A stiffness equation transfer method for natural frequencies of structures

A stiffness equation transfer method is proposed for obtaining vibration frequencies of structures. This method is an extension of the finite element-transfer matrix (FE-TM) method. In the present method, the transfer of state vectors from left to right in the ordinary FE-TM method is changed into the transfer of stiffness equations of every section from left to right. This method reduces the propagation of round-off errors produced in the ordinary transfer matrix method. Furthermore, the drawback that the number of degrees of freedom on the left boundary must be the same as that on the right boundary in the ordinary FE-TM method, is now avoided. Besides, this method finds out the values of the frequency by Newton-Raphson iteration method, so no plotting of the value of the determinant versus assumed frequency is necessary. An IFETM—W program based on this method for use on an IBM PC586 microcomputer is developed. Finally, numerical examples are presented to demonstrate the accuracy as well as the potential of the proposed method for free vibration analysis of structures.     

A method for crack sizing using Laser Doppler Vibrometer measurements of Surface Acoustic Waves

The goal of non-destructive testing (NDT) is to determine the position and size of structural defects, in order to measure the quality and evaluate the safety of building materials. Most NDT techniques are rather complex, however, requiring specialized knowledge. In this article, we introduce an experimental method for crack detection that uses Surface Acoustic Waves (SAWs) and optical measurements. The method is tested on a steel beam engraved with slots of known depth. A simple model to determine the cracks size is also proposed. At the end of the article, we describe a possible application: fatigue crack sizing on a damaged slat track. This technique represents a first step toward a better understanding of the crack growth, especially in its early stages (preferably when the cracks can still be repaired) and when it is possible to assume a linear propagation of the crack front. The ultimate goal of this research program is to develop a useful method of monitoring aircraft components during fatigue testing.     

Hybrid holographic-numerical method for modal analysis of complex structures

This paper presents a hybrid holographic-numerical method for modal analysis of complex structures. A continuous structure is first lumped into a number of discrete elements to form an elastically connected lumped linear system. The matrix of influence coefficients of the lumped linear system are then determined by exerting a static load to the element centers and measuring the corresponding whole-field displacement using digital holographic interferometry. The eigenvalues and eigenvectors of the influence coefficients, which in a physical sense represent the natural frequencies and mode shapes of the structure, are then calculated using the numerical method. A major advantage of the proposed hybrid method is that it is not necessary to know the Young's modulus, the Poisson's ratio of the material and the boundary conditions, as the displacement field measured by the optical method has automatically reflected the real boundary conditions and the material properties, which makes this method particularly useful for studying objects made from anisotropic materials such as composites. Another advantage of the proposed method is that structures of any complex and irregular shape will not increase the complexity of the characterization process. The proposed is also suitable for experimentally validating the modal analysis results from finite element method models.     

Cooling analysis of welded materials for crack detection using infrared thermography

Infrared thermography offers a wide range of possibilities for the detection of flaws in welding, being the main difference among them the thermal excitation of the material. This paper analyzes the application of an inexpensive and versatile thermographic test to the detection of subsurface cracks in welding. The procedure begins with the thermal excitation of the material, following with the monitoring of the cooling process with IRT (InfraRed Thermography). The result is a sequence of frames that enables the extraction of thermal data, useful for the study of the cooling tendencies in the defect and the non-defect zone. Then, each image is subjected to a contour lines algorithm towards the definition of the morphology of the detected defects. This combination of data acquisition and processing allows the differentiation between two types of cracks: toe crack and subsuperficial crack, as defined in the quality standards.     

一种铁磁材料探伤的方法

介绍一种利用LC振荡电路的磁回路探测材料表面和内部伤痕的方法．实验证明,该方法能应用于高压容器的深层探伤,且效果很好．     

Design of interdigital transducers for crack detection in plates

Interdigital transducers (IDT) for non-destructive evaluation (NDE) of cracks in plates are designed based on an analytical model established previously. Key considerations include mode selectivity, excitation strength, collimation of wave and cost. The advantage of mode selectivity of IDT over PZT patch is presented both analytically and experimentally. Effects of parameters, namely finger spacing, width, length, number of fingers, and the size of IDT, on the excitation strength and mode selectivity are considered. This led to the design of a mobile double-sided IDT as an efficient device where excitation strength is strong and focused. The device was fabricated in-house using commercially available piezoelectric ceramics and used to develop a procedure for accurate identification of the direction and extent of cracks in plates. Three aluminum plates, one with a linear deep crack, another with a piecewise linear shallower crack and the third with a curved crack, were used to illustrate the accuracy and efficiency of both the proposed device and procedure for effective NDE.     

