Digital Image Correlation System Research Articles

Veneer-wrapped steel columns: Proof-of-concept experimental and numerical investigations

This paper presents and discusses the structural behaviour of a novel type of steel–timber composite column to be used in residential building applications. It consists of a circular hollow section wrapped by multiple beech veneer sheets and where the composite action is ensured by a structural bi-component glue applied over the contact surfaces. As part of a proof-of-concept study, five short and three long columns with lengths of 0.27 m and 2.00 m, respectively, were tested under vertical concentric load with fibres parallel to the load direction and inclined by 15°. The global response of the specimens in terms of load-displacement behaviour and failure modes was studied, along with localised strain measurements through strain gauges and a digital image correlation (DIC) system. Results showed a significant increase in buckling resistance due to the stiffness added by the veneer layers. Already with the still-moderate veneer layer thicknesses used in this study, an increase in strength of over 25 % could thus be achieved. This enhancement highlights the effectiveness of veneer wrapping in stabilizing the inner core and delaying the occurrence of flexural buckling failure. A 3D non-linear finite element model (FEM) validated these findings, showing a good correlation with experimental results in resistance and stiffness. A parametric study was conducted to explore different geometries and materials, thus creating an extended FEM-based database. A relatively good agreement with modest deviation between the FEM predictions and analytical formulations available in the European Standards was found, considering the appropriate introduction of the mechanical properties of timber.

Clustering classifier of FRP strengthened concrete beams using superpixels and principal component analysis

Nowadays, the application of Fiber Reinforced Polymer (FRP) materials in concrete structures is more and more extended because of their advantages. However, premature and brittle failure caused by debonding of FRP materials from the concrete surface may be critical and needs to be detected at its earliest stages to avoid possible catastrophic events. Digital image correlation (DIC) has been used in the past for the detection of abnormalities from surface digital images. In this work, an efficient computer-aided diagnosis (CAD) system for damage identification for this type of FRP strengthening is developed. It is based on the combination of cells of strain maps built with a DIC system and an automated clustering classifier optimized with the use of superpixels and principal component analysis. Additionally, preprocessing of the images is also done with some filters to enhance the accuracy of the methodology. The procedure has been validated with experimental results on a progressively damaged FRP externally strengthened beam. Results show that the proposed detection scheme achieves an effective automated classification method for the damage detection of this kind of strengthened specimens. Furthermore, the optimal choice of the subset size, number of superpixels and principal components contributed to the success of the method. Simultaneously, an attempt to quantify damage from the proposed method and its validation with electromechanical impedance method has been developed.

Load transfer behavior of 3D printed concrete formwork for ribbed slabs under eccentric axial loads

3D printed concrete (3DPC) has primarily been used for non-structural applications, with limited exploration into its potential for structural load-bearing applications. This is mainly due to the layered structure of 3DPC and the lack of compatibility of conventional reinforcement strategies to be integrated within the 3D concrete printing process. The post-tensioning of 3DPC structures presents an effective solution to improve the load-carrying capacity and cracking behavior of 3DPC. However, understanding the load transfer behavior and failure modes of 3DPC structures under a particular post-tensioning configuration are crucial to determine the permissible post-tensioning load for a structure without premature failure and to understand the distribution of stresses within the concrete structure under post-tensioning loads. The current study aims to investigate the load transfer behavior and failure mode of a 30 mm thick 3DPC formwork designed for one-way ribbed slabs when subjected to end-anchorage post-tensioning. For this purpose, a series of mechanical characterization and load transfer experiments were conducted on ribbed 3DPC formwork. The mechanical characterization investigations involved examining the compressive and flexural strength, as well as the elastic modulus of 3DPC, under various loading orientations relative to the print path. In the load transfer experiments, the end anchorage post-tensioning system was simulated by applying the axial load to the specimen via end plates bonded to the specimen. The variable parameters of the load transfer experiments were the boundary conditions, the eccentricity of the axial load from the neutral axis, and the topology of the ribbed 3DPC formwork. A digital image correlation system was used to study the axial and transverse strain evolution during the load transfer experiments. The experimental results showed that, regardless of the eccentricity of the applied load, the ribbed 3DPC formwork specimens exhibited higher axial load capacity when the load was applied near the bottom flange rather than the top flange. This was because of the reduced effective cross-sectional area for compression when the load was positioned near the top flanges, a consequence of the selected formwork topology, where top flanges were discontinuous to allow for pouring of the cast concrete within the formwork.

