Search

Additively manufactured materials usually exhibit mesoscale heterogeneities. Mesoscale fluctuations of strain fields in notched samples made of 17-4PH Stainless steel and loaded in tension are investigated. Regularized digital image correlation enables for the analysis of strain fluctuations at different length scales. Five tests on specimen fabricated with different printing parameters are studied. It is shown that the strain fluctuations have no characteristic length scale and are essentially independent of the probed processing parameters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The mechanical behavior of fibrous metamaterials is mainly determined by the interactions between the fibers composing the architecture. These interactions are usually of two different kinds: those directly depending on the positions of the fibers and those that need mediators, usually consisting of hinges, either inelastic or perfect, inducing restrictions on the kinematics of the fiber joints. In cases of interest, it has been observed that hinges can either have a certain torsional stiffness or behave as perfect joints, simply ensuring that the fibers remain interconnected, but not applying any constraint on the relative rotations between them. Here the effect of torsional stiffness of inelastic hinges is studied in two shear tests for a selected fibrous metamaterial. It is shown that the stiffness of hinges can be tailored to avoid, or at least reduce, out-of-plane deformations. Moreover, it is shown that, after reaching a threshold, permanent deformations are observed. This phenomenon is treated in a simplified way, by introducing damage in the continuum model. [ABSTRACT FROM AUTHOR]

This paper presents and compares two combined experimental‐numerical techniques for the investigation of fracture properties of additively manufactured polymer parts using digital image correlation (DIC) measurements. The first method only uses measured kinematic fields, and the second is based on finite element simulations driven by measured boundary conditions. A mini single edge notched tensile sample manufactured by fused filament fabrication with ABS is studied. It is shown that both methods locally extract J‐integrals, and the crack tip is accurately located by the FE‐based method. By comparing computed displacements to those measured via DIC, it is possible to locally check the validity of the numerical model. The initiation and propagation stages are analyzed independently thanks to two different magnifications of acquired image series. [ABSTRACT FROM AUTHOR]

Digital image correlation (DIC) is a full-field measurement technique. In instantaneous approaches (i.e., registering two images), DIC only gives access to displacement (or velocity) fields. Consequently, acceleration fields are not one of the primary measured variables. To evaluate acceleration fields, a regularization scheme has to be used. The latter may be either heuristic or mechanically motivated. The key idea of the paper is to use spatiotemporal analyses in order to explicitly measure acceleration fields. Various regularization schemes will be assessed, and their relative merits will be studied when performing uncertainty quantifications. Pyrotechnic cutting simulations will provide a set of artificial pictures to be studied in order to validate the new implementations. This analysis enables the measurement performances to be evaluated for the new implementations. [ABSTRACT FROM AUTHOR]

Bi-pantographic fabrics are composed of two families of pantographic beams and correspond to a class of architectured materials that are described in plane as second-gradient 2D continua. On a discrete level, a pantographic beam is a periodic arrangement of cells and looks like an expanding barrier. The materialization of a bi-pantographic fabric made from polyamide was achieved by additive manufacturing techniques. Starting from a discrete spring system, the deformation energy of the corresponding continuum is derived for large strains by asymptotic homogenization. The obtained energy depends on the second gradient of the deformation through the rate of change in orientation and stretch of material lines directed along the pantographic beams. Displacement-controlled bias extension tests were performed on rectangular prototypes for total elastic extension up to 25%. Force–displacement measurements complemented by local digital image correlation analyses were used to fit the continuum model achieving excellent agreement. [ABSTRACT FROM AUTHOR]

Measured displacement fields constitute a rich database for the identification of mechanical behavior. A variety of methods are available today to address this problem. A formalism is proposed showing that they all revert to the minimization of a quadratic norm (or semi-norm) between measured and computed displacement fields. However, they differ in the chosen metric. Transporting image noise through Digital Image Correlation and identification procedures, the uncertainty in the measured displacement fields (i.e., their full covariance) and the resulting uncertainty in the estimated constitutive parameters is assessed. Consequently, an optimal choice of metric (with respect to image noise) is proposed so that the resulting identification uncertainty is minimized. [ABSTRACT FROM AUTHOR]

• dynamic Brazilian tests are analyzed with images acquired at 5 and 10 million fps. • Displacements up to 1 mm, velocities up to ± 250 m/s, and accelerations as high as ± 4 × 107 m/s2 are measured with spacetime DIC. • uncertainty quantifications on displacements, velocities, acceleration, strains and strain rates. Two dynamic Brazilian tests are analyzed via digital image correlation for acquisition rates equal to 5 and 10 million frames per second. Displacements of the order of 1 mm, velocities of the order of ± 250 m/s, and accelerations as high as ± 4 × 107 m/s2 are measured with spacetime DIC. Uncertainty quantifications enable spacetime DIC to be compared with instantaneous analyses. The gains provided by the temporal regularization are very significant for the acceleration fields. The observed levels are consistent with a priori estimates. [ABSTRACT FROM AUTHOR]

