准脆性材料黏聚阻裂的计算与实验

工程常用材料铸铁混凝土等,在裂缝前端区域有微观及宏观缺陷的变化及能量耗散,表现为准脆性损伤断裂特点.若把该阻止裂缝扩展与张开的损伤发展区简化成虚拟裂纹,将存在裂纹黏聚分布力与变形或张开位移;讨论了与此相关的黏聚裂纹力学模型及方程解答说明.相应于黏聚张开位移有微细观测图与实验测试数据,给出黏聚裂纹模型的解析计算方法,并将理论计算与实验测试数据进行了对比.以双K断裂准则和黏聚模型为基础,给出对含裂纹结构的承载力的预估计算.理论计算比较符合实验结果.     

混凝土断裂过程的力学模型与数值模拟

混凝土是一种典型的非均匀准脆性材料,其断裂过程是非常复杂的.评述了混凝土断裂以及断裂过程研究的进展状况,主要总结了一些研究混凝土断裂的宏观力学方法和模型,介绍了从混凝土的细观层次出发模拟其断裂过程的数值模型.最后,简单介绍了作者用细观数值方法模拟混凝土宏观断裂过程的研究成果.      

双材料界面断裂力学模型与实验方法

纤维增强聚合物(FRP)质轻、高强, 可提高结构的刚度、强度、抗震性能和耐久性, 近年来在结构加固及工程改造中得到广泛应用. FRP与传统复合材料之间形成双材料黏结界面, 界面断裂特性是决定双材料结构性能的关键因素. 对双材料界面裂纹尖端应力场理论、界面裂纹模型、黏结界面I型、II型及混合型断裂试验及理论研究现状进行综合评述和分析. 界面模型主要有经典梁/板理论和刚性节点模型、考虑剪切变形的双亚层理论和半刚性节点模型、基于双亚层理论的柔性节点模型、考虑剪切变形的多层亚层理论和多亚层柔性节点模型、弹性地基梁模型以及黏聚模型. 还介绍了双材料界面断裂力学在FRP-混凝土研究中的应用.      

混凝土双K断裂参数计算的半解析有限元法

混凝土裂缝扩展的双$K$断裂准则，用于描述混凝土结构裂缝的起裂、稳定扩 展和失稳断裂. 其相应的双$K$断裂参数(起裂断裂韧度$K_{\rm IC}^{\rm ini} $和失 稳断裂韧度$K_{\rm IC}^{\rm un}$)一般通过简便的试验和基于虚拟裂缝扩展粘 聚力的解析方法确定. 利用平面扇形域哈 密顿体系的方程，通过分离变量法及共轭辛本征函数向量展开法，以解析的方法推导出基于混 凝土虚拟裂缝扩展线性粘聚力模型的平面裂缝解析元列式. 将该解析元与有限元相结合，构成 半解析的有限元法，可求解任意结构几何形状的混凝土平面裂缝双$K$断裂参数的计算问题. 数值计算结果表明半解析有限元法对该类问题的求解是十分有效的.     

基于扩展有限元的地质聚合物混凝土断裂过程研究

扩展有限元法是基于常规有限元框架分析裂纹等不连续力学问题的一种有效数值方法,在常规的有限元位移表达式中,增加了能够反映位移不连续性的跳跃函数和渐进缝尖位移场函数来对不连续结构附近的节点自由度进行局部加强。本文介绍了扩展有限元法及粘聚力模型的基本原理,给出了基于扩展有限元法的地质聚合物混凝土断裂过程分析方法。分别采用四种不同的软化曲线对I型缺口地质聚合物混凝土梁从裂纹萌生、扩展直至断裂破坏的全过程进行了模拟,并基于双K断裂准则分析了其断裂韧性。结果表明,Petersson模型与试验结果吻合较好,最后基于模拟结果进一步揭示了断裂过程区的演化过程。     

