3D-DIC System Research Articles

Multiple-view 3D digital image correlation based on pseudo-overlapped imaging.

The restriction of the field of view (FOV) enlargement and spatial resolution increase during optical monitoring was investigated. Traditional optical instruments usually have a fixed FOV in one test; thus, they have low accuracy for small samples under large motions/deformations. To improve the spatial resolution in a relatively large FOV of an optical instrument, a multiple-view 3D digital image correlation (3D-DIC) method based on pseudo-overlapped imaging is proposed. Using a set of optical components, pseudo-overlapped imaging can image two FOVs with the same camera, which converts one pair of cameras to four virtual cameras. Each virtual camera has the same whole pixels. Therefore, compared with the conventional 3D-DIC system, the proposed method simultaneously enlarges FOVs and increases spatial resolutions by two times. The efficiency, accuracy, and feasibility of the technique were validated through experiments.

Coaxial panoramic 3D-DIC system for full inner surface deformation measurement

A coaxial panoramic three-dimensional digital image correlation (3D-DIC) system was developed for full 360o inner surface deformation measurement. The panoramic observation was realized by using a convex mirror. Two cameras were arranged coaxially to ensure the largest overlapping observation area. After the coordinate transformation and correlation calculation, the 3D displacement and strain distributions were reconstructed. The principles of the system were presented in detail, and the internal morphology and deformation of pipes subjected to compression loading were described to verify the effectiveness, practicality, and accuracy of the proposed system.

Dynamic impact experimental and global cohesive element method to shale fracture characterization

The fracture behavior and various of mechanical mechanisms of shale are greatly influenced by the bedding surface. To study the effects of bedding angles on the crack propagation patterns and dynamic fracture properties, the shale notched semi-circular bend (NSCB) specimens were studied by the modified split Hopkinson pressure bar (SHPB) device, three-dimensional digital image correlation (3D-DIC) system with the high-speed (HS) photography and ABAQUS finite element software. The research results showed that at the same impact velocity, the 30°, 45° and 60° beddings played an obvious deflexion effect on the shale crack propagation. However, the 0° and 90° bedding did not deflect the crack propagation. The dynamic fracture toughness of shale NSCB specimens increased with the increase of impact loading rate but the increase magnitude decreased with the higher impact rate. The kinetic energy during specimen fracture was calculated by the extracting displacement data from the 3D-DIC, and it was found that the kinetic energy accounted for less than 6% of the absorbed energy during the dynamic compression of the shale. The crack propagation characteristics of ABAQUS simulation was approximately consistent with the experimental results. It is conducive to the study of shale gas fracturing and the formation of complex seam networks in shale and it is of great research significance for the efficient extraction of shale gas.

Comparison of different optical techniques to measure skin vibrations

We test different possible setups to measure skin vibrations in the abdominal region which could, in the future, enable quick and comfortable monitoring of aortic aneurysms in the abdominal region. For this study, a simple mechanical model is used that simulates the human tissue and a large blood vessel, the aorta. The model consists of a plastic box filled with a gelatin as used for ballistic experiments. A rubber hose goes straight through the gel block and, with a pump that is connected to the hose, one can create a pulsatile flow of water through the flexible hose leading to traveling waves along the artificial vessel. Surface vibrations on the gelatin block are then measured using a Laser-Doppler vibrometer (LDV), a commercial 3D DIC system and a simple camera setup with only one camera and an additional mirror for 3D capabilities. While the LDV offers the best signal quality, it can only measure one point simultaneously. We discuss implications how this will influence the possible data analyses compared to more noisy, but full-field measurements provided by the camera-based systems.

Uncertainty quantification for 3D digital image correlation displacement measurements using Monte Carlo method

Three-dimensional digital image correlation (3D-DIC) is a leading optical measurement technique for measuring full-field shape, displacement, and deformation of solid materials and structures. However, compared with the well-developed and well-accepted measurement technique, uncertainty quantification (UQ) of 3D-DIC measurements is less advanced and less widely practiced. There still lacks a simple and effective method for quantifying the uncertainty of displacements measured by a specific 3D-DIC system for a specific speckle pattern. This is because the 3D-DIC practice involves a complex and long measurement chain and the propagation of uncertainty is highly nonlinear, which makes the uncertainty quantitation tricky. This work proposes a Monte Carlo-based method to quantify the uncertainty of 3D-DIC displacement measurements. The method is based on a theoretical analysis of random error in image matching and theoretical estimation of camera calibration uncertainty. It can estimate the uncertainty in 3D-DIC displacement measurements without additional experimental procedures. Validation experiments reveal that the estimated uncertainty agrees well with the actual situation. Based on the proposed method, the effects of the speckle pattern quality, subset size, and camera calibration quality on 3D digital image correlation displacement measurement uncertainties are also examined. The results reveal that, compared with image matching, camera calibration has a more pronounced influence on the uncertainty of displacement measurements. The proposed method can be integrated with existing 3D-DIC software to quantify the metrological performance of 3D-DIC measurements and therefore better interpret the measurement results.