Identification of damage in timber I-beams using continuous wavelet transform of deflection measured with digital image correlation

The article presents a method for detecting damage in timber I-beams based on the continuous wavelet transform (CWT) of their static deflection, measured using the ARAMIS digital image correlation (DIC) system. So far, no detailed uncertainty analysis for this type of identification has been carried out. Filling this gap was the main motivation for the authors to conduct this research. For this purpose, analytical and experimental static deflection CWT analyses were performed for the timber I-beams with softwood flanges and oriented strand board webs, in which local cross-sectional weakenings were artificially introduced for verification purposes. A detailed analysis of the CWT uncertainty resulting from measurement errors and numerical data processing allowed its expression as a function of the beam axis position and the scale parameter. Based on the research, a new methodology was formulated for the diagnostic defect identification in timber beams related to bending stiffness reduction.

Bond-slip behavior of Aluminum Alloy (AA) bars for near-surface mounted (NSM) technique

An experimental study consisting of 22 pull-out tests was carried out to investigate the bond-slip performance and load transfer mechanism between near-surface mounted (NSM) Aluminum Alloy (AA) bars and concrete when the AA bars are inserted into pre-cut grooves on the surface of the host structure and bonded with an appropriate bonding agent. The effects of several design parameters were evaluated including bond length, diameter of AA bars, bar surface treatment, and adhesive type. A digital image correlation (DIC) system was used to measure the slips and surface strain and four failure modes were observed in the tests, being concrete splitting failure, epoxy splitting, debonding at the bar-epoxy interface, and AA bar rupture. Results showed that an increase in bond length could make the failure mode more ductile, while AA bars with rough surfaces exhibited a relatively satisfactory bond-slip behavior when a suitable epoxy was used. Finally, the test data from this study were used to examine three bond-slip models based on fiber reinforced polymers (FRP) for the NSM technique. A new bond-slip model considering the influence of the bar external surface was proposed and validated by the test results from both this study and literature.

Short humeral stem in total shoulder arthroplasty does not jeopardize primary implant stability

BackgroundThe trend of the modern humeral components in total shoulder arthroplasty (TSA) is towards shorter and shorter humeral stems. However, the question remains whether short uncemented stems can provide the same implant stability as long stems. This study aimed to evaluate and compare the torsional primary stability, and the pull-out extraction force of both a long and a short version of the same stem. Materials and MethodsTen humeral components (five long-stem and five short-stem) were press-fitted into ten synthetic composite humeri. A torsional load was applied to generate the most critical loading condition. The specimens were loaded with 100 cycles between 2 Nm and 10 Nm, at 1 Hz. A 3D Digital Image Correlation system was used to measure the relative displacement between the prosthesis and the host bone during the test. After completing the torsional test, the pull-out force was measured. Differences between the long and short-stem on the biomechanical parameters (permanent migrations, inducible micromotion, and extraction force) were tested with the non-parametric Mann-Whitney test (p<0.05). ResultsThe main rotational inducible micromotion was around the craniocaudal axis. No significant differences were found between the rotational permanent migrations of the long- and short-stem around the craniocaudal (p=0.421), anteroposterior (p=0.841), and mediolateral axes (p=0.452). No significant differences were found between the rotational inducible micromotions of the long- and short-stem around the craniocaudal (p=0.222), anteroposterior (p=0.420), and mediolateral axes (p=0.655). No significant differences were found between the permanent translations of the long- and short-stem the along craniocaudal (p=0.341), anteroposterior (p=0.420), and mediolateral (p=0.429) directions. No significant differences were found between the translations of the long- and short-stem in terms of inducible translation in the craniocaudal (p=0.547), anteroposterior (p=0.999), and mediolateral axis (p=0.285). Similar extraction force (p=0.35) was found. Discussion and ConclusionNo statistically significant difference was found between the long-stem and short-stem implants. These results show that short uncemented stems can provide adequate primary mechanical stability. As the long-stem version of this stem is already clinically used, the present findings suggest that the short-version can be reasonably expected to deliver similar outcomes in terms of implant stability.

Curing deformation compensation in bonding assembly for high-precision linear displacement encoder sensors

Abstract Linear displacement encoder sensors extensively appear in high-precision instruments, in which the core scale is usually bonded with the scale base by adhesives. However, the curing shrinkage of the adhesives often causes a warping deformation of the moving ruler, which in turn leads to a decrease in measurement accuracy. To improve the accuracy of the bonding assembly, this paper proposes a novel deformation compensation method. A force control fixture is first designed, and a tensile force is applied to tense the scale base. The scale is then bonded to the scale base, and the tensile force is maintained until the adhesive is cured. The optimal tensile force is determined to minimize the warping deformation by combining a finite-element simulation and an artificial fish swarm algorithm. The optimal result is verified experimentally, where the warping deformation is measured using a homemade three-dimensional digital image correlation system. The experimental results demonstrate that the warping deformation of the moving ruler is reduced by 89.2% using the proposed method.

