三维数字图像测量系统稳定性的实验研究

基于双目立体视觉原理的三维数字图像相关方法已被广泛应用于平面或曲面物体的表面形貌和三维变形测量。然而因相机自热、环境温度变化等内外部因素的影响,同一三维数字图像相关测量系统在不同时间,其内部参数会不可避免地发生微小改变,从而或多或少地引起额外的形貌和变形测量误差。为获得高精度的测量结果,需对由相机自热或环境温度变化引起的变形测量误差进行表征,并采用措施予以减小或消除此类误差。本文以石英玻璃平板为研究对象,利用三维数字图像相关测量系统对其位移和应变分布进行了连续5小时的测量,分析了测量结果随相机温度变化的关系曲线。研究结果显示,本文所用相机通电后其机壳温度一般约上升8~10℃甚至更高,所搭建的三维数字图像相关测量系统因相机温度变化所引起的应变测量误差在50~200με(约15~21με/℃)之间。本文还给出了相应建议来减小和消除相机或环境温度变化对三维变形测量的影响。     

基于数字图像体相关的物体内部三维位移场分析

在二维数字图像相关算法的基础上,推导了三维数字图像体相关算法,并应用于物体内部的三维位移场分析。用计算机模拟方法对SR-CT重建的物体内部三维图像施加已知变形,对变形前后三维数字图像进行体相关运算,获得三维位移场。在计算亚像素位移时,本文提出了一种三维的基于灰度梯度的算法,给出了梯度函数,阐述了数字模拟三维体相关的计算过程,通过数字模拟实验证明了算法的正确性和可靠性。     

使用单彩色相机的单相机三维数字图像相关方法

介绍了一种基于单个彩色相机的新型全靶面、单相机三维数字图像相关(3D-DIC)方法。借助于设计巧妙的颜色分光光路,被测物体表面图像可以通过两条不同的光路达到相机靶面,采集的标定靶和实验件表面的彩色图像可以分离得到蓝色和红色子图像。通过使用3D-DIC分析标定靶和实验件表面分离后的蓝色和红色子图像,可以获得物体表面的三维形貌和变形。形貌测量、面内和离面平移、以及静动态三维变形实验验证了该单彩色相机3D-DIC方法的有效性和测量精准度。由于可避免双相机同步,且能实现无分辨率损失的全靶面三维形貌和变形测量,本文方法在需要实现瞬态位移和变形测量的爆炸、冲击、振动等领域中具有广阔重要的应用前景。     

基于三维数字图像相关的三维形貌拼接研究

三维数字图像相关(DIC)方法已在众多工程技术领域中得到了日益广泛的应用.当该方法用来测试表面曲率较大的物体的变形时,受双相机之间夹角的限制,不能实现全场的三维形貌测量.采用在两个(及以上)不同位置拍摄,分别得到被测物的局部形貌,然后用三维DIC算法计算局部坐标系之间的坐标变换关系,将局部形貌转换至同一坐标系下并拼接得到被测物全场三维形貌.实验结果验证了该方法的可行性和可靠性.     

数字图像相关方法中散斑图的质量评价研究

在利用数字图像相关方法测量物体表面变形时,被测物体表面必需覆盖有灰度随机分布的散斑场,该散斑场作为试件表面变形信息的载体随试件一起变形。在实际情况下,不同的散斑场会显示出完全不同的灰度分布特征,并对数字图像相关方法的测量结果有着重要影响。因此如何定量评价散斑图的优劣是数字图像相关方法中一个重要的基本问题,也是该方法的使用者非常关心的问题。基于最近数字图像相关方法基本理论研究的进展,本文提出平均灰度梯度这一新参数用于散斑图质量的评价。为证实该参数的有效性,本文对五幅明显不同的散斑图进行了精确平移,并将数字图像相关方法测量的位移与预加的平移量进行比较,分析了位移测量结果的均值误差和标准差。结果显示位移测量结果的均值误差和标准差均与散斑图的平均灰度梯度有关,一个好的散斑图应该具有较大的平均灰度梯度。     

多相机3D-DIC及其在高温变形测量中的应用

将基于双目视觉的三维数字图像相关方法 (Three-dimension digital image correlation,3D-DIC)与多相机同步采集系统相结合,形成基于多相机的3D-DIC系统。依据三维空间误差(Three-dimensional residual,3Dresidual)最小原则,确定各点对应的最佳双目视觉系统,获得物体全场三维变形。以四相机3D-DIC系统为例,与测量精度达10~20nm的电子散斑干涉测量系统同时对平板的离面位移进行测量,并对测量得到的离面位移最大值进行了对比分析。结果显示,荷载较小时,四相机3D-DIC与电子散斑干涉测量系统误差稍大,最大达到2.7%;荷载增大,物体变形增大时,两种测量系统结果基本相同。文中讨论了四相机测量系统的不稳定对实验结果的影响。利用该四相机3D-DIC系统对镍合金不锈钢材料在高温场中的变形进行测量,获得了物体的三维变形场,并分析了材料的膨胀系数,得到了试件的热应变-温度曲线和膨胀系数随温度变化的关系式。     

