大体积砼温度裂缝的预防

大体积砼所产生的裂缝的主要形式，为温度裂缝。其裂缝的根本原因是砼的变形。因此，制造合适的允许温差，加强施工中的温度观测，采取适当的温度控制措施，提高砼的极限拉伸强度，以预防大体积砼的温度裂缝。     

大体积混凝土温度应力造成裂缝的控制

分析大体积砼温度裂缝 ,提出降低温度应力、防止大体积砼裂缝几项措施     

大体积混凝土裂缝产生原因及防治措施综述

本文简单地分析了大体积混凝土为何产生裂缝，概括介绍了防治裂缝的措施，可在大体积砼施工中参考应用。     

大体积混凝土裂缝产生原因及防治措施综述

本文简单地分析了大体积混凝土为何产生裂缝,概括介绍了防治裂缝的措施,可在大体积砼施工中参考应用。     

地下室底板大体积砼裂缝控制措施

所谓大体积砼,是指砼结构实体最小几何尺寸不小于1m,体积大于1000m3,或预计会因砼中水泥水化热引起的温度和收缩而导致有害裂缝产生的砼工程.     

浅论大体积砼结构温度裂缝控制技术

我国在建筑设计、施工技术等方面,随着国内高层建筑的兴起,都有了飞速的进展,但也出现了许多问题,如大体积砼的温度裂缝控制问题.近年来,大体积硅的裂缝问题越来越引起重视.本文着重以高层建筑中地下室砼底板为主要研究对象,通过预测与实测的紧密配合,采取控制砼温度的变化的措施来达到防止温度裂缝产生,便于工程人员掌握并在工程实践中运用.     

砼温度应力计算理论在地下室裂缝控制中的应用

结合工程实际，运用砼温度应力计算理论，对大体积浇筑地下室底板砼，在凝结养护过程中所产生的温度收缩应力的计算。估量砼浇筑后可能产生的最大温度收缩应力，及自然养护条件下7天时的温度收缩应力，并采取有效措施，控制温度收缩应力对地下室底板砼裂缝的影响。     

大体积混凝土污水处理工程的施工技术

为防止大体积砼产生裂缝，采用HE微膨胀剂与LNF泵送剂行之有效。同时应进行温度应力与浇筑长度验算和精心施工。     

谈确保大体积砼的施工安全

随着建筑业的快速发展,高层建筑物,高耸结构及大型设备基础大量的出现大体积砼已经被广泛应用,大体积砼与普通钢筋砼相比,具有结构厚,体形大,钢筋密,砼数量多,工程条件复杂等特点,因此对施工技术提出了更高的要求,只有重视大体积砼的施工问题,避免裂缝的产生,才能确保施工质量.本文从材料的选用、施工方法、提高施工质量的途径等方面进行简单的论述.     

浅谈大体积混凝土施工

大体积混凝土的施工控制已经成为建筑工程施工不可回避的重要问题之一。与一般的钢筋砼结构相比,大体积砼经常出现的问题,不是力学上的结构强度问题,而是如何控制砼温度变形裂缝。因此提高砼的抗渗、抗裂、抗侵蚀性能力,以提高建筑结构的耐久年限就成为突出任务。     

蠕变条件下完整岩石的起裂和扩展准则

The mechanism and criterion of crack initiation and propagation of rocks were investigated by many researchers,And the creep behaviour of rocks was also theoretically and experimentally studied by some scientists and engineers.The characteristics of crack initation and propagation of rocks under creep condition.however,are very improtant for rock engineering and still not paid enough attention by researchers,In this paper,the criterion and mechanism of crack initiation and propagation under creep condition were investigated using specimens collected from sandstone rock formations outcropping in the Emei Mountain,the Sichuan Province of China.Cuboid specimens under three point bending were used in this investigation.All specimens were classified into four sorts and used for Mode-I fracutre of creep frcture tests.The experimental result shows that due to creep deformation.rock crack will inevitably initialt and propagate under a load of KI,which is less than fracture toughness KIC but not less than a constant(marked as KIC2),KIC2 indicates the ability of rock to resist crack initiation and propagation under creep conditions and is less than fracture tough ness KIC.defined as creep fracture toughness in this paper,KIC2 should be considered as an importnat parameter on design and computation of rock engineering.The microstructureal mechanism for crack initiation and propagation of rock materials under creep condition was introduced based on competitive model between softening effect and hardening effect,and the validity of test result was explained.The test result was also verified in rheological theory.When KI is more than KIC2 but less than KIC,rock crack will initiate and propagate after a time interval of sustained loading under creep condition.In order to find the relation between duration of sustained lading.which can lead to crack initiation and propagation,and the initial stress intensity factor KI,an unequal0interval time sequence forecasting and predicting model was introduced,and the relation was obtained for homogeneous and isotropic fine-grained red sandstone.Finally a modified fracture toughness formula was given,in which the influence of fracture process zone(FPZ) was fully considered.     

Fatigue and fracture behavior of nickel-based superalloy Inconel 718 up to the very high cycle regime

The fatigue and fracture behavior of nickel-based superalloy Inconel 718 was investigated up to the very high cycle regime under rotary bending tests at room temperature. It was found that this superalloy can still fracture after exceeding 107 cycles. Fractographic analysis revealed that there was a transition from fatigue crack initiation at multi-sites to single initiation with decreasing stress levels. The fracture surface can be divided into four areas according to the appearance, associated with fracture mechanics analysis of the corresponding stress intensity factors. The fracture mechanism dominant in each area was disclosed by scanning electron microscope examination and analyzed in comparison with those obtained from the crack growth tests. Subsequently, life prediction modeling was proposed by estimating the crack initiation and propagation stage respectively. It was found that Chan （2003）＇s model for initiation life and the Paris law for growth life can provide comparable predictions against the experimental life.     

