堆石料三维形状量化及其对颗粒破碎的影响

筑坝堆石料由于尺寸较大,必须对其按一定比例缩尺后才能用来开展室内三轴试验。但缩尺前后颗粒形状难免会有差异,如何评价颗粒形状变化对堆石料变形特性的影响是十分重要的。引入了高精度的三维激光扫描技术对紫坪铺面板坝筑坝堆石料2.5～5、5～10、10～20 mm以及20～40 mm 4个粒径组的颗粒进行了空间形状分析,在此基础上进一步开展了单一粒组的三轴试验,研究了4个粒组的颗粒形状指标与颗粒破碎率的相关性。试验表明,紫坪铺堆石料颗粒破碎率随着其平均球度的增加而减小,并且呈近似半对数线性关系;随着围压的增加,颗粒形状对颗粒破碎的影响逐渐减弱,颗粒强度的影响逐渐增大。紫坪铺堆石料的颗粒强度随着颗粒尺寸的增加逐渐减小,但其破碎率反而随着颗粒尺寸的增加而逐渐减小,主要是因为试验所采用的紫坪铺堆石料颗粒尺寸越小时,其形状越不规则。因此,研究缩尺效应对颗粒破碎率的影响时,要同时考虑颗粒尺寸和颗粒形状。     

颗粒形状对人工模拟堆石料强度和变形特性影响的试验研究

颗粒形状对堆石料的强度和变形特性有着明显的影响。采用水泥净浆浇筑方法,制备了相同体积不同形状的粗颗粒近似模拟堆石料颗粒。通过三轴剪切试验,研究了不同围压条件下颗粒形状对人工模拟堆石料的强度和变形特性的影响。结果表明:颗粒形状对人工模拟堆石料的强度和剪胀特性有影响,随着颗粒球度的增大,人工模拟堆石料峰值强度增大,低围压下剪胀特性越明显;极限应力比随着颗粒球度的增大而减小;初始切线模量和切线体积模量随着颗粒球度的增大而增大;初始内摩擦角受颗粒球度影响较小,内摩擦角增量受颗粒球度的影响相对较大,随着颗粒球度的增大而减小。     

颗粒材料的堆积密度与均匀性研究进展

颗粒是一种常见的工业与工程材料,其堆积密度与均匀性是影响产品和工程质量的重要因素。本文全面回顾和总结了国内外学者在颗粒堆积密度和均匀性方面的研究成果和最新进展。结果表明,颗粒材料的堆积密度与颗粒自身性质（颗粒大小、形状、表面粗糙度等）、容器性质（容器尺寸、形状和内表面粗糙度等）、堆积方法（下落高度、振动条件、装料强度和装填顺序等）、粒径比和粒径级配有关;堆积不均匀性（离析程度）随颗粒粒径差异、密度差异和振动加速度的增大而增大,随粒径范围、粒组数量和振动频率的增大而减小,而受颗粒形状的影响相对较小。对于作为高放废物深地质处置库缓冲/回填材料的膨润土颗粒,科学、高效的颗粒原位填充技术是今后值得深入研究的方向。     

堆石料单颗粒劈裂试验破碎强度随机性与尺寸效应的三维离散元模拟

堆石单颗粒形状不规则,强度随机且存在尺寸效应,开发堆石颗粒模型是开展堆石料颗粒流室内数值试验的基础。文中提出一种随机裂缝模型,并对模型参数的设置进行分析讨论,模型在合理的参数设置的情况下能反映真实堆石颗粒的脆性劈裂、强度随机性Weibull分布、强度尺寸效应,是深入开展筑坝材料破碎及缩尺效应颗粒流模拟分析的前提。单颗粒数值试验表明,颗粒形状对破碎模式影响显著;在相同岩性材质和裂缝分布情况下形状对颗粒强度影响较大;颗粒越不规则,其强度离散性越大。     

颗粒形态对砂土抗剪强度影响的试验和离散元数值模拟

为研究颗粒形态对砂土抗剪强度的影响，制备了5组不同粒径的球形石英砂和颗粒形态不规则的实验室标准砂，并进行一系列的直剪试验；离散元法能有效地模拟砂土的离散性和不均匀性，基于高性能离散元分析软件MatDEM，通过二次开发，建立直剪试验三维模拟器，对直剪试验过程进行了数值模拟，并对比分析了试验和数值模拟结果。数据表明:(1)石英砂试样的自然休止角与其抗剪强度有着正相关性；(2)球形石英砂较实验室标准砂能更快达到峰值强度，但其值相对较低；标准砂存在着一定的黏聚力；(3)在直剪过程中，直剪盒中部颗粒的位移较周围颗粒不同，形成了明显的剪切带，并且球形石英砂试样的剪切带内颗粒的位移更小；(4)砂土直剪试验中配位数与剪应力有着良好的正相关性。本文结果表明，在相同的荷载条件下不规则颗粒形态的石英砂的抗剪强度要明显大于球形石英砂。     

