Three-dimensional Digital Image Correlation Research Articles

An optical-based technique to obtain operating deflection shapes of structures with complex geometries

Abstract The dynamic characteristics of structures are conventionally obtained by exciting the structure using an impulse hammer or a mechanical shaker and measuring the response using uniaxial or multi-axial accelerometers. However, contact-based sensors can mass-load the structure and do not provide full-field data. Hence, obtaining the true dynamics of the structure using conventional sensors can be challenging. That makes test engineers seek different non-contact techniques that can provide full-field data without mass-loading the structure. Recently, stereo-photogrammetry and three-dimensional digital image correlation (3D DIC) have been adopted to collect operating data for structural analysis. These non-contact optical techniques provide a wealth of distributed data over the entire structure. However, the stereo-camera system is limited by the field-of-view of the cameras; a single pair of DIC cameras may not be able to provide deformation data for the entire structure. Hence, it is challenging to obtain the vibration characteristics of the entire structure. In the current work, a multi-view 3D DIC approach is used and validated to predict the vibrational characteristics of an automotive muffler with a complex structure. A pair of DIC cameras travels over the entire structure to capture the deformation of each field of view. The measured data includes the geometry and displacement data, which is later mapped into a global coordinate system. The measured data in the time domain for each field-of-view is transformed to the frequency domain to extract the operational deflection shapes and resonant frequencies for each field of view. The obtained deflection shapes are stitched together in the frequency domain to extract the operating deflection shapes and resonant frequencies of the automotive body panel with a complex structure.

Digital image correlation method for measuring deformations of vinyl chloride-coated metal multilayer sheets

A three-dimensional (3D) digital image correlation (DIC) method is presented for measuring the deformations of vinyl chloride-coated metal (VCM) multilayer sheets and their composites. The calculations and the principle of strain and deformation measurements using the DIC method are described. A VCM multilayer sheet consists of a substrate [steel plate cold commercial (SPCC) and steel plate cold elongation (SPCE)] and a clad (a VCM film). The corresponding deformations of VCM deep-drawing multilayer sheets (SPCE as a substrate and a VCM film as a clad), VCM nondeep-drawing multilayer sheets (SPCC as a substrate and a VCM film as a clad), nondeep-drawing substrates (SPCC), deep-drawing substrates (SPCE) and clads (VCM films) were captured along the x- and y-directions in uniaxial tension experiments and using the DIC method. The maximal measured strains along the x-direction for the VCM deep-drawing multilayer sheets, VCM nondeep-drawing multilayer sheets, nondeep-drawing substrates, deep-drawing substrates and clads were, respectively, 637.835%, 132.210%, 31.688.632%, 107.102%, and 118.937%. The maximal measured strains along the [Formula: see text]-direction were 739.028%, −11.174%, −9.678%, −13.273% and 12.120%, respectively. These data show that the mechanical properties of VCM multilayer sheets are better than those of their substrates and clads. The effectiveness and accuracy of the presented DIC method for VCM multilayer sheet measurements were confirmed in a series of experiments.

Experimental testing of the tensile elastic properties of cellulose braided composites

Braided composites are a class of composite materials that feature an inter-woven structure that improves structural stability and damage tolerance. Presently, braided composites under tension and torsion loading have been studied individually. Mechanical behaviour of braided composites under combined tension–torsion loading is common and therefore requires investigation. In this study, mechanical properties of carbon fibre, fibreglass and aramid 2D tubular braided composites (TBCs) were assessed and compared under coupled tension–torsion loading. The plane stress theory investigated the failure mechanism of braids. A contact-free three-dimensional digital image correlation (3D DIC) technique was used to derive detailed and continuous strain maps and understand the buckling behaviour of TBCs.

