Advanced Image-Processing Techniques for Automatic Nondestructive Evaluation of Adhesively-Bonded Structures Using Speckle Interferometry

In conventional optical nondestructive evaluation (NDE) of structures using shearography or electronic speckle pattern interferometry (ESPI), results are typically provided in the form of fringe patterns or deformation contour plots. However, in order to fully automate the process of defect detection, it is desirable to obtain simpler results which are easier to interpret. We present here one such optical system based on additive–subtractive shearography/ESPI. This system processes additive–subtractive fringe patterns and provides the sizes and locations of defects such as disbonds in adhesively-bonded composite structures. This is achieved by exciting the structure under inspection using an acoustic stressing mechanism which sweeps a range of vibration frequencies of the structure. Since the defective areas of the structure have different mechanical properties from their neighboring regions, varying and complex fringe patterns are obtained at different stressing frequencies. We propose an algorithm which enables the automatic identification and selection of relevant additive–subtractive fringe patterns that pertain only to localized deformations associated with defects, and which excludes images that pertain to any overall modes of the entire structure. The algorithm also includes a pixel-by-pixel adjustable thresholding scheme which compensates for intensity variations due to nonuniform reflectivity from unpainted and dirty test objects. Morphological processing is then performed to extract the shapes of the defect from the processed fringe clusters. Various structures, from simple aluminum specimens with simulated defects to a complex honeycomb-based aviation repair patch specimen, have been successfully evaluated using this system.     

Parametric study on design of composite–foam–resin concrete sandwich structures for precision machine tool structures

In this paper, sandwich structures for micro-EDM machines are optimized by using parametric study varying composite geometries and parameters like stacking sequence, thickness and rib geometry. The structures are composed of fibre reinforced composites for skin material and resin concrete and PVC foam (Closed cell, Divinycell) for core materials. Column structure was designed by a beam with cruciform rib and performance indices such as static bending stiffness (EI) and specific bending stiffness (EI/ρ) for dynamic stability are examined by controlling the thickness and stacking sequence of composites. For the machine tool bed, which usually has a plate shape, was designed to have high stiffness in two directions at the same time controlling stacking sequence and rib geometry; that is, rib thickness and number of ribs. The sensitivity of design parameters like rib thickness and composite skin thickness was examined and the optimal condition for high stiffness structure was suggested. Finite element analysis was also performed to verify the static and dynamic robustness of the machine structure. L-shaped joint for combining bed and column of the micro-EDM machine was proposed and fabricated using adhesive bonding. The dynamic performance such as damping characteristics was investigated by vibration tests. From the results optimal configuration and materials for high precision micro-EDM machines are proposed.     

斜靠式拱肋系侧倾失稳临界荷载计算方法

该文以近年来发展起来的一种新型空间拱桥——斜靠式拱桥拱肋系为研究对象,基于最小势能原理,综合考虑斜靠式拱肋系主拱肋、稳定拱肋、主拱肋与稳定拱肋间横撑的变形能、作用于拱肋系的外力势能,首次推导出了斜靠式拱肋系侧倾失稳临界荷载的解析解,提出了斜靠式拱肋系侧倾失稳临界荷载的计算方法,并通过与有限元计算结果比较,证明了该文所提出计算方法的正确性。在此基础上研究了稳定拱肋倾角变化、及其侧向抗弯和抗扭刚度,主拱肋与稳定拱肋间横撑抗弯刚度对临界荷载的影响,研究结果表明稳定拱肋倾角和侧向抗弯刚度是控制斜靠式拱桥侧向稳定性的主要因素。     

Optical evaluation of the buckling behavior of stiffened composite laminates

Digital fringe projection profilometry (FPP) is suitable to inspect the surface profiles of objects with some apparent advantages such as being non-contact, making full-field measurement, using simple apparatus, being of high precision and enabling real-time profile automation. With our technique, four phase-shifted sinusoidal fringe patterns were circularly projected onto the surface of a composite laminate panel reinforced with crossed ribs. A phase shifting method was applied to inspect the surface profile variations during compression. The profile results revealed that different deflection directions of local buckling appeared at different crossed ribs, with the wave valleys located at the middle and the wave peaks occurring at the composite panel ends. From the details of the deflection analysis, it has been seen that the local buckling couple with the whole bending of the composite panel. The local buckling deflections agree well with finite element method (FEM) simulations.     

