Two-dimensional Digital Image Correlation Research Articles

3D strain heterogeneity and fracture studied by X-ray tomography and crystal plasticity in an aluminium alloy

Strong correlations between measured strain fields and crystal plasticity finite element (CP-FE) predictions based on the real microstructure are found for a plane strain tensile specimen made of 6016 T4 aluminium alloy. This is achieved using multimodal X-ray lab tomography giving access to both the initial grain structure and the strain evolution. The real microstructure of the central region of interest (ROI) of the undeformed specimen is obtained non destructively using lab-based diffraction contrast tomography (DCT) and meshing. An in situ tensile test, using absorption contrast tomography (ACT) is then performed for twelve loading increments up to fracture. Taking advantage of the plane strain condition, the evolution of the internal strain field is measured by two-dimensional digital image correlation (DIC) in the material bulk using the natural speckle provided by intermetallic particles. Early strain heterogeneities in the form of slanted bands, that are spatially stable over time, are revealed and the fracture path – determined from the post mortem scan – is found to coincide with the bands exhibiting maximum strain. CP-FE simulations are performed on the meshed microstructure of the specimen acquired by DCT and are compared with image correlation measurements. The measured strain fields are well described by 3D CP-FE predictions, whilst it is shown that neither a macroscopic anisotropic plasticity model nor a CP-FE simulation with random grain orientations could reproduce the measurements.

Strain rate sensitivity of rotating-square auxetic metamaterials

This study provides an in-depth analysis of the mechanical behavior of rotating-square auxetic structures under various strain rates. The structures are fabricated using stereolithography additive manufacturing with a flexible resin. Mechanical tests performed on structures include quasi-static, intermediate, and high strain rate compression tests, supplemented by high-speed optical imaging and two-dimensional digital image correlation analyses. In quasi-static conditions (5 × 10–3 s-1), multiscale measurements reveal the correlation between local and global strains. It is shown that cell hinges play a significant role in structural deformation and load-bearing capacity. In drop tower impact conditions (intermediate strain rate of ca. 200 s-1), the auxetic structures display significant strain rate hardening compared to loading at quasi-static rates. The thin-hinge structures maintain a Poisson's ratio of approximately -0.8, showing higher auxeticity than slow-rate compression tests. High strain rate conditions (ca. 2000s-1) activate additional deformation mechanisms, including a delayed state of equilibrium exemplified by a heterogeneous distribution of lateral strains, possibly due to stress wave interactions and inertial stresses. The study further reveals nonlinear correlations between Poisson's ratio, strain, and strain rate, indicating reduced auxeticity at higher strain rates. These observations are discussed in terms of complex wave interactions and the strain rate hardening characteristics of the base polymer.

Low-cost two-dimensional digital image correlation system

A low-cost digital image correlation (DIC) system for measuring two-dimensional displacement fields is described. Using an off-the-shelf camera, lens, and open-source DIC analysis software, displacement fields in aluminum and steel dog-bone tensile specimens were measured. Correction for out-of-plane motion is used to obtain strains from the displacement fields. Proposed DIC strain measurement system is validated using conventional thin-film strain gauges for strains below 1%. Average strain measurements from the DIC system were found to be within 1% of the measurement obtained using strain gauges. For strains between 1% and 10%, strain histories from the DIC systems displayed excellent correlation with strain measurements obtained using an extensometer.

Adaptive matching strategies for 3D digital image correlation in strain measurement of an aerostat envelope

To address the limitations of two-dimensional digital image correlation (2D-DIC) in measuring strain on the aerostat envelope, the more precise 3D-DIC has been introduced to handle curved surfaces. However, the increased computational load of 3D-DIC requires more efficient correlation strategies. This paper evaluates three basic matching strategies and introduces two adaptive strategies to enhance the efficiency of 3D-DIC. The experimental results show that the adaptive composite matching (ACM) strategy automatically switches strategies based on deformation, improving the matching correlation. Meanwhile, the adaptive grouping matching (AGM) strategy dynamically adjusts image groups based on real-time data, optimizing the computational speed and enhancing measurement flexibility. These strategies provide crucial support for the application of 3D-DIC in the monitoring aerostat envelope strain, especially in managing significant or uneven deformations.

