Use Of Digital Image Correlation Research Articles

Vibration analysis of a partially submerged structure using the DIC technique

The continuous expansion of offshore wind turbine sites has been garnering increasing attention, particularly in countries where available land is limited. Nevertheless, efficiently allocating resources for the maintenance and repair of offshore wind turbines presents a formidable challenge. This challenge is compounded by the complex and often unpredictable weather conditions, as well as various business-related issues specific to the offshore environment. To tackle these challenges, one effective solution is to implement vibration analysis through the use of digital image correlation (DIC) methods. This approach has a well-established theoretic background dating back to approximately 2008 and has demonstrated its effectiveness in the realm of structural health monitoring for large structures like bridges and towers. Relevant information on the upper structure of the wind turbines has been obtained using numerical models in our previous work. Changes in the structural feature frequencies of wind turbines are the primary focus in vibration analysis. However, changes in feature frequencies can occur for various reasons, and relying solely on frequency shifts can make it challenging to pinpoint the precise areas of damage within wind turbine structures, especially without the benefit of long-term observations. In this study, the DIC method is employed on scaled cylindrical models to gain insights into the frequency variations stemming from the erosion of non-cohesive soils beneath offshore wind turbine foundations. Changes in modal curvature within the upper structure of the wind turbine will also be analyzed. This additional investigation will enable a clear differentiation between defects in the upper structure and frequency shifts resulting from erosion or problems with the underwater foundation.

Residual Stress Determination with the Hole-Drilling Method on FDM 3D-Printed Precurved Specimen through Digital Image Correlation

The hole-drilling method (HDM) is a common technique used for the determination of residual stresses, especially for metal alloy components, though also for polymers. This technique is usually implemented with strain gages, though other methods for determining the fields of displacements are quite mature, such as the use of digital image correlation (DIC). In the present paper, this combined methodology is applied to a 3D-printed PLA precurved specimen that is flattened in order to impose a bending distribution which can be considered known with a reasonable accuracy. The back-calculated stress distribution is in agreement with the expected (imposed) bending stress, however, a converging iterative procedure for obtaining the solution is introduced and discussed in the paper.

Effect of the Energy Director Material on the Structure and Properties of Ultrasonic Welded Lap Joints of PEI Plates with CF Fabric/PEI Prepreg

To estimate the possibility of using both low-melting TecaPEI and neat PEI films as energy directors (EDs) for ultrasonic welding (USW) of carbon fiber (CF) fabric–polyetherimide (PEI) laminates, some patterns of structure formation and mechanical properties of their lap joints were investigated by varying the process parameters. The experiment was planned by the Taguchi method with the L9 orthogonal matrix. Based on the obtained results, USW parameters were optimized accounting for maintaining the structural integrity of the joined components and improving their functional characteristics. The use of the low-melting EDTecaPEI film enabled US-welding the laminates with minimal damage to the fusion zone, and the achieved lap shear strength (LSS) values of ~7.6 MPa were low. The use of EDSolverPEI excluded thermal degradation of the components as well as damage to the fusion zone, and improved LSS values to 21 MPa. With the use of digital image correlation (DIC) and computed tomography (CT) techniques, the structural factors affecting the deformation behavior of the USW lap joints were justified. A scheme was proposed that established the relationship between structural factors and the deformation response of the USW lap joints under static tension. The TecaPEI film can be used in USW procedures when very high interlayer adhesion properties are not on demand.

Precision of Measurements of Delayed Slip in Structural High-Strength Assemblies by Contact and Optical Methods

Structural bolted connections of steel plates must fulfill the requirements specified by the standard EN 1090-2:2018 in the European Union. For the slip-resistant joints, the standard establishes the experimental procedure to obtain the delayed slip under a constant loading, which should not exceed 2 μm from the first five minutes to three hours. However, the usual extensometers available in laboratories, like LVDTs, lack the necessary precision to obtain a reliable measurement of such a small displacement because, even if their resolution is 1 μm, the scatter of repeated measurements of the same distance is normally higher. In contrast, the resolution and precision in optical methods like Digital Image Correlation (DIC) can be easily set under 1 μm depending on the number of pixels covering the studied area and the performed subpixel interpolation. This work introduces the use of DIC for delayed slip tests. In comparison with an LVDT, it is verified that this technique is a valid alternative to measure displacements at that scale with a simpler experimental setup.

