Comportement dynamique de panneaux raidis pour des superstructures de frégate

International audienceLes matériaux composites ont connu un grand succès dans le domaine de la construction surtout dans les structures avancées. Toutes les structures en question exigent le maximum de satisfaction au niveau des propriétés des matériaux utilisés. Dans les navires et les avions, la possibilité d'avoir des explosions ne peut pas être mise à l'écart au moment de la conception d'une telle structure. Il est donc très important d'étudier et de tester les structures sous des chargements dynamiques. Un des objectifs est le développement des structures raidies améliorées pour les superstructures de navires à base de composites à renforts fibreux. Parmi les différents types de panneaux raidis dans une superstructure nous nous intéressons à des panneaux sandwichs avec des raidisseurs de type oméga. La structure raidie se compose d'un panneau sandwich à âme balsa joints par le remplisseur formant une transition douce (rayon de 25 millimètres), et au-dessus un raidisseur oméga stratifié de la même épaisseur que la peau du sandwich. Ces structures ont été testées sous trois vitesses d'impact : 5, 10 et 15 m/s. Pour réaliser ces essais, une machine de choc hydraulique dont dispose l'ENSTA Bretagne a été employée. Les réponses dynamiques d'un raidisseur type Ω collé, par un adhésif, sur un panneau composite de type sandwich ont été étudiées dans ce travail, sous différentes vitesses d'impact dans l'objectif d'avoir le maximum d'informations en ce qui concerne les effets de la vitesse d'impact sur de telles structures. L'analyse des résultats pour les trois vitesses d'impact appliquées aux structures, nous permet d'énoncer des conclusions sur l'évolution de la raideur dynamique, de l'effort maximal et de la cinétique de l'endommagement avec la vitesse d'impact

Modélisation du délaminage par la méthode de la zone cohésive et problèmes d'instabilité = Modelling of delamination by the cohesive zone method and problems of instability

National audienceCe travail concerne la modélisation par éléments finis du phénomène du délaminage dans les matériaux composites stratifiés. La faiblesse des zones riches en résine entre les plis peut entraîner leur décollement. Nous avons choisis la modélisation de ce phénomène par l'utilisation de la méthode de la zone cohésive qui est basée sur la mécanique de la rupture par fissuration et la mécanique de l'endommagement. Les avantages majeurs de ce modèle est de tenir compte du couplage existant entre les différents modes de rupture en mécanique linéaire de fissuration ajoutant la capacité à prédire l'initiation du délaminage. Au travers d'une formulation thermodynamique, nous avons proposé quelques variantes. L'introduction de la variable d'endommagement, dépendante du saut de déplacement local, ajoute une dimension physique au modèle. L'implémentation numérique du modèle cohésif a été réalisée. Des problèmes ont été soulevés lors de l'utilisation des éléments cohésifs qui sont principalement liés à la convergence du modèle. On s'est donc intéressé ensuite à l'étude de l'instabilité du modèle cohésif. L'étude a été menée en quasi-statique dans un code éléments finis implicite. La résolution numérique du système non-linéaire a été effectuée par la méthode de Newton-Raphson qui s'est révélée incapable de venir à bout de l'instabilité matérielle induite par le comportement adoucissant du modèle cohésif. On montre avec un essai de traction simple composé de deux barres reliées par un élément cohésif, que la solution du problème discrétisé par élément finis peut diverger (solution jumps). Cette étude a permis de déterminer et de traiter l'instabilité du système en introduisant une viscosité régulatrice dans le modèle. Cette énergie supplémentaire ajoutée au modèle peut régler complètement le problème d'instabilité pour un incrément de temps adéquat et des méthodes de résolutions numériques de type Newton-Raphson. Des simulations numériques de l'essai sur éprouvettes DCB sont présentées et comparées aux essais pour illustrer les performances du modèle

Mechanical properties of carbon black/poly (ε-caprolactone)-based tissue scaffolds

