Crack status analysis for concrete dams based on measured entropy

The integrity and safety of concrete dams are seriously affected by the existing cracks in dam bodies, and some serious cracks may cause dam failure or disaster. The propagation of cracks in concrete dams is accompanied by changes in energy distribution, which can be represented by changes in the structure’s system entropy. Therefore, the entropy theory can be used in analyzing the behavior of dam cracks. Due to the randomness and locality of crack propagation, it is difficult to predict the location of cracks by traditional monitoring methods. To solve this problem, the influence of spatial positions of monitoring points on inspection zones is represented by a weight index, and the weight index is determined by the distance measure method proposed in this paper. Through the weighted linear fusion method, the entropy of multiple monitoring points is obtained for analyzing the behavior of dam cracks in the selected zones. Meanwhile, the catastrophe theory is used as the variation criterion of an entropy sequence in order to predict the instability time of dam cracks. Case studies are put forward on a high arch dam, and the fusion entropy is calculated according to the monitoring data from strain gauges. Results show that the proposed method can effectively predict the occurrence time and location of dam cracks regardless of the layout of monitoring instruments, and it is a new way to analyze the occurrence and propagation of dam cracks.     

紧水滩拱坝裂缝稳定性分析

为研究紧水滩拱坝已监测出的主要坝面裂缝的存在或发展对拱坝正常运行的影响,进行了裂缝稳定性数值计算,其中坝体混凝土采用理想弹塑性本构关系和Mohr-Coulomb屈服准则,含裂缝的部分区域采用子模型技术,裂缝内采用动态接触状态的分析模型.分析结果表明:已有裂缝对拱坝整体工作性态无影响,裂缝的稳定性与其位置和缝面水压作用形式有关.验证了4种工况下裂缝的稳定性,提出应对其中的3处裂缝采取防渗止水措施防止其继续发展.     

基于裂缝扩展准则的混凝土重力坝裂缝扩展全过程数值分析

将裂缝扩展准则应用于混凝土重力坝裂缝扩展全过程分析,结合虚拟裂缝模型计算了经典混凝土重力坝模型的断裂特性,并与其他数值方法进行了对比。结果表明：利用该数值方法得到的混凝土重力坝模型的外荷载一裂缝口张开位移曲线及裂缝扩展路径与试验结果均吻合良好;当给定了混凝土材料的起裂断裂韧度、断裂能、抗拉强度等参数后,即可采用该数值方法对混凝土重力坝裂缝扩展全过程进行分析。     

混凝土重力坝的裂缝跟踪分析

通过对混凝土重力坝在外荷载作用下裂缝问题的分析，提出了一种估算应力的方法．该方法能加速在运用有限元法寻找坝体裂缝开展长度时的迭代过程，节省大量机时，以实现对混凝土重力坝在外荷载作用下裂缝开展的跟踪分析     

脆性材料在压缩载荷作用下的断裂破坏准则

目前多裂隙材料在压缩载荷作用下的力学行为问题已成为当前断裂力学及岩石力学等学科的重要课题.选取位于同一直线上的多裂隙体,考虑裂隙(纹)表面间的摩擦力并应用复变应力函数,推出了裂纹应力强度因子的解析表达式.当只有两个裂纹或一个裂纹存在时,给出了准确的应力强度因子解析解.最后依据共线双裂纹模型和单裂纹模型及传统的裂纹断裂准则,建立了脆性材料在压缩载荷作用下的断裂破毁准则.在一些特殊情况下, 这个准则可以简化,并且其简化后的准则具有应用价值,特别是当材料不含有宏观裂纹时,这个准则就和著名的库伦 - 摩尔准则相一致.     

On-line diagnosis method of crack behavior abnormality in concrete dams based on fluctuation of sequential parameter estimates

Research on on-line diagnosis method of crack behavior abnormality in concrete dams can provide support for timely grasping abnormality state of the crack itself and achieving real-time monitoring of the dam safety.Considering that samples of crack effects in concrete dams increase actually over monitoring time,a superiority criterion for the on-line diagnosis is determined so as to detect the abnormality moments timely and reliably.By integrating the safety monitoring statistical model of crack effect variable with change point theory,a fluctuation method of regression coefficients is established for the on-line diagnosis.In addition,each abnormality moment is detected by the cumulative sum of regression model residuals.Results indicate that abnormality of crack behavior in concrete dams can be characterized by structural instability of crack monitoring model.And causes of crack behavior abnormality can be analyzed by the established method,which will play an important role in dam safety monitoring.Further,taking the crack in a concrete gravity-arch dam as an example,the scientific rationality and validity of the established on-line diagnosis method are confirmed.     

