A Study of Large Plastic Deformations in Dual Phase Steel Using Digital Image Correlation and FE Analysis

Large plastic deformation in sheets made of dual phase steel DP800 is studied experimentally and numerically. Shear testing is applied to obtain large plastic strains in sheet metals without strain localisation. In the experiments, full-field displacement measurements are carried out by means of digital image correlation, and based on these measurements the strain field of the deformed specimen is calculated. In the numerical analyses, an elastoplastic constitutive model with isotropic hardening and the Cockcroft–Latham fracture criterion is adopted to predict the observed behaviour. The strain hardening parameters are obtained from a standard uniaxial tensile test for small and moderate strains, while the shear test is used to determine the strain hardening for large strains and to calibrate the fracture criterion. Finite Element (FE) calculations with shell and brick elements are performed using the non-linear FE code LS–DYNA. The local strains in the shear zone and the nominal shear stress-elongation characteristics obtained by experiments and FE simulations are compared, and, in general, good agreement is obtained. It is demonstrated how the strain hardening at large strains and the Cockcroft–Latham fracture criterion can be calibrated from the in-plane shear test with the aid of non-linear FE analyses. An erratum to this article can be found at     

基于微分求积法的钢筋混凝土梁静力分析

采用广义微分求积法(GDQM)对钢筋混凝土(RC)梁进行了准静力分析,得到了其抗静载的强度特性．首先,基于虚功原理导出了考虑钢筋和混凝土材料的非线性的RC梁准静力分析控制微分方程,并根据广义微分求积法对其离散,从而得到有限自由度的非线性代数方程组,进而采用Newton-Raphson迭代求解格式,建立了荷载增量法数值分析模型．其次,通过本文GDQM与有限元法分析结果比较,表明了新建算法的正确性;与有限元法的收敛性对比表明本文算法较有限元法有优越性．     

数字图像相关方法在桥梁裂缝变形监测中的应用

本文结合工程实际背景分析了数字图像相关方法应用在桥梁裂缝变形监测中的可行性及优越性。根据桥梁裂缝变形的特点采用数字图像处理技术中的位移梯度法进行所选区域的相关匹配搜索并对相关系数的计算公式加以改进。使实验过程得以简化。计算结果全场化、直观化。本文对数字图像处理软件的测量精度和在实际测量中的测试误差进行了分析和研究。证明该软件能够更加精确地测定裂缝在各种荷载作用下的变形量值．实际的桥梁测试也验证了该方法的可行性和优越性。若图像记录设备的分辨率是12．47pixel／mm(0．0802ram／pixel)。本文所涉及到的桥梁裂缝边缘的位移测试精度为0．000802mm。     

Operational Modal Analysis of a Helicopter Rotor Blade Using Digital Image Correlation

A novel procedure to perform operational modal analysis on a reduced-scale, 2 m diameter helicopter rotor blade is described. Images of the rotor blade rotating at 900 RPM are captured by a pair of high-speed digital cameras at a sampling rate of 1000 frames per second. From these images, the out-of-plane bending deformation of the rotor blade is measured using Digital Image Correlation, with a spatial resolution of 7.2 mm and an accuracy of 60 μm, or 0.006 % of the rotor radius. Modal parameters including natural frequencies and mode shapes are determined from the bending deformation through application of the Ibrahim Time Domain method. The first three out-of-plane bending modes were identified at each rotational speed and compared to an analytical finite element model of the rotor blade. The experimental and analytical natural frequencies agreed to within 0.2 % in the best case and 10.0 % in the worst case. The experimental mode shapes were also found to closely match the analytical predictions. The results of this study demonstrate the ability of this procedure to accurately determine the modal parameters of rotating helicopter rotor blades.     

散斑图像相关数字技术原理及应用

研究图像处理技术在散斑测量中的应用，提出了一种散斑图像相关数字技术，该方法引进了亚像素技术，采用重心算法计算特征斑的重心，避免了数字散斑相关法计算相关系数的繁复过程；应用位移和应变的有关公式，可以获得物体变形实验曲线，实验结果表明，该方法在工程实际现场、振动过程以及变形测量的自动化等方面有着广泛的应用潜力，从而为光测力学拓展应用领域、实现自动化测量展现了新的前景。     

Residual Stress Analysis of Orthotropic Materials Using Integrated Digital Image Correlation

Measuring residual stress in an orthotropic material is a difficult task due to the complex behavior of the material. Recently, two different approaches based on Smith’s simplified real value formulation and the general solution developed by Lekhnitskii have been proposed. Both solutions assume the measurement of the displacement field via interferometric optical methods and estimate stress values through solving an inverse problem. However, the high sensitivity to vibrations of interferometric techniques makes their use difficult outside optical laboratories; standard Digital Image Correlation could be used, but its low sensitivity and relatively high standard deviation of displacements severely affect the reliability of estimates. In this work we propose to integrate the residual stress displacement functions related to orthotropic materials into the shape functions of Digital Image Correlation. This makes it possible overcome most of the problems related to low sensitivity and large standard deviation because a single large patch can be used for the measurement, thus providing an accurate and reliable algoritm for the measurement of residual stress.     

Measurement of Local Thermal Deformations in Heterogeneous Microstructures via SEM Imaging with Digital Image Correlation

It is challenging to measure accurately and with high spatial resolution the local thermal strains in heterogeneous microstructures due of the complex nature of the thermal deformations and local boundary conditions. In the enclosed study, a digital image correlation (DIC) based, thermal strain mapping technique is described that is able to probe thermal deformations with sub-micron spatial resolution and sub-nanometer displacement accuracy for both homogeneous and heterogeneous materials, including cross-sections of IC packages. The full-field thermal deformation maps of different materials within a nanostructured IC chip cross-section are established from room temperature up to 160 °C, uncovering the heterogeneous nature of the specimen while accurately measuring the highly non-uniform displacement and strain fields across the multiple material constituents. As described in this work, the DIC-enabled technique is capable of high resolution mapping of local thermo-mechanical deformations in heterogeneous materials, providing a methodology that can improve our understanding of complex material systems under controlled thermal-environmental conditions.     

