3D Digital Image Correlation Research Articles

Tests on reinforced concrete U-shaped walls subjected to torsion and flexure

This data paper describes an experimental campaign on two reinforced concrete (RC) U-shaped walls, including details of the test units, materials, test setup, loading protocol, instrumentation, primary observations and response of each unit during testing, organization of the test data in the public open source research data repository (DOI: 10.14428/DVN/FDJ4EU), and examples of derived data. The two wall units represent the lower stories of a half-scale 6-story prototype core wall that was designed for medium-to-high ductility. The tests conducted aimed at assessing the flexural and torsional response and capacity of RC U-shaped walls. The first wall unit, UW1, was tested for axial-flexure and subjected to quasi-static reverse-cyclic bending about its weak axis until failure. Furthermore, UW1 was subjected to an additional overturning moment to increase the shear span. The second wall unit, UW2, was tested for axial-torsion and subjected to a quasi-static reverse-cyclic rotation about its vertical axis until failure. Both wall units were subjected to a constant axial load ratio of 5%. A range of conventional instrumentation was used to capture salient global and local measurements of the wall, including three-dimensional (3D) digital image correlation on multiple faces. High-definition fiber-optic sensors were also utilized to capture the entire strain profile of several longitudinal reinforcing bars in each wall unit.

Energy absorption characteristics of thin-walled tube filled with liquid suspension of hollow glass microspheres under axial crushing

The deviation between the high initial peak force and low mean crushing force of thin-walled tubes has significantly limited its energy absorption efficiency. To tackle this challenge, a liquid suspension of hollow glass microsphere (SHGM) is employed as a new type of filling material. The axial crushing behavior of SHGM-filled tubes is evaluated by a series of quasi-static compression and dynamic drop weight impact tests. Results show that the specific energy absorption of the composite tube is increased more than 40 %. The energy absorption efficiency of SHGM-filled tube is increased to 0.65–0.70 compared to that of empty tube (0.37–0.43). This promoted energy absorption performance of the composite tube is due to the much-enhanced SHGM liquid filler-tube wall interaction, which is validated from the increased strengthening coefficient of 4.0. This enhanced filler-tube wall interaction is visualized by using 3D digital image correlation technique. These results demonstrate that the SHGM is a superior filling material for thin-walled structures and facilitate the design of next generation lightweight composite structures for energy absorption purposes.

Characterization of concrete behavior under cyclic loading using 2D digital image correlation

The development of plastic deformations of concrete during loading-unloading cycles and post-cracking behavior must be carefully controlled for accurate modeling of concrete structures. This study aims to validate the use of digital image correlation (DIC) for characterizing concrete behavior under loading-unloading conditions. Experimental tests were conducted on cubic and cylindrical concrete specimens, employing strain gauges and DIC methods. Uniaxial compressive static and loading-unloading tests were performed on two series of concrete with different compressive strengths to establish stress-strain relationships.The experimental results demonstrated good agreement between strain gauges and DIC measurements in the elastic range for both cylindrical and cubic specimens. Analytical and numerical modeling further confirmed the accuracy of permanent strains obtained through DIC for the cubic specimens subjected to cyclic loading beyond the elastic domain. Based on these findings, it can be concluded that the DIC technique is reliable for tracking strain evolution up to the point of material failure under uniaxial loading-unloading compression.

Failure behavior of single-lap CFRP hybrid joints fastened using micro-bolt

This study examines the mechanical response and failure behavior of a hybrid joint fastened using multiple micro-bolts subjected to quasi-static tensile loading numerically and experimentally. The hybrid joint fastened using a single standard bolt and only adhesively bonded joint configuration are also evaluated for a comparative purpose. These joint performances are also further analyzed under tensile-tensile fatigue loading experimentally. Carbon fiber reinforced plastic (CFRP) laminates with [0/+45/90/−45]s lay-up sequence are used to make adherends. In this work, 3D digital image correlation (DIC) technique is used to capture the whole field deformation and strain field over the overlap region. An acoustic emission (AE) technique is employed to capture the damage evolution upon loading qualitatively. After failure, a scanning electron microscope (SEM) and optical microscope are used to capture the extent of damage and failure modes within the adherends. A finite element method (FEM) based analysis is performed involving a progressive damage model (PDM) to understand the damage evolution in CFRP adherend upon static tensile loading. From the observations, it has been found that the hybrid joint fastened using micro-bolts deformed greater at the peak load before the adhesive layer got disbanded, thereby demonstrating a greater fatigue life with gradual stiffness degradation behavior as compared to the other two joint configurations.

