数字体图像相关方法中基于迭代最小二乘法的物体内部变形测量

本文提出一种基于迭代最小二乘法的亚体素位移测量算法.该算法模型结合了符合实际情况的线性灰度变化模型和线性位移映射函数,采用仅需变形后体图像一阶灰度梯度的迭代最小二乘法计算亚体素位移和位移梯度.由于该算法采用的模型符合实际情况,因而具有更高的位移测量精度和更广泛的适用性.此外,由于仅需变形后体图像的一阶灰度梯度,无需计算...     

Micro- and Nanoscale Deformation Measurement of Surface and Internal Planes via Digital Image Correlation

The digital image correlation (DIC) technique is successfully applied across multiple length scales through the generation of a suitable speckle pattern at each size scale. For microscale measurements, a random speckle pattern of paint is created with a fine point airbrush. Nanoscale displacement resolution is achieved with a speckle pattern formed by solution deposition of fluorescent silica nanoparticles. When excited, the particles fluoresce and form a speckle pattern that can be imaged with an optical microscope. Displacements are measured on the surface and on an interior plane of transparent polymer samples with the different speckle patterns. Rigid body translation calibrations and uniaxial tension experiments establish a surface displacement resolution of 1 μm over a 5×6 mm scale field of view for the airbrushed samples and 17 nm over a 100×100 μm scale field of view for samples with the fluorescent nanoparticle speckle. To demonstrate the capabilities of the method, we characterize the internal deformation fields generated around silica microspheres embedded in an elastomer under tensile loading. The DIC technique enables measurement of complex deformation fields with nanoscale precision over relatively large areas, making it of particular relevance to materials that possess multiple length scales.     

应用数字图像相关方法测量含缺陷试样的全场变形

利用数字图像相关方法测量表面带孔洞、裂纹、缺口等缺陷试样的全场变形是许多实际测量任务中经常遇到的问题。就此问题，本文阐述了一种先对要避免计算的缺陷区域进行标记，在随后进行的相关计算中直接避免这些标记区域的方法。在已计算得到全场位移的情况下，文中提出了基于局部位移场最小二乘拟合的方法来计算区域边界、孔洞、裂纹或缺口附近等区域应变。最后对单侧边带半圆缺口试样的单向疲劳拉伸实验的计算结果充分显示本文方法的有效性和可靠性。     

变形测量中的数字散斑相关搜索方法

本文用仿射变换阐述了数字散斑相关方法的基本原理,通过分析散斑位移及其导数对物体变形前后散斑图的相关性的影响,对相关算法作了简化,提出了相关搜索方法.典型的实验证实了该方法的可行性,同时就散斑颗粒尺寸、子区大小对相关识别的影响等问题进行了讨论,初步给出了相关计算的有关最佳参数.     

Overview of High-temperature Deformation Measurement Using Digital Image Correlation

Background

Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures.

Objective

This overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC.

Methods

We comprehensively summarize challenges and recent advances in high-temperature DIC techniques.

Results

Fundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided. 

Conclusions

We expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.

     

Quantification of Three-Dimensional Surface Deformation using Global Digital Image Correlation

A novel method is presented to experimentally quantify evolving surface profiles. The evolution of a surface profile is quantified in terms of in-plane and out-of-plane surface displacements, using a Finite Element based Global Digital Image Correlation procedure. The presented method is applied to a case study, i.e. deformation-induced surface roughening during metal sheet stretching. The surface roughness was captured in-situ using a confocal optical profiler. The Global Digital Image Correlation method with linear triangular finite elements is applied to track the three-dimensional material movement from the measured height profiles. The extracted displacement fields reveal the full-field kinematics accompanying the roughening mechanism. Local deviations from the (average) global displacements are the result of the formation, growth, and stretching of hills and valleys on the surface. The presented method enables a full-field quantitative study of the surface height evolution, i.e. in terms of tracked surface displacements rather than average height values such as Root-Mean-Square or height-height correlation techniques. However, the technique does require that an initial surface profile, i.e. contrast, is present and that the contrast change between two measurements is minimal.     

用于物体表面形貌和变形测量的三维数字图像相关方法

使用单个摄像机的二维数字图像相关方法通常仅局限于平面物体的面内变形测量,而使用两个摄像机基于双目立体视觉原理的三维数字图像相关方法克服了这一局限,可对平面和曲面物体表面的三维形貌和载荷作用下的三维变形进行测量。本文介绍了三维数字图像相关方法的基本原理及其关键技术,并用两个典型的实验验证了该方法的有效性。     

