含表面裂纹简支梁的非线性振动分析

对含表面裂纹简支梁在大幅振动下的几何非线性进行了理论分析,从建立了梁的非线性振动的半解析解.用Rayleigh方法将振型函数表示为线性模型振型函数的组合,建立了梁非线性振动的第一阶振型函数的显式表达式,数值模拟计算了不同的裂纹深度和给定不同第一函数系数a1对梁最大位移的影响.建立的显式方程简单,易于工程应用.     

机器视觉疲劳裂纹扩展试验摄像头自动聚焦方法

在机器视觉疲劳裂纹扩展试验中,为了能够满足对裂纹宏观观察裂纹和准确定位裂纹尖端的要求,需要采用变焦镜头放大和缩小采集图像范围,针对于摄像头变焦后的聚焦问题,提出了摄像头的自动聚焦方法。首先建立图像采集系统,采集裂纹图像至计算机,通过中值滤波对图像预处理,去除噪声干扰,选取裂纹区域作为聚焦窗口,采用Laplace算子法作为清晰度评价函数,并提出一种变步长穷举法进行聚焦搜索。最后设计了以ARM7为核心的摄像头运动控制器。实验表明,所提出的方法能够实现疲劳裂纹扩展试验中摄像头在各种情况下的准确自动聚焦,为下一步精确测量疲劳裂纹扩展参数奠定基础。     

基于扩展有限元法的黏聚性裂缝模型的混凝土梁断裂过程模拟

为研究混凝土梁的断裂过程,提出用基于二维扩展有限元法（eXtended Finite ElementMethod,XFEM）的黏聚性裂缝模型模拟混凝土简支梁在集中载荷作用下的断裂过程.推导考虑近裂尖奇异性的基于XFEM的黏聚性裂缝模型,得出裂缝开度随裂缝长度的变化曲线;对上述模型与相关文献用有限元结合节点释放技术对相同时间的裂缝扩展的计算结果进行比较,二者结果吻合良好,并与实际裂缝扩展过程相符.计算结果证实基于XFEM的黏聚性裂缝模型能有效进行混凝土梁的断裂过程模拟.     

Mixed mode fracture analysis of rectilinear anisotropic plates using singular boundary elements

This paper reports a multidomain boundary element formulation for direct calculation of stress intensity factors in rectilinear anisotropic plates subjected to arbitrary in-plane loading. The √r displacement and 1/√r traction behaviour near a crack tip is correctly represented in crack elements and transition elements. The use of these singular boundary elements is investigated for mode I and mixed mode crack problems.     

基于概率断裂力学的管道疲劳寿命分析

采用奇异单元模拟裂纹尖端应力场的奇异性,计算裂纹尖端的应力强度因子和张开应力.以概率论为基础,结合确定性疲劳断裂力学估算方法,考虑参数的不确定性和随机性,应用蒙特卡洛模拟法分析管道的疲劳寿命.结果表明：通过J积分计算得到的裂纹尖端张开应力与计算得到的管道工作应力基本相等.采用蒙特卡洛模拟法进行的一定可靠度和置信度下的疲劳寿命预测能反映评定参数的不确定性,较传统的断裂力学计算结果更安全.     

The Dirichlet-to-Neumann map for two-dimensional crack problems

A coupled analytic/finite-element method is presented for two-dimensional crack problems. Two classes of problems are studied. The first considers problems where non-linear constitutive processes occur in a region near the crack tip and the remotely applied loading can be characterized by the linear elastic K-field and perhaps the T-stress. In this case, the finite-element method is applied in a circular region around the crack tip where non-linear constitutive response is occurring, and stiffness contributions associated with a numerically implemented Dirichlet-to-Neumann map are imposed on the circular boundary to account for the large surrounding elastic domain and the remote applied loading. The second class of problems considers entirely linear elastic domains with irregular external boundaries and/or complex applied loadings. Here, the discrete Dirichlet-to-Neumann map is used to represent a circular region surrounding the crack tip, and finite-elements are used for the external region. In this case the mixed mode stress intensity factors and the T-stress are retrieved from the map.     

基于Abaqus柔度标定法的Q235材料断裂韧性仿真

为简便、准确地获得Q235材料的应力强度因子值和J积分值,用Abaqus对Q235材料进行有限元仿真,得到三点弯曲试样及其裂纹尖端区域的应力分布情况;针对裂纹尖端的奇异性,引入折叠单元进行裂纹尖端单元的奇异性建模.不同尺寸试件应力强度因子仿真值与试验值基本一致,表明该方法可以准确预测材料断裂参数.     