High temperature fracture behavior of 316L stainless steel-Inconel 718 functionally graded materials manufactured by directed energy deposition: Role of interface orientation and heat treatment

Investigation of functionally graded component consisting of stainless steel 316L (SS316L) and Inconel 718 superalloy (IN718) deposited by laser directed energy deposition (LDED) is presented here. Ambient as well as high temperature mechanical behavior is observed for as-deposited (AD) and for heat-treated (HT) states. The gradient sample was manufactured with the interface orientation parallel and perpendicular to the loading direction to understand the impact of this factor on the materials properties. The evolution of chemical composition, grain structure, phase transformations, and tensile properties at room and elevated temperatures, as well as creep behavior before and after heat treatment was determined. With use of digital image correlation (DIC) analysis, the effects of interface orientation and heat treatment on deformation and fracture mechanism was revealed. Results show that heat treatment smooths the chemical composition and mechanical deformation process, as characterized by the formation of an enhanced gradient diffusion zone in the IN718-rich region near the interface. Heat treatment impacts mechanical properties based on the orientation of the interface. Specimens with interfaces parallel to the load direction combine the strengths of both materials effectively. After HT, they exhibit enhanced mechanical performance, achieving an ultimate tensile strength of 973 MPa and 14% elongation at room temperature. Additionally, at elevated temperatures, the specimen demonstrates exceptional creep resistance, sustaining structural integrity for over 1700 h at 650 °C and 225 MPa. Sample orientation is shown a decisive effect on deformation and fracture mechanism, with the interface not consistently being the weakest point across various gradient samples.

The Contribution of Digital Image Correlation for the Knowledge, Control and Emergency Monitoring of Earth Flows

Earth flows are complex geological processes that, when interacting with linear infrastructures, have the potential to cause significant damage. Consequently, conducting comprehensive investigations and continuous monitoring are becoming paramount in designing effective mitigation measures and ensuring sustainable risk management practices. The use of digital image correlation (DIC) algorithms for the displacement monitoring of earth flows is a relatively new and growing field with a limited number of dedicated works in the literature. Within this framework, the present paper contributes by evaluating DIC techniques at several earth flow sites. These assessments were founded upon data obtained from both low-cost and multi-platform sensors, used to assess the displacement field, pinpointing the most active sectors, and gauging their rates of movement during different kinematic phases. In this regard, the highest localized movement velocities reached in the Pietrafitta (0.01 m/day) and Grillo (0.63 m/day) earth flows experience different ongoing kinematic processes. The versatility of these techniques is notable, manifesting in the ease of installation, cost-effectiveness, and adaptability to various platforms. The proven reliability in terms of maximum accuracy (1 pixel) and sensitivity (0.05 m) lays the foundation for the extensive use of the presented techniques. The amalgamation of these attributes positions DIC techniques as competitive, adaptable, and readily deployable tools in earth flow studies.

Recommendations on Use of DIC in Experimental Geotechnics

Abstract Image-based deformation measurement techniques, like digital image correlation (DIC), are used to measure the displacement of soil particles and thereby to investigate the behavior of geo-structure in experimental geotechnics. The precision of the measurement from the DIC technique depends on the position of the camera and light panels, the color temperature of light, size of the region of interest (RoI), type and relative density of sand, seeding ratio, type of camera, and strain rate used in experiments. In the paper, the effect of above-mentioned parameters is investigated experimentally in 1-g model tests. The studies show that the precision in DIC measurements is dependent on the position of camera and light panels, size of RoI, texture of the sand, and seeding ratio. However, the relative density of sand and strain rate do not significantly affect the results from the DIC analysis. The efficacy in the measurement of the large deformations and suitability of smartphone cameras using DIC are also addressed. Last, we developed recommendations to obtain precise results from DIC in experimental geotechnics.

