Digital Correlation Technique Research Articles

Experimental study of mechanical properties of steel 40Cr in the necking area of specimen during the postcritical deformation

The work is devoted to the experimental study of the evolution of mechanical properties of the structural steel 40Cr during postcritical deformation of solid cylindrical specimens in conditions of strain localization formation in the form of neck at tension. The methodical issues of providing tests by ‘specimen from specimen’ scheme are observed. Different levels of preliminary postcritical deformation of initial samples are realized, and the results of testing specimens cut from the initial samples with a neck are obtained. To measure the displacement and strain fields in the gauge length of specimens with the neck the noncontact 3D video system Vic 3D, based on the digital images correlation technique, was used. On the base of test results, the stiffness and strength of steel 40Cr were evaluated in a necked specimen at various stages of postcritical deformation. The hardness distribution on the specimen after necking was analyzed too. It is shown that the material in the peripheral areas of the gauge length of the sample is in a strengthened state, which does not depend on the degree of previously achieved postcritical deformation, and the strength of the material in the neck-forming zone on the initial specimens is thereby increased.

Regularities of mechanical behavior of steel 40Cr during the postcritical deformation of specimens in condition of necking effect at tension

The work is devoted to the experimental study of mechanical behavior regularities of the structural steel 40Cr under postcritical deformation of specimens in conditions of strain localization formation in the form of neck during extension. The results of testing specimens cut from the initial samples with a neck are given. Various levels of preliminary postcritical deformation of initial samples are implemented. Using a noncontact 3D video system for recording displacement and strain fields of Vic 3D Correlated Solutions, based on the digital images correlation technique, the displacement and strain fields in the gauge length of both the original specimens with the neck and after the groove were recorded in two different schemes to eliminate geometrical heterogeneity. On the base of obtained results, the stiffness and strength of 40Cr steel were evaluated in a necked specimen at various stages of postcritical deformation. It is shown that the material in the peripheral areas of the gauge length of the sample is in a strengthened state, which does not depend on the degree of previously achieved postcritical deformation, and the strength of the material in the neck-forming zone on the initial specimens is thereby increased.

Microstructural Consequences of Blast Lung Injury Characterized with Digital Volume Correlation

This study focuses on microstructural changes that occur within the mammalian lung when subject to blast and how these changes influence strain distributions within the tissue. Shock tube experiments were performed to generate the blast injured specimens (cadaveric Sprague-Dawley rats). Blast overpressures of 100 kPa and 180 kPa were studied. Synchrotron tomography imaging was used to capture volumetric image data of lungs. Specimens were ventilated using a custom-built system to study multiple inflation pressures during each tomography scan. This data enabled the first digital volume correlation (DVC) measurements in lung tissue to be performed. Quantitative analysis was performed to describe the damaged architecture of the lung. No clear changes in the microstructure of the tissue morphology were observed due to controlled low to moderate level blast exposure. However, significant focal sites of injury were observed using DVC, which allowed detection of bias and concentration in the patterns of strain level. Morphological analysis corroborated the findings, illustrating that the focal damage caused by a blast can give rise to diffuse influence across the tissue. It is important to characterise the non-instantly fatal doses of blast, given the transient nature of blast lung in the clinical setting. This research has highlighted the need for better understanding of focal injury and its zone of influence (alveolar inter-dependency and neighbouring tissue burden as a result of focal injury). Digital volume correlation techniques show great promise as a tool to advance this endeavour, providing a new perspective on lung mechanics post-blast.

Effects of sedimentation on rift segment evolution and rift interaction in orthogonal and oblique extensional settings: Insights from analogue models analysed with 4D X-ray computed tomography and digital volume correlation techniques

