A numerical model for reinforced concrete structures

This paper describes a two-dimensional approach to model fracture of reinforced concrete structures under (increasing) static loading conditions. The first part is dedicated to the concrete material. The concrete is described in compression by a non-local isotropic damage constitutive law. In tension, a fictitious crack/crack band model is proposed. The influence of biaxial stress states is incorporated in the constitutive relations. In the second part a bond model is described. It accounts for different failure mechanisms, a pullout failure and a splitting failure. This approach is applied to prestressed concrete beams with different failure mechanisms. The numerical results are compared to experimental data and show good agreement.     

基于3D⁃DIC技术的约束岩石裂缝扩展研究

采用 RMT-301B岩石与混凝土力学实验系统对施加预应力值为峰值强度 20 %、40 %、60 %的泥质白云 岩进行单轴压缩实验,并借助三维数字图像相关技术(3D-DIC)测试系统观测受约束岩石试样的变形和破坏形 态,研究泥质白云岩在约束条件下的位移场和 Tresca应变场的演化过程．结果表明：(1) 岩石的破坏过程是由 内向外扩展,当应力积累超出岩石承受范围后,岩石出现裂纹,随后裂纹尖端继续扩展,应力也发生了重分 布,岩石内部多组裂纹开始联合扩展、贯穿,直到试件发生完全破坏．(2) 含约束岩石在单轴压缩下一共有轴 向劈裂破坏、沿片理破坏、Y形破坏、共轭剪切破坏四种破坏形态,且以轴向劈裂破坏为主     

Application of some anisotropic damage model to the prediction of the failure of some complex industrial concrete structure

The aim of this paper is to present a new model for the anisotropic damage of concrete and apply it to the numerical simulation of the failure of some complex structure. This model considers damage as anisotropic in tension but isotropic in compression, and incorporates asymmetry between tension and compression. In spite of its complexity, it is expressed in a format fit for numerical calculations by the finite element method, and involves only six material parameters (in addition to the usual elastic constants), which makes it suitable for applications to large industrial structures. The material parameters can be identified from simple uniaxial and biaxial experiments. The application envisaged relates to the failure of some cylindrical nuclear containment vessel subjected to some excessive internal pressure. This structure is made of reinforced concrete containing both passive (initially stress-free) steel armatures and pre-stressed cables. The numerical results illustrate the importance of incorporation of both asymmetry between tension and compression and anisotropy of damage in the simulation to accurately predict the resistance of the structure to fracture.     

Micromechanisms of deformation and fracture of polypropylene nonwoven fabrics

The micromechanisms of deformation and fracture in tension were analyzed in a commercial polypropylene nonwoven geotextile material in a wide range of strain rates. Two different loading scenarios (smooth and notched specimens) were considered to study how these mechanisms are modified in presence of a stress concentration. The nonwoven fabric presented significant deformability and energy-absorption capability, which decreased with the strain rate, together with a high level of strength, which increased with strain rate. In addition, the material was notch-insensitive as the stress concentration around the crack tip was relieved by marked nonlinear behavior, which induced crack blunting. Different experimental techniques (standard mechanical tests, in situ testing within the scanning electron microscope, digital image correlation, etc.) were used to establish the sequence of deformation and failure processes and to link these micromechanisms with the macroscopic behavior.     

热障涂层弹性模量和残余应力测试研究

采用数字图像相关法实验研究了热喷涂制作的热障涂层的弹性模量和残余应力。首先,采用三点弯测试方法对热障涂层试件进行加载,并利用二维数字图像相关方法对热障涂层试件加装过程中的弯曲变形进行了精确的测量,进而获得了热障涂层在受拉和受压两种状态下的弹性模量,结果表明,受拉时热障涂层试件陶瓷层的弹性模量为31GPa,而受压时其弹性模量为34GPa。其次,基于内力平衡,推导了考虑曲率变化的涂层残余应力计算公式;利用三维数字图像相关法测量了喷涂前后基体曲率的变化,进而获得了涂层残余应力的大小,结果表明,热喷涂后的热障涂层残余应力为压应力,大小为-86^-70MPa。     