回转体三维扫描形貌测量及其应用

本文提出一种测量三维形貌的扫描系统,该系统可被用于测量回转体的三维形貌。激光光刀被投射在被测物体表面,由于被测物体表面的形貌而改变了光刀的弯曲方式,摄像机将变形后的图像采集到计算机中。当被测物体放置在转台上旋转一周时,相应的一系列光刀变形的图片被采集到计算机中,这一系列图片包含了物体表面形貌的全部信息。通过连续扫描采集系列图片并计算,最终可以获得整个被测物体的三维形貌。当被测物体直径在一百毫米左右尺寸范围内的测量灵敏度可到0.01mm,其它尺寸范围要更换成像系统,才能达到相应的灵敏度。本文给出了两个较为复杂模型的三维形貌测量结果。该结果表明,本系统在对回转体表面形貌的测量方面具有一定的技术难度和广泛的工程需求,本文的研究具有重要的参考价值。     

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...     

用白光进行散斑图的信息处理

引言近几年来散斑法获得很大发展。这种方法具有非接触测量、不需要破坏被测物体、对被测物体材料没有特殊要求、测量灵敏度高和设备简单等优点。用散斑法能测量平面变形和三维问题,测量物体内部应变,研究静、动态应力分 ...     

基于时序变形预测的数字图像相关加速方法

利用数字图像相关求解连续变形物体的位移场和应变场时,会遇到处理速度非常慢的问题,原因是相关算法计算量大且忽略了物体在时间轴上的变形规律。本文提出了一种基于变形预测的数字图像相关方法,该方法利用物体在时间轴上的变形规律,通过已经得到的变形值来预测后面时刻的变形初值。首先分析了物体在时间轴上的变形规律,然后结合实际应用对其进行修正,得到物体下一步变形的初值,最后通过NR方法(Newton-Raphson Method)得到物体的位移场和应变场。通过计算机模拟和金属试件拉伸实验并结合GPU(Graphic Processing Unit)编程验证了该方法的有效性。计算机模拟和金属试件拉伸实验的计算结果表明,该方法能够在计算精度保持不变的情况下,使计算速度提高4~7倍。     

三维DIC在铸铁拉伸试验中的应变测量精度研究

基于三维数字图像相关方法(3D-DIC)的拉伸实验研究了铸铁的拉伸力学性能,分别得到了应力-应变曲线、弹性模量、抗拉强度、延伸率等拉伸力学性能参数．将3D-DIC应力-应变的测试结果与目前实验中使用广泛的接触式引伸计方法的实验结果作了对比,得到两者的应力-应变曲线基本重合,弹性模量相差不超过4%．此外,还对3D-DIC和机械引伸计的应变绝对误差和相对误差作了详细比较,实验表明3D-DIC在铸铁拉伸力学性能测试中具有足够的应变测量精度,完全可以取代传统引伸计成为一种有效的非接触式变形测量手段．     

基于三维DIC方法的高强钢拉伸力学性能测定

高强度钢在建筑等工程领域发挥着极为重要的作用,因此准确测定其力学性能具有至关重要的意义．鉴于传统机械引伸计在小尺寸试样变形测试中的不便性,利用三维数字图像相关(3D-DIC)方法,对8.8级螺栓和Q690钢这两类试样在单轴拉伸试验全过程中的变形进行了测试,分别得到了应力-应变曲线、弹性模量、屈服强度、强度极限、断后延伸率和断面收缩率,由于试样在屈服阶段应变增加而应力基本不变,因此同时研究了该阶段中试样从弹性变形演化到塑性变形的发展规律．实验结果表明三维DIC在小尺寸试样力学性能测试方面具有很强的优越性,可用来灵活地测量变形并研究变形的演化规律．     

Leveraging Full-Field Measurement from 3D Digital Image Correlation for Structural Identification

Within the domain of structural health monitoring (SHM) measurement techniques have primarily relied on discrete sensing strategies using sensors physically attached to the structural system of interest. These sensors have proven effective in describing both global and local phenomena, but are limited to providing discrete response measurements of these systems. With the introduction of novel imaging tools and image analysis techniques, such as digital image correlation (DIC), the ability to measure the full-field response of these systems provides a novel approach to refining structural identification (St-ID) approaches used in SHM. This paper explores this proposed concept through a case study on a series of structural test specimens analyzed using 3D digital image correlation (3D-DIC) for St-ID. Finite element model updating (FEMU) was used as the technique for the structural identification. For the identification process, ABAQUS was interfaced with MATLAB to converge on the optimal unknown/uncertain system parameters of the experimental setup. 3D-DIC results provided a rich full-field dataset for the identification process, which was compared against measurements derived from traditional physical in-place sensors typically used in SHM. In this work a Hybrid Genetic Algorithm (HGA), which combines the genetic algorithm as a global optimization and a gradient-based method as a local optimization, was used for the FEMU based on 3D-DIC results of structural specimen subjected to variable loading. To minimize the error between the full field 3D-DIC measurements and FEA model updating results, an objective function was introduced that included the full-field contributions of strains and deformation response. The evolution of this objective function illustrated satisfactory convergence of the identified parameters and the excellent agreement of the experimental and numerical strain and displacement responses after the model updating process confirmed the success of the proposed approach. The results of this study highlight the advantage of this hybrid approach and provide the foundation for effective deployment of the proposed strategy for large-scale structural systems.     