Investigation of the 3D-DIC testing method for composite shell in a deep-water high-pressure environment

The investigation of pressure-bearing capacity design for underwater structures is of significant importance. This study focuses specifically on composite shell structures and conducts stress–strain experiments using strain gauges and 3D-DIC (digital image correlation) in a high-pressure underwater environment of 15 MPa, utilizing a 3D-DIC underwater shield. Numerical simulations were carried out to verify the feasibility of the 3D-DIC testing system in underwater high-pressure environments, and a scientific underwater DIC testing method was proposed. The experimental and numerical results show that strain gauges have limited accuracy and random measurements due to the large size ratio (5.5) between the strain gauges and small structural components, and the high strain gradient (41.0 μɛ/mm) at the small structural components, with limited measurement points. On the other hand, the 3D-DIC system has a minimum measurement unit that is 0.12 times smaller than the size of small structural components, accurately measuring stress–strain results over a larger measurement area. Based on the DIC and numerical results, this study explores the mechanical performance of the composite shell structure. The stress and strain at the ribs were relatively small compared to the adjacent ribs, with a difference of 71.3 MPa in average Mises stress and 479.2 μɛ in average strain, effectively improving the structural anti-deformation ability and enhancing structural strength.

Experiment study of dynamic buckling for stiffened panels under longitudinal impact

The stiffened panels of marine structures are usually suffered from various dynamic loads. However, there are very few reports about the dynamic buckling experiment for stiffened panels. In this paper, stiffened panels are designed to carry out a series of impact experiments on a falling hammer platform. The high-speed deflection fields during the extremely short impact time are recorded using 3D-DIC (Digital Image Correlation) system in these experiments. Thus, the dynamic deflections are acquired to analyze the dynamic buckling behavior for stiffened panels. Based on the experiment results, the application of dynamic buckling criteria is studied to determine the critical impact velocity for the experiment models. At last, the FE simulation is used to compare with the experiment results of dynamic response for stiffened panels, and the influence of initial imperfection modes are also discussed. The analysis results show that, the stiffened panels tend to have dynamic bucking modes with more half-waves of deformation in longitudinal direction, and the dynamic response can be significantly influenced by the initial imperfection modes.

On the Measurement of Nakazima Testing Based Out-of-Plane Forming Limit Curves using 2D Digital Image Correlation

The strain compensation method for measuring in-plane forming limit curves (FLCs) using 2D digital image correlation developed previously [A method of measuring in-plane forming limit curves using 2D Digital Image Correlation, SAE Int. J. Mater. Manf., 2023] was modified and extended to more versatile and popular out-of-plane FLCs. The current study introduces a straightforward strain compensation technique for measuring Nakazima testing based out-of-plane FLCs utilizing an affordable single-camera (2D) DIC system. In this study, forming tests are performed on two automotive-grade sheet metal alloys: DP980 steel and a 6xxx series aluminum alloy using the Nakazima test method. The experiments are conducted on a customized setup that allows for simultaneous optical strain measurements using both a stereo DIC and a 2D DIC system. The FLCs are obtained by applying a temporal FLC computation approach to the two measurement sets. The results show that 2D DIC FLC points match those obtained by stereo DIC for both the materials after applying the proposed strain correction method.

In situ measurement of full-field deformation for arc-based directed energy deposition via digital image correlation technology