悬链线斜靠式拱肋系侧倾失稳临界荷载

基于能量分析方法,推导了主拱肋和稳定拱肋拱轴线为悬链线的斜靠式拱肋系侧倾失稳临界荷载计算公式,并通过与有限元数值计算结果比较,证明了该计算公式的正确性,进一步验证了所提出的拱肋系侧倾失稳时横撑切向和径向力学模型的适用性,并将拱轴线为悬链线和圆弧曲线的斜靠式拱肋系的侧倾失稳临界荷载进行了比对分析,阐明了圆弧曲线代替悬链线斜靠式拱肋系侧倾失稳临界荷载的适用条件。研究结果表明：拱轴线的线型对斜靠式拱肋系的侧倾失稳临界荷载有一定的影响;当稳定拱肋倾角较小时,悬链线拱肋系的侧倾失稳临界荷载与圆弧曲线拱肋系的差异不超过5%;当矢跨比较小时,圆弧曲线拱肋系代替悬链线拱肋系的侧倾失稳临界荷载产生的误差在10%以下,但矢跨比较大时,二者的差异较大,不可等同。     

Laser direct writing of rotationally symmetric high-resolution structures

We present a laser direct writing system for the efficient fabrication of high-resolution axicon structures. The setup makes use of scanning beam interference lithography incorporated with a fringe locking scheme for tight fringe phase control and allows us to fabricate large area structures with a period down to 450 nm.     

Dual-Polarized Near-Field Microwave Reflectometer for Noninvasive Inspection of Carbon Fiber Reinforced Polymer-Strengthened Structures

Carbon fiber reinforced polymer (CFRP) composites are increasingly being used to strengthen various components of civil, aerospace, and automotive infrastructures. Subsequent to the application of CFRP, evaluation of the structural integrity of the strengthened members becomes a critical issue. Microwave noninvasive inspection techniques have been successfully used for this purpose. To this end, a novel near-field microwave inspection system that employs a dual-polarized reflectometer for detecting defects, such as disbonds between CFRP laminates and strengthened structures, was developed. It is shown that this system is capable of automatic removal of the influence of undesired standoff distance (or surface roughness) variations. It can simultaneously generate three images of a defect: two at orthogonal polarizations and one after the influence of standoff distance variations is removed using the information provided by the two images. This paper discusses the unique features of a unidirectional CFRP laminate that enabled the design of this unique system, as well as the design of the dual-polarized reflectometer at the X-band (8.2-12.4 GHz). The detail design of the reflectometer, as well as the results of several measurements conducted on an actual bridge, whose members were strengthened with CFRP laminates, are also presented, illustrating the practical utility of this system.     

基于试验设计与有限元仿真的辐射井水平钻机承载部件优化设计

为了提升辐射井水平钻机承载部件的安全性与稳定性，运用试验设计（design of experiment，DOE）与有限元仿真技术对承载部件进行优化设计。通过正交试验法结合ANSYS Workbench有限元静力学仿真对由回转平台、立支架及横支架组成的承载系统的加强筋板布置形式进行优化设计，获得承载系统加强筋板的最优布局类型。在此基础上，运用基于试验设计的响应面优化法对回转平台的台面厚度、筋板高度及筋板厚度进行优化设计，进一步减小回转平台的最大等效应力、最大变形及质量，提升承载系统的安全性与稳定性。结果表明承载系统加强筋板的最优布局类型为：回转平台采用纵横筋板，立支架采用X形筋板，横支架采用V形筋板；回转平台的最优参数组合为：台面厚度为11.2 mm，筋板高度为31.9 mm，筋板厚度为12.3 mm。集成运用试验设计与有限元仿真技术对水平钻机承载部件进行优化设计的方法可为水平钻机实体样机设计提供一定的参考。     

网格式整体壁板增量成形有限元模拟

将机加后的平面外形的壁板毛坯成形为具有复杂曲面外形的壁板零件具有很大难度,为此,采用有限元方法进行壁板成形工艺模拟,分析壁板加工过程中的应力应变分布与失稳开裂现象.研究表明:网格式整体壁板增量成形过程中,塑性变形主要集中于筋条处,成形后的壁板主要是由于筋条发生塑性变形而导致壁板外形的改变.在成形过程中,筋条容易出现失稳与开裂现象.采用分段逐点循环模拟的方法,实现了网格式整体壁板结构件增量成形过程的数值模拟.     

