A mesoscopic approach to study the influence of aggregates spatial arrangement on concrete dynamic behavior

International audienceThe strength variability of the concrete-like materials is usually represented, in pheno-menological macroscopic models, using stochastic tools that need to be identified with a large number of experimental tests. The variability phenomenon is then usually linked to the non-uniform characteristics of the material at the mesoscopic scale, due to the various phases of the concrete. An alternative to a macroscopic model with numerous parameters is to represent these phases explicitely. In this work, we intend to explore the influence of the heterogeneous meso-structure of the concrete (aggregates in a cement paste matrix) on its dynamic properties. For this purpose, we use FE simulations with a cohesive approach and an explicit representation of the meso-structure. A 2D geometrical model of concrete consisting of aggregates, in-terfacial transition zones and a matrix is used. That kind of approach has been previously been used to give a better understanding of the fracture mechanisms ([1], [2]) and gave relevant results on the Dynamic Increase Factor (DIF) experimentaly observed. However, if the scale of the numerical specimen is large enough, that means that the material can be seen as homogeneous, and we are not able to reproduce the experimental variability on the tensile strength [3]. The influence of the spatial disposition of the elements of the mesostructures is still open for discussion. Specifically, the impact of the clustering of aggregates on crack initiation and propagation. In our study, we propose to analyse the effect of such groups of aggregates later called clusters. In order to detect and mesure the clustering, we developped a statistical indicator inspired from a global estimator, Ripley's function [4]. Numerical results coupling the mesure of the clustering with its effect on the global fracture process will be presented

Influence of the meso-structure in numerical simulation of dynamic tensile fracture of concrete

International audienceThe macroscopic modeling of the tensile response of concrete in dynamic has considerably evolved and the current constitutive laws are able to represent more and more complex phenomena. Nevertheless, in these approaches the different scales present in the structure of the material are not represented despite their strong influence on the macroscopic behaviour. In this work the influence of the heterogeneous meso-structure of concrete on the dynamic tensile response is analyzed. For this purpose a 2D geometrical model of concrete consisting of aggregates, interfacial transition zones and a matrix is modeled. We use a finite element framework with cohesive elements to explicitly represent the crack nucleation and growth. The debonding process in the cohesive elements is controlled by a traction separation law based on the popular linear extrinsic irreversible law proposed by Camacho and Ortiz [1]. This numerical approach has proved its efficiency on brittle material like ceramics [2] without an explicit meso-structure representation. We validate our model by simulating a direct dynamic tension test of a concrete specimen. The role of the meso-structure and the influence of the loading rate are then analyzed. We especially focus on the evolution of the stress peak and the energy dissipated. We can observe that with a traction separation law independent of the strain rate we are not able to reproduce the macroscopic rate effect experimentally observed while the global dissipated energy is correctly simulated. We will also show the respective influences of the meso-structure and the loading rate on the variability of the stress peak in tension

A testing technique for concrete under confinement at high rates of strain

International audienceA testing device is presented for the experimental study of dynamic compaction of concrete under high strain rates. The specimen is confined in a metallic ring and loaded by means of a hard-steel Hopkinson pressure bar (80 mm diameter, 6 m long) allowing for the testing of specimens large enough regarding the aggregate size. The constitutive law for the metal of the ring being known, transverse gauges glued on its lateral surface allow for the measurement of the confining pressure. The hydrostatic and deviatoric responses of the specimen can then be computed. The proposed method is validated by several numerical simulations of tests involving a set of four different concrete-like behaviours and different friction coefficients between the cell and the specimen. Finally, three tests performed with the MB50 concrete at three different strain rates are processed with the method and are compared with literature results for the same material under quasi-static loadings

Delay-active damage versus non-local enhancement for anisotropic damage dynamics computations with alternated loading

International audienceAnisotropic damage thermodynamics framework allows to model the concrete-like mate- rials behavior and in particular their dissymmetric tension/compression response. To deal with dynamics applications such as impact, it is furthermore necessary to take into account the strain rate effect observed experimentally. This is done in the present work by means of anisotropic visco-damage, by introducing a material strain rate effect in the cases of posi- tive hydrostatic stresses only. The proposed delay-damage law assumes no viscous effect in compression as the consideration of inertia effects proves sufficient to model the apparent material strength increase. High-rate dynamics applications imply to deal with wave prop- agation and reflection which can generate alternated loading in the impacted structure. In order to do so, the key concept of active damage is defined and introduced within both the damage criterion and the delay-damage evolution law. At the structural level, strain localization often leads to spurious mesh dependency. Three-dimensional Finite Element computations of dynamic tensile tests by spalling are presented, with visco-damage and either without or with non-local enhancement. Delay- damage, as introduced, regularizes the solution in fast dynamics. The location of the macro-crack initiated is found influenced by non-local regularization. The strain rate range in which each enhancement, delay-damage or non-local enhancement, has a regularizing effect is studied

