Residual Stress Measurement Using Hole Drilling and Integrated Digital Image Correlation Techniques

The effectiveness of optical (mostly interferometric) methods for the measurement of residual stresses is largely demonstrated in literature. Nevertheless, these techniques are still confined to optical laboratories due to their high sensitivity to vibrations which makes it very difficult to perform the measurement in an industrial environment. Digital Image Correlation (DIC) has recently been proposed as a possible solution to this problem: this non-interferometric technique is much less affected by vibrations, but its sensitivity is relatively low, thus negatively affecting the accuracy of results. This work proposes to use a variant of Digital Image Correlation, known as Integrated DIC (iDIC), in combination with the hole drilling technique. Since iDIC directly incorporates in its formulation the displacement field related to hole drilling, it overcomes most of the problems of standard DIC; in this way it is possible to obtain accurate results without using interferometric techniques.     

On the Implementation of the Integral Method for Residual Stress Measurement by Integrated Digital Image Correlation

Experimental Mechanics - The Integrated Digital Image Correlation method (iDIC) is a simple and effective approach for residual stress measurement. iDIC differs from Digital Image Correlation...     

Residual Stress Determination Using Hole Drilling and 3D Image Correlation

In recent years, the hole drilling method for determining residual stresses has been implemented with optical methods such as holographic interferometry and ESPI to overcome certain limitations of the strain rosette version of hole drilling. Although offering advantages, the interferometric methods require vibration isolation, a significant drawback to their use outside of the laboratory. In this study, a 3D image correlation approach was used to measure micron-sized surface displacements caused by the localized stress relief associated with hole drilling. Residual stresses were then found from the displacements using non-dimensional relations previously derived by finite element analysis. A major advantage of image correlation is that it does not require interferometric vibration isolation. Experiments were performed to check the ability of this new approach for uniaxial and equi-biaxial states of stress. Stresses determined by the approach were in good agreement with computed values and those determined by hole drilling using holographic interferometry.     

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.     

网格数字图像相关方法测量位移场的研究

本文将数字图像相关方法与古老的网格法相结合,给出了一种测量面内位移场的新方法——网格数字图像相关方法,实现了高精度的测量,精度可达0.02亚像素．在该方法中网格点采用圆标记点,使该方法特别适用于小线应变位移场的研测．本文运用Windows下的新的图像采集方法测量物体位移场,突破了Dos系统下对内存操作的限制,并使每一帧图像的像素数由512×512增至768×576,提高了图形的分辨率．     

Improvements in Residual Stress Measurement by the Incremental Centre Hole Drilling Technique

This paper presents results which advance and improve the usefulness, accuracy and efficiency of incremental centre hole drilling as a method of measuring near surface residual stress fields. Particular emphasis is placed on providing optimal values for the number of drilling step increments to be used and their corresponding size. Guidelines on the optimal values for the number and size of steps to use during measurements are presented for various ratios of hole radius to strain gauge rosette radius in the form of tabulated data. These guidelines are subsequently incorporated into a new data analysis program which permits very near surface residual stress fields to be accurately determined in real components. The benefits of the new approach are highlighted by reporting the results of measurements made on three industrial components, each of which has been subjected to a well-known engineering process. These components are a shot-peened spring-steel, a friction stir welded aluminium alloy, and a titanium alloy subjected to three different machining processes. The results reveal that the improvements to the incremental centre hole drilling technique can provide measured residual stresses from depths ranging from about 10 m to 1 mm.     

数字图像相关分析法增量位移场测试技术

利用位移场的连续性,对亚像素位移场的算法进行了一些改进,设计了一套分步计算位移场、应变场的测量计算方法,较好地解决了数字图像相关分析法计算精度和效率.采用增量位移场叠加的方法计算大应变位移场,采用局部平面拟合的方法计算应变场.通过对高分子材料拉伸试验位移场的测量和结果标定,说明该方法具有较强的实用性和计算精度.同时,由于避免了对亚像素点的搜索,大大提高了计算效率.     

