Measurement of Structural Stresses by Hole-Drilling and DIC

Evaluation of stresses in structures such as bridges, buildings, pipelines and railways is challenging because the loads cannot easily be manipulated to allow direct measurements. This paper focuses on the development of a method that combines the hole-drilling technique with Digital Image Correlation (DIC) to evaluate these difficult-to-measure structural stresses. The hole-drilling technique works by relating local displacements caused by the removal of a small amount of stressed material to the original stresses within the drilled hole. Adaptation of this method to measure structural stresses requires scaling up the hole size and modifying the calculation approach to measure deeper into a material. DIC provides a robust means to measure full-field displacements that can easily be scaled to different hole sizes and corrected for typical artifacts that occur in practical on-site measurements. There are two primary areas of investigation: the adaptation of the DIC/hole-drilling method to measure structural stresses and the development of a correction method to remove coexisting stresses such as residual and machining stresses from the measurement. Experimental measurements are made to demonstrate the measurement method on different structure types including the example practical problem of measuring thermally induced stresses in railroad tracks.     

Incremental Computation Technique for Residual Stress Calculations Using the Integral Method

The Integral Method for determining residual stresses involves making surface deformation measurements within a sequence of small increments of material removal depth. Typically, the associated matrix equation for solving the residual stresses within each depth increment is ill-conditioned. The resulting error sensitivity of the residual stress evaluation makes it essential that data measurement errors are minimized and that the residual stress solution method be as stable as possible. These two issues are addressed in this paper. The proposed method involves using incremental deformation data instead of the total deformation data that are conventionally used. The technique is illustrated using an example ESPI hole-drilling measurement.     

Residual Stress Determination Using Cross-Slitting and Dual-Axis ESPI

Hole-drilling and Electronic Speckle Pattern Interferometry (ESPI) are used to measure residual stresses in metal specimens. The slitting method is chosen as an alternative to the more commonly used hole-drilling method because it involves less material removal and leaves large areas of highly deformed material available to be measured. However the conventional single-slitting method is sensitive only to the stress component perpendicular to the slit direction, and thus has a strong directional bias. Conventional ESPI has a similar bias because it responds to surface displacements in a specific sensitivity direction. In this paper, a novel cross-slitting method with dual-axis ESPI measurements is proposed to address both directional biases. Cross-slitting is introduced as a means of releasing all in-plane stress components. The dual-axis ESPI system uses diagonal-mirror and shutter devices to provide surface displacement measurements in orthogonal in-plane directions. The combination of the cross-slit and dual-axis measurement gives isotropic sensitivity to the in-plane residual stress components. Experimental measurements are described that illustrate the capability and effectiveness of the cross-slitting/ESPI technique.     

Comparison Between Surface and Near-Surface Residual Stress Measurement Techniques Using a Standard Four-Point-Bend Specimen

Background

Determination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties.

Objective

This study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI).

Methods

These measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties.

Results

The XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation.

Conclusion

The ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method.

     

A New Procedure to Measure Near Yield Residual Stresses Using the Deep Hole Drilling Technique

Mechanical strain relief techniques for estimating the magnitude of residual stress work by measuring strains or displacements when part of the component is machined away. The underlying assumption is that such strain or displacement changes result from elastic unloading. Unfortunately, in components containing high levels of residual stress, elastic-plastic unloading may well occur, particularly when the residual stresses are highly triaxial. This paper examines the performance of one mechanical strain relief technique particularly suitable for large section components, the deep hole drilling (DHD) technique. The magnitude of error is calculated for different magnitudes of residual stress and can be substantial for residual stress states close to yield. A modification to the technique is described to allow large magnitudes of residual stress to be measured correctly. The new technique is validated using the case of a quenched cylinder where use of the standard DHD technique leads to unacceptable error. The measured residual stresses using the new technique are compared with the results obtained using the neutron diffraction technique and are shown to be in excellent agreement.     

Augmented Lagrangian Digital Image Correlation

Experimental Mechanics - Digital image correlation (DIC) is a powerful experimental technique for measuring full-field displacement and strain. The basic idea of the method is to compare images of...     