A biomimetic chiral auxetic vertebral meta-shell

Abstract The work presents a novel thin-walled biomimetic auxetic meta-shell for patient-specific vertebral orthopedic implants. The proposed design stemmed from the concept of an intrinsically multiple curved auxetic meta-structure, which is created by folding a 2D bio-inspired chiral geometry according to the morphology of human vertebral cortical bones. Through a multi-view stereo Digital Image Correlation (DIC) system, we investigated the mechanical response of a bio-grade titanium (Ti6Al4V ELI) additively manufactured prototype of the meta-structure under compressive loadings. In addition, we analyzed the morphology of the prototype using a scanning electron microscopy (SEM) and an optical image dimension measurement system both before and after compressive tests. An accurate Finite Element model, which exactly reproduced the geometry of the 3D printed meta-shell, was implemented and calibrated against experimental results, obtaining a precise prediction tool of its mechanical response. The findings of this work demonstrate that the designed meta-shell shows a peculiar auxetic behavior, a targeted stiffness matching to that of human vertebral bone tissues and a higher global elastic strain capability compared to those of monolithic traditional vertebral body replacements.&amp;#xD;&amp;#xD;

A novel methodology to determine tensile properties in the heat-affected zone of tailor welded blanks with different base material thicknesses

When testing the mechanical properties of a tailor welded blank (TWB) under uniaxial tension, the localized bending moment generated by eccentric loading, due to the change in thickness between the welded sheets, causes the experimentally measured stress-strain behavior to not match the actual stress-strain behavior. In this paper, a new method to calibrate the tensile stress-strain behavior in the heat affected zone (HAZ) of a TWB with different thicknesses is proposed based on indentation testing and tensile test with multi-camera digital image correlation (DIC). The nominal stress-strain curve is calculated for calibration purposes. The nominal strain is obtained by performing multiple linear regression analysis on the front and back surface strains of the HAZ provided by the multi-camera DIC system. Experiments were conducted using this method on a TWB of the same material (DP1180) and different thicknesses (1 mm and 1.4 mm). The validation of the corrected stress-strain curves demonstrated the reliability and effectiveness of the proposed method.

Shear Behavior and Modeling of Short Glass Fiber- and Talc-Filled Recycled Polypropylene Composites at Different Operating Temperatures

The present paper aims to broaden the field of application of the phenomenological model proposed by the authors in a previous study (ICP model) and to assess the shear properties of a recycled 30 wt.% talc-filled polypropylene (TFPP) and a recycled 30 wt.% short glass fiber-reinforced polypropylene (SGFPP), used in the automotive industry. The materials were produced by injection molding employing post-industrial mechanical shredding of recycled materials. In particular, Iosipescu shear tests adopting the American Standard for Testing Materials (ASTM D5379) at three different operating temperatures (−40, 23 and 85 °C) were performed. The strain was acquired using a Digital Image Correlation (DIC) system to determine the map of the strain in the area of interest before failure. Lower operating temperatures led to higher shear chord moduli and higher strengths. Recycled SGFPP material showed higher mechanical properties and smaller strains at failure with respect to recycled TFPP. Finally, the ICP model also proved to be suitable and accurate for the prediction of the shear behavior of 30 wt.% SGFPP and 30 wt.% TFPP across different operating temperatures.

Low-cost two-dimensional digital image correlation system

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

Accuracy of digital image correlation system with telecentric lens for compression tests of wood

Accuracy of digital image correlation system with telecentric lens for compression tests of wood

Experimental and numerical investigation on failure behaviour of aluminum-polymer friction stir composite joints

Friction stir composite joints between an aluminum alloy (AA6082-T6) and reinforced polymer (NorylTM GFN2) were studied regarding their failure characteristics. A comprehensive analysis on the mechanical behaviour of the joints was carried out, assessing the resulting deformed shape, the associated strain field and the observed failure modes using a digital image correlation (DIC) system and comparing with a finite elements method (FEM) model. The numerical model evidenced a good fitness to the experimental results since both the displacements and strain fields were found to be very similar. Both numerical and experimental strain fields in the longitudinal direction exhibited a localized spot of highly strained material, clearly indicating the nucleation site. Since secondary bending occurs in structural elements under tension due to geometrical and stiffness asymmetries, as is the case of overlap composite joints, a bending stress is induced, increasing the local stress compared to the nominal tension stress values. The ultimate tensile stress of the joints was found to range between 60.2 and 70.4 MPa, leading to an average joint efficiency of 82.6 %. The bending factor (kb) at the failure region was found to be 2.0, which means that the bending stress accounts for 2/3 of the total stress, acting as main failure catalyst.

