Global Digital Image Correlation Research Articles

Convolution finite element based digital image correlation for displacement and strain measurements

This work presents a novel global digital image correlation (DIC) method, based on a newly developed convolution finite element (C-FE) approximation. The convolution approximation can rely on the mesh of linear finite elements and enables arbitrarily high order approximations without adding more degrees of freedom. Therefore, the C-FE based DIC can be more accurate than the usual FE based DIC by providing highly smooth and accurate displacement and strain results with the same element size. The detailed formulation and implementation of the method have been discussed in this work. The controlling parameters in the method include polynomial order, patch size, and dilation. A general choice of the parameters and their potential adaptivity have been discussed. The proposed DIC method has been tested by several representative examples, including the DIC challenge 2.0 benchmark problems, with comparison to the usual FE based DIC. C-FE outperformed FE in all the DIC results for the tested examples. This work demonstrates the potential of C-FE and opens a new avenue to enable highly smooth, accurate, and robust DIC analysis for full-field displacement and strain measurements.

Unified digital image correlation under meshfree framework

AbstractThe implementation of digital image correlation (DIC) involves several different problems, such as image prefiltering, image intensity gradient calculation, image intensity interpolation at subpixel positions, shape function construction and strain calculation. This paper offers a unified insight into the nature of several key problems in DIC technique. We treat all the problems involved in the former mentioned key steps as fundamentally the same problem, that is, reconstructing an analytical description from a discrete and noisy sampling of a signal, such as discrete image intensity and displacement at some scattered nodes. From the reconstructed analytical description, the gradient and physical value at subpixel or integral pixel position can be analytically calculated without extra error. Here, we solve all these problems using the same mathematical tool, the meshfree method, leading to a unified DIC (U‐DIC) method. This method introduces errors solely during the steps involving the determination of the continuous description. However, it effectively avoids errors in the remaining steps. It holds the best balance between spatial resolution and measurement resolution for both displacement and strain measurements compared to 28 state‐of‐the‐art DIC algorithms based on the benchmark tests on DIC Challenge 2.0. It also holds all the unique advantages of the advanced meshfree DIC (MF‐DIC) compared to conventional local DIC and global DIC. The novel concept and excellent balance between spatial resolution and measurement resolution make U‐DIC an attractive replacement for conventional DIC methods. Additionally, the consistency of the implementation procedures in U‐DIC can simplify the parameter selection in DIC, which is of potential for the standardization of DIC technique.

Element-removal global digital image correlation for accurate discontinuous deformation field measurement in fracture mechanics

We propose an element-removal (ER) global digital image correlation (DIC) method to improve the measurement accuracy of discontinuous deformation fields, such as crack propagation. The occurrence of cracks in materials or structures inevitably deteriorates the tracking accuracy, and, consequently, the strain field accuracy obtained by regular subset and global DIC. The proposed ER-global-DIC algorithm iteratively identifies and removes all the elements covering the crack, during the updating of displacement fields. In the remaining elements, the continuous shape function is applicable for accurate deformation measurement. In principle, although elements that contain the cracks are removed, the algorithm preserves the same number of nodes since the nodes are retained by the remaining elements. Synthetically deformed images based on analytical discontinuous displacement fields validate the effectiveness and accuracy of the proposed method. The ER-global-DIC is further applied to measure the discontinuous displacement fields containing a crack deflection, generated from a finite element model with a cohesive zone model. The results demonstrate the potential of the proposed method for discontinuous deformation measurement on advanced materials, e.g., fiber-reinforced composites.

Challenges and solutions of environmental scanning electron microscopy characterisation of biomaterials: Application to hygro‐expansion of paper

AbstractMost methodologies to measure the moisture‐induced deformation (hygro‐expansion) of paper microconstituents, including fibres and interfibre bonds, are low resolution or time‐consuming. Hence, here, a novel method is proposed and validated to measure high‐resolution full‐field strain maps of paper microconstituents during hygro‐expansion, based on environmental scanning electron microscopy (ESEM). To this end, a novel climate stage enables accurate control of the relative humidity (RH) near the specimen in the ESEM from 0%–100%. The fibre surface, which is decorated a priori with a microparticle pattern, is captured during RH change. Subsequently, correlating the fibre surface using a dedicated global digital image correlation algorithm enables high‐resolution hygro‐expansion strain maps. Method optimisation involved performing contrast enhancement, scan‐correction to reduce ESEM artefacts and a background correction, resulting in a strain resolution of . Method validation revealed that the fibres' crystallinity is affected by the electron beam, even for minimal invasive electron beam settings. Interestingly, however, the fibres consistently exhibit conventional hygro‐expansion behaviour during the drying slopes. Using the optimised procedure, hygro‐expansion characterisation of two interfibre bonds and four interfibre bond cross‐sections revealed the competition between the low longitudinal and large transverse fibre hygro‐expansion in the bonded area.

