The pseudo-overlapped imaging system was proposed to resolve the fundamental trade-off between a large field of view (FOV) and high spatial resolution in 3D digital image correlation (3D-DIC). To overcome the limitations of such systems in synchronous acquisition, this paper introduces PONet, a deep neural network designed to separate complex nonlinear overlapped images from different FOVs on the sensor. The network incorporates Atrous Spatial Pyramid Pooling (ASPP) to enhance multi-scale feature extraction, and employs a dual-output head architecture to ensure independent and clear reconstruction of both sub-views. Integrated into the imaging setup, PONet enables an end-to-end synchronous acquisition and back-end decoupling framework, allowing single-shot measurements as well as measurements of large specimens spanning both fields of view. Experimental results demonstrate that the reconstructed 3D displacement fields show good agreement with benchmark data. A continuous full-field displacement map was successfully generated by stitching these fields together.

Full-field three-dimensional (3D) deformation measurement of flexible structures undergoing large reconfigurable deformation remains a significant challenge in experimental solid mechanics, particularly when pronounced surface curvature and large out-of-plane displacements exceed the depth-of-field limitations. Local defocusing leads to incomplete displacement fields and deteriorated measurement accuracy in stereo digital image correlation (DIC). To address the challenge, this article presents a single-camera stereo DIC system integrated with an electrically tunable lens (ETL) to enable large-depth-of-field 3D deformation measurement. High-quality speckle pattern imaging is achieved by dynamically adjusting the focal state. To address the intrinsic inconsistency of geometric parameters induced by ETL-driven focal variation, a cross-current calibration and coordinate-unification framework is developed, allowing 3D reconstructions obtained under different focal states to be accurately mapped into a unified physical coordinate system. A periodic calibration target and a defocus-robust frequency-domain feature extraction strategy are employed to ensure reliable reference-point localization under defocused conditions. The performance of the proposed system is validated through experiments, including repeatability assessment under repeated focal switching, large-range axial translation measurement over 200 mm, and full-field deformation measurement of a flexible multistable thin-shell structure undergoing configuration transitions. The results demonstrate that the proposed approach achieves continuous and physically consistent 3D displacement fields across an extended depth range, with high repeatability and low measurement error. The developed method provides an effective experimental tool for investigating large deformation and reconfiguration behaviors of flexible structures.

Conventional binocular stereo digital image correlation (stereo-DIC) faces challenges in balancing measurement area and measurement uncertainty, particularly in scenarios involving complex surfaces and large fields of view (FOVs). In response to this challenge, the multi-camera stereo-DIC method, based on synchronous capture and a unified coordinate system, has emerged. Throughout its development, this technique has not only shown excellent performance in large FOV and panoramic deformation measurements but also demonstrated substantial potential in multi-scale, high-dynamic-range, and high-speed measurements, greatly expanding the application prospects of vision-based deformation measurements. This paper commences with a concise overview of the technical foundations of multi-camera stereo-DIC, detailing the advancements in key technologies from reducing measurement uncertainty, expanding the measurement area, to achieving multi-scale, high-dynamic-range, and high-speed measurements. Subsequently, the method is introduced with typical applications in large FOV, panoramic, multi-scale, high-temperature, and high-speed measurements. Finally, the paper outlines current challenges and highlights future development goals for this method. This review aims to provide a valuable resource for researchers and engineers striving to enhance the accuracy and precision of deformation measurement techniques.

Two-dimensional digital image correlation (2D-DIG) is indispensable for in-plane deformation measurement owing to its simple configuration and high computational efficiency. However, its measurement accuracy is severely compromised by the defocus blur effect and virtual deformation induced by out-of-plane motion. Existing compensation methods typically address only one of these two issues and tend to fail when severe defocus prevents correlation computation entirely. To address this challenge, this study introduces a compensation method based on an electrically tunable lens (ETL). The mechanism by which the ETL eliminates the influence of out-of-plane displacement and achieves autofocus is first analyzed. Technically, a climbing search algorithm based on DIG measurement strategy rapidly modulates the ETL focal length by targeting stripe spacing, maintaining both image clarity and constant magnification during the image acquisition stage to compensate for defocus blur effect and virtual deformation caused by out-of-plane motion. Multiple experimental validations confirm the effectiveness of this approach. This approach reduces substantial virtual strain to noise levels while simultaneously resolving defocus issues in measurements. This comprehensive compensation fundamentally improves the quality and reliability of the raw speckle images, thereby providing a solid foundation for accurate deformation measurement. The ETL-based method effectively expands the application potential of 2D-DIG for characterizing flexible materials and holds promise for future extension to microscopic deformations. (c) 2026 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