Abstract During the early evolution of rift systems, individual rift segments often develop along pre-existing crustal weaknesses that are frequently non-continuous and laterally offset. As extension progresses, these initial rift segments establish linkage in order to develop a continuous rift system that might eventually lead to continental break-up. Previous analogue and numerical modelling efforts have demonstrated that rift interaction structures are influenced by structural inheritances, detachment layers, magma bodies, rate and direction of extension, as well as the distance between rift segments. Yet to date, the effects of syn-tectonic sediments have been largely ignored or only modelled in 2D. In this study we therefore assess the influence of sedimentation on rift segment and rift interaction zone evolution in orthogonal and oblique extensional settings, by means of 3D brittle-ductile analogue models, analysed with 4D X-ray computed tomography (XRCT or CT) methods and digital volume correlation (DVC) techniques. Our models show that syn-rift sedimentation does not significantly influence the initial large-scale evolution of rift segments and rift interaction zones. Nevertheless, syn-rift sedimentation can strongly affect rift-internal structures: sedimentary loading reinforces the rift wedge, decreasing rift wedge faulting and increases subsidence within the rift basin. These effects are strongest in areas where most accommodation space is available, that is, along the main rift segments. In contrast, rift segments that undergo high degrees of oblique extension develop less accommodation space and are therefore less influenced by sedimentary loading. Rift interaction structures are least affected by sediment influx, as they experience relatively low amounts of subsidence so that little accommodation space is available. Our conclusions are valid for the early stages of rift development, when a high sediment influx could delay continental break-up, as other processes are likely to become dominant during later stages of continental extension. Finally, state-of-the-art DVC analysis of CT data proves to be a powerful tool to extract and fully quantify 3D internal model deformation in great detail and could be useful for comparing and calibrating analogue and numerical models.

Femoral fracture type can be predicted from femoral structure: A finite element study validated by digital volume correlation experiments.

Proximal femoral fractures can be categorized into two main types: Neck and intertrochanteric fractures accounting for 53% and 43% of all proximal femoral fractures, respectively. The possibility to predict the type of fracture a specific patient is predisposed to would allow drug and exercise therapies, hip protector design, and prophylactic surgery to be better targeted for this patient rendering fracture preventing strategies more effective. This study hypothesized that the type of fracture is closely related to the patient-specific femoral structure and predictable by finite element (FE) methods. Fourteen femora were DXA scanned, CT scanned, and mechanically tested to fracture. FE-predicted fracture patterns were compared to experimentally observed fracture patterns. Measurements of strain patterns to explain neck and intertrochanteric fracture patterns were performed using a digital volume correlation (DVC) technique and compared to FE-predicted strains and experimentally observed fracture patterns. Although loaded identically, the femora exhibited different fracture types (six neck and eight intertrochanteric fractures). CT-based FE models matched the experimental observations well (86%) demonstrating that the fracture type can be predicted. DVC-measured and FE-predicted strains showed obvious consistency. Neither DXA-based BMD nor any morphologic characteristics such as neck diameter, femoral neck length, or neck shaft angle were associated with fracture type. In conclusion, patient-specific femoral structure correlates with fracture type and FE analyses were able to predict these fracture types. Also, the demonstration of FE and DVC as metrics of the strains in bones may be of substantial clinical value, informing treatment strategies and device selection and design. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:993-1001, 2018.

Trabecular deformations during screw pull-out: a micro-CT study of lapine bone

The mechanical fixation of endosseous implants, such as screws, in trabecular bone is challenging because of the complex porous microstructure. Development of new screw designs to improve fracture fixation, especially in high-porosity osteoporotic bone, requires a profound understanding of how the structural system implant/trabeculae interacts when it is subjected to mechanical load. In this study, pull-out tests of screw implants were performed. Screws were first inserted into the trabecular bone of rabbit femurs and then pulled out from the bone inside a computational tomography scanner. The tests were interrupted at certain load steps to acquire 3D images. The images were then analysed with a digital volume correlation technique to estimate deformation and strain fields inside the bone during the tests. The results indicate that the highest shear strains are concentrated between the inner and outer thread diameter, whereas compressive strains are found at larger distances from the screw. Tensile strains were somewhat smaller. Strain concentrations and the location of trabecular failures provide experimental information that could be used in the development of new screw designs and/or to validate numerical simulations.

