Digital Image Correlation Method Research Articles

In-plane biaxial low-cycle dwell fatigue behavior of CP-Ti based on DIC method

This study investigates the biaxial dwell fatigue behavior of commercial pure titanium (CP-Ti) under different load ratios and dwell times. The evolution of mean strain, ratcheting strain, and creep strain was systematically analyzed. The findings reveal that with the decrease of load ratio or the increase of dwell time, both ratcheting and creep strain exhibit an upward trend. The interaction between ratcheting strain and creep strain promotes crack initiation and propagation. The statistical analysis of crack paths by the digital image correlation (DIC) method indicates that crack initiation occurs between 0.7Nf and 0.8Nf, and fatigue failure is governed by the maximum principal strain. Moreover, the electron backscatter diffraction (EBSD) analysis results indicate that as the load ratio increases, the activity of prismatic slip systems decreases, while the activities of other slip systems and twinning modes increases. To enhance predictive accuracy, a modified SWT model by incorporating time-dependent creep damage and anisotropy is proposed for life prediction under biaxial dwell fatigue conditions.

Poisson function and volume ratio of soft anisotropic materials under large deformations

Accurate transverse deformation measurements are required for the estimation of the Poisson function and volume ratio. In this study, pure silicone and soft composite specimens were subjected to uniaxial tension, and the digital image correlation method was used to measure longitudinal and in- and out-of-plane transverse stretches. To minimize the effects of measurement errors on parameter estimation, the measured transverse stretches were defined in terms of the longitudinal stretch using a new formulation based on Poisson's ratios and two stretch-dependent parameters. From this formulation, Poisson functions and volume ratio for soft materials under large deformations were obtained. The results showed that pure silicone can be considered isotropic and nearly incompressible under large deformations, as expected. In contrast, Poisson's ratio of silicone reinforced with extensible fabric can exceed classical bounds, including negative value (auxetic behavior). The incompressibility assumption can be employed for describing the stress-stretch curve of pure silicone, while volume ratios are required for soft composites. Data of human skin, aortic wall, and annulus fibrosus from the literature were selected and analyzed. Except for the aortic wall, which can be considered nearly incompressible, the studied soft tissues must be regarded as compressible. All tissues presented anisotropic behavior.

Effects of moisture content on the behaviour of Scots pine heartwood and sapwood under impact

The material properties of sapwood and heartwood vary within various wood species and even they can show significant differences within a single tree. Scots pine (Pinus sylvestris L.), a species that plays a crucial role in timber production for joinery and building construction applications, is among those that show a notable distinction between its heartwood and sapwood. To examine the influence of moisture content (MC) on the impact behaviour of the sapwood and heartwood of pine, we tested specimens with two distinct moisture levels: a low moisture content (LMC) group with 12% MC and a high moisture content (HMC) group with 45% MC. In our study, we investigated deflection, normal strain and force development of the specimens during the short period of an impact, and also calculated the impact bending strength (IBS) of samples, using an impact testing machine equipped with a high-speed camera and digital image correlation method. Our results indicate that the differences between sapwood and heartwood at LMC were insignificant in the case of maximum deflection and normal strain, thus there is no need for differentiation; however, these differences became more pronounced, and non-negligible, with an increase in MC. We also evaluated the IBS of both heartwood and sapwood and found that, at LMC, heartwood had greater impact bending strength than sapwood, making it a preferable choice as a material subjected to impact loadings. Conversely, at HMC, both heartwood and sapwood would be equally strong against impacts, indicating that pine green wood shows no sensitivity to the ratio of sapwood to heartwood in the tree.

Correlation between deflection and crack propagation in reinforced concrete beams

An experimental study was conducted to investigate the correlation between crack propagation and deflection of reinforced concrete beams. A four-point loading test was performed on specimens having varying reinforcement ratios and concrete cover thicknesses. Digital image correlation (DIC) and traditional measuring instruments were used to evaluate and compare the deflection and crack width results. The DIC method accurately measured the deflection and single crack width progression. However, the total crack width was overestimated owing to elastic deformation. The experimental data revealed a linear relationship between the sum of the crack widths and the deflection of beams. Curvature idealization was utilized to propose a model that estimates the deflection based on the crack width measurements obtained by using the DIC method for serviceability evaluation. The results of the proposed model were comparable with the experimental results; the maximum error between them was 8.35%. The proposed model simplified the serviceability evaluation for deflection.