3D analysis for pit evolution and pit-to-crack transition during corrosion fatigue

This paper presents a deterministic model to predict the pit evolving morphology and crack initiation life of corrosion fatigue. Based on the semi-ellipsoidal pit assumption, the thermodynamic potential including elastic energy, surface energy and electrochemical energy of the cyclically stressed solid with an evolving pit is established, from which specific parameters that control the pit evolution are introduced and their influence on the pit evolution are evaluated. The critical pit size for crack nucleation is obtained from stress intensity factor criterion and the crack nucleation life is evaluated by Faraday’s law. Meanwhile, this paper presents a numerical example to verify the proposed model and investigate the influence of cyclic load on the corrosion fatigue crack nucleation life. The corrosion pit appears approximately as a hemisphere in its early formation, and it gradually transits from semicircle to ellipsoid. The strain energy accelerates the morphology evolution of the pit, while the surface energy decelerates it. The higher the stress amplitude is, the smaller the critical pit size is and the shorter the crack initiation life is.     

J-Integral of Interfacial Crack Between Metal-Base Ceramic Coating and Steel

The definition ofJ-integral of interfacial crack was introduced.The three.point bending tests were carried out to obtain the criticaI lOading values when the interfacial crack initiation occurred between coatings and substrates.The finite element analysis(FEA)was adopted to analyze the stress distribution in the specimens and compute the J-integral of the interfaciaI crack between LX88A coating and Chinese 0345 steel.The results showed that the average value of critical J-integral iS 0.70 N/m.which can be taken as the fracture parameter to evaluate the interfaciaI fracture behavior for the three-point bending specimens of LX88A coating/Q345 steel system.     

Calculation Model of Equivalent Strength for Induced Crack Based on Double-K Fracture Theory and Its Optimizing Setting in RCC Arch Dam

By means of fracture testing on roller-compacted concrete (RCC) three-point bending beams with two different specimen sizes, the P-CMOD complete curve for RCC was gained. Furthermore, by applying double-K fracture theory, KIC^ini,KIC^un, as well as the critical effective crack length and the critical crack tip opening displacement, were evaluated. Based on the double-K fracture parameters above, the calculation model of equivalent strength for induced crack was established, thus the calculation method on its initiation, stable propagation and unstable fracture was ascertained. Moreover, the finite element simulation analysis of stress field in ShaPai arch dam and the on-site observational splaying points of induced crack at different altitudes validated the reliability of the model. Finally, crack inducer‘s optimal setting in RCC arch dam was studied. It improves the design level of induced crack in RCC arch dam and satisfies the necessity of engineering practice.     

混凝土坝裂纹扩展算法(英文)

混凝土建筑物中的裂纹扩展追踪计算是确定结构安全度的重要手段。本文提出用虚拟裂纹单元及断裂力学准则(应变强度因子)追踪和模拟裂缝初始断裂、扩展并最终导致破坏的过程,给出一条逐段扩展的裂纹线,本方法计算结果与某些混凝土坝实测资料及模型试验结果吻合较好。     

裂纹岩体贯通机制的数值模拟研究

裂隙岩体在荷载作用下导致其内部裂纹的相互贯通,从而在裂纹尖端形成较大的塑性破坏区域,岩体工程的失稳大多是由于其内部节理、裂隙等缺陷发展导致的,应用断裂力学的方法,可以追踪岩体中节理裂隙的起裂、扩展至相互贯通使岩体局部破坏的过程,从而揭示岩体失稳的渐进破坏机制.本文采用有限元方法模拟了岩体中不同倾角的压剪裂纹的破坏问题,根据不同倾角裂纹的力学机理分析了其受力差异,依据最短塑性区距离判据分析了相近裂尖的扩展距离,得出了不同倾角的两共线裂纹相近裂尖破坏后岩桥之间的有效距离.     

Acoustic Emission Monitoring of Fatigue Crack Growth in a Slender Web of Bridge Girder

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复杂飞机结构裂纹萌生寿命确定的工程方法

采用静拉伸应变测量试验的方法,确定了典型飞机复杂结构在载荷作用下的应变分布,并准确判断出其疲劳危险部位。对典型飞机复杂结构模拟件进行了等幅加载疲劳试验,并在疲劳试验过程中采用着色探伤方法测得了疲劳裂纹扩展数据。在此基础上,提出并建立了一种适用于确定复杂结构疲劳裂纹萌生寿命的工程方法,包括疲劳裂纹萌生寿命反推法和含有不确定寿命样本的统计推断方法。根据该方法对试验样本疲劳数据进行处理得出了裂纹萌生中值寿命及其在95%置信水平下的单侧置信下限。     

J-Integral of interfacial crack between metal-base ceramic coating and steel

The definition of J-integral of interfacial crack was introduced. The three-point bending tests were carried out to obtain the critical loading values when the interfacial crack initiation occurred between coatings and substrates. The finite element analysis (FEA) was adopted to analyze the stress distribution in the specimens and compute the J-integral of the interfacial crack between LX88A coating and Chinese Q345 steel. The results showed that the average value of critical J-integral is 0.70 N/m, which can be taken as the fracture parameter to evaluate the interfacial fracture behavior for the three-point bending specimens of LX88A coating/Q345 steel system. Supported by Tianjin Natural Science Foundation (No.08JCYBJC09100), New Teacher Research Fund for the Doctoral Program of Higher Education of China (No.20070056096) and New Century Outstanding Talented Person Plan of China. XU Lianyong, born in 1975, male, Dr.