带边角裂纹悬臂Mindlin 板的振动特性研究

本文基于Mindlin板理论,应用Ritz法研究带边角裂纹Mindlin板的振动特性,分析了不同裂纹参数如裂纹位置,裂纹长度,裂纹角度对悬臂Mindlin板的固有频率和模态的影响.利用Ritz法求解固有频率和模态函数,本文构造了一个特殊的模态函数,其模态函数由两部分构成,一部分是用梁函数组合法得到的无裂纹理想完整矩形板的振型,另一部分是利用裂纹尖端奇异性理论,构造描述裂纹附近位移和转角不连续的角函数.通过高精度的数值计算软件Maple得出结果,并与有限元软件ANSYS分析的结果进行对比,验证本文计算结果的准确性.     

一种新的故障检测与诊断方法及其应用研究

给出了在故障检测与诊断中采用经验模式分解与希尔伯特变换相结合的方法。经验模式分解不同于小波变换、KL变换、奇异值分解(SVD)等信号分解方法,它把数据序列分解为能够表示数据中嵌入的不同振荡的本征模式函数。首先介绍方法的原理与特点,然后将该方法用于齿轮故障的探测与诊断,结果显示,这种方法能准确识别出裂缝故障的特征频率。     

The dynamic stresses induced by the propagation of skew cracks

Skew crack propagation in an elastic medium due to the application of transient in-plane and anti-plane loads is investigated. For the anti-plane loading the motion of the crack is purely of mode III type, whereas in the case of in-plane loading a mixed mode type of fracture takes place in which both modes I and II occur. The method of solution is numerical and is based on a certain time-dependent transformation which maps the physical plane of the crack into an auxiliary plane in which the crack propagates collinearly with its propagating tip appearing always at the origin of the moving coordinate system. The transformed equations of motion are approximated by an implicit, three-level, finite difference system of equations of second-order accuracy, whose stability analysis is discussed. The reliability of the proposed method of solution is examined in several situations in which analytical results are known, and satisfactory agreement is achieved. Extension to smoothly curving cracks is discussed.     

A finite element analysis of the finite width interface mixed mode bi-material fracture problem

One aspect of the terminal crack, mixed mode bi-material fracture mechanics problem is investigated using finite elements. The influence of a finite width bond line interface is considered for one representative material pair combination (E₂/E₁ = 0.10). The stress intensity factors for an inclined crack terminating at a variable thickness interface are established as a function of crack inclination. Since the order of the stress singularity is not the typical r^−1/2 associated with LEFM problems, variable power singular finite elements are used to model the terminating crack tip. Crack tip stress distributions and probable angles of crack extension are presented as a function of crack inclination and bond line thickness. Crack tip stress distributions assuming an interfacial debonding criterion are presented as a function of crack inclination and bond line thickness.     

Elastodynamic fields near running cracks by finite elements

A finite element method is developed for the computation of elastodynamic stress intensity factors at a rapidly moving crack tip. The method is restricted to bodies whose surfaces and two-material interfaces are either parallel to the direction of propagation or are sufficiently remote. The crack tip starts to move at the instant that it is struck by an incident wave. The finite element grid moves undeformed with the crack tip. The main result consists in the fact that the method of non-singular calibrated crack tip elements, in which the stress-intensity factor is determined from its statically calibrated ratio to the crack opening displacement in an adjacent node, is extended to dynamic problems with moving cracks, for both in-plane and anti-plane motions. The dependence of the calibration ratio on the crack tip velocity is established from previously developed analytical solutions for the near-tip displacement fields. Numerical results compare favorably with known analytical solutions for cracks moving in an infinite solid. The grid motion causes an apparent asymmetric additional damping matrix.     

Computational dynamics approach to the effect of damping on stability of a cantilevered column subjected to a follower force

The paper discusses on the basis of computational approach the effect of small damping on the theoretical flutter bounds of a cantilevered column subjected to a follower force. A cantilevered viscoelastic column having a tip mass is considered. The impulse responses of the column are studied by the mode superposition method to observe the growth rate of the motion. It is established that the growth rate of the motion of the nonconservative column with a larger but small size of damping is greater than that with the smaller. It is demonstrated that the gap between the flutter predictions made by considering and neglecting damping can be bridged by assuming the practical concept of stability, the stability in a finite time interval.     

A finite element analysis of the mixed mode bi-material fracture mechanics problem

Two aspects of the mixed mode bi-material fracture mechanics problem are investigated using finite elements. The stress intensity factors for an inclined crack at various distances from a bi-material interface are established as a function of inclination for two material pair combinations. The probable angle of crack extension is established for this problem using the maximum hoop stress criterion. The inclined terminal crack problem is studied using variable power singular elements at the interface. Crack tip stress distributions and probable angle of crack extension are presented as functions of crack inclination and material pair combinations. Crack tip stress distributions assuming an interfacial debonding criterion are also presented as functions of crack inclination and material pair combinations.     