Determination of fracture parameters for propellant/liner interface affect zone through measuring J-integral

The debonding of propellant/liner interface is one of the primary failure modes that impact the structural integrity of solid rocket motors. In order to study the mechanical properties of propellant/liner interface, tensile tests on rectangular specimens with various prefabricated crack lengths were conducted at different temperatures and loading rates, as well as uniaxial tensile tests on the propellant. Utilizing the digital image correlation(DIC) method, the strain field including the whole rectangular specimen and the propellant part was analyzed, and the change law of the crack length of the rectangular specimen with time was extracted by programming batch processing at different temperatures. Meanwhile, the fracture toughness of the NEPE propellant/liner interface under different loading conditions was obtained using J-integral theory. The results show that the area near the propellant/liner interface forms an interface affect zone (IAZ), where cracks grow steadily in a continuous and smooth way without significant fluctuations. The effect of temperature and loading rate on J-integral is interactive. The lower the temperature, the greater the fracture toughness of the interface and the lower the crack propagation rate. Finally, based on the bi-linear cohesive model, the numerical simulation of the interface affect zone (IAZ) in the rectangular specimen is carried out, which shows that the J-integral can effectively represent the energy required for the interface affect zone failure. A method to determine the fracture parameters for the propellant/liner interface affect zone is proposed.

Multiscale homogenization of thermo-mechanical viscoelastic response of 3D orthogonal composites with time-dependent CTEs

Viscoelastic behavior of the matrix affects the thermomechanical response of polymer composites. This study investigated the structural effects of the time-dependent thermal expansion response of 3D orthogonal woven composite (3DOWC) by combining experiments and finite element modeling (FEM), incorporating the viscoelasticity of its constituents. The deformation characteristics at three orthogonal cross-sections, in the presence of structure-related strain fields at various temperatures, were investigated using digital image correlation method. Findings reveal heterogeneity and localization of the thermal strain, characterized by periodic stripes with alternating high strain regions on the resin and low strain regions on the yarn. This localization intensifies at higher temperatures, but diminishes with increased distance from the interface. Furthermore, the composites exhibit time-dependent thermal expansion behavior derived from the time-independent thermal expansion of its constituents, which cannot be captured in classic elastic material model. FEM reveals that thermal stresses are concentrated at the interfaces between constituents with significant differences in their coefficients of thermal expansion (CTE), particularly at the interface between the resins and axial yarns. The maximum thermal stress occurs in the binder yarns over the considered temperature range due to their low volume content, which should be considered in practical applications.

Stress–strain behavior of Cu on the AMB‐Si3N4 substrate undergoing thermal cycles via in situ strain measurement

AbstractThe active metal brazing of a Si3N4 substrate with Cu has been evaluated for excellent reliability, demonstrating durability up to 1000 cycles in a cycling test ranging from −40°C to 250°C. While the finite element method (FEM) is commonly used for predicting thermal stress–strain in cyclic tests, experimental data on the measurement of thermal stress–strain on metallized substrates have remained limited. In this study, a digital image correlation (DIC) method was employed for the in situ measurement of thermal strain on a fully Cu‐coated Si3N4 substrate (AMB‐SN substrate) in various consecutive thermal cycles, ranging from 1–2 to 199–200. The thermal strains exhibited hysteresis curves that expanded slightly with cycles. By incorporating the coefficients of thermal expansion (CTE) of plain Cu and Si3N4, both the thermal stress and strain of Si3N4 and Cu on the AMB‐SN substrate were computed. The stress–strain curves of Cu revealed that the yield stress of Cu increased with the number of cycles, attributed to the cyclic hardening of the Cu layer. The Cu stress–strain curve calculated in this work showed a good agreement with the previous results obtained from compression/tension test of Cu at room temperature, which indicates the stress–strain curve of Cu on the composite was not sensitive to the temperature during the thermal cycle.