The multi-view three-dimensional (3D) reconstruction method is extensively employed across diverse scientific and engineering domains due to its inherent advantages of flexible system configuration and comprehensive view information. While previous research has extensively studied error and uncertainty in the multi-view system, there is a notable gap in understanding the influence of the number of views on the precision of 3D reconstruction. This paper presents a pioneering investigation of the relationship between the number of views and the precision of 3D reconstruction in multi-view systems. Concise precision estimation formulas are derived in normal and convergent camera configurations. Our most significant contribution lies in the novel discovery of the negative power relationship between the precision of 3D reconstruction and the number of views, substantiated by theoretical derivation, numerical simulations, and real experiments. Consequently, our precision estimation formulas and precision law enable the estimation of 3D reconstruction precision without cumbersome calibration processes. This advancement enhances the flexibility and designability of the multi-view 3D reconstruction technique, enabling it to better meet the diverse measurement demands encountered in various fields.

Deep-water vision-based three-dimensional (3D) measurement is often confronted with extreme environmental challenges, significantly compromising the accuracy and efficiency of such methods. We developed an underwater 3D camera array capable of high-precision 3D reconstruction at short working distances with a large field of view in deep-water environments. The camera array comprises multiple binocular camera subsystems. A refractive camera model and calibration method are used to eliminate refraction-induced error in reconstruction. The extrinsic parameters of the subsystems are unified to ensure continuity of the measurement results. Near-infrared random speckles facilitate dense image matching in underwater environments. The proposed method greatly improves the measurement range and guarantees measurement accuracy and imaging clarity. The measurement accuracy is verified experimentally for both a single deep-water binocular system and a multi-camera stitching. The 3D camera array was mounted on an underwater robot and successfully applied in underwater structural defect detection. The proposed method overcomes refraction challenges and employs a modular design that allows for flexible reconfiguration of subsystems, enabling close-range, large-field-of-view 3D measurement in turbid underwater environments.

To address the limitations of Digital Image Correlation (DIC) technology in terms of single-view and resolution capabilities, multi-camera systems have emerged, expanding the measurement area by increasing the number of cameras. However, traditional multi-camera systems continue to face challenges in the calibration of global external parameters and error control, particularly when uniform calibration between camera subsystems is not achieved, making the calibration results difficult to evaluate and analyze. In response, this paper systematically investigates the sources of error in multi-camera system calibration and proposes a novel precision evaluation method. This method qualitatively analyzes the registration results of subsystems by examining the determinant of the rotation matrix used to transform local coordinate systems to the global coordinate system and quantitatively assesses the registration accuracy based on the alignment error of targets after registration. Furthermore, a series of experiments were conducted to validate the proposed evaluation method, with results demonstrating that the method not only offers high effectiveness in precision evaluation but also provides reliable technical support in complex engineering measurements.

Underwater structural inspection is essential for ensuring the safety and longevity of bridges. To improve the efficiency and accuracy of these inspections, this paper presents a method for measuring the morphology of bridge piers through refraction correction and multi-camera calibration. Using an underwater visual inspection platform with appropriate lighting, the measurement equipment mitigates low visibility challenges. A coplanar camera refraction parameter calibration method based on encoded markers is proposed to reduce the effects of refraction, along with the development of a multi-refraction correction model. Additionally, a novel multi-camera extrinsic calibration method is introduced to stitch point clouds. A comparative analysis of the two extrinsic calibration methods, conducted both in air and underwater, has been performed to validate the accuracy and efficiency of the proposed approach. Finally, the circular cross-section shape of the underwater bridge pier was successfully measured, and the results of defect localization were effectively presented.

