【作者】 董伟；

【导师】 吴智敏；

【作者基本信息】 大连理工大学 ， 结构工程， 2008， 博士

【摘要】 混凝土的受力特性与裂缝的发展密不可分,混凝土裂缝发展机理及其定量描述是混凝土结构理论的一个基础性课题。然而,由于混凝土材料的不均质性和非线性,其破坏过程并不像脆性材料那样简单,断裂过程表现为微裂缝萌生、扩展、贯通、直到宏观裂缝的产生而导致混凝土失稳断裂的过程。同时由于结构或荷载的不对称及材料各向异性等原因,实际结构中的裂缝通常是处于复合应力场下,这就使得裂缝尖端附近的应力场不是单纯的Ⅰ型（张开型）、Ⅱ型（滑移型）或Ⅲ型（撕裂型）,而是Ⅰ、Ⅱ型甚至Ⅰ、Ⅱ、Ⅲ型同时存在,这种裂缝称为复合型裂缝。在实际工程中,又以Ⅰ-Ⅱ复合型裂缝最为常见。因此Ⅰ-Ⅱ复合型裂缝的起裂、稳定扩展、失稳扩展全过程及各阶段所对应的断裂参量成为人们普遍关心的内容。针对这些问题,本文通过试验研究和数值模拟,开展了如下的研究工作:1、Ⅰ-Ⅱ复合型裂缝起裂准则的试验研究采用尺寸分别为900×200×200mm、1300×300×200mm、1700×400×200mm、2100×500×200mm、2500×600×200mm,75个共13m³混凝土试件,进行了三点弯曲（纯Ⅰ型）和四点剪切（Ⅰ-Ⅱ复合型）的断裂试验研究。根据所测得的各个试件起裂荷载,确定四点剪切状态下较为合理的荷载布置方式,通过有限元法计算开裂状态下的K_Ⅰ、K_Ⅱ分量,进而提出了与以往不同的起裂断裂准则,并研究了该准则的尺寸效应。2、Ⅰ-Ⅱ复合型裂缝扩展全过程的光弹性试验研究在混凝土断裂机理研究中,特别是观察原型试件的裂缝发展过程,光弹贴片具有实时、直观、准确、便于保存等优点。本文采用光弹贴片法对五种尺寸,共19个试件进行了四点剪切试验研究。通过光弹贴片表面彩色条纹的变化,观测Ⅰ-Ⅱ复合型裂缝的起裂、稳定扩展、失稳扩展全过程;依据光弹贴片的光学参数及试件的基本力学参数,计算了各试件贴片中每一级条纹代表的应变值;得到了各个试件P-Δα曲线,并描述了裂缝扩展过程中裂缝尖端主应变的变化情况。结果表明,对于尺寸相同的一组试件,随着Ⅱ型分量的增加,起裂荷载逐步增大,且临界裂缝扩展量有增大的趋势;对于开缝位置相同,尺寸不同的试件,随着尺寸的增大,起裂荷载相应增加。3、Ⅰ-Ⅱ复合型裂缝扩展全过程的数值分析首先提出了Ⅰ-Ⅱ复合型裂缝扩展准则,即当外荷载引起的裂缝尖端应力强度因子与粘聚力引起的裂缝尖端应力强度因子的差值满足起裂断裂方程时,裂缝开始扩展。结合根据试验结果拟合的起裂断裂准则,借助ANSYS有限元计算软件对复合型断裂整个裂缝扩展过程进行数值模拟。根据以上算法,可以在已知起裂准则、混凝土抗拉强度、弹性模量、泊松比的条件下,完整模拟Ⅰ-Ⅱ复合型断裂全过程,获得P—CMSD全曲线及起裂荷载、最大荷载P_max、裂缝临界扩展量△a_c,预测裂缝扩展轨迹。该方法也可应用于Ⅰ型断裂过程的数值分析,得到P—CMOD全曲线、起裂荷载、最大荷载P_max、裂缝临界扩展量△a_c等断裂参量及K_R阻力曲线。两种断裂形式的数值结果均与试验结果进行了对比,吻合良好,验证了所提出计算方法的可行性。更多还原

