Digital Correlation Technique Research Articles

Particle tracking-aided digital volume correlation for clay–sand soil mixtures

In this study, a novel, interdisciplinary method is introduced that merges fundamental geomechanics with computer vision to develop an advanced hybrid feature-aided digital volume correlation (DVC) technique. This technique is specifically engineered to measure and compute the full-field strain distribution in fine-grained soil mixtures. A clay–sand mixture specimen composed of quartz sand particles and kaolinite was created. Its mechanical properties and deformation behaviour were then tested using a mini-triaxial apparatus, combined with micro-focus X-ray computed tomography (μCT). The CT slices underwent image processing for denoising, segmentation of distinct phases, reconstruction of sand particles and feature extraction within the soil specimen. The proposed approach incorporated a two-step particle tracking method, which initially uses particle volume and surface area features to establish a preliminary matching list for a reference particle and then use the iterative closest point method for precise target particle matching. The soil specimen's initial displacement field was then mapped onto the DVC method's grid, and further refined through sub-voxel registration by way of a three-dimensional inverse compositional Gauss–Newton algorithm. The proposed method's effectiveness and efficiency were validated by accurately calculating the displacement and strain fields of the soil mixture sample, and comparing the results with those from a traditional DVC method. Given the soil's compositional and microstructural characteristics, these image-matching techniques can be integrated to create a versatile, efficient, and robust DVC system, suitable for a variety of soil mixture types.

Investigation of mechanical characterization and damage evolution of coral reef sand concrete using in-situ CT and digital volume correlation techniques

Coral reef sand concrete (CRSC) plays an indispensable role in island reef projects, but limited knowledge of its damage poses safety and stability concerns for coral reef sand concrete structures. Therefore, the in-situ computed tomography (CT) is employed to investigate the strength and deformation, and pore structure evolution of coral reef sand concrete under uniaxial load. Additionally, the crack propagation and three-dimensional strain field evolution under different stress states are revealed by the digital volume correlation (DVC) techniques. The results demonstrate that when the stress is below 35.79 MPa, the porosity gradually decreases as the stress increases. However, when the stress exceeds 35.79 MPa, the local porosity increases with the rising stress, and the porosity at 39.89 MPa is higher than the initial porosity. Moreover, stresses evidently concentrate at areas of local maximum porosity, resulting in the initiation and development of cracks in those regions. During the failure process, CRSC experiences splitting damage firstly caused by longitudinal cracks, followed by the extension of these cracks into oblique cracks, resulting in shear damage. Coral aggregate is the weak phase in CRSC, exhibiting transgranular fractures. Additionally, CRSC exhibits significant strain in the pores and coral aggregates, leading to crack development along the direction of the pores or coral aggregates. Therefore, this study offers a deeper understanding of the damage mechanisms of CRSC, providing theoretical support for its further research and practical application in far-sea projects.

Pressure effects on electrochemical and crack driving forces in aluminium-doped LLZO-based all-solid-state lithium metal batteries

This study employed a combination of computational models and experimental techniques to investigate the effects of stacking pressure on the mechanical deformation and electrochemical performance of All-Solid-State Li metal batteries (ASSLBs). The effects of the stacking pressure were studied in Al-doped LLZO-based electrolytes at the electrode/electrolyte interface and within the electrodes. Optical microscopy and digital imaging correlation (DIC) techniques were used to study the strain distributions induced in the Al-LLZO-based electrolyte at different stack pressures. The results show that minimal levels of stack pressure result in low strains within the electrolyte, whereas intermediate increasing pressures increase the interfacial contacts and electrochemical performance of the cells. However, higher stacking pressures result in higher induced levels of local stresses and strains, which are sufficient to cause cracking and degradation of the electrochemical performance. The implications of the results are discussed for the pressure-assisted manufacturing of all-solid-state lithium metal batteries.

Three-dimensional ranging system based on Fresnel incoherent correlation holography.

We proposed a three-dimensional (3D) ranging system based on Fresnel incoherent correlation holography (FINCH). Distinct from the displacement measurement based on coherent digital holography (DH), our system simultaneously achieves a 3D range measurement using incoherent illumination. The observation range is obtained by the holographic reconstruction, while the in-plane range is determined using the two-dimensional digital imaging correlation (2D-DIC) technique. Experimental results on the resolution target demonstrate precise 3D ranging determination and improved measurement accuracy.

