Numerical study of inter‐particle bond failure by 3D discrete element method

The cohesive‐frictional nature of cementitious geomaterials raises great interest in the discrete element method (DEM) simulation of their mechanical behavior, where a proper bond failure criterion is usually required. In this paper, the failure of bond material between two spheres was investigated numerically using DEM that can easily reproduce the failure process of brittle material. In the DEM simulations, a bonded‐grain system (composed of two particles and bond material in between) was discretized as a cylindrical assembly of very fine particles connecting two large end spheres. Then, the bonded‐grain system was subjected to compression/tension, shear, rolling and torsion loadings and their combinations until overall failure (peak state) was reached. Bonded‐grain systems with various sizes were employed to investigate bond geometry effects. The numerical results show that the compression strength is highly affected by bond geometry, with the tensile strength being dependent to a lesser degree. The shear, rolling and torsion strengths are all normal force dependent; i.e., with an increase in the normal force, these strengths first increase at a declining rate and then start to decrease upon the normal force exceeding a critical value. The combined actions of shear force, rolling moment and torque lead to a spherical failure envelope in a normalized loading space. The fitted bond geometry factors and bond failure envelopes obtained numerically in this three‐dimensional study are qualitatively consistent with those in previous two‐dimensional experiments. The obtained bond failure criterion can be incorporated into a future bond contact model. Copyright © 2015 John Wiley & Sons, Ltd.     

延脆性变化对隧道掘进机刀具破岩过程及其破坏模式影响的颗粒元模拟分析

在全断面岩石掘进机（TBM）刀具破岩的颗粒元模拟中颗粒参数的选取至关重要,其中颗粒间平行黏结切向、法向强度是关键控制性参数之一,它们之间的比值关系直接决定所模拟试样的延脆性质,影响刀具破岩过程及其破岩效果。为探讨延脆性对刀具破岩模式的影响,（1）建立9种采用不同平行黏结强度比值的数值模型,分别进行单轴压缩及巴西劈裂模拟,研究不同延脆性试样的力学行为及破坏模式的变化。（2）对9种模型进行双刀破岩,并监控其裂缝的发展情况及刀具的受力状况。（3）为减小随机性对模拟结果的影响,通过改变随机数,每种模型重复模拟5次,综合分析5次的计算结果。模拟分析发现,随着切向和法向黏结强度比值( )的增大,试样的脆性增加,破坏模式逐渐从剪切破坏转变为脆性张拉破坏,刀具破岩压碎区范围减小,张拉裂缝更容易在刀具间贯通延伸从而切割出块体更大的岩渣;随着试样脆性的增加,归一化比能减小,刀具破岩的效率增加;平行黏结强度比值相同的条件下采用不同随机数种子生成的模型中,试样的具体破坏情况有一定的差别,但总体破坏模式相似。     

An experimental and numerical investigation on the cyclic response of a portal frame pier belonging to an old reinforced concrete viaduct

This paper deals with the assessment of the seismic response of a portal frame pier belonging to an old reinforced concrete viaduct. A series of tests, consisting of cyclically imposed displacements, were carried out on three 1:4 scale mock‐ups. The objective of the experimental campaign is twofold: (1) identification and evaluation of the local failure mechanisms and (2) calibration of a numerical model including all observed nonlinear phenomena. The experimental results show that the shear strength of the transverse beam and of the beam–column joints characterizes the post‐elastic behavior of the piers. Other phenomena, like bond‐slip and buckling of the longitudinal bars of the columns, typical of old reinforced concrete structures have also been observed. Finally, a numerical model, built in OpenSEES, was calibrated to reproduce in a satisfactory way the experimental results and to provide a reliable tool for the evaluation of the seismic response of the pier. Copyright © 2011 John Wiley & Sons, Ltd.     

水泥胶结颗粒的微观力学模型试验

土体粒间胶结是建立天然结构性土本构模型的决定性因素之一,将胶结颗粒理想化为两铝棒,在指定位置处形成胶结。采用水泥作为胶结材料,对胶结颗粒进行了一系列简单加载试验（包括拉伸、压缩、压剪、压扭）和复杂应力路径试验,并与蒋明镜等所做的环氧树脂胶结试验进行对比。结果表明：胶结材料对胶结颗粒的力学性能存在一定影响,但基本规律符合蒋明镜等所提出的理想颗粒间胶结模型。水泥胶结抗拉强度低于环氧树脂胶结抗拉强度,但延性相对较好,抗压特性均呈塑性软化现象;二者的抗剪强度初始均随法向压力的增加而增大,当法向压力超过一定值时,又随法向压力的增加而减小（该压力称为界限法向压力）,但水泥胶结的界限法向压力明显高于环氧树脂胶结,扭转试验规律与剪切试验规律类似。在三维应力空间中（法向压力-扭矩-剪力）水泥胶结的强度包线呈橄榄球状,环氧树脂胶结强度包线呈水滴状。     

Numerical simulation of kinetic friction in the fracture process of rocks in the framework of General Particle Dynamics

A numerical simulation of kinetic friction function in the fracture process of rocks in the framework of General Particle Dynamics (GPD) is performed in this paper. The frictional algorithm is implemented into the General Particle Dynamics code (GPD) to describe frictional behavior of particles, where frictional forces among discrete particles are formulated using the principle of balance of two forces based on ideal plastic contact between two surfaces of solids. In General Particle Dynamics code (GPD), interaction among discrete particles is formulated using the virtual-bond method. Fractures of virtual bonds among particles are determined through the Hoek-Brown damage evolution law of rock materials. Three numerical cases are to verify the stability and accuracy of the numerical algorithm. Then, the numerical results are compared with analytical solutions and experimental results. It is found that the numerical results are in good agreement with the experimental ones.     