A novel sensitivity-based method for damage detection of structures under unknown periodic excitations

This paper presents a technique for damage detection in structures under unknown periodic excitations using the transient displacement response. The method is capable of identifying the damage parameters without finding the input excitations. We first define the concept of displacement space as a linear space in which each point represents displacements of structure under an excitation and initial condition. Roughly speaking, the method is based on the fact that structural displacements under free and forced vibrations are associated with two parallel subspaces in the displacement space. Considering this novel geometrical viewpoint, an equation called kernel parallelization equation (KPE) is derived for damage detection under unknown periodic excitations and a sensitivity-based algorithm for solving KPE is proposed accordingly. The method is evaluated via three case studies under periodic excitations, which confirm the efficiency of the proposed method.     

一种直接测量压电换能器串联谐振频率和并联谐振频率的新方法

本文提出了一种利用补偿原理直接测量压电换能器串联谐振频率fs和并联谐振频率fP的新方法一补偿法。在这种方法测量原理的基础上，进行了实际测量，并设计了一种实用的测量电路。采用此方法测量的测量结果与电桥法的结果相符得很好。     

Bidirectional beam propagation method based on the method of lines for the analysis of photonic band gap structures

The bidirectional beam propagation method based on the method of lines is proposed as an innovative and efficient algorithm to investigate the optical properties of photonic band gap (PBG) structures. A few examples illustrate the application of this technique to the modeling of passive, lossy and active one-dimensional and index confined PBG structures. The algorithm results are validated by comparison with those obtained via the transfer matrix method, the mode-matching method and the finite difference time domain method. With respect to these methods, the present algorithm exhibits accurate results with reduced computer resources.     

A dimension-wise method for the static analysis of structures with interval parameters

A novel method for the static analysis of structures with interval parameters under uncertain loads is proposed, which overcomes the inherent conservatism introduced by the conventional interval analysis due to ignoring the dependency phenomenon. Instead of capturing the extremum of the structural static responses in the entire space spanned by uncertain parameters, their lower and upper bounds are calculated at the minimal and maximal point vectors obtained dimension by dimension with respect to uncertain parameters based on the Legend orthogonal polynomial approximation, overcoming the potential engineering insignificance caused by the optimization strategy. After performing its theoretical analysis, both the accuracy and applicability of the proposed method are verified.     

水下网络突发分组前导码检测的自适应阈值算法

本文针对水声信道的特点,提出了一种适于水下传感器网络突发通信分组前导码检测的自适应阈值算法。该算法根据估计的信号瞬时功率自适应地调整阈值,以取代固定阈值算法中的预设常数阈值。进行了理论分析和仿真,证明该算法具有恒虚警的特点,检测概率比固定阈值算法有明显的提高。针对检测到相关函数旁瓣的情况,加入了滑动窗口。该算法经过了湖试验证,性能良好,具有实用价值。     

凝视观测图像空间弱小目标检测方法

针对凝视观测模式下光测图像中弱小目标的检测问题,提出一种多帧频域特征累积的目标检测方法。利用恒星匹配、临帧差分等方法消除背景变化的影响,通过图像时频域特征分析,推导出递归形式频域特征累积方程,并由此检测图像中的弱小目标。实验结果表明采用多帧频域特征累积方法能将图像信噪比提高3倍以上,有利于检测出弱小目标。     

A laser beam deflection technique for the quantitative detection of ultrasonic Lamb waves

Ultrasonic Lamb waves have been measured with a quantitative optical beam deflection (OBD) technique. Calibration of the technique is derived for two cases of laser probe beam cross-section, one with a uniform beam and the other with a Gaussian beam intensity distribution. Expressions for angular beam deflection angle are derived for both cases in terms of the total light intensity falling on the photodetector, assumed to have a square-law voltage response. Using a Gaussian beam intensity distribution from a HeNe laser, the OBD measurements were compared with those from a Michelson interferometer to show that measurements were self-consistent.     

