In situ 3D characterization of fatigue cracks displacement fields

The three dimensional growth of fatigue cracks in samples of nodular graphite cast iron ischaracterized using laboratory X-ray computed tomography. The cracks grow from laser machined artificialdefects, their development is monitored in situ using laboratory X-ray computed tomography (lab. CT) andDigital Volume Correlation (DVC). The combination of both techniques gives access to the 3D displacementfield at the tip of the crack (mainly mode I opening)

Identification d'une loi cohésive à partir d'images numériques : cas du PMMA

Depuis les travaux de Dugdale et Barenblatt, le concept de zone cohésive a été largement utilisé tant du point de vue matériaux que du point de vue numérique. Néanmoins, les échelles mises en jeu dans ces modèles (ouverture critique de quelques microns, longueur de zone cohésive d'une vingtaine de microns pour le PMMA) font de l'identification de ces modèles un problème délicat. Nous proposons sur la base d'observations optiques à l'échelle de la zone cohésive et de corrélation d'images, une stratégie permettant d'identifier les paramètres d'une loi cohésive pour le PMMA

Suivi par corrélation d'images du benchmark FATACRACK

Dans le cadre du benchmark du GDR 3651, des essais ont été réalisés au GeM à l'Ecole Centrale de Nantes. Un traitement des essais par corrélation d'images est effectué. Le suivi de la position de la fissure au cours des cycles et la mesure des facteurs d'intensité des contraintes sont mis en place. Des mesures aux crêtes sont effectuées de manière périodique. Des mesures à l'intérieur des cycles sont également mises en place régulièrement pour essayer de capter les effets de fermeture et leur évolution

Damage law identification of a quasi brittle ceramic from a bending test using digital image correlation

International audienceAlthough ceramics are generally considered as to be elastic brittle solids, some of them are quasi brittle. These ceramics show a non linear mechanical behaviour resulting most of the time in a difference between their tensile and compressive stress-strain laws. The characterization of their fracture strengths might be biased if elastic linear formulae are used to analyze classical tests like bending tests. Based on Digital Image Correlation (DIC), an efficient technique to measure full field displacements, a methodology is proposed to characterize and model materials with dissymmetric behaviours between tension and compression. Applying specific basis functions for DIC displacement decompositions for bending, compressive and tensile tests, a stress-strain model and its damage law are identified and then validated for aluminium titanate, a damageable micro cracked ceramic at room temperature. This identification method using DIC can obviously be applied to other quasi brittle materials

Molecular imaging for stem cell therapy in the brain

In this work, we suggest a method, based on the data driven computational mechanics framework introduced by Kirchdoerfer and Ortiz, to extract representative strain-stress couples from a collection of non-homogenous full field measurements corresponding to different loading conditions

Mixed-mode crack propagation using a Hybrid Analytical and eXtended Finite Element Method

International audienceA Hybrid Analytical and eXtended finite element method (HAX-FEM) is proposed to study the propagation of curved mixed-mode cracks. A Double Edge Notched test on a concrete sample is simulated. A specific treatment is proposed to account for crack initiation from a finite width notch. The crack morphology is in good agreement with previously published experimental results

Estimation du facteur d'intensité des contraintes à partir d'un champ cinématique mesuré

On propose une technique de réduction de l'influence du bruit de mesure sur la détermintation des facteurs d'intensité des contraintes. L'intégrale d'interaction est utlisée dans les développements présentés. Le champ d'extension virtuelle associé à cette intégrale est optimisé pour réduire l'influence d'un bruit affectant la mesure du champ de déplacement par corrélation d'images numériques étendue. Il est alors possible de prendre en compte la texture des images et les caractéristiques des fonctions de discrétisation grâce à un noyau de corrélation spécifique. Un exemple illustre les améliorations obtenues

Vibrational Properties of Quasi-periodic beam structure

Quasi-periodic structures have been widely studied for its atom dynamics, Photonic, magnetic and electronic wave propagation(Ashraff and Stinchcombe (1989), Vekilov et al. (1999), Szallas and Jagannathan (2008)). These structures exhibit frequency ranges in which no propagative wave exist(Bayindir et al. (2001) and Engel et al. (2007)). Band gap can lead to interesting applications in various domain (Florescu et al. (2009)). The recent progress in the additive manufacturing open new possibilities for using quasi-periodic structures by allowing the printing of complex meta-materials, in a consistent manner. Additive manufactured meta material can be designed to exhibit unusual macroscopic behavior due to their internal structure as in (Bückmann et al. (2012) and Claeys et al. (2014)). Therefore the possibility of creating meta materials having the same properties than the quasi periodic atomic structures can be highly interesting and allow to get rid of unwanted complex behaviors (Engel et al. (2007)). Such meta material could create band gap in their virbational mechanical response while being isotropic.  The mechanical and vibrational properties of quasi-periodic and of amorphous structures are related to complex mathematical problems due to the impossibility of periodic simplifications. Therefore, in order to solve these problems, big size matrix problems have to be dealed with. In this paper a beam structure based on a octohedric quasi-periodic Pensore lattice approximant is studied. The octohedric Penrose tilling is chosen for his ability to create a periodic approximant to the quasi periodic tilling thus allowing the use of periodic boundary conditions as suggested in (Duneau (1989)).It has been shown in Sørensen et al. (1991) that, for ferromagnetic properties, the approximant with periodic boundary conditions closely mimic the in finite lattice properties. Numerical methods for the vibrational study of big systems have been applied from the atomic vibration domain up to finite elements modelization. Kernel Polynomial Method (KPM) is used herein to calculate the Vibrational Density Of States (VDOS) and the Dynamical Structure Factor (DSF) without exact diagonalization of the dynamical matrix. The KPM method is detailled by Lin et al. (2016) and was adapted to the study of to the vibrational properties of large-size systems by Beltukov et al. (2016b). These methods allow better understanding of vibrational response of quasi-periodic structures

From pictures to extended finite elements: extended digital image correlation (X-DIC)

International audienceAn image correlation algorithm accounting for discontinuities is proposed. It is based on a decomposition of the displacement field onto a regular finite element basis supported by a uniform mesh, enriched with suitable functions to describe accurately discontinuities, paralleling recent developments of extended finite elements. This algorithm is applied to a bolted assembly where interface slip is observed upon loading