On the obstacles and way to assess the seismic catastrophe for high arch dams

To prevent possible seismic catastrophe,naturally,its assessment is deeply concerned over in China as a series of arch dams of about 300 m high will be constructed in the severe seismic regions.In this paper the major obstacles to the seismic catastrophe assessment of high arch dams which focused on clearly defining the Maximum Credible Earthquake (MCE) and reasonably selecting its site-specific seismic input parameters as well as quantitatively evaluating the limit state of dam-breach for designers are emphasized.Some breakthrough progress with pending problems is presented,such as to adopt more reasonable seismic input parameters based on seismic hazard evaluation of dam site; to develop model and method more fit in with the reality for non-linear seismic analysis of dam-foundation-reservoir system.The ideals of further improvement both in evaluating the MCE and defining the quantitative index of its performance objective are discussed,including how to use semi-empirical method of simulating strong ground motion near fault,how to solve the long-standing problem of stress singularity at dam heel,and how to investigate dynamic behaviors of fully-graded damconcrete through dynamic tests and 3-dimensional meso-mechanics analysis checked by CT technique.     

Cell size effect analysis of the effective Young’s modulus of sandwich core

In this paper, different methods of predicting the effective Young’s modulus of periodic cellular materials are taken into account and their abilities on revealing the size effect of the unit cell are analyzed. The bending energy method (BEM) is proposed to compare with the homogenization method (HM) and the G-A meso-mechanics method (G-A MMM). A variety of tests are carried out for different cell configurations and discussions are fully made to show its advantage. It is proved that the HM and G-A MMM results only compared to the situation of the scale factor n → ∞ and cannot reflect the size effect of the unit cell. The BEM results can perfectly reveal the underlying relationship between the effective Young’s moduli and the cell size and they are consistent with the HM and G-A MMM results for n → ∞.     

基于细观尺度的混凝土热膨胀性能研究

用细观力学方法对混凝土有效热膨胀系数和温度应力进行研究,提出了有效热膨胀系数理论公式.同时假定混凝土为骨料和砂浆组成的两相复合材料,用代表性体积单元(RVE)建立混凝土三维随机椭球形骨料模型,并对此模型进行有效热膨胀系数和温度应力的数值计算.将所提出的有效热膨胀系数理论公式、其他经典理论公式与数值计算结果对比后发现:对于混凝土有效热膨胀系数的预测,所提出的混凝土有效热膨胀系数理论公式预测精度最高,自洽法次之,而稀疏解法精度较低且只适用于骨料体积分数较小的情况.提出的理论解与数值计算结果吻合较好,其误差随骨料体积分数的增大而增大;当骨料与砂浆的弹模比小于1时,理论解小于数值计算结果,当弹模比大于1时,理论解大于数值计算结果.     

不同受力模式下的沥青混合料接触力链分布特征

为了研究拉压弯剪不同受力模式下沥青混合料内部的接触力链传递分布情况,明确骨料砂浆不同组分抵抗荷载时的各自贡献水平,基于工业CT与数字图像处理技术获取了骨料颗粒模型,利用PFC3D软件构建了SMA13级配的沥青混合料三维数字化试件并进行了不同受力模式下的虚拟试验,对沥青混合料内部接触力链的传递分布特征以及砂浆骨料不同组分的接触力组成分布情况进行可视化及量化分析,以力链概率作为量化指标对不同受力模式下各类接触力链的分布特征进行统计分析。研究结果表明:利用离散元方法建立的三维数字化模型可以较好地模拟沥青混合料低温力学行为;单轴压缩受力模式下各类接触的强力链占比均为4种受力模式当中的最高水平,骨料间的嵌挤作用为抵抗外载的主要因素,骨料所传递的接触力水平占总接触力的69.9%;间接拉伸与半圆弯拉载荷模式下,沥青砂浆内部与界面位置的强力链占比显著高于骨料间的强力链占比,沥青砂浆所传递的接触力分别占总接触力的50.1%和57.4%,沥青砂浆间的黏结力为抵抗破坏的主导因素;单轴贯入受力模式下沥青砂浆与界面位置的强力链占比略高于骨料间接触的强力链占比,骨料所传递的接触力占总接触力的64.7%。强弱力链占比可以较好地反映沥青混合料不同组分在不同受力模式下的传递载荷能力,依据研究结果可以从细观结构角度为深入了解沥青混合料的力学行为提供理论依据。     

混凝土细观力学模型研究:非均质影响

本文在改进的混凝土随机骨料生成技术的基础上,基于连续介质力学的方法,采用非线性损伤本构关系描述混凝土细观各相组成--骨料、砂浆与界面的性能,建立了随机骨料数值模型与相应的细观单元参数选取方法;在算法上,通过对比二维与三维有限元隐式和显式解法求解混凝土细观力学问题所需内存及CPU时间,表明基于有限元显式解法可有效地减少内存及机时的耗费.在此基础上,引入细观各相组分力学性能的Weibull概率统计分布以表征材料的非均质性,进行了系统的研究.结果表明,细观各相组分的非均质性对混凝土力学性能具有重要影响,主要表现特征为材料力学性能的离散性与强度差异性.     