Applications of 3D scanning and digital image correlation in structural experiments

AbstractThe primary aim of the present study is to explore the use, advantages and potential of two advanced data acquisition techniques – 3D scanning and digital image correlation (DIC), in the testing of metallic structures. 3D scanning is a novel method for capturing the full outer geometry of an object. It is being increasingly used in experimental studies for the determination of dimensional parameters and the characterisation of local and global imperfections, examples of which are given and discussed. Digital image correlation (DIC) is a rapidly evolving technology for the full‐field measurement of the displacements and strains over a surface. A brief introduction to DIC and its workflow is firstly provided. Example applications of DIC in various scales of experiments, including material tests, structural element tests and full‐scale structural system tests, are subsequently discussed. These new‐generation data acquisition techniques enable structural researchers to gain a deeper insight into the behaviour of metallic structures in experimental research.

Experimental and numerical assessment of two reconstructive techniques for the fragility fractures of the pelvis type Ia.

Anterior pelvic ring fractures are common in geriatric patients. The Supraacetabular External Fixator (SEF) is a relatively simple and effective surgical procedure. On the other hand, there is the option of a Subcutaneous Iliopubic Plate (SIP) osteosynthesis. Only limited comparative biomechanical data of these two devices are available. Therefore, this biomechanical study's objective was to compare the stabilizing effect of the SEF versus the SIP in a model of Fragility Fractures of the Pelvis (FFP) type Ia. A test stand for pelvic biomechanics testing that emulates the gait loading cycle with physiological relevance was used. The osteotomy on the right pelvic ring was stabilized either with the SEF or the SIP. Strain gauges were used to measure strain in the pelvic ring. The osteotomy's spatial interfragmentary displacement (SID) was monitored using a 3D digital image correlation system. The SEF stabilization reduced the SID by approximately 10%, whereas the locking SIP could reduce displacement by about 62%. Additionally, the SIP reduced the stress/strain levels by 67% in the posterior pelvic ring. We could demonstrate that the SIP is superior to SEF in treating FFP type Ia as it significantly reduced the osteotomy's SID and the strain in the posterior pelvic ring.

Study on the bending and shear properties of quasi-honeycomb sandwich structures considering the variable-density core design

The quasi-honeycomb cell has better coplanar bearing capacity and stability in comparison with a honeycomb cell. The topology optimization can achieve the best distribution of materials within a core composed of cells. Hence, this paper makes an effort to provide a clear understanding about the enhancement effect of the core’s variable-density topology optimization on the mechanical properties of a quasi-honeycomb sandwich structure. The SR (i.e., the initials of SIMP and RAMP) interpolation function, obtained by weighting the Solid Isotropic Material with Penalization (SIMP) and Rational Approximation of Material Properties (RAMP) functions, is modified by adding the minimum elastic modulus term. The Optimal Classification (OC) algorithm is revised by adopting the volume constraint of Bi-directional Evolutionary Structural Optimization (BESO). And then taking into account the mapping between the topology elements and cells, the material distribution optimization of the quasi-honeycomb core is performed to minimize the structural compliance. The failure behavior, strength, stiffness, and energy absorption of three-dimensional (3D) printed uniform and variable-density quasi-honeycomb sandwich structures are studied by three-point bending tests, and the evolution processes and distribution laws on the displacement and strain of cores are analyzed using the 3D Digital Image Correlation (DIC) system. The comparisons between the experimental and numerical results are also carried out for the displacement and energy absorption of the sandwich structure, the bending strength of the face sheet, and the displacement and strain fields of the core. The results show that the bending strength, bending stiffness, shear strength, and energy absorption are enhanced after the variable-density topology optimization of the core, and the numerical values agree well with the experimental data. This study provides new ideas and important guidance for the variable-density design of quasi-honeycomb cores and the mechanical performance improvement of sandwich structures.

Improving the structural integrity of foam-core sandwich composites using continuous carbon fiber stitching

This study aims to improve the structural integrity of foam-core sandwich structures by embedding carbon-fiber/epoxy stitched reinforcements with fiber-volume fraction (Vf) as high as 55 %. Sandwich panels with 1.5 mm thick carbon fiber skins and 30 mm thick PVC foam-core were stitched with 0, 12 k and 24 k carbon fiber yarns. X-ray computed tomography (CT) was utilized to determine the “as-manufactured” quality of the stitched panels. Flatwise-, edgewise-compression and 3-point bending tests were conducted with in-situ 3D Digital Image Correlation (DIC) to record the deformation and analyze the structure’s mechanical response. It was observed that compared to non-stitched samples - under flatwise compression loading, the reinforced samples with a stitch Vf of 0.34 and 0.55 showed an increase in stiffness and strength by 130 % and 163 %, and 29 % and 53 %, respectively. For edgewise-compression and 3-point bending loading cases, stiffness and strength of stitched samples showed an increase of ∼20 % over the benchmark. Using DIC, X-ray CT and a fractography analysis, the external and internal damage as well as failure mechanisms were evaluated. Analytical solution was employed to understand the parametric influence of the stitch Vf and stitch density per unit area in pursuit of an optimized sandwich structure.