Building blocks towards progressive fatigue damage modeling of a whole wind turbine blade

This paper presents a concise summary of the recent fatigue damage modeling research in the Mechanics of Materials and Structures (MMS) research group at Ghent University, which utilizes physics-based models and microscopic experiments to develop components for a progressive fatigue damage model of laminated composite structures. The paper discusses the evolution of ply cracking and delamination under cyclic loading and highlights distinct modeling elements for stress analysis and fatigue damage evolution characterization. It also explores the experiments required for model calibration and validation, including the use of digital image correlation to quantify damage mechanisms. Additionally, the paper discusses the use of neural networks for efficient and robust modeling of damage effects and outlines the remaining steps required to develop a comprehensive fatigue design tool. Finally, the recent developments with the BladeMesher software of the research group provide a platform to integrate this fatigue design tool for complete wind turbine blades.

Quasi-static tensile failure mechanism analysis of CFRP/Al countersunk electromagnetic riveted joints with different rivet-hole clearances

CFRP/Al countersunk riveting can meet the requirements of aircraft aerodynamic shape and lightweight joining. This paper reports an experimental study on the forming mechanism and quasi-static tensile failure of single-lap electromagnetic riveted joints with varying rivet-hole clearances. The aim is to quantitatively evaluate their deformation characteristics and mechanical properties. The failure process and fracture mechanism are revealed through the use of digital image correlation (DIC) and scanning electron microscope (SEM). The results demonstrate that the rivet-hole clearance has a significant impact on the initial damage and mechanical properties of riveted joints. The interference is not uniformly distributed in the Al sheet and the CFRP sheet. Increasing the clearance can delay the occurrence of damage of the laminate under external load. As the clearance increases, the riveted joint failure mode changes from rivet sheared off to manufactured head fracture and rivet pull-off, and the rivet fracture shows a brittle- ductile mixed fracture mode. Additionally, the initial riveting damage of the joint does not determine the final extent of damage to the laminate after failure, and the bearing damage of CFRP laminates increases with the increase in the load-carrying capacity of the joint.

Making use of Digital Image Correlation to identify the true character of the applied load

Digital Image Correlation became one of the dominant optical full-field techniques used in experimental mechanics. It does not require sophisticated equipment to evaluate the full displacement vector field of the specimen under test with great accuracy. In this paper, we would like to put the reader's attention to the DIC technique as a solution that may identify the genuine character of the load applied to the object under test. Our experience with testing wind turbine blades using DIC proves that possibility. Full Text: PDF References E.M.C Jones, M.A. Iadicola, "A good practises guide for digital image correlation" International Digital Image Correlation Society, (2018), CrossRef B. Pan, "Digital image correlation for surface deformation measurement: historical developments, recent advances and future goals", Meas. Sci. Technol. 29, 082001 (2018), CrossRef B. Pan, L.P. Yu, Q.B. Zhang, "Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement", Sci. China Technol. Sci. 61, 2 (2018), CrossRef S. Sharma, N. Iniyan Thiruselvam, S.J. Subramanian, G.S. Kumar, "Computation of strains from stereo digital image correlation using principal component analysis", Meas. Sci. Technol. 32, 105201 (2021), CrossRef IEC 61400-5:2020 Wind energy generation systems - Part 5: Wind turbine, DirectLink IEC 61400-23:2014 Wind turbines - Part 23: Full-scale structural testing of rotor blades, DirectLink

An Acoustic Device for Ultra High-Speed Quantification of Cell Strain During Cell-Microbubble Interaction.