Measurement of Orthogonal Stress Gradients Due to Impact Load on a Transparent Sheet using Digital Gradient Sensing Method

A full-field optical method called Digital Gradient Sensing (DGS) for measuring stress gradients due to an impact load on a planar transparent sheet is presented. The technique is based on the elasto-optic effect exhibited by transparent solids due to an imposed stress field causing angular deflections of light rays quantified using 2D digital image correlation method. The measured angular deflections are proportional to the in-plane gradients of stresses under plane stress conditions. The method is relatively simple to implement and is capable of measuring stress gradients in two orthogonal directions simultaneously. The feasibility of this method to study material failure/damage is demonstrated on transparent planar sheets of PMMA subjected to both quasi-static and dynamic line load acting on an edge. In the latter case, ultra high-speed digital photography is used to perform time-resolved measurements. The quasi-static measurements are successfully compared with those based on the Flamant solution for a line-load acting on a half-space in regions where plane stress conditions prevail. The dynamic measurements, prior to material failure, are also successfully compared with finite element computations. The measured stress gradients near the impact point after damage initiation are also presented and failure behavior is discussed.     

混凝土单轴压缩下细观损伤特性的CT研究

混凝土是一种非均质的材料,在细观层次上将混凝土看作由骨料、砂浆和两者之间的界面组成的三相复合材料.本文利用CT技术对混凝土的细观损伤过程进行实时扫描观测,获得了混凝土试件实时静力压缩CT图像,提取出图像上各点的CT数并根据分区理论定义了一个基于CT数的统计损伤变量.通过对图像和CT数以及损伤变量的分析表明混凝土试件在静力压缩条件下经历了压密阶段、扩容阶段,损伤急剧增大到破坏的细观损伤演化全过程,以此反映了混凝土细观损伤演化的特性.进而通过细观损伤对混凝土材料裂纹的扩展、贯通以及最后失稳破坏的破坏实质进行了有益的探索.     

混凝土破坏理论研究进展

本文综述了混凝土的材料性质、破坏特点及机理,回顾了断裂力学与损伤力学在混凝土材料破坏研究中的应用,对当前混凝土破坏研究的主要问题和今后方向进行了讨论与展望。      

玄武岩纤维高延性水泥基复合材料的动态力学性能

利用玄武岩纤维和水泥基材料，通过一定配比融合制成了在静态拉伸试验中呈现多缝开裂、应变硬化、极限拉伸应变0.5%以上的玄武岩纤维高延性水泥基复合材料(basalt fiber engineered cementitious composites, BF-ECCs)。用分离式霍普金森压杆(split Hopkinson pressure bar, SHPB)装置对不同玄武岩纤维掺量的水泥基复合材料进行动态压缩和动态劈裂试验。结果表明:(1)在压、拉两种应力状态下,玄武岩纤维对水泥基复合材料的静态强度、动态强度均有增强，且高应变率下玄武岩纤维对抗压强度动态增幅较小，对劈裂强度动态增幅较大；(2) BF-ECC的抗压强度和劈裂强度均随应变率升高而显著提高，两者均可以采用动态增强因子(dynamic increase factor, DIF)反映动态强度的增幅，但劈裂强度的应变率敏感性强于抗压强度；(3)依据试验得到的普通水泥混凝土速率敏感性的CEB-FIP方程(2010)不适用于BF-ECCs。     

基于颗粒流的混凝土材料数值实验研究

混凝土材料是一种典型的非均质材料,其力学行为和破坏过程很复杂.本文利用PFC颗粒流软件的内嵌fish语言编写了骨料生成程序、边界条件控制程序,程序模拟了MTS伺服功能,实现了虚拟的混凝土单轴压缩试验.相对于一般有限元程序投放的骨料,文中骨料可以破裂.在生成的数值试件基础上,设计了单级配和全级配混凝土单轴压缩试验,并对其破裂形态、裂纹扩展过程、应力应变曲线和破裂过程能量变化规律进行了研究.利用该虚拟试验平台,可对混凝土的动、静力学特性进行系统研究.     