Gray level residual field: an effective metric for pixelwise matching quality evaluation in local digital image correlation

Gray level residual (GLR), also known as correlation residual field or image difference, reflects the intensity differences between reference and deformed images. In finite element-based global digital image correlation (DIC), GLR field can be readily computed and has been successfully used for crack or damage detection. The computation of pixelwise GLR field requires dense displacement results, which can be obtained using mesh interpolation in global DIC. To obtain the GLR field in subset-based local DIC, in which results are obtained at discrete calculation points, a simple, efficient and practical computation method for GLR field is presented, which accounts for the influence of illumination variations unavoidably occurred in real DIC tests. The proposed method can be easily implemented on both central process unit and graphics process unit to realize real-time visualization. Experiments showed that the GLR field can quantitatively and intuitively evaluate the pixelwise matching quality and provide clearer and finer locations for new components or textures (e.g. cracks) presented in deformed images. By fully leveraging the pixelwise matching quality information provided by the GLR field, the capability and robustness of local DIC is expected to be further enhanced.

Meshfree Digital Image Correlation Using Element Free Galerkin Method: Theory, Algorithm and Validation

BackgroundThe association of advanced digital image correlation (DIC) and numerical simulation has been widely used for inverse parameter identification.ObjectiveIt is attractive to develop an accurate DIC method sharing the common features with numerical simulation, which can lead to better synergy between experiments and simulations.MethodsA new meshfree digital image correlation (MF-DIC) using element free Galerkin method (EFGM) is proposed for deformation measurement. The EFGM is a classical meshfree method in numerical studies, and it is directly used to construct the shape function in MF-DIC from a set of scattered nodes for image matching. The MF-DIC is principally different from the classical local DIC and global DIC since it does not rely on the concept of a subset or an element.ResultsIn MF-DIC, the {C}^{1}-continuous displacement for every point is constructed based on a group of scattered nodes in a small support domain surrounding it. The continuous strain map can then be directly derived from the displacement, instead of using an additional smoothing technique as required in classical local DIC or post-processing used in global DIC. A performance assessment based on the Metrological Efficiency Indicator (MEI), as defined in DIC Challenge 2.0, shows that the proposed MF-DIC yields an excellent balance between spatial resolution and measurement resolution for both displacement and strain measurements.ConclusionsGiven that the proposed MF-DIC shares common features with the classical meshfree method in computational mechanics, it paves the way for an enhanced synergy between experiments and simulations required for robust inverse parameter identification methods.

Photometric DIC: a unified framework for global Stereo Digital Image Correlation based on the construction of textured digital twins

An innovative approach allowing to rigorously address surface curvature and lighting effects in Digital Image Correlation (DIC) is proposed. We draw inspiration from the research work in Computer Vision (CV) regarding the physical modelling of a camera and adopt it to bring novel and significant capabilities for full-field measurements in experimental solid mechanics. It gives rise to a unified framework for global stereo DIC that we call Photometric DIC (PhDIC). It is based on the irradiance equation that relies on physical considerations and explicit assumptions, which stands for a clear breakthrough compared to the usual grey level conservation assumption. Most importantly, it allows to define a Digital Twin of the Region of Interest, which makes it possible to compare a model with different observations (real images taken from different viewpoints). This results in a consistent formalism throughout the framework, suitable for large-deformation and large-strain displacement measurements. The potential of PhDIC is illustrated on a real case. Multi-view images are first used to measure (or scan) the shape and albedo (sometimes called intrinsic texture) of an open-hole plate. The kinematic basis considered for the displacement measurement is associated to a Finite-Element mesh. Results for the shape and albedo measurement are compared for two completely different sets of pictures. Eventually, a large displacement of the structure is measured using a well-chosen single image.