In the last decade, the exotic properties of pantographic metamaterials have been investigated and different mathematical models (both discrete or continuous) have been introduced. In a previous publication, a large part of the already existing literature about pantographic metamaterials has been presented. In this paper, we give some details about the next generation of research in this field. We present an organic scheme of the whole process of design, fabrication, experiments, models and image analyses. [ABSTRACT FROM AUTHOR]

Highlights • The manuscript entitled "On the analysis of heat haze effects with space- time DIC," proposes a new space time implementation of global DIC to analyze heat haze effects in high temperature experiments. In addition, a denoising technique is proposed in the framework of spacetime analyses. To the authors' knowledge, all these implementations were never done before. • It is shown that the use of a fan allows the bias induced by the heat haze to be reduced in an experiment where a ceramic matrix composite is heat with a laser beam for temperature greater than 1200 °C. Abstract Long image series (i.e., movies) can be analyzed via digital image correlation (DIC) techniques. A new spacetime implementation is introduced to extend instantaneous DIC with minimal implementation complexity. With this procedure, a denoised reference image is built considering a series of more than 200 images. Such spacetime approach is very interesting when high temperature experiments are performed since it provides a very effective temporal regularization. The analysis of heat haze effects is carried out on a thermomechanical test in which the sample made of ceramic matrix composite is subjected to temperatures greater than 1200 °C in the heat affected zone of the laser beam. It is shown that the addition of a small fan reduces the spurious displacement fluctuations. [ABSTRACT FROM AUTHOR]

An inverse method for the identification of a set of crystal plasticity parameters is introduced and applied to in situ tensile tests on steel polycrystals. Various mean grain sizes are obtained with different conditioning of AISI 316LN austenitic stainless steel. Identification is based on a weighted Finite Element Model Updating (FEMU) using both displacement fields at the microscale and macroscopic load levels. The values of the identified parameters depend on the factor weighing the contributions of the microscopic and macroscopic quantities. On the one hand, surface displacement fields are measured by Digital Image Correlation (DIC). On the other hand, they are simulated by resorting to Finite Element calculations with a local crystal plasticity law ( i.e., Méric-Cailletaud’s model) and a 2D simulation of the microstructure. The parameters associated with isotropic hardening are calibrated for the different mean grain sizes. The benefits of considering full-field measurements are manifest for instance in the excellent Hall-Petch trend captured at the microstructural scale. The identification procedure is also applied to the estimation of hardening parameters describing the interaction between slip systems. [ABSTRACT FROM AUTHOR]

A mesh-based framework is developed by extending global stereocorrelation techniques to faceted surfaces with three-noded elements. A two-step self-calibration procedure is followed to determine the projection matrices of the stereo-rig and to update the nominal surface model to match the surface of interest. To prove the feasibility of mesh-based stereocorrelation, two different test parts are analyzed with the present techniques and compared to already validated optical procedures. [ABSTRACT FROM AUTHOR]

In this work 3D strain and damage analyses are performed in the immediate vicinity of the notch root of a flat CT-like specimen made of aluminum alloy. Experimental data, partially exploited by Morgeneyer et al. (Acta Mat 69:78-91, ), were obtained by using synchrotron laminography and the 3D reconstructed volumes are subsequently analyzed via Digital volume correlation. These data enable for in situ assessments of strain fields and ductile damage in the zone where the stress triaxiality evolves from elevated to lower levels, which is accompanied by flat-to-slant crack transition. The measured strain field patterns in this area are analyzed herein in a systematic manner by studying the incremental strain activity during several loading steps. It is shown that from the very beginning of the loading history multiple slant strained bands appear in front of the notch root while the corresponding damage growth sets in at later loading stages and higher strains. The activity of the different strained bands at the notch root is alternating between different locations over the loading history. However, the band leading to final rupture is always active. The region where slant fracture occurs is identified to be in plane strain condition with respect to the crack propagation direction. [ABSTRACT FROM AUTHOR]