Health monitoring of concrete structures strengthened with advanced composite materials using piezoelectric transducers

An active damage interrogation (ADI) method which uses an array of piezoelectric (PZT) transducers attached to a structure was used to detect and localize disbonds and delaminations of advanced composite reinforcement from concrete structures. The ADI system provides the ability to detect, localize, and estimate the extent of the disbond by actively exciting the structure with PZT transducers and processing the structural response as measured by the PZT transducers. The ADI system makes use of both amplitude and phase information from various actuator/sensor transfer functions, and also provides a unique method for determining when the transducer/structure bond has degraded. This paper investigates the feasibility of using the ADI method for health monitoring of concrete structures repaired with composite materials, and the advantages and limitation of this method are discussed.     

Detection of stiffness degradation in laminated composite plates by filtered noisy impact testing

The purpose of this paper is to detect damage (stiffness degradation) of laminated composite plates from noisy impact response data. The combined finite element method (FEM) with five degrees of freedom (DOF) and the advanced noise filtering algorithm described in this paper may allow us not only to detect the deteriorated elements but also to find their locations and the extents. A first order shear deformation theory (FSDT) is used to predict the structural behavior and to detect damage of laminated composite plates. The filtering procedure is designed by means of a wavelet decomposition together with a selection of the measuring points, and the optimization criterion is constructed on an estimate of the probability of detection using genetic algorithms. All these techniques are applied for the first time to composites. The effects of filtered noise associated with the uncertainty of measurements due to the complex nature of composites are considered for different layup sequences, number of layers, and length–thickness ratios. Several numerical results show that the noise filtering system is computationally efficient in identifying stiffness degradation for complex structures such as laminated composites.     

Near-field microwave image formation of defective composites utilizing open-ended waveguides with arbitrary cross sections

Many nondestructive testing techniques fail to detect the presence of defects inside composite materials due to their complex nature. Microwave near-field nondestructive testing utilizing open-ended rectangular waveguides showed promising results in detecting defects inside these composite structures. Carbon-loaded composites are lossy materials, which, in general, reduce the range of frequency that can be used to inspect these kinds of composites due to the high attenuation. Thus far, open-ended rectangular waveguide based probes have been used to inspect the structural integrity and homogeneity of these structures. These probes were found to be suitable for detecting defects in these composites. However, due to the linear polarization of the fields radiated out of open-ended rectangular waveguide probes and the linear nature of the carbon fibers, high attenuation of the radiated signal is inevitable. Consequently, only close to the surface defects were detected. Circular waveguides have not been used in near-field NDT. Circular waveguides offer many attractive features such as the circular polarization of the radiated waves. This fact might be used in penetrating deeper in high-loss composites. In this paper, the potential of utilizing rectangular and circular waveguides in inspecting defects in carbon-loaded composites will be theoretically investigated. An air void will be introduced inside both types of composites and theoretical images will be formed. Interpretation of the captured images will be done through power patterns in different planes.     

Estimating the depth and width of arbitrarily-oriented disbonds in laminates using shearography

Disbonds in a laminated plate are readily revealed from anomalies in the fringe pattern of a shearogram. The shearographic fringes represent loci of constant displacement derivatives of the deformed surface of the plate when subjected to a load increment such as vacuum stressing. In this investigation, a simple method is developed for easy estimation of the size and depth of arbitrarily-oriented square disbonds in laminated plates from a shearogram. The theory and experimental method presented may also be extended to the assessment of disbonds of arbitrary shapes.     

基于一阶剪切变形理论和移动最小二乘近似的加肋板屈曲临界荷载求解

针对加肋板屈曲临界荷载的求解,提出了一种基于一阶剪切变形理论和移动最小二乘近似的无网格方法。该方法将加肋板的肋条和平板分开考虑,肋条用梁模型来模拟,按照一阶剪切变形理论和移动最小二乘近似给出平板和肋条的无网格近似位移场,再利用板和肋条交界上的位移协调条件推导出将肋条的节点参数转换成板节点参数的公式,最后通过转换公式,将板和肋条的势能叠加,由最小势能原理得到描述整个加肋板线性屈曲行为的控制方程。该文方法相对有限元的优势在于加肋板肋条不必沿网格线布置,即使肋条位置改变也不需要网格重构。文末通过几个算例比较了该文方法解和采用实体单元的ANSYS 有限元解,两者较为接近,证明了该文方法的准确性。     

The impedance method of non-destructive inspection

The physical basis of the impedence method of non-destructive testing has been investigated. A theoretical study has been backed up by impedance measurements on structures with deliberately introduced disbonds. It has been shown that defects such as disbonds and delaminations may be modelled as a spring beneath which is the undamaged structure, the spring stiffness being infinite if no defect is present. The impedance at the top of the spring is a strong function of the spring stiffness, so impedance measurements may be used to detect damage. The technique is most sensitive when the defect is close to the surface and the base structure is relatively stiff. The limiting factor on the sensitivity of the technique is the stiffness of the dry point-contact between the impedance transducer and the structure. The results show that the technique is potentially a simple and rapid means of non-destructive testing.     