橡胶材料弹性模量数字图像相关测定法

利用数字图像相关方法测量了小应变下柔性橡胶的弹性模量.用CCD相机记录单轴压缩实验中圆柱体橡胶试样表面人工散斑图像,作为数字图像相关测量技术中的变形信息载体.分析了镜头畸变对位移测量的影响,运用数字图像相关法得出小应变范围内像胶的应力应变曲线,计算出橡胶的弹性模量.并与采用千分表所得到的结果进行了比较,两者符合较好.实...     

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.

     

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.     

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。     

Residual Stress Determination by Hole Drilling Combined with Optical Methods

An overview is provided of the use of eight different optical methods with hole drilling to determine residual stresses. The methods considered are: brittle and photoelastic coatings, Moire interferometry, holographic interferometry, electronic speckle pattern interferometry, interferometric strain rosette, digital image correlation and shearography. A number of applications are summarized, such as the use of hole drilling with holographic interferometry to investigate stresses in rock structures accessed by deep boreholes and to determine manufacturing-induced residual stresses in fillets of small radii.     

钻孔法测量残余应力过程中钻孔附加应变

本文叙述了钻孔法测量残余应力过程中的附加应变.研究应力水平对附加应变的影响是在单向应力条件下进行的,结果表明,钻孔条件、材料状态以及残余应力达到一定值时,附加应变为零.     

钻孔法测量残余应力过程中钻孔附加应变的研究（Ⅱ）

本文应用弹塑性理论研究了被测试样的屈服强度、应力场对钻孔法测量残余应力过程中钻孔附加应变的影响,使用简化的理论模型推导了低速旋转钻头钻孔产生附加应变的一般表达式.结果表明钻孔引入的附加应变与被测试样的原始残余应力状态有关,随应力水平增加而增大,在压应力减小到某一临界值σ时,钻孔附加应变为零。在 Ly12铝合金上的实验测量结果与理论结果一致。     

云纹干涉与钻孔法测量残余应力的实验方法与系统

应变片钻孔法是工程中应用最广的残余应力测量方法之一，由于应变片只能得到其长度范围内的平均应变，测量误差比较大，本文提出用云纹干涉法测量的位移信息代替应变片测量的应变信息来确定残余应力，用有限元建立位移与残余应力之间的关系，基于以上理论，开发了一种可以进行现场残余应力测量的便携式云纹干涉钻孔系统，并用该仪器进行了铝合金激光焊接接头的残余应力测量，得到了焊缝中心残余应力值。     

On the Use of NURBS Functions for Displacement Derivatives Measurement by Digital Image Correlation

In this paper, we propose to investigate the potential improvement of using Non-Uniform Rational B-Spline (NURBS) functions for displacement measurements by digital image correlation (DIC). The aim is at improving the performance of DIC to capture with low uncertainty and low noise levels not only the displacement field but also its derivatives. Indeed, when the displacement field is used to feed constitutive law identification procedures, displacement derivatives are required and thus may be measured with robustness. Two examples illustrate the potential of NURBS for DIC: a compressive test on a wood sample and a bending test on a steel beam. For the latter, beam kinematics are adopted and NURBS are used in order to capture the variation of the curvature (second derivative of the displacement) along the beam axis. For these two examples, an error study based on a decomposition of the error into the correlation error and the interpolation error, is carried out and shows the great potential of NURBS functions for DIC.     

二维数字图像相关测量中离面位移引起的误差分析

二维数字图像相关方法作为一种非接触全场变形测量的光学方法,在工程上有着广泛的应用.由于其假设物体只发生面内位移,而实验中往往存在离面位移,从而引起测量误差.本文针对这一问题利用了针孔模型,从理论上分析了离面位移对普通镜头测量结果的影响,通过铝块平移实验验证了理论分析的正确性.利用提出的理论模型修正橡胶材料的变形测量结果,并与远心镜头测量结果进行对比,发现修正后的普通镜头测量的结果更接近远心镜头测量的结果.最后,本文根据理论分析和实验验证,给出了减小或消除离面位移造成误差的方法.     

数字图像相关测量的误差分析及其改进措施

本文对数字图像相关测量方法的测量误差进行了系统分析，并对该方法的实验技术进行了研究，提出了通过实验技术消除和抑制误差的有效措施。