Revealing crack initiation and extension of brittle rock via time-lapse acoustic attenuation: Insights from active ultrasonic experiments

Predicting brittle rock failure is essential for rock mechanics research and early disaster alerts in deep rock engineering. Successful prediction relies on identifying crack initiation and extension. However, currently, there is no universally accepted method for accurately identifying and tracking crack evolution to predict brittle failure of the surrounding rock. In this study, the propagation parameters of transmission ultrasonic waves were used to characterize crack behaviors at the laboratory scale. The differential acoustic attenuation term was proposed by modifying the spectral ratio method to evaluate the dissipation capability of the corresponding medium to the transmitted ultrasonic waves. In addition to acoustic attenuation, velocity and amplitude attenuation were introduced to characterize the damage evolution during brittle failure. The indicators were analyzed by conducting a uniaxial compression test on a sandstone sample with a single flaw. During compression, active ultrasonic surveys were conducted to attain the propagation parameters. The two-dimensional digital image correlation method was simultaneously applied to capture the full-field strain on the front surface of the sample. The shear and tensile strain profiles indicated that the shear damage initially accumulated at the tips of the flaw, causing a reduction in the velocity and amplitude of the transmitted ultrasonic waves. The acoustic attenuation indicator in the flaw-tip regions increased before the shear strain began. As the damage developed further, the acoustic attenuation increased overall, with numerous local fluctuations in the flaw-tip regions due to stress rearrangement and damage development, while the velocity and amplitude remained consistently low. In contrast, intact regions that were far away from the tips of the flaw exhibited different characteristics. The velocity and amplitude variations were more staged, while the acoustic attenuation changed gradually. The results indicated that the variations in the propagation parameters exhibited specific patterns before the damage was initiated, offering a potential for predicting crack initiation. The variations in the acoustic attenuation, velocity, and amplitude attenuation allow for tracking the crack propagation.

A novel experimental technique to determine the tack development during automated placement of uncured thermoset carbon/epoxy tows

In-situ bond strength toughness (IBST), commonly referred to as tow tackiness, is a first order property affecting the adherence quality and defect formation (e.g., wrinkles, folds) during automated tow placement (ATP) processing of uncured thermoset polymer matrix composite (PMC) tows. Tow-tow IBST develops over characteristic millisecond timescales (tc≤50ms) due to the rapid tow placement velocity of ∼1 m/s. In this paper, an experimental technique is presented to determine millisecond timescale tow-tow mode-I IBST in terms of fracture mechanics-based traction-separation relationship. The test specimen consists of two tows bonded to rigid platens with a pre-crack between them to induce crack initiation. Experiments are performed using IM7-G/8552 carbon/epoxy prepregs for both long and short timescales at a constant debonding rate of 5 mm/s, contact pressure of 0.23 MPa, and bonding temperature of 40 °C. In addition, high-speed two-dimensional digital image correlation (2D-DIC) is used to image and measure the tow-tow interfacial deformation during debonding. The peak traction and apparent energy release rate are found to significantly decrease (about 60 %) at the short millisecond timescale contact hold times compared to longer (i.e., second) timescale.

Experimental Study of Strain Measurement using 2D Digital Image Correlation on Fixation Plate and Calcaneus Bone Fracture

The plate and screw internal fixation technique is widely used for the treatment of fractured bones. Determining the mechanical behavior of bone plates under load conditions remains challenging, as it is anisotropic, non-linear, and inhomogeneous. Bone strain is frequently measured using strain gages, but they can only measure the strain at a single point. The optical method known as digital image correlation (DIC) provides the displacement and consequently the strain over the entire region of interest on the bone surface. The objective of this study was to investigate the strain changes in the surface of a calcaneus plate fixation during load compression using the digital image correlation method. In this study, a two-dimensional digital image correlation (2D-DIC) and strain gauges-based experimental methodology for calculating calcaneal plate strains is presented. A 500 N static load was applied to a synthetic calcaneus that was both plate-covered and intact. A Sanders type II fractured calcaneus was stabilized with stainless steel (316L) plate. At the same locations on the calcaneus plate, displacement and strains were recorded. According to the results of the DIC method, the maximum strain values along the x, y, and shear directions were 0.008, 0.010, and 0.012 mm, respectively, while the measurement result for the strain gauge was 0.0015 mm. The experimental result had a slightly lower strain gauge than the DIC method’s output. The results of the experiment and the DIC were thought to be in good agreement.