不同失效模式下轴压管状结构的吸能特性比较

圆管、方管等管状结构受轴向的压缩作用,是工程上常见的一种结构受力形式.由于几何尺寸、边界条件和材料特性的不同,管状结构可以发生5种不同的失效模式:渐进屈曲、整体失稳、翻转、膨胀和劈裂.本文综述了近年来管状结构在轴压作用下不同失效模式的理论建模、实验测试和数值仿真研究结果,并对5种失效模式的力学响应、能量吸收特性进行了对比与分析.     

混凝土细观随机骨料结构与有限元网格剖分

在细观层次上,混凝土被认为是一种由粗骨料、水泥砂浆及二者间的粘结带所组成三相非均质复合材料。本文首先基于蒙特卡罗随机抽样原理,用“取和放”方法在计算机上产生形状、尺寸和骨料颗粒分布与真实混凝土相似的随机骨料结构,再使用有限元分析软件ANSYS对骨料区域及砂浆区域分别划分网格,并编程在骨料和砂浆之间生成三角形三结点可控制厚度粘结单元,从而使三相网格缝合为一个整体,为混凝土非线性有限元分析提供可靠的细观计算模型。最后利用建立的模型进行混凝土轴心受拉和轴心受压的仿真模拟,在细观层次研究的基础上揭示出混凝土的宏观力学性能。     

A non-local model with tension and compression damage mechanisms

A non-local isotropic damage model is proposed which can be used to predict the behaviour of rock-like materials up to failure. Two isotropic damage variables account for the progressive degradation of mechanical properties under stress states of prevailing tension and compression and two internal lengths, one for tension and the other for compression, are introduced as localisation limiters. A linear bifurcation analysis highlights the regularisation properties of the non-local model. An iterative scheme for the numerical solution of the finite-step problem consisting of a linear global predictor, an averaging phase and a non-linear local corrector is presented. Some illustrative examples of tension and splitting tests show the effectiveness of the proposed model.     

基于能量守恒理论的带肋钢筋-混凝土粘结滑移本构关系研究

针对钢筋-混凝土粘结滑移的劈裂破坏模式,将整个破坏过程分为未开裂的弹性阶段和带裂缝阶段。弹性阶段采用弹性厚壁圆筒模型,带裂缝阶段采用考虑混凝土软化特性的厚壁圆筒模型。基于这两种模型,研究了粘结滑移劈裂破坏过程的能量变化规律,推导出了两种模型的能量计算公式。利用能量守恒定律建立了钢筋-混凝土粘结滑移本构关系的微分方程,并通过数值积分方法得到了粘结滑移本构模型。该本构模型能够体现混凝土与钢筋材料参数和几何参数的影响,对不同形状的粘结滑移关系曲线具有较好的适应性。最后,将得到的本构关系与文献的试验结果进行对比,并分析了各参数的变化规律。     

Determination of the intrinsic behavior of polymers using digital image correlation combined with video-monitored testing

Three methods for the determination of the large-strain behavior of ductile polymers are compared in both tension and compression. Each method relies on some (non-contact) measurement of the strain and some approximations in the calculation of stress. The strain measurement techniques include digital image correlation (DIC) and two techniques of video-based extensometry: marker tracking and area variation monitoring. Since the specimens are inevitably subject to structural plastic instabilities (necking in tension, barreling in compression) the strain and stress states are no longer uniform in the gauge section after the peak load. Under such circumstances, it is demonstrated that the three experimental methods can lead to significant differences. It is inferred from the comparative analysis that the method based on vertical marker tracking is not reliable. Validated by DIC, video-based area variation is shown to be a simple alternative way to obtain an excellent estimate of the intrinsic true stress–strain behavior of the polymer.     

