基于裂缝扩展准则的混凝土重力坝裂缝扩展全过程数值分析

将裂缝扩展准则应用于混凝土重力坝裂缝扩展全过程分析,结合虚拟裂缝模型计算了经典混凝土重力坝模型的断裂特性,并与其他数值方法进行了对比.结果表明:利用该数值方法得到的混凝土重力坝模型的外荷载-裂缝口张开位移曲线及裂缝扩展路径与试验结果均吻合良好;当给定了混凝土材料的起裂断裂韧度、断裂能、抗拉强度等参数后,即可采用该数值方法对混凝土重力坝裂缝扩展全过程进行分析.     

带裂缝服役混凝土结构力学性能的多尺度模拟方法

为了研究既有裂缝及其扩展对服役混凝土结构力学性能的影响,建立了以随机骨料和黏结单元为基础的细观结构黏聚区模型,并进一步建立了带裂缝混凝土梁的一致多尺度等效模型。在开裂区域引入符合牵引分离准则的黏结单元来考虑裂缝的随机扩展,通过随机骨料投放来模拟骨料、砂浆等混凝土的细观结构组成。采用已有试验结果对细观结构黏聚区模型进行了验证,该方法能够模拟混凝土裂缝的随机扩展及其应力-应变全过程曲线。然后,对不同裂缝深度的混凝土试件进行单轴拉伸试验的数值模拟,采用一致多尺度模型对不同裂缝深度的混凝土梁进行模拟,并对其计算效率和精度进行了比较。结果表明：对于初设裂缝深度小于0.3h(h为混凝土试件高度),试件尺寸对混凝土抗拉强度的影响很小,而当初设裂缝深度大于0.3h时,试件尺寸对混凝土抗拉强度的影响明显;随着初设裂缝深度的增加,混凝土的抗拉强度和弹性模量以及混凝土梁的抗弯刚度、受弯承载力逐渐减小;在不同区域采用不同控制尺度建立的一致多尺度细观结构等效模型能够模拟混凝土梁的裂缝及其随机扩展,并且能在不显著影响其计算效率的情况下保证其计算精度。     

混凝土I-型细观断裂模型及其在材料层次#br# 尺寸效应中的应用

混凝土材料的断裂破坏本质上是内部微细裂纹在荷载作用下不断萌生、扩展以及贯通的结果,断裂裂缝在细观层次上则是由砂浆裂缝、界面裂缝以及骨料裂缝3部分组合而成。文章基于细观力学和断裂力学基本理论,建立一类能够同时考虑细观裂缝在混凝土材料内部扩展过程中绕过骨料和穿透骨料发展的混凝土I 型细观断裂模型。与已有试验数据对比表明,文章模型能够有效预测混凝土断裂能等宏观力学参数随细观组分力学性能的变化规律。进而,基于建立的细观断裂模型,初步分析混凝土材料层次的强度尺寸效应,结果表明：当砂浆力学性能确定时,混凝土材料的名义强度与骨料强度和界面强度正相关;界面的力学性能能够显著影响混凝土材料名义强度等宏观力学参数随骨料尺寸的变化规律;高性能混凝土强度随骨料尺寸增大而增大,普通性能混凝土强度随骨料尺寸增大而减小。文章模型分析方法以期为基于性能设计的混凝土配合比研究奠定理论基础。     

橡胶混凝土断裂性能试验研究

开展了橡胶混凝土三点弯曲梁断裂试验,获得了不同试件尺寸和橡胶掺量下的裂缝断裂全过程曲线,对橡胶混凝土断裂能GF及裂缝扩展K-R阻力曲线进行研究。基于数字图像相关方法,获得了橡胶混凝土试件垂直于裂缝扩展方向的全场位移和应变信息,对裂缝扩展的损伤断裂演化过程进行研究。结果表明:随试件尺寸增加和橡胶掺量提高,断裂能均呈现出上升趋势;K-R阻力曲线随试件尺寸变化表现出明显尺寸效应,橡胶掺量在0%~14%设计范围内时,K-R阻力曲线与橡胶掺量无关。     

