•The first open source library with full functions of 2D, stereo DIC and DVC.•The library is developer-friendly and facilities the method study of DIC.•The modules demonstrate superior accuracy, ...robustness, and efficiency.•Novel and powerful DIC methods are developed using OpenCorr as good showcases.
OpenCorr is an open source C++ library made for research and development of digital image correlation (DIC) technology. It provides full-function modules of 2D and stereo DIC, as well as digital volume correlation (DVC). In contrast with most of current open source DIC software which is application-oriented, the sophisticated design of OpenCorr makes it developer-friendly and facilitates the method study. Users can create DIC software quickly for specific applications by assembling the modules in this library and realize new algorithms readily by modifying the modules or incorporating additional modules. Three examples are given as the showcases for the capability of OpenCorr. The first one is an image feature guided DIC method, which can accurately measure large strain over 35%. The second is a stereo DIC method combining image feature guided DIC with an epipolar constraint aided algorithm to achieve enhanced robustness. The third is a novel self-adaptive DIC method, which dynamically optimize the size and shape of subset at each point of interest (POI) according to the nearby image features. The computation speed of DIC and DVC modules is evaluated through a few tests, which demonstrate superior running efficiency of the library. OpenCorr has attracted rapidly growing interest from both academic and industrial communities. It is expected to stimulate the advance and spread of DIC technology.
•A novel MLP-SRCNN composite neural network is introduced to enhance the scanning images from THz-TDS system.•Unlike conventional super-resolution enhancement algorithms that require an extensive set ...of images for training, this network training algorithm minimizes the need for such a large dataset by utilizing the relative displacement fields as the loss function.•An adaptive method is proposed to convert the time domain waveform to a pixel intensity, instead of using a fixed component from the waveform.
A enhancement algorithm for scanning images from terahertz time domain spectral system is proposed. The algorithm is a machine learning algorithm based on a multi-layer perceptron (MLP) network and a super-resolution convolutional neural network (SRCNN). The MLP network is utilized to convert a time domain waveform vector into a scalar pixel intensity adaptively; the SRCNN network aims to achieve the spatial enhancement of the output from the MLP network. The new algorithm needs only two scanning images to achieve the enhancement effect. The two images are captured before and after a rigid body translation of the specimen. Then, a classic relative deformation algorithm, digital image correlation (DIC), is used to compute the displacement fields between the two images. The difference between the calculated and actual displacement fields is taken as the loss function of the MLP-SRCNN composite network. The effectiveness of the proposed algorithm is demonstrated by a typical validation experiment. A considerable enhancement on the quality of the scanning images from THz-TDS is achieved under the no need of additional training data.
•A stereo-vision calibration method is proposed for large FOV measurement based on the close-range photogrammetry.•No calibration board is required and both intrinsic parameters and extrinsic ...parameters can be calibrated on the experimental site.•The proposed calibration method has been successfully applied to large scale seismic simulation shaking table test.•The displacement measured by stereo-DIC with the proposed calibration method is in good agreement with the results of displacement meter.
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.
The full-field deformation characterizations of a metal undergoing large plastic strains are difficult to be obtained from the conventional true stress-strain curves. In this study, we proposed using ...a simple non-contacting technique, digital image correlation (DIC) with laser speckles, to evaluate large plastic deformations of metals, especially for thin metal foils and for the forming at high temperatures. The feasibility and accuracy of this DIC technique were confirmed by conducting uniaxial tensile tests using different materials. As results, the measured strains indicated a good accuracy with an error range of 6% in measuring the large plastic deformation at room temperature and 10% in measuring a large strain up to 0.6 at a high temperature of 400 °C. The strain distribution measured by DIC with laser speckles and with artificial speckles showed good agreement, confirming the feasibility of this DIC technique for measuring large plastic deformations. By using this technique, the evolution of plastic zone was successfully investigated for thin metal sheets with a thickness of 0.5 mm. Furthermore, the strain fields involving localizations of 50 μm thick metal foils forming at room temperature and a large plastic deformation of metal tubes forming at 400 °C were evaluated qualitatively and quantitatively.