Effect of Prolonged Ultraviolet Radiation Exposure on the Blast Response of Fiber Reinforced Composite Plates

An experimental study with corresponding numerical simulations was conducted to evaluate the blast response of composite plates after prolonged exposure to ultraviolet radiation. Two composite materials were used in this study, namely carbon fiber/epoxy and glass fiber/vinyl ester. The composite materials consisted of four unidirectional fiber plies oriented in a [± 45°]s layup. The plates were placed in a QUV Accelerated Weathering Tester for 500 h of exposure on each face, totaling 1000 h of ultraviolet radiation exposure, equivalent to 2.5 MJ/m2 of energy. The QUV Accelerated Weathering Tester was equipped with eight UVA-340 lamps to provide real service life exposure conditions. Material characterization experiments were performed on virgin specimens, as well as specimens exposed to ultraviolet radiation for 1000 h to determine its effect on the mechanical properties of the composites. Blast experiments were conducted on the composite plates to investigate the dynamic response before and after exposure to ultraviolet radiation. The plates were clamped on all edges and subjected to an air blast using a shock tube apparatus. Three-dimensional digital image correlation was coupled with high-speed photography to obtain full-field displacements of the specimens during blast loading. Furthermore, three piezoelectric pressure transducers were mounted on the shock tube apparatus and utilized to measure the pressure history of the shock wave loading. The quasi-static material characterization showed that the tensile modulus E1 had minimal change after exposure to ultraviolet radiation for 1000 h. However, matrix-dominated properties such as the in-plane shear modulus G12 increased after exposure to ultraviolet light for both the carbon and glass fiber composites, which resulted in lower out-of-plane displacements during blast loading. For the carbon fiber plates, the forced vibration frequency induced by the transverse blast load was higher in plates exposed to ultraviolet radiation when compared to virgin specimens. However, for the glass fiber plates exposed to ultraviolet radiation, the forced vibration frequency decreased when compared to virgin specimens. Finally, the finite element simulations were in good agreement with the experimental results. The simulations revealed that the glass fiber plates exposed to ultraviolet radiation had higher failure strains at the boundary than virgin specimens, despite having higher stiffness. A parametric study in which higher loading was simulated also showed that the glass fiber plates experienced more damage after 1000 h of ultraviolet radiation exposure.

Dual-biprism-based digital image correlation for defect detection of pipelines

A dual-biprism-based stereo camera system, with an imaging model derived from geometrical optics analysis, is proposed for detection of defects on the external and internal surfaces of pipelines. The proposed system, which comprises two biprisms, a lens, a single camera, and common light sources, overcomes the difference in the camera specifications and synchronization mechanism of conventional two-camera systems. Moreover, different fields of views can be obtained by adjusting the angles of the two prisms and the distance between the dual-biprism and the camera. Measurements can be made even for regions in narrow and small areas using a coaxial illumination and speckle-based three-dimensional digital image correlation method. After a typical experiment performed to verify the efficacy of the proposed system, three different types of defects were accurately detected: (1) a local indentation on the external pipe wall; (2) a weld defect in the pipeline that was measured, and the shape and location retrieved after image processing; and (3) a crack on the internal surface of the pipe determined based on strain calculation. These results validate the proposed system as an efficient and convenient tool for defect detection.

A Framework for Measuring the Time-Varying Shape and Full-Field Deformation of Residual Limbs Using 3-D Digital Image Correlation.

This study proposes a novel framework for the measurement and analysis of residual limb shape and deformation, using a high-resolution and low-cost system. A multi-camera system was designed to capture sets of simultaneous images of the entire residuum surface. The images were analyzed using a specially developed open-source three-dimensional digital image correlation (3D-DIC) toolbox, to obtain the accurate time-varying shapes as well as the full-field deformation and strain maps on the residuum skin surface. Measurements on a transtibial amputee residuum were obtained during knee flexions, muscle contractions, and swelling upon socket removal. It was demonstrated that 3D-DIC can be employed to quantify with high resolution time-varying residuum shapes, deformations, and strains. Additionally, the enclosed volumes and cross-sectional areas were computed and analyzed. This novel low-cost framework provides a promising solution for the in vivo evaluation of residuum shapes and strains, as well as has the potential for characterizing the mechanical properties of the underlying soft tissues. These data may be used to inform data-driven computational algorithms for the design of prosthetic sockets, as well as of other wearable technologies mechanically interfacing with the skin.