Dynamic Behavior of Twisted UHMWPE Yarns

Twisting is an effective method to reduce defects during fabric weaving and improve the impact resistance of the armor material. This study examined the dynamic behavior of twisted ultra-high molecular weight polyethylene (UHMWPE) yarns. Material characterization was conducted using three-dimensional X-ray tomography and scanning electron microscopy (SEM). Dynamic mechanical experiments were carried out on the twisted yarns using a Kolsky bar, with the yarn's failure process captured in real time through high-speed imaging. Additionally, the digital imaging correlation (DIC) technique was used to calibrate the strain measured by the Kolsky bar. Post-fracture analysis was performed using SEM. The effects of twisting extent, strain rate, and gauge length on the dynamic behavior of twisted yarns were studied. It was observed that all twisted yarns underwent elongation, shrinkage, fiber breakage, untwisting, and yarn rupture. Furthermore, as the twisting degree and strain rate increased, and the gauge length decreased, the fracture area became more concentrated. The tensile strength peaked at a twisting degree of 250 t/m and decreased thereafter. Moreover, twisting was found to alter the sensitivity of the yarns' stress–strain curves and tensile properties to the strain rate and gauge length.

Energy evolution and strain localization in fractured sandstone under freeze-thaw cycling and uniaxial loading-unloading

In cold regions, combined cyclic freeze-thaw (F-T) and loading-unloading can cause rock slope instability and sliding. The macro-failure of rock masses originates from the expansion of strain localization areas driven by energy. Therefore, a deep understanding of the energy evolution and strain distribution can reveal the instability process. Along these lines, in this work, intact and intermittently jointed sandstones were subjected to a cyclic F-T environment and subsequently carried out uniaxial graded loading-unloading to obtain their mechanical parameters and X-ray computed tomography images. By investigating the energy density, it was found that the input, elastic, dissipated, and damage energies all increased with loading according to a quadratic polynomial function, with the energies obeying a linear allocation law. Meanwhile, F-T changed the energy storage capacity and energy dissipation coefficient of the rock by causing meso-damage. To measure the meso-scale strain localization characteristics, the advanced digital volume correlation technique was also used to obtain three-dimensional strain fields. In terms of local strain, the failure behavior of intact specimens under the application of low and high numbers of F-T cycles was controlled by potential fracture bands and frost-heave cracks, respectively, while fractured specimens were mainly dependent on the macro-fracture arrangement. However, both frost heaving and thawing transformed sandstone from brittle to ductile. In terms of statistical strain, the equivalent strain (εeq) showed a log-normal distribution and continued to deviate to the right due to loading-unloading. Meanwhile, the normal strain (εxx, εyy, εzz) exhibited a normal distribution, where fractured sandstone mainly developed along with the slip direction. Finally, the intrinsic correlations among the energy parameters, meso-damage, and macro-failure were discussed. From our analysis, three stages of energy allocation and transformation were revealed, namely the uniform allocation of damage energy, banded accumulation of damage energy, and fracture caused by the elastic energy release.

Tensile damage evolution and mechanical behaviour of SiCf/SiC mini-composites through 4D in-situ micro-CT and data-driven modelling

Damage evolution of SiCf/SiC ceramic matrix mini-composites (CMMCs) was characterised by using a 4D in-situ micro-computed tomography (CT) tensile test and digital volume correlation (DVC) technique. Additionally, a CT damage data-driven shear lag model was developed to predict its tensile stress-strain response. A 4D in-situ X-ray CT tensile test of a unidirectional SiCf/SiC mini-composite was first carried out. Then two ad-hoc deep-learning image segmentation models were developed to automatically identify its microstructure and damages induced by tension, respectively. Damage evolution was quantitively characterised by visualising the initiation and propagation of matrix cracks in three-dimensions (3D). A two-step approach was employed to evaluate its 3D internal strain distributions at various loading levels, which further revealed strain concentrations and helped establishing the tensile stress-strain response of the CMMCs. It was observed that transverse cracking is the predominant damage mode, and the average crack opening displacement increases with loading. A high-fidelity X-ray CT data-driven shear lag model was developed, incorporating inputs of transverse matrix crack spacing calculated by the 4D in-situ CT test data. The predicted stress-strain response showed a good correlation with the experimental results.