【Abstract】 The mechanical behaviors of concrete are closely associated with crack propagation and the research on crack propagation theory has become a foundational subject for concrete. However,owing to the heterogeneities and non-linear behaviors of concrete,different from the ideal brittle materials,the fracture process of concrete is complex and shows three typical stages:crack initiation,crack stable propagation and unstable propagation.In addition,due to the asymmetry of the structure or load and the anisotropic behaviors of the materials,the crack in structure is usually under combined stress condition,which means the stress field at the crack tip is not absolutely classified as modeⅠ(opening),or modeⅡ(sliding),or modeⅢ(tearing),but a combined effect of both modeⅠand modeⅡ,or even that of the three.This kind of crack is defined as mixed mode crack.In engineering,theⅠ-Ⅱmixed mode crack occurs more frequently.Therefore,the whole process of crack initiation,stable propagation and unstable propagation of the mixed mode fracture,as well as its corresponding fracture parameters,has been widely concerned.Then the work is done in this dissertation using experimental and numerical simulation methods as follows:1、Experimental investigation on initial cracking criterionWith the total number of 75 specimens for concrete fracture,13m~3 in volume,and 900×200×200mm,1300×300×200mm,1700×400×200mm,2100×500×200mm, 2500×600×200mm respectively in sizes,the fracture tests of three-point bending beams and four-point shearing beams were carried out.Based on the initial load measured in the test, the corresponding K_Ⅰand K_Ⅱcan be calculated using finite element method.Besides,a more rational load distribution under four-point shearing condition was obtained and the initial cracking criterion for the specimens with five different sizes is given.Finally,a crack initial cracking criterion forⅠ-Ⅱmixed mode crack is proposed and its size effect is discussed.2、Study on crack propagation process ofⅠ-Ⅱmixed mode crack in concrete by means ofphotoelastic coatingsFor the research of the concrete fracture theory,especially for the observation of the crack propagation in specimens,the photoelastic coatings have many advantages,such as lossless,live,intuitive,precise,easier to reserve,etc.19 four-point shearing specimens with five different sizes were utilized to systematically investigate the crack initiation,stable propagation and unstable failure of concrete using photoelastic coatings.The complete crack propagation process of concrete appeared in the form of interference fringes on the photoelastic coatings and was recorded using camera.On this basis,the propagation lengths in different loading stages were obtained.The whole process of stable propagation and unstable development of the main crack was recorded by photographic pictures,and the strain distribution around the crack tip was also obtained based on the colored strain sequence.Test results show that both the initial load and the critical crack propagation length are increased with the increasing of theΠmode portion for those specimens with the same size.While for those specimens with the same crack position but different sizes,the initial load is increased with the increasing of the size.3、Numerical simulation of crack propagation forⅠ-Ⅱmixed mode in concreteA crack propagation criterion is proposed.Once the difference between the stress intensity factor caused by the applied load and the one caused by the cohesive force satisfies the initial fracture equation,the crack begins to propagate.Based on the method,only if the uniaxial tensile strength,the elastic modulus,poisson’s ratio and the initial fracture toughness of concrete are obtained,the whole process ofⅠ-Ⅱmixed mode fracture propagation,the complete P-CMSD curve,the initial load,the maximum load,the critical crack propagation length and the crack trajectory in concrete can be given respectively using the proposed method.The same method can be applied into numerical simulation ofⅠmode fracture,and hence the complete P-CMOD curve,the initial load,the maximum load,the critical crack propagation length and the KR-curve can be also obtained.Upon the comparison with the experimental results,the calculated ones show good agreement for these fracture parameters. Therefore,the proposed method in this dissertation is feasible.更多还原