Error estimation of 3D reconstruction in 3D digital image correlation

Three-dimensional digital image correlation (3D-DIC) is widely used in various measurement fields. Once a DIC setup is established, different parameters, such as lens focal length, pixel size, stereo angle, and working distance, are easy to know or measure; the range of the matching error is generally 0.01–0.02 pixels. It would be very convenient if the measurement error could be estimated based on the parameters above without performing the entire DIC calculation process. In this study, a numerical simulation was conducted to study the variation of the 3D reconstruction error with the stereo angle in different measurement areas. It was found that the error of the center point was closely related to the error of the full field. Therefore, using simple parameters to describe the 3D reconstruction error of the center point is of great significance for estimating the overall error of a DIC system. The general solution of the 3D reconstruction equation was simplified on the premise that high order terms of pixel coordinates could be omitted. The derived error estimation formulas had a simple form and described the 3D reconstruction error of the center point in three directions. Through a set of static experiments, the 3D reconstruction error of a planar object under different stereo angles (from 10° to 120°) was studied. The matching error was calculated by a 2D-DIC software and inserted into the proposed formulas to obtain the estimated error. The estimated error was compared with the results of a 3D-DIC software, and the effectiveness of the proposed method was verified.

Damage and failure mechanism of thin composite laminates under low-velocity impact and compression-after-impact loading conditions

Impact resistance and damage tolerance are of great significance in the design of composite structures. This study investigated the damage and failure mechanism of thin composite laminates under low-velocity impact and compression-after-impact (CAI) loading conditions. Four levels of impact energy were included in the test matrix. Delamination induced by low-velocity impact was captured using ultrasonic C-scan, and a three-dimensional (3D) digital image correlation (DIC) system was employed to measure full-field displacement during the CAI tests. Infrared thermography was also used to online monitor the thermal field variation of the test specimen during the impact and CAI process. The cross sections of typical tested specimens were inspected using an optical microscope and a scanning electron microscope (SEM). A 3D damage model that considers both interlaminar and intralaminar damage was proposed to study the complex damage and failure mechanism. Excellent correlation was obtained between the experimental results and the numerical results. The experimental results obtained from various tests and the results from the numerical simulation were combined to provide a new and deep insight of damage evolution and failure mechanisms under low-velocity impact and CAI loading conditions.

Experimental and numerical studies on the behaviour of fibre-reinforced composites subjected to soft impact loading

This paper presents detailed experimental and numerical studies on the behaviour of composite laminates subject to impact loading by soft objectives. In this research, gas-gun experiments employ woven carbon fibre reinforced poly (ether-ether ketone) (CF/PEEK) and gelatine projectiles to study the response of composite laminates under soft impact loading. A high-speed camera is employed to capture the deformation of the projectiles and a three-dimensional (3D) Digital Image Correlation (DIC) system is used to record the deformation of the impacted composite specimens. A numerical finite-element (FE) model, in which Hashin’s theory is employed for composite modelling and Smooth Particle Hydrodynamics (SPH) method is used for modelling the gelatine projectile and is developed to simulate the soft impact on the composite laminates. The capability of the developed model is validated using the gas-gun experimental results. Good agreement has been shown between the predictions from using the FE model and the experimental results.

Progressive failure behaviors and crack evolution of rocks under triaxial compression by 3D digital image correlation

In the study, a series of monotonic triaxial compression tests were carried out on Emochi andesite to evaluate the effects of confining pressure on progressive failure behaviors. The three-dimensional digital image correlation (3D-DIC) system consisting of six cameras was used to capture the images based on the transparent pressure cell. The effects of confining pressures on stress thresholds, energy evolution, field strain patterns and crack evolution were investigated qualitatively or quantitatively. The crack initiation (σci), crack damage (σcd) and peaks stress (σc) increased with confining pressure and the ratios σci/σc and σcd/σc respectively varied from 57.74% and 75.11% to 64.02% and 85.33%. Total energy U, elastic energy Ue, and dissipation energy Ud at different characteristic points increased with confining pressures. Field strain patterns indicated that localization developed rapidly in the post-peak region due to the acceleration of crack propagation and damage evolution. The failure mechanisms of Emochi andesite under the uniaxial and triaxial condition are respectively tension-shear destruction and shear destruction. Furthermore, the stress-strain response inside and outside the localization zone are characterized by strain accumulation or inelastic unloading. Finally, a method of confirming the stress level of localization initiation is proposed based on the DIC technique. The stress level decreases with confining pressure. The study provides a useful reference for understanding the failure process of rocks and improving the stability and safety of underground structures.

