discrete element method (DEM) with bonded particles is investigated to simulate the mechanical properties of sea ice in uniaxial compressive and three-point bending tests. Both the uniaxial compressive strength and flexural strength of sea ice are related to the microparameters in DEM simulation including particle size, sample size, bonding strength, and interparticle friction coefficient. These parameters are analyzed to build the relationship between the material macrostrengths of sea ice and the microparameters of the numerical model in DEM simulations. Based on this relationship, the reasonable microparameters can be calculated by given macrostrengths in the applications of simulating the failure processes of sea ice. In this simulation, both uniaxial compressive strength and flexural strength of ice increase with the increasing ratio of sample size and particle size. The interparticle friction coefficient is directly related to the compressive strength but has little effect on the flexural strength. In addition, numerical simulations are compared with experimental data to show the performance of the proposed model, and a satisfactory agreement is achieved. Therefore, this microparameter validation approach based on macrostrengths can be applied to simulate the complicated failure process of sea ice interacting with offshore platform structures.     

不同应力路径下堆石料的动力变形特性试验研究

采用大型多功能静动三轴试验机,对堆石料分别进行了常规三轴循环加载、等p循环加载和等q循环加载三种不同应力路径下的动力变形试验研究,探讨了不同应力路径下围压﹑固结应力比及动应力比等因素对体应变和偏应变的影响规律及其变形机制。试验结果表明:在循环荷载作用下,不同应力路径对堆石料体应变和偏应变的发展规律影响较大;在等p循环加载作用下引起的往返体应变为负值,即发生剪胀现象,且在轴向剪切时达到最大剪胀值,反向剪切时基本不发生剪胀;在等q循环加载作用下产生较大的残余体应变和残余偏应变,两者均不可忽略不计,主要与堆石料的各向异性和颗粒破碎有关。