Micro-CT based finite element models of cancellous bone predict accurately displacement once the boundary condition is well replicated: A validation study

Non-destructive 3D micro-computed tomography (microCT) based finite element (microFE) models are used to estimate bone mechanical properties at tissue level. However, their validation remains challenging. Recent improvements in the quantification of displacements in bone tissue biopsies subjected to staged compression, using refined Digital Volume Correlation (DVC) techniques, now provide a full field displacement information accurate enough to be used for microFE validation. In this study, three specimens (two humans and one bovine) were tested with two different experimental set-ups, and the resulting data processed with the same DVC algorithm. The resulting displacement vector field was compared to that predicted by microFE models solved with three different boundary conditions (BC): nominal force resultant, nominal displacement resultant, distributed displacement. The first two conditions were obtained directly from the measurements provided by the experimental jigs, whereas in the third case the displacement field measured by the DVC in the top and bottom layer of the specimen was applied. Results show excellent relationship between the numerical predictions (x) and the experiments (y) when using BC derived from the DVC measurements (UX: y=1.07x−0.002, RMSE: 0.001mm; UY: y=1.03x−0.001, RMSE: 0.001mm; UZ: y=x+0.0002, RMSE: 0.001 mm for bovine specimen), whereas only poor correlation was found using BCs according to experiment set-ups. In conclusion, microFE models were found to predict accurately the vectorial displacement field using interpolated displacement boundary condition from DVC measurement.

A Parametric Study on the Influence of Internal Speckle Patterning for Digital Volume Correlation in X-Ray Tomography Applications

Use of the Digital Volume Correlation (DVC) technique has grown steadily in the mechanics community as a way of quantifying internal mechanical response of complex microstructures under loading. The DVC technique has been combined with internal imaging methodologies such as X-ray microcomputed tomography (X-ray microCT), confocal microscopy, and photoelasticity enabling many full field studies of bone, foam, rock, polymers, and metals. Despite all these efforts, many DVC limitations remain unknown such as those of “optimal” subset size, step size, and the quality of an internal speckle pattern. Here we investigate in detail the effects of internal speckle patterning on DVC. To help determine the optimum setup for DVC, we develop internal speckle patterns using markers ranging from 5 to 50 μm particles and study these over different resolution length scales. A correlation of pattern size and quality was determined using baseline (i.e., no deformation) and rigid body motion experiments, resulting in a recommended pattern quality parameter. We then performed a uniaxial compression experiment using the optimal pattern determined from the parametric study where we compare accuracy using DVC calculated strains and displacements with theoretical values. An in situ uniaxial compression test demonstrated DVC capabilities resulting in a 2 % error in strain computations.

Strain mapping and correlative microscopy of the alveolar bone in a bone-periodontal ligament-tooth fibrous joint.

This study details a method to calculate strains within interradicular alveolar bone using digital volume correlation on X-ray tomograms of intact bone-periodontal ligament-tooth fibrous joints. The effects of loading schemes (concentric and eccentric) and optical magnification on the resulting strain in alveolar bone will be investigated with an intent to correlate deformation gradients with data sets from other complementary techniques. Strain maps will be correlated with structural and site-specific mechanical properties obtained on the same specimen using atomic force microscopy and atomic force microscopy-based nanoindentation technique. Specimens include polydimethylsiloxane as a standard material and intact hemi-mandibles harvested from rats. X-ray tomograms were taken at no-load and loaded conditions using an in situ load cell coupled to a micro X-ray computed tomography unit. Digital volume correlation was used to calculate deformations within alveolar bone. Comparison of strain maps was made as a result of different loading schemes (concentric vs eccentric) and at different magnifications (4× vs 10×). Virtual sections and strain maps from digital volume correlation solutions were aligned with structure and reduced elastic modulus to correlate datasets of the same region within a specimen. Strain distribution between concentrically and eccentrically loaded complexes was different but illustrated a similar range. Strain maps of homogeneous materials (polydimethylsiloxane) resulting from digital volume correlation at different magnifications were similar. However, strain maps of heterogeneous materials at lower and higher magnification differed. The digital volume correlation technique illustrated a dependence on optical magnification specifically for heterogeneous materials such as bone. The results at a higher optical magnification highlight the potential for extracting deformation at higher resolutions. Correlation of data spaces from different complementary techniques is plausible and could provide insights into biological and physicochemical processes that lead to functional adaptation of tissues and joints.