高速3D-DIC测试技术在装甲钢贯穿试验中的应用

数字图像相关（digital image correlation, DIC）技术作为一种非接触、非干涉的全场无损光学量测技术,可获取材料表面的动态变形信息和破坏过程。为了评估装甲钢的抗弹性能并探索高速三维数字图像相关（3D-DIC）技术在钢板贯穿试验测试中的应用,基于氢氧爆轰驱动弹道枪开展了7发15 mm口径可变形弹体以不同速度（255～568 m/s）冲击不同厚度（5、8和10 mm）高强高硬装甲钢板的试验,并结合帧率为144 000 s?1的高速3D-DIC测试技术获取了靶板的离面位移和应变时程。随后,基于前期标定并验证的装甲钢本构模型参数,对上述试验进行了数值模拟。通过对比弹体残余速度和长度验证了有限元分析方法的可靠性。进一步通过对比试验与数值模拟得到的靶背离面位移时程曲线和不同时刻靶背的应变云图,验证了高速3D-DIC测试结果的准确性。最后,对比分析了靶板最大离面位移与弹体冲击速度和装甲钢板厚度的关系。高速3D-DIC测试技术的应用可为相关试验测试提供参考,靶板最大离面位移分析结果可为屏障类防护结构的分析验证和优化设计提供试验依据。     

三维DIC在GFRP锚杆拉伸力学性能测试中的应用

随着玻璃纤维增强塑料(GFRP)广泛运用于轨道交通及边坡加固中,其力学性能的测试显得尤为重要。基于三维数字图像相关(DIC)方法,研究了三种不同加载速率下GFRP锚杆的拉伸力学性能,分别得到了应力-应变曲线及弹性模量、抗拉强度、最大力总延伸率等参数。实验结果表明,螺旋状变截面锚杆的轴向应变呈非均匀分布,由三维DIC得到的锚杆弹性模量结果一致性优于传统的接触式引伸计方法,GFRP锚杆几乎呈线性分布的应力-应变曲线表明该材料为典型的脆性材料。     

High-Accuracy 2D Digital Image Correlation Measurements with Bilateral Telecentric Lenses: Error Analysis and Experimental Verification

By comparing two digital images of a test planar specimen surface recorded in different configurations, two-dimensional digital image correlation (2D-DIC) provides full-field displacements to sub-pixel accuracy and full-field strains in the recorded images. For the 2D-DIC systems using an optical lens, a simple pinhole imaging model is commonly used to describe the linear relationship between the measured sensor plane displacements and the actual displacements in the object surface. However, in a practical measurement, various unavoidable disadvantageous factors, such as small out-of-plane motion of the test object surface occurred after loading, small out-of-plane motion of the sensor target due to the self-heating or temperature variation of a camera, and geometric distortion of the imaging lens, may seriously impair or slightly change the originally assumed linear correspondence. In certain cases, these disadvantages may lead to significant errors in displacements and strains measured by 2D-DIC. In this work, the measurement errors of 2D-DIC due to the above three disadvantageous factors are first described in detail. Then, to minimize the errors associated with these disadvantages, a high-accuracy 2D-DIC system using a bilateral telecentric lens is established. The performance of the established 2D-DIC system and other two 2D-DIC systems using a conventional lens and an object-side telecentric lens are investigated experimentally using easy-to-implement stationary, out-of-plane and in-plane rigid body translation tests. A detailed examination reveals that a high-quality bilateral telecentric lens is not only insensitive to out-of-plane motion of the test object and the self-heating of a camera, but also demonstrates negligible lens distortion. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively compare the axial and transversal strains measured by the proposed 2D-DIC system and those measured by strain gage rosettes. The perfect agreement between the two measurements further verifies the accuracy of the established 2D-DIC system.     

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.     

Recent Progress in Digital Image Correlation: Background and Developments since the 2013 W M Murray Lecture

Since presentation of the 2013 Murray Lecture focusing on developments in digital image correlation (DIC), the methods have continued to expand internationally and their use has begun to grow in fields where there was less activity in the past. First, a brief history of digital image correlation methods is presented from the perspective of the first author, followed by a discussion of recent trends associated with the use of digital image correlation methods in academics, governmental laboratories and industrial settings. In the remainder of the article, new results are provided in three areas where DIC methods have seen rapid growth; application of StereoDIC or three-dimensional DIC (3D-DIC) to the study of wall structures in civil engineering; the use of Volumetric DIC or Digital Volume Correlation (DVC) to quantify the internal response of a specially-designed composite material and in the area of model validation for another application in civil engineering; transfer length measurements in pre-stressed concrete beams.