Development of a mechanical characterisation device for intracranial aneurysms: Calibration on polymeric phantom arteries

Intracranial aneurysm is a major health issue related to biomechanical arterial wall degradation. Currently, no method allows predicting rupture risk based on in vivo quantitative mechanical data. This work is part of a large-scale project aimed at providing clinicians with a non-invasive patient-specific decision support tool, based on the in vivo mechanical characterisation of the aneurysm wall. Thus, the primary objective of the project was to develop a deformation device prototype (DDP) of the artery wall and to calibrate it on polymeric phantom arteries. The deformations induced on the phantom arteries were quantified experimentally using a Digital Image Correlation (DIC) system. The results indicated that the DIC system was able to measure the small displacements generated by the DDP. We also observed that the flow mimicking the blood flow did not significantly disturb the measurements of the artery wall displacement caused by the DDP. Finally, a limit displacement value generated by the DDP was evaluated. This value corresponds to the lowest displacement value detectable by the clinical imaging system that will be tested on animals in the future (Spectral Photon Counting CT).

Strength and Fracture Toughness of TIG- and Laser-Welded Joints of Low Carbon Ferritic Steels.

This paper presents the results of experimental testing of joints welded using conventional TIG and laser methods. The welded components were sheets of the low-carbon steels 13CrMo4-5 and 16Mo3. Welded joints were made using different levels of linear welding energy. In the case of laser welding, a bifocal beam with longitudinal positioning of the focal lengths in relation to the welding direction was used. Experimental tests on welded joints included a bending test and determination of hardness distribution, mechanical properties, and fracture toughness, as well as microstructural research in the material of the various joint zones. Based on the determined strength characteristics, the true stress-strain relationships were defined, and a numerical model of the laser joints was developed in Abaqus 6.12-3. The modelled joint was subjected to loading to determine the most stressed areas of the joints. The numerical results were compared with those obtained using GOM's Aramis 3D 5M digital image correlation system. The system used made it possible to record displacements on the surface of the analysed joints in real time. Good agreement was obtained between the strain fields calculated numerically and those recorded using the Aramis 3D 5M video system. The numerical calculations provided information on the strains and stresses occurring inside the analysed joint during loading. It was found that the welded joints were characterised by increased hardness and high strength properties in relation to the base material. The bending test of the laser-welded joints gave a positive result-no cracks were observed on the face or root of the weld. The fracture toughness of the joint zones is slightly lower in relation to that of the base material, but no brittle fracture was observed.

An Investigation of Tensile, Fatigue, and Fracture Behavior of 3D-Printed Polymers

Abstract The progression of manufacturing technology has significantly benefited from the adoption of 3D printing techniques, which enable the production of parts with intricate geometries. However, it is important to acknowledge that components fabricated through this additive manufacturing method frequently manifest defects and are prone to failure under severe conditions. Therefore, a thorough examination of the mechanical properties of these parts is essential to effectively reduce the failure. This study aimed to explore the mechanical properties of two prevalently used 3D-printed polymers, specifically Onyx and acrylonitrile butadiene styrene (ABS), by integrating computational and experimental analyses. The experimental study utilized a material testing system and digital image correlation (DIC) technology, while the computational analysis covered the finite element (FE) modeling of the 3D-printed samples. The research focused on evaluating the tensile strength and fatigue resistance of the specimens printed in various orientations, alongside a detailed investigation of their fracture behavior. The crack propagation analysis was carried out using the DIC system and the separating morphing and adaptive re-meshing technology (SMART) scheme in ansys. It was observed that upright build orientation produced the weakest samples for axial loading and specimens with notches failed earlier than those without. Moreover, Onyx was found to have a higher resistance to fracture or failure compared to ABS. The FE modeling results demonstrated strong agreement with the experimental results, validating their accuracy and reliability in characterizing the critical mechanical response of 3D-printed parts rapidly and cost effectively.

Method for computer tomography voxel-based finite element analysis and validation with digital image correlation system

Understanding the mechanical behavior of heterogeneous materials is becoming increasingly crucial across various fields, including aerospace engineering, composite materials development, geology, and biomechanics. While substantial literature exists on this topic, conventional methods often rely on commercial software packages. This study presents a framework for computed tomography (CT) scan-based finite element (FE) analysis of such materials using open-source software in most of the workflow.Our work focuses on three key aspects:1.Mesh generation that incorporates spatially varying mechanical properties and well-defined boundary conditions.2.Validation of the FE results through comparison with digital image correlation (DIC) system measurements.3.Open-source software utilization throughout the entire process, making it more accessible and cost-effective.This work aims to demonstrate the effectiveness of this framework for analyzing heterogeneous materials in various fields, offering a more accessible and affordable approach.