基于多级拟合的道路病害自动检测与识别

传统的基于人工视觉检测裂缝的方法愈来愈不能适应高速公路发展的要求:其耗人力、耗时、不精确、影响交通、危险、花费高的缺点要求路面状况调查的自动完成。而现有的检测识别方法仅适合某些情况并仅在小样本集上测试过。论文介绍了一种基于多级拟合机制的道路裂缝自动检测与识别方法。大量实验结果表明该方法具有很强的通用性和准确性。     

A singular ES-FEM for plastic fracture mechanics

The stress and strain fields around the crack tip for power hardening material, which are singular as r approaches zero, are crucial to fracture and fatigue of structures. To simulate effectively the strain and stress around the crack tip, we develop a seven-node singular element which has a displacement field containing the HRR term and the second order term. The novel singular element is formulated based on the edge-based smoothed finite element method (ES-FEM). With one layer of these singular elements around the crack tip, the ES-FEM works very well for simulating plasticity around the crack tip based on the small strain formulation. Two examples are presented with detailed comparison with other methods. It is found that the results of the presented singular ES-FEM are closer to reference solution, which demonstrates the applicability and the effectiveness of our method for the plastic field around the crack tip.     

Stable and realistic crack pattern generation using a cracking node method

This paper presents a method for simulating surface crack patterns appearing in ceramic glaze, glass, wood and mud. It uses a physically and heuristically combined method to model this type of crack pattern. A stress field is defined heuristically over the triangle mesh of an object. Then, a first-order quasi-static cracking node method (CNM) is used to model deformation. A novel combined stress and energy combined crack criterion is employed to address crack initiation and propagation separately according to physics. Meanwhile, a highest-stress-first rule is applied in crack initiation, and a breadth-first rule is applied in crack propagation. Finally, a local stress relaxation step is employed to evolve the stress field and avoid shattering artifacts. Other related issues are also discussed, such as the elimination of quadrature sub-cells, the prevention of parallel cracks and spurious crack procession. Using this method, a variety of crack patterns observed in the real world can be reproduced by changing a set of parameters. Consequently, our method is robust because the computational mesh is independent of dynamic cracks and has no sliver elements. We evaluate the realism of our results by comparing them with photographs of real-world examples. Further, we demonstrate the controllability of our method by varying different parameters.     

Investigation of different isoparametric axisymmetric crack tip elements applied to a complete circumferential surface crack in a pipe

Standard isoparametric finite elements can be used as special crack tip elements in fracture mechanical computations by appropriately shifting the middle nodes in the neighbourhood of the crack tip. Such elements have already been applied to several plane and three-dimensional problems so that this method can be considered as commonly well accepted. In this paper the application of isoparametric axisymmetric elements as crack tip elements to a particular axisymmetric problem is studied. For that reason a complete circumferential crack at the inner surface of a pipe under axial tension is considered. The calculated stress intensity factors are compared with results from the literature. The general purpose finite element programs ASKA and ADINA have been used. In the first case triangular and quadrilateral elements were investigated, in the latter case quadrilateral and collapsed quadrilateral elements. In spite of the rather coarse grids good results for the stress intensity factor were found with the only exception of the collapsed quadrilateral elements.     

基于曲率滤波和N-P准则的路面裂缝识别方法

针对沥青路面裂缝不连续问题,提出一种基于曲率滤波和N-P准则的裂缝识别方法。通过组合最小矩形切平面和最小三角切平面及修正正则能量项的改进型曲率滤波算法,消除随机噪声并平滑纹理。采取二次分割策略提取疑似裂缝目标和运用裂缝几何特性去除块状或点状噪声,实现裂缝定位和获取裂缝片段。在此基础上,融合裂缝片段的位置和方向信息,提出利用N-P准则连接裂缝片段的端点,从而获得完整裂缝数据。研究结果表明,提出的方法对横裂、纵裂、块裂及龟裂等裂缝都具有较好的检测效果和较高的检测精度,裂缝检测的完整性达到90.5%以上。     

基于多尺度布朗运动模型的路面破损检测

路面裂缝的自动检测与识别是路面病害信息系统的关键环节,裂缝检测结果的优劣直接影响路面损坏程度的评估。基于布朗运动模型,采用不同尺度下的分数布朗运动特征参数矢量描述图像块特征,并运用k聚类与投票机制产生最终结果。通过与经典算子（canny）及单一尺度布朗运动矢量的比较结果证明,该算法在检测路面破损弱信息时更有效,检出的结果更准确、更接近原始裂缝位置,具有实际应用价值。