大型飞机机身双曲度蒙皮纵向拉伸成形加载优化设计与试验研究

目的针对C919大型客机机身双曲度蒙皮进行纵向拉形的加载轨迹优化与试验研究。方法应用蒙皮拉形工艺设计制造软件,采用优化设计的方法,对机身典型蒙皮零件进行纵向拉形加载轨迹设计;进行拉形过程的有限元仿真,判断设计的加载轨迹是否满足成形要求,将优化后的加载轨迹应用于实际零件成形;进行实际零件拉形试验,对毛料伸长与局部应变进行测量,与有限元仿真进行对比,验证有限元仿真的准确性。结果实际零件的毛料伸长与轨迹设计和有限元模拟的伸长量十分接近;将优化设计后的加载轨迹应用于实际零件的成形,获得了满足零件交付要求的蒙皮零件。结论通过加载优化设计与试验,验证了有限元模拟有较好的精度;通过加载轨迹优化与有限元模拟结合的拉形工艺设计方法,可以用于零件的实际生产。     

基于王莲叶脉分布的机床横梁筋板结构仿生优化

利用结构仿生的方法对Lin MC6000型龙门加工中心横梁的筋板结构进行了优化设计.通过对王莲叶脉构型规律的分析,提出了3种仿生构型并利用Optistruct软件进行了有限元仿真验证.结果表明,相对于传统的平行均布筋板,最优仿生型筋板的结构比刚度提高了17.36%,前4阶固有频率平均提高7.39%,体现了材料的优化分布和最大结构效能,并总结出横梁内部筋板设计的仿生学规律.这为突破机床构件的传统设计思路、实现结构轻量化提供了一种新方法.     

Some examples of nondestructive flaw detection by shearography

This paper presents some examples of nondestructive flaw detection using an optical method based on speckle shearing interferometry called shearography. In the method, a structure under study is illuminated by laser and imaged by a special image-shearing camera. After suitable processing, a fringe pattern which represents loci of surface displacement derivatives, i.e., strains, is observed in the image. Since defects in structures usually induce strain concentrations around them and since strain concentrations usually cause perturbations on the surface of structures, shearography reveals defects from anomalies in the recorded fringe pattern. In this work, the technique has been applied to the nondestructive detection of various flaws in plain and welded pipes, composite plates, and other engineering components. A simple model was also developed for the estimation of the depth of disbonds in glassfiber reinforced plastic sheets. Results obtained are good and demonstrate the usefulness of the method as a complement to other conventional NDT techniques.     

Computational study on effects of rib height and thickness on heat transfer enhancement in a rib roughened square channel

A computational study was carried out for the heat transfer augmentation in a three-dimensional square channel fitted with different types of ribs. The standard k–ε model and its two variants (RNG and realizable) were used for turbulence modeling. The predictions were compared with available experimental and computational results. Three rib configurations were used in the present study: 90° continuous attached ribs, 60° V-shaped broken attached thick and thin ribs. It was observed that the maximum heat transfer occurs at the normalized rib spacing (p/e) = 10 in the case of 90° attached ribs. The effects of the blockage ratio and rib thickness were investigated for 60° V-shaped broken ribs with Re = 10,000–30,000 and p/e = 10. It was observed that the average Nusselt number decreases with an increase in the Reynolds number for almost all configurations studied in the present study. For the 60° V-shaped broken ribs, increasing the blockage ratio had an adverse effect on the heat transfer. It was also observed that thin ribs perform better than thick ribs.     

基于硅与非硅复合方法的异平面空心微针制作

为了解决制备空心微针工艺复杂且成本高的问题,本文提出了采用湿法刻蚀、光刻和电镀结合的方法制备低成本空心金属微针.首先采用湿法刻蚀硅,得到280μm深的倒四棱锥锥坑;然后在锥坑上甩200μm厚的负胶SU-8填充锥坑,并通过曝光显影负胶形成微针的形貌;最后在曝光显影后的负胶上电镀50μm厚的镍得到所需形状的空心金属微针.用此方法制备的空心微针高度为350μm、壁厚为50μm,其针尖形状为三棱锥和四棱锥.通过有限元仿真分析微针强度与微针结构尺寸的关系.用此方法加工出的微针具有锥形尖,改善了刺入皮肤的效果.