Measuring both thermal and kinematic full-fields using a single CMOS camera during high temperature tests

In numerous domains, both kinematic and thermal fields are sought to study temperature dependent phenomena. They are usually obtained by digital image correlation and infrared thermography, respectively. Although these techniques are well mastered, their combination is not straightforward since they need radically different conditions to work efficiently. This paper presents a procedure to measure accurately thermal and kinematic full-fields simultaneously using a single CMOS camera in a chosen region of interest. The kinematic field is measured by global digital image correlation and the thermal field by near-infrared thermography for temperatures above 700 ∘C. A procedure is detailed to identify the radiometric model and compute the thermal field without being affected by the heterogeneous emissivity of the sample surface caused by the speckle pattern. A methodology showing how to manage temperature measurements during high thermal transients is also presented. The proposed approach aims to be widely accessible since it does not need strong knowledge in DIC algorithms and only requires a standard camera. The method is first validated on a virtual test case, before being used on real experiments performed at high temperature with a stereo setup.

2D Numerical Simulation of Auxetic Metamaterials Based on Force and Deformation Consistency

This work showcases a novel phenomenological method to create predictive simulations of metallic lattice structures. The samples were manufactured via laser powder bed fusion (LPBF). Simulating LPBF-manufactured metamaterials accurately presents a challenge. The printed geometry is different from the CAD geometry the lattice is based on. The reasons are intrinsic limitations of the printing process, which cause defects such as pores or rough surfaces. These differences result in material behavior that depends on the surface/volume ratio. To create predictive simulations, this work introduces an approach to setup a calibrated simulation based on a combination of experimental force data and local displacements obtained via global Digital Image Correlation (DIC). The displacement fields are measured via Finite Element based DIC and yield the true local deformation of the structure. By exploiting symmetries of the geometry, a simplified parametrized simulation model is created. The simulation is calibrated via Response Surface Methodology based on nodal displacements from FE-DIC combined with the experimental force/displacement data. This method is used to create a simulation of an anti-tetrachiral, auxetic structure. The transferability and accuracy are discussed, as well as the possible extension into 3D space.

Determination of residual stress evolution during repair welding of high-strength steel components

During the assembly of steel structures, unacceptable weld defects may be found. An economical solution is local thermal gouging of the affected areas and re-welding. Due to high shrinkage restraints of repair weld and surrounding structure, high global and local welding stresses superimpose, and may lead to cracking and component failure, especially in connection with the degraded microstructure and mechanical properties of high-strength steels during the repair process. Component-related investigations of high-strength steels (FOSTA P1311/IGF20162N) focus on welding residual stress evolution during local thermal gouging and rewelding. In this study, repair welding of S500MLO (EN 10225) is carried out using in-situ digital image correlation (DIC) and ex-situ X-ray diffraction (XRD) to analyse strains and stresses. Self-restrained slit specimen geometries were identified representing defined rigidity conditions of repair welds of real components, which were quantified using the restraint intensity concept. The specimens were rewelded with constant welding heat control and parameters. Weld specimens exhibited significantly increased transverse residual stresses with higher transverse restraint intensities, in the weld metal, and in the heat affected zone. Transverse stresses along the weld seam decrease at the weld seam ends leading to different stress state during gouging and welding. XRD analysis of the longitudinal and transverse local residual stresses after cooling to RT showed a good comparability with global DIC analyses.

Residual Stress Maps Determination with global Digital Image Correlation

Aeronautic workpieces are produced thanks to heavy processes and undergo heat treatments to exhibit high mechanical properties. These operations generate residual stress within the material which are released during machining steps. Thus, workpieces deform to satisfy the internal equilibrium. The knowledge of residual stress maps of raw materials is an important data to control final part deflection. In this paper, a method relying on global Digital Image Correlation is proposed in order to measure the residual stress distribution and induced deformations during the machining of a specimen. It ensures to measure deformation part fields without unclamping unlike classical methods.

