A novel grating matching method for 3D reconstruction

Automatic 3D surface reconstruction has been an important research topic in digital photogrammetry for many years. Shaping from stereoscopic information is one of the widely studied topics in computer vision. Its central part is to solve the stereo matching problem automatically. Most algorithms used to solve the matching problem can be categorized as either area-base techniques or feature-based techniques. Feature-based techniques have gained more and more popularity for and it is the method that supports activities in object recognition and image understanding. But none of the former research can ensure the 100% matching exactness, thus, cannot complete high precision 3D surface measurement. In order to improve the precision of 3D measurement, we design a novel grating matching method, which can ensure 100% matching accuracy. It does not need any other assistant symbol or flag, only select one of the gratings which have to be projected to the objects. On the beginning of the measurement, the background of the object is captured by a CCD camera. Later gratings include single grating and group gratings will be projected to the object in sequence. All the images include gratings will make a subtraction with the background. The difference of the two images will be treated as 3D cues to acquire the 3D shape. Because no recognizing work is needed, the location of the single grating and each one in the group can be positioned exactly. From a lot of experiments, the proposed grating matching method is proved and it is a technique with high precision, low costs, easy operation, and an automatically matching method. Furthermore, it can be widely used in most of 3D vision recovery systems.     

可见光和近红外熔池图像融合方法

为了获取质量信息更佳的熔池图像，设计用分束镜将采集的同一路光源平均传输给两个相机，利用同步触发装置，建立了可见光和近红外熔池图像同路光源同步采集系统. 由于两个相机的图像之间具有不同的形变，提出了基于仿射变换的熔池图像配准方法，配准了可见光和近红外熔池图像. 在此基础上，提出了基于主成分分析的图像融合方法对可见光和近红外熔池图像进行图像融合. 结果表明，与加权平均融合，亮度色度饱和度融合和小波融合等传统图像融合方法相比，主成分分析图像融合方法最有效，得到的熔池图像质量最好.     

基于双光栅修正测力传感器温感量的研究

光纤光栅测力传感器存在温变量与应变量交叉影响光纤光栅中心波长的现象,为了补偿温变量对光纤光栅中心光波长的影响,提出双光栅法,以修正温变量引起的中心光波长偏转量。在不同受力、不同温度下进行光纤光栅中心光波长测量实验,结果表明：应变量的测量结果与理论计算结果基本一致,表明双光栅法制备测力传感器是可行的。     

Porosity reduction in CO2 laser welding of aluminium alloys ‐ Influence of penetration,joint, oxygen gas and oxide films

This study aims to ensure the safety of nuclear power plants. The accidents involving leaks from the welded zones at the pipe penetration part of a reactor vessel or at a coolant pipe are reported at home and abroad. One of the main causes is the welding residual stress. So, it is important to know the welding residual stress for maintaining high safety of the plants, the estimation of plant's life cycle and the plan of maintenance. The welded joints of the nuclear power plants have complex shapes, and the welding residual stresses also have complex distributions three-dimensionally. In this study, the inherent strain method combined with finite element method is used to measure the welding residual stresses accurately.

The mock-up is idealized for the welded joint at the pipe penetration part of the actual reactor vessel. The inherent strain method is applied to measure the residual stresses. In this method, the inherent strains are unknowns. When the residual stresses are distributed complexly in a three-dimensional stress-state, the number of unknowns becomes very large. So, the inherent strains are expressed with some functions to decrease the number largely. The theory, the experiment process and the analysed results are explained. The characteristics of the distributions of residual stresses and their production mechanisms are discussed. The inherent strain method gives the most probable values and the deviations of the residual stresses. The deviations are small enough for the most probable values. It assures the high reliability of the estimated results.     

钨极氩弧焊熔池表面高度测量中变形条纹图的获取

提出一种基于条纹反射和傅立叶变换的测量GTAW熔池表面高度的新方法.利用激光器将光栅条纹投影在GTAW熔池表面,反射光轴上设置双透镜系统以转移熔池物面,利用空间滤波的方法阻挡弧光成分,在输出面放置白屏接收反射的熔池变形激光条纹,用CCD对屏上变形条纹实时成像.分析了焊枪位置、熔池的反射性能、弧光辐射等对变形条纹图获取的...     