Multi-thread two-dimensional digital image correlation for full-field displacement and rotation angle measurement

Multi-thread two-dimensional digital image correlation for full-field displacement and rotation angle measurement

Global and Local Deformation Analysis of Mg-SiC Nanocomposites: Digital Image Correlation (DIC) and Representative Volume Element (RVE) Techniques

Improving the ductile deformation behavior of Mg-SiC nanocomposites without compromising strength is critical to enhancing their mechanical properties. Mg-SiC nanocomposites are produced through mechanical milling, cold isostatic pressing, sintering, and hot extrusion processes. This study investigates the uniaxial stress–strain response and deformation behavior of the Mg-SiC nanocomposite compared to pure Mg samples with and without the milling process. The deformation behavior was investigated by two-dimensional (2D) digital image correlation (DIC) at two macroscopic and microscopic scales, employing light micrographs and in situ loading samples, respectively, in the scanning electron microscope. Compared to the pure Mg samples, the mechanical test results demonstrated a significant improvement in strength (80 MPa) and fracture strain (23.5%) of the Mg-SiC nanocomposite. The three-dimensional (3D) representative volume element (RVE) model revealed the particle dispersion effect on the mechanical properties of the nanocomposite. The RVE results demonstrate ductile deformation behavior in the sample with homogenous dispersion of SiC particles compared with the heterogeneous dispersion of SiC particles in Mg-SiC nanocomposite. The results demonstrated a good agreement between DIC and RVE predictions for Mg-SiC nanocomposites across macro- and microscales.

Performance of delaminated CFRP composite under pure bending using two-dimensional digital image correlation

ABSTRACT Delamination is an interlaminar damage which may be pre-existing in multilayered fibre reinforced polymer composite parts due to manufacturing imperfections or generated by out of plane loading during service life. Its existence leads to significant reduction of structural integrity, strength and stiffness of the material. Hence, to study the effect of induced delamination on the performance of a composite under pure bending, a circular polytetrafluoroethylene (PTFE) film was introduced between the second and third plies of a sixteen layered carbon/epoxy laminate during fabrication. The bending tests were conducted in displacement control mode. The displacements and strains were measured through the thickness of the composite using two-dimensional digital image correlation. The damage modes were captured using CCD camera and the causes were explained. The flexural strength and modulus of composite were 329.74 MPa and 104.58 GPa respectively. The delaminated composite behaved as a single member without any crack within the elastic range. The existence of PTFE drastically reduced flexural stiffness of composite beyond the stress at 75.81% of its ultimate strength. The ± 45º plies offered low mechanical performance against the induced off axis loading. Hence, in these plies fibre breakage was higher. In the portion of PTFE, the strains were higher due to larger outward displacements produced by local buckling.

Crack Detection of Asphalt Concrete Using Combined Fracture Mechanics and Digital Image Correlation

Cracking is a common failure mode in asphalt concrete (AC) pavements. Many tests have been developed to characterize the fracture behavior of AC. Accurate crack detection during testing is crucial to describe AC fracture behavior. This paper proposes a framework to detect surface cracks in AC specimens using two-dimensional digital image correlation (DIC). Two significant drawbacks in previous research in this field were addressed. First, a multiseed incremental reliability-guided DIC was proposed to solve the decorrelation issue due to large deformation and discontinuities. The method was validated using synthetic deformed images. A correctly implemented analysis could accurately measure strains up to 450%, even with significant discontinuities (cracks) present in the deformed image. Second, a robust method was developed to detect cracks based on displacement fields. The proposed method uses critical crack tip opening displacement (δc) to define the onset of cleavage fracture. The proposed method relies on well-developed fracture mechanics theory. The proposed threshold δc has a physical meaning and can be easily determined from DIC measurement. The method was validated using an extended finite-element model. The framework was implemented to measure the crack-propagation rate while conducting the Illinois-flexibility index test on two AC mixes. The calculated rates could distinguish mixes based on their cracking potential. The proposed framework could be applied to characterize AC cracking phenomenon, evaluate its fracture properties, assess asphalt mixture testing protocols, and develop theoretical models.