Fast Adaptive Augmented Lagrangian Digital Image Correlation

Digital image correlation (DIC) is a powerful experimental technique for measuring full-field displacement and strain. The basic idea of the method is to compare images of an object decorated with a speckle pattern before and after deformation in order to compute the displacement and strain fields. Local Subset DIC and finite element-based Global DIC are two widely used image matching methods; however there are some drawbacks to these methods. In Local Subset DIC, the computed displacement field may not satisfy compatibility, and the deformation gradient may be noisy, especially when the subset size is small. Global DIC incorporates displacement compatibility, but can be computationally expensive. In this thesis, we propose a new method, the augmented-Lagrangian digital image correlation (ALDIC), that combines the advantages of both the local (fast and in parallel) and global (compatible) methods. We demonstrate that ALDIC has higher accuracy and behaves more robustly compared to both Local Subset DIC and Global DIC. DIC requires a large number of high resolution images, which imposes significant needs on data storage and transmission. We combined DIC algorithms with image compression techniques and show that it is possible to obtain accurate displace- ment and strain fields with only 5 % of the original image size. We studied two compression techniques – discrete cosine transform (DCT) and wavelet transform, and three DIC algorithms – Local Subset DIC, Global DIC and our newly proposed augmented Lagrangian DIC (ALDIC). We found the Local Subset DIC leads to the largest errors and ALDIC to the smallest when compressed images are used. We also found wavelet-based image compression introduces less error compared to DCT image compression. To further speed up and improve the accuracy of DIC algorithms, especially in the study of complex heterogeneous strain fields at various length scales, we apply an adaptive finite element mesh to DIC methods. We develop a new h-adaptive technique and apply it to ALDIC. We show that this adaptive mesh ALDIC algorithm significantly decreases computation time with no loss (and some gain) in accuracy.

2D numerical simulation of auxetic metamaterials based on global DIC

This work discusses a novel approach to simulate metallic auxetic structures manufactured via Selective Laser Melting (SLM). SLMmanufactured metamaterials are difficult to simulate accurately based on nominal geometry and bulk material behaviour. The geometry after printing is different from the nominal CAD geometry. Artefacts due to the printing process such as pores yield a material behaviour which depends on the surface/volume ratio. We investigate a phenomenological approach to obtain a simulation model calibrated with experimental data and Digital Image Correlation (DIC). Finite Element based global DIC as suggested by Hild [1,2] allows for obtaining accurate displacement fields, consistent with the true deformation of the lattice structure. Based on the nominal CAD geometry, a simplified parametrized simulation model is created, exploiting the abundant symmetries of lattice structures. Using nodal displacements from DIC in combination with the expected forces from an experiment, the model is calibrated via LS-OPT. The approach is applied to an antitetrachiral, auxetic structure. Furthermore, we discuss the accuracy of the approach, its applicability to other structures and possible extension into 3D space.

On the benefits of correcting brightness and contrast in global digital image correlation: Monitoring cracks during curing and drying of a refractory castable

The gray level conservation is the underlying hypothesis of Digital Image Correlation (DIC). However, it may be challenging to enforce in some experimental configurations. Brightness and contrast corrections (BCCs) may be added to the registration procedure. Different types of BCCs were implemented for global DIC, and their benefits were analyzed for localized and diffuse sources of brightness changes. As a case study to apply BCCs, a refractory castable was placed inside a climatic chamber, and cracks were generated due to localized expansions during its curing and drying. To choose the best BCC for this case, two sets of images were considered. The first one allowed the noise floor levels to be evaluated. The second one dealt with the development of a crack network. The BCCs significantly reduced gray level residuals enabling cracks with small openings to be detected. The coarse discretization was effective in correcting lighting changes and avoided its coupling with the measured kinematic fields and other local phenomena.

Characterization at high spatial and angular resolutions of deformed nanostructures by on-axis HR-TKD

Global Digital Image Correlation (DIC) is applied on the electron diffraction patterns acquired by the “on-axis” Transmission Kikuchi Diffraction (TKD) technique. High-angular resolution (HR-TKD) mappings of the grain internal disorientations and the associated geometrically necessary dislocation densities are then derived at a nanoscale resolution. Tailored for the fine characterization of nanomaterials in the scanning electron microscope (SEM), the method is illustrated on a nanostructured high-purity aluminium processed by severe plastic deformation (SPD) and its performances are discussed in the light of imaging by transmission electron microscopy (TEM) and by SEM using a forescatter electron detector (FSD).