Deformation measurement and control is a critical challenge in metal additive manufacturing. The metal deposition and solidification occur under the condition of high temperature gradient and large restriction accompany with complex metallurgical phenomena and stress evolutions. High stresses may result in deformation over yield strength of material, or even crack and fracture over the ultimate strength. Thus, it is necessary to monitor the deformation during additive manufacturing process. This paper focus on an in situ measurement of full-field deformation for arc-based directed energy deposition process via digital image correlation (DIC) technology. The measurement qualities were evaluated from the following aspects including system error, speckle pattern, influence of arc light and comparison in 2D-DIC and 3D-DIC. Results indicated that the relative system error of the measured deformation is no more than 0.12 % within a total deformation of 12 mm. Both the artificial speckle and natural speckle can meet the requirement of 2D-DIC system. The image qualities for artificial speckle is higher than that of natural speckle. For 3D-DIC, however, artificial speckle must be applied. The moving arc light will weaken the image qualities, which must be shielded in order to prevent the interference of intense light and the damage of speckle. An L-type shield was proposed and proved to be practical. Finally, DIC measuring was applied in both single-wall and cylindrical AM parts, comparing with numerical simulation. Results indicated that the evolution of the measured deformations showed good agreement with that of numerical simulation. Deformation evolution mechanism was revealed through numerical simulation and the evolution process was divided into three stages. For the cylindrical part, the final outer contour measured by DIC was consistent with that of 3D laser scanning and numerical simulation. As a conclusion, DIC technology was proved to be a feasible approach for in situ monitoring of full-field deformation of arc-based directed energy deposition process.

Adaptive modal identification of honeycomb thin-walled composite structures with pit defects under thermal modal testing using variational mode decomposition technique based on digital image correlation

The active imaging blue light high-speed three-dimensional digital image correlation (3D-DIC) system was combined with a transient aerodynamic heating system and random pulse excitation technology to establish a high-frequency thermal vibration optical test system capable of multi-temperature zone testing at 900 °C to accurately obtain the thermal modal parameters of thin-walled structures. The full-field temperature distribution of a novel honeycomb thin-walled composite structure (HTWCS) with pit defects under non-uniform temperature was accurately simulated through reinforcement learning. The first six modal frequencies of honeycomb thin-walled composite structure with pit defects at room temperature increased compared with the non-destructive honeycomb thin-walled composite structure, and the third and fourth modal shapes interchanged. An adaptive modal parameter identification method for full-field three-dimensional vibration measurements under high-temperature environments was developed by combining the successive multivariate variational mode decomposition (VMD) with the modal superposition method. This method can effectively identify close modes. The area ratio-based approach was used to evaluate the damping of the measured response. The finite element model (FEM) of the HTWCS with pit defects at different temperatures was updated using a multi-state step-by-step model updating method and the temperature-dependent material properties were established. The results showed that the introduction of a damping ratio improved the accuracy of the updated FEM of HTWCS with pit defects. The proposed technique is anticipated to provide an effective method for high-frequency thermal vibration optical measurement and modal identification of thin-walled structures under multi-temperature regions.

Interfacial fatigue debonding retardation in wrapped composite joints: Experimental and numerical study

Debonding crack propagation at the composite-to-steel interface has been found to be an important failure mechanism for wrapped composite joints under static and fatigue loads. Friction at the interface behind the crack tip may deviate fatigue debonding of the joints from the linear-fracture-mechanics behaviour. This paper presents static and fatigue tests of axial wrapped composite joints. 3D DIC and optical fiber system is employed to monitor displacements and crack propagation. A finite element model is established and validated against static and fatigue test results, where friction is considered at the cracked interface. Through FE modelling, it is proved that the friction at the interface significantly reduce the strain energy release rate (SERR) at the crack tip, leading to retardations of crack growth and stiffness degradation. Parametric study is conducted finally to investigate the influence of friction coefficient, failure modes as well as Paris relationship parameters on the predicted fatigue behaviour of wrapped composite joints.

Experimental determination of the reversed plastic zone size around fatigue crack using digital image correlation

This paper provides an experimental methodology to determine the reversed plastic zone size at the crack tip under cyclic loading through high-resolution DIC measurements. Due to the small size of this region, a 3D-DIC system based on two cameras in a stereoscopic setup with an optical microscope was used in the experimental setup. Fatigue crack growth tests were performed on AISI 1020 steel specimens to evaluate crack tip plasticity. The experimental results obtained with the proposed methodology demonstrate that the reversed plastic zone is a potential parameter to describe the fatigue crack propagation process and related phenomena.