Fast, Robust and Accurate Digital Image Correlation Calculation Without Redundant Computations

High-efficiency and high-accuracy deformation analysis using digital image correlation (DIC) has become increasingly important in recent years, considering the ongoing trend of using higher resolution digital cameras and common requirement of processing a large sequence of images recorded in a dynamic testing. In this work, to eliminate the redundant computations involved in conventional DIC method using forward additive matching strategy and classic Newton–Raphson (FA-NR) algorithm without sacrificing its sub-pixel registration accuracy, we proposed an equivalent but more efficient DIC method by combining inverse compositional matching strategy and Gauss-Newton (IC-GN) algorithm for fast, robust and accurate full-field displacement measurement. To this purpose, first, an efficient IC-GN algorithm, without the need of re-evaluating and inverting Hessian matrix in each iteration, is introduced to optimize the robust zero-mean normalized sum of squared difference (ZNSSD) criterion to determine the desired deformation parameters of each interrogated subset. Then, an improved reliability-guided displacement tracking strategy is employed to achieve further speed advantage by automatically providing accurate and complete initial guess of deformation for the IC-GN algorithm implemented on each calculation point. Finally, an easy-to-implement interpolation coefficient look-up table approach is employed to avoid the repeated calculation of bicubic interpolation at sub-pixel locations. With the above improvements, redundant calculations involved in various procedures (i.e. initial guess of deformation, sub-pixel displacement registration and sub-pixel intensity interpolation) of conventional DIC method are entirely eliminated. The registration accuracy and computational efficiency of the proposed DIC method are carefully tested using numerical experiments and real experimental images. Experimental results verify that the proposed DIC method using IC-GN algorithm and the existing DIC method using classic FA-NR algorithm generate similar results, but the former is about three to five times faster. The proposed reliability-guided IC-GN algorithm is expected to be a new standard full-field displacement tracking algorithm in DIC.     

基于DIC方法的残余应力快速测量系统

结合数字图像相关(Digital Image Correlation,DIC)方法与钻孔法,开发了残余应力快速测量系统。该系统可分为两部分:适用于现场测量的便携式机械系统与针对残余应力测量而改进的基于DIC算法的程序。在四点弯曲加载平台上对工件进行载荷释放前后的残余应力测量试验,通过与应变片测量结果进行对比,该残余应力测量系统的精度达到了应变片测量的同等精度。同时,该测量系统解决了传统应变片测量系统对心误差大、操作繁琐、效率低和测量结果稳定性差等问题,具有较高的工程应用价值。     

Towards Criteria Characterizing the Metrological Performance of Full-field Measurement Techniques

Users of full-field measurement methods like Digital Image Correlation (DIC) often aim to perform measurements with the best trade-off between spatial resolution, bias and measurement resolution. Whenever two full-field methods are compared, it is essential that these criteria are taken into consideration. Recently a metrological efficiency indicator for full-field measurements has been proposed and discussed. This indicator combines measurement resolution and spatial resolution. It has been shown to be invariant to the subset size in the case of Local DIC. The goal of this article is to discuss a method, which determines both the spatial and the measurement resolutions for a given bias for two different DIC methods, in order to obtain the metrological efficiency indicator for each of these methods. The benefit of this indicator is that it does not depend on setting parameters such as the subset size, which are chosen by the user. As such, it can be considered as intrinsic to each technique, thus enabling fair comparison. Local DIC and triangular finite element based Global DIC will be the subject of this investigation. With this setting, their respective subset and triangular element sizes will be related to the spatial resolution of both methods for a given acceptable bias. By using the metrological efficiency indicator, the performance of the two methods will be compared and discussed to a new level of detail. Generally speaking, the indicator shows that the metrological performance of both methods is similar, confirming their popularity. However, it will be shown that, depending on the choice of what an acceptable bias is, one of the method may be preferred to another. The results show that for the specific DIC versions used in the study, for cases for which a significant bias is acceptable, Local DIC outperforms Global DIC, while the opposite is true in the case for which the bias requirements are more stringent. Finally, the quadratic versions of both DIC versions are shown to significantly outperform their respective linear versions.

     

基于退相关DIC的疲劳裂纹全局动态测量方法

工程材料和结构在反复荷载长期作用下容易发生疲劳开裂, 疲劳裂纹测量对于开展科学试验研究和工程问题分析都至关重要, 但现有方法无法实现高精度的疲劳裂纹全局动态测量. 本文基于数字图像相关(digital image correlation, DIC)技术, 合理利用DIC的退相关效应, 提出一种疲劳裂纹全局动态测量及可视化方法. 该方法首先在相机采集得到的裂纹图像内, 建立具备拓扑关系的目标点云结构, 并运用DIC亚像素算法得到裂纹区域位移场, 再基于零均值归一化互相关(zero-mean normalized cross correlation, ZNCC)计算结果剔除退相关的DIC目标点(灭点). 进一步通过“三生点”算法提取得到裂纹离散边界, 并采用最小二乘法将离散边界拟合为连续裂纹边界, 实现裂纹形态的几何重构, 最终自动计算得到裂纹长度和宽度的动态变化过程. 该方法原理清晰、理论简单, 易于实现. 开展数值模拟和钢节点疲劳试验, 对相关算法和图像采集参数进行了验证, 结果表明本文方法对疲劳裂纹边界的数字化重构误差在0.5个像素内, 基于重构结果计算得到的裂纹长度和宽度误差分别为0.46像素和0.08像素(类同于0.06 mm和0.01 mm), 并成功实现了对疲劳试验裂纹扩展形态的精细化动态测量及可视化. 研究成果证明了DIC技术用于疲劳裂纹全局动态测量及可视化的有效性, 并在测量精度、效率、成本等方面具有显著优势, 可在实验室测量和工程现场测试中推广应用.      