Shaking of reinforced concrete structures induced by a transient dynamic load

International audienceThe purpose of our study consists in the research of new ways of designing reinforced concrete structures submitted to commercial aircraft impact. We will particularly focus on shaking resulting from such load case. The cutoff frequency of this kind of stress is within the range of 40 to 100 Hz which should correspond to medium frequency [1]. Therefore, industrial structure design implies dynamic studies. The response, especially during the transient stage, cannot be completely described using classical finite element method associated with explicit numerical schemes. Indeed, the medium frequency range is often ignored unless the calculation is carried out with a very refined mesh and consequently, a refined time discretization. This would lead to a prohibitive computation time. The linear behaviour is not questioned outside the impact area, however, the non-linearity of the portion of the impacted structure can have a significant influence. A new multiscale computational strategy, the Variational Theory of Complex Rays [2], is developed for the analysis of the vibration of structures in the medium frequency regime. Using two-scale shape functions which verify the dynamic equation and the consecutive relation within each substructure, the VTCR can be viewed as a means of expressing the power balance at the different interfaces between substructures in variational form. The solution is searched as a combination of propagative and evanescent waves. Only the amplitude of these waves, which are slowly varying quantities of the solution, are discretized. This leads to a numerical model with few degrees of freedom in comparison with a Finite Element model. The method consists in an initial decomposition FFT (Fast Fourier Transform) of the signal loading. The VTCR ensures the transfer of the decomposed signal into the structure. The obtained signals are then processed by inverse FFT (IFFT) to reconstruct a time signal and a response spectrum [3]. The aim is to develop a robust method to get mid-frequency spectra generated by an aircraft impact on a simplified containment [4]. References [1] Hervé G., « Simulation du comportement sous impact de structures en béton armé », PhD thesis, ENS Cachan, 2005 [2] Ladeveze P., Arnaud L., Rouch P. and Blanze C., « The variational theory of complex rays for the calculation of medium-frequency vibrations », Engineering Computations, Vol. 18, pp 193-214, 2001. [3] Chevreuil M., « Sur une nouvelle approche en calcul dynamique transitoire, incluant les basses et les moyennes fréquences », PhD thesis, ENS Cachan, 2005. [4] Thai Nam Quan, « Etude de la réponse en moyenne fréquences d'une enceinte de confinement par la Théorie Variationnelle des Rayons Complexes », Master Thesis, ENS Cachan, 2009

Anisotropic 3D delay-damage model to simulate concrete structures

International audienceHigh dynamic loadings lead to material degradation and structural failure. This is even more the case for concrete structures where the parts initially in compression break in ten- sion due to waves propagation and reflection. The dissymmetry of the material behavior plays a major role in such cases, dissymmetry mainly due to damage induced anisotropy. Loading induced damage is most often anisotropic and one proposes here to take advantage of such a feature to build a damage model for concrete, dissymmetric in tension and in compression, 3D, suitable for dynamic computations. A single 2nd order tensorial damage variable D is consid- ered with a damage law ensuring a damage rate proportional to the square of the positive part of the strain tensor. One focus in the present work on viscous regularizations for the anisotropic damage model proposed, regularizations of Norton-Perzyna type. Numerical examples of dy- namic failures illustrate the ability and the efficiency of the model to deal with 3D structures

Calcul de perméabilité en milieu fissuré : approche méso-macro

International audienceIn this paper, a sequential multi-scale framework to solve mass (air or water) transfer problems is described. Numerical results are checked against mechanical and permeation experimental datas from a reinforced concrete specimen under tensile load designed by C. Desmettre and J.P. CharronCe papier présente une approche multi-échelle séquentielle permettant de résoudre des problèmes de transfert de masse (air ou eau). Les résultats numériques sont confrontés à des données expérimentales obtenues par C. Desmettre et J.P. Charron [DES 11]. Ces derniers ont mesuré le débit traversant un tirant en béton armé sous différents paliers de chargement

Dimensionnement de structures sous impacts : du comportement des matériaux à la simulation numérique