Three-dimensional Full-field Measurements of Large Deformations in Soft Materials Using Confocal Microscopy and Digital Volume Correlation

A three-dimensional (3-D) full-field measurement technique was developed for measuring large deformations in optically transparent soft materials. The technique utilizes a digital volume correlation (DVC) algorithm to track motions of subvolumes within 3-D images obtained using fluorescence confocal microscopy. In order to extend the strain measurement capability to the large deformation regime (>5%), a stretch-correlation algorithm was developed and implemented into the Fast Fourier Transform (FFT)-based DVC algorithm. The stretch-correlation algorithm uses a logarithmic coordinate transformation to convert the stretch-correlation problem into a translational correlation problem under the assumption of small rotation and shear. Estimates of the measurement precision are provided by stationary and translation tests. The proposed measurement technique was used to measure large deformations in a transparent agarose gel sample embedded with fluorescent particles under uniaxial compression. The technique was also employed to measure non-uniform deformation fields near a hard spherical inclusion under far-field uniaxial compression. Introduction of the stretch-correlation algorithm greatly improved the strain measurement accuracy by providing better precision especially under large deformation. Also, the deconvolution of confocal images improved the accuracy of the measurement in the direction of the optical axis. These results shows that the proposed technique is well-suited for investigating cell-matrix mechanical interactions as well as for obtaining local constitutive properties of soft biological materials including tissues in 3-D.     

数字图像相关方法的准确度与速度增强

数字图像相关方法已经成为近几年来最为广泛使用的实验力学方法之一.本文从算法的角度提出了增强该方法准确度与速度的几个技术.由于插值算法在数字图像相关方法测量中扮演着重要的角色,文中采用了几种基于B样条表示的递归插值算法来提高数字图像相关方法的分析准确度与计算速度.在实践中,三维的测量在许多数字图像相关方法的应用中成为主导...     

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.

     

关于数字图像相关中曲面拟合法的几点讨论

曲面拟合法求解亚像素位移是数字图像相关亚像素位移定位中的一种重要方法,它具有抗噪声能力较强、精度高、计算效率高等优点,在实际应用中多被采用.本文就该算法中影响亚像素位移定位精度的各要素进行了详细讨论,并用计算机生成的模拟散斑图和金属试件的刚体平移实验进行了验证,结果表明,整像素搜索时选取不同的计算窗口对计算结果的影响最大,而文中所列的几种相关函数的选取对计算结果的影响则可以忽略.     

Full-Field Deformation Measurements in Liquid-like-Solid Granular Microgel Using Digital Image Correlation

This paper presents the experimental characterization of the in-plane deformation field at any depth within a granular support medium (GSM) called Carbomer 940 using digital image correlation (DIC) and particle image velocimetry (PIV). A method was developed to produce a 2D plane of randomly shaped speckles within the GSM for DIC. Four different needle diameters and four different speeds were used as test specimens representative of those utilized for 3D printing of soft matter in the GSM. The results can be used to determine dimensional tolerances and assessing interactions between multiple injection needles and acceptable spacing. The displacements in the direction of needle motion (u) and transverse (v) were obtained. Subsequently, the magnitudes were determined as a function of distance from the needle path and time history. Results show that near the needle there is a region of yielded/fluidized material and away from the needle path the material acts like a viscoelastic solid. Permanent deformation decreases with increased distance from the path and recovery is enhanced by reversing back through the path.     

Experimental Analysis of the Shot Peening Particle Stream Using Particle Tracking and Digital Image Correlation Techniques

Experimental Mechanics - The conventional air pressure shot peening process consists of multiple impacts of particles propelled with pressurized air through a nozzle at the surface of mechanical...     

Mechanical Characterization of Coatings Using Microbeam Bending and Digital Image Correlation Techniques

A new technique for characterizing end-supported microbeams of coating materials is presented. Microbeams are fabricated using micro-EDM machining to isolate the material under investigation from the underlying substrate. Three- and four-point bending is realized by a custom-built microspecimen testing system, and digital image correlation is employed to capture full-field strains and displacements in theses microbeams. These experiments provide the foundation for the use of finite element modeling and inverse methods to determine the mechanical properties (elastic moduli, strength, interfacial toughness) of the coatings. Here, the experimental details of the microbeam bending experiments are explained, discussed and illustrated through application to a multilayered metal/oxide/ceramic thermal barrier coating system commonly used in aero-turbines.     

结合数码显微镜的数字散斑相关方法精度分析及应用

结合数字散斑相关方法和一种新型的显微镜数码显微镜,提出了一种测量多晶材料晶粒尺度面内变形的新方法,并通过零变形校准实验、重聚焦实验和平移实验等一系列验证实验分析了该方法的精度和实用性.作为应用实例,对一种镍基合金试件进行了单向拉伸和疲劳实验,得到了晶粒尺度下具有较大应变梯度的应变分布图像.结果表明,该方法能够得到精确的位移测量数据,是一种理想的测量晶粒尺度变形的光测方法.     

Full-field Ultrahigh-speed Quantification of Dynamic Shear Ruptures Using Digital Image Correlation

Experimental Mechanics - Producing dynamic ruptures in the laboratory allows us to study fundamental characteristics of interface dynamics. Our laboratory earthquake experimental setup has been...