Dynamic impact mechanical properties of red sandstone based on digital image correlation method

ABSTRACT In the process of underground blasting excavation, the safety and stability of rock mass are significantly influenced by dynamic load. To study the full-field strain and displacement behaviour of rock materials subjected to dynamic compressive force, the impact tests were done on red sandstone specimens using an electromagnetically driven Hopkinson pressure bar. At the same time, the data were collected by a high-speed camera, and non-contact measurements of the red sandstone were carried out with the digital image correlation method. The resulting images were screened and analysed, based on which the stress-strain curve, full-field strain cloud diagram and displacement cloud diagram were obtained. Results show that the dynamic peak stress increases with the increase of impact velocity. During the impact process, substantial internal uneven deformation occurs in the red sandstone close to the transmission bar’s end face. Under the impact load, the cracks initiate from the upper and lower sides near the transmission bar’s end face, followed by formation of cracks in the middle part. These cracks expand throughout the rock to form through-cracks, eventually resulting in complete failure. For rock materials, the evolution of their strain field during the failure process can reflect the initiation and propagation of their internal cracks. Therefore, the use of digital image correlation method proves effective for studying the deformation and failure mechanisms of rock. The conclusions obtained in this study provide a valuable reference for the macro-meso deformation and failure mechanisms of geotechnical materials.

Real-time motion-induced error reduction for phase-shifting profilometry with projection points tracking method

Fringe projection profilometry is a significant method for three-dimensional measurement due to its non-contact and high accuracy. However, the motion-induced error will lead to the loss of measurement accuracy in dynamic scenes due to the disruption of the phase measurement process. In this paper, we introduce a novel motion error model that considers object motion causes the misalignment of the projection points on the camera, and propose the projection points tracking method to reduce the motion-induced error. First, the speckle pattern is added to projection sequences, after which the projection point displacements are determined using the digital image correlation (DIC) method between the adjacent speckle patterns. Finally, the fringe patterns are corrected using the image remapping method to calculate the 3D shape. Quantitative analysis and dynamic measurement experiments verify the feasibility of the proposed method. Different from 3D-DIC, we use one camera-projector system to realize high-accuracy measurement in dynamic scenes.

A semi-empirical life-prediction model for multiaxial ratchetting-fatigue interaction of SUS301L stainless steel tubular welded joint

Based on the strain measured in the SUS301L stainless steel tubular welded joint in the asymmetrically stress-controlled multiaxial low-cycle fatigue tests by using the digital image correlation (DIC) method, the dependence of fatigue life on the loading level and loading path is well explained by discussing the multiaxial ratchetting-fatigue interaction in the fatigue failure zone of welded joint. To reflect the contribution of mean stress to the non-proportionality of loading path, a new non-proportionality factor is defined. And then the path-dependent damage parameters of pure fatigue damage and additional ratchetting one are proposed in a strain form and validated. Finally, a multiaxial life-prediction model is constructed through addressing the concurrent efforts of additional ratchetting damage and pure fatigue one to predict the multiaxial fatigue life of welded joint. The results demonstrate that the predicted multiaxial fatigue lives are well expected and all life data fall in the twice error bands regarding to the experimental ones.

Study on crack propagation characteristics of rocks with different lateral pressure based on joint monitoring of DIC and AE

During the process of rock failure, the characteristics of crack propagation affect the fracture characteristics and macroscopic mechanical behavior of rocks, indirectly affecting the safety and stability of rock engineering. In order to study the evolution characteristics of cracks during rock failure under different lateral pressure, based on an improved digital image correlation (DIC) and acoustic emission (AE) signal recognition method, a visual biaxial servo loading device was developed to conduct biaxial compression tests on mudstone with prefabricated cracks of the same inclination angle. The research results indicate that the stages of crack propagation include microcracks propagation, crack tip formation, stable macroscopic cracks propagation, and unstable macroscopic cracks propagation. As the lateral pressure increased, the initiation frequency of cracks decreased, the quantity of propagation decreased, and the propagation path shortened, indirectly increasing the bearing strength of rocks. The initiation stress, peak stress, and elastic modulus of pre-cracked rocks with lateral pressure ≤ 2 MPa were lower than those of pre-cracked rocks with lateral pressure > 3 MPa, with the minimum reduction amplitude of 14.1%, 21.2%, and 12.6%, respectively. As the lateral pressure decreased, the dispersion of the AE main frequency distribution increased and accelerated its downward expansion. The surface temperature curves of rocks were prone to fluctuations and rapid upward evolution characteristics corresponding to crack tip formation and crack propagation, respectively. The research results provide theoretical and engineering references for the mining of weak coal seams.