颗粒形态对离散介质剪切强度的影响

自然条件下,颗粒介质大多以非规则单元形态存在。非规则几何形态对颗粒介质的宏观力学性能有很大影响。针对颗粒单元的不同几何形态,采用团颗粒单元对离散介质的直剪试验过程进行了离散元数值计算,详细地讨论了颗粒形态对离散介质剪切强度的影响。该非规则颗粒由不同形态、不同数目、镶嵌尺寸、组合方位和颗粒大小的球形颗粒进行随机构造,其在局部与整体坐标之间的转动、力矩和方位关系通过4元素方法进行确定,基本球体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。通过构造7种具有相同的质量概率分布的不同形态的团颗粒,在不同法向应力下,对团颗粒的直剪试验进行了离散元模拟,分析了不同形态颗粒的剪切强度。通过对不同形态颗粒介质剪切强度的数值分析,进一步揭示了非规则颗粒间的咬合互锁效应,为分析非规则颗粒的宏观动力特性提供了依据。     

颗粒形状对粒状材料破碎演化规律及强度准则影响

为探讨颗粒形状对粒状材料的颗粒破碎演化规律及强度特征的影响,提出了一个新的粒状材料颗粒形状量化参数,设计了一种考虑三维颗粒形状的人工试样制备方法,随即进行了常规三轴压缩试验,并分析了颗粒破碎和强度特征,最终建立了一个二元介质强度准则,具体的研究成果为：建立颗粒形状量化参数——球形模量GM,在此基础上制备了5种不同形状的可破碎粒状材料三轴试样,并发现球形模量影响着粒状材料的三轴压缩强度特征;通过筛分确定试样的颗粒破碎情况,对试样的颗粒破碎演化规律和临界状态进行探讨,发现颗粒形状通过影响颗粒破碎规律而控制着宏观强度的非线性演化特征;以二元介质理论为基础,建立了考虑颗粒形状的可破碎粒状材料强度准则,并通过试验对其适用性进行了验证。     

颗粒破碎的三维离散元模拟研究

采用PFC等离散元方法研究岩土材料的颗粒破碎已经成为热点。采用考虑局部应力集中的点荷载破碎准则,利用阿波罗填充和膨胀法保证破碎前、后颗粒之间的种群平衡,并引入尺寸因子来表征不同粒径的颗粒强度。在此基础上,开展了石英砂、钙质砂和萨克拉门托河砂3种不同破碎难易程度材料的数值试验,并与室内试验结果进行对比。结果表明:建立的三维颗粒破碎模型能够很好地描述破碎难易程度不同的颗粒材料的压缩特性;考虑应力集中效应的点荷载破碎准则比基于平均应力Mohr-Coulomb理论的颗粒破碎准则更能真实地反应颗粒材料的破碎现象。同时,所建立的模型能够揭示破碎对颗粒材料各向异性消散和级配曲线演化的影响规律。     

颗粒离散元法的颗粒碎裂研究进展

颗粒离散元法(distinct element method,DEM)是被不同学科和工程领域广泛应用的方法,各学科结合各自的理论基础和研究范围,对颗粒离散元法进行改进、完善,因此各学科间的发展各有侧重,可进行相互借鉴。本文针对颗粒离散元法中有关\     

Discrete element modeling of crushable sands considering realistic particle shape effect

This study proposed a novel approach for generating crushable agglomerates with realistic particle shapes in discrete element modeling (DEM). The morphologies of sand particles were obtained by X-ray micro-computed tomography scanning and image processing. Based on the particle surface reconstructed by spherical harmonic analysis, the crushable agglomerates with realistic particle shapes can be generated in DEM simulations. The results of single particle crushing tests showed that particle shapes significantly influence the fracture patterns and crushing strengths of sand particles. Furthermore, two one-dimensional compression tests were conducted to investigate the particle shape effect on micro- and macro-mechanical behaviors of crushable sands.     

