167 research outputs found
A procedure for specimen optimization applied to material testing in plasticity with the virtual fields method
No full textA simulator to optimize the experimental set-up for elasto-plastic material characterization
No full textEffect of DIC Spatial Resolution, Noise and Interpolation Error on Identification Results with the VFM
No full textThe use of experimental tests that involve full-field measurements to characterize mechanical material properties is becoming more
widespread within the engineering community. In particular digital image correlation (DIC) on white light speckles is one of the most used tools,
thanks to the relatively low cost of the equipment and the availability of dedicated software. Nonetheless the impact of measurement errors on
the identified parameters is still not completely understood. To this purpose, in this paper, a simulator able to numerically simulate an
experimental test, which involves DIC is presented. The chosen test is the Unnotched Iosipescu test used to identify the orthotropic elastic
parameters of composites. Synthetic images are generated and then analysed by DIC. Eventually the obtained strain maps are used to identify
the elastic parameters with the Virtual Fields Method (VFM). The numerical errors propagating through the simulation procedure are carefully
characterized. Besides, the simulator is used to compare the performances of DIC and the grid method in the identification process with the
VFM. Finally, the influence of DIC settings on the identification error is studied as a function of the camera digital noise level, in order to find the
best testing configuration.sponsorship: Professor Pierron gratefully acknowledges the support from the Royal Society and the Wolfson Foundation through a Royal Society Wolfson Research Merit Award. (Royal Society, Wolfson Foundation through a Royal Society Wolfson Research Merit Award)status: Publishe
Impact of experimental uncertainties on the identification of mechanical material properties using DIC
No full textThis paper is concerned with an in-depth study of the interactions between full-field measurements errors and material identification. It is a further step in a research plan that aims to create a simulation procedure of actual experiments, with the final goal of using the simulator to optimise the test set-up in terms of specimen shape, measurement technique, applied load etc. In particular, here, Digital Image Correlation (DIC) is used as a full-field technique to obtain strain and displacement fields. These maps are used as input in an inverse methodology as, for instance, the virtual fields method (VFM) to obtain the material parameters introducing uncertainties in the characterization. The purpose of this contribution is to bridge the gap between experiments and simulations, in order to obtain predictions as close as possible to reality in terms of identification error. That will be used, as final goal of the general study, to optimize numerically a test set-up configuration, giving a priori the best parameters to use to experimentally identify a specimen. In the present contribute, the operating procedure is to perform real experiments and then to reproduce them numerically. Experimental uncertainties such as noise, lighting conditions, in-plane and out-of-plane motions are treated separately and introduced in the simulator. As such, their impact on the identified material properties can be unambiguously investigated. Here, focus is on the elastic properties of aluminium specimens, i.e. the Young’s modulus and the Poisson ratio and their specific variances due to the aforementioned errors. The simulator predicts reality to a large extent.status: Publishe
Experimental and Numerical Study of Clinched Connections
No full textManufacturing thin-walled structures and products inevitably involves the selection of suited joining techniques. The ongoing search for new, sustainable and innovative lightweight materials puts high demands on joining skills. In order to integrate these new materials in structures, products or vehicles, appropriate joining techniques have to be available. In particular for the joining of lightweight metallic sheets, which can be coated and/or dissimilar, alternative joining techniques have emerged in recent years. Amongst those, clinching -or press-joining- can assemble sheet metal parts by solely relying on local plastic deformation of the combining sheets. Unlike traditional joining techniques, clinching does not use additional material inserts such as a rivet or a bolt, and, as a consequence, the mechanical strength of such a connection highly depends on the final geometry after forming. This work focuses on the so called single stroke round clinch process with a closed die. In addition, the quasi-static mechanical response of joints produced by this technology is studied. This thesis can be divided in three major parts:1. Since clinch forming involves severe plastic deformation, the first part embarks on the identification of plastic material properties at large plastic strains. Two alternative sheet metal material tests are presented. The purpose of these tests is to reveal the plastic material behaviour hidden in the post-necking regime of thin metal sheet.2. In the second part, the complex metal flow during forming is studied with the aid of finite element techniques. Since friction plays an important role in this forming process, a strategy to identify the elements of the tribological system in clinch forming is presented. The predicted final geometry of the joint after forming is systematically compared with experiments.3. The third part concerns the mechanical response of clinched joints to quasi-static loading conditions. To be specific, the capability of finite element techniques to reproduce the experimental results obtained with pull-out tests, single lap shear tests and multi-axial loading of clinched joints is investigated. In addition, analytical models to estimate the shear and pull-out strength of a clinched connection are presented.status: Publishe
Onzekerheidsbepaling van digitale beeldcorrelatie en de impact op materiaalidentificatie
No full textThe digital image correlation (DIC) technique is a full-field non-contact optical deformation measurement tool. The technique correlates images of a specimen before and after deformation, so that the deformation can be retrieved. The correlation is obtained by comparing the similarity of the image features (intensity of pixels) before and after deformation to establish the correspondence between undeformed images and deformed images. Since it was proposed in the 1980s, the technique is largely improved and widely applied in different fields. However, as a measurement method, the accuracy of DIC is still a problem. Plenty of studies have been done concerning the performance of DIC, and yet there are no solid solutions to assess the accuracy of the measurement.