Tensile performance of aluminum alloy roof panel-member clamp connection

In aluminum alloy reticulated shell structures with gusset joints, the roof panel is effectively connected with the structural members through a clamp connection, enabling the roof panels to participate in the force resisting system. The performance of the connection between the roof panels and the members is the foundation for studying the collaborative action of the roof panels and the members. Relevant studies have shown that when the roof panels participate in the force resisting system, there is mainly a tensile force in the roof panels perpendicular to the axis of the members. Therefore, this paper experimentally studied the mechanical performance of the clamp connection between the roof panels and the members under the tensile force in the roof panels, and employed two-dimensional plane digital image correlation (DIC) to measure the deformation of the specimen section during the entire loading process, thus exploring the force transmission mechanism of the connection. The tensile capacity of the connection can be divided into four stages: roof panel sliding, roof panel tension–shear, lower limb bending of the strip, and roof panel bending stages. The mechanical index of the connection is primarily determined by the tension–shear stage of the roof panel, and the ultimate strength and stiffness can be determined as the peak load and linear segment stiffness of the connection at this stage. The refined finite element model was further established, and it was verified that the model could efficiently simulate the mechanical properties of the connection through comparison with the test. Parametric analysis shows that with an increase in the friction coefficient between materials, roof panel thickness, angle between roof panel and flange, and bolt length adjustment, the restraint on the roof panel increases, and the failure mode of the clamp connection changes. Based on the parametric analysis, the recommended values of the friction coefficient between the materials, minimum panel thickness, minimum bolt screw depth, and maximum bolt spacing are provided. Finally, combined with the theoretical analysis, calculation formulas for the tensile strength and stiffness of the connection were proposed. A comparison with the test results indicates that the prediction accuracy of the formula proposed in this study is good.

Projectile Impact on Plain and Reinforced Concrete Slabs

Reinforced concrete is a material frequently used in protective structures and infrastructure exposed to extreme loading. In this study, the ballistic perforation resistance of 100 mm thick plain and reinforced concrete slabs impacted by 20 mm ogive-nose steel projectiles was investigated both experimentally and numerically. Two different reinforcement configurations were used to investigate the effect of rebar diameter and spacing. Concrete with nominal unconfined compressive strength of 75 MPa was used to cast material test specimens and slabs. Ballistic impact tests were performed in a compressed gas gun facility. The mechanical properties of the concrete were found using standardised tests and two-dimensional digital image correlation, and the constitutive relation was described by a modified version of the Holmquist-Johnson-Cook model. Finite element models in LS-DYNA reproduced the projectile residual velocity in good agreement with the experimental results. The primary objective of the study was thus to validate a rather simple constitutive relation intended for large scale numerical simulations of concrete structures exposed to ballistic impact loading, while a secondary objective was to investigate the effect of reinforcement on the ballistic perforation resistance of concrete slabs both experimentally and numerically since the literature is somewhat inconsistent on this matter.

Full-Field Vibration Measurements by Using High-Speed Two-Dimensional Digital Image Correlation

This work developed a method that uses a single monochrome high-speed camera without sacrificing the spatial resolution to measure both in-plane and out-of-plane full-field vibrations. By using the high-speed camera and a two-dimensional digital image correlation (2D-DIC) algorithm, the method first extracts the out-of-plane displacement field from the measured virtual in-plane strains. Then it retrieves the in-plane displacement field after eliminating the out-of-plane motion-induced virtual component. For validation, in-plane and out-of-plane translation tests and single-frequency vibration experiments were carried out. The measurement results show good agreement with the reference values, indicating the effectiveness of the proposed high-speed 2D-DIC (HS-2D-DIC). Further, the natural frequencies and mode shapes of a rectangular cantilever panel were also measured successfully, exhibiting the method’s effectiveness in practical applications. Since the HS-2D-DIC requires only a single monochrome camera, no complex optical setup, and no complicated calibration process, the method can be developed as a competitive tool for full-field vibration characterizations.