混凝土坝裂缝变形的不确定性分析模型

混凝土坝裂缝的发生和发展受到不确定因素影响。将大坝系统及其安全监控看作一个受诸多因素影响的不确定性系统,结合裂缝实测资料,应用近似推理模型和信息分配法,建立了裂缝变形的不确定性分析模型。     

微观结构疲劳短裂纹生长行为的描述方法

通过对裂纹损伤区位错平衡条件的分析,建立了循环应力下的疲劳短裂纹生长位错模型．结合对疲劳短裂纹损伤区与晶界交互作用分析,由模型给出了一个振荡形式发展的微观结构疲劳短裂纹生长速率表达式,其范围包络线可以较好地覆盖裂纹生长速率的试验数据．     

晶须增强铝合金中短裂纹的疲劳扩展速率

应用微观力学方法研究晶须增强铝合金中短裂纹的扩展速率,为预报短裂纹寿命提供依据。通过晶须增强铝合金中疲劳短裂纹扩展过程的分析,利用裂纹扩展的能量释放率给出短裂纹的疲劳扩展速率方程,数值计算结果与实验结果相吻合,裂纹扩展速率具有波浪式向前发展的特点。     

高碾压混凝土坝快速施工过程中的温度控制措施

提出了高碾压混凝土坝施工期温度时空动态控制方法,该方法的核心在于通过调整水管冷却的5个要素(冷却开始时刻、通水时间、水温、流量和水流方向)来动态控制混凝土的温度,进而达到温控防裂的要求.基于ANSYS平台开发了大体积混凝土施工期温度场、应力场三维有限元仿真程序,将温度时空动态控制方法应用于官地高碾压混凝土重力坝的施工过程中,并做跟踪监测和反演计算.实践证明,该方法易于操作,且在官地大坝的施工过程中未出现危害性的裂缝,有效解决了其温控防裂问题,给类似工程提供了借鉴和参考.     

混凝土重力拱坝极限抗震能力评价方法初探

从基于性能的抗震设防标准及理论出发,坝体某个关键性能破坏便可能影响大坝的正常运行,单一的评价准则已经不能满足对大坝极限抗震能力评估的要求。针对混凝土重力拱坝的结构特点,基于混凝土弥散裂缝模型,利用地震动超载时域分析方法,从收敛性、地震位移响应突变、塑性区贯通、坝体开裂破坏等方面,对国内某重力拱坝极限抗震能力进行了探讨和研究。研究表明,该重力拱坝的极限抗震能力为0.51g,顶拱中部是坝体相对薄弱的高地震响应区。     

小湾导流中孔闸墩开裂成因及工作性态分析

针对小湾拱坝在施工过程中导流中孔闸墩会产生裂缝问题,建立了一套基于弹粘塑性势理论的数值分析系统,对其开裂的原因及工作性态进行了计算分析.结果表明:开裂原因并非单一因素所致,而是温控措施、锚索张拉及蓄水的累计作用,其中温控措施产生的拉应力为最主要原因;导流中孔若继续参与导流工作,裂缝充水时,其尖端的应力强度因子将超过混凝土的断裂韧度,在水力劈裂作用下裂缝将继续扩展.该研究成果不仅为制定小湾拱坝导流方案提供了依据,对类似工程也具有一定的借鉴和参考价值.     

Simulation of hydraulic fracture utilizing numerical manifold method

A 2nd order numerical manifold method(NMM) based method is developed to simulate the hydraulic fractures propagating process in rock or concrete. The proposed method uses a weak coupling technique to analyze the fluid phase and solid phase. To study the seepage behavior of the fluid phase, all the fractures in solid are identified by a block cutting algorithm and form a flow network. Then the hydraulic heads at crack ends are solved. To study the deformation and destruction of solid phase, the 2-order NMM and sub-region boundary element method are combined to solve the stress-strain field. Crack growth is controlled by the well-accepted criterion, including the tension criterion or Mohr-Coulomb criterion for the initialization of cracks and the maximum circumferential stress theory for crack propagation. Once the crack growth occurs, the seepage and deformation analysis will be resolved in the next simulation step. Such weak coupling analysis will continue until the structure becomes stable or is destructed. Five examples are used to verify the new method. The results demonstrate that the method can solve the SIFs at crack tip and fluid flow in crack network precisely, and the method is effective in simulating the hydraulic facture problem. Besides, the NMM shows great convenience and is of high accuracy in simulating the crack growth problem.     

