Un cadre pour la réduction de modèle dans la simulation de structures assemblées

Les assemblages boulonnés influencent fortement l'amortissement et la rigidité des structures. Malheureusement, leur comportement mécanique reste difficile à prédire, principalement parce que les phénomènes physiques impliqués se produisent avec des échelles très différentes, taille des zones du contact réel et longueur d'onde des modes de vibration. Cela rend la méthode des éléments finis classiques difficile à utiliser parce que dans les zones de contact, le maillage doit être fin, alors qu'il peut être grossier ailleurs. Pour surmonter ces difficultés, le but de cet article est d'utiliser une décomposition en deux sous-domaines qui permet de travailler avec deux niveaux de discrétisation spatiale différents. La première idée consiste à résoudre alternativement les problèmes écrits sur les deux sous-domaines jusqu'à convergence. Ceci est fortement inspiré de la méthode LATIN mais adapté à l'étude des vibrations périodiques en utilisant la méthode de l'équilibrage harmonique (HBM) couplée à celle de l'algorithme ""alternating frequency time"" (AFT). La deuxième idée de ce travail est de réduire l'ordre du modèle en utilisant une projection sur base réduite (méthode de Ritz) et des méta-modèles couvrant le sous-espace qui contient les solutions

Analyse multi-échelle expérimentale et numérique du comportement d'une liaison

Les vibrations des structures assemblées sont très dépendantes du comportement mécanique des liaisons. La rigidité et l'amortissement induits par ces liaisons dépendent essentiellement des conditions de contact / décollement et d'adhérence / glissement. La mesure des champs de déformations dans les zones de liaisons et à proximité de celles-ci constitue donc un enjeu important pour la mise au point de modèles prédictifs. Ce travail porte sur l'utilisation de la vélocimétrie laser en trois dimensions pour la caractérisation du comportement de ces liaisons. Un assemblage constitué de deux poutres boulonnées en plusieurs points a été testé lors d'essais vibratoires sous excitations aléatoire et harmonique. La mesure du champ de déplacement global permet d'évaluer l'évolution des paramètres modaux en fonction de l'amplitude des vibrations. Des mesures du champ de déplacement local permettent d'évaluer les conditions de contact et de frottement dans chaque assemblage. Des corrélations avec les modèles, via l'étude de l'erreur en relation de comportement, et des études de sensibilité de ces modèles aux différents paramètres sont également proposées

Erratum to: Intercomparison of the Arctic sea ice cover in global ocean–sea ice reanalyses from the ORA-IP project

In the original publication of the article, Fig. 11 was published incorrectly. The correct version of Fig. 11 is provided her

Vibration couplings in assembly dynamics

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Analyse de champs par vibrométrie laser sur un assemblage boulonné

International audienceAnalyse de champs par vibrométrie laser sur un assemblage boulonn

Design of an assembly for nonlinear vibration reduction

International audienceThe lightness of the space and aerospace structures causes their vulnerability to vibrations. The cold temperatures do not allow using polymer materials. Active and semi-active control using piezos embedded on the structure can be used efficiently instead of polymers but they induce energy consumption. Friction damping is less efficient but it does not depend on temperature and it is energetically passive. Unfortunately its efficiency depends on the vibration amplitude as well as on the tightening force. The damping is very low for the lowest amplitudes and the largest tightening loads and increase up to an optimal value. This optimum damping is adjustable thanks to the tightening force. The purpose of our work is to evaluate the efficiency of the control of the tightening force in bolted joints to reduce the vibration of the assembled structure. In the first part, we present a original setup and a very detailed design of experiments that highlights the optimal sets of parameter in order to get a good control of the vibrations according to the frequency and the magnitude of the load. To conclude, we propose to share experimental data with the attendees for further discussions

Performances of an architected composite plate with nonlinear stick-slip interactions for vibration reduction

International audienceMetamaterials are architectured structures that have recently shown convincing performances for vibration reduction. Two main phenomena are commonly coupled to achieve these performances, local resonances and Bragg scattering that allow to develop interesting frequency bandgaps where waves cannot propagate. The performances for vibration reduction of a lattice metamaterial introduced as a core in a composite structure are here experimentally and numerically studied. This structure is composed of a series of slits with rotational symmetries and is placed between two aluminium plates : the planar configuration with periodic resonant patterns is propitious to generate bandgaps. Different parameters are adjusted to achieve good performances as the number of patterns, the width of the slots, or the length of the internal beams. Nonlinear effects consisting in stick-slip interactions inside patterns are evaluated as favorable aspects to increase the obtained globaldamping

Clustering acoustic emission data streams with sequentially appearing clusters using mixture models

International audienceThe interpretation of unlabeled acoustic emission (AE) data classically relies on general-purpose clustering methods. While several criteria have been used in the past to select the hyperparameters of those algorithms, few studies have paid attention to the development of dedicated objective functions in clustering methods able to cope with the specicities of AE data. We investigate how to explicitly represent clusters onsets in mixture models in general, and in Gaussian Mixture Models (GMM) in particular. We propose the rst clustering method able to provide, through parameters estimated by an expectation-maximization procedure, information about when clusters occur (onsets), how they grow (kinetics) and their level of activation through time. This new objective function accommodates continuous timestamps of AE signals and, thus, their order of occurrence. The method, called GMMSEQ, is experimentally validated to characterize the loosening phenomenon in bolted structure under vibrations. A comparison with four standard clustering methods on raw streaming data from five experimental campaigns shows that GMMSEQ not only provides useful qualitative information about the timeline of clusters, but also shows better performance in terms of cluster characterization

Clustering acoustic emission data streams with sequentially appearing clusters using mixture models

International audienceThe interpretation of unlabeled acoustic emission (AE) data classically relies on general-purpose clustering methods. While several criteria have been used in the past to select the hyperparameters of those algorithms, few studies have paid attention to the development of dedicated objective functions in clustering methods able to cope with the specicities of AE data. We investigate how to explicitly represent clusters onsets in mixture models in general, and in Gaussian Mixture Models (GMM) in particular. We propose the rst clustering method able to provide, through parameters estimated by an expectation-maximization procedure, information about when clusters occur (onsets), how they grow (kinetics) and their level of activation through time. This new objective function accommodates continuous timestamps of AE signals and, thus, their order of occurrence. The method, called GMMSEQ, is experimentally validated to characterize the loosening phenomenon in bolted structure under vibrations. A comparison with four standard clustering methods on raw streaming data from five experimental campaigns shows that GMMSEQ not only provides useful qualitative information about the timeline of clusters, but also shows better performance in terms of cluster characterization

A new concept of vibration damper based on friction induced dissipation between magnetic materials

International audienceThis paper presents a new concept for passive vibration mitigation in sandwich structures. The test structure in the present work is composed of permanently magnetized MagnetoActive Elastomers (MAEs) and ferromagnetic sheets representing the core and the skins of the sandwich respectively. The magnetic forces which develop between the core and the skins in the contact interfaces are sufficient to maintain the structure assembled without the need of glue. The sandwich structure is subjected to harmonic vibrations around a single resonance mode with different excitation magnitudes. A nonlinear evolution of the frequency response function’s (FRF) amplitude are stated. This non-linearity induces on the one hand a decrease of the resonance frequency. On the other hand, the damping evolves in a non-monotonic way according to the excitation amplitude. This is very characteristic of frictional damping