混凝土Ⅰ-型细观断裂模型及其在材料层次尺寸效应中的应用

混凝土材料的断裂破坏本质上是内部微细裂纹在荷载作用下不断萌生、扩展以及贯通的结果,断裂裂缝在细观层次上则是由砂浆裂缝、界面裂缝以及骨料裂缝3部分组合而成。文章基于细观力学和断裂力学基本理论,建立一类能够同时考虑细观裂缝在混凝土材料内部扩展过程中绕过骨料和穿透骨料发展的混凝土I-型细观断裂模型。与已有试验数据对比表明,文章模型能够有效预测混凝土断裂能等宏观力学参数随细观组分力学性能的变化规律。进而,基于建立的细观断裂模型,初步分析混凝土材料层次的强度尺寸效应,结果表明:当砂浆力学性能确定时,混凝土材料的名义强度与骨料强度和界面强度正相关;界面的力学性能能够显著影响混凝土材料名义强度等宏观力学参数随骨料尺寸的变化规律;高性能混凝土强度随骨料尺寸增大而增大,普通性能混凝土强度随骨料尺寸增大而减小。文章模型分析方法以期为基于性能设计的混凝土配合比研究奠定理论基础。     

材料对混凝土结构裂缝的影响分析

从混凝土的组成材料方面,对混凝土结构产生裂缝的原因进行了分析,指出了混凝土中水泥、骨料、外加剂、掺合料及配合比对混凝土结构裂缝的影响,并提出了控制和减少混凝土结构裂缝的材料措施,以防止混凝土结构裂缝的产生。     

民用建筑混凝土温度裂缝的预防策略探究

在建筑工程中，混凝土是工程结构的重要组成部分，影响着建筑结构的使用与安全。混凝土是由多种外加剂和骨料等组成的人工材料，受多种因素的影响容易产生不同程度的温度裂缝。本文主要就民用建筑混凝土温度裂缝的预防提出建议。     

浅淡混凝土裂缝的成因及预防处理

混凝土裂缝有微观裂缝和宏观裂缝两种。微观裂缝的宽度一般在0．05mm以下，主要有三种：即沿着骨料周围出现的骨料与水泥石粘结面上的粘着裂缝；分布于骨料之间水泥浆中的水泥石裂缝和存在于骨料本身的骨料裂缝。宏观裂缝是指宽度不小于0．05mm的裂缝。微裂缝通常是一种无害裂缝，对混凝土的承重、抗渗及其他一些使用功能不产生危害。但是在混凝土受到荷载、温差等作用后，微裂缝就会不断的扩展和连通，最终形成我们肉眼可见的宏观裂缝，也就是混凝土工程中的裂缝。     

极端高温天气对连续梁桥施工的影响分析

某特大桥引桥的一部分采用三跨一联现浇预应力混凝土箱梁结构,在桥面沥青混凝土铺装层施工结束后,在极端温度(40℃)作用下,发现某几个桥梁墩柱出现了横向或斜向裂缝。在对开裂桥墩及桥面进行详细的现场勘查后,针对裂缝的成因进行了模拟计算和分析,预测了已有裂缝对未来成桥运营的影响,同时也为施工单位提出了裂缝处理建议。     

混凝土结构的断裂模型分析

本文介绍了多个评定混凝土裂缝的断裂模型，包括应力强度因子法，能量法，虚拟裂缝模型，裂缝带模型，双参数模型，有效裂缝模型，尺寸效应模型，KR阻力曲线模型，双K断裂模型，双G断裂模型，为混凝土结构的裂缝监控和危害预测提供参考。     

Correlation between tension softening relation and crack extension resistance in concrete

Changes of the material fracture energy consumption with crack propagation can be described by a crack extension resistance curve, one of the fundamental fracture criteria in crack mechanics. Recently, experimental observation of the fracture behavior in concrete was used to develop a new fracture criterion, the crack extension G_R resistance curve, to analyze crack propagation during the entire concrete fracture process. The variation of the crack extension resistance is mainly associated with the energy consumption in the fracture process zone ahead of the stress-free crack tip. The crack extension resistance is then a function of the softening curve, which is a basic mechanical property in the fracture process zone. The relationship between the softening curve and the crack extension G_R resistance curve is then analyzed based on results of three-point bending beams tests. The results indicate that the characteristic points of the crack extension resistance G_R curve is closely related to the characteristic point on used tension softening curve.     