Crack propagation process and lifetime prediction method of concrete-rock interface under constant loading

This study investigated the crack propagation processes and established the lifetime prediction methods of the concrete-rock interfaces under constant loading. Firstly, the constant loading tests were performed under three-point bending loading on the composite concrete-rock beams with three kinds of interfaces, i.e. natural, 4 × 4, and 7 × 7 interfaces, and under three constant load levels, i.e. initial cracking load, 80 % and 97 % of the peak loads. Then, the clip gauge method and digital image correlation method were employed to detect the crack lengths under constant loading. Finally, the effects of constant load levels on fracture properties of the concrete-rock interfaces were analyzed and the crack propagation processes under constant loading were discussed. The results indicated that the clip gauge method was an available and accurate method to detect the crack lengths of the concrete-rock interfaces under constant loading. Crack propagation processes and crack mouth opening processes under constant loading exhibited the three-stage feature, i.e. deceleration stage, uniform stage and acceleration stage. In addition, the logarithms of the constant loading lifetimes showed an approximate linear relationship with the logarithms of the crack mouth uniform opening rates, and a quantitative relationship was established by using the linear regression analysis to establish the lifetime prediction methods. This study proposed a practical method to predict the service life of concrete structure-bedrock interface. When the crack mouth uniform opening stage was recognized, the lifetime of concrete structure-bedrock interface in service can be predicted by substituting the crack mouth uniform opening rate into the pre-established prediction model. This study can provide the theoretical support for the safety assessment of the concrete structure-bedrock interface in service.

Finite-discrete element modelling of fracture development in bimrocks

Block-in-matrix rocks (bimrocks) are a complex geological feature composed of hard blocks in a weak matrix bonded together via a weak interface. The failure mechanism and deformation behavior of the bimrocks is quite complex due to the simultaneous existence of blocks with different shapes, strengths and proportions in a matrix. Therefore, it is of great importance to characterize the bimrocks under different situations. This study employs the combined finite-discrete element method (FDEM) to characterize the failure pattern and load carrying capacity of the bimrocks considering changes in various parameters, including block size, volumetric block proportion (VBP), the strength ratio of block-matrix interface to matrix, and the shape of the blocks (angular, circular, and elliptical shapes). First, the robustness and feasibility of the FDEM in modeling fracture development is verified against an analytical solution as well as a series of experimental tests accompanied with the digital image correlation (DIC) method. Then, the verified model is used to model disc specimens with various block shapes and properties. The results indicate the remarkable influence of VBP, block size and block-matrix interface characteristics on the bimrock mechanical behavior under the same VBP. The results are assessed based on macroscopic observations to investigate the influence of determinative physical properties of bimrocks. The results are also evaluated based on the mode of failure, mode I, II, or mixed mode, to qualitatively elucidate the influence of key factors on the formation of different cracks in different places, including the interface of matrix-blocks or intergrain or intragrain. The results show that since the loading is a quasi-static scheme, all the cracks are intergrain and no intragrain crack is observed for any VBP. The simulations outcomes shows that the block sizes determine the failure pattern, failure mode, and load–displacement behavior, where the larger blocks exhibiting the lowest load carrying capacity due to experiencing more interface breakage in shear. It is found that the VBP is a crucial parameter in the deformation of the bimrocks, showing that the strength of the bimrocks significantly increases with a decrease in the VBPs and the ones with greater VBPs experience more breakage and thus less resistance to loading. The block-interface strength is found to be the most critical factor in the deformation of the bimrocks (regardless of the value of the VBP). Finally, the effect of block shape is shown to have a considerable impact on bimrocks with the higher VBPs (more than 70%).

