【作者】 刘问；

【导师】 徐世烺；

【作者基本信息】 大连理工大学 ， 结构工程， 2012， 博士

【摘要】 超高韧性水泥基复合材料(Ultra-high toughness cementitious composites,简称UHTCC)是由水泥、水、增强纤维、精细骨料、以及粉煤灰和硅灰等活性矿物掺合料组成的新型工程材料,具有拉伸应变硬化能力与多缝开裂特征。该材料的静态力学性能与耐久性十分突出,由此,也寄希望于该材料能够改善地震、车载等动荷载作用环境中的混凝土结构,这就有必要对其动态力学性能进行研究。本文结合国家自然科学基金重点项目(50438010)和南水北调工程建设重大关键技术研究及应用(JGZXJJ2006-13),对UHTCC在冲击与疲劳荷载下的性能进行了试验研究,同时,对其疲劳裂纹扩展性能及理论基础——断裂性能进行了探索性的计算分析。论文主要研究内容如下：(1)研究了UHTCC的抗冲击性能,结果显示,UHTCC冲击试件产生的大量裂缝能有效延缓主破坏面的出现,提高其抗冲击能力,防止试件离析。对UHTCC的弯曲疲劳性能进行了试验研究,得出：在弯曲疲劳荷载作用下,UHTCC仍表现出多缝开裂特征,随应力水平的降低,裂缝数目减少,二者之间近似呈双线性关系；UHTCC试件的疲劳破坏为延性破坏,其S-N曲线具有双线性特征,原因是PVA纤维在高应力水平下发挥作用高于低应力水平的情况,且疲劳荷载下纤维的破坏方式与金属质材料类似。(2)研究UHTCC在弯曲疲劳荷载下的损伤模型,UHTCC的疲劳损伤为韧性损伤,可采用试件底面塑性应变εp作为损伤变量,计算损伤量D,基于弹塑性各向同性损伤模型,建立UHTCC的疲劳损伤演变方程。(3)对预制单边切口UHTCC梁的弯曲断裂性能进行了试验研究,并基于非线性断裂力学,采用J积分方法评价其断裂韧性。结果表明,可用开裂与失效J积分(JIC-JIF)作为评价UHTCC断裂韧性的两个指标,当流入预制切口前端塑性区的能量大于JIC时,初始裂缝出现,随后,裂缝稳定扩展,当流入塑性区的能量大于JIF时,主裂缝出现,材料进入失稳发展阶段。用宏观裂缝覆盖面积发展量△A代替单裂缝的长度扩展量△a作为描述UHTCC裂缝发展的参量,研究其JR阻力曲线：在主裂出现前,UHTCC的J-AA存在两阶段线性关系,分界点即为宏观裂缝出现点；在稳定变形阶段,UHTCC的J-△A呈线性关系,即相同面积的裂纹发展量消耗的能量相同。(4)基于描述疲劳裂纹扩展规律的Paris公式,对UHTCC的疲劳裂纹扩展性能进行了探讨性研究,用宏观裂缝覆盖面积A作为描述其裂缝发展的参量,并通过试验验证其适用性。结果表明,疲劳荷载作用下,预制单边切口UHTCC弯曲试件有多条裂缝产生,其位置、形状、长度皆随机分布,破坏时组成一个从切口根部开始发散最终汇集到加载点处的橄榄球形状；UHTCC疲劳裂纹扩展速率的计算公式为dA/dN=C(△J)m,随纤维掺量的增加,其疲劳裂纹扩展速率呈递减趋势,此外,纤维掺量对疲劳裂纹扩展速率的影响随J积分值增大而愈加明显；UHTCC存在疲劳裂纹扩展门槛值,即当疲劳过程中的疲劳荷载产生的断裂能小于某一临界值△Jth时,疲劳裂缝不扩展。(5)对UHTCC/混凝土(UC)复合梁进行三点弯曲疲劳试验,有以下发现：疲劳荷载作用下,UC梁跨中截面变形符合平截面假定,高应力水平下UC复合梁的受压区高度要小于低应力水平下相应荷载循环时的受压区高度,同一应力水平下,受压区高度随荷载循环次数的增加而降低,疲劳破坏时的受压区高度约占截面高度的0.05-0.10；复合梁的UHTCC层产生若干条可见裂缝,数目随应力水平降低而减少,混凝土层裂缝数目为1-3条,复合梁的疲劳变形曲线表现出延性特征,随荷载循环率呈现三阶段发展,且变形能力随应力水平减小而降低；分析其疲劳过程可知,UC复合梁的疲劳破坏是由UHTCC层的完全损伤所致,应用文中回归的UHTCC的疲劳损伤方程计算的复合梁变形曲线略小于试验测量值,尤其在低应力水平下吻合良好。更多还原