Bulge test technique and digital image correlation for the determination of the biaxial behavior of polymeric films

Abstract Materials can display multiple mechanical behaviors when they are biaxially stressed. In this research work, films obtained through the blowing process were investigated in order to compare the mechanical properties and behavior of the film under uniaxial and biaxial stress conditions. Bulge test coupled with a continuous full-field strain measurement was undertaken in order to characterize polymeric film in a biaxial stress state. As far as this study is concerned, a laboratory-made pneumatic bulge test machine was used for the determination of the biaxial behavior of a 50 μm thickness PLSF (modified PLA) film. 3D-digital image correlation (3D-DIC) technique was used in order to compute the strain field continuously during the test. Tensile test was equally performed on the film along its machine direction (MD) and transverse direction (TD). Strain field distribution in the film sample was analyzed and compared in both tests. The tensile test results displayed an isotropic behavior of the film in both directions. However, a difference in the strain at break was noticed because of the thickness instability along the TD. The comparison between uniaxial and biaxial properties resulted in almost the same properties with respect to the Young Modulus as well as the tensile strength.

The Calibration Process and Setting of Image Brightness to Achieve Optimum Strain Measurement Accuracy Using Stereo-Camera Digital Image Correlation

Combining the drilling method with the digital image correlation (DIC) method is becoming more common to speed up the measurement and evaluate the strains relieved at several locations. However, to obtain the most accurate results, it is necessary to be aware of the influence of possible aspects that could adversely affect the results of the strain/stress analysis carried out using DIC. The paper describes several analyses to assess the influence of the 3D DIC system’s calibration procedure for strain/stress analysis of the specimen with a hole loaded with four different levels of tensile force. In addition, the paper also deals with the analysis of the influence of the image brightness, which was modified by changing the exposure time of the cameras. Based on the results of strain/stress analyses performed on small areas (approx. 25 × 25 mm) of a specimen with a hole by a stereo-camera DIC system, it can be concluded that both analysed factors can negatively influence the results. The most accurate results are ensured using the calibration target of very high manufacturing precision sized similarly to the field of view observed in correlation mode. The optimal image brightness is adjusted when the mean grey value of the image is from the range of 56 up to 171 with as evenly distributed image point intensities as possible.

Experimental investigation of single and butt-welded GFRP filament winding tubes bending behavior based on conventional and Digital Image Correlation method

Composite joints have become increasingly popular in pipeline assembly, replacing conventional fasteners. However, butt-welded joint composite techniques still need further structural study. This paper aims to study the flexural mechanical performance of the above mentioned composite joint method. Four-point bending tests were conducted on both unassembled (single) and two butt-welded tube types (based on NFT and DIN standard methods). In this context, the 2D digital image correlation (DIC) technique was applied during these tests to analyze the damage scenario based on full-field strain mapping. To further visualize the microscopic damage modes, X-ray tomography observations were conducted on both single and butt-welded specimens. The experimental results show that single tubes have the highest mechanical properties, with quasi-elastic behavior until the occurrence of fracture. The NFT butt-welded composite tubes present the maximum flexural stiffness and strength compared with the DIN tubes. It has also been shown that the strain distribution depends on the specimen configuration. The DIC analysis results show that the damage scenario is divided into two events: the onset of fracture and then the fracture propagation. These results were confirmed through microscopic inspections.

Application of the bulge test combined with 3D-DIC for biaxial characterization of PLA films and defect detection

The bulge test is a versatile mechanical testing technique widely applied in materials science, engineering, and biomedical research to understand the mechanical behavior of materials and structures. In this study, PLA blown films were characterized through the bulge test, and the corresponding properties were compared to those obtained from uniaxial t!esting. To achieve more accurate and precise deformation measurements, 3D digital image correlation (3D-DIC) was used, offering superior capabilities in terms of accuracy, precision, and non-destructive testing. This technique can detect defects such as cracks, voids, and delamination in materials and structures. During the bulge testing, 3D-DIC was utilized to analyze the strain field evolution within the test sample and identify regions of reduced film thickness. Notably, the results indicated zones with strain concentration aligned parallel to the film machine direction, suggesting localized thickness reduction in those areas.

