Une nouvelle procédure d’identification des paramètres de lois cohésives

L’objectif est ici d’introduire un nouvel outil de caractérisation de lois cohésives, modèle le plus couramment utilisé pour décrire et simuler le phénomène de délaminage. Cet outil est basé sur une méthode de corrélation d’image globale, où la définition de l’espace de recherche cinématique est adéquatement choisie. Après avoir présenté le principe de cette méthode, on s’intéresse à sa validation, ainsi qu’à son comportement au bruit. Cette validation est réalisée numériquement, (i) en construisant un ensemble d’images à l’aide d’un outil de simulation évolué du délaminage et (ii) en identifiant à partir de ces images les caractéristiques du comportement de l’interface

In-situ measurement of machining part deflection with Digital Image Correlation

International audienc

FPGA-based architecture for bi-cubic interpolation: the best trade-off between precision and hardware resource consumption

International audienc

Une approche en dissipation pour l’identification de propriétés matériaux hétérogènes à partir de mesures de champs

Une piste privilégiée pour l’identification rapide de comportements complexes ou très hétérogènes est l’emploi de mesures de champs (cinématiques, thermiques, etc.). De par leur richesse par rapport à des mesures ponctuelles classiques, ces mesures permettent d’envisager l’identification de lois d’évolution complexes sur la base de peu d’essais très hétérogènes. Plusieurs méthodes ont été proposées en ce sens, principalement en élasticité linéaire. Nous détaillons ici l’application de l’erreur en relation de comportement, en vue de l’identification de modèles élastiques linéaires. L’extension naturelle de l’erreur en relation de comportement, au travers d’une méthode de saut à la dissipation, est proposée pour l’identification de phénomènes dissipatifs. Des exemples de référence 2D concluent cette publication

Une approche en dissipation pour l’identification de propriétés matériaux hétérogènes à partir de mesures de champs

Une piste privilégiée pour l’identification rapide de comportements complexes ou très hétérogènes est l’emploi de mesures de champs (cinématiques, thermiques, etc.). De par leur richesse par rapport à des mesures ponctuelles classiques, ces mesures permettent d’envisager l’identification de lois d’évolution complexes sur la base de peu d’essais très hétérogènes. Plusieurs méthodes ont été proposées en ce sens, principalement en élasticité linéaire. Nous détaillons ici l’application de l’erreur en relation de comportement, en vue de l’identification de modèles élastiques linéaires. L’extension naturelle de l’erreur en relation de comportement, au travers d’une méthode de saut à la dissipation, est proposée pour l’identification de phénomènes dissipatifs. Des exemples de référence 2D concluent cette publication

Introducing Virtual DIC to Remove Interpolation Bias and Process Optimal Patterns

International audienceBackground: Digital Image Correlation (DIC) is an image-based measurement technique routinely used in experimental mechanics, which provides displacement and strain maps of an observed surface/volume. The metrological performance of DIC has reached its limit which is directly determined by the texture of the imaged surface/volume. Objective : This paper proposes a novel DIC strategy, which relies on a virtual image. This image, noiseless and of infinite resolution, is moreover optimized for providing measurements with the best metrological performance. Methods : The so-called Virtual DIC retrieves the displacement fields by comparing this virtual image to the experimental images. No interpolation is required andprocessing optimal textures such as checkerboards is possible. Results : Virtual DIC is first applied on synthetic images for comparison purposes with a usual DIC approach. Outstanding metrological performance is observed thanks to the possibility of processing checkerboard patterns. Conclusions : The proposed Virtual DIC is twofold: (i) thanks to the use of a closed-form expression, built-in DIC operators are elaborated without recurring to noisy and poorly defined real images. Interpolation is therefore avoided; (ii) it makes possible it to process checkerboard patterns, which offers the best metrological performance

Towards standardized mechanical characterization of microbial biofilms: analysis and critical review