Radius of curvature measurements for laser beams: A simple method

A simple method for measuring the radius of curvature of laser beams is introduced. It has been developed to estimate the astigmatic aberration of a diode laser. Compared with the interferometry, this method is convenient and straightforward.     

像散法离焦检测中的像散透镜参数设计

推导出了离焦量与聚焦误差信号功率的关系式。利用MATLAB软件仿真,详细分析了像散透镜系统各个参数对离焦量、灵敏度、光斑大小等的影响,得到了像散圆透镜与像散柱透镜焦距之间的距离关系。结果发现聚焦物镜和探测器参数对最小离焦量、灵敏度、精度有决定性的影响。提出了像散透镜系统各光学参数选择的原则和方法,并以德国Silicon Sensor公司的QP50-6型四象限探测器为例,详细分析了离焦量与聚焦误差信号功率的线性关系。这些工作对建立满足应用对象要求的像散法离焦检测系统具有重要的指导意义。     

A subfringe integration method for multiple-beam Fizeau fringe analysis

Some aspects concerning the subfringe integration method in interferogram analysis have been investigated and modified. The modified algorithm, introduced in this paper, is capable of reconstructing the phase in the presence of noise or errors in carrier frequency. The subfringe integration method was applied to analyze two computer simulated patterns of equispaced Fizeau fringes using N bucket integration. Also, it is used to analyze the multiple-beam Fizeau fringe. The refractive index profile of polyethylene fiber is obtained by using two methods, subfringe integration method, and Fourier transform method. A comparison between the obtained results using the maintained methods is presented.     

Laser computed tomography of opaque industrial products measured by coherent detection imaging method

The coherent detection imaging method with a low-power He–Ne laser as the source is used to obtain transillumination laser computed tomographic and two-dimensional (2D) images of opaque industrial products such as fluorescent lamps and electric light bulbs. This method is principally based on the optical heterodyne detection technique that has the highest sensitivity along with excellent selectivity in terms of coherence, polarization and high directionality. Structures of the filament and electrode enclosed in the opaque glass were clearly identified with submillimeter resolution.     

一种测量无规则裂缝宽度的量化模型与实现

建立了一种测量无规则裂缝宽度的量化模型,提出了一种真伪裂缝区域分割新方法.该方法通过边缘检测并利用形态学膨胀运算,在待测裂缝的两侧边缘区域选取近似等量的目标像素和背景像素后,设置门限以自适应选取最佳分割阈值,提取出裂缝区域,进而提取裂缝的走势及边缘像素,用建立的量化模型,即可实现宽度测量.该测量方法能有效克服裂缝图像的噪音较大、裂缝不规则等弊端,去除裂缝图像的伪裂缝区域,测量结果的重复准确度在0.1 mm以内.实验结果表明,该方法有效地实现了裂缝宽度的准确测量,具有较强的适应性.     

A novel method for the detection of plasma jet boundaries by exploring DNA damage

The coupled-channel optical method (CCOM) is implemented in this work to study the scattering of electron on rubidium atom at 20 eV. In order to provide a realistic calculation, the continuum effect of the scattering system is accounted by incorporate an ab initio optical potential into the CCOM calculation. The differential cross sections (DCS), as well as the reduced Stokes parameters of 5p excitation are reported and compared to the available experimental and theoretical data.     

超声阵列方法在层状结构损伤检测中的应用*

层状钢筋混凝土结构常见的裂缝、层间离缝等损伤具有尺度小、隐蔽的特点,常规检测方法无法对其精确查明与定量评价。以高铁层状无砟轨道结构为研究对象,使用阵列式超声装备采集半矩阵数据,利用全聚焦算法对轨道内部结构及损伤成像,通过线性旅行时插值射线追踪算法计算超声波在层状介质中传播的最短到达时间代入全聚焦成像公式,具有深度准确、分辨率高、聚焦性好的特点。提出一种层间密实度评价方法,以表面超声直达波能量为基准分析反射波能量差异计算密实指数,可定量分析无砟轨道层间状态。文中层状介质的检测和层间损伤定量评价方法有效查明层状无砟轨道层内裂缝和层间离缝,对使用超声波检测层状结构及层间损伤具有借鉴意义。