内聚力模型的形状对胶接结构断裂过程的影响

内聚力模型被广泛应用于粘接结构的断裂数值模拟过程中,为深入分析不同形状内聚力模型与胶黏剂性质和粘接结构断裂之间的关系,本文分别采用脆性和延展性两种类型胶黏剂,对其粘接的对接试件进行了单轴拉伸、剪切实验,以及其粘接的双臂梁试件进行了断裂实验.3种类型的内聚力模型(抛物线型、双线型和三线型)分别模拟了以上粘接结构的断裂过程,并与实验结果进行对比.结果发现:双线型的内聚力模型适用计算脆性胶黏剂的拉伸与剪切的断裂过程;指数型内聚力模型较适合计算延展性胶黏剂的拉伸和剪切的断裂过程,临界应力、断裂能和模型的形状参数是分析拉伸和剪切的重要参数;双臂梁试件的断裂过程模拟结果发现,断裂曲线与胶黏剂性质有关,内聚力模型形状参数也有影响.通过实验与计算结果分析,双线型内聚力模型更适合脆性胶黏剂粘接的双臂梁断裂计算,而三线型更适合计算延展性胶黏剂粘接的双臂梁断裂过程,此研究结果对胶黏剂的使用和粘接结构的断裂分析有很重要意义.     

混凝土Ⅰ-Ⅱ复合型裂纹断裂准则研究方法概述

I—II复合型断裂是混凝土断裂中最常见的一种形式．断裂力学基本上有两种分析裂缝稳定性的方法:应力强度因子法和能量法,在混凝土断裂性能的描述上两者应该是等效的．本文介绍了不同种断裂准则的基本假定、判别标准、适用范围,并与实际试验结果相比较,指出各种断裂准则的优缺点．     

双K断裂模型粘聚韧度K_(Ic)~c实用插值计算方法

为了配合<水工混凝土断裂试验规程>在工程界的推广使用,提出了双K断裂模型中在临界状态时分布在虚拟裂缝上的粘聚力σ(x)产生的粘聚韧度K_(Ic)~c的二元拉格朗日插值计算方法,进而由准脆性材料韧度三参数定律计算了混凝土起裂韧度K_K~(ini).与粘聚韧度K_(Ic)~c积分计算和简化计算方法的比较,表明此插值计算方法具有很高的精度,计算过程简化,使用计算器即可实现,便于工程人员实际使用.     

木材Ⅰ/Ⅱ复合型断裂破坏研究进展

复合型应力状态是木材断裂破坏的主要受力模式,本文从复合断裂模型和试验方法两个方面回顾了木材复合断裂研究的发展历程.在介绍了木材复合断裂的经验模型和物理模型后,通过一组试验数据进行了这些断裂模型的比较,总结了各自的特点.同时,总结了确定这些模型而进行的各类试验方法,主要是Ⅰ/Ⅱ复合断裂模式,即断裂模式Ⅰ和断裂模式Ⅱ均存在的情况.最后阐述了木材复合断裂研究中的一些问题,并对今后的研究方向提出了建议.     

双K断裂模型粘聚韧度KcIc实用插值计算方法

为了配合《水工混凝土断裂试验规程》在工程界的推广使用,提出了双K断裂模型中在临界状态时分布在虚拟裂缝上的粘聚力σ(x)产生的粘聚韧度KcIc的二元拉格朗日插值计算方法,进而由准脆性材料韧度三参数定律计算了混凝土起裂韧度KiInci。与粘聚韧度KcIc积分计算和简化计算方法的比较,表明此插值计算方法具有很高的精度,计算过程简化,使用计算器即可实现,便于工程人员实际使用。     

Analytical representation of the non-square-root singular stress field at a finite angle sharp notch

The stress field near the tip of a finite angle sharp notch is singular. However, unlike a crack, the order of the singularity at the notch tip is less than one-half. Under tensile loading, such a singularity is characterized by a generalized stress intensity factor which is analogous to the mode I stress intensity factor used in fracture mechanics, but which has order less than one-half. By using a cohesive zone model for a notional crack emanating from the notch tip, we relate the critical value of the generalized stress intensity factor to the fracture toughness. The results show that this relation depends not only on the notch angle, but also on the maximum stress of the cohesive zone model. As expected the dependence on that maximum stress vanishes as the notch angle approaches zero. The results of this analysis compare very well with a numerical (finite element) analysis in the literature. For mixed-mode loading the limits of applicability of using a mode I failure criterion are explored.     

