【作者】 张秀芳；

【导师】 徐世烺；

【作者基本信息】 大连理工大学 ， 结构工程， 2007， 博士

【摘要】 对混凝土乃至钢筋混凝土结构来说,其受力破坏过程与裂缝的蔓延滋生有着密切的关系,是一个裂缝扩展问题。断裂力学作为专门研究带裂缝固体强度及裂缝扩展规律的科学,它应该是一种有效且合理的分析混凝土结构响应的工具。然而,由于混凝土的不均匀性和非完全线性行为,其断裂破坏的过程并不像理想脆性材料简单,表现为裂缝起裂,稳定扩展,失稳扩展三个阶段。构成混凝土的骨料组份所发挥的粘聚咬合作用,造成了裂尖非线性断裂过程区的出现及发展,被认为是混凝土非线性断裂破坏主要的物理力学机理。结合这些断裂特征,迄今以应力强度因子为工具已建立了一些典型的断裂模型,很好地描述了混凝土的裂缝稳定扩展。然而,以能量为出发点定量分析整个断裂全过程材料的裂缝阻止能力以及能量的相互转化关系的工作却远远不够。鉴于此不足,本文尝试从能量的观点来进一步理解混凝土整个断裂过程所表现的断裂行为。1.裂缝扩展阻力曲线很好地描述了随裂缝扩展材料裂缝抵制力的变化,是判定裂缝在其任意扩展时刻稳定与否的一种断裂准则。本文首先回顾了Irwin在上世纪六十年代提出的传统裂缝扩展阻力曲线概念以及在此概念基础上发展的两种确定R阻力曲线的半解析半经验方法,然后结合混凝土断裂的现象学观测,在两个基本假设的基础上从复合材料的观点构建了描述裂缝扩展阻力变化的G_R阻力曲线模型。在这个模型中,认为裂缝的扩展阻力由两部分组成:一部分是基体水泥凝胶材料的贡献,称为起裂韧度;另一部分是骨料咬合作用的贡献,称为骨料粘聚韧度。对于前者,根据Griffith脆性断裂理论,其大小是一常数,认为接近于水泥净浆材料的断裂韧度。但对于后者,随着裂缝的扩展是变化的。接着根据Hillerborg虚拟裂缝模型,本文给出了局部断裂能的计算公式及物理力学解释。在此基础上,使用Reinhardt非线性软化本构,根据等效裂缝长度a及其特征长度a_w0的大小,分两种情况着重推导了骨料粘聚韧度计算的解析公式,从而给出了材料整个断裂过程中裂缝扩展阻力的表达式。2.浇注了最大尺寸分别为2000mm×500mm×200mm的三点弯曲梁和1440mm×1200mm×200mm的楔入劈拉试件共七个立方米的混凝土断裂试件,并对完好无损试件完成了断裂试验。根据本文构建的G_R阻力曲线模型,针对试验的两种几何形式,详细给出了确定任意裂缝扩展时刻裂缝扩展阻力的步骤和相应的计算公式。使用试验获得的P-CMOD曲线,得到了G_R阻力曲线和动力曲线,观察了G_R阻力曲线的基本特征。通过比较荷载裂缝嘴张开位移曲线,解释了它们相交点的物理意义。并讨论了G_R阻力曲线的尺寸效应和形状效应问题以及与使用软化本构曲线的相关性。3.软化曲线描述了裂尖非线性断裂过程区的物理力学行为,不同的软化曲线对骨料粘聚性能的定量不尽相同,由此而确定的G_R阻力曲线也有所差异。为此,本文采用广泛使用的双线性软化曲线和非线性软化曲线,在假定Hillerborg定义的断裂能相同的情况下,以数值模拟的三点弯曲梁为研究对象,计算了G_R阻力曲线,研究了不同软化曲线对其结果的影响。4.类似于理想脆性材料,混凝土的断裂过程也是一个能量相互转化的过程。在这个断裂过程中,外力功主要以弹性应变能和断裂表面能两种方式所消耗。在以往的研究中,大多针对的是试件断裂的最后时刻,在这个时刻外力功被假设完全被断裂窄条所吸收,而对裂缝发展的其它时刻却很少涉及。鉴于此,本文使用试验的楔入劈拉试件结果,研究了裂缝扩展的整个过程中弹性应变能和断裂表面能的变化规律,从而进一步深入地理解了混凝土的断裂行为。同时,计算了裂缝扩展任意时刻形成单位面积裂缝所需要的能量,对获得的结果进行了分析讨论。更多还原