Microbubbles utilize high-frequency oscillations under ultrasound stimulation to induce a range of therapeutic effects in cells, often through mechanical stimulation and permeabilization of cells. One of the largest challenges remaining in the field is the characterization of interactions between cells and microbubbles at therapeutically relevant frequencies. Technical limitations, such as employing sufficient frame rates and obtaining sufficient image resolution, restrict the quantification of the cell's mechanical response to oscillating microbubbles. Here, a novel methodology was developed to address many of these limitations and improve the image resolution of cell-microbubble interactions at high frame rates. A compact acoustic device was designed to house cells and microbubbles as well as a therapeutically relevant acoustic field while being compatible with a Shimadzu HPV-X camera. Cell viability tests confirmed the successful culture and proliferation of cells, and the attachment of DSPC- and cationic DSEPC-microbubbles to osteosarcoma cells was quantified. Microbubble oscillation was observed within the device at a frame rate of 5 million FPS, confirming suitable acoustic field generation and ultra high-speed image capture. High spatial resolution in these images revealed observable deformation in cells following microbubble oscillation and supported the first use of digital image correlation for strain quantification in a single cell. The novel acoustic device provided a simple, effective method for improving the spatial resolution of cell-microbubble interaction images, presenting the opportunity to develop an understanding of the mechanisms driving the therapeutic effects of oscillating microbubbles upon ultrasound exposure.

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.

Use of Digital Image Correlation in Predicting Mechanical Properties of Parts Made by Additive Manufacturing

Abstract The publication is focused on the anisotropic properties of products of additive manufacturing using a Fused Filament Fabrication technology (FFF). Five different materials used in technical practice were tested. These materials are PLA (polylactic acid), PETG/CF (polyethylene terephthalate-glycol with carbon fibre, composite material), PETG (polyethylene terephthalate-glycol), ABS (acrylonitrile butadiene styrene), and ASA (acrylic styrene acrylonitrile). Mechanical properties are verified by tensile testing according to ASTM D638. Testing is complemented by a digital image correlation method to locate the areas with the highest load transfer. The results have shown clear anisotropic properties of the test specimens. Anisotropic mechanical properties were also confirmed by digital image correlation (DIC).

High-throughput evaluation of mechanical properties of biomedical Ti alloys by digital image correlation and acoustic emission

Acoustic emission and digital image correlation (DIC) were used to determine mechanical properties of six biomedical alloys from a single compression deformation experiment. Layered sample of six Ti-(20–35)Nb-7Zr-0.2O alloys was prepared by spark plasma sintering. All six layers were deformed in compression at once. Advanced use of DIC allowed us to separate the deformation in each layer and determine the flow curve of each alloy. Simultaneous use of acoustic emission provides insight into active plastic deformation mechanisms. Nb content in the studied alloys significantly affects ductility of the material as well as shape of the compression flow curve suggesting different plastic deformation mechanisms in the studied alloys. Presented methods provide high-throughput determination of mechanical properties allowing efficient optimization of alloy composition.

Fracture mechanics behavior of coarse-grained MgO–C at room and high temperature

The fracture mechanical properties of coarse-grained MgO–C were investigated in four-point bending tests at single-edge V-notched beams (SEVNB) at room and high temperature. For measurement of the crack mouth opening displacement (CMOD) an optical system was used. Room temperature tests were accompanied with microstructural observations of the crack path with the use of digital image correlation to investigate the mechanisms of crack propagation and to determine the crack length. MgO–C showed stable crack propagation at room temperature as well as up to 1500 °C. Increased fracture toughness KIc and work of fracture were observed in the temperature range from 700 °C to 1200 °C. This behavior was correlated with crack closure, quasi-plastic deformation as well as the activation of visco-plastic creep mechanisms above 1200 °C. Additionally, the results were discussed in terms of Hasselman's thermal shock parameters.

A novel hydraulic bulge test in hot forming conditions

A novel hydraulic bulge test device was developed to evaluate high temperature biaxial stress–strain curves of quenchable boron steel sheets. The work mainly focuses on the resistance heating designed to assure a homogeneous temperature field in the dome area of the circular blank where the hydraulic pressure will be applied. The practical hot stamping conditions, including the heating and cooling steps, were reproduced to perform hydraulic bulge tests on the Usibor ®1500P steel for a temperature range between 700 to 900 °C, after an austenization step at 900 °C. Stress–strain curves were obtained from these expansion tests using the data extracted with a laser profilometer, due to the difficulties associated with the use of Digital Image Correlation at such high temperatures. Although the profilometer is a compromise solution, the comparisons between tensile and biaxial stress–strain results enable to verify the feasibility of the new device for the evaluation of the stress–strain curves at a high temperature in a biaxial state. The results point-out that, besides the difficulties with the acquisition of the strain fields and in the strain-rate control, there are also challenges in the interpretation of the metallurgical evolutions that can occur during the tests, which can affect the biaxial flow curves.