基于数字散斑相关实验测量的材料构型力的计算方法

材料构型力学主要研究材料中的缺陷(夹杂、空穴、位错、裂纹、塑性区等)的构型(形状、尺寸和位置)改变时,所引起的系统自由能的变化。本研究将基于数字散斑相关技术,实验测量材料试件的位移场分布,随后通过材料构型力的定义式,计算求得弹塑性材料中缺陷构型力的分布。其方法概括如下:位移场通过数字图像相关技术测得;应变及位移梯度场利用三次样条拟合获得;线弹性材料应力通过简单线弹性本构方程获取,而塑性材料的表面应力场通过Ramberg-Osgood本构方程计算求得;弹塑性应变能密度分布则由应力-应变曲线数值积分获得。该方法对普通弹性材料或者弹塑性材料均适用,可以用于各种不同的缺陷及缺陷群的材料构型力测量。     

Three-Dimensional Study of Graphite-Composite Electrode Chemo-Mechanical Response using Digital Volume Correlation

A custom built reusable cell for in situ lithiation and mechanical deformation studies while in an X-ray tomograph was demonstrated, and the strain and volume changes of a composite graphite anode were computed from 3D X-ray microcomputed tomography data via Digital Volume Correlation (DVC). The test anode was a composite electrode comprised of a porous compliant matrix, graphite as the Li⁺ host material, 5-μm ZrO₂ marker particles for use with DVC, and active carbon black to enhance electrical conductivity. The composite electrodes were hot-pressed to control their porosity, and in turn the mechanical integrity of the resulting material. This composite anode was included in a half-cell and lithiated in situ while in a tomograph, and intermittent 3D data were collected at different lithiation levels up to full gravimetric capacity. Strain measurements by DVC demonstrated relatively uniform expansion of the freestanding electrode with average normal strains in the three directions varying by 20%, while the internal shear strains were found to be negligible. The average experimental strains were about 75% of the theoretical value, as estimated by the rule of mixtures, which implies that ~25% of the normal strains in graphite, due to lithiation, are accommodated by the surrounding matrix.     

数字时间序列图像相关方法在动态变形测量中的应用

本文结合微机械陀螺的简谐振动测试提出了一种改进的数字图像相关方法--数字时间序列图像相关方法,并编制了相应软件.通过对作高速简谐振动的微机械陀螺进行相关分析,可以得到试件全场的位移时程曲线;进一步对其作傅里叶变换,可以得到试件的频率等参数.通过该软件得到的微陀螺的振动频率相对误差为0.12%,从而为微结构动力学研究提供了可靠的分析手段.     

Measurement of Creep Deformation across Welds in 316H Stainless Steel Using Digital Image Correlation

Spatially resolved measurement of creep deformation across weldments at high temperature cannot be achieved using standard extensometry approaches. In this investigation, a Digital Image Correlation (DIC) based system has been developed for long-term high-temperature creep strain measurement in order to characterise the material deformation behaviour of separate regions of a multi-pass weld. The optical system was sufficiently stable to allow a sequence of photographs to be taken suitable for DIC analysis of creep specimens tested at a temperature of 545 °C for over 2000 h. The images were analysed to produce local creep deformation curves from two cross-weld samples cut from contrasting regions of a multi-pass V-groove weld joining thick-section AISI Type 316H austenitic stainless steel. It is shown that for this weld, the root pass is the weakest region of the structure in creep, most likely due to the large number of thermal cycles it has experienced during the fabrication process. The DIC based measurement method offers improved spatial resolution over conventional methods and greatly reduces the amount of material required for creep characterisation of weldments.     

3D Deformation Measurement of Soft Material under Indentation Using Improved Diffraction-Assisted Image Correlation

In inverse finite element-based analysis, complete experimental data collection is critical for multi-parameter identification and physical modeling of all kinds of materials. In this paper, diffraction-assisted image correlation (DAIC) is improved and proposed for the deformation measurement of a soft material under indentation with no blind area. A simple and convenient image-based 3D calibration method was developed, and more accurate formulations for 3D displacement measurement based on a more rigorous imaging model were derived. Using the improved DAIC, a newly developed imaging device with indenter-fixed loading and no blind area is proposed that allows 3D displacements of the whole upper surface of a soft silica gel specimen to be retrieved. The experimental results demonstrate that the proposed method is an accurate, efficient and convenient tool with a simple structure for 3D indentation deformation measurement and illustrate its capabilities to capture deformation in indentation tests with tough testing requirements, such as in situ measurement with limited access (high integration level) and dynamic testing (capturing of synchronously stereo images).     

Digital Volume Correlation: Review of Progress and Challenges

3D imaging has become popular for analyzing material microstructures. When time lapse series of 3D pictures are acquired during a single experiment, it is possible to measure displacement fields via digital volume correlation (DVC), thereby leading to 4D results. Such 4D analyses have been performed for almost two decades. The present paper aims at reviewing the achievements of and challenges faced by such measurement technique. Ex-situ and in-situ experiments are discussed. A general and unified DVC framework is introduced. Various sources of measurement bias and uncertainties are analyzed. The current challenges are studied and some propositions are given to address them.     

A Parametric Study on the Influence of Internal Speckle Patterning for Digital Volume Correlation in X-Ray Tomography Applications

Experimental Techniques - Use of the Digital Volume Correlation (DVC) technique has grown steadily in the mechanics community as a way of quantifying internal mechanical response of complex...     

