Body to Rayleigh wave mode-conversion at steps and slots

The efficient excitation of Rayleigh waves at surface discontinuities due to body wave incidence has potential as a method for the detection and sizing of surface breaking defects. In this paper, the mode-conversion phenomenon at several types of surface features is studied in detail, using numerical models which employ finite-difference methods. The emphasis is on examining the spectral content of the Rayleigh wave field in order to arrive at a method for relating the spectral information to the defect dimensions. The numerical results are backed up by experimental observations.     

A Guided Wave Approach to Defect Detection in Switch Rail

An approach to detect switch rail defect based on ultrasonic guided wave technology is studied. Eight typical cross-sections are chosen from the switch rail, and each cross-section's theoretical dispersion curves and wave structures are derived using the semianalytical finite-element method. The dispersion curve of eight sections has 262 modes at 60 kHz. Based on k-means clustering analysis algorithm, 25 kinds of classification results are obtained from 262 modes. According to the mean and variance of energy, an optimal set of modes with uniform amplitude over the entire section of the switch rail is selected. The optimal excitation point is determined based on the vibration energy. The defect echo signal is obtained by excitation of these guided wave modes. Field experiment results show that by comparing the sum of the differences between the collected and basic data and setting a certain threshold, the internal defect detection of the switch guide rail can be realized. The research results herein are valuable for analyzing the dispersion characteristics and realizing the nondestructive testing of the variable cross-section waveguide.     

用有限元亏量校正求特征值下界

对解2阶椭圆特征值问题的线性有限元法,本文考虑了一种计算简单的有限元亏量校正方案。基于插值校正和Rayleigh商给出了新的校正特征值。理论分析表明该校正特征值或者达到二次元的精度阶或者当网格直径充分小时下逼近准确特征值,并用数值实验验证了理论结果。     

利用泄漏瑞利波成像检测陶瓷表面缺陷

介绍了利用泄漏瑞利波成像检测陶瓷表面缺陷的方法，并通过实例讨论了采用高频聚集探头激励泄漏瑞利波的特性以及泄漏瑞利波成像的规律。     

扫描激光激发瑞利波探测材料表面微缺陷

提出了一种无损探测材料表面微小缺陷的新方法,利用移动扫描激光线源在材料表面激发的超声瑞利波,结合自行改进的差分式光偏转探测系统接收该超声波信号,并对该探测系统的测试原理进行了讨论;在利用系统进行实验研究时观察到：当激发激光源接近材料表面的人工缺陷时,探测信号的幅度会发生显著变化．通过对信号幅度变化规律的特征分析,可以对材料表面的微小缺陷进行诊断,亦证实了扫描激光线源技术在材料表面微小缺陷无损检测方面具有的应用价值．     

作大范围运动矩形薄板的建模理论和有限元离散方法

研究了作大范围运动薄板的耦合动力学建模理论和离散化方法。对作大范围运动的薄板建立了耦合动力学模型，计及了在结构动力学中对薄板动力学特性影响很小的二次耦合变形量。用有限元方法对秉性薄板进行离散，基于Jourdain速度变分原理导出了作大范围运动薄板的动力学方程。计算了作旋转运动的薄板的变形，将仿真结果与不计二次耦合变形量的传统方法进行比较表明，随着转速的提高，仿真结果出现明显的差异。此外，将本文有限元与假设模态法的计算结果进行比较，揭示了高速旋转时假设模态法的局限性，表明取无大范围运动的高阶模态可以提高假设模态法的计算精度。     

Modeling and evaluating the scattering of Rayleigh waves by a linear distribution of surface cracks

A numerical evaluation model of the reflection coefficient of Rayleigh waves for a linear distribution of surface cracks is developed using the weight function estimation method. The numerical evaluation of the reflection coefficients varying with respect to the crack depth ratios, frequencies, and the number of the cracks for several commonly used engineering materials is performed. The results show that the model can effectively be used to evaluate the reflection coefficient for a linear distribution of the cracks. The crack depth which can be evaluated has been extended substantially and the evaluation results have been improved.     