Three-dimensional digital image correlation method based on a light field camera

With the recent development of computer vision and photographic technology, many three-dimensional (3D) modelling methods have emerged. This paper introduces a new method that combines digital image correlation (DIC) with a light field camera to reconstruct the surface of an object. Before experimentation, the surface of the object was sprayed with a speckle pattern. The parameters of the light field camera, such as the micro-lens, focus, and lens distortion, were pre-calibrated. During the experiments, raw images of the samples were recorded by a light field camera, then each was extracted to two sub-aperture images. Using the digital image correlation method, feature point matching of the corresponding sub-images was executed and the 3D space coordinates of all of the matching points were calculated using the binocular vision principle with surface topography information. The experimental results demonstrate that the method can realise 3D reshape and 3D deformation measurements using a single camera. The system has the advantage of a simple structure, large depth of field, and wide measurement range for out-of-plane deformation. The light field camera is also applied to 3D-DIC.

Full-field analysis of epicardial strain in an in vitro porcine heart platform

The quantitative assessment of cardiac strain is increasingly performed to provide valuable insights on heart function. Currently, the most frequently used technique in the clinic is ultrasound-based speckle tracking echocardiography (STE). However, verification and validation of this modality are still under investigation and further reference measurements are required to support this activity.The aim of this work was to enable these reference measurements using a dynamic beating heart simulator to ensure reproducible, controlled, and realistic haemodynamic conditions and to validate the reliability of optical-based three-dimensional digital image correlation (3D-DIC) for a dynamic full-field analysis of epicardial strain.Specifically, performance assessment of 3D-DIC was carried out by evaluating the accuracy and repeatability of the strain measurements across multiple cardiac cycles in a single heart and between five hearts. Moreover, the ability of this optical method to differentiate strain variations when different haemodynamic conditions were imposed in the same heart was examined.Strain measurements were successfully accomplished in a region of the lateral left ventricle surface. Results were highly repeatable over heartbeats and across hearts (intraclass correlation coefficient = 0.99), whilst strain magnitude was significantly different between hearts, due to change in anatomy and wall thickness. Within an individual heart, strain variations between different haemodynamic scenarios were greater than the estimated error of the measurement technique.This study demonstrated the feasibility of applying 3D-DIC in a dynamic passive heart simulator. Most importantly, non-contact measurements were obtained at a high spatial resolution (~ 1.5 mm) allowing resolution of local variation of strain on the epicardial surface during ventricular filling. The experimental framework developed in this paper provides detailed measurement of cardiac strains under controlled conditions, as a reference for validation of clinical cardiac strain imaging modalities.

Cornea Full-field Displacement and Strain Measurement in Vivo Using Three-dimensional Digital Image Correlation.

Corneal biomechanics are becoming increasingly important for clinical evaluation and diagnosis, such as in refractive surgery, glaucoma, and keratoconus. We developed and used a new technique to measure the full-field displacement and strain of the cornea under elevated intraocular pressure in vivo. In this study, we propose a three-dimensional digital image correlation method for the determination of corneal biomechanical properties. Corneal deformation change on the rabbit eyeball was investigated under different inflation conditions with intraocular pressure levels of 2.3, 3.3, 5.3, and 6.9 kPa in vivo. The proposed method was able to measure the displacement of the corneal surface and to construct real-time full-field three-dimensional deformation vector fields and strain mapping. The results show that the strain distribution is not uniform on the corneal surface at each intraocular pressure level. Compression strains, rather than extensile strains, are dominant at the corneal apex region. This technique has the potential to be used as an assistive tool for the determination of corneal biomechanical properties in ophthalmologic investigations.

Postpeak Deformability Parameters of Localized and Nonlocalized Damage Zones of Rocks under Cyclic Loading

Previous studies on the effects of cyclic loading on the strength and deformability behavior of rocks under uniaxial conditions, i.e., strength damage together with the degradation of rocks’ mechanical properties, have solely taken into account the postpeak stress strain regime. Nevertheless, rock may experience cyclic loading history in the postpeak regime as well. In this study, the effects of cyclic loading on the deformational characteristics of both the nonlocalized damage zone (NLDZ) and the localized damage zone (LDZ) of rock in the postpeak regime are investigated. A series of postpeak damage-controlled tests were carried out on three different rock types, including sandstone, limestone, and granite specimens. Three-dimensional digital image correlation was implemented to measure the strain development in the surface of the specimens. Strain localization was found to be more significant in the lateral direction and less occurrent in the axial direction. In addition, it was observed that the material located in the NLDZ experiences much less damage in contrast to material located in the LDZ. Both large irreversible strain accumulation and Young’s modulus degradation were found to take place progressively in the LDZ, initially occurring at a lower rate and then faster as the number of loading-unloading cycles increased. In this regard, the extent of irreversible deformation and stiffness degradation in the LDZ is considered to be a major contributor to the global loss of strength of the entire specimen.