Suction Cup Quality Predication by Digital Image Correlation

<div class="section abstract"><div class="htmlview paragraph">Vacuum suction cups are used as transforming handles in stamping lines, which are essential in developing automation and mechanization. However, the vacuum suction cup will crack due to fatigue or long-term operation or installation angle, which directly affects production productivity and safety. The better design will help increase the cups' service life. If the location of stress concentration can be predicted, this can prevent the occurrence of cracks in advance and effectively increase the service life. However, the traditional strain measurement technology cannot meet the requirements of tracking large-field stains and precise point tracking simultaneously in the same area, especially for stacking or narrow parts of the suction cups. The application must allow multiple measurements of hidden component strain information in different fields of view, which would add cost. In this study, a unique multi-camera three-dimensional digital image correlation (3D-DIC) system was designed and applied to measure the strain concentration of the suction cups while the cups were running the pulling progress. In this technique, a multiplexed quad-cameras DIC system which contains two sets of 3D-DIC system (4 cameras) with different field of view or different measurement directions enables simultaneous measurement of full-filed and hidden parts under the same calibration progress. The first two cameras built a sub-group of the 3D-DIC system, which was used to measure the local strain of the narrow or stacked prats. The other system was used to acquire the strain fields of the entire suction cup. In addition to the experimental test, the fatigue test to see the cracks appeared location. The results of DIC were compared to the fatigue data, and the DIC experimental data validated the crack location. This project aims to help designers and operators thoroughly understand the performance of vacuum cups by investigating the strain concentration and crack location.</div></div>

A Method for Measuring In-Plane Forming Limit Curves Using 2D Digital Image Correlation

<div>With the introduction of advanced lightweight materials with complex microstructures and behaviors, more focus is put on the accurate determination of their forming limits, and that can only be possible through experiments as the conventional theoretical models for the forming limit curve (FLC) prediction fail to perform. Despite that, CAE engineers, designers, and toolmakers still rely heavily on theoretical models due to the steep costs associated with formability testing, including mechanical setup, a large number of tests, and the cost of a stereo digital image correlation (DIC) system. The international standard ISO 12004-2:2021 recommends using a stereo DIC system for formability testing since two-dimensional (2D) DIC systems are considered incapable of producing reliable strains due to errors associated with out-of-plane motion and deformation. This work challenges that notion and proposes a simple strain compensation method for the determination of FLCs using a low-cost single-camera (2D) DIC system. In this study, formability tests are performed on an automotive-grade 6xxx series aluminum alloy using the Marciniak in-plane FLC testing method. The tests are performed on a custom setup that enables simultaneous optical strain measurements using a stereo DIC as well as a 2D DIC system. The results show how 2D DIC FLC points match those obtained by stereo DIC using two popular FLC approaches: ISO 12004-2 section-based spatial method and a time-dependent linear best fit (LBF) method.</div>

Single-Camera Three-Dimensional Digital Image Correlation with Enhanced Accuracy Based on Four-View Imaging

Owing to the advantages of cost-effectiveness, compactness, and the avoidance of complicated camera synchronization, single-camera three-dimensional (3D) digital image correlation (DIC) techniques have gained increasing attention for deformation measurement of materials and structures. In the traditional single-camera 3D-DIC system, the left and right view images can be recorded by a single camera using diffraction grating, a bi-prism, or a set of planar mirrors. To further improve the measurement accuracy of single-camera 3D-DIC, this paper introduces a single-camera four-view imaging technique by installing a pyramidal prism in front of the camera. The 3D reconstruction of the measured points before and after deformation is realized with eight governing equations induced by four views, and the strong geometric constraints of four views can help to improve the measurement accuracy. A static experiment, a rigid body translation experiment, and a four-point bending experiment show that the proposed single-camera 3D-DIC method can achieve higher measurement accuracy than the dual-view single-camera 3D-DIC techniques and that the single-camera 3D-DIC method has advantages in reducing both random error and systematic error.

Characterizing Complex Deformation, Damage, and Fracture in Heterogeneous Shale Using 3D-DIC

Safe drilling and effective fracturing are constant challenges for shale formations. One of the most important influencing factors is the accurate characterization of the deformation and damage caused by inherent lamination and natural fractures. Furthermore, shale formations exhibit fine-scale heterogeneity, which conventional laboratory methods (linear variable differential transformer (LVDT), strain gauges, etc.) cannot distinguish. To overcome these constraints, this research aims to investigate the damage and deformation characteristics of shale samples using three-dimensional digital image correlation (3D-DIC). Under uniaxial and diametrical compression, samples of Wolfcamp, Mancos, and Eagle Ford shale with distinct lamination and natural fractures are evaluated. The 3D-DIC system is utilized for image processing, visualization, and analysis of the shale damage process under varying loads. DIC made quantitative full-field strain maps with load (tension, compression, and shear), showing all the damage process steps and strain localization zones (SLZs). DIC maps are used to quantify damage variables in order to investigate sample damage. Damage variables are used to categorize the damage evolution process of shale specimens into four stages: initial damage, linear elastic, elastic–plastic, and plastic damage. Characterizing shale damage evolution with a strain localization line is more effective because there is more damage there than in the whole sample. Damage variables based on major strain and its standard deviation from the DIC strain map for all tested shale samples follow a similar trend, though diametrical compression variables are greater than uniaxial compression. In both uniaxial and diametrical compression, the Wolfcamp shale was reported to have the highest damage variable, which was measured at 0.37, while the Eagle Ford shale was reported to have the lowest damage variable. This image-based technique is more effective not only for understanding the laminated and naturally fractured rocks but also for predicting the hydraulic fractures that will occur during the stimulation process.