Adaptive Isogeometric Digital Height Correlation: Application to Stretchable Electronics

AbstractA novel adaptive isogeometric digital height correlation (DHC) technique has been developed in which the set of shape functions, needed for discretization of the ill‐posed DHC problem, is autonomously optimized for each specific set of profilometric height images, without a priori knowledge of the kinematics of the experiment. To this end, an adaptive refinement scheme is implemented, which refines the shape functions in a hierarchical manner. This technique ensures local refinement, only in the areas where needed, which is beneficial for the noise robustness of the DHC problem. The main advantage of the method is that it can be applied in experiments where the deformation mechanisms are unknown in advance, thereby complicating the choice of suitable shape functions. The method is applied to a virtual experiment in order to provide a proof of concept. A second virtual experiment is executed with stretchable electronics interconnects, which entail localized buckles upon deformation with complex kinematics. In both cases, accurate results were obtained, demonstrating the beneficial aspects of the proposed method. Moreover, the technique performance on profilometric images of a real experiment with stretchable interconnects was demonstrated.

Influence of pores on crack initiation in monotonic tensile and cyclic loadings in lost foam casting A319 alloy by using 3D in-situ analysis

Lost Foam Casting (LFC) process is replacing the conventional gravity Die Casting (DC) process in automotive industry for the purpose of geometry optimization, cost reduction and consumption control. However, due to lower cooling rate, LFC produces in a coarser microstructure that reduces fatigue life. In order to study the influence of the casting microstructure of LFC Al-Si alloy on damage micromechanisms under monotonic tensile loading and Low Cycle Fatigue (LCF) at room temperature, an experimental protocol based on the three dimensional (3D) in-situ analysis has been set up and validated. This paper focuses on the influence of pores on crack initiation in monotonic and cyclic tensile loadings. X-ray Computed Tomography (CT) allowed the microstructure of material being characterized in 3D and damage evolution being followed in-situ also in 3D. Experimental and numerical mechanical fields were obtained by using Digital Volume Correlation (DVC) technique and Finite Element Method (FEM) simulation respectively. Pores were shown to have an important influence on strain localization as large pores generate enough strain localization zones for crack initiation both in monotonic tensile and cyclic loadings.

Full-field measurement with a digital image correlation analysis of a shake table test on a timber-framed structure filled with stones and earth

This paper aims at presenting a digital correlation technique to capture the full-field displacement thanks to a high-speed camera of a full scale structure tested on a shaking table. The challenges are both the measurements at a full scale to visualize damages versus the resolution of pictures and the dynamical loading that requires a large number of pictures. The final goal is a better understanding of the seismic behavior of timber-framed structures with infill to help at modeling such structures and predicting their seismic vulnerability. For this purpose, results of shake table tests carried out on a full-scale one-story timber-framed house filled with stones bonded by an earth based mortar are presented and discussed. DIC full-field measurements allow deriving displacements and accelerations on shear walls as well as lateral forces applied on them. The experimental results presented herein allow analyzing the influence of bracing and might be used to propose optimized aseismic constructions based on cheap technological solutions. These results demonstrate the seismic-resistant behavior of timber-framed structures with infill and constitute a key issue for the promotion of such constructions in developing countries.

Constitutive model of friction stir weld with consideration of its inhomogeneous mechanical properties

In practical engineering, finite element(FE) modeling for weld seam is commonly simplified by neglecting its inhomogeneous mechanical properties. This will cause a significant loss in accuracy of FE forming analysis, in particular, for friction stir welded(FSW) blanks due to the large width and good formability of its weld seam. The inhomogeneous mechanical properties across weld seam need to be well characterized for an accurate FE analysis. Based on a similar AA5182 FSW blank, the metallographic observation and micro-Vickers hardness analysis upon the weld cross-section are performed to identify the interfaces of different sub-zones, i.e., heat affected zone(HAZ), thermal-mechanically affected zone(TMAZ) and weld nugget(WN). Based on the rule of mixture and hardness distribution, a constitutive model is established for each sub-zone to characterize the inhomogeneous mechanical properties across the weld seam. Uniaxial tensile tests of the AA5182 FSW blank are performed with the aid of digital image correlation(DIC) techniques. Experimental local stress-strain curves are obtained for different weld sub-zones. The experimental results show good agreement with those derived from the constitutive models, which demonstrates the feasibility and accuracy of these models. The proposed research gives an accurate characterization of inhomogeneous mechanical properties across the weld seam produced by FSW, which provides solutions for improving the FE simulation accuracy of FSW sheet forming.