Moiré interferometry determination of residual stresses in the presence of gradients

The full-field technique of high-sensitivity moiré interferometry in conjunction with a multiple-hole-drilling procedure is applied to residual-stress measurements in the presence of gradients. The method arrives at residual-stress estimates starting from in-plane displacement components. Successful applications of the method to problems simulating the nonuniform transverse residual stresses of welded joints are reported.Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, NM on June 3–6.     

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 measurement of residual-stress gradients in aluminum-alloy specimens

The assessment or prediction of fatigue life or strength improvement due to residual stresses requires knowledge of their magnitude and distribution. This paper presents an extension of the modified hole-drilling technique (MHDT) to the measurement of stress gradients in a biaxial-residual-stress field. This is achieved by taking a series of ‘point’ measurements and evaluating the stress profile with due consideration to the effects of hole location, the interaction between holes and the redistribution of stress due to hole drilling. An application to the measurement of residual stresses induced in 2024-T3 aluminum-alloy specimens by edge-dimpling technique is described and the method of compensation for the effect of redistribution of stress is explained. The experimental results are shown to be in good agreement with those obtained elsewhere by an analytic-numerical solution.     

Diffraction Assisted Image Correlation: A Novel Method for Measuring Three-Dimensional Deformation using Two-Dimensional Digital Image Correlation

Digital Image Correlation (DIC) provides a full-field non-contact optical method for accurate deformation measurement of materials, devices and structures. The measurement of three-dimensional (3D) deformation using DIC in general requires imaging with two cameras and a 3D-DIC code. In the present work, a new experimental technique, namely, Diffraction Assisted Image Correlation (DAIC) for 3D displacement measurement using a single camera and 2D-DIC algorithm is presented. A transmission diffraction grating is placed between the specimen and the camera, resulting in multiple images which are then used to obtain apparent in-plane displacements using 2D-DIC. The true in-plane and out-of-plane displacements of the specimen are obtained from the apparent in-plane displacements and the diffraction angle of the grating. The validity and accuracy of the DAIC method are demonstrated through 3D displacement measurement of a small thin membrane. This technique provides new avenues for performing 3D deformation measurements at small length scales and/or dynamic loading conditions.     

A Procedure to Measure Biaxial Near Yield Residual Stresses Using the Deep Hole Drilling Technique

Deep Hole Drilling (DHD) is a mechanical strain relief technique used to measure residual stresses within engineering components. Such techniques measure strains or displacements when part of the component is machined away and typically assume elastic unloading. However, in components containing high levels of residual stress, elastic–plastic unloading can occur which may introduce substantial error. For the case of the DHD technique, a modification to the technique referred to here as the incremental or iDHD technique has been developed to allow such high levels of residual stress to be measured. Previous work has demonstrated the accuracy of the iDHD technique, although only for axisymmetric residual stress distributions. In the present investigation, the application of the iDHD technique has been extended to the general case of biaxial residual stress fields. Finite element simulations are first carried out to demonstrate the ability of the iDHD technique to measure biaxial residual stress. Experimental measurements were then made on shrink fit components and ring welded components containing biaxial residual stress to investigate the performance of the technique in practice. Good agreements between iDHD measurements, neutron diffraction measurements and FE predictions of the residual stresses were obtained, demonstrating the generally improved accuracy of the iDHD technique compared to the standard DHD approach.     

Grid Method for Microscale Discontinuous Deformation Measurement

The objective of this paper is to explore both grid method and Digital Image Correlation (DIC) technique for microscale and discontinuous displacement measurements, such as those associated with crack tips. First, the principle of the grid method is revisited. The grid method and DIC technique are then applied to computer generated images to calculate the displacement field around crack tips. Finally, the grid method is applied to actual experimental images of fracture tests which are conducted inside a Scanning Electron Microscope (SEM) chamber. A new technique is developed to generate microscale pattern that is suitable for both grid method and DIC technique. The displacement fields calculated from grid method are compared with those from DIC technique to identify the strengths and weaknesses of each technique for the microscale and discontinuous displacement measurements. It has been determined that grid method can obtain data closer to the discontinuity than DIC; however, DIC produces smoother displacement fields at the far field. Using this new pattern generation technique, both grid method and DIC technique can be applied to the fracture test at the microscale to complement with each other to achieve the best experiment results.     