Failure analysis of CFRP-aluminum electromagnetic self-pierce riveting-bonded joints subjected to highspeed loading

The mechanical performance of dissimilar material riveted joints is important for passenger safety during a crash situation. Hence, in this paper, the mechanical properties of CFRP-aluminum electromagnetic self-pierce riveting-bonded (E-SPR-bonded) joints under quasi-static and dynamic loading was investigated. First, the hybrid joints were manufactured under five discharge energies to explore the best process parameter, then, the effect of shearing speed on the mechanical properties were studied. Digital image correlation (DIC) method was carried out to further understand the surface strain distribution. Results indicated that the evolution of shear tests could be divided into two stages, including two peak loads, named peak A and peak B, respectively. The hybrid joint at 7.7 kJ had the best peak load during static load. Under the dynamic loading condition, the peak load increased with ascending load speed, especially the peak B, under 4, 8 and 12 m/s, were 189.0 %, 205.5 %, and 215 % higher than that of 2 mm/min, respectively. The hardening effect of sheets was increased under high shear speeds, which dominated the fracture behaviors of the bonding layer. The hybrid joints failure behavior was the CFRP fracture, and the rivet remained in the aluminum sheets. The results could provide theoretical guidance for the structural design of automobiles and promote its engineering application.

Experimental study of the fatigue failure behavior of aluminum alloy 2024-T351 under multiaxial loading

Multiaxial fatigue has been drawing much attention because it is more applicable to engineering practice. In this paper, an experimental study of aluminum alloy (AA) 2024-T351 was carried out under multiaxial loading with an aim to assess the damage evolution process and failure mechanism under in-phase and out-of-phase loadings. Firstly, fatigue life and stress response under multiaxial cyclic loads were obtained, and it found that although there is non-proportional hardening, the fatigue life subjected to proportional loading is significantly shorter than that of under a nonproportional loading, which was tried to be explained by the ductility of the material. Secondly, a detailed analysis of the damage evolution process based on the degradation of the elastic modulus and the fatigue failure process based on the digital image correlation (DIC) method was provided. Next, a micro-analysis of the specimens’ fracture appearance was conducted to obtain the fracture characteristics and found that AA2024-T351 presents a dominant shear fracture mode under proportional loading and a mixed mode of tensile and shear fracture under non-proportional loading. Last but not least, The Fatemi-Socie criterion was modified by considering the material’s ductility and the interaction between normal stress and shear stress acting on the critical plane. The multiaxial life prediction results of the modified FS model for AA2024-T351 in this paper were all within the scatter bands of 3 on life.

Coded speckle target: Design, analysis and applications

In this paper, a novel encoding feature based on speckle patterns. It comprises three components: a pre-positioning ring, localization speckle area, and encoding speckle area. The pre-positioning ring identifies potential regions of interest. Shape function of the localization speckle area can be estimated by formulating the elliptical equation of the pre-positioning ring. Next, digital image correlation method is employed to achieve precise positioning and refine the shape function. Through the high-precision shape function, the encoded speckle area can be determined. Encoding technique utilizes a reference encoding scheme, in which each coded value region either same or is inverse to the reference region. Zero-mean normalized cross correlation relative to the reference region can be used to obtain a highly robust decoding value. Experimental results demonstrate that, despite challenges such as noise, defocus, variations in lighting intensity, and lighting uneven, the coded speckle target maintains high robustness and precise positioning accuracy. Moreover, the structure from motion and displacement measurement experiments further verifies its high accuracy in practical applications.

Assessment of Digital Image Correlation Effectiveness and Quality in Determination of Surface Strains of Hybrid Steel/Composite Structures.

The application of the digital image correlation (DIC) contactless method has extended the possibilities of reliable assessment of structure strain fields and deformations throughout the last years. However, certain weak points in the analyses using the DIC method still exist. The fluctuations of the results caused by different factors as well as certain deficiencies in the evaluation of DIC accuracy in applications for hybrid steel/composite structures with adhesive joints are one of them. In the proposed paper, the assessment of DIC accuracy based on the range of strain fluctuation is proposed. This relies on the use of a polynomial approximation imposed on the results obtained from the DIC method. Such a proposal has been used for a certain correction of the DIC solution and has been verified by the introduction of different error measures. The evaluation of DIC possibilities and accuracy are presented on the examples of the static tensile tests of adhesively bonded steel/composite joints with three different adhesives applied. The obtained results clearly show that in a non-disturbed area, very good agreement between approximated DIC and FEM results is achieved. The relative average errors in an area, determined by comparison of DIC and FEM strains, are below 15%. It is also observed that the use of approximated strains by polynomial function leads to a more accurate solution with respect to FEM results. It is concluded that DIC can be successfully applied for the analyses of hybrid steel/adhesive/composite samples, such as determination of strain fields, non-contact visual detection of faults of manufacturing and their development and influence on the whole structure behavior during the strength tests, including the elastic response of materials.