不同胶结宽度粒间胶结特性试验研究

胶结砂土中水泥含量、能源土中水合物的含量会导致其宏观力学特性的差异,从微观层面可以解释为颗粒之间胶结物含量的不同所导致的粒间力学性质的差异所致。为研究不同胶结物含量的胶结颗粒的力学特性,进行了不同胶结宽度的粒间胶结试验,试验结果表明：（1）峰值压缩荷载随胶结宽度的减小呈非线性变化,宽高比对峰值荷载有明显影响;（2）峰值拉伸荷载随胶结宽度减小而线性减小,宽高比对其影响不大;（3）峰值剪切荷载和峰值扭矩由两部分组成,即胶结部分和摩擦部分。且其变化趋势相似,随着法向荷载的增大,峰值荷载先随之增大,在达到临界应力比后,峰值荷载开始减小,当应力比达到1,即胶结破坏时,胶结部分不再发挥作用,此时粒间荷载由摩擦部分提供;（4）在压-剪-扭试验中,不同初始偏心距情况下得到的峰值荷载在剪力-扭矩平面内呈椭圆状。     

平行黏结模型宏细观力学参数相关性研究

采用变量控制法较全面地分析了各细观参数与宏观参数的定量关系,表明：弹性模量E随颗粒模量Ec、黏结模量 、平行黏结半径乘子 呈线性增长,随颗粒刚度比kn /ks、黏结刚度比 呈对数减小;泊松比则主要受kn /ks和 的影响,两者之间呈对数关系;颗粒键的黏结强度决定了材料的强度,室内材料黏聚力c和抗拉强度 主要受法向平行黏结强度 、平行黏结强度比 的影响,随 线性增长,随 对数减小;摩擦角 主要受颗粒摩擦系数u影响,两者呈对数关系。分析裂隙扩展特征,表明材料法向黏结强度 和切向黏结强度 的相对大小决定裂纹分布规律,随 增大,岩样的拉破坏区域减少,而压剪破坏区域增加,破坏面由剪切破坏向共轭破坏发展;材料的强度离散性越小,岩样破坏趋于集中,破坏面明显,强度均值标准差比值 >3.5为宜; 增加,宏观破坏形式向共轭破坏发展。细观参数的选取除了匹配强度参数,同时还需要考虑破坏形式的一致,考虑多参数相互影响,建立了宏细观参数之间的经验公式,对细观参数进行优化选择,并做了实例验证。室内试验和数值模拟获得的峰值荷载、变形参数、剪切强度等数值接近,应力-应变演化规律相同,破坏形态一致,表明细观参数结果是可靠的。     

云南松侧根摩擦型根土粘合键的破坏机制及模型

以云南松侧根为研究对象，探讨了侧向牵引情况下，云南松测根与土壤之间摩擦型根土粘合键的力学作用机制，阐述了侧根从土中拔出时，摩擦型根土粘合健破坏的实质，并基于莫尔-库仑破坏准则建立了极限平衡条件下的摩擦型根土粘合键破坏模型。该模型除了可以描述摩擦型根土粘合键的破坏原理外，还可用于定量地估算、预测摩擦型根土粘合健能够提供的最大牵引阻力。最后，通过实测与模型理论计算结果进行了比较，结果表明，所建立的模型具有一定的精度和适用性。     

Laboratory Tests on Shotcrete-Rock Joints in Direct Shear, Tension and Compression

Summary. A series of laboratory tests was performed on cemented shotcrete-rock joints to investigate the strength and stiffness of the interfaces, while simulating field conditions as close as possible. The direct shear test formed the core of the experimental work, while the tension and compression tests were complementary. To simulate loading conditions experienced in practical cases the direct shear tests were performed under fairly low normal stresses. In most practical cases when shotcrete is used with rock bolts, the normal load on shotcrete lining seldom exceeds 0.2 to 0.5 MPa. The direct shear test results show that, for such normal load range the shear strength is determined by the bond strength for genuinely bonded shotcrete-rock interfaces. For higher normal stresses (σ_n > 1.0 MPa), which rarely exist at the shotcrete-rock interface, the shear strength is largely influenced by friction resulting in the cohesive strength being less significant. Assessment of the shear surface revealed that the steel fibres in the shotcrete appeared to contribute significantly to the frictional component. The shear and normal stiffnesses of the interface were also determined, which were in principal the stiffnesses of the bond. An interesting observation was the complex interaction at the interface and the mechanisms that controlled the peak shear strength which depended on the surface roughness, the existence of natural flaws and the normal load.