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•Large deformations of metals are successfully evaluated by using digital image correlation (DIC) with laser speckles.•Stress-strain curves of thin foils and for the forming at high temperatures are measured by this DIC technique accurately.•Full-field strain distribution obtained by DIC with laser and artificial speckles shows good agreement.•The evolution of plastic zone of thin metal sheets indicates different paths for different materials.•Pure Ti foils indicate non-uniform plastic deformation, while deformation of A1050 at 400 °C is relatively uniform.
•A novel UV stereo-DIC system combining UV lighting and UV cameras was proposed for shape measurement in ablation tests conducted in arc-heated wind tunnel tests.•The UV stereo-DIC technique can ...effectively suppress the strong thermal radiation encountered high-temperature wind tunnel tests.•The natural texture on the test object illuminated by the UV lighting is used to reconstruct the 3D shape of blunt cones.•3D ablation shapes and recessions of blunt cones subjected to arc-heating with a highest temperature up to 1868 °C were successfully obtained.
In-situ three-dimensional (3D) ablation shape and recession measurements of ablative materials of hypersonic vehicles in high-temperature wind tunnel tests are essential to understand the ablation performance of these materials, which can provide necessary information for better thermal protection system (TPS) design. In this work, a novel ultraviolet stereo-digital image correlation (UV stereo-DIC) technique is proposed for in-situ measurements of 3D ablation shapes and recession in ablation tests. Specifically, a self-developed UV stereo-DIC system, combing a monochromatic UV illumination, two bandpass filters and two UV cameras, is first established. Using the UV stereo-DIC system, strong thermal radiation emitted from the heated sample and the heating device can be effectively suppressed. Natural textures formed from illuminating the rough sample surface by UV light are used as speckle patterns in stereo-correlation. In-situ 3D ablation shapes of a blunt cone subjected to arc heating were measured with a temperature range of 1000 °C to 1868 °C, and the UV stereo-DIC measurements showed good agreement with post-test measurement of the recession of the test articles. Based on the experimentally measured 3D shapes, several key parameters, such as full-field recession, dynamic recession rate, were calculated. The results demonstrate the efficacy and potential of UV stereo-DIC for 3D shape and recession measurements in ablation tests.
Metals with a hexagonal close packed structure can deform by several different slip modes with different Critical Resolved Shear Stresses, which provides a great deal of complexity when considering ...mechanical performance of Mg, Ti and Zr alloys. Hence, an accurate but also statistically meaningful analysis of active slip systems and their contribution to plasticity is of great importance for the understanding of deformation mechanism. In the present study, a correlative scanning electron microscopy-based method of slip trace analysis has been utilised to provide statistical, accurate information of slip behaviour in a weakly textured Ti–6Al–4V alloy with a plastic strain of ∼2%. This is achieved through grain orientation mapping by Electron Backscatter Diffraction and strain mapping by High Resolution Digital Image Correlation. The initial identification of slip mode was performed by comparing the slip trace captured in the high-resolution effective shear strain map with all theoretical slip planes with an angle acceptance criterion of ±5°. Ambiguity in slip mode identification was further resolved using the Relative Displacement Ratio method, which enables the determination of the Burgers vector directly from the displacement data. The correctness of the identified slip modes has been confirmed by detailed dislocation analysis using Bright Field Scanning Transmission Electron Microscopy on thin foils extracted from specific grains employing Focused Ion Beam. This detailed investigation demonstrates the robustness of the slip trace analysis based on grain orientation and high-resolution strain mapping.
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•Ductility of cementitious backfill composites is improved by 3D printed polymeric lattice (3D PPL).•Several types and shapes of 3D PPL were tested by agreeing the digital image correlation ...method.•Flexural strengths of ordinary and nylon resin 3D PPL reinforced CBC were the best among others.•3D PPL has great potential for turning brittle CBC into ductile material without limiting its strength.