Uncertainty quantification for 3D digital image correlation displacement measurements using Monte Carlo method

Three-dimensional digital image correlation (3D-DIC) is a leading optical measurement technique for measuring full-field shape, displacement, and deformation of solid materials and structures. However, compared with the well-developed and well-accepted measurement technique, uncertainty quantification (UQ) of 3D-DIC measurements is less advanced and less widely practiced. There still lacks a simple and effective method for quantifying the uncertainty of displacements measured by a specific 3D-DIC system for a specific speckle pattern. This is because the 3D-DIC practice involves a complex and long measurement chain and the propagation of uncertainty is highly nonlinear, which makes the uncertainty quantitation tricky. This work proposes a Monte Carlo-based method to quantify the uncertainty of 3D-DIC displacement measurements. The method is based on a theoretical analysis of random error in image matching and theoretical estimation of camera calibration uncertainty. It can estimate the uncertainty in 3D-DIC displacement measurements without additional experimental procedures. Validation experiments reveal that the estimated uncertainty agrees well with the actual situation. Based on the proposed method, the effects of the speckle pattern quality, subset size, and camera calibration quality on 3D digital image correlation displacement measurement uncertainties are also examined. The results reveal that, compared with image matching, camera calibration has a more pronounced influence on the uncertainty of displacement measurements. The proposed method can be integrated with existing 3D-DIC software to quantify the metrological performance of 3D-DIC measurements and therefore better interpret the measurement results.

Deformation mechanisms in PBT at elevated temperatures

Plastic deformation mechanisms of Polybutylene terephthalate (PBT) are investigated at 120 °C, 150 °C and 180 °C using tensile experiments, 2D digital image correlation, wide and small angle X-ray scattering measurements.Plots of nominal stress vs true strain (ε), true stress vs ε and Haward-Thackray representation reveal the same I, II, III regimes in strain ranges virtually independent of temperature. Between ε = 0 and the yield strain εY ≈ 0.3 (I) the layer morphology is preserved, the interlamellar distance increases in the drawing direction and the strain is localized in amorphous regions. Between ε ≈ 0.3 and ε ≈ 1.0 (II), amorphous chains remain describable as gaussian coils while transition from spherulitic to fibrillar morphologies occurs. Beyond ε ≈ 1.0 (III) the chains are taut, the fibrillar morphology is established, additional stress causes the transitions between polymorphs, respectively from isotropic amorphous to smectic and from α to β crystalline phases.

Contribution of high-temperature 3D-Digital Image Correlation to investigate the creep behaviour of refractory materials

Potential contributions of 3D-Digital Image Correlation are investigated to understand the dilatant behaviour of a refractory concrete during high-temperature creep experiments. An experimental analysis of high-temperature image quality is performed. The measurement uncertainty is assessed and compared to the sensitivity of computed kinematic fields to the parameters of the Drucker-Prager model that control the dilatancy. Promising results are found i) concerning deflection and neutral axis location measurement in bending creep tests and ii) contactless measurement of axial strain in compression creep tests, iii) to support the assumption of dilatant behaviour from bending and diametral compression tests and iv) to provide data for a rough parameter identification from compression creep tests.

Three-Dimensional Shape and Deformation Measurements Based on Fringe Projection Profilometry and Fluorescent Digital Image Correlation via a 3 Charge Coupled Device Camera.

We propose a novel hybrid FPP-DIC technique to measure an object's shape and deformation in 3D simultaneously by using a single 3CCD color camera, which captures the blue fringe patterns and red fluorescent speckles within the same image. Firstly, red fluorescent speckles were painted on the surface of the specimen. Subsequently, 12 computer-generated blue fringe patterns with a black background were projected onto the surface of the specimen using a DLP projector. Finally, both the reference and deformed images with three different frequencies and four shifted phases were captured using a 3CCD camera. This technique employed a three-chip configuration in which red-green-blue chips were discretely integrated in the 3CCD color camera sensor, rendering independent capture of RGB information possible. Measurement of out-of-plane displacement was carried out through the implementation of Fringe Projection Profilometry (FPP), whereas the in-plane displacement was evaluated using a 2D Digital Image Correlation (DIC) method by leveraging a telecentric-lens-based optical system. In comparison to the traditional FPP-DIC hybrid methodology, the present approach showed a lower incidence of crosstalk between the fringe patterns and speckle patterns while also offering a corrective for the coupling of the in-plane displacement and out-of-plane displacement. Experimental results for the in-plane cantilever beam and out-of-plane disk comparisons with the traditional 3D-DIC method indicated that the maximum discrepancy obtained between FPP-DIC and 3D-DIC was 0.7 μm and 0.034 mm with different magnifications, respectively, validating the effectiveness and precision of the novel proposed FPP-DIC method.