A finite element analysis for the characteristics of temperature and stress in micro-machining considering the size effect

In this paper, a finite element method for predicting the temperature and the stress distributions in micro-machining is presented. The work material is oxygen-free-high-conductivity copper (OFHC copper) and its flow stress is taken as a function of strain, strain rate and temperature in order to reflect realistic behavior in machining process. From the simulation, a lot of information on the micro-machining process can be obtained; cutting force, cutting temperature, chip shape, distributions of temperature and stress, etc. The calculated cutting force is found to agree with the experiment result with the consideration of friction characteristics on the chip–tool contact surface. Because of considering the tool edge radius, this cutting model using the finite element method can analyze micro-machining with a very small depth of cut, almost the same size of tool edge radius, and can observe the ‘size effect' characteristic. Also, the effects of temperature and friction on micro-machining are investigated.     

Orientation relationship,shape change and their traces in electron diffraction patterns and high-resolution transmission electron microscopy images

This paper outlines a straightforward method that can be used to predict the total number of orientation relationships between a pair of lattices that can be related by an invariant plane strain transformation and the shape change associated with each of the orientation relationships. This paper also provides a simple technique for calculating the shape strain and crystallography directly from electron diffraction patterns or Fourier-transformed digital patterns of high-resolution transmission electron microscopy images. It demonstrates that many of those diffusional phase transformations that are traditionally classified as non-displacive exhibit in fact a remarkable shape change and that the shape change can even be a simple shear.     

Weld pool boundary and weld bead shape reconstruction based on passive vision in P-GMAW

A passive visual sensing system is established in this research, and clear weld pool images in pulsed gas metal arc welding （ P-GMA W） can be captured with this system. The three-dimensional weld pool geometry, especially the weld height, is not only a crucial factor in determining workpiece mechanical properties, but also an important parameter for reflecting the penetration. A new three-dimensional （3D） model is established to describe the weld pool geometry in P-GMAW. Then, a series of algorithms are developed to extract the model geometrical parameters from the weld pool images. Furthermore, the method to reconstruct the 3 D shape of weld pool boundary and weld bead from the two-dimensional images is investigated.     

Homogeneous and heterogeneous deformation mechanisms in an austenitic polycrystalline Ti–50.8 at.% Ni thin tube under tension. Investigation via temperature and strain fields measurements

An initially austenitic polycrystalline Ti–50.8 at.% Ni thin-walled tube with small grain sizes has been deformed under tension in air at ambient temperature and moderate nominal axial strain rate. Temperature and strain fields were measured using visible-light and infrared digital cameras. In a first apparently elastic deformation stage, both strain and temperature fields are homogeneous and increase in tandem. This stage is followed by initiation, propagation and growth of localized helical bands inside which strain and temperature increases are markedly higher than in the surrounding regions. During the first apparently elastic stage of the unloading, both strain and temperature fields are homogeneous and decrease. The temperature and strain fields evolutions are then analysed in order to determine the deformation mechanisms (types and extents of phase transformations, variants (de)twinning, macroscopic banding) involved during the homogeneous and heterogeneous stages of deformation throughout the whole tube. The findings have significant implications for the understanding and modelling of superelastic behaviour of NiTi shape memory alloys.     

Relationship Between the Depth of Drilling and Residual Strain Relief in Fiber Reinforced Composite Materials

The mechanical properties of fiber are quite different from matrix in composite laminates, and parameters, such as the angle of ply, and the temperature and pressure during composite forming, would play an important role in residual stress in the composite. The residual strains between layers depend on the forming parameters and their mechanical properties. In this article, grating rosette, Moiré interferometry, and step drilling-hole method are used to measure the relief strains when drilling each single laminate step by step, and find some relationship between the depth of drilling and residual strain relief in fiber reinforced composite material.     

The effect of non-uniformity on the in-plane modulus of honeycombs

An approximate analysis is presented of the Young's modulus of two-dimensional (isotropic) cellular materials with a distribution of cell dimensions and cell wall thicknesses. Two approaches are used to estimate the modulus: one is based on an energy balance and the other involves a separate calculation of stress and strain. The estimated moduli depend on the moments of the distributions of wall widths and wall thicknesses and are affected by an eventual correlation between the two distributions. Actual values for three-connected foams are calculated for particular distributions.     