Multi-scale Measurement of (Amorphous) Polymer Deformation: Simultaneous X-ray Scattering,Digital Image Correlation and In-situ Loading

This paper presents a method to investigate the behaviour of polymers on different scales during deformation using simultaneously collected synchrotron X-ray scattering, digital image correlation (DIC) and tensile loading. The method is demonstrated through experiments made on specimens of amorphous polycarbonate. Deformation is measured in-situ, simultaneously across different scales from the macroscopic deformation, measured using sensors on the tensile machine, to the full-field mesoscopic deformation, measured using DIC, down to the deformation of the nano-scale structure, studied using small and wide angle X-ray scattering (SAXS/WAXS). The DIC reveals highly inhomogeneous deformations that render conventional techniques for measuring deformation, such as extensiometers, virtually useless. The X-ray scattering is measured in several spatial points during continuous loading giving the evolution of the microstructure with respect to both spatial location and load level. The spatial mapping of the scattering reveals characters that would not be observed when only measuring at the centre point or measuring on a large area of the specimen, e.g. wide beam SAXS/WAXS or small angle neutron scattering (SANS). With these data, the macroscopic and the mesoscopic deformation can be correlated to the behaviour of the microstructure providing relevant information when developing micro-mechanical based constitutive models. The experimental results shown here indicate a direct correlation between the major principal strain direction and the maximum anisotropy direction of the SAXS patterns. The current approach can be extended to any kind of polymeric materials or polymer-based nano-composites.     

散斑图像相关数字技术在薄壁钢管混凝土长柱变形测量中的应用

薄壁钢管混凝土构件作为新发展的组合构件,对其研究具有非常重要的意义。文中首先介绍了散斑图像相关数字技术的基本原理,然后给出了变形计算公式。为弥补传统测量方法的不足,在薄壁钢管混凝土长柱偏压破坏实验中,引入了散斑图像相关数字技术,对长柱跨中截面的全过程变形(位移)进行了测量,并对测量结果进行了分析与讨论,同时还与传统测量方法所得结果进行了对比分析。实验结果与分析表明:该方法具有较高的精度和工程可用性,可克服传统测量方法中的一些弊端,且对实验设备和测量环境要求不高,很适合现场非接触测量。     

Assessment of Digital Image Correlation Measurement Errors: Methodology and Results

Optical full-field measurement methods such as Digital Image Correlation (DIC) are increasingly used in the field of experimental mechanics, but they still suffer from a lack of information about their metrological performances. To assess the performance of DIC techniques and give some practical rules for users, a collaborative work has been carried out by the Workgroup “Metrology” of the French CNRS research network 2519 “MCIMS (Mesures de Champs et Identification en Mécanique des Solides / Full-field measurement and identification in solid mechanics, http://www.ifma.fr/lami/gdr2519)”. A methodology is proposed to assess the metrological performances of the image processing algorithms that constitute their main component, the knowledge of which being required for a global assessment of the whole measurement system. The study is based on displacement error assessment from synthetic speckle images. Series of synthetic reference and deformed images with random patterns have been generated, assuming a sinusoidal displacement field with various frequencies and amplitudes. Displacements are evaluated by several DIC packages based on various formulations and used in the French community. Evaluated displacements are compared with the exact imposed values and errors are statistically analyzed. Results show general trends rather independent of the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the uncertainty with the parameters of the algorithms, such as subset size, gray level interpolation or shape functions, is discussed.     

Digital Image Correlation Study of Plastic Deformation and Fracture in Fully Martensitic Steels

Plastic deformation and fracture of two grades of fully martensitic steel are investigated with a miniature tensile stage, a custom image acquisition algorithm and digital image correlation. The image acquisition algorithm controls the camera framing rate according to user defined load, displacement and timing thresholds. This provides a greater number of images captured during periods of rapid load change over small displacements. True stress–true strain curves reveal substantial differences in material ductility and failure behavior. Fracture surfaces are examined using scanning electron microscopy and energy dispersive spectroscopy to provide insight into differences in the tensile behaviors observed for these steels.     

热-力联合作用下柱壳结构变形的计算分析

本文研究了激光连续加热和恒内压作用下柱壳结构的变形规律。以4340钢材料作为研究对象，其本构方程选用热粘塑性本构模型（Johnson-Cook模型）；选取某一应变率为临界值（本文中临界应变率取为1s^-1)。考察了激光功率密度，预载荷大小，激光作用时间等对结构变形规律的影响。主要结论有：结构失稳是导致激光辐照下充内压柱壳破坏的重要原因；激光功率密度和内压越高，结构破坏时间越短；根据辐照时间的长短，预测到了结构急速破坏，延迟破坏和不破坏三种模式。本工作对于深入认识激光作用下预载结构的热－力联合破坏有一定意义。