Strain evaluation of axially loaded collateral ligaments: a comparison of digital image correlation and strain gauges

The response of soft tissue to loading can be obtained by strain assessment. Typically, strain can be measured using electrical resistance with strain gauges (SG), or optical sensors based on the digital image correlation (DIC), among others. These sensor systems are already established in other areas of technology. However, sensors have a limited range of applications in medical technology due to various challenges in handling human soft materials. The aim of this study was to compare directly attached foil-type SG and 3D-DIC to determine the strain of axially loaded human ligament structures. Therefore, the medial (MCL) and lateral (LCL) collateral ligaments of 18 human knee joints underwent cyclic displacement-controlled loading at a rate of 20 mm/min in two test trials. In the first trial, strain was recorded with the 3D-DIC system and the reference strain of the testing machine. In the second trial, strain was additionally measured with a directly attached SG. The results of the strain measurement with the 3D-DIC system did not differ significantly from the reference strain in the first trial. The strains assessed in the second trial between reference and SG, as well as between reference and 3D-DIC showed significant differences. This suggests that using an optical system based on the DIC with a given unrestricted view is an effective method to measure the superficial strain of human ligaments. In contrast, directly attached SGs provide only qualitative comparable results. Therefore, their scope on human ligaments is limited to the evaluation of changes under different conditions.

Space–time evolution law of progressive failure area and mechanical behaviour of rock under different seepage conditions

In engineering geological disasters, the presence of water can accelerate rock mass fracture, and the state of the seepage field has a varying influence on the elastic behaviour of rock mass. A three-dimensional digital image correlation (3D-DIC) system is used to conduct triaxial compression tests on sandstone under different seepage conditions. The variation law of rock mechanical behaviour and the space–time evolution law of the surface radial strain field in the progressive failure process are analysed. The results show that, as the seepage pressure increases, the rock peak stress decreases and the Young's modulus increases, and the mechanical properties of the rock are easily affected by the varying seepage conditions. Meanwhile, a constitutive equation based on the Gumbel distribution, which can accurately reflect the axial stress–strain relationship in the rock failure process is established. By analysing the spatial distribution of virtual data points with large radial strain values at different times on the sample surface, a patent strain localisation phenomenon is identified in the progressive failure process of the rock. The localisation degree and extent of the failure area can be quantified using the average nearest-neighbour analysis (ANNA) and user-defined parameter S, which is used to normalise the average distance from the virtual data points to the fitted fracture surface. The results show that, with the continuous compression of samples under different seepage conditions, ANN decreases as S increases, and the large strain regional distribution is increasingly concentrated. The research results can be applied to predict surrounding rock failure areas in underground spaces and provide monitoring references with respect to engineering geological disasters under different seepage conditions.

Experimental Investigation of Double-Sided Strain of Composites in Tensile Testing Using MultiCamera DIC

Composite materials are widely used in engineering because of their high strength, high modulus, and corrosion resistance. Accurately obtaining the mechanical properties of composites is helpful for their further development. However, in a uniaxial tensile test, the strain distribution between the front and back of the composite sheet sample may differ due to its heterogeneity, and its stress-strain curve depends on which side the strain is measured. To report the stress-strain curve more accurately, a multicamera digital image correlation (DIC) system was employed to measure the double-sided strain in this work. Two sets of 3D digital image correlation (3D-DIC) systems were placed in front of and behind the specimen to record the tensile process of the specimen, and a double-sided calibration technique was implemented to connect the two sets of 3D-DIC systems into a unified coordinate system. Young’s modulus, percent bending, and local strain history are analyzed based on the measurements of strain on both sides. The results show that the strain on both sides of the composite is not exactly same, especially in the strain concentration area. As a comparison, the strain distribution on both sides of the metal material is almost identical.

A comparison of elasto-plastic parameters of S355 steel obtained in tensile tests using an extensometer, a strain gauge and an ARAMIS 3D DIC system

A comparison of elasto-plastic parameters of S355 steel obtained in tensile tests using an extensometer, a strain gauge and an ARAMIS 3D DIC system