Global DIC approach guided by a cross-correlation based initial guess for HR-EBSD and on-axis HR-TKD

Digital image correlation (DIC) techniques usually performed on deforming speckle patterns are applied here on electron diffraction patterns (EDP) in order to map disorientations with high angular resolution, as well as geometrically necessary dislocation densities and elastic strains. The proposed approach relies on a DIC analysis which is conducted independently from the microscope calibration. It registers EDP as a whole through a single and large region of interest whose relative deformation with respect to a reference EDP is described by a first-order homography. Subpixel registration is performed iteratively in the spatial domain using an inverse-compositional Gauss-Newton (IC-GN) algorithm which integrates the correction of the optical distortions. Its robustness against large orientation changes as well as its computational efficiency are improved by mean of an automated and path-independent initial guess which fairly captures the effects of large rotations on the EDP. Using successive Fourier-Mellin and Fourier transforms based cross-correlation techniques, the initial guess measures the in-plane rotation and translation between the reference and the target EDP, respectively. The performances of the technique are illustrated in markedly plastically deformed steels using two electron diffraction techniques in the scanning electron microscope. A 15% deformed interstitial free steel is investigated by means of electron backscattered diffraction (EBSD) while a quenched and tempered oxide dispersed strengthened steel subjected to martensitic transformation is characterized using a recently developed on-axis Transmission Kikuchi Diffraction (TKD) configuration.

Towards Criteria Characterizing the Metrological Performance of Full-field Measurement Techniques

Users of full-field measurement methods like Digital Image Correlation (DIC) often aim to perform measurements with the best trade-off between spatial resolution, bias and measurement resolution. Whenever two full-field methods are compared, it is essential that these criteria are taken into consideration. Recently a metrological efficiency indicator for full-field measurements has been proposed and discussed. This indicator combines measurement resolution and spatial resolution. It has been shown to be invariant to the subset size in the case of Local DIC. The goal of this article is to discuss a method, which determines both the spatial and the measurement resolutions for a given bias for two different DIC methods, in order to obtain the metrological efficiency indicator for each of these methods. The benefit of this indicator is that it does not depend on setting parameters such as the subset size, which are chosen by the user. As such, it can be considered as intrinsic to each technique, thus enabling fair comparison. Local DIC and triangular finite element based Global DIC will be the subject of this investigation. With this setting, their respective subset and triangular element sizes will be related to the spatial resolution of both methods for a given acceptable bias. By using the metrological efficiency indicator, the performance of the two methods will be compared and discussed to a new level of detail. Generally speaking, the indicator shows that the metrological performance of both methods is similar, confirming their popularity. However, it will be shown that, depending on the choice of what an acceptable bias is, one of the method may be preferred to another. The results show that for the specific DIC versions used in the study, for cases for which a significant bias is acceptable, Local DIC outperforms Global DIC, while the opposite is true in the case for which the bias requirements are more stringent. Finally, the quadratic versions of both DIC versions are shown to significantly outperform their respective linear versions.

一种针对裂尖变形场测量的正则化全局DIC方法

一种针对裂尖变形场测量的正则化全局DIC方法

Global digital image correlation up to very high temperatures with grey level corrections

The characterisation of high temperature materials for extreme environment implies experimental challenges. Displacement measurements using digital image correlation (DIC) have proven to be an efficient non-contact method even at high temperatures. This paper uses global DIC and grey level corrections to analyse very high temperature tests in which inversion of the grey level distribution may occur. The benefits of grey level corrections are shown on a coated ceramic matrix composite heated up to 1380 °C. The inversion of the grey level distribution is then accounted for to study various ceramics subjected to temperatures up to 1860 °C.

A diurnal carbon engine explains 13C-enriched carbonates without increasing the global production of oxygen

In the past 3 billion years, significant volumes of carbonate with high carbon-isotopic ([Formula: see text]C) values accumulated on shallow continental shelves. These deposits frequently are interpreted as records of elevated global organic carbon burial. However, through the stoichiometry of primary production, organic carbon burial releases a proportional amount of [Formula: see text], predicting unrealistic rises in atmospheric [Formula: see text] during the 1 to 100 million year-long positive [Formula: see text]C excursions that punctuate the geological record. This carbon-oxygen paradox assumes that the [Formula: see text]C of shallow water carbonates reflects the [Formula: see text]C of global seawater-dissolved inorganic carbon (DIC). However, the [Formula: see text]C of modern shallow-water carbonate sediment is higher than expected for calcite or aragonite precipitating from seawater. We explain elevated [Formula: see text]C in shallow carbonates with a diurnal carbon cycle engine, where daily transfer of carbon between organic and inorganic reservoirs forces coupled changes in carbonate saturation ([Formula: see text]) and [Formula: see text]C of DIC. This engine maintains a carbon-cycle hysteresis that is most amplified in shallow, sluggishly mixed waters with high rates of photosynthesis, and provides a simple mechanism for the observed [Formula: see text]C-decoupling between global seawater DIC and shallow carbonate, without burying organic matter or generating O2.