Dynamic fracture behavior of rock under impact load using the caustics method

We studied the dynamic fracture mechanical behavior of rock under different impact rates. The fracture experiment was a three-point bending beam subjected to different impact loads monitored using the reflected caustics method. The mechanical parameters for fracture of the three-point bending beam specimen under impact load are analyzed. The mechanism of crack propagation is discussed. Experimental results show that the dynamic stress intensity factor increases before crack initiation. When the dynamic stress intensity factor reaches its maximum value the crack starts to develop. After crack initiation the dynamic stress intensity factor decreases rapidly and oscillates. As the impact rate increases the cracks initiate earlier, the maximum value of crack growth velocity becomes smaller and the values of dynamic stress intensity factor also vary less during crack propagation. The results provide a theoretical basis for the study of rock dynamic fracture.     

10Cr9Mo1VNb钢冲击断裂机理

采用小能量示波冲击法和金相法，研究了10Cr9Mo1VNb钢冲击断裂时裂纹的萌生、生长与扩展．结果表明：10CrgMo1VNb钢裂纹生长机理是裂纹前沿和孔洞之间以颈缩聚合的方式相连，裂纹生长速率缓慢；裂纹扩展其机理是裂纹前沿和孔洞之间以剪切裂纹的方式相连，裂纹扩展速率很快．裂纹的临界破断为尺寸2mm左右，临界冲击功为114J左右。     

结构裂缝的分布式光纤监测方法及试验研究

结构裂缝监测是评估结构安全性的重要依据之一,分布式光纤裂缝监测技术可有效避免点式检测空间不连续造成的漏检现象,且易于实现自动化监测。该文提出了布里渊光频域分析计（BOTDA）结合斜交光纤组的裂缝监测方法,通过几何分析得到了光纤应变和裂缝宽度及开展方向的理论模型,并采用标定试验建立了由光纤测试应变反算裂缝宽度及开展方向的数值方程。同时开展了光纤裂缝传感器标距和预拉力大小对测量精度影响的试验研究,并标定了300mm标距的光纤裂缝传感器实测应变和裂缝宽度及夹角的定量关系。最后通过裂缝模拟试验对裂缝开展进行了跟踪监测,结果表明斜交光纤组能有效得到光纤与裂缝之间夹角值并实时监测裂缝宽度变化。     

基于裂缝计的碾压混凝土"金包银"结合部性态分析

结合裂缝计监测资料,通过建立数学模型对某碾压混凝土"金包银"坝型的常态混凝土与碾压混凝土结合部的裂缝进行定量分析,分析表明:该混凝土坝裂缝计实测资料能反映出坝体测点部位混凝土裂缝状况,其变化规律和分布规律符合该类型混凝土大坝的一般物理规律,运行期该类裂缝实测开度主要受上下游水压和温度的影响,时效分量已趋于稳定,坝体运行良好.     

Modeling stationary and moving cracks in shells by X-FEM with CB shell elements

The continuum-based (CB) shell theory is combined with the extended finite element method (X-FEM) in this paper to model crack propagation in shells under static and dynamic situations. Both jump function and asymptotic crack tip solution are adopted for describing the discontinuity and singularity of the crack in shells. Level set method (LSM) is used to represent the crack surface and define the enriched shape functions. Stress intensity factors (SIFs) are calculated by the displacement interpolation technique to prove the capability of the method and the maximum strain is applied for the fracture criterion. Also, an efficient integration scheme for the CB shell element with cracks is proposed.     

复合型裂纹扩展的形状改变比能准则

为了预测金属材料裂纹在小范围屈服时的延性断裂，以复合型裂纹为研究对象，将裂纹前缘塑性屈服区内的总形状改变比能用于建立复合型裂纹的断裂准则。该准则表明了金属材料裂纹在扩展过程中，起决定作用的是形状改变比能，而不是整个应变能。能成功地预测了复合型裂纹的启裂角度以及临界荷载，将其结果与Sih的S准则和实验数据结果进行了比较。结果表明，复合型裂纹扩展的形状改变比能准则，在预测裂纹启裂角方面优于S准则，并且预测的临界荷载偏保守。在工程上，将其应用于复合型裂纹断裂判定是安全的。     

Excel在大坝监测数据分析中的应用

为确保大坝安全运行,必须定期对大坝进行安全监测并及时准确地对监测数据进行整编分析.运用Excel函数,可以大大提高监测数据分析的效率及准确性.按照大坝监测资料整编规范要求,文章简要介绍了Excel函数在大坝安全监测数据整编分析中的应用.