Three-dimensional Digital Image Correlation System Research Articles

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.

Creep behavior of sandstone under the coupling action of stress and pore water pressure using three-dimensional digital image correlation

Understanding the damage evolution and time-dependent property of rock creep is of great significance for predicting geohazards and evaluating the long-term stability of geotechnical structures. In this study, a three-dimensional digital image correlation system was adopted to investigate the creep behavior of sandstone under the coupling action of stress and pore water pressure. The apparent strain fields, deformation characteristics of the localization zone, and micromorphology of the fracture surface were analyzed. The results demonstrated that when the applied deviatoric stress level was above σci (crack initial stress) or σcd (crack damage stress), the increase in pore water pressure promoted creep deformation evidently, improved the creep rate significantly and shortened the time-to-failure of the rock obviously. In the radial strain field, the localized development of substantial microcracks on the rock surface was concentrated in the steady-state creep, while the microcracks interconnected to form macroscopic shear cracks that dominated the accelerating creep, and this damage evolution characteristic can be used as a precursor and early warning of rock creep failure. Besides, increasing the pore water pressure also would cause the divergence point of strain curves inside and outside the localization zone to appear earlier at the secondary creep, and produce a wider localization zone at the tertiary creep. The creep fracture surface of the rock was dominated by intergranular microcracks. Increasing the pore water pressure would result in the deterioration of the cemented structure and breakage of the cemented matrix more seriously, thus stimulating the generation of more microcracks.

Study on the Compressive Properties of an Elastomeric Porous Cylinder Using 360° Three-Dimensional Digital Image Correlation System.

To study the compressive properties of an elastomeric porous cylinder, a 360° 3D digital image correlation (DIC) system is proposed. This compact vibration isolation table system captures different segments of the object from four different angles and fields of view, enabling a comprehensive measurement of the full surface of the object. To increase the stitching quality, a coarse-fine coordinate matching method is presented. First, a three-dimensional rigid body calibration auxiliary block is employed to track motion trajectory, which enables preliminary matching of four 3D DIC sub-systems. Subsequently, scattered speckle information characteristics guide fine matching. The accuracy of the 360° 3D DIC system is verified through a three-dimensional shape measurement conducted on a cylindrical shell, and the maximum relative error of the shell's diameter is 0.52%. A thorough investigation of the 3D compressive displacements and strains exerted on the full surface of an elastomeric porous cylinder are investigated. The results demonstrate the robustness of the proposed 360° measuring system on calculating images with voids and indicate a negative Poisson's ratio of periodically cylindrical porous structures.

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.

Omnidirectional 3D-DIC method for determining inner deformation in pipelines

An omnidirectional three-dimensional digital image correlation (DIC) system was developed for full 360-degree inner surface deformation measurements using a pyramid reflector to realize panoramic observation. After the coordinate transformation, the 3D displacement and strain distributions were reconstructed. The basic principles, optical arrangements, and experimental procedures were described in detail. Morphology and rigid-body displacement experiments were conducted to evaluate the efficiency and accuracy of the 3D shape and displacement determination. The experimental results agree well with the actual values and the data obtained by multiple measurements using the traditional 3D-DIC method. Moreover, the proposed method was successfully applied to evaluate the inner deformation distributions of pipes subjected to compression loading. The experimental results further validated the applicability of the proposed method.

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>

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 into the response of GFRP S-shaped foldcore sandwich panels to impulsive loading

S-shaped foldcore sandwich structures have potential applications as protection structures under impact loading. In this paper, the S-shaped glass fiber composite foldcore sandwich structure was fabricated by hot pressing method and its impact resistance against the aluminum foam projectiles was studied using a one-stage gas gun.The three-dimensional digital image correlation system was used to measure the dynamic deformation of the sandwich panels. During the experiments, the dynamic response of the structure was studied when the projectile impacted the node and base locations of the sandwich structure, where the node location refers to the top platform of the core and the base location refers to the middle position between the nodes. In addition, the influence of the face sheet thickness and core thickness on the impact resistance of the sandwich structure was also investigated. The results show that the sandwich structure is dominated by tensile fracture of the face sheet, crush fracture or extrusion fracture of the core, and debonding failure between the face sheet and the core. The impact resistance of the node location is better than that of the base location for the same configuration. Moreover, the maximum deflection of the rear sheet increases linearly with the increase of the impact loading when impacting the base location. And increasing the face sheet thickness or core thickness can better reduce the structure deflection and improve the impact resistance of the structure.