Prediction of bending rupture strength of non-linear materials with different behavior in tension and compression

Many materials have quite different stress-strain relations in tension and compression. Examples include such diverse materials as rock, cast iron, concrete, tire cord-rubber and soft biological tissues. It is shown by analysis in this paper that DBTC (different behavior in tension and compression) has a profound effect on the flexural strength as predicted by application of fundamental continuum mechanics relations. The theory is applied to a non-linear material model which is shown to be applicable to two widely different materials: concrete, which has more strength degradation in tension than in compression, and steel cord-rubber, which has strength which is enhanced in tension by cord strengthening and degraded in compression by cord microbuckling.     

Analysis of strain localization during tensile tests by digital image correlation

This paper presents an imaging technique developed to study the strain localization phenomena that occur during the tension of thin, flat steel samples. The data are processed using digital speckle image correlation to derive the two in-plane components of the displacement vectors. The authors observe that the calculation of the intercorrelation function reveals a systematic error and propose a numerical method to limit its influence. Plastic incompressibility and thin-sheet assumptions are used to derive the third displacement component and, hence, the various strain and strain rate components. Numerous checks are presented at each step in processing the data to determine the final accuracy of the strain measurements. It is estimated that this accuracy is quite sufficient to track the inception and the development of localization. Examples of possible application are presented for mild steels whose strain localization mechanisms appear to be precocious and gradual.     

结构破坏的尺度律

文中综述了结构破坏的尺度律和尺寸效应的研究进展,尤其将重点放在准脆性材料的分析上,因为它们的尺寸效应是重要和复杂的．在回顾了尺寸效应研究的悠远发展史以后。着重讨论了三种主要类型的尺寸效应,即由于强度随机性引起的统计尺寸效应、能量释放的尺寸效应和由于微裂纹或断裂的分形特性可能引起的尺寸效应．得出了这些理论应用的明确结论．之后讨论了如何运用已知的尺寸效应律来测量材料的断裂特性,并采用内聚裂纹模型（cohesivecrackmodel）、非局域化有限元模型和离散元模型等对尺寸效应进行模化．文中还进而分析了尺寸效应在压缩失效和车相关材料行为下的有关问题,并讨论了在断裂扩展区描述含微裂纹材料所需的损伤本构关系．最后也讨论了尺寸效应对准脆性材料的多种应用,这些材料包括,如混凝土、海冰、纤维复合材料、岩石和陶瓷等．本文包含了参考文献377篇     

Recent Progress in Digital Image Correlation: Background and Developments since the 2013 W M Murray Lecture

Since presentation of the 2013 Murray Lecture focusing on developments in digital image correlation (DIC), the methods have continued to expand internationally and their use has begun to grow in fields where there was less activity in the past. First, a brief history of digital image correlation methods is presented from the perspective of the first author, followed by a discussion of recent trends associated with the use of digital image correlation methods in academics, governmental laboratories and industrial settings. In the remainder of the article, new results are provided in three areas where DIC methods have seen rapid growth; application of StereoDIC or three-dimensional DIC (3D-DIC) to the study of wall structures in civil engineering; the use of Volumetric DIC or Digital Volume Correlation (DVC) to quantify the internal response of a specially-designed composite material and in the area of model validation for another application in civil engineering; transfer length measurements in pre-stressed concrete beams.     

胀锚型锚栓锚固破坏及承载力研究

混凝土基材锥体受拉破坏是后锚固锚栓受拉的基本破坏形式,采用混凝土破坏准则和弹性力学方法,推导了胀锚型锚栓基于混凝土基材破坏的抗拔极限承载力简化计算公式．通过对不同混凝土强度的极限状态数值分析,发现基材锚固破坏锥面底部直径R与有效锚固深度he为近似比值关系。与试验的对比结果表明,简化计算方法精度满足工程需要,和多种试验测试结果相当吻合。