Residual stresses in ceramic metal assembly after brazing process

The framework of this study is the thermo-mechanical analysis of the brazing process of ceramic metal assemblies. The thermal expansion gradient between ceramic and metallic materials leads to the development of residual stresses during the cooling phase of the brazing process which induce consequently an important reduction of the strength of these composite structures. In the present work, numerical simulations are performed in order first to predict the residual stresses distribution after the brazing process and in a second step, to study their influence on the tensile strength of metallized ceramic seals. Results obtained are compared with experimental tests

Fatigue behaviour of impacted composite structures

The aim of this study was to compare the mechanical behavior of different impact-damaged composite materials. Three composite materials were realized using the Liquid Resin Infusion process (LRI) accord- ing to three different cycles of polymerization. Thus the temperature of the glass transition of the resin was controlled and the influence of this parameter was then determined. In accordance with the aeronautical use of composite materials, the plates were subjected to Barely Visible Impact Damage (BVID). Two experimental methods were developed to ensure that impact condi- tions were respected. The first method, using the numerical digitalization of surfaces made it possible to reconstruct the defect. Thus the residual indentation could be measured. This indentation had to be lower than 0.3 mm. The second method, using passive infrared thermography, enabled the thermal cartography on the surface to be determined. The size of the damage area was then obtained. Through the use of these two methods, the best cycle of polymerization was determined and fatigue tests were carried out to estimate the evolution of the defect. For this purpose and in compliance with impact and fatigue testing standards, a new and adapted sample was imagined (FAI sample). The fatigue testing conditions were determined to avoid buckling effects and to limit the rise of the temperature of the sample. An experimental method using infrared thermography was then developed to follow the size evolution of the defect. This real time evolution was then compared to the loss of rigidity of the sample

The detection of aeronautical defects in-situ on composite structures using Non Destructive Testing

A study of three Non Destructive Testing methods (Ultrasonic Testing, InfraRed Thermography and Speckle Shearing Interferometry, known as Shearography) was carried out on different specific types of composite specimens having a variety of defects. The aim of this study is to evaluate the efficiency of these NDT methods in the detection of in-situ defects resulting from Barely Visible Impact Damages (BVID) or in-service damages to complex surfaces such as wings or rods. The size and position of all the defects were determined by GVI (General Visual Inspection): GVI being the reference. The evaluation of the three NDT techniques enabled conclusions to be drawn regarding defect detection and size. The first part of the study deals with determining and measuring defects. It appears that only the ultrasonic method enables the depth of a defect to be determined. In the second part of the study, the results obtained by the three NDT methods are compared. Finally, the feasibility and the time taken to set up the experimental protocol are analyzed. The study shows that all the defects were revealed by, at least, one of the three NDT methods. Nevertheless it appears that InfraRed Thermography and Shearography produced results very quickly (in about 10 s) compared to Ultrasonic Testing

Identification du comportement en fatigue bi-axiale de composite stratifié

La fatigue est un enjeu majeur dans la conception des structures en composites. Elle correspond à l'application d'un chargement répété sur la structure. Il a été montré à de nombreuses reprises dans la littérature, qu'une pièce mécanique pouvait rompre en fatigue alors que le chargement appliqué est inférieur à la charge critique à rupture statique. La durée de vie de la structure est ainsi dépendante de l'intensité du chargement appliqué, ainsi que du nombre de cycles réalisés. La difficulté de prédiction de l'évolution de l'endommagement sous une sollicitation cyclique pousse généralement les concepteurs à sur-dimensionner les structures, en utilisant des coefficients de sécurité importants. De nombreuses études ont donc été menées pour prédire l'endommagement des composites face à de telles sollicitations. Divers modèles et critères de rupture en fatigue ont été élaborés et sont couramment utilisés pour le dimensionnement de structures. Pour étudier le comportement d’un composite plastique renforcé de fibres de carbone sous un chargement bi-axial, un nouveau type d’éprouvette cruciforme a été conçu. Cette éprouvette est soumise à un chargement bi-axial statique et cyclique en fatigue. Les essais seront suivis simultanément en utilisant la stéréo-corrélation d’images numériques (DIC), et la thermographie infrarouge. Une comparaison entre les mesures expérimentales et un modèle éléments finis est réalisée pour valider le design de l’éprouvett