Structural Integrity Assessment of Defected Gas Pipelines Using a Simplified Ductile Damage Model

Abstract The finite element method using the damage model has been increasingly used to predict the failure of various structures. Thus, various damage models were presented, and recently, a phenomenological model called the local fracture strain model was presented, making it easy and accurate to predict the damage of the structure. This model has the advantage of defining fracture strain as a function of stress triaxiality with only a few notched tensile tests but has a limitation because it does not consider the damage evolution because of the void growth. This study presents an enhanced damage model that improves the accuracy of the failure simulation of defected structures by adding a parameter that considers stiffness degradation according to void growth to the damage model based on the fracture strain. Therefore, loading–unloading tests were conducted and the damage index of fracture was identified using a three-dimensional digital image correlation system. The failure simulation results using the proposed damage model were compared with experimental, such as notched tensile, single edge notched tension (SENT), and full-scale burst tests.

Effect of thermal treatment on the fracture toughness and subcritical crack growth of granite in double-torsion test

An understanding of the fracture behavior and subcritical crack propagation of hard rock under different temperatures is necessary for the design and stability of nuclear waste disposal projects. The aim of this study was to understand the thermal effect on the fracture behavior of granite, which had been heated at 25 (untreated), 100, 200, 400, and 600 °C, respectively. A series of uniaxial compressive tests and double-torsion tests were performed on granite specimens. The three-dimensional digital image correlation technique was used to investigate the strain evolution and fracture pattern in granite specimens during subcritical crack propagation. According to the results, the uniaxial compression strength, Young’s modulus and fracture toughness of the granite specimens increased and then decreased with an increase in treatment temperature. These parameters dropped sharply as the treatment temperature exceeded 400 °C, mainly because of the thermally-induced destruction of the quartz crystal structure and a generation of wider intergranular fractures. A comparison of the subcritical crack length by theoretical calculations and experimental observations from digital image correlation showed that the two values were consistent. At a treatment temperature of 400 °C and 600 °C, the velocity of the subcritical crack propagation was accelerated, so the length of the subcritical crack during the relaxation test was 4.14 times and 5.25 times that at 25 °C, respectively. The subcritical crack growth and fracture process of the double-torsion tests were visible by the three-dimensional digital image correlation system. A subcritical crack growth index (n) can be used to characterize the susceptibility of the subcritical crack propagation in rock relaxation tests, which helps to evaluate the long-term stability of nuclear waste disposal projects.

Ex situ Synthesis and Characterization of High Strength Multipurpose Bacterial Cellulose-Aloe vera Hydrogels.

The innate structural and functional properties of bacterial cellulose (BC) have been greatly improved by developing its composites with other materials for its applications in different fields. In the present study, BC-Aloe vera (BCA) gel composite with high tensile strength was ex situ developed and characterized for its potential applications in environmental and medical fields. FE-SEM micrographs showed the impregnation of Aloe vera gel into the fibril network of BC. The dry weight analysis showed the addition of 40 wt.% Aloe vera contents into the BC matrix. The addition of Aloe vera resulted in a 3-fold increase in the mechanical strength of BCA composite. The critical strain or stress concentration points were accurately identified in the composite using a three-dimensional digital image correlation (3D-DIC) system. The BCA composite retained water for an extended period of up to 70 h. The BCA composite effectively adsorbed Cu, Co, Fe, and Zn metals. Moreover, the BCA composite supported the adhesion and proliferation of MC3T3-E1 cells. The findings of this study suggest that the developed BCA composite could find multipurpose applications in different fields.

Bond resistance of prestressed CFRP strips attached to concrete by using EBR and EBROG strengthening methods

Strengthening reinforced concrete (RC) beams and slabs with prestressed carbon fiber-reinforced polymer (CFRP) composites is of high interest since the FRP material can be used more efficiently. Developing a non-mechanical prestressed strengthening system entails evaluating the bond resistance of prestressed FRP to the substrate. To increase the bond resistance of prestressed CFRP strips to the concrete substrate, a new method called externally bonded reinforcement on grooves (EBROG) was investigated and compared with the conventional externally bonded reinforcement (EBR) method. For this purpose, prestress force-release tests were performed to study the bond behavior of prestressed FRP to concrete. In-plane and out-of-plane deformations were measured through a three-dimensional digital image correlation (3D DIC) system. All the tests were carried out at Empa Structural Engineering Laboratory in Dübendorf, Switzerland.Experimental results showed that the average bond resistances of EBR and EBROG joints in prestress force-release experiments were 34.4 and 81.4 kN, respectively; indicating an increased bond resistance for the EBROG method with a factor of 2.4. By using the EBROG method, the failure plane incorporated massive crack development in concrete bulk and therefore, the fracture energy and bond resistance of prestressed joints increased significantly. Besides, it was shown that prestressed joints experienced large out-of-plane deformations, emphasizing the fact the bond behavior of prestressed FRP to concrete should be treated as a mixed shear/tension fracture mode in the future studies.