Digital image correlation through surface patterning of skins to monitor knee motion

Digital Image Correlation (DIC) systems have been used to measure surface displacement fields by tracking a pattern applied to a surface. Offering many benefits in comparison to conventional strain-measurement devices, the use of DIC has been reported in the study of tissue biomechanics and performance of joints underneath. In this study, a new method was developed to apply a quality speckle pattern on the skin of knees. Temporary tattoo paper provided a safe, easy, and quick way for pattern application to the skin without hindering its natural behavior. A pattern was developed for analyzing the knee during a squatting motion in which the field of view, system set up, and curvature of the knee were contributing factors to the development. This research is designed to prove the feasibility of using tattoo papers as patent indicators for DIC measurement. The software was able to map the entire region of the knee and measure the displacement of each area independent of other parts, thus making it easy to identify twisting and bending of the joint. Experimental results indicated that it is an effective method to accurately analyze the motion of the knee without the need of sophisticated equipment. This methodology can be used to understand how strain affects the knee during action and thus will be useful for various sports-related activities such as training and injury management.

Use of Digital Image Correlation (DIC) to Improve the Critical Strain-Annealing (CSA) Method for Producing Large Crystals: Application to A1050 Commercially Pure Aluminum

The aim of this work is to present an efficient procedure for growing large metallic single crystals that associates a classical growth technique (namely, the critical strain-annealing—CSA—method) with advanced methods of accurate full-field strain measurements based on digital image correlation (DIC) technique and of sample geometry design using finite element analysis. Measuring the critical plastic strains with an accuracy better than 0.1% resulted in a significantly improved construction of the recrystallization diagram. Applying this “DIC-assisted CSA method” to an A1050 commercially pure aluminum allowed obtaining in less than two days (26 h to 30 h) large multi-crystal samples with a half dozen of large grains. Their length between 35 mm and 100 mm was in full agreement with the obtained recrystallization diagram.

Digital image correlation and numerical analysis of CFRP-poplar timber interface subjected to modified single shear test

This paper presents the results of a digital image correlation (DIC) and finite element (FEM) study of the interface between carbon-fiber-reinforced polymers (CFRP) and poplar timber. The interfacial behavior between CFRP and poplar timber is investigated. Specimens with different bond lengths were manufactured. The DIC was applied to capture the full strain field from two views of the specimens. The complementary use of DIC and FEM allowed us to obtain the strain field in the CFRP and lateral side of the poplar, capturing the detachment between the two materials. The results were compared in terms of shear strain, shear stress, slip distribution, and failure modes, derived experimentally and numerically. Once the FEM simulations were calibrated, a good agreement with the experimental results was found, proving the relevance of the presented FEM models in predicting the behavior of the interface between the two materials tested. The proposed test set-up revealed that the entire bond length is active from the onset until the debonding, regardless of the bond length. Although an identical failure mode was obtained for all bond lengths, the final strength of the interface was found to be dependent on the bond length.

Digital image correlation of cross-ply laminates in tension to reveal microcracking

The use of Digital Image Correlation (DIC) to reveal microstructural damage in cross-ply laminates was investigated. Matrix toughness plays a key role in governing microcracking at the tow level in near-surface plies. Experiments revealed that using a tough epoxy polymer as the matrix of the laminate resulted in increased laminate moduli in the principal directions. DIC provides insights into cross-ply laminate failure; the increase in modulus is attributed to microcrack formation in transverse plies. Early onset of matrix cracking around the tows is revealed by variations in the strain along the gauge length. The use of a tough epoxy polymer delays the load at which this cracking occurs. When an untoughened epoxy polymer is used as the matrix, microcracking can be observed at the beginning of the test, suggesting processing induced damage. The use of toughened polymers as the matrix of composite laminates is recommended to mitigate against this.