Fatigue damage represents a significant risk to the structural integrity of engineering components. However, current experiments on fatigue crack propagation struggle to fundamentally elucidate the mechanisms governing crack initiation and propagation. Based on the Digital Image Correlation (DIC) method, this article investigates techniques that effectively measure the displacement field at the fatigue crack tip. Methodologically, the traditional DIC displacement mode has been refined to introduce incomplete second-order displacement shape functions, findings indicate that a suitable incomplete second-order displacement function closely approximates the second-order shape function in terms of measurement accuracy, yielding an additional 12

In recent years, numerous techniques and methods for large field-of-view (FOV) camera calibration have been proposed for applications in aerospace and civil engineering. However, it remains challenging to delve into the specifics of a calibration method and compare its accuracy with that of other methods. This study comprehensively examined five representative large-FOV calibration methods and analyzed their differences in calibration parameters, epipolar distance, reprojection error, three-dimensional coordinate error, and displacement measurement error. Additionally, it focuses on evaluating the accuracy of the camera’s intrinsic parameters across several calibration methods and discusses the impact of two types of corresponding points used for extrinsic parameter calculation. The paper concludes by summarizing the key findings in large-FOV calibration and presenting the advantages and disadvantages of the selected methods.

Due to their flexible configuration and lightweight characteristics, film structures have gained significant attention in the field of aerospace engineering. The scales of film structures typically range from several meters to over ten meters. Stereo-digital image correlation (stereo-DIC) methods offer distinct advantages for obtaining full-field measurement results. However, challenges persist in fabricating high-quality speckle patterns and addressing the problem of imaging reflections, particularly for large-scale transparent or semi-transparent film structures. This paper presents an experimental measurement method for large-scale, transparent thin-film structures. The method focuses on fabricating high-quality digital speckle patterns without altering the vibration characteristics of thin film, as well as addressing the problem of imaging reflections. A combined large-scale backlighting system and transmission imaging are introduced to solve the problem of reflections. To avoid altering the characteristics of the thin film, a single-particle transfer printing technique is developed. A large umbrella thin-film structure with a diameter of 6 meters is selected to validate the effectiveness of the proposed method. The structure is composed of multiple steel trusses and fan-shaped films. With high-quality speckle patterns and solving the problem of reflections, the full-field displacement results of the umbrella thin-film structure are measured. The first-order and second-order natural frequencies along with corresponding mode shapes are further obtained. The effectiveness of the experimental method is demonstrated through rotational and vibration tests conducted on the large umbrella thin-film structure. This method provides a powerful means for studying the mechanical behavior and vibration characteristics of large-scale thin-film structures.

Localised corrosion in steel bars has been a long-standing issue in the durability of reinforced concrete structures, but a comprehensive scheme for the analysis of pitting corroded steel bars, especially with respect to the deformation capacity, is not currently available. In this study, the morphological characteristics of 27 pitting steel bars were captured using a 3D scanner. The measured data were used to establish the probability distribution model of the cross-sectional areas of the corroded bars. Uniaxial tensile tests were conducted, and the evolving deformation field of the corroded bars was recorded through Digital Image Correlation (DIC). Based on the 3D reconstructed model and DIC results, an analytical method for evaluating the mechanical properties of pitting steel bars was developed and validated. The results show that the two-component Gaussian mixture distribution model outperforms conventional unimodal distribution models. Comparison of the analytical results with experimental data demonstrates that the proposed procedure is capable of predicting not only the ultimate strength but also the gauge length-dependent ultimate strain of corroded bars. Additionally, there exists a strengthening effect in the ultimate stress at the critical sections and this effect should not be ignored for accurate predictions.