粗骨料和钢纤维对混凝土中裂纹扩展模式的影响

以混凝土断面上粗骨料断裂分数作为评价手段,研究了粗骨料和钢纤维对混凝土中裂纹扩展模式的影响。研究结果表明:对于轻骨料混凝土和高强混凝土,裂纹的扩展模式主要是以穿过粗骨料的方式进行;对于低强混凝土,裂纹的扩展模式主要是以沿过渡区的方式进行。掺入钢纤维后,显著改变了轻骨料混凝土和高强混凝土中裂纹的扩展模式,使得部分裂纹的扩展由穿过粗骨料进行转变为沿过渡区进行;掺入钢纤维后,轻骨料混凝土断面上粗骨料断裂分数降低了39.6%,高强混凝土断面上粗骨料断裂分数降低了26.0%。     

Correlation between tension softening relation and crack extension resistance in concrete

Changes of the material fracture toughness with crack propagation can be described by a crack extension resistance curve, one of the fundamental fracture criteria in crack mechanics. Recently, experimental observation of the fracture behavior in concrete was used to develop a new fracture criterion, the crack extension G _R resistance curve, to analyze crack propagation during the entire concrete fracture process. The variation of the crack extension resistance is mainly associated with the energy consumption in the fracture process zone ahead of the stress-free crack tip. The crack extension resistance is then a function of the softening curve, which is a basic mechanical property in the fracture process zone. The relationship between the softening curve and the crack extension G _R resistance curve is then analyzed based on results of three-point bending beams tests. The results indicate that the characteristic points of the crack extension resistance G _R curve is closely related to the characteristic point on used tension softening curve. __________ Translated from Journal of Tsinghua University (Science and Technology), 2008, 48(3): 316–320 [译自: 清华大学学报(自然科学版)]     

Fracture resistance on aggregate bridging crack in concrete

Fracture toughening exhibited in quasi-brittle materials such as concrete is often mainly related to the action of aggregate bridging, which leads to the presence of a fracture process zone ahead of stress-free cracks in such materials. In this investigation, the fracture resistance induced by aggregate bridging, denoted by G _I-bridging, in the primary focus. In order to quantitatively determine it, a general analytical formula is firstly developed, based on the definition of fracture energy by Hillerborg. After this, we further present the calculated procedures of determining this fracture resistance from the recorded load vs. crack opening displacement curve. Then, both numerical simulations and fracture experiments are performed on concrete three-point bending beams. Utilizing the obtained load against crack opening displacement curve, the value of G _I-bridging at any crack extension as well as the change of G _I-bridging with the crack extension is examined. It is found that G _I-bridging will firstly increase with the development of crack and then stay constant once the initial crack tip opening displacement reaches the characteristic crack opening displacement w ₀. The effects of material strength and specimen depth on this fracture resistance are also investigated. The results reveal that the values of G _I-bridging of different specimens at any crack propagation are strongly associated with the values of fracture energy of specimens. If the values of fracture energy between different specimens are comparable, the differences between G _I-bridging are ignored. Instead, if values of fracture energy are different, the G _I-bridging will be different. This shows that for specimens with different strengths, G _I-bridging will change greatly whereas for specimens that are different in depth, whether G _I-bridging exhibits size effect depends on whether the fracture energy of specimens considered in the calculation of G _I-bridging is assumed to be a size-dependent material parameter.     

斜井水力压裂过程中的缝间干扰力学机理研究

根据斜井水力压裂过程中的多裂缝扩展规律的调研,通过建立不同裂缝缝长和不同裂缝分布关系的模型,将位移不连续方法应用于斜井水力压裂过程中的力学干扰分析研究,同时在裂缝扩展准则上运用了修正了的能量释放率准则(G准则)--F准则进行裂缝间的裂缝扩展影响规律研究。根据不同裂缝缝长和不同裂缝分布关系的内外裂尖缝间干扰因子的模拟计算,结果表明:裂缝间距离越近受干扰程度越大,裂缝间的干扰不但有促进裂缝扩展的干扰也有阻止裂缝扩展的干扰。     

Deformation field and crack analyses of concrete using digital image correlation method