Digital image correlation through image registration in the frequency domain

The present work analyses displacement field computations for several digital image correlation test cases obtained through the application of image registration in the Fourier domain. The applied method relies on the properties of the discrete Fourier transform with regard to the cross-correlation function to find the rigid displacement between two subsets. The computed displacement fields were compared to their analogous values obtained through a commercial digital image correlation code. The test cases cover both two-dimensional and three-dimensional digital image correlation and make use of the ability of the developed method to calculate the translation and rotation between subsets from images at different loading states, as well as its capacity to match subsets from two cameras of a stereo rig. The presented analysis aims to contribute to the investigation of the use of correlation in the frequency domain as the basis for an alternative to conventional digital image correlation which could provide advantages in terms of robustness and efficiency, particularly in field measurements, where the lack of control over lighting and other factors impacting image quality requires a system which is optimised towards low dependence on the optimal quality of these conditions.

Short-term bond behavior and debonding capacity of prestressed CFRP composites to steel substrate

In this study, the short-term bonding behaviors of prestressed CFRP plates to steel substrates and their debonding capacities have been investigated. For this purpose, single lap-shear and prestress release tests were performed on adhesively bonded CFRP-to-steel joints. The feasibility of accelerated curing (AC) of the adhesive by heating was also investigated based on the conducted tests. Moreover, bond tests with partial prestress release and subsequent lap-shear loading were conducted to examine the feasibility of prestressed strengthening of steel members using AC. A three-dimensional (3D) digital image correlation (DIC) technique was utilized to monitor the bond behavior of CFRP-to-steel joints. Experimental results demonstrated that a mixed-mode I/II (tensile/shear) fracture governs the debonding failure of CFRP-to-steel joints during the prestress release. However, given that the steel substrate cannot undergo tensile failure, a relatively high prestressing force can be transferred to the steel substrate prior to debonding. The experimental results also revealed that the AC of the epoxy adhesive can be an advantageous alternative to the conventional room temperature curing (RTC) for strengthening steel members with prestressed bonded CFRP plates.

Fracturing Behavior Study of Three-Flawed Specimens by Uniaxial Compression and 3D Digital Image Correlation: Sensitivity to Brittleness

Brittleness has significant effects on the fracturing behaviors of preflawed rock-like materials. In this study, uniaxial compressive experiments are conducted on rock-like specimens containing three flaws, and a three-dimensional digital image correlation technique is applied to capture the real-time displacement and strain fields on the rock-like specimens’ surface. The present experimental study focuses on the effects of the brittleness of rock-like materials on crack initiation, propagation, and coalescence behaviors in three flaw-contained specimens. The experimental results show that the crack initiation mode transforms from the tensile mode to the shear mode with a decrease in the brittleness index of rock-like materials. Meanwhile, crack coalescence modes transform from the tensile crack coalescence mode to the mixed tension–shear coalescence mode, then to the shear coalescence mode as the brittleness index of rock-like materials decreases. Moreover, the ratio of the crack initiation stress to the peak stress increases, while the ratio of the crack coalescence stress to the peak stress decreases as the brittleness index decreases. This paper also addresses the influences of the rock bridge angle on the mechanical and cracking behaviors of three flaw-contained specimens. For specimens with the lower brittleness index, the ratio of the crack initiation stress to the peak stress decreases, while the ratio of the crack coalescence stress to the peak stress increases as the rock bridge angle increases. However, for specimens with the higher brittleness index, the ratio of the crack initiation stress to the peak stress and the ratio of the crack coalescence stress to the peak stress decrease with increasing the rock bridge angle. The experimental results provide a better understanding of the sensitivity of mechanical and cracking behaviors of three flaw-contained specimens to the brittleness of rock-like materials under uniaxial compression.