Deflection, as an intuitive index, plays a pivotal role in assessing the load-bearing capability and structural integrity of complex and sizable steel structures. Notwithstanding, the conventional contact measurement is limited to static deflection and necessitates work stands and manual readings. Therefore, it hinders the attainment of dynamic deflection and fails to cater to the engineering demands of real-time and prolonged monitoring. By leveraging the advancements in computer vision technology, we propose an innovative system for real-time deflection monitoring of complex and sizable steel structures, particularly suitable for monotonic deformation induced by prolonged loading. Specifically, the off-axis-based displacement measurement method was adopted to surmount the constraint of requiring the optical axis of the camera to be perpendicular to the target. Additionally, the inverse compositional Gauss-Newton (IC-GN) algorithm and parallel computation based on seed point diffusion were exploited to boost the matching and computing efficiency for attaining real-time monitoring of multi-points. To validate the efficacy of the proposed system, we conducted static load monitoring tests on a Bailey beam, with a height of 29.5 m and length of 16.5 m per span, as part of the Alibaba Jiangsu headquarters project in Nanjing. The collected test data were compared with the results from a laser displacement sensor and the ABAQUS model. The outcomes substantiate that the system is capable of non-contact, expeditious, and straightforward installation, besides achieving high accuracy and real-time deflection monitoring. This system serves as a sophisticated solution for health monitoring in constructing complex and sizeable steel structures and further contributes to realizing construction intelligence.

For deep learning-based stereo-digital image correlation technique, the initial speckle position is crucial as it influences the accuracy of the generated dataset and deformation fields. To ensure measurement accuracy, an optimized extrinsic parameter estimation algorithm is proposed in this study to determine the rotation and translation matrix of the plane in which the speckle is located between the world coordinate system and the left camera coordinate system. First, the accuracy of different extrinsic parameter estimation algorithms was studied by simulations. Subsequently, the dataset of stereo speckle images was generated using the optimized extrinsic parameters. Finally, a dual-branch convolutional neural network, named Displacement and Strain Network (DAS-Net), was established to simultaneously reconstruct the displacement and strain fields. The simulation and experimental results demonstrate that the optimized extrinsic parameters can reduce the relative displacement errors to less than 2%. Furthermore, the DAS-Net algorithm accurately measures the displacement and strain fields as well as their morphological characteristics.

High-accuracy panoramic three-dimensional (3D) dynamic deformation measurements are of considerable significance for studying the mechanical properties of large or ultra-large structures. As a simple, non-contact, and highprecision full- field deformation measurement technology, three-dimensional digital image correlation ( 3D-DIC) can provide an effective measurement method for large- scale structural deformation measurement. This paper introduces the basic principles of 3D-DIC measurement and presents key technical advancements in the panoramic deformation measurement of large structures with multiple cameras. These advancements include large-scale speckle fabrication, 3D calibration of large field- of-view, coordinate unification of multi-camera systems, and real-time calibration of camera extrinsic parameters. Furthermore, this paper presents practical applications of high- accuracy 3D dynamic deformation measurements for large structures in civil engineering and aerospace engineering fields, such as the assessment of the cassette structure in seismic shaking table tests, panoramic high- speed deformation measurement of suspension cable dome structures in progressive collapse tests, and panoramic deformation measurement of cabin structures of launch vehicles in load tests. Using highaccuracy 3D dynamic deformation measurements for large structures provides a reliable experimental method for seismic performance analysis, mechanical modeling of large structures, and in-depth study of failure mechanisms.

The traditional stereo vision system with an air-based imaging model is unsuitable for multi-layer imaging conditions, where refraction effects can introduce significant errors in the measurement outcomes. Although numerous theoretical studies have developed refraction correction algorithms, it may be more beneficial and desirable to reduce the error by optimizing the system parameters in some cases. To address this dilemma, in this study, we first build a three-layer stereo vision model by forward projection and backward reconstruction. Then, the impact of various factors on the errors of reconstructed coordinates in the conditions of air-glass-air and airglass-water is demonstrated through simulations. Real experiments are further carried out to verify the validity of the simulation. Finally, a practical guide for conducting experiments and tests is presented to assist in the design of more effective system parameters.

Refraction-induced errors affect the accuracy of three-dimensional visual measurements in deepwater environments. In this study, a binocular camera refractive imaging model was established, and a calibration method for the refraction parameters was proposed for high-accuracy shape and deformation measurements in deep-water environments. First, an initial estimate of the refractive axis was obtained using a three-dimensional calibration target. Then, the errors in the distance between the spatial point pairs and the reprojection errors are taken as the dual optimization objectives, and the Non-dominated Sorting Genetic Algorithm II is applied to optimize the refraction parameters. To efficiently calculate the reprojection error, an improved numerical computation method is proposed to accelerate the calculation of the analytical forward projection. Underwater experiments were conducted to verify the method's effectiveness. The results showed that the average error of the absolute position of the reconstructed points was less than 1.1 mm and the average error of the displacement was less than 0.04 mm. This study provides a sound solution for accurate three-dimensional visual measurement in deep-water environments.