Corrosion damages of reinforced concrete characterized by X-ray CT and DVC techniques

X-ray microcomputed tomography (X-ray CT) technique was performed to monitor the whole corrosion cracking process of a cylinder reinforced concrete specimen non-destructively. Based on gray threshold separation and 3D reconstruction technology, the spatial distributions of each substance were obtained. Combined with Digital Volume Correlation (DVC) technique, the development of corrosion depth, generation and migration of corrosion products, variation of internal strain field and crack propagation during the corrosion process were all analyzed. The experiment results indicate that the electric current accelerated method can lead to a non-uniformity corrosion at the early age. The distribution fields of rusts and strain are affected by the micro- and meso- structure of concrete. A turning point of maximum principle strain along the cracking path can be identified as the critical point at which micro-cracks of concrete connect and propagate into macro-cracks. The statistical analysis reveals that the gray values of mortar matrix, corrosion products, steel bar and coarse aggregates in corroded concrete follow a Garrison distribution, while the pores do not.

Evaluation of compressive behaviour of porous structures under large deformation using micro‐CT, DVC and micro‐FE

AbstractCompressive behaviour of open and closed cell polyurethane foam samples under large deformation is studied using micro‐Computed Tomography (micro‐CT), Digital Volume Correlation (DVC) technique and micro‐Finite Element (micro‐FE) modelling. The micro‐CT images of the foam samples at different compression strains are used to determine anisotropy in the foams, to obtain qualitative information on deformation mechanisms, to quantify the deformation and strains using a local DVC approach and to generate images for micro‐FE modelling of the foam samples. Micro‐FE modelling predicts the deformation using an elastoplastic material model coupled with continuum damage mechanics. Two different types of boundary conditions, experimentally derived (ExBC) and interpolated from DVC (IPBC), were implemented to evaluate the displacements in the micro‐FE models. A reduced integration scheme in micro‐FE analysis resulted in high artificial energy and was discarded in favour of full integration. The displacement predicted by IPBC matched with DVC displacement contours for closed cell foam. The ExBC‐predicted axial displacement (W) showed a better agreement with DVC than transverse displacements (U, V) contours. However, a significant statistical comparison (R2 > 0.70) of all displacements was obtained for both IPBC and ExBC. For open cell foam, both boundary conditions predicted a significant difference in the displacement contours with respect to DVC measurements. Still, the axial displacements of ExBC and IPBC showed a better statistical significance (R2 > 0.70).

The multiscale characterization and constitutive modeling of healthy and type 2 diabetes mellitus Sprague Dawley rat skin.

In type 2 diabetes mellitus (T2DM), elevated glucose level impairs the biochemistry of the skin which may result in alteration of its mechanical and structural properties. The several aspects of structural and mechanical changes in skin due to T2DM remain poorly understood. To fill these research gaps, we developed a non-obese T2DM rat (Sprague Dawley (SD)) model for investigating the effect of T2DM on the in vivo strain stress state, mechanical and structural properties of skin. In vivo strain and mechanical anisotropy of healthy and T2DM skin were measured using the digital imaging correlation (DIC) technique and DIC coupled bulge experiment, respectively. Fluorescence microscopy and histology were used to assess the collagen and elastin fibers microstructure whereas nanoscale structure was captured through atomic force microscopy (AFM). Based on the microstructural observations, skin was modeled as a multilayer membrane where in and out of plane distribution of collagen fibers and planar distribution of elastin fibers were cast in constitutive model. Further, the state of in vivo stresses of healthy and T2DM were measured using model parameters and in vivo strain in the constitutive model. The results showed that T2DM causes significant loss in in vivo stresses (p < 0.01) and increase in anisotropy (p < 0.001) of skin. These changes were found in good correlation with T2DM associated alteration in skin microstructure. Statistical analysis emphasized that increase in blood glucose concentration (HbA1c) was the main cause of impaired biomechanical properties of skin. The presented data in this study can help to understand the skin pathology and to simulate the skin related clinical procedures. STATEMENT OF SIGNIFICANCE: Our study is significant as it presents findings related to the effect of T2DM on the physiologic stress strain, structural and mechanical response of SD rat skin. In this study, we developed a non-obese T2DM SD rat model which mimics the phenotype of Asian type 2 diabetics (non-obese). Several structural and mechanical characterization techniques were explored for multiscale characterization of healthy and T2DM skin. Further, based on microstructural information, we presented the constitutive models that incorporate the real microstructure of skin. The presented results can be helpful to simulate the realistic mechanical response of skin during various clinical trials.