【Abstract】 Ultra-high toughness cementitious composites (UHTCC) is a new type of engineering material which is composed of Portland cement, fiber, water, fine aggregate, and active mineral admixtures such as fly ash, silica fume and so on. The static mechanics and durability of this material are outstanding, due to its typical characteristics of strain hardening and multiple cracking under uniaxial tension. Then it is hoped that UHTCC can be used to improve the performance of concrete structures, which are under dynamic and cyclic loads, e.g. seismic load, burials load, vehicle load, et al. As a result, it is necessary to investigate the dynamic mechanics of this material. For this purpose, this thesis took an experimental study on the performances of impact resistance and flexural fatigue of this material. Moreover, exploratory research on its fatigue crack propagation law and the theoretical basis, fracture property, was carried out based on some experiments. The research is supported by the Key Program of the National Natural Science Foundation of China (No.50438010) and the Research and Application Program of Key Technologies for Major Constructions in the South-North Water Transfer Project Construction in China (JGZXJJ2006-13). The detailed contents of this thesis are as follows(1) The impact resistance property of UHTCC was studied. The results showed that multiple cracks were generated in UHTCC impact specimens, which delayed the development of the failure crack, prevented the fragmentation, and improved the whole ability of impact resistance. Four-point fatigue bending tests were conductedto study its flexural fatigue property. Multiple cracks were produced during the fatigue progress, and the total number of cracks dropped with the decrease of fatigue stress levels, for which, a bi-linear relationship was found. Due to these multiple cracks, a ductile fatigue failure exhibited for UHTCC, and its S-N bi-logarithm relation was found to be bi-linear, which proved that the effect of PVA fibers was less obvious at low stress levels.(2) The damage mode of UHTCC under flexure fatigue was investigated. A ductile damage happened for UHTCC. The plastic tensile strain, εp, was adopted to calculate the damage quantity, D. Then a fatigue damage evolution function of UHTCC was constructed, on the basis of elastic-plasitic isotropic damage model.(3) Three-point flexural fracture tests were carried out on UHTCC specimens with a single notch, and the non-linear fracture mechanics was employed here to study the fracture property of this material. Through analysis, the double J integral, initial cracking value, JIC, and failure value, JIF, could be used to evaluate UHTCC’s fracture toughness. That was, macrocacks began to develop if the energy absorbed by the plastic zone was larger than while the failure crack began to develop if that was larger than JIf. The growth of macrocracking coveraging area, AA, here was selected here as the parameter to describe the development of multiple cracks. Based on the experimental results, a simplified JR resistance curve was introduced. From this curve, a good bi-linear relationship existed between J integral and△A before the failure crack initiated, with JIc as the dividing point. Consequently, it could be considered that the energy in need for the same amount of cracking area propagation was equal during the stable developing stage.(4) Based on the Paris law of fatigue crack propagation rate, a theoretical and experimental investigation was undertaken to study the fatigue crack propagation mechanism of UHTCC. During the experiment, it was observed that multiple cracks were formed, with a random distribution of their locations, shapes and lengths. According to some discussion, Paris law was applicable for this material, with the function form as, dA/dN=C(△J)m, in which, the coveraging area of multiple cracks, A, was applied in instead of the length of a single crack, a; meanwhile, the J integral substituted for the stress singularity intensity, K. A fatigue crack propagation threshold was found in the experiment, that is to say, the cracks did not develop while the fatigue range of J integral value,△J, was smaller than the critical value,△Jth. It was found that, the fatigue crack propagation rate slowed down with the increases of PVA fiber fraction. Furthermore, the influence of PVA fiber on the propagation rate was found to become obvious with the increase of J integral.(5) The flexural fatigue tests on UHTCC/concrete composite beam (UC composite beam) were carried out to study the effects of UHTCC layer on concrete layer. The results showed that, the section deformation of the composite beam fitted to the plane section assumption for both static and fatigue loading. In addition, the depth of compression zone at higher stress levels was smaller than that of lower stress levels for the same load cycles. Good bond strength was realized and no relative slip happened under fatigue loading. During the fatigue process, several visible cracks generated on the UHTCC layer, and the number reduced with the decrease of stress levels, while, about1-3cracks were produced on the concrete layer. Due to the multiple-cracking characteristic, ductile deformation was found for UC beams under fatigue load. Three stages of deformation existed, while the deformation ability became weaker with the decrease of stress levels. Through analysis, the fatigue life of a UC composite beam was defined by the complete damage of UHTCC layer. The regression function on UHTCC’s flexural fatigue damage was applied to calculate the evolution process of maximum tensile strain, and the calculated results were a little smaller than the experimental results, expecially at lower stress levels, the two results were in good agreement.更多还原