RGB Colour Encoding Improvement for Three-Dimensional Shapes and Displacement Measurement Using the Integration of Fringe Projection and Digital Image Correlation.

Three-dimensional digital image correlation (3D-DIC) has become the most popular full-field optical technique for measuring 3D shapes and displacements in experimental mechanics. The integration of fringe projection (FP) and two-dimensional digital image correlation (FP + DIC) has been recently established as an intelligent low-cost alternative to 3D-DIC, overcoming the drawbacks of a stereoscopic system. Its experimentation is based on the colour encoding of the characterized fringe and speckle patterns required for FP and DIC implementation, respectively. In the present work, innovations in experimentation using FP + DIC for more accurate results are presented. Specifically, they are based on the improvement of the colour pattern encoding. To achieve this, in this work, a multisensor camera and/or laser structural illumination were employed. Both alternatives are analysed and evaluated. Results show that improvements both in three-dimensional and in-plane displacement are obtained with the proposed alternatives. Nonetheless, multisensor high-speed cameras are uncommon, and laser structural illumination is established as an important improvement when low uncertainty is required for 2D-displacement measurement. Hence, the uncertainty has been demonstrated to be reduced by up to 50% compared with results obtained in previous experimental approaches of FP + DIC.

A Novel Single Camera Robotic Approach for Three-Dimensional Digital Image Correlation with Targetless Extrinsic Calibration and Expanded View Angles

Three-Dimensional Digital Image Correlation (3D-DIC) is a technique used widely in both academic and industrial settings to measure object morphology, velocity, and full-field strain. However, conventional methods require the use of two synchronized cameras and a lengthy calibration process of taking thirty or more images of specialized target, after which the cameras must be kept fixed. This paper proposes a new approach for obtaining 3D-DIC measurements using only a single camera moved between different positions by a robotic arm, with the extrinsic calibration parameters obtained by manipulation of the internal position and orientation information known to the robot, obviating the need for a calibration target in order to calculate the extrinsic matrix for various positions (after the intrinsic matrix has been established). The specifications of a prototype are presented, and the method is quantitatively and qualitatively compared to the traditional 3D-DIC approach to determine relative accuracy and reliability in shape measurements. By correcting for errors in the reported orientation of the robotic arm, it was possible to achieve the same accuracy as the traditional 3D-DIC approach for stereo angles up to 30o, beyond which the loss of image overlap results in linear increasing errors that are up to an order of magnitude greater. The method is also further extended to allow for images from more than two positions to be combined in a single analysis, which enables an expanded view of up to 360° using only a single camera and without the additional calibrations required by current multi-camera expanded view systems. The integration of this new 3D-DIC approach into future robotic systems will facilitate new capabilities for performing vital tasks, such as material analysis, infrastructure assessment, and human-robot interactions.

Surface deformation of walnut burl veneer on aircraft sandwich panels assessed by three-dimensional digital image correlation

The three-dimensional digital image correlation method (3D-DIC) was used to study the surface deformation of aircraft interior sandwich panels as a result of a water vapor adsorption treatment. The effects of a fire-retardant treatment and wood burl structure were evaluated. Unvarnished and varnished panels made with walnut burl veneer (Juglans hibdsii L.) were selected and analyzed separately. Half of the samples from each type of panel received a fire-retardant treatment (phosphate based) on all three layers of the decorative plywood. The other half had the two inner layers treated and the outer layer untreated. Swirl grain and bud traces areas were identified on the burl pattern of veneer surfaces. Samples preconditioned to 20 °C and 40% relative humidity underwent an adsorption (25 °C, 90% RH) treatment. Changes in moisture content were measured after adsorption. Full-field swelling strains (in-plane) and Z-displacements (out-of-plane) were obtained from each figure type in samples after the adsorption conditioning. The application of the 3D-DIC method revealed that the fire-retardant treatment increased the swelling strains and Z-displacements on unvarnished and varnished surface panels. This treatment also caused a significant differentiation of Z-displacements on swirl zones compared to bud trace zones in varnished panels. Thus, the degree of surface deformation depended on the burl wood structure and the fire-retardant treatment. The 3D-DIC method was suitable for evaluating local swelling strains and Z-displacements on unvarnished and varnished wood veneer surfaces.