【Abstract】 For concrete or reinforced concrete structures,their failure process when subjected to external load is intimately associated with the appearance and propagation of crack and hence is regarded as a process of crack development.Fracture mechanics,where the strength of cracked solid and the general rules of crack development in cracked solid are major-cared, should be an effective and reasonable tool to analyze the mechanical response of cracked structures.However,owing to the heterogeneity of concrete and non-linear behavior exhibited in concrete,different from the ideal brittle materials,the fracture process of concrete material is complex and shows the typical three stages:crack initiation,crack stable propagation and unstable propagation.It had been commonly accepted that the bridging cohesive property of aggregate reinforcements is primary governing mechanism which results in the presence and development of fracture process zone（FPZ） and thereby resulting non-linear fracture behavior. So far,after considering the influence of fracture process zone on concrete fracture behavior, many fracture models applicable for concrete had been proposed in last several years. However,most of models were based on stress intensity factor approach not energy approach. It is very reason that the researches on the crack resistance determined using energy approach as well as studies regarding the transformed relationship between different energy consumption patterns during the whole fracture process in concrete are rather lack.As a consequence,in this paper the energy approach is attempted with the major purpose of obtaining further deeper understanding of fracture behaviors exhibited in concrete.1.Crack extension resistance curve better describes the change of crack resistance capacity of material with crack development and is used as a criterion to judge whether crack is stable at certain crack propagation.In this paper,firstly,a state-of-the-art retrospection on the conception of traditional crack extension resistance curve proposed by Irwin in 1960s and two different semi-analytical and semi-experimental methods to construct R-curve are made.Then,combining with the phenomenology observations from fracture experiments of concrete,and based on the two basic assumptions,new G_R crack extension resistance curve model which describes the change of crack resistance with crack propagation is constructed from the view of the composite materials.In this model, the crack extension resistance is composed of two parts.One part is contribution produced by hardening cement-gel matrix and is called mitiation fracture toughness. Another part is contribution caused by bridging cohesive action of aggregates and is named as cohesive toughness.For the front,according to the Griffith’s brittle fracture theory,the value is a constant and is assumed to be equal to the fracture toughness of cement paste.While for the latter,the value is changeable.To determine it,the calculation equation of local fracture energy is firstly deduced based on the Hillerborg’s fictitious crack model.After this,using Reinhardt’s non-linear softening curve,the two different equations for evaluating cohesive toughness are obtained respectively corresponding to two different cases defined in terms of the values of crack length a and crack characteristic length a_w0.Finally,the equations for crack extension resistance at any time during the whole fracture process are obtained.2.A total of 7m³ concrete fracture specimens are cast.Of which,two typical geometries,i.e. three-point bending beam（TPB） and wedge-splitting（WS）,are used.The maximum dimension for TPB and WS is 2000mm×500mm×200mm and 1440mm×1200mm×200mm,respectively.Fracture tests are performed on the intact concrete specimens.Then, according to the proposed G_R crack extension resistance curve model,the detailed calculation steps and corresponding calculation equations are further introduced for two geometries,respectively.Using the obtained P-CMOD curve,G_R crack extension resistance curve and crack driving curve are determined for every tested specimen.The basic features shown on G_R crack extension resistance curve are pointed out.Moreover, the implied physical meaning behind the intersection points of crack resistance curve and crack driving curve is clarified by comparing them with the P-CMOD curve.At last,the size effect and geometry effect of new G_R crack extension resistance curve together with the correlation between it and used softening curve are discussed.3.Softening curve gives a description of physical mechanical behavior occurred in non-linear fracture process zone ahead of crack tip.It can be said that the bridging cohesive properties determined by the different softening curve are distinguished.Hence, the calculated G_R crack extension resistance curve will show some differences,too.In this paper,the bilinear softening curves and non-linear softening curve are adopted.Also, the fracture energy determined from them according to the definition by Hillerborg is assumed to be a constant in all used softening curves.In such case,G_R crack extension resistance curves for seven numerical simulated TPB are computed.The impact of different softening curves on the obtained G_R crack extension resistance curves is highlighted.4.Similar to ideal brittle materials,the fracture process of concrete is a process of energy transformation,too.During this process,the work done by external load is consumed in two prime manners,i.e.,elastic strain energy and fracture surface energy.However,last extensive studies put more interests in the finally completely fracturing time at which the external work caused by applied load is assumed to totally flow into fracture narrow zone. But,for others any crack propagation time,the related studies are seriously few.Because of this,in the present paper,the emphasis of the work is put on the whole fracture process. Then,two kinds of energy consumption of external force work,namely,elastic strain energy and fracture surface energy at any picked crack propagation are determined using the data obtained from wedge-splitting experiments.The calculated results provide new insights into the understanding of concrete fracture behavior.Meanwhile,the energy needed for creating unit area crack propagation at any crack propagation is computed,too. The corresponding discussions are made.更多还原