Behavior analysis of concrete with recycled tire rubber as aggregate using 3D-digital image correlation

Owing to the rapid growth of world population and the usage of cars, scrap tires are causing serious environmental problems and ecological threats, which can be solved by recycling them, e.g., by using them as aggregates in concrete. In this study, an experimental investigation is established to understand the compression behavior of concrete by replacing natural aggregates with recycled tire rubber at different percentages, ranging from 10% to 50% by volume. In experimental testing, several compression tests were performed on cylindrical concrete samples, and a three-dimensional digital image correlation (3D-DIC) system was used to analyze the compression behavior of concrete structures during deformation. The experimental results indicated that the behavior of concrete changed from brittle to ductile and their ability to absorb compression energy (compression toughness) improved by increasing the crumb rubber percentage. Using 3D-DIC system which has a potential to replace the conventional tools, cracks onset locations were detected, failure mechanism and crack progress were tracked, and critical strain values which are usually at the interface of rubber particles and concrete were located. Although the cost of rubber crumbs is high compared with that of natural aggregates, incorporating recycled tire rubber in concrete structures can solve ecological problems.

Modal Characteristics of Carbon Fiber Reinforced Composite Structure at High Temperature

Flight vehicles that fly at high Mach speeds will be exposed to a severe aerodynamic heating problem, which leads to high surface temperatures and large temperature gradients. Thermal load will induce thermal stress, thermal deformation, and even thermal buckling. Therefore, the dynamic stiffness and the modal characteristics will be affected. In this paper, several tests are carried out for a plain woven carbon-fiber-reinforced silicon carbide (C/SiC) composite plate under elevated temperature. The plate is heated by a quartz-lamp radiation heating system. A three-dimensional digital image correlation system is first employed to obtain the relationship between displacements and temperature to get the critical buckled temperature. Then, the thermal modal tests are performed by a laser scanning vibrometer to investigate the effect of thermal loads on modal parameters: especially the effect of the critical buckled temperature. A specific fixture with water cooled is designed in order to avoid the influence of the uncertainty of the boundary condition since the modal parameters are very sensitive to the boundary condition. Finally, the numerical simulation process of the high-temperature modal tests for the buckled plate is presented to investigate the contributions from temperature-dependent material properties and thermal stresses. As the temperature increases, the modal frequencies reduce. After the critical buckled temperature, the modal frequencies increase. A parametric study is further performed using the presented numerical model to investigate the thermal modal characteristics of C/SiC composite plates with various thickness and boundary conditions.

Measurement of Zircaloy-4 cladding tube deformation using a three-dimensional digital image correlation system with internal transient heating and pressurization

In this study, multi-dimensional (Multi-D) deformation of a Zircaloy-4 cladding tube under internal transient heating and pressurization conditions was measured by experimental setup named ‘Experimental setup to investigate high temperature deformation of cladding tube under internal pressurizing and heating (EIGEN)’ and three-dimensional digital image correlation (3D DIC) system. The ‘EIGEN’ experimental setup was designed to investigate high temperature deformation of the cladding tube under inert gas atmosphere. The 3D DIC system based on stereovision was further developed to be applied to high temperature by using high temperature resistant speckle pattern and bandpass filter. To evaluate measurement accuracy of the 3D DIC system, validation works of 3D DIC calibration were performed with proposed methodology. Additionally, a 3D finite element model (FEM) of the ‘EIGEN’ experiment was developed to evaluate the results of the 3D DIC. Finally, ‘EIGEN’ experiment was conducted under several fill pressure conditions such as 30, 50, 60, 70 bars. As a result, the established 3D DIC system could successfully measure in-situ and real-time large deformation such as ballooning and rupture as well as full-field multi-D deformation of the cladding tube at the high temperature under the inert atmosphere. Multi-D strains from the 3D DIC showed good agreement with calculated ones of the 3D FEM, and they were also compared with 3D laser scanning results for validation.