The Elastic Constants Measurement of Metal Alloy by Using Ultrasonic Nondestructive Method at Different Temperature

The ultrasonic nondestructive method is introduced into the elastic constants measurement of metal material. The extraction principle of Poisson’s ratio, elastic modulus, and shear modulus is deduced from the ultrasonic propagating equations with two kinds of vibration model of the elastic medium named ultrasonic longitudinal wave and transverse wave, respectively. The ultrasonic propagating velocity is measured by using the digital correlation technique between the ultrasonic original signal and the echo signal from the bottom surface, and then the elastic constants of the metal material are calculated. The feasibility of the correlation algorithm is verified by a simulation procedure. Finally, in order to obtain the stability of the elastic properties of different metal materials in a variable engineering application environment, the elastic constants of two kinds of metal materials in different temperature environment are measured by the proposed ultrasonic method.

TRNA acceptor-stem and anticodon bases embed separate features of amino acid chemistry

abstractThe universal genetic code is a translation table by which nucleic acid sequences can be interpreted as polypeptides with a wide range of biological functions. That information is used by aminoacyl-tRNA synthetases to translate the code. Moreover, amino acid properties dictate protein folding. We recently reported that digital correlation techniques could identify patterns in tRNA identity elements that govern recognition by synthetases. Our analysis, and the functionality of truncated synthetases that cannot recognize the tRNA anticodon, support the conclusion that the tRNA acceptor stem houses an independent code for the same 20 amino acids that likely functioned earlier in the emergence of genetics. The acceptor-stem code, related to amino acid size, is distinct from a code in the anticodon that is related to amino acid polarity. Details of the acceptor-stem code suggest that it was useful in preserving key properties of stereochemically-encoded peptides that had developed the capacity to interact catalytically with RNA. The quantitative embedding of the chemical properties of amino acids into tRNA bases has implications for the origins of molecular biology.

Expanded digital volume correlation for ex situ applications

Full-field deformation/strain measurements are important to experimental mechanics, and the digital volume correlation (DVC) technique is one of the most important techniques for full-field measurement. However, traditional DVC has difficulty in distinguishing deformation displacements and rigid-body displacements in ex situ applications. To expand the applications of traditional DVC, two expanded DVC methods are proposed, based on outer and inner image registration respectively. These methods would remove the rigid motions in ex situ applications. Their performances are thoroughly evaluated. These methods have similar accuracy and precision to traditional DVC. One proposed method is further applied on a low-velocity impact deformation investigation, which would be impossible using traditional DVC. The high accuracy and precision of the methods demonstrate their reliability and feasibility, and the case study on low-velocity impact deformation shows their potential in ex situ applications.

Analysis of Piles Under Oblique Pullout Load Using Transparent-Soil Models

This paper presents an experimental investigation of internal deformation in transparent sand caused by a pile moving under oblique pullout loads using the digital image correlation (DIC) technique. Transparent sand used in this study is manufactured with fused quartz and a pore fluid (mixture of Norpar 12 and white mineral oil) with a matching refractive index. An optical system consisting of a linear laser, a charge-coupled device (CCD) camera, an optical platform, a frame grabber, and a computer is employed. During the test process, the speckle (which is the interaction between the transparent-soil matrix, impurities, entrapped air, and laser) is generated. The laser speckle images before and after soil deformation are used to measure the relative displacement field using the DIC technique. The belled wedge pile, conventional tapered pile, and equal section pile with the same volume, slenderness ratio L/D = 16.9, and oblique pullout loads at angles α = 0°, 45°, 60°, and 90° are conducted in transparent sand. The load-displacement response, oblique ultimate pulling resistances, internal deformation field, surface heaving, and failure mechanisms have been studied. The normalized values in this paper were compared with previously measured values. The results indicate that the optical system and transparent sand are suitable for studying soil deformation caused by pile–soil interaction under oblique pullout loading.