基于有限元方法的双层材料激光超声数值模拟

采用有限元方法数值模拟了单脉冲激光热弹激发双层材料中的超声波,得到了材料的上下表面的应力场分布。通过对铝板中的波形进行快速傅里叶变换得到频率信号,计算出波包群速度,判断出材料中兰姆波的模态。结果表明：受基底影响,在双层材料的上层铝板中得到了三种模态的兰姆波。该研究为复合材料激光超声的应用研究提供了新的途径。     

Impulse Modeling of the Rayleigh Wave Propagation Generated by a Spherical Focused Transducer

In order to characterize materials locally by means of the Rayleight wave analysis, a new modeling of acoustic microscopy in the case of an impulse excitation is presented. The usualV(z) representation established for a given frequency component, is extended to the case of a broadband excitation of the transducer. Therefore, the time-dependant acoustic response of the material,s(z,t), is mainly composed of two echoes: the specular and the Rayleigh contributions which are resolved in time. In the first part, we demonstrate that the acoustic response can be represented by the time convolution product between the acoustic signal detected at the focus on an ideal reflector, and a function depending onz andt variables. This last function is connected to the tranducer emission profile, which includes the diffraction effects, and the reflectance of the material. In the second part, the modeling is discussed with regard to experimental data on several materials and takes into account the attenuation phenomena. Experimental results and computations are shown to be in a good agreement.     

Stabilized Finite Element Approximation of the Stationary Magneto-Hydrodynamics Equations

In this work we present a stabilized finite element method for the stationary magneto-hydrodynamic equations based on a simple algebraic version of the subgrid scale variational concept. The linearization that yields a well posed linear problem is first identified, and for this linear problem the stabilization method is designed. The key point is the correct behavior of the stabilization parameters on which the formulation depends. It is shown that their expression can be obtained only on the basis of having a correct error estimate. For the stabilization parameters chosen, a stability estimate is proved in detail, as well as the convergence of the numerical solution to the continuous one. The method is then extended to nonlinear problems and its performance checked through numerical experiments.     

扩展有限元法在裂纹扩展问题中的应用

扩展有限元法(Extended finite element method,XFEM)是近几年发展起来的数值方法,属于传统有限元法的扩展,具有区别于传统有限元法的优点,在求解不连续断裂问题上具有更高的精度及效率。本文针对影响裂纹扩展的主要因素进行探讨,继而介绍扩展有限元的基本原理,并对其在裂纹扩展中的应用进行综述,同时对该方法的下一步研究进行了展望。     

激光声表面波检测铝板表面凹痕的数值研究

采用平面应变的有限元模型数值模拟了热弹机制下线型脉冲激光辐照金属铝板表面激发高频声表面波,及声表面波经过表面矩形凹痕时发生反射的过程.计算结果表明: 声表面波中的瑞利波经过表面凹痕时发生明显的反射,并产生两个相继出现且具有相同的传播速度的反射表面波成分;随着凹痕深度的增加,两个反射瑞利脉冲出现的时间间隔将增大;数值计算从理论的角度有力地证实了前一反射瑞利波产生于凹痕的顶端,而后者源于其底部的论断,从而为定量检测金属表面缺陷的深度提供了理论依据.     

波浪入射角及地形对浮体水动力学特性影响的有限元分析

利用有限元方法分析了二维情况下波浪入射角及地形对浮体水动力学特性的影响，计算了不同情况下浮体的水动力学系数；通过与简单地形情况下解析解的比较，验证了数值方法的正确性；并在分析中采用了BiCGStab算法，高效地求解离散形成的线性方程组，大大地提高了分析的效率。研究结果为海上结构物的设计提供了关键的水动力学数据。     

天津软土地区地铁运营沿线地面振动响应实测与建模分析

首先利用CMG-5TCDE一体化强震仪,对天津地铁5号线职业大学站-淮河道站区间沿线进行场地地表振动强度测试;接下来基于实际工程数据,利用ABAQUS建立轨道-道床-隧道-土层三维有限元模型,数值模型通过Py-thon程序实现黏弹性边界的设置,并应用DLOAD子程序实现列车轮载的移动施加,最终采用动力隐式分析方法模拟计...     

Size Sensitivity for the Reliability Index in Stochastic Finite Element Analysis of Damage

The direct differentiation method is applied to the estimation of statistical size effect behaviour in quasi-brittle solids. The scale factor is included in the finite element model and the autocorrelation function. Particular attention is paid to the proper differentiation of the Nataf transformation, which has been chosen to convert the basic random variables into a set of uncorrelated, standard normal variables. The predictive possibilities of the presented algorithm provide a valuable insight in the actual mechanisms responsible for failure. It can be evaluated to what extent the scale factor sensitivity of the failure probability is influenced by the phenomena related to the material disorder or the deterministic size effect.     