The role of crystallographic grain size and microstructure of an austenitic stainless steel (A316LN) is studied via in-situ mechanical testing in an SEM and digital image correlation to measure displacement and strain fields. The latter ones are directly measured on a mesh supported by the grain boundaries as imaged by EBSD. This experimental analysis is applied to five microstructures of the same cast material but subjected to different heat treatments. Differences in the strain distributions that can be attributed to the grain size are observed. The full-field measurements allow in turn for a direct, local and quantitative comparison with crystal plasticity finite element simulations of the same experiments based on the same mesh. A weak correlation between the plastic strain and the Schmid's factors, which are computed with macroscopic or even local stress evaluations, emphasizes the role of inter-grain strain incompatibilities or grain boundaries. However, a good agreement in the major slip system evaluation between experimental observation and computations is observed. [ABSTRACT FROM AUTHOR]

A new displacement measurement technique is proposed in a stereovision setup, which uses the object of interest as the support of the correlation process. This procedure leads to a global approach to stereocorrelation. The method is presented in its general formulation and is then particularized to the case of non uniform rational B-splines (NURBS). The displacement field is directly measured as a 3D field expressed in a NURBS basis consistent with the existing geometric model. The kinematic measurements are validated against prescribed displacements of a machined Bézier patch. The feasibility in an industrial context is shown with the analysis of 3D displacement fields of a 2- m automotive roof panel during a welding operation. [ABSTRACT FROM AUTHOR]

The glass industry requires the use of innovative ceramics that enable for long lifetimes. At very high temperatures, one of the key parameters for ceramics is their creep resistance. The characterization of the creep behavior, usually assessed through flexural tests, can be complex when an asymmetry appears between tension and compression. To detect and quantify such asymmetrical behaviors, the use of Digital Image Correlation (DIC) is proposed. First, several challenges are to be tackled for DIC at high temperature, namely, the random pattern stability, the radiation filtering and the heat haze. They are exacerbated by the limited possibilities to heat ceramics, the nonuniform strain fields and their low levels. Beyond several experimental developments, the strain uncertainties are decreased thanks to the use of two global approaches of DIC based on ad hoc finite-element kinematics. Last, the proposed methodology is applied to the analysis of asymmetric creep at 1350°C of an industrial zircon ceramic designed for its high creep resistance. [ABSTRACT FROM AUTHOR]

Abstract: Optical aberrations are one of the biases affecting images. Their correction can be performed as a transformation of the image. In this paper, it is proposed to use digital image correlation to calculate the distortion fields. A correlation calculation is performed between a known numerical pattern and a picture of the same pattern printed or etched on a plate. This new procedure is applied to determine the distortion fields of a camera lens. First, a parametric description of the distortion field is used to reduce the degrees of freedom required to describe optical distortions. Second, a non-parametric model based upon splines is used. The distortion fields found by both methods are compared. A resolution analysis is performed for two different procedures (removing the lens between pictures or not). The standard displacement resolution is found to be of the order of 10-2 pixel in the first case, and pixel in the second case. [Copyright &y& Elsevier]

Surface patterning by e-beam lithography and scanning electron microscope (SEM) imaging distortions are studied via digital image correlation. The global distortions from the reference pattern, which has been numerically generated, are first quantified from a digital image correlation procedure between the (virtual) reference pattern and the actual SEM image both in secondary and backscattered electron imaging modes. These distortions result from both patterning and imaging techniques. These two contributions can be separated (without resorting to an external caliper) based on the images of the same patterned surface acquired at different orientations. Patterning distortions are much smaller than those due to imaging on wide field images. [ABSTRACT FROM AUTHOR]

Abstract: In the context of regular inspections of structures, the presence of cracks can sometimes be revealed. It is therefore interesting to know whether the structure can still be used or if a degraded mode of operation should be considered. It is of concern to assess the scatter of the remaining life of such cracked parts due to the uncertainties on the parameters of the prediction model. Thus for the purpose of the present study, a special attention has been given to the quantification of the uncertainty of each collected data (e.g., material properties, crack measurements) to be integrated in the crack propagation model. Three families of uncertainties have been studied, namely, material properties, geometrical and loading uncertainties. A linear elastic fracture mechanics (LEFM) based approach has been used to predict crack propagation in a widely used pressure vessel steel. The model uses classical Paris’ law where all the controlling parameters have been replaced by statistical distributions obtained from experiment, namely, crack growth tests, hardness tests, tensile tests and Charpy tests. Crack growth tests have also been carried out on thick notched specimen submitted to uniaxial cyclic load to obtain two dimensional cracks that can be representative of those to be found in industrial structures. During these tests, crack lengths have been measured simultaneously by time of flight diffraction (TOFD) ultrasonic method, digital image correlation (DIC) and some markings have been performed to estimate the crack length and crack front shape after the specimen failure. The comparison of data obtained from different observation techniques allowed for the quantification of the crack measurement uncertainty of the industrial TOFD technique. A sensitivity analysis has been carried out on the parameters of the model to evaluate and classify the influence of all sources of uncertainty on the residual life prediction. Last, results from crack growth tests on notched specimen have been used to assess the accuracy of the model prediction. [Copyright &y& Elsevier]