Damage evolution and full-field 3D strain distribution in passively confined concrete

The damage evolution of concrete under multiple stress states is complicated because the triaxial stress on the heterogeneous concrete affects mesoscale interactions between aggregates and mortar, which further alters the internal cracking of concrete and its load bearing mechanism. Hence, probing the internal cracking evolution of concrete under a complex stress state is the key to better understanding the characteristics and failure mechanism of concrete material. This study conducted a series of X-ray computed tomography (X-CT) tests on fiber-reinforced polymer (FRP) confined concrete with varied in situ axial load levels. Different cross-sections of concrete specimens—circular, square with round corners, and square with sharp corners—were designed to achieve uniform or nonuniform passive confinement conditions. The obtained volumetric images from X-CT clearly exhibited the internal damage evolution of concrete, where crack growth, shear banding, and failure localization were displayed for the concrete under varying triaxial stress states. The development of damage to concrete due to cracking with increasing load was also quantified by image segmentation. In addition, the digital volumetric correlation technique (DVC) was employed to produce the volumetric displacement field and strain distribution in the confined concrete according to the image variation of natural speckles. A three-dimensional internal strain analysis revealed that strain localization was mainly caused by mesoscale concrete heterogeneity and nonuniform confinement. More importantly, this study provides the first database of full-field strain tensors for FRP-confined concrete under different axial load levels. These data are valuable for the development of concrete mechanics, failure criteria of materials, and computational modeling of concrete materials and their structures, especially for concrete under complex stress states.

Deformation mode in 3-point flexure on pantographic block

Pantographic blocks are metamaterials made of a finite number of parallel pantographic sheets interconnected by cylindrical pivots. In this paper, a pantographic block subjected to 3-point flexure, where the prescribed displacements are parallel to the pantographic plane, shows essentially monoclastic deformation (i.e., one of the principal curvatures of the top surface is found to be negligible wrt. the other one). Pantographic blocks are modeled herein with a second gradient 3-dimensional continuum model that is valid at the length scale of pantographic cells. This reduced order model allows for predictive numerical simulations whose computational burden is relatively small. Second gradient effects (i.e., higher-order terms contributing to the strain energy) are limited to the second derivatives along the fibers of their transverse displacements. Digital Volume Correlation (DVC) techniques are employed to measure deformed shapes of pantographic blocks. A model-driven initialization procedure of DVC is followed to quantify the shape of such pantographic blocks in large displacements and strains.

Experimental Investigation of Damage Evolution Characteristics of Coral Reef Limestone Based on Acoustic Emission and Digital Volume Correlation Techniques

Experimental Investigation of Damage Evolution Characteristics of Coral Reef Limestone Based on Acoustic Emission and Digital Volume Correlation Techniques

Comparing Acute IOP-Induced Lamina Cribrosa Deformations Premortem and Postmortem.

Lamina cribrosa (LC) deformations caused by elevated intraocular pressure (IOP) are believed to contribute to glaucomatous neuropathy and have therefore been extensively studied, in many conditions, from in vivo to ex vivo. We compare acute IOP-induced global and local LC deformations immediately before (premortem) and after (postmortem) sacrifice by exsanguination. The optic nerve heads of three healthy monkeys 12 to 15 years old were imaged with spectral-domain optical coherence tomography under controlled IOP premortem and postmortem. Volume scans were acquired at baseline IOP (8-10mmHg) and at 15, 30, and 40mmHg IOP. A digital volume correlation technique was used to determine the IOP-induced three-dimensional LC deformations (strains) in regions visible premortem and postmortem. Both conditions exhibited similar nonlinear relationships between IOP increases and LC deformations. Median effective and shear strains were, on average, over all eyes and pressures, smaller postmortem than premortem, by 14% and 11%, respectively (P's < 0.001). Locally, however, the differences in LC deformation between conditions were variable. Some regions were subjected premortem to triple the strains observed postmortem, and others suffered smaller deformations premortem than postmortem. Increasing IOP acutely caused nonlinear LC deformations with an overall smaller effect postmortem than premortem. Locally, deformations premortem and postmortem were sometimes substantially different. We suggest that the differences may be due to weakened mechanical support from the unpressurized central retinal vessels postmortem. Additional to the important premortem information, comparison with postmortem provides a unique context essential to understand the translational relevance of all postmortem biomechanics literature.