Cyclic testing and diagonal strut modelling of different types of masonry infills in reinforced concrete frames designed for modern codes

This article presents an experimental study on cyclic testing of full-scale reinforced concrete (RC) frames designed for modern codes with three different masonry infills: burnt clay brick, fly ash brick, and autoclaved aerated concrete (AAC) block masonry. A digital image correlation (DIC) system was used to study, in detail, the failure pattern and the complex interaction between the infill and the surrounding frame. The experimental results were compared with the pre-existing analytical models for strength and stiffness to determine the best-fit model. The experimental results indicated that the masonry-infilled frames had higher strength, stiffness, and energy dissipation capacity compared to the bare frame, and brittle modes of failure were avoided due to the design for modern codes. The masonry infill altered the RC frame’s deformation pattern by constraining the end portions of the columns from moving laterally, resulting in widespread flexure cracks throughout the column’s height, unlike the bare frame, where cracking was limited to the end portions. The modification was minimal in the case of AAC block masonry infill. The AAC block infilled frame reached different damage states at lower drifts than the other two infill types. The strength and stiffness, separately, were found to be in good agreement with some of the existing models; however, none of the considered models could predict both stiffness and strength accurately.

Efficiency of FRPU strengthening of a damaged masonry infill wall under in-plane cyclic shear loading and elevated temperatures

This paper presents results of in-plane shear tests carried out at the ZAG laboratory in Ljubljana (Slovenia) on a RC frame with masonry infill made of clay blocks (KEBE OrthoBlock). The frame was loaded with constant vertical loads at the top of the columns and then by gradually increasing horizontal cyclic loads at the top beam level. Acquired forces and measured displacements allowed capturing hysteretic behavior for determination of dissipation energy. In addition, two Digital Image Correlation (DIC) systems, Aramis and the CivEng Vision, were used to visualize the behavior of the tested specimens, with an emphasis on computing locally required information about the behavior of highly deformable interfaces. Three types of specimens were tested in-plane: the reference specimen in form of plain RC frame, the reference specimen with constructed masonry infill without any strengthening and the specimen, previously damaged and then strengthened on both sides using glass mesh bonded to the infill and the RC frame using flexible adhesive made of polyurethane matrix (Glass Fiber Reinforced PolyUrethane - GFRPU system). The strengthening process, allowed the specimen to withstand additional cyclic loads, reaching a maximum drift of 3.6 % without serious damage disqualifying the structure from further exploitation. The GFRPU strengthening system was found to be highly effective in preventing infill collapse of damaged masonry infill wall during in-plane loading. Additionally, the results of extended thermal analysis of PU are presented as polymers are, in general, a material, poorly resistant to heat. However, the analyzed PU manifested stable properties up to 200 degrees Celsius, which makes this material promising in civil engineering applications at elevated temperatures.

Evaluation of the errors of measuring residual stresses by the hole drilling method

A sample configuration with a specified distribution of residual stresses in depth was developed to elaborate modes for studying residual stresses using mechanical and physical methods. The sample formation technique is based on non-uniform plastic deformation of an aluminum beam of rectangular cross section according to the pure bending scheme. The control of the deformed state during loading was performed on the end surface of the sample by the field of normal deformations obtained using a digital image correlation system. The depth of the plastically deformed layer was 1.3 mm. The theoretical distribution of residual stresses obtained as a result of unloading of a plastically deformed sample was determined from the results of a numerical calculation of a finite element model, with allowance for the physical and mechanical characteristics, elastic-plastic hardening, as well as the deformation curve in true coordinates obtained as a result of uniaxial tension on elementary samples. A study of residual stresses inhomogeneous in depth was carried out by drilling holes in accordance with ASTM E837 on two opposite sides of the sample: the region of tension under loading and the area of compression, respectively. The control of the deformation response resulted from drilling was recorded using three-axis strain gauge rosettes. The results of the actual measurement of the longitudinal component of residual stresses were compared with their theoretical distribution obtained by a numerical method. The root-mean-square error in measuring residual stresses, relative to their theoretical distribution, for an aluminum alloy sample reaches 18.7 MPa. It should be noted that largest measurement errors were recorded at small depths, since they are characterized by small values of deformations comparable to the value of shot noise.