In this research, effort is contributed to the analysis of the error of DIC both theoretically and experimentally. A comprehensive theoretical analysis is presented for the measurement errors on the displacement field and the strain field obtained by subset-based DIC. In general, the systematic measurement error can not be estimated, but the random measurement error is well studied. For the displacement error, a general solution is provided to assess the random measurement error due to the imaging noise. The solution indicates that the random displacement error is determined by the imaging noise level, the intensity gradient of the interested image, the subset size, the sub-pixel part of displacement, and the interpolation scheme. For the strain error, the analysis suggests that the random error relates to the DIC implementations (i.e. subset size, step size and strain window size), the extent of deformation, and the random error of displacement measurement.
As the theoretical analysis can not fully estimate the measurement error, an experimental approach is proposed to study the performance of DIC with virtual tests associated with finite element analysis (FEA). The idea of this approach is to numerically simulate a real DIC test, and assess the measurement error of DIC afterwards. In this approach, the interested experiment is firstly simulated by FEA with prescribed material properties. The obtained deformation fields are extracted from FEA, and are utilized to deform a reference image numerically. DIC is then performed to measure this deformation by correlating the numerically deformed image and the reference image. By comparing the DIC measured deformation with the FEA results, one is enabled to assess the accuracy of the DIC measurements. In the current stage, this approach could be generalized to any 2D DIC applications.
Amongst possible applications of DIC, an important one is the study of the the mechanical behaviour of materials, which is often described by phenomenological constitutive equations (e.g. hardening law and yield criterion). The constitutive equations are one of the basis of FEA for material simulations. To obtain the parameters for the constitutive equations, inverse modelling methods can be adopted associated with full field measurements such as DIC. Inverse modelling methods include several different categories, among which the finite element model updating (FEMU) method is intuitive to understand. The FEMU method combined with DIC has been well developed for years. However, the accuracy of DIC measurements is rarely considered. This is rather dangerous since misinterpretation of the measurements leads to meaningless results. In this thesis, the uncertainty of DIC measurement is studied. As such, the measurement error can be taken into account by using a weighted cost function in FEMU. The mechanical properties of steel DC06 are studied. The results suggest that the identified parameters are sensitive to the measurement errors, and more appropriate parameters are expected when the measurement error and the strain profile are considered.
The accuracy of phenomenological constitutive equations is limited: indeed they are only valid for the experiments to which they are calibrated. As such, the identified parameters will generally depend on the type of experiments used to calibrate them. Therefore, FEA of the material under specific deformation might not be acceptable when the parameters are identified for another deformation state. Consequently, it is necessary to identify the parameters from tests in accordance with the interested deformation state so that FEA is capable to provide optimized simulations. As out-of-plane deformation are generally involved in the modern manufacturing fields, material properties obtained from 2D tests might not be sufficient for accurate FEAs of these processes. In literature, however, FEMU with stereo tests is seldom discussed. In this work, a method is proposed to associate FEMU with stereo DIC in an Erichsen bulging test. It is found that the optimized constitutive parameters for the Erichsen test are different from the ones obtained from 2D tests, which proves the necessity of the stereo FEMU method.status: Publishe
Numerical modelling of dynamic ductile fracture propagation in different lab-scale experiments using GTN damage model
No full textsponsorship: The author gratefully acknowledges the support of the Research Foundation Flanders (FWO) via PhD fellowship grant 1SB6420N.. (Research Foundation Flanders (FWO) via PhD fellowship|1SB6420N)status: Published onlin
Impact of motion blur on stereo-digital image correlation with the focus on a drone-carried stereo rig
No full textStereo-digital image correlation (DIC) is a wide-spread technique in the field of experimental mechanics for measuring shape, motion, and deformation and it is frequently used for material identification by using inverse methods (e.g., virtual fields method and finite element model updating). New applications emerge due to the reached maturity level of the technique, which poses new challenges towards reaching a desired level of accuracy in operating conditions. In this work, the possibility of a drone carrying an in-house-made portable DIC setup is explored, and the effect of the drone-induced vibrations on the accuracy of stereo-DIC for shape and strain measurement is evaluated. During acquisition, the relative motion between the camera system and the measured item generates motion-blurred images. The effect of this phenomenon on the precision of stereo-DIC is further evaluated in this paper.status: Publishe
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