Nondestructive residual-stress measurement on the inside surface of stainless-steel pipe weldments

The instrumentation, technique, and procedures are described for the nondestructive measurement of residual stresses on the inside surface of pipe as small as 10 in. in diam. The instrument is based upon a unique position-sensitive scintillation X-ray detector which provides for the most compact X-ray stress-measurement instrument available since the introduction of film cameras four decades ago. This instrument is capable of applying the single-exposure technique of X-ray stress measurements which results in unprecedented rapidity of stress measurement consistent with excellent precision and accuracy. The results of testing the precision and accuracy of the instrument on a zero-stress powder and four-point-bend specimen are given. Residual stresses in four austenitic stainless-steel girth-welded pipes are presented illustrating the effects of the different welding procedures. The results from the pipes confirm the beneficial residual-stress condition of heat-sink-welding procedures.     

Measurement of Orthogonal Stress Gradients Due to Impact Load on a Transparent Sheet using Digital Gradient Sensing Method

A full-field optical method called Digital Gradient Sensing (DGS) for measuring stress gradients due to an impact load on a planar transparent sheet is presented. The technique is based on the elasto-optic effect exhibited by transparent solids due to an imposed stress field causing angular deflections of light rays quantified using 2D digital image correlation method. The measured angular deflections are proportional to the in-plane gradients of stresses under plane stress conditions. The method is relatively simple to implement and is capable of measuring stress gradients in two orthogonal directions simultaneously. The feasibility of this method to study material failure/damage is demonstrated on transparent planar sheets of PMMA subjected to both quasi-static and dynamic line load acting on an edge. In the latter case, ultra high-speed digital photography is used to perform time-resolved measurements. The quasi-static measurements are successfully compared with those based on the Flamant solution for a line-load acting on a half-space in regions where plane stress conditions prevail. The dynamic measurements, prior to material failure, are also successfully compared with finite element computations. The measured stress gradients near the impact point after damage initiation are also presented and failure behavior is discussed.     

Surface Decoration for Improving the Accuracy of Displacement Measurements by Digital Image Correlation in SEM

In-situ straining experiments and residual stress evaluations by micromachining require accurate measurement of surface displacements. Such measurements can be conveniently done using Digital Image Correlation (DIC). Three surface decoration techniques are presented to enhance surface deformation and residual stress measurement capabilities on micron-scale samples within a Scanning Electron Microscope—Focused Ion Beam (SEM-FIB) instrument. They involve the use of Yttria-stabilized-zirconia nano particles applied chemically, nano platinum dots applied using FIB, and Focused Electron Beam (FEB) assisted deposition. The three decoration techniques create distinctive, random surface features that can be used with Digital Image Correlation to provide full field maps of surface displacements at high magnifications. A series of experiments using a FEGSEM-FIB demonstrated the effectiveness of the three surface decoration techniques for FEGSEM imaging at magnifications from 2,000× to 60,000×. The precision of the image correlation is substantially enhanced by the surface decoration, with displacement standard deviations reduced to the 0.005–0.03 pixel range, depending on the patch size used. By means of an example application, the use of surface decoration for microscopic hole-drilling residual stress measurements within a FIB-SEM is presented. The same trends in DIC uncertainty observed in the analysis of the surface decoration patterns carried through to the example application. Guidelines are given for appropriate choice of decoration method to suit various practical applications.     

A Novel Cutting Strategy for Measuring Two Components of Residual Stresses Using Slitting Method

An exact knowledge of residual stresses that exist within the engineering components is essential to maintain the structural integrity. All mechanical strain relief (MSR) techniques to measure residual stresses rely on removing a section of material that contains residual stresses. Therefore, these techniques are destructive as the integrity of the components is compromised. In slitting method, as a MSR technique, a slot with an increasing depth is introduced to the part incrementally that results in deformations. By measuring these deformations the residual stress component normal to the cut can be determined. Two orthogonal components of residual stresses were measured using the slitting method both experimentally and numerically. Different levels of residual stresses were induced into beam shaped specimens using quenching process at different temperatures. The experimental results were then compared with those numerically predicted. It was shown that while the first component of residual stress was being measured, its effect on the second direction that was normal to the first cut was inevitable. Finally, a new cutting configuration was proposed in which two components of residual stresses were measured simultaneously. The results of the proposed method indicated a good agreement with the conventional slitting.