The compression properties of cementitious backfill composite (CBC) is strong, but their tension properties are weak and presents little ductility because of their low fatigue crack growth rate and resistance. Thus, proper reinforcement techniques are needed to improve CBC’s ductility without rupture. This paper deals a system for boosting CBC’s ductility by reinforcing it with several types (e.g., ordinary resin OR, transparent resin TR, and nylon NY) and shapes (e.g., hexagon, cube, and rhombus) of 3D printed polymeric lattice (3D PPL) by agreeing the digital image correlation method. Specimens were subjected to three-point bending and SEM experiments to study their flexural and microstructural behavior. Results shown that: the flexural strengths of OR and NY 3D PPL reinforced CBC specimens were larger than TR and non-3D-PPL (N-3D-PPL) reinforced ones. But TR 3D PPL reinforcement effect was not so vibrant. For an ideal flexural strength, the best type and shape of 3D PPL were OR and rhombus, respectively. The deflection of 3D-PPL reinforced CBC specimens was larger than those N-3D PPL reinforced ones. For an ideal deflection value, the optimal shape and 3D PPL types were cube and OR. 3D PPL reinforced CBC specimens exhibited great ductility and toughness while N-3D PPL reinforced ones exhibited obvious brittleness. The most abundant elements were oxygen, calcium, and silicon, which gathered in the large amounts around C-S-H gels’ location. Finally, the outcomes of this work state that 3D PPL has great potential for converting brittle CBC into a ductile material without dropping its hardening performance.
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•Microstructurally small fatigue crack growth (FCG) rate is studied by DIC.•FCG rate retardation is associated with shear strain localisations.•Grain boundaries control interaction ...between strain localization and FCG.•Shear strain localisations lead to formation of grain sub-structures.•Intense cross slip facilitates the overcoming of the strain localisation zone.
Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.
•An optimal path search method (OPSM) with error control for full-field strain measurement of material is proposed.•The OPSM is implemented by optimizing the algorithm of the traditional digital ...image correlation method.•The validity of the OPSM is verified by two numerical model experiments and a quasi-static compression experiment of 3D printed cellular structure.•The OPSM method improves the calculation accuracy of the strain field in the boundary, hole, and zone with an abrupt change in displacement.
Digital image correlation (DIC) is one of the main means of optical measurement of the strain field. However, there has been little discussion on the calculation errors at positions with abrupt change. This study seeks to address how to reduce the full-field strain calculation errors and improve the strain accuracy. An optimal path search method (OPSM) with error control for a full-field strain solution is proposed by optimizing the algorithm of the traditional DIC method, using the material displacement field with abrupt change. The strain calculation considering the calculation zone boundary, hole, and shock wave front is discussed. A comparison of the results using the proposed method with the theoretical results shows that this method can effectively obtain full-field strain information, especially at positions with abrupt change in displacement. The application of OPSM to the cellular structure in a quasi-static compression experiment again verifies the validity. This method not only is comparable with the original method in the noise suppression effect but also ensures the approximation of the results to the true values in the displacement zone with abrupt change.
•The intensity of saturated pixel is estimated by a proposed strategy which is based on feature matching and standard DIC method.•The intensities of all the pixels in a large saturated speckle can be ...estimated.•The simulations and experiments indicate that the accuracy and correlation coefficient have been greatly increased.
Accuracy of digital image correlation (DIC) is deeply affected by illumination variation, especially the local pixel saturation exists. A simple and efficient gray intensity adjustment strategy based on feature matching and standard DIC method is presented for estimating the intensities of saturated pixels. Principle of the strategy based on surrounding pixels around the saturated pixel and the principle of the new proposed strategy are first detailed. Simulations and experiments are performed to verify the efficiency of the proposed strategy. The results indicate that the DIC measurements based on the proposed strategy are able to provide more reliable and accurate results as compared to the existing strategy, even though the large local speckles are saturated.
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