ANISOTROPIC DEFORMATION BEHAVIOR OF CONTINUOUS COLUMNAR-GRAINED CuNi10Fe1Mn ALLOY

In continuous unidirectional solidification process, an unidirectional heat transfer condition can be established to control grain growth direction along the solidification direction(SD). By this method, continuous columnar- grained(CCG) polycrystalline alloys without transverse grain boundary can be obtained, which possess high orientated texture and straight grain boundary morphology. High orientated texture can significantly improve the consistency among the grains, and the straight grain boundaries reduce the number of coordinated strain compo-nents, resulting in high plasticity and excellent extension behavior along the SD in the CCG alloys. For example,the CCG polycrystalline Cu Ni10Fe1 Mn alloy has a high tensile elongation(40%). However, as described above,the CCG polycrystalline alloy has an extremely anisotropic microstructure. In order to improve its performance, select the appropriate processing methods, and establish a reasonable process, its mechanical properties and deformation behavior were investigated with tensile direction along the SD or perpendicular to the solidification direction(PD) in this work. The electron back- scatter diffraction(EBSD) and digital image correlation(DIC) techniques were introduced to study the effects of microstructure anisotropy on the mechanical properties and deformation behavior. The results indicate that both SD and PD samples have [100] preferred orientation. All grains in SD samples(Taylor factor m=2.17) are nearby [100], while some grains in PD samples(Taylor factor m=2.93) scatter among [001]- [011]. Microstructure characteristics of low orientation dispersion and no horizontal grain boundary in SD samples contribute to the uniform stress distribution and consistent deformation behavior in each grain along the tensile direction. The yield strength, tensile strength and elongation are 85 MPa, 215 MPa and 42%, respectively. Compared to SD samples, PD samples appear to grain boundary stress concentration and zigzag surface morphologies due to the orientation dispersion and horizontal grain boundaries. As a result, the yield strength markedly increases to 115 MPa, and the elongation decreases to 36%. The SD and PD samples occur ductile and mixed fracture, respectively. The anisotropic deformation behavior of CCG polycrystalline Cu Ni10Fe1 Mn alloy is attributed to the anisotropic grain orientation and the grain boundary distribution.

Santa Maria di Collemaggio Church (L’Aquila, Italy): Historical Reconstruction by Non-Destructive Testing Techniques

The main goal of this work was the non-destructive testing (NDT) of an ancient fresco (15th century) preserved in the Santa Maria di Collemaggio Church (L’Aquila, Italy) and damaged after the 2009 earthquake. Active infrared thermography (IRT), near-infrared (NIR) reflectography and ultraviolet imaging (UV) were used. In addition, the state of the fresco prior to the earthquake was analyzed by electronic speckle pattern interferometry (ESPI), digital speckle correlation (DSC), raking light, tap, and chemical NDT techniques. The use of these techniques was important for the monitoring of new damages and for a comparison between the results over the years. Square heating thermography (SHT) data were processed using principal component thermography (PCT) and pulsed phase thermography (PPT) algorithms, in order to improve the defects’ signature and to reduce the impact of non-uniform heating and emissivity variations due to the painting’s pigments. A multi-analysis approach, segmentation operators and a specific data correlation method emphasize the overall study of the fresco. Furthermore, the facade and the high altar area were inspected by passive thermography and ground-penetrating radar (GPR), respectively. In the present case, the combined use of NDT techniques was useful to fill in the gaps in the construction history of the building.

Development of a Railway Track Displacement Monitoring by Using Digital Image Correlation Technique

To avoid an unnecessary catastrophic accident due to a failure of a railway track, it is important to have a reliable condition monitoring system for the railway track. The integrity of the railway track can be assessed by monitoring the displacement field of the track, which can then be used to determine the strain and stress field. By knowing the stress history of the track and the S–N curves of the track material, the remaining life of the railway track can be predicted. In the present work, a simple system to monitor and record the displacement field of the railway track has been developed by using Digital Image Correlation (DIC) technique. The set–up to monitor the displacement field of the railway track was developed using a high speed video camera of Nikon J1 to capture the image of the railway track when the train passing through. The DIC technique was then employed off line to measure the displacement field of the 2D image captured. The results showed that the full field displacement measured by using DIC technique gives a good agreement compared to the finite element results. The full field displacement can be used to calculate the strain-stress field, and later on the remaining life assessment can be conducted based on the results.