Theoretical development and experimental validation of a thermally dissipative cohesive zone model for dynamic fracture of amorphous polymers

A thermally dissipative cohesive zone model is developed for predicting the temperature increase at the tip of a crack propagating dynamically in a nominally brittle material exhibiting a cohesive-type failure such as crazing. The model assumes that fracture energy supplied to the crack tip region that is in excess of that needed for the creation of new free surfaces during crack advance is converted to heat within the cohesive zone. Bulk dissipation mechanisms, such as plasticity, are not accounted for. Several cohesive traction laws are examined, and the model is then used to make predictions of crack tip heating at various crack propagation speeds in the nominally brittle amorphous polymer PMMA, observed to fail by a crazing-type mechanism. The heating predictions are compared to experimental data where the temperature field surrounding a high speed crack in PMMA was measured. Measurements are made in real time using a multi-point high speed HgCdTe infrared radiation detector array. At the same time as temperature, simultaneous measurement of fracture energy is made by a strain gauge technique, and crack tip speed is monitored through a resistance ladder method. Material strength can be estimated through uniaxial tension tests, thus minimizing the need for parameter fitting in the stress-opening traction law. Excellent agreement between experiments and theory is found for two of the cohesive traction law temperature predictions, but only for the case where a single craze is active during the dynamic fracture of PMMA, i.e. crack tip speed up to approximately 0.2c_R. For higher speed fracture where subsurface damage becomes prominent, the line dissipation model of a cohesive zone is inadequate, and a distributed damage model is needed.     

轻质泡沫混凝土的劈裂破坏力学行为分析

利用平台巴西圆盘加载方式和钢质压条加载方式,对两种厚度为25mm和50mm、不同密度的轻质泡沫混凝土(400～1000kg/m3)进行巴西圆盘劈裂试验,研究密度和厚度对泡沫混凝土裂纹宽度、劈裂强度、断裂韧度、断裂能的影响规律。结果表明,在橡胶垫平台巴西圆盘和钢质压条加载方式下,其劈裂断裂特征大致分为四个阶段:线性弹性段、非线性弹性段、起裂阶段、失稳阶段。同样加载率下最大裂纹宽度随着泡沫混凝土密度增加逐渐减小,劈裂拉伸强度、断裂韧度、断裂能呈幂函数形式增加。借鉴Reinhardt非线性软化曲线,对不同密度泡沫混凝土的应力软化关系进行曲线拟合,建立基于拉伸强度、断裂韧度等控制参数的应力-裂纹宽度关系三段式模型。基于试验结果,对理想多孔材料细观力学预测模型进行修正,获得泡沫混凝土孔隙率与拉伸强度的半经验公式。     

混凝土拉伸软化曲线折线近似的逆解方法

研究基于Hillerborg的虚拟裂纹模型，利用有限元分析方法，求得折线近似的拉伸软化曲线的逆解方法。对弹性模量，初始开裂应力的决定方法进行了研究。以双直线模型的计算结果为算例进行了逆推分析，算例符合得很好。也较好地从实验得到的荷载位移曲线再现了拉伸软化曲线。这对于研究混凝土的断裂能，尺寸效应等问题很具意义。     

Molecular-dynamics simulation-based cohesive zone representation of intergranular fracture processes in aluminum

A traction-displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics (MD) simulations. An MD model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat Σ99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with very little or no dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is consistent with the Rice criterion for cleavage vs. dislocation blunting transition at the crack tip. The preference for twinning to dislocation slip is in agreement with the predictions of the Tadmor and Hai criterion. A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a Cohesive-Zone-Volume-Element—an atomistic analog to a continuum cohesive zone model element—the results from the MD simulation are recast to obtain an average continuum traction-displacement relationship to represent cohesive zone interaction along a characteristic length of the grain boundary interface for the cases of ductile and brittle decohesion.     

