二维数字图像相关(two-dimensional digital image correlation，2D-DIC)在测量过程中不可避免地会出现相机光轴与测量表面非垂直，由此产生的离面位移而将导致较大的测量误差，同时在视场受限的环境中难以通过单台相机完成大范围的变形测量。有鉴于此，该文开发了基于双反射镜的2D-DIC变形测量系统，使用双反射镜成像缓解离面运动对2D-DIC的影响，通过可移动相机实现小视场下的图像采集，提出基于频域移位的高精度图像拼接方法，并改进了融合函数，最终获得试样的高分辨率图像。单轴拉伸实验结果表明，轴向应变的平均相对误差相比传统2D-DIC方法降低12。82％，测量分辨率提高约34。92％，验证了测量系统的可行性和有效性。
Development of a 2D-DIC deformation measurement system based on a double reflector
[Objective]Two-dimensional digital image correlation(2D-DIC)often encounters out-of-plane displacement issues during strain measurement when the camera's optical axis is not perpendicular to the measurement surface,leading to significant measurement errors.Additionally,in environments with a limited field of view,large-scale deformation measurements are challenging to complete with a single camera.To address these problems,this article developed a 2D-DIC deformation measurement system based on dual mirrors,using dual mirror imaging to alleviate the effects of off-plane motion on 2D-DIC and achieving image acquisition in a small field of view with a movable camera.Meanwhile,a high-precision image stitching method based on the frequency domain shift theorem was proposed to achieve high-precision image stitching and reduce the impact of stitching algorithms on DIC measurement results.[Methods]A sub-pixel compensation method based on subset correlation and frequency domain shift theorem was used to find accurate stitching positions.High-precision image stitching was performed on speckle images presented by dual reflectors.A sine trigonometric function replaced the linear weight curve of the traditional fade-in and fade-out fusion function to eliminate the second type of discontinuity generated by the curve at the boundary of overlapping areas.A simulation experiment was designed to generate two sets of speckle images(four in total)with uniform axial displacement and sub-pixel size overlapping areas,as well as two original speckle images,to verify the performance of image stitching and fusion algorithms and conduct quantitative analysis.A uniaxial tensile experiment was designed to validate the measurement system in practice:using a tensile force of 1,200 N as a reference,the deformation of specimens under five different tensile states was measured.The experimental results were compared with strain gauges and traditional 2D-DIC methods.[Results]In the simulation experiment,the results of the three sets of displacement data were consistent,with relative errors less than 0.20％,average errors and root mean square errors less than 0.0040 pixels,indicating minimal impact of the proposed image stitching and fusion method on the DIC measurement results.In real experiments,the axial strain calculated by the proposed method closely matched the measurement results of the strain gauge,showing the smallest absolute error.The minimum and maximum absolute errors were 1.54 μεand 34.53 με,respectively.The average relative error was 4.43％,which is 12.82％lower than traditional 2D-DIC,and the measurement resolution improved by about 34.92％.[Conclusions]The 2D-DIC measurement system developed in this article,combining dual mirror imaging and a movable camera,achieved high-precision quasi-static full-field deformation measurement in a small field of view.The feasibility and effectiveness of the measurement system were verified through simulation experiments and uniaxial tensile tests.The current implementation only deployed two image acquisition positions,suggesting that the measurement range could theoretically be further expanded,offering certain application potential.However,the system is currently not suitable for measurement situations outside of 2D-DIC,its feasibility in material bending experiments is not yet known,and it suffers from low computational efficiency.These aspects will be addressed in future research.
万方数据
维普