International audienceDeveloping reliable anti-biofilm strategies or efficient biofilm-based bioprocesses strongly depends on having a clear understanding of the mechanisms underlying biofilm development, and knowledge of the relevant mechanical parameters describing microbial biofilm behavior. Many varied mechanical testing methods are available to assess these parameters. The mechanical properties thus identified can then be used to compare protocols such as antibiotic screening. However, the lack of standardization in both mechanical testing and the associated identification methods for a given microbiological goal remains a blind spot in the biofilm community. The pursuit of standardization is problematic, as biofilms are living structures, i.e., both complex and dynamic. Here, we review the main available methods for characterizing the mechanical properties of biofilms through the lens of the relationship linking experimental testing to the identification of mechanical parameters. We propose guidelines for characterizing biofilms according to microbiological objectives that will help the reader choose an appropriate test and a relevant identification method for measuring any given mechanical parameter. The use of a common methodology for the mechanical characterization of biofilms will enable reliable analysis and comparison of microbiological protocols needed for improvement of engineering process and screening

Designing Patterns for DIC with Poisson Image Editing

International audienceBackground: The pattern used in DIC directly influences the quality of the measurement obtained with this technique, so it is of prime importance to optimize the geometry of the features characterizing the pattern to achieve the best metrological performance. Objective: The primary objective of this study is to quantify the influence of the collinearity between the actual displacement and the pattern image gradient on the systematic and random errors affecting displacement fields measured by DIC. Methods: Poisson Image Editing (PIE), a technique borrowed from the image processing community where it was introduced to perform seamless cloning of images, has been employed here to render various patterns whose gradient is globally collinear to the displacement field. Directly integrating the displacement field considered as the image gradient does not lead to a sufficiently contrasted pattern. A texture is therefore superposed to the displacement field before integration. Results: Different patterns obtained or not by PIE are compared, and it is shown with suitable simulations performed with synthetic images that rendering a pattern where the image gradient is globally collinear to the displacement significantly reduces the total error affecting the measurements. The value of this improvement depends on the texture added to the displacement field before applying PIE. Conclusion: 1 This study demonstrates that the collinearity between the speckle image gradient and the displacement field reduces the error affecting the displacement fields measured by DIC. It also proposes a route to account for this displacement in the design of an optimized pattern when a priori knowledge on the direction of the displacement is available. If not, it is shown that a small checkerboard inlayed in a larger one leads to a metrological performance much better than that obtained with a classic random pattern, and close to that achieved with an optimized pattern for which the displacement field is accounted for

Interface debonding characterization by image correlation integrated with Double Cantilever Beam kinematics

A procedure is proposed for the identification of spatial interfacial traction profiles of peel loaded Double Cantilever Beam (DCB) samples, from which the corresponding traction-separation relation is extracted. The procedure draws upon recent developments in the area of non-contact optical techniques and makes use of so-called Integrated Digital Image Correlation (I-DIC) concepts. The distinctive feature of the I-DIC approach proposed herein is that the unknown degrees of freedom are not displacements or rotations, but the set of interfacial fracture properties describing the traction profile. A closed-form theoretical model is developed to reconstruct a mechanically admissible displacement field representing the deformation of the adhering layers during debonding in the DCB fracture test. The proposed modeling accounts for the spatial traction profile along the interface between the adherends using few degrees of freedom, i.e. crack tip position, maximum stress and size of the process zone. By minimizing the correlation residual with respect to the degrees of freedom, the full set of interfacial fracture properties is obtained through a one-step algorithm, revealing a substantial gain in terms of computational efficiency and robustness. It is shown that the identified traction profile can be effectively combined with the crack opening displacement to extract the corresponding traction-separation relation, i.e. the key input data for any cohesive zone model (CZM). The proposed procedure is validated by post-processing virtually deformed images generated through the finite element method. The robustness with respect to noisy data, as well as the low sensitivity to the initial guess, are demonstrated. © 2014 Elsevier Ltd. All rights reserved