Numerical analysis of the shape effect on the tensile strength evaluation of axisymmetric specimens

This paper analyses the shape effect on the tensile strength of axisymmetric rock specimens by an improved adaptive dynamic relaxation method. It is shown that the present approach is convenient and efficient. Numerical results for spheroidal specimens with different shapes reveal the variation of stress distributions along the loading axes and suggest that loading along the shorter principal axis is preferable in strength tests. Furthermore, an empirical formula based on the failure criterion of maximum tensile stress is proposed to evaluate the tensile strength of specimen materials.     

Discrete element simulation of the effect of particle size on the size of fracture process zone in quasi-brittle materials

Experimental tests performed on quasi-brittle materials show that a process zone develops ahead of a crack tip. This zone can affect the strength and the deformation pattern of a structure. A discrete element approach with a softening contact bond model is utilized to simulate the development of the fracture process zone in the three-point bending tests. Samples with different dimensions and particle sizes are generated and tested. It is shown that as the material brittleness decreases, the width of the process zone becomes more dependent on the specimen size. Furthermore, the increase in the particle size, results in increase in the width of the process zone. A dimensional analysis together with the numerical results shows that the width of process zone is a linear function of particle size (radius). This finding is discussed and compared with published experimental data in the literature.     

Discrete element modelling of the influence of cover strength on basement-involved fault-propagation folding

A discrete element model is used to investigate the influence of sedimentary cover strength on the development of basement-involved fault-propagation folds. We find that uniformly weak cover best promotes the development of classical, trishear-like fault-related folds showing marked anticlinal thinning and synclinal thickening, with cover dips increasing downwards towards the fault tip. Uniformly strong cover results in more rounded fold forms with only minor hinge thickening/thinning and significant basement fault-propagation into the sedimentary cover. Heterogeneous, layered, cover sequences with marked differences in strength promote the development of more complex and variable fold forms, with a close juxtaposition of brittle and macroscopically ductile features, which diverge from the predictions of simple kinematic models. In these structures the upper layers are often poor indicators of deeper structure. In addition, we find that in layered cover sequences fault-propagation into the cover is a complex process and is strongly buffered by the weaker cover units.     

Discrete element analysis of pile instability

Recently, instability of slender piles during installation has been analysed as a non-conservative buckling process. For these results to be of potential use in practice, simple procedures are necessary whereby piles of arbitrary cross-section installed in ground with arbitrarily varying properties can be treated. This paper lists discrete element matrices which enable the prediction of the onset of instability in piles assuming conservative or non-conservative ground resistance. The same matrices can then be used to incorporate effects of incipient instability in driveability analyses.     

Effect of particle shape of glass beads on the strength and deformation of cemented sands

Acta Geotechnica - Few studies have focused on the influence of particle shape on the mechanical properties of cemented sand. To address this lack of information, this study investigated the...     

Discrete element modelling of fragmentable geomaterials with size dependent strength

This paper presents the application of a discrete element technique to the analysis of the dynamic indentation of either a purely brittle or a brittle viscoplastic geomaterial which can experience fragmentation resulting in fragments with size dependent strength characteristics.     

岩石单轴抗拉强度的几何形状效应研究

提出了一种模拟岩石破坏过程的细胞自动机模型,以力、位移作为基本状态变量,基于基本的力学规律构造了细胞自动机模型的矢量局部更新规则,克服了以往规则确定随意性以及标量化的缺点。利用该模型模拟了岩石的单轴破坏过程,研究了非均质性对单轴直接拉伸破坏过程的影响,并研究了几何形状（长宽比）对单轴拉伸强度的影响,研究结果加深了对岩石单轴抗拉强度的了解,对试验有一定的指导作用,应进一步加强岩石的直接拉伸试验研究。     

各向异性结构性砂土离散元分析

天然沉积砂土力学特性受各向异性及结构性影响明显,实际工程中不能忽视。为探究二者的影响,首先在二维离散元程序NS2D中采用椭圆颗粒模拟了重力场中颗粒长轴主方向为水平的各向异性净砂样,随后基于结构性砂土胶结厚度分布规律及室内试验提出了一个新的微观胶结接触模型并将其引入各向异性净砂样以模拟天然各向异性结构性砂土,最后对该离散元试样进行了双轴试验模拟,将模拟结果与室内试验结果对比以验证该模型的适用性,并对其微观力学特性变化进行研 究。分析结果表明,随着剪切进行,各向异性结构性砂土呈明显应变软化及剪胀现象;胶结接触逐渐减少,且主方向始终为竖向方向;胶结破坏速率及胶结破坏率变化情况与宏观力学响应较一致,且胶结物多为拉剪破坏;土颗粒排列主方向始终为水平向,且水平向排列颗粒所占比例略微增大。