Abstract: A novel methodology to identify a fatigue crack propagation law is proposed. It is based on an optimization scheme that directly interrogates the images from the sought law rather than using a sequence of fitting schemes. Two procedures are presented and tested first on artificial data with a known law, and second on an actual image series obtained from a fatigue test performed on a thin titanium sheet. The results demonstrate the ability of the proposed procedures to deal with inherently noisy data, and yet obtain an accurate propagation law. As a side result “admissible” time series of stress intensity factors and crack advance are obtained that fulfill the identified law and are constrained by the registration of actual images, rather than being simply filtered from the data in a post-processing step. [Copyright &y& Elsevier]

Abstract: Digital Image Correlation (DIC) is an ill-posed problem. To circumvent this difficulty, the needed regularization is often introduced implicitly through the choice of a kinematic basis used to embed the sought solution for displacement fields. Alternatively, a priori information on the mechanical behavior of the studied specimen is often available. This presentation is focused on different ways of supplementing DIC with mechanical information to achieve enhanced fidelity and robustness. Adapted unstructured mesh is used herein as an example of such a coupling. [Copyright &y& Elsevier]

Most often, digital image correlation is used to analyse a sequence of images. Exploitation of an expected temporal regularity in the displacement fields can be used to enhance the performances of a digital image correlation analysis, either in terms of spatial resolution, or in terms of uncertainty. A general theoretical framework is presented, tested on artificial and experimental image series. [ABSTRACT FROM PUBLISHER]

Two different mechanical tests are performed on a laminated composite coupon to induce an anisotropic damage affecting essentially shear modulus softening. The first test is a uniaxial tension loading on a straight coupon, which is used to evaluate the damage law using a conventional approach, while the second contains a notch that enhances dramatically the strain (and hence damage) heterogeneity. A global digital image correlation approach is used to quantify the kinematic fields all along the loading path of the second experiment. Displacement fields are hence evaluated based on a finite element type discretization. A further exploitation based on the reconditioned equilibrium gap method (and without any further information) gives access to a quantitative measurement of the damage law. The latter approach makes use of a finite element model based on the very same mesh and element shape function. This full-field-based identification method compares very well with traditional techniques, up to the stage where macroscopic localization prevents their subsequent exploitations. Moreover, it is shown that neither the type of mechanical test, nor the discretization of the displacement field, affects the identification of the damage law. [ABSTRACT FROM PUBLISHER]

Abstract: In this study, a crack in commercially-pure Titanium (CP-Ti) is observed with optical means during its propagation in fatigue. Digital image correlation is then used to extract directly, with no post-processing step, the crack tip location and the corresponding stress intensity factor amplitude to evaluate the parameters of Paris’ law, the T-stress component and an estimate of the plastic zone shape. [Copyright &y& Elsevier]

Abstract: It is proposed to identify the behavior of steel beams prior to and after the inception of local buckling by using digital image correlation. Full-field measurements are used to evaluate kinematic and static fields for determining constitutive laws. It enables for the detection of local buckling inception and the evaluation of the post-buckled behavior. Constitutive models are tuned by using measured Euler–Bernoulli kinematics. [ABSTRACT FROM AUTHOR]

Abstract: It is proposed to develop a digital image correlation procedure that is suitable for beams whose kinematics is described by an Euler–Bernoulli hypothesis. As a direct output, the degrees of freedom corresponding to flexural and axial loads are directly measured. The performance of the correlation algorithm is evaluated by using a picture of a cantilever beam experiment. One load level is analyzed with the present algorithm. The latter is validated by comparing the displacement field with that given by a finite element based correlation algorithm. It is also shown that a locally buckled zone is detectable with the present procedure. [Copyright &y& Elsevier]

Abstract: Finite elements have been used recently to solve the optical flow conservation principle invoked to determine displacement fields by digital image correlation. Inspired by these recent advances, and by the computational effort that has been accomplished during the past 10 years for the simulation of discontinuities by the extended finite element method (XFEM), an extended correlation technique is introduced for capturing shear-band like discontinuities from images of real mechanical tests. Because of the specific information at hand (i.e., gray level images), this extended finite element approach is included in a non-linear multi-grid solver. The performances of the proposed approach are discussed on two examples, namely the analysis of a bolted assembly and the formation of Piobert–Lüders bands. [Copyright &y& Elsevier]

Copyright of Canadian Journal of Civil Engineering is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)