碳纳米管力学性能分析的分子结构有限元模型

基于分子结构力学理论,将C—C共价键等效比拟为空间梁单元,利用ANSYS的二次开发工具APDL语言编程创建能够恰当模拟碳纳米管原子间相互作用势的分子结构有限元模型,应用该模型详细研究碳纳米管在简单拉伸和纯扭转时的力学性能及其尺度效应,即手性角和管径变化对碳纳米管杨氏模量、泊松比和剪切模量的定量影响。计算结果表明,杨氏模量和剪切模量与大部分相关文献的结果吻合,但泊松比的数值结果和尺度依赖性规律在国内外文献中尚未获得统一观点,且在弹性模量、泊松比和剪切模量之间未发现明确的定量关系。     

钛/瓷连接界面残余应力有限元分析

采用有限元法分析了钛/瓷、钛/ZrO2/瓷连接冷却过程中界面残余应力形成的大小和分布特征。结果表明,钛/瓷试样在结合界面处存在较大的残余应力。在平行于界面的方向上,陶瓷表面受拉应力的作用,易导致表面裂纹的产生。钛/瓷界面的边缘处存在较大的应力集中,尤其是结合界面位于Z向棱边的顶角处,应力集中较为明显,易导致瓷崩和瓷裂。ZrO2中间层可抑制钛表面过度氧化,缓和钛/瓷界面应力集中,提高钛/瓷结合强度,降低陶瓷层裂纹倾向。     

Finite element interface models for the delamination analysis of laminated composites: mechanical and computational issues

The finite element analysis of delamination in laminated composites is addressed using interface elements and an interface damage law. The principles of linear elastic fracture mechanics are indirectly used by equating, in the case of single‐mode delamination, the area underneath the traction/relative displacement curve to the critical energy release rate of the mode under examination. For mixed‐mode delamination an interaction model is used which can fulfil various fracture criteria proposed in the literature. It is then shown that the model can be recast in the framework of a more general damage mechanics theory. Numerical results are presented for the analyses of a double cantilever beam specimen and for a problem involving multiple delamination for which comparisons are made with experimental results. Issues related with the numerical solution of the non‐linear problem of the delamination are discussed, such as the influence of the interface strength on the convergence properties and the final results, the optimal choice of the iterative matrix in the predictor and the number of integration points in the interface elements. Copyright © 2001 John Wiley & Sons, Ltd.     

The Determination of Stress Distribution and Elastic Properties for Heterogeneous Materials with Hybrid Finite Element

This paper describes a numerical method for evaluating the micro and macromechanical response of two-dimensional heterogeneous materials with both periodic and random distribution of the second phase. The proposed method uses two different numerical programs, based on the Voronoi Cell Finite Element Method, developed by the authors. Various numerical examples are executed for validating the effectiveness of the proposed method in evaluating the overall elastic constants of heterogeneous materials, and different comparison with analytical models and experimental results are shown. In order to validate the accuracy of the programs for predicting the stress micro-fields around the inclusion, some comparisons with a FEM commercial numerical code are performed.     

Determination of the Normal Contact Stiffness and Integration Time Step for the Finite Element Modeling of Bristle-Surface Interaction

In finite element modeling of impact, it is necessary to define appropriate values of the normal contact stiffness, K_n, and the Integration Time Step (ITS). Because impacts are usually of very short duration, very small ITSs are required. Moreover, the selection of a suitable value of K_n is a critical issue, as the impact behavior depends dramatically on this parameter. In this work, a number of experimental tests and finite element analyses have been performed in order to obtain an appropriate value of K_n for the interaction between a bristle of a gutter brush for road sweeping and a concrete surface. Furthermore, a suitable ITS is determined. The experiments consist of releasing a steel bristle that is placed vertically at a certain distance from a concrete surface and tracking the impact. Similarly, in the finite element analyses, a beam is modeled in free fall and impacting a surface; contact and target elements are attached to the beam and the surface, respectively. The results of the experiments and the modeling are integrated through the principle of conservation of energy, the principle of linear impulse and momentum, and Newton’s second law. The results demonstrate that, for the case studied, K_n and the impact time tend to be independent of the velocity just before impact and that K_n has a very large variation, as concrete is a composite material with a rough surface. Also, the ratio between the largest height of the bristle after impact and the initial height tends to be constant.