Effect of Plate-Bone Distance and Working Length on 2.0-mm Locking Construct Stiffness and Plate Strain in a Diaphyseal Fracture Gap Model: A Biomechanical Study.

The aim of this study was to determine the effect of plate-bone distance (PBD) and working length on 2.0-mm locking compression plate (LCP) stiffness and strain in four-point bending and torsion in a diaphyseal fracture gap model. A total of 54 LCP with three screws per fragment were assigned to one of nine combinations of working length (WL; short, medium, and long), and PBD (1, 1.5, and 3 mm) for a sample size of six per construct configuration. Stiffness was measured under quasistatic, nondestructive four-point compression bending and torsion. Plate surface strain was recorded using three-dimensional (3D) digital image correlation during four-point compression bending. WL had a significant effect on overall construct stiffness in both compression bending and in torsion, with shorter WL constructs having higher stiffness (p < 0.0001). PBD had no effect on construct stiffness in compression bending; however, a significant reduction in stiffness was noted in torsion (p = 0.047) as PBD incrementally increased. WL had a significant effect on plate strain in compression bending, with shorter WL constructs having lower plate strain (p < 0.0001). PBD had no effect on plate strain in compression bending except for lower plate strain recorded in long WL constructs with 1-mm PBD, compared with 1.5- and 3-mm PBD constructs (p < 0.0001). Longer WL constructs, regardless of PBD, had lower stiffness in compression bending, while in torsion, some modulation of this effect was noted with incremental decreases in PBD. Longer WL resulted in high plate strain, regardless of PBD.

High temperature creep model development using in-situ 3-D DIC techniques during a simulated LOCA transient

In-situ strain measurements of fuel cladding can enable high-throughput data collection and validation to support accelerated qualification of cladding materials. In this work, 3D digital image correlation was used to map strain for both Zircaloy-4 (Zry-4) and Cr-coated Zry-4 during two types of cladding rupture experiments: isobaric temperature ramp tests at 5 °C/s and isothermal pressure jump tests at 600 °C. Zry-4 strain data initially showed a temperature dependence expected for creep deformation, yet a shift to a new plastic deformation mechanism not reported in literature dominated during the finals seconds prior to rupture. Cr-coated Zry-4 did not show the change in deformation mechanism at the end of life and showed a delay in measurable creep deformation. Stress dependences were similar for Zry-4 and Cr/Zry-4 during pressure jump tests. Cr-coatings were found to decrease the strain rate during both testing scenarios. Creep parameters were calculated to support modelling efforts regarding design basis accidents.

Tracking the Observations of Volume Deformation of Concrete under Sulfate Attack through 3D Digital Image Correlation

Abstract In this study, three-dimensional digital image correlation was used to characterize the nonuniform volume deformation of concrete under sulfate attack. Based on the test and results, it was determined that the displacement distribution of the concrete exposed to sulfate attack was inhomogeneous, and the expansion of the concrete in the sodium sulfate (Na2SO4) solution was greater than that in magnesium sulfate (MgSO4). Thus, the distribution of nonuniform expansion and the mechanisms of sulfate attack were identified. Notably, sample expansion did not increase linearly with the content of hydrates such as ettringite and gypsum. The volume expansion of the concrete in the Na2SO4 solution grew slowly when the content of corrosion products was low and developed rapidly with further increases in corrosion products. The volume expansion of the concrete in the MgSO4 solution grew slowly with low corrosion product content, and then the volume expansion slowed down with the accumulation of corrosion products.

Capturing fracture initiation and crack propagation of car windshields

To facilitate the development of accurate material models for glass, there is a need for reliable experimental procedures that capture the stochastic nature of the material. The experimental setup must be easy to model numerically and output relevant data for validation of important material characteristics. We conducted an experimental study on three types of windshields exposed to quasi-static loading. A robust and repeatable testing procedure was designed with the aim of revealing the stochastic behavior of windshields. 3D-Digital Image Correlation (3D-DIC) was used to monitor the windshield displacement field and we were able to capture fracture initiation locations and crack propagation data by using high-speed cameras combined with a pose estimation-based post-processing tool implemented in Python. We performed finite element (FE) simulations in LS-DYNA to estimate major principal stress values at first fracture. The results from the experimental study provided new insight and valuable data for the development of numerical models of the problem at hand.