Application of fused visual and thermal images in monitoring and evaluation of welding processes

This article presents the application results of different methods of merging images recorded using vision cameras and thermal imaging cameras. The images were recorded during the evaluation of a welding process. The object of observation was the welding arc and the welded joint during its cooling phase. Images recorded during GMA welding on two different welding stations have been taken into account. The observations have been conducted using cameras equipped with different optics systems. This article presents an analysis of the influence of different parameters of the optics systems on the resulting fused images. The analysis also considers the pre-processing method to ensure correct matching of images. The study shows that the fused image contains aggregated information useful in online monitoring and quality evaluation of the welding process.     

一种新型经济断面钢格板

针对现有矩形和Ⅰ型断面钢格板的形状设计不利于多规格生产的问题,设计了一种新型经济断面钢格板,介绍了新型钢格板的形状、特点,及其几何尺寸、力学参数与现有钢格板的对比情况。     

Remote laser welding applications for car bodies

The fundamental objective of this study is to ensure the safety of nuclear reactors. A few accidents involving leaks from welded zones at the pipe penetration part of reactor vessels or at coolant pipes have been reported at home and abroad. One of the main causes is welding residual stress. Therefore, it is very important to know the welding residual stress in order to maintain the high safety of the plant, estimate the plant life cycle and design an effective maintenance plan. Welded joints of nuclear reactor vessels have complex shapes, and the welding residual stresses also have three-dimensional (3D) complex distributions. In this study, inherent strain-based theory and method are applied to measure the welding residual stresses. The inherent strain method is an analytical method as an inverse problem, using the least squares method, based on the finite element method. So the method gives the most probable value and deviation of residual stress. The reliability of the estimated result is discussed. In this method, inherent strains are unknowns. When residual stresses are distributed complexly in a 3D stress-state, the number of unknowns becomes very large. So, the inherent strain distribution is expressed with an appropriate function to decrease largely the number. A mock-up is idealized for a welded joint at the pipe penetration part of an actual reactor vessel. The inherent strain method is applied to the measure the residual stress of the joint. In this paper, the applicability of the inherent strain distribution function is diagnosed. Ten kinds of functions are applied to estimate the residual stress, and the accuracy and reliability of the analysed results are judged from three points of view, i.e. residuals, unbiased estimate of variance of errors and welding mechanics. The most suitable function is selected, which brings the most reliable result.     

Development of in situ measurement system for welding deformation using digital cameras

Abstract

A three-dimensional (3D) deformation (in plane and out of plane deformations) measurement method is developed using digital cameras, which require no special equipment. This method is a non-contact method, and it can sequentially measure over the entire photographed image. Furthermore, since image analysis is based on the technique of image matching, the method is applicable even when the deformation to be measured is large. In addition, since it is possible to use all pixels as measuring points, the number of available measuring points at one time is the same as the number of effective pixels of the camera. In this study, the proposed method is applied to the sequential measurement of displacement under strong lighting levels in arc welding. Through the comparison of the results measured by a 3D shape measurement system (LAT-3D) using a laser displacement gauge and digital caliper, the quantitative validity of the proposed method is also verified.     

Precise measurement of strain accommodation in austenite matrix surrounding martensite in ferrous alloys by electron backscatter diffraction analysis

Local strain distributions in austenite matrix that is deformed to accommodate shape strain associated with formation of martensite were investigated by means of electron backscatter diffraction (EBSD) analysis for various morphologies of lath, lenticular and thin plate martensite in ferrous alloys. By detecting small changes in EBSD patterns through image analysis of the patterns, components of both strain and rotation tensors in austenite matrix adjacent to martensite were measured quantitatively. In the austenite matrix surrounding thin plate martensite, the magnitude of components of strain tensor is nearly as large as those of rotation tensor, implying that shape strain of thin plate martensite is accommodated by elastic deformation of austenite. On the other hand, in the austenite matrices surrounding lenticular and lath martensite, components of strain tensor are found to be much smaller than those of rotation tensor even near the austenite/martensite interface. This indicates that most of the shape strain associated with the formation of lenticular and lath martensite is accommodated by plastic deformation in the austenite matrix. The misorientation axis of austenite adjacent to lenticular and lath martensite coincides well with that predicted from the phenomenological theory of martensite crystallography.     