Carbon black (CB) spherical particles were added to poly(ε-caprolactone) (PCL) polymer to produce strong synthetic tissue scaffolds for biomedical applications. The objective of this paper is to study the mechanical behavior of CB/PCL-based nanocomposites using experimental tests, multi-scale numerical approaches, and analytical models. The mechanical properties of CB/PCL scaffolds were characterized using thermal mechanical analysis and results show a significant increase of the elastic modulus with increasing nanofiller concentration up to 7 wt%. Conversely, finite element computations were performed using a simulated microstructure, and a numerical model based on the representative volume element (RVE) was generated. Thereafter, Young's moduli were computed using a 3D numerical homogenization technique. The approach takes into consideration CB particles’ diameters, as well as their random distribution and agglomerations into PCL. Experimental results were compared with data obtained using numerical approaches and analytical models. Consistency in the results was observed, especially in the case of lower CB fractions

Real-time strain monitoring and damage detection of composites in different directions of the applied load using a microscale flexible Nylon/Ag strain sensor

Composites are prone to failure during operating conditions and that is why vast research studies have been carried out to develop in situ sensors and monitoring systems to avoid their catastrophic failure and repairing cost. The aim of this research article was to develop a flexible strain sensor wire for real-time monitoring and damage detection in the composites when subjected to operational loads. This flexible strain sensor wire was developed by depositing conductive silver (Ag) nanoparticles on the surface of nylon (Ny) yarn by electroless plating process to achieve smallest uniform coating film without jeopardizing the integrity of each material. The sensitivity of this Nylon/Ag strain sensor wire was calculated experimentally, and gauge factor was found to be in the range of 21–25. Then, the Nylon/Ag strain sensor wire was inserted into each composite specimen at different positions intentionally during fabrication depending upon the type of damage to detect. The specimens were subjected to tensile loading at a strain rate of 2 mm/min. Overall mechanical response of composite specimens and electrical response signal of the Nylon/Ag strain sensor wire showed good reproducibility in results; however, the Nylon/Ag sensor showed a specific change in resistance in each direction because of the respective position. The strain sensor wire designed not only monitored the change in the mechanical behavior of the specimen during the elongation and detected the strain deformation but also identified the type of damage, whether it was compressive or tensile. This sensor wire showed good potential as a flexible reinforcement in composite materials for in situ structural health monitoring applications and detection of damage initiation before it can become fatal

Nanotechnology and Development of Strain Sensor for Damage Detection

Composite materials, having better properties than traditional materials, are susceptible to potential damage during operating conditions, and this issue is usually not found until it is too late. Thus, it is important to identify when cracks occur within a structure, to avoid catastrophic failure. The objective of this chapter is to fabricate a new generation of strain sensors in the form of a wire/thread that can be incorporated into a material to detect damage before they become fatal. This microscale strain sensor consists of flexible, untwisted nylon yarn coated with a thin layer of silver using electroless plating process. The electromechanical response of this sensor wire was tested experimentally using tensile loading and then verified numerically with good agreement in results. This flexible strain sensor was then incorporated into a composite specimen to demonstrate the detection of damage initiation before the deformation of structure becomes fatal. The specimens were tested mechanically in a standard tensometer machine, while the electrical response was recorded. The results were very encouraging, and the signal from the sensor was correlated perfectly with the mechanical behavior of the specimen. This showed that these flexible strain sensors can be used for in situ structural health monitoring (SHM) and real-time damage detection applications

Effect of Graphene Nano-Additives on the Local Mechanical Behavior of Derived Polymer Nanocomposites

International audienceIn this study, indentation tests of graphene-based polymer nanocomposites were carried out to determine the local elastic mechanical properties. The samples consist of epoxy matrix with graphene additives. Additives were added at levels of 0% as a control, 0.5%, 1%, 2.5%, 5% and 10% by weight. The local elastic properties such as moduli and hardness were calculated. After each indentation, the prints were characterized using scanning electron microscopy (SEM). It seems that the local mechanical properties of nanocomposite samples were improved as the amount of nano-additives increased. Based on the curve displacement and surface imaging, we can conclude that the nano-additives influenced the overall plastic mechanical behavior of the samples. For simulating micro-indentation test, a finite element analysis model was developed using ABAQUS software and compared to experimental tests. Good correlation was observed