Enhanced Voids Growth Model for Ductile Fracture Prediction of High-Strength Steel Q690D under Monotonic Tension: Experiments and Numerical Simulation

Accurate prediction of progressive damage and failure of structural steels is a nontrivial issue in practical design and assessment of steel structural buildings. It appears far more important in the fracture-oriented scenarios under extreme environmental actions or accidental overloading conditions because, in these cases, the ultimate strength of steel structures is directly governed by material failure. However, previous studies on ductile fracture of structural steels were relatively limited, especially for high-strength steels (HSSs) like Q690D steel. In this paper, an enhanced fracture model based on the voids growth model (VGM) is proposed. The normalized maximum shear stress is introduced to make it capable of capturing ductile fracture at low and moderate stress triaxialities. To calibrate and verify the newly proposed model, a series of tensile tests of smooth and notched round bar specimens and shear specimens with different shear angles were carried out by using Q690D steel. Bridgeman’s theory was applied to identify the postnecking constitutive relationship with the help of a three-dimensional digital image correlation (3D-DIC) system. The good agreements between numerical simulations and experimental results finally verified the applicability and reliability of the enhanced model in ductile fracture prediction.

The behaviour of fibre-reinforced composites subjected to a soft impact-loading: An experimental and numerical study

The present paper presents experimental and numerical studies on the behaviour of composite laminates subject to impact loading by soft projectiles to represent the impact of a small bird or hail-stone. In this research, gas-gun experiments are performed to study woven carbon-fibre reinforced poly (ether-ether ketone) (CF/PEEK) composites subjected to an impact by soft-gelatine projectiles. In addition, woven carbon-fibre reinforced epoxy (CF/epoxy) composite specimens are also evaluated using gelatine projectiles to investigate the effect of the matrix system on the impact response of the composites. A high-speed camera is employed to capture the deformation of the projectiles and a three-dimensional (3D) Digital Image Correlation (DIC) system is used to record the deformation of the impacted composite specimens. A Finite Element (FE) model is developed to simulate the impact by a soft projectile on the composite specimens. Good agreement is shown between the predictions from using the FE model and the experimental results.

Strain field measurements over 3000 °C using 3D-Digital image correlation

With the development of aerospace and fusion engineering, understanding the mechanical behavior of materials under high-temperature conditions has become increasingly important. However, few studies are devoted to the ultra-high temperature range of 2000–3000 °C. In this study, with the aim of developing non-contact measuring techniques of mechanical deformation under ultra-high temperature, a high heat flux (~300 MW) comprehensive experimental platform is established, which includes a vacuum chamber, a three-dimensional digital image correlation (3D-DIC) system, infrared radiation thermometers and an electron beam heating system. Using the electron beam heating technique, the tungsten specimen can be heated to over 3000 °C. Owing to the use of a vacuum chamber, the thermally induced airflow disturbance at high temperature can be completely removed. Tantalum carbide (TaC) powder is chosen as the speckle material and speckle fabrication technology is developed to adapt ultra-high temperatures under vacuum conditions. In order to suppress the blackbody radiation at high temperature, three schemes based on blue light sources, self-radiating light sources and a dual wavelength optical filter technique are designed for three temperature ranges from room temperature to 3067 °C. Afterwards, full-field thermal deformation of the tungsten specimen above 3000 °C was determined based on the above strategies using the 3D-DIC technique. The feasibility and accuracy of the proposed methods are verified by comparing the measurement results with the thermal expansion strain data and model from available databases and literature. The standard deviations in different temperature intervals are 50 με for 25–1200 °C, 100–200 με for 1200–1800 °C and less than 500 με for 1800–3067 °C. The proposed methods and technologies are expected to lay a foundation for further developments in strain field measurements at ultra-high temperature.

The Impact Performance of Woven-Fabric Thermoplastic and Thermoset Composites Subjected to High-Velocity Soft- and Hard-Impact Loading

The present paper investigates the impact performance of woven-fabric carbon-fibre composites based upon both thermoplastic- and thermoset-matrix polymers under high-velocity impact loading by conducting gas-gun experiments at impact velocities of up to 100 m.s−1. The carbon-fibre reinforced-polymers (CFRPs) are impacted using soft- (i.e. gelatine) and hard- (i.e. aluminium-alloy) projectiles to simulate either a soft bird-strike or a hard foreign-body impact (e.g. runway debris), respectively, on typical composites employed in civil aircraft. The out-of-plane displacements of the impacted composite specimen are obtained by means of a three-dimensional Digital Image Correlation (DIC) system for the soft-projectile impact on the composites and the extent of damage is assessed both visually and by using portable C-scan equipment. The perforation resistance and energy absorbing capability of the composites are also studied by performing high-velocity impact experiments using the hard-projectile and the resulting extent and type of damage are identified. In addition, a Finite Element (FE) model is also developed to investigate the interaction between the projectile and the composite target.