High-speed digital image correlation: application to bi-axial fatigue

To study the behavior of a carbon fiber reinforced plastic laminate under a bi-axial stress state, a new type of cruciform specimen has been developed. This specimen is loaded bi-axially under static and cyclic fatigue loadings. The experiments are monitored simultaneously using Digital Image Correlation with two High-speed cameras, and infrared thermography. A comparison between the measurements and finite elements model is used to validate the design of the sample

Study of the behavior of bi-axially loaded composite laminates

Composite materials are increasingly used in the industry. Combining light weight and high resistance, they represent a good alternative to metals for weight saving in structures, such as airplanes. The behavior of those materials is intensively studied by researchers all over the world. Since a structure is usually submitted to complex loading, the study of the bi-axial behavior of those materials is a key issue to the understanding of damage progression of in service composite structures. Very few experiments have been done on bi-axial loading. The particular shape of the specimen and the absence of standard specimen make the task complex and the study remains open

High-Speed Stereo Digital Image Correlation: Application to Biaxial Fatigue

To study the behaviour of a carbon-fibre reinforced-plastic laminate under a biaxial stress state, a new type of cruciform specimen has been developed. This specimen is loaded biaxially under static and cyclic fatigue conditions. The experiments are monitored simultaneously using digital image correlation with two high-speed cameras and infrared thermography. A comparison between the measurements and a finite element model is used to validate the design of the sample

Paraben exposure through drugs in the neonatal intensive care unit: a regional cohort study

Background and objectives: Environmental factors influence the development of very preterm infants (VPIs, born at less than 32 weeks of gestation). It is important to identify all potential sources of paraben exposure in these vulnerable infants. We aimed to quantify paraben exposure via drug administration in a cohort of VPI cared for in neonatal intensive care units (NICUs).Methods: A prospective, observational study was carried out over a five-year period in a regional setting (two NICUs using the same computerized order-entry system). The main outcome was exposure to paraben-containing drugs. The secondary outcomes were: time of the first exposure, daily intake, number of infants exceeding paraben acceptable daily intake (ADI: 0–10 mg/kg/d), duration of exposure, and cumulative dose.Results: The cohort consisted of 1,315 VPIs [BW 1129.9 (±360.4) g]. Among them, 85.5% were exposed to paraben-containing drugs. In 40.4% of infants, the first exposure occurred during the second week of life. Mean paraben intake and duration of exposure were, respectively, 2.2 (±1.4) mg/kg/d and 33.1 (±22.3) days. The cumulative paraben intake was 80.3 (±84.6) mg/kg. The ADI was exceeded in 3.5% of exposed infants. Lower GA was associated with higher intake and longer exposure (p < 0.0001). The main molecules involved in paraben exposure were: sodium iron feredetate, paracetamol, furosemide, and sodium bicarbonate + sodium alginate.Conclusion: Commonly used drugs are potential source of parabens, and ADI can be easily exceeded in VPIs cared for in NICUs. Efforts are needed to identify paraben-free alternative formulations for these vulnerable infants

Modélisation numerique et expérimentale du comportement de structures sandwich

Le but de ce travail dans le thème "Modélisation de structures composites" est la modélisation en flexion de structures sandwiches. Les étapes principales de cette étude sont : la détermination des constantes élastiques du matériau par une homogénéisation numérique à l'aide du logiciel "Samcef" ; le développement analytique et l'implémentation dans le logiciel de calcul formel "Mathematica" de modèles raffinés de la flexion 3 et 4 points ; la validation expérimentale des différents modèles développés. Il est important de noter la mise en œuvre de nouveaux moyens de mesure tels que la stéréocorrélation d'images et l'interférométrie laser pour la mesure de déformations, des déplacements et des contraintes

Étude du comportement d'une liaison mécanique par éléments de fixation

International audienceThe origin of this work takes place in the framework of an industrial collaboration in order to study the behaviour of a mechanical join between two sheets, used in the industry of railway structures. A static experimental study, was carried out in order to understand the phenomena brought into play when a structure of reference is submitted to various conditions, with an aim of being able to model in finite elements and to optimize a more complex structure multi joins.L’origine de ce travail se situe dans le cadre d’une collaboration industrielle afin d'étudier le comportement d'une liaison mécanique entre deux tôles, utilisée dans la confection de structures ferroviaires. Une campagne d'essais statiques, a été effectuée afin de comprendre les phénomènes mis en jeu lors de la sollicitation d'une structure de référence sous différentes conditions, dans le but de pouvoir modéliser en éléments finis et optimiser par la suite une structure multi liaisons plus complexe