Electro-thermal–mechanical performance of a sensor based on PAN carbon fibers and real-time detection of change under thermal and mechanical stimuli

Structural health monitoring (SHM) is a vastly growing field consisting of sensors embedded in or attached with the structure which respond to the strain or other stimuli to monitor the deformation in real-time. In this study, a carbon fiber (CF) sensor was developed using unidirectional Polyacrylonitrile (PAN) carbon filaments aligned straightly together and its sensitivity was calculated experimentally, with the gauge factor (GF) in 10.2–10.8 range. The electro-thermal behavior of this CF sensor showed distinct performance and detected the change in the surrounding temperature. There is a good reproducibility in the results in both piezoresistive and electro-thermal behavior of the CF sensor and its electrical performance showed real-time detection of both mechanical and thermal stimuli. The results established that the CF exhibited good potential as a flexible strain sensor for in-situ monitoring of damage or energy release during the failure of composites

Multi-mode real-time strain monitoring in composites using low vacuum carbon fibers as a strain sensor under different loading conditions

Structural health monitoring is a vastly growing field consisting of sensors embedded in or attached with the structure which respond to the strain or other stimuli to monitor the deformation in real-time. In this study, a multi-mode strain detection is carried out in composites using nanomaterial-based sensor technology. A Carbon fiber (CF) sensor was developed using unidirectional carbon filaments aligned straightly together and its sensitivity was calculated experimentally, with gauge factor (GF) in 10.2–10.8 range. Then, this CF sensor is embedded gradually at different directions i.e. 0°, +45°, 90°, −45° between the plies of composite for real-time/in-situ strain monitoring. The composite specimen was then cut in star profile, each leg demonstrating the direction of the CF sensors. These composite samples are then tested under tensile and flexural cyclic loading. There is a good reproducibility in the results and the mechanical response of composite correlated perfectly with the electrical resistance of the CF sensor. It can also be noted that the sensors, depending on their respective position, manage to faithfully reproduce the mechanical behavior of the specimen tested (traction/compression). The results established that the CF exhibited good potential as flexible reinforcement for in-situ monitoring of composites and can provide detection over large sections and unapproachable locations. This study also showed that direction and position of the sensor plays a vital role in the detection, identification (whether its tensile or compressive) and quantification of the deformation experienced by the structure under different loading conditions

Study of mechanical performance of polymer nanocomposites reinforced with exfoliated graphite of different mesh sizes using micro-indentation

The first phase of this work aims to use the right additive nano-fillers choices, such as exfoliated Graphite (ExG), increasing the mechanical, electrical, and thermal performances. In this work, we are interested in quantifying the effect particles' size on a polymer matrix's performance. For this, three sets of exfoliated polymers filled with Graphite, characterized by three particle sizes, called meshes 50, 100, and 150, were investigated. In this analysis, exfoliated Graphite reinforced polymers were subjected to indentation tests to define local mechanical properties. The sample is an epoxy 862 matrix reinforced with exfoliated graphite additives. For each specific size, the additives are mixed in percentages of 0% in the act of control, 0.5%, 4%, 8%, and 16% by weight. Matching pure polymers, polymers reinforced by exfoliated Graphite have proven to have significant improvements in local elastic properties (such as modulus, hardness, stiffness, etc.). Results showed that the reinforced epoxy's local mechanical properties are affected by the size and the percentage of nano-additives. Through the inspection of the load-displacement curve, it can be concluded that the nano-additive has a significant influence on the plastic mechanical properties of the sample. Therefore, the size of nanoparticles has significantly improved in material properties