Study on the strength mechanism of the wooden round-end mortise-and-tenon joint using the digital image correlation method

Abstract The aim of this study was to reveal the strength mechanism of the mortise-and-tenon (M–T) joint at a deeper level. The effects of tenon fit on bending and withdrawal load resistances, and strain distributions outside and inside beech (Fagus sylvatica) wooden round-end M–T joints were experimentally investigated using mechanical testing synchronizing digital image correlation method (DICM). The results showed that (1) the tenon fit had greater significance on withdrawal properties than that of bending properties of M–T joints; (2) the bending load resistance was linearly proportional to withdrawal load resistance based on both theoretic analysis and regression methods; (3) strain distributions outside M–T joints during the loading process were not sufficient to evaluate the mechanical behaviors of the M–T joint; (4) strain distributions inside M–T joints showed that the maximum strains on top and bottom parts of the tenon were significantly greater than that of middle part, but the difference decreased with the growth of tenon fit; (5) the method of determining the optimal tenon fit of the M–T joint based on the DICM was proposed, and optimal tenon fit of beech wooden round-end M–T joint evaluated ranged from 0.4 to 0.5 mm.

Deformation and failure behavior of 2024-T42 sheet under impact loading

The deformation and failure behavior of 2024-T42 aluminum alloy sheet was investigated through a combined experimental-numerical method. Nine types of specimens were designed to cover a wide range of stress triaxialities and strain rates. Quasi-static and dynamic tests were carried out by electronic testing machine and split Hopkinson tensile bar respectively, and digital image correlation (DIC) method was introduced to measure the deformation. The phenomenological damage model GISSMO (generalized incremental stress state dependent damage model) and Johnson-Cook model were adopted to simulate all of the above tests and the load-displacement curves through numerical simulation were derived. An optimization method was developed to obtain all the model parameters by matching the load-displacement curves from simulation with those from tests by LS-OPT. Finally, drop weight tests and tensile tests of the central-hole plate were conducted. The applicability and accuracy of the damage models were verified by comparing the simulation results with the experiments, which indicates that GISSMO model predicts better the tests than the Johnson-Cook failure model.

Using unsupervised learning based convolutional neural networks to solve Digital Image Correlation

Digital Image Correlation (DIC) is a robust, non-contact method for full-field deformation measurement widely used in photomechanics. Based on subset image matching and an iterative optimization architecture, traditional DIC deformation field analysis requires manual selection of such calculation parameters as subset size, rendering it challenging to balance spatial resolution and measurement resolution in non-uniform field measurements. The requirement of manual parameter adjustment is lowered by a recently emerged supervised learning-based neural network DIC method, as it establishes a direct correlation between images and deformation fields using deep neural networks (DNN). However, its generalization and accuracy are contingent on the scope and variety of the dataset because it requires training with a large number of simulated speckle images of deformation fields with known standards. This paper proposes an unsupervised learning neural network based DIC measurement method that circumvents the need for standard deformation fields and achieves high precision analysis of deformed fields. It constructs a loss function based on the photometric consistency assumption of speckle images before and after deformation and allows the neural network model to directly extract displacement information from the gray data of the “reference image-deformed image” pair. Meanwhile, the model is enhanced accordingly by further introducing displacement continuity constraints to enhance accuracy and mitigate noise interference. Validation through both simulated and real experiments shows that the proposed approach surpasses traditional subset DIC in measuring non-uniform deformation fields with higher precision. It also offers superior flexibility and adaptability over supervised learning-based neural network DIC methods by eliminating the need for a large training dataset, allowing for immediate application in displacement field analysis.