Tensile failure study of 3D printed PLA using DIC technique and FEM analysis

The paper presents the experimental and numerical study of the failure behaviour of Fused Filament Fabricated (FFF) Polylactic Acid (PLA) samples subjected to tensile load. The examined samples are printed in flat orientation with 0°, 45° and 90° raster angles. During the experiments the deformation of the specimens is continuously scanned with the 3D Aramis measuring system utilizing the digital imaging correlation technique, enabling the determination of strain and stress distribution. In the modelling, it is assumed that each printed layer is a homogeneous transversely isotropic medium with the raster direction treated as the favoured one. The finite element models are developed in the Abaqus-Standard package. A two-dimensional equivalent single-layer approach is utilized to describe the deformation and stress state of the samples. The failure progress of the material is simulated by making use of the Hashin damage algorithm with energy-based softening, whereas the non-linear in-plane shear behaviour is included.

Investigation on mechanical behaviors of Self-compacting concrete containing reclaimed asphalt pavement

In order to investigate the effect on mechanical properties of self-compacting concrete (SCC) with reclaimed asphalt pavement (RAP), SCC samples with RAP content of 0%, 30%, 60%, 90%, and 100% by weight were prepared. The stress–strain relationship and crack propagation mechanisms were researched by the uniaxial compression test and digital speckle correlation technique. Furthermore, a compression damage constitutive model was established. The results show that RAP has an apparent negative influence on workability and compressive strength of SCC with RAP content ranging from 0% to 100%. Peak stress and elastic modulus of SCC samples decrease with RAP content increase while peak strain of samples increases. The cracks number of the specimens increases significantly and energy absorption ability is improved with RAP content increase. The proposed damage constitutive model effectively describes the strain–stress characteristics of self-compacting concrete with different RAP contents and the results agreed well with experimental tests. The change rate of damage variable constantly decreases with RAP content increase.

Effects of Cutting Edge Radius on Surface Integrity in Machining of Nickel-Based Cast Superalloy: An In Situ Imaging Approach

Abstract Cutting edge radius is a crucial factor affecting surface integrity during metal machining, which determines product performance. However, the exact mechanism of how the cutting edge radius affects machined surface has not yet been understood, especially lacking in situ evidence during the material removal process. In this article, effects of cutting edge radius on surface roughness, subsurface deformation, and work hardening of nickel-based cast superalloy are studied through an in situ imaging approach. Based on continuous high-speed filming and digital imaging correlation (DIC) techniques, detailed chip formation and quantitative subsurface plastic deformation under various cutting edge radii are analyzed, and the formation of built-up edge (BUE) is observed when using a large edge radius. Furthermore, when the cutting edge radius is greater than the uncut chip thickness (h), the thickness of plastic deformation increases dramatically. On the other hand, the machined surface roughness can be improved when the cutting edge radius is between 30% and 60% of h. The sharp cutting tools or the cutting edge radius higher than 60% of h result in a poor surface quality on the machined surface during nickel-based cast superalloy machining. The effects of cutting edge radius on machined surface generation are systematically categorized as cutting with chipping, cutting with significant plowing, and cutting with plowing accompanied by BUE formation.