Finite Element Analysis of Split Sleeve Cold Expansion Process on Multiple Hole Aluminum Alloy.

Multiple cold expansion holes are widely used in connection areas of aircraft structures, in order to achieve uniform load transfer of the skin or connection parts. Split sleeve cold expansion (SSCE) is widely used to enhance the fatigue life of fastener holes by applying compressive residual stresses around the holes. In this study, the finite element method (FEM) was used to research the distribution and variation of residual stresses along the hole edges of 7075AA single-hole and multi-hole cold expansion (CE) specimens. Full-field strain measurements of single-hole and multi-hole specimens were performed using two-dimensional digital image correlation (DIC), and the residual stress and strain at the hole edge of the specimens measured by FEM and DIC were compared. FEM results shows that the maximum circumferential and radial residual stresses of three-hole specimens with three-hole spacing are increased by 5.37% and 31.53% compared with single-hole specimens. The maximum circumferential residual stress of three-hole specimens with four-hole spacing increases by 7.25% compared with a single hole, but the radial residual stress decreases by 12.98%. In addition, for three-hole specimens with hole spacing three times the hole diameter, the strengthening effect of SSCE in the order of middle hole, then left hole, and, finally, right hole is better than that of SSCE in the order of left to right hole. FEM and DIC full-field strain results are basically consistent.

Digital Image Correlation Characterization of Deformation Behavior and Cracking of Porous Segmented Alumina under Uniaxial Compression

In this study, the two-dimensional digital image correlation (DIC) technique has been applied to sequences of images taken from the surfaces of porous, segmented alumina samples during uniaxial compression tests. The sintered alumina was structurally composed of polycrystalline alumina grains with interior ~3–5-μm pores, a network of discontinuities that subdivided the sample into ~230 μm segments, and ~110 μm pores located at the discontinuity network nodes. Bimodal pore structure and the segment boundaries were the results of the evaporation and the outgassing of the paraffin and ultra-high-molecular-weight polyethylene admixed with alumina powder via slip casting. Only partial bonding bridges between the segments were formed during a low-temperature sintering at 1300 °C for 1 h. A special technological approach made it possible to change the strength of the partial bonding bridges between the segments, which significantly affected the deformation behavior ceramics during compression. The subpixel accuracy of the DIC results was achieved using an interpolation scheme for the identification functional. The vector fields obtained in the experiment made it possible to characterize the processes of deformation and destruction of a porous, segmented alumina using the strain localization in situ maps, cardinal plastic shear, and circulation of vector fields. The use of these characteristics made it possible to reveal new details in the mechanisms of deformation and destruction of segmented ceramics. The localizations of damage were identified and related to the characteristic structural heterogeneities of the tested porous segmented ceramics.

Mechanical Performance Study of Pipe-Liner Composite Structure Based on the Digital Image Correlation Method

Repair of reinforced concrete drainage pipes is often performed using polyethylene (PE) lining. To investigate the bearing capacity characteristics of a repaired composite structure, this study conducted experiments on three groups, namely, PE pipes, reinforced concrete pipes, and composite-reinforced concrete PE pipes (count = 9). The cracking and ultimate loads of the three groups under plate loads were evaluated and analyzed. A displacement monitoring method based on two-dimensional digital image correlation (2-D-DIC) and three-dimensional digital image correlation (3-D-DIC) techniques is proposed, and the quantitative laws of strain distribution at the end face of the specimen are obtained. Furthermore, the damage mechanism of the reinforced concrete pipe and the interface spalling damage mechanism of the composite-reinforced concrete PE under load were investigated to reveal the effect of PE lining improvement on the bearing capacity of the reinforced concrete pipe. The maximum deviation of the specimen vertical displacement results obtained using the new noncontact 3-D-DIC method is 2.6% compared with that of the traditional physical sensor method, and this method can successfully capture the deformation and strain data of the target specimen. Therefore, the noncontact DIC method has a broad application prospect in municipal pipeline early warning monitoring systems.