飞行器在服役中面临着严酷的高温环境,飞行器构件高温力学性能的研究对于其结构设计与可靠性保障十分重要.数字图像相关方法作为一种常用的非接触式光学测量方法,具有测量精度高、环境要求低、全场测量等优势,但在高温环境下的测量仍存在诸多技术难题.本文针对高温导致的退相关现象、热流扰动消除、高温散斑制备技术、力热解耦困难及特殊场景下光路遮挡受限等问题技术难题介绍了对应的改进方法,总结了国内外研究人员的优化思路与解决方案,最后针对飞行器力热参数测量需求,对高温数字图像相关方法的技术发展趋势进行展望.

A steel-fiber reinforced polymer (FRP) composite bar (SFCB) is a novel reinforcement composed of an inner steel bar and outer continuous FRP. Moreover, SFCBs have excellent corrosion resistance and stable positive post-yield stiffness and can be used to control structural damage. However, the relatively small fracture strain of the outer FRP may limit the deformation ability of the concrete members reinforced with SFCBs. Bundled reinforcement can weaken the interfacial adhesion to improve the structural deformation capacity and reduce the connections of the reinforced precast structures to enhance the construction efficiency and quality. In this paper, a quasistatic test of six concrete columns was conducted, and the deformation of the column foot was observed by the digital image correlation (DIC) method to investigate the effects of the reinforcement type (steel bar and SFCB), the number of bars in each bundle (1, 2, and 3), and the construction method (cast-in-place, general precast, and precast with full-length grouting) on the deformation capacity. The crack spacing of the precast concrete column reinforced with single -bar SFCBs increased by approximately 10% compared to the precast reinforced concrete (RC) column, and the crack spacing decreased with an increasing number of bars within each bundle. The proportion of the lateral displacement caused by rigid body rotation due to strain penetration of the longitudinal reinforcement at the precast column foot in total lateral drift was approxi-mately 35%; the smaller growth rate of residual rotation in concrete columns reinforced with SFCBs produced a superior post-disaster resetting ability. The curvature distribution range of columns reinforced with SFCBs was wider and more uniform than that of RC columns. Overall, the utilization of SFCBs as longitudinal reinforcement improved the equivalent plastic hinge length by approximately 120% compared with RC columns, thus greatly reducing the curvature requirement of the column bottom. Moreover, the bundled reinforcement reduced the growth rate of plasticity. The precast specimen with full-length grouting altered the failure mode and ach-ieved a considerable deformation capacity with the longest equivalent plastic hinge length of approximately 435 mm, which is 2.77 times that of the conventional RC column.

Calibration of a stereo-digital image correlation (stereo-DIC) system is essential for three-dimensional (3D) shape and deformation measurement. Although the traditional planar calibration method is flexible in most application scenarios, it still has difficulties in large field of view (FOV) calibration due to the limited size of calibration board. In this paper, a stereo-vision calibration method is proposed for large FOV measurement based on the close-range photogrammetry. Specifically, a certain number of coded targets are first arranged in the area to be measured. Then, the left and right cameras are used to capture images at the calibration and measurement positions. With these images, accurate camera intrinsic parameters can be obtained by close-range photogrammetry methods through encoded targets. The extrinsic parameters can be obtained by the 3D coordinates of these corresponding points and further optimized using corresponding points obtained from both coded targets and speckle patterns. Laboratory experiments with field-of-view ranges of 25 cm by 25 cm and 2.8 m by 2.8 m demonstrate that this method can obtain precise calibration parameters and has good deformation measurement accuracy. Compared to traditional planar calibration methods, this method does not require high-quality or large-sized calibration objects, and all parameters can be calibrated on-site. A large shaking table experiment demonstrated the potential of this method in the field of engineering measurements.