基于改正二维主成分分析的焊缝表面缺陷检测

机器人焊接因零件形状不规则和焊接工艺复杂不可避免带来各种焊缝缺陷. 针对二维主成分分析应用于焊缝表面缺陷检测时面临计算复杂度高、分类准确率低及无法进行增量学习等问题,提出了一种基于均值更新的增量二维主成分分析(mean updated incremental two-dimensional principal component analysis,MUI2DPCA)算法,并将MUI2DPCA和前馈神经网络( feedforward neural network,FNN)相结合进行焊缝表面缺陷在线检测. 首先,对相机捕获的视频帧图像进行预处理得到焊缝局部块图像. 然后,利用MUI2DPCA在线提取局部块图像的模式特征. MUI2DPCA对图像的特征主成分进行增量迭代估计,降低计算复杂度,并且能够增量更新当前的样本均值,减少无关特征变化对主成分收敛性的影响. 最后,利用FNN建立提取的模式特征与焊缝类别之间的联系,实时返回焊缝表面缺陷的检测信息. 试验结果表明,该检测方法平均分类准确率为95.40%,平均处理速度可达29帧/s,能够满足焊缝在线检测的实时性要求.     

Comparison between X-ray micro-computed tomography and optical scanning tomography for full 3D strain measurement by digital volume correlation

In this study, we present three-dimensional (3D) measurements by digital volume correlation (DVC) with images from X-ray micro-computed tomography (XμCT) and optical scanning tomography (OST). In this article, we compare both techniques and we define their field of application by evaluating their measurement uncertainty in the case of rigid body translations, homogeneous strain tests and localised compression tests. For displacement, measurement uncertainty is around 0.037 voxel for OST and 0.049 voxel for XμCT which correspond to a measured strain about 0.1%. For larger strain, uncertainty increases with strain intensity but relative error remains constant about 10%. To resume, measurement uncertainties given by DVC are similar to the ones which we obtain generally for a two-dimensional (2D) study with DIC.     

Plastic and elastic strains in short and long cracks in Alloy 600 studied by polychromatic X-ray microdiffraction and electron backscatter diffraction

The microscopic strain distributions were studied for stress corrosion cracks produced electrochemically in C-rings of Alloy 600 (0.65 Ni, 0.16 Cr, 0.08 Fe). The strain data were obtained using polychromatic X-ray microdiffraction (PXM) and (in part) by electron back-scatter diffraction (EBSD). PXM was used to measure plastic and elastic strain distributions around the tip of a short crack, along with the changes to the direction and shape of the diffraction spots (ellipticity). For a sample with a short (30 μm) crack, the misorientation map showed a well-defined region of plastic deformation along the grain boundary in advance of the crack tip, extending to the next triple point. For the large crack sample, plastic and elastic stains as well as crystalline order could be measured in high detail with respect to the crack path. However, no correlation between these could be obtained, except for a notable degradation of crystalline order near the crack mouth. A comparable EBSD misorientation map shows strong correlation between misorientation and the crack edges; this may in part reflect the role of sharp edges in the more surface-sensitive approach.     

INFLUENCES OF EDGING ROLL SHAPE ON THE PLASTIC STRAIN DISTRIBUTION OF SLAB DURING MULTI-PASS V-H ROLLING PROCESS

Multi-pass slab vertical-horizontal （V-H） rolling process with variable edging roll shape have been simulated with explicit dynamic finite element method and updating geometric method. The distributions of plastic strain contour in slab daring rolling process with different edging roll and under different rolling stage have been obtained. The results show that there exist two thin strain assembling zones in slab when the flat edging roll is used, and there just exist one strain assembling zone in slab when the edging roll with groove is used. And compared the deformation equality between flat edging roll and edging roll with groove, the lateris better than the former, which supplies the theory prove to the slab deformation distribution during V-H rolling process and is helpful for predicting the slab texture.