Rotational Extension Promotes Coeval Upper Crustal Brittle Faulting and Deep‐Seated Rift‐Axis Parallel Flow: Dynamic Coupling Processes Inferred From Analog Model Experiments

AbstractThe lower parts of warm, thick continental crust can flow in a ductile fashion to accommodate thinning of the upper brittle crust during extension. Naturally occurring continental rifts with a rift‐axis parallel deformation gradient imply an underlying rotational component. In such settings, rift‐parallel crustal flow transports material perpendicular to the direction of rifting. We use analog experiments to investigate rotational rifting and coeval crustal flow. To test the effect of rift‐axis parallel flow on rift evolution, we use different gravitational loads resulting in a range of horizontal pressure gradient magnitudes which drive horizontal lower‐crustal flow. The use of (three dimensional) 3D Digital Volume Correlation techniques on X‐ray CT data combined with 3D Digital Image Correlation techniques applied to topographic stereo images provides detailed insights on the contemporaneous evolution of ductile flow patterns and brittle rift structures, respectively. Our results depict a complex flow field in the ductile lower crust during rotational rifting with: (a) extension‐parallel horizontal inward flow and vertical upward flow that compensates thinning of the brittle upper crustal layer; (b) rift‐axis parallel lateral flow, that compensates greater amounts of thinning further away from the rotation axis; and (c) different degrees of mechanical coupling between the brittle and viscous layers that change during rift propagation. Our analog experiments provide insights into ductile lower crustal flow patterns during rift evolution. The results emphasize the three dimensionality of rifting, which is an important effect that should be considered when estimating the amount of crustal extension from two dimensional (2D) cross sections.

Trochanteric Nails for the Reduction of Intertrochanteric Fractures: A Biomechanical Analysis Based on Finite Element Analysis and DIC System

PurposeIntertrochanteric fractures are common among femoral fractures in the elderly population. The trochanteric nail is a standard internal fixator used in treating femoral intertrochanteric fractures. The technique of femoral fracture reduction affects the postoperative outcome. Here, we applied finite element analysis (FEA) to study mechanical effects of different reduction approaches using the trochanteric nail in treating both stable and unstable intertrochanteric fractures.MethodsWe combined FEA and in vitro experiments using a digital imaging correlation (DIC) technique to study effects of different alignment conditions after treating 4 cases of intertrochanteric fractures using the trochanteric nail system. A downward force of 2250 N was applied to the femoral head, and the distal end of the femur was fixed. The observed indicators were the femur displacement, together with the stress on the femur and trochanteric nail system. In addition, the displacement distribution was analyzed using DIC.ResultsIn the case of space reduction, the force was transmitted by the trochanteric nail system, resulting in greater stress imposed on the femur or the trochanteric nail system. In the case of closed reduction, the stress was much smaller. In the case of unstable fracture reduction, closed reduction was associated with a smaller contact area at the fracture site, resulting in greater stress on both trochanter and the trochanteric nail system.ConclusionWhen the trochanteric nail system was used for fixation, the fracture site was well aligned, reducing the stress on the femur or the trochanteric nail.

Pore and permeability properties of reservoir sandstone under a uniaxial compression CT test

For underground gas storage projects that use depleted gas reservoirs, cyclic gas injection will change the stress fields and cause rock deformation. In-depth knowledge of the stress-induced deformation of reservoir rocks is important for secure gas storage. In this study, we performed a uniaxial compression computed tomography (CT) test with a reservoir sandstone sample to investigate the influence of rock deformation on the pore and permeability properties. Based on scanned 3D images, we analyzed the axial strain evolution by using the digital volume correlation (DVC) technique and the variations in pore properties with the microstructures and permeability properties that were extracted by the pore network modeling method. The DVC analysis shows that strain localization develops gradually with increasing axial stress. The initial porosity distribution with sample height is inhomogeneous. The local porosity variations are not completely consistent with the global porosity variations under increasing axial stress. The level of inconsistency will decrease with increasing axial stress in both the volumetric compression and dilation stages. Increases in axial stress mainly induce the variations in pore numbers and pore volumes. The variations in pore connectivity are not significant. For the permeability evolution, X-ray CT scanning combined with pore network modeling can simulate the permeability decreases during volumetric compression. For high porosity sandstones (>15%), the permeability still decreases in the volumetric dilation stage due to grain size reductions and tortuosity increases. However, this technique has limitations in analyzing the permeability reductions in the volumetric dilation stage. The microcracks that are induced inside the grain particles are too small to be recognized. The variations in the relative gas and water permeabilities at different axial stresses are not significant.