L'objectif de ce mémoire est de faire une présentation synthétique des activités de recherche que j'ai menées au sein du LMT Cachan depuis neuf ans et de mettre en avant le fil conducteur du travail réalisé. Ce travail est la synthèse de collaborations avec différents chercheurs, industriels, doctorants et stagiaires de M2.Trois thèmes principaux seront développés ici. Ils concernent, comme le titre du mémoire l'indique, la modélisation mécanique et numérique de structures soumises à un impact. Afin de pouvoir proposer des méthodes prédictives de dimensionnement, je me suis attaché à aborder les trois éléments de bases : la modélisation du matériau, l'expérimentation sur structures et la modélisation numérique. En plus de ma notice individuelle, ce document présente donc les principaux résultats obtenus dans chacune des trois thématiques suivantes : Développement de lois de comportement adaptées à la dynamique Calculs sur structures industrielles Caractérisation expérimentale Pour chacun de ces trois thèmes, les motivations du travail et les principaux apports sont présentés en compléments de la copie d'un article représentatif des activités menées. La première partie est consacrée au premier thème de recherche développé, à savoir la modélisation du comportement des matériaux, tout particulièrement le béton, soumis à des sollicitations dynamiques. Je présenterai dans celui-ci un bref rappel du modèle développé lors de ma thèse de doctorat et qui a servi de base de travail pour une thèse que j'ai co-encadrée. Dans un deuxième temps, je présenterai un modèle d'endommagement anisotrope qui a fait l'objet de nombreux développements et publications ces dernières années. Enfin, je finirai par présenter un modèle utilisé pour les matériaux composites et que j'ai été amené à utiliser. Le seconde chapitre vise à présenter mes principales contributions concernant le développement de méthodes multiéchelles pour la représentation des matériaux et/ou de structures sous chargement sévère. Ainsi, je commencerai par présenter une approche ou le matériau n'est plus considéré comme homogène mais constitué d'une matrice et d'inclusions plus ou moins rigides. Ensuite je m'attarderai un peu plus sur deux méthodes numériques développées pour la modélisation de structure. Ces deux méthodes sont le fruit de mon passage dans le secteur ''Structures et Systèmes'' du LMT Cachan pendant 4 ans. Ceci m'a permis d'étoffer le champs de mes connaissances dans les matériaux composites et dans la méthode LATIN développée au laboratoire. La troisième partie est liée au développement d'outils de calcul prédictifs pour les problèmes d'impact. C'est pourquoi je commencerai par présenter des essais sur tour de chute que nous avons développés en collaboration avec le CEA Saclay. Ces essais nous ont ensuite été utiles pour montrer les capacités de prédictions des différentes modélisations développées. Ceci a été confirmé par la simulation d'essais de souffle et d'impact de réacteur d'avion sur des dalles en béton armé réalisés dans les années 90. Finalement, quelques perspectives sur mes activités de recherche sont proposées à la fin de ce document. Cette dernière partie présente notamment les activités de recherche et d'encadrement qui débute cette année. L'annexe correspond à mon dossier professionnel. Il contient, en plus de mon CV, mon implication dans divers contrats de recherche, mes activités d'encadrement, ainsi que la liste de mes publications scientifiques

Prévision de la rupture des ouvrages en béton sollicités en dynamique rapide

This study deals with the aspects of the concrete behaviour under dynamic loading such as impacts and explosions. For this kind of solicitations concrete experiences compaction, a decrease of porosity, plastic strains in compression and also cracking in tension. It is relevant to be able to represent, at the same time, the phenomena of compaction of material, as well as the rupture by phenomenon of extension. The objective of this study is initially to test a constitutive model for concrete, developed within a static framework, for loading where the strain rate becomes significant. In that goal, original experiments of dynamic concrete compaction were developed. These experiments are based on the split Hopkinson test and were carried out on confined concrete samples. These tests made it possible to highlight a significant effect of the strain rate on the spherical behaviour of concrete. Following these experimental observations, a new viscoplastic and visco-damage model was developed. This model is based on Perzyna viscoplasticity associates with a modified Gurson yield function and on a Dubé visco-damage model. This new constitutive model for concrete is able to represent the rate effect experimentally observed. The two models presented in this work were implemented in the finite elements code LS-DYN3D. Simulations of tests carried out on structures allowed to validate the numerical implementation, as well as the "viscous'' model for fast dynamic loading.Ce travail concerne la prise en compte des aspects du comportement du béton sous des chargements dynamiques tels que les impacts ou les explosions. Pour ce type de sollicitations, il est important d'être capable de représenter à la fois les phénomènes de compaction du matériau, ainsi que la rupture par phénomène d'extension. L'objet de ce travail est dans un premier temps de tester une loi de comportement, développée dans un cadre statique, pour des chargements où la vitesse de déformation devient importante. Pour cela des expérimentations originales sur la compaction dynamique des bétons ont été développées. Ces expérimentations s'appuient sur la technologie des barres de Hopkinson et ont été réalisés sur des échantillons de béton confiné. Ces essais ont permis de mettre en évidence un important effet de la vitesse de déformation sur le comportement sphérique d'un béton. Suite à ces observations expérimentales, un nouveau modèle viscoplastique visco-endommageable est proposé. Ce modèle s'appuie sur une viscoplasticité de type Perzyna, associée à une surface seuil de Gurson pour les déformations plastiques et à un visco-endommagement de type Dubé associé à une nouvelle surface seuil. Ce modèle devient donc capable de représenter les effets de la vitesse de chargement observés expérimentalement. Les deux modèles de comportement présentés dans ce travail ont été implantés dans le code de calcul aux éléments finis LS-DYNA3D. Des simulations numériques d'essais réalisé sur des structures ont permis de valider l'implantation numérique, ainsi que le modèle "visqueux'' pour les chargements de dynamique rapide