The study on the deformation distribution and crack propagation of concrete under axial compression was conducted by the digital image correlation (DIC) method. The main parameter in this test is the water-cement (W/C) ratio. The novel analysis process and numerical program for DIC method were established. The displacements and strains of coarse aggregate, and cement mortar and interface transition zone (ITZ) were obtained and verified by experimental results. It was found that the axial displacement distributed non-uniformly during the loading stage, and the axial displacements of ITZs and cement mortar were larger than that of coarse aggregates before the occurrence of macro-cracks. The effect of W/C on the horizontal displacement was not obvious. Test results also showed that the transverse and shear deformation concentration areas (DCAs) were formed when stress reached 30%–40% of the peak stress. The transverse and shear DCAs crossed the cement mortar, and ITZs and coarse aggregates. However, the axial DCA mainly surrounded the coarse aggregate. Generally, the higher W/C was, the more size and number of DCAs were. The crack propagations of specimens varied with the variation of W/C. The micro-crack of concrete mainly initiated in the ITZs, irrespective of the W/C. The number and distribution range of cracks in concrete with high W/C were larger than those of cracks in specimen adopting low W/C. However, the value and width of cracks in high W/C specimen were relatively small. The W/C had an obvious effect on the characteristics of concrete deterioration. Finally, the characteristics of crack was also evaluated by comparing the calculated results.     

土坝裂缝开展的无单元法数值模拟

结合压实黏土拉伸状态下的脆性断裂模型和弥散裂缝理论,提出了点插值无单元法模拟土石坝坝体裂缝开展过程的数值计算方法。当压实黏土达到其抗拉强度后,在每个积分点建立各向异性刚度矩阵,并构造了平面应变条件下考虑压实黏土脆性开裂的无单元与有限元耦合的计算模式。开发的结点自动加密技术对坝体的拉应力区域进行自动结点加密,并根据可视准则确定无单元结点的影响域。以最小结点间距为开裂步长,通过对坝体裂缝离心模型试验的模拟计算,检验了脆性断裂模型和无单元算法对土体拉伸裂缝发展过程的适用性。     

类岩石裂纹压剪流变断裂与亚临界扩展实验及破坏机制

进行双轴压缩条件下类岩石裂纹的压剪流变断裂实验,采用双扭试件的常位移松弛法对类岩石材料进行亚临界裂纹扩展与断裂韧度试验。在实验室尺度上证实了类岩石裂纹流变断裂现象的存在,并且得到了翼形裂纹–翼形裂纹贯通、翼形裂纹–原生裂纹贯通和翼形裂纹–翼形裂纹–剪切裂纹贯通的 3 种流变断裂贯通模式。类岩石材料的流变断裂是一种稳定的裂纹扩展,其本质原因是类岩石裂纹的亚临界扩展。以黏弹性断裂力学、流变力学和能量准则为理论依据,推导以应力强度因子、翼形裂纹长度和时间为内变量的相应势函数,建立多种破坏机制的 压剪岩石裂纹的流变断裂 判据和计算模型。 利用 流变断裂 实验对计算模型进行验证,得出裂纹流变 贯通的 理论时间与实验时间较为吻合,当翼形裂纹的扩展方向与最大压应力方向偏离较大时实验结果与理论模型误差较大。提出的计算方法和理论判据为研究岩石裂纹的流变断裂的细观机理及岩体工程流变破坏的宏观机制提供了一个新而实用的研究手段。     

模拟岩体失效全过程的连续–非连续变形体离散元方法及应用

 岩石、混凝土等材料,由于存在细观构造的非均匀性,其损伤–破碎发展过程及破损特征与材料中含有的微裂隙分布相关。基于有限–离散耦合分析的思路,在变形体离散元法的基础上引入基于弹塑性断裂力学的黏聚力模型,在细观单元之间插入界面单元,并对界面单元的刚度等参数进行推导,通过界面单元的起裂、扩展和失效,实现岩体开裂扩展的数值模拟。以岩石单轴压缩、单个颗粒破碎以及边坡失稳3个算例对该方法进行验证,结果表明：所提出的方法能较好地模拟计算裂纹的开裂萌生与扩展,反映岩石介质从小变形到大变形再到破坏的全过程。