Characterizing the Tensile Behavior of Double Wire-Feed Electron Beam Additive Manufactured “Copper–Steel” Using Digital Image Correlation

The paper presents the results of the evaluation of the mechanical characteristics of samples of multi-metal “copper-steel” structures fabricated by additive double wire electron beam method. The global and local mechanical characteristics were evaluated using uniaxial tensile tests and full-field two-dimensional digital image correlation (DIC) method. DIC revealed the peculiarities of the fracture stages: at the first stage (0.02<ε≤0.08) the formation of V-shaped shear lines occurs; at the second stage (0.08<ε≤0.15) transverse shear lines lead to the formation of a block structure; at the third stage (0.15<ε≤0.21) the plasticity resource ends in the central part of the two necks cracks are formed, and the main crack is the cause of the fracture of the joint. It is found that shear lines are formed first in copper and then propagate to steel. Electron microscopy proves that uniformly distributed iron particles could always be found in the “Fe-Cu” and “Cu-Fe” interfaces. Additionally, the evolution of average strain rates and standard deviations were measured (calculated) in the regions of necks in copper and steel regions. New shear approach shows that the most of angles for parallel shears components are ±45°, rupture angles are about 0°, and combined account of these two types of shears provides us additional discrete angles.

Study on dynamic shear deformation behaviors and test methodology of sawtooth-shaped rock joints under impact load

Shear slip of rock joints under dynamic disturbance is a major factor causing disasters such as landslides and cavern instability. To explore the dynamic shear deformation behaviors of rock joints, an impact-induced direct shear test applicable for jointed rock specimen is conducted. A biaxial Hopkinson pressure bar (BHPB) equipment is employed, which can simulate intensive disturbances encountered during engineering construction in rock masses. Tangential and normal dynamic stresses on rock joint can be measured simultaneously through stress wave acquisition. Dynamic displacements are obtained based on a two-dimensional digital image correlation (2D-DIC) technique. Firstly, the feasibility of the experimental method is verified by trial tests on metallic specimens. The measurement technique for obtaining the dynamic stress and displacement of joint is proposed. Then, a series of dynamic shear experiments are carried out on jointed and intact granite specimens, respectively. The dynamic shear deformation of the specimens is obtained, and the effects and mechanisms of undulating angle and initial normal stress on the dynamic shear deformation are discussed. The test results show that, due to the variable input force and normal stress, the shear rate varies. The dynamic slip of the joint can be divided into three sub-stages: slip initiation, secondary stress accumulation, and post-slip. The shearing process of intact rock includes two stages: shear stress accumulation and post-peak damage. Deformation rebounding is also observed in the jointed and intact granite specimens. The maximum shear displacement is negatively correlated with the normal stress and the undulating angle. Based on the distribution and evolution of displacement field in rock specimen, the effects of undulating angle and normal stress on the shear strength and the maximum shear displacement are revealed. Finally, the damage modes of rock joint under different conditions are discussed.

Effect of support conditions on the performance of B70 sleeper overlying reinforced ballast and sub-ballast layer: Experimental and analytical investigation

It is well known that the moment distribution on the sleepers is very sensitive to deformations of the ballast caused by cyclic axle load over time and other environmental factors. To limit the non-uniform deformations of the ballast, reinforcement of the ballast or sub-ballast with geosynthetics can be a cost-effective solution. In this study, B70 prestressed concrete sleepers were tested under static loading conditions in a special test box developed to simulate the in-situ condition of a railroad track in the laboratory. The main test parameters were the degree of settlement of the ballast and the type of geosynthetics used. Four different settlement levels and two reinforcement types (sub ballast with geonet and ballast with geocells) were considered. The deflections and surface strain maps of the concrete sleepers were measured using the two-dimensional digital image correlation (2D-DIC) method. Following the experimental part, an analytical study was also carried out to investigate the ballast pressure distribution and consequently the moment distribution on the concrete sleeper. Based on the vertical deflection shape of the sleeper, a new algorithm was developed to derive the ballast pressure distribution under the sleeper by back calculation. The experimental results showed that using geocells to reinforce the ballast layer increased the stiffness of the ballast against the vertical deflection of the sleeper. From the analytical results, the magnitude of the negative bending moment increased significantly with increasing settlement level.