Intersonic crack propagation in homogeneous media under shear-dominated loading: theoretical analysis

The mechanics of cohesive failure under mixed-mode loading is investigated for the case of a steadily propagating subsonic and intersonic dynamic crack subjected to a follower tensile and shear distributed load. The cohesive failure model chosen in this study is rate independent but accounts for the coupling between normal and tangential damage. Special emphasis is placed here on mixed-mode cases with predominantly shear loading. The analysis shows that the size of the mixed-mode cohesive zone is smaller than that obtained in the pure shear case. The relative extent of the shear and tensile cohesive damage zones depends on the crack speed and the mode mixity. In the intersonic regime, the failure process takes place exclusively in shear, even under remote mixed-mode loading conditions.     

分子动力学模拟含不同初始缺陷单晶镍的断裂行为与应力演化

断裂是一个跨尺度复杂的物理过程,对宏观尺度的断裂行为已有深入的研究和发展,然而对微观尺度的断裂行为及断裂过程中应力场的变化缺乏深入的理解。本文通过分子动力学模拟,研究了具有不同初始缺陷(尖锐裂纹、钝裂纹和孔洞)的单晶镍的断裂行为和应力分布特征。结果表明,不同的初始缺陷导致了不同的断裂机制、断裂强度和抗断裂性能。含初始孔洞的单晶镍样品有最高的断裂强度和最强的抗断裂性能,这与孔洞扩展过程中堆积层错的形成密切相关。其次是含初始钝裂纹的样品,在裂纹扩展过程中出现由[100]超位错发射引起的裂尖钝化;含尖锐裂纹的样品表现为脆性断裂,裂尖原子没有出现微结构的变化,其强度和抗断裂性能最低。此外,不同的初始缺陷也会导致断裂过程中应力分布的变化,对含有尖锐裂纹的脆性断裂试样,高应力(拉伸应力、平均应力和米塞斯应力)总是出现在扩展裂纹的裂尖。而对于含有钝裂纹或孔洞的韧性断裂试样,高应力不仅分布在裂尖,也分布在位错发射和堆积层错形成的区域,在裂纹/孔洞扩展之前,应力随着加载时间的增加而迅速增加,而一旦裂纹或孔洞开始扩展,应力增加非常缓慢或几乎不增加,但拉伸应力值始终大于平均应力和米塞斯应力值。这表明,在I型...     

拉伸载荷下混凝土体分形内聚模型

在综合考虑混凝土试件微断裂面分形分布及断面分形演化基础上,提出了体分形内聚模型,用于描述混凝土试件在准静态拉伸载荷作用下的破坏行为。数值计算结果与现有实验数据吻合较好。此外,采用该模型分析了混凝土骨料级配对材料软化性质的影响。结果表明,在拉伸载荷作用下,混凝土骨料级配越均匀,材料的软化特征越明显。     

An isolated cohesive crack in tension

The shape of the back-calculated stress-separation curves obtained from the in-situ fracture of first-year sea ice in the Arctic and Antarctic is convex, and radically different from those for concrete. In these tests, the process zone size changes with crack growth, but the nature of this change differs with the test conditions, load-control or displacement control. These results prompted a closer examination of the cohesive crack model using the simplest cracked configuration, a finite cohesive crack in an infinite elastic medium loaded in tension by a uniform stress at infinity. Different types of strain softening are examined: rectangular softening, linear softening, prescribed cohesive stresses, and prescribed cohesive crack-opening displacements. For each of these cases, crack nucleation is examined; close attention is paid to test control conditions, be they load-control or fixed-grip. The test control conditions alter the fracture, crack nucleation, crack growth, and process zone size behavior significantly. Accurate approximate solutions to linear softening are presented and examined.