岩土材料破损特性的颗粒流研究

基于颗粒流理论,利用颗粒接触胶结本构模型,建立结构性岩土材料的颗粒流模型。通过颗粒流数值模拟试验,对结构性岩土材料破损的细观机理作了初步的研究。试验中采用胶结在一起的小圆盘去代替一个可破碎颗粒,再在可破碎颗粒之间施加适当的胶结,形成结构性岩土材料试样。在试验过程中通过记录胶结破损的数目和空间位置,就可以直观反映结构性岩土材料破损特性。试验表明试样在低围压下压缩试验中应力应变曲线表现为软化型,伴随剪切带的形成,有必要区分体积破损率和面积破损率。试样在高围压下压缩试验应力应变曲线表现为硬化型,没有明显的剪切带形成,表现为体积破损。分析表明颗粒流方法是研究岩土材料破损特性的一个有力工具。     

大坝混凝土试件三维细观力学并行计算研究

基于混凝土材料基本特性及其有关试验研究结果,给出了混凝土材料的损伤演化关系、及其抗拉强度和弹性模量的应变率强化定律,建立了既考虑细观损伤弱化又计及应变率强化效应的混凝土非线性有限元静动力学虚功方程。根据FEPG有限元语言的规则,编写了混凝土试件细观静、动力学方程的描述文件及位移、应力场的算法文件,并基于PFEPG软件平台开发了混凝土试件的三维细观数值并行计算程序。利用三维随机骨料随机参数模型对混凝土试件弯折损伤破坏过程进行了细观数值模拟。其计算结果与试验结果相吻合,显示了该算法及所编并行程序的有效性。     

3D meso-mechanical analysis of concrete specimens under biaxial loading

In recent years, the mechanics of materials group at ETSECCPB‐UPC has developed an approach for meso‐mechanical analysis of concrete using zero‐thickness interface elements in 2D and more recently in 3D. In this methodology, the meso‐structure is generated with in‐house developed computer programs based on Voronoï/Delaunay theory. In the analysis, continuum elements are assumed linear elastic. Non‐linearity and fracture phenomena are made possible by the systematic use of zero‐thickness interface elements inserted on a priori determined potential fracture planes. In this paper, the results obtained for a 3D specimen under biaxial loading are presented. The results turn out to be very satisfactory and, in particular, it is observed that even the specimens which contain a reduced number of aggregates (14 in the present calculations) lead to a realistic failure envelope under biaxial loading, and they also capture the tendencies of cracking and fracture orientations observed in experiments for different rates of biaxial loading. The special limit case of biaxial loading under restrained out‐of‐plane deformations is also analysed, leading to practically elastic behaviour as shown by available experimental evidence.     

静力触探试验的离散元数值模拟研究

由于静力触探试验的可重复性及可靠性,它在现场试验中被广泛应用。对静力触探试验进行颗粒流模拟,有助于从细观上了解静力触探试验机理,使之更好地服务于工程实际。颗粒流方法在模拟土体大变形方面具有一定的优势,本文利用颗粒流理论及其程序,首先在双轴试验的基础上确定了土样细观参数,然后建立静力触探的数值模型,模拟研究了贯入过程及锥尖土体的细观力学特征,重点分析了静力触探过程中砂土的位移运动规律,砂土的水平和垂直应力场及主应力的偏转。通过施加不同的应力边界条件,分析了模型不同的应力条件及超固结比(OCR)对锥尖贯入阻力的影响。同时,分析了锥尖尺寸效应对锥尖贯入阻力的影响规律。数值模拟结果与已有的试验规律具有较好的一致性,从而验证了颗粒流方法模拟静力触探试验的可行性,所得结论具有一定的理论价值和工程意义。     

上拔荷载作用下桩的颗粒流数值模拟

对桩进行了承受上拔荷载作用的数值模拟试验并与物理实物试验实测值作了对比。应用颗粒流理论及其PFC2D程序,模拟研究了承受上拔荷载作用的桩及桩周土的细观力学特征,桩的上拔位移和颗粒的分布和速度。随荷载增加土颗粒的受影响区由大到小,且影响区边界逐渐明显,颗粒排列发生变化,形成了锥体状的受影响区域。根据土颗粒的分布、速度和位移的分布和变化,确定土中滑裂面为锥体状;数值模拟了上拔荷载、桩上拔位移与时步的关系,并与宏观物理实物试验的实测结果作了对比分析;综合确定了桩的极限上拔承载力。桩承受荷载过程中土颗粒细观结构变化的颗粒流仿真,是关于土体的细观力学特征与宏观力学响应相关联的初步研究。