Discrete ply model of circular pull-through test of fasteners in laminates

In aeronautical structures, assemblies with thin laminates are becoming increasingly usual, especially for fuselage design. In these structures, out-of-plane loads can appear in bolted joints and can lead to progressive punching of the fastener’s head in the laminate resulting, in some cases, in a failure mode called pull-through [1]. This complex phenomenon, which occurs in assemblies, was studied firstly by using a simplified ‘‘circular’’ pull-through test method. Qualitative micrographic examinations showed damage very similar to that observed in impacted specimens. The research presented here extends the Discrete Ply Model Method (DPM) developed by Bouvet et al. [2] to this case. The finite elements model is based on a particular mesh taking ply orientations into account. Cohesive elements are placed at the interfaces between solid elements to represent matrix cracks and delamination, thus allowing the natural coupling between these two damage modes to be represented. The model shows good correlation with test results, in terms of load/displacement curve, and correct prediction of the damage map until failure, including the splitting phenomenon

Vers une industrialisation du modèle de frottement dynamique de LuGre. Application à des essais de glissement en condition de frottement sec

National audienceDans une démarche d’amélioration de la prédiction du comportement en glissement d’assemblages boulonnés, des essais de glissement en condition de frottement sec sous chargement statique ont été réalisés. Pour cela, une éprouvette en aluminium AU4G non revêtu a été boulonnée à précharge contrôlée avec une éprouvette en acier inoxydable 316L non revêtu. Un chargement transverse a alors été introduit dans l’assemblage à l’aide d’une machine de traction. Le glissement inter-éprouvette induit et l’évolution de l’effort normal dans l’assemblage ont été mesurés. Les résultats mettent notamment en évidence le phénomène d’instabilité dynamique dit de « stick-slip » (ou de « broutement »). Dans un 1er temps, un état de l’art des modèles théoriques de frottements disponibles dans la littérature est présenté. Le modèle dynamique de LuGre a été sélectionné en vue d’une possible utilisation industrielle au sein des méthodes de pré-dimensionnement d’assemblages vissés actuelles. Dans un second temps, une simulation numérique des essais de glissement est présentée. Le modèle correspondant est statique non-linéaire et a été réalisé sous Abaqus v6.13. Il est appelé à être amélioré par la suite afin de prendre en compte un maximum de mécanismes physico-mécaniques intervenant dans le phénomène de glissement. Par la suite, les résultats des essais de glissement réalisés sont présentés. Ils ont permis d’identifier les différents paramètres du modèle théorique de frottement et sont comparés aux résultats issus du modèle numérique. En conclusion, les perspectives envisagées à la lumière de ces premiers résultats sont énoncées

Comportement en matage de composites à matrice céramique oxyde/oxyde

Le comportement mécanique des liaisons boulonnées en composite à matrice céramique oxyde/oxyde a été caractérisé à l’aide d’essais de matage et d’une modélisation numérique. Les endommagements observés lors de la campagne d’essais sont similaires à ceux se développant lors du matage des composites à matrice organiques : flambement des fibres, ruptures matricielles, délaminages et bandes de cisaillement. Des essais interrompus ont montré que ces différents endommagements sont présents dès le pic de matage. La simulation numérique permet de modéliser à l’échelle du pli la dégradation des propriétés mécaniques du matériau. Les lois d’endommagements ont été implémentées sous le logiciel Abaqus, les paramètres mécaniques nécessaires étant identifiés par des essais de caractérisation matériau. Les résultats de la simulation montrent une bonne corrélation avec les essais. En effet, la modélisation permet de retrouver les différentes phases du matage sur la courbe de comportement, et les cartographies d’endommagement sont cohérentes avec les observations expérimentales

Bearing damage identification in oxide/oxide ceramic matrix composite with a new test design

In this paper, the mechanisms of damage of an oxide/oxide ceramic matrix composite bearing are studied with a new experimental setup, the balanced quarter hole device (BQH). This test was designed to allow direct observation of the damage development on a material subjected to bearing failure. In a standard bearing experiment, real time monitoring is extremely difficult to set up, and post mortem observations of the bearing plane can be biased by the cutting operation. High speed cameras were used to take pictures of the bearing plane so that the damage development could be studied and a damage chronology established. The validity of the setup was verified by comparing the results obtained with those of a standard bearing test. Two different stacking sequences were studied, and the influence of the material microstructure and composite machining was investigated. It was shown that the first critical damage, matrix cracks, appeared before the load drop, and then led to delamination and kink bands, causing the final failure of the material

La ma\ⁱtrise du transfert de charge dans les éléments de liaison aéronautiques : quelle définition de modèles et d'outils d'assistance de prédimensionnement en phase de conception ?

International audienceno abstrac

Study of the behavior of a complex revolute joint of aircraft landing gear

International audienceComplex mechanical joints are modeled by basic joints, but their architectures make the study of behavior difficult. A numerical study raises the issue of compatibility during the transfer between different simulation tools. The low compatibility of digital data when transferring information between multiple simulation software is a real problem. It could lead to the loss of information or an increase of the time needed to produce a numerical model. This study focuses on the experimental analysis of behavior of a revolute joint overhang called “complex” as applied to the real case of a torque link of a landing gear. The effect of simple and combined loading was used to determine the overall stiffness of the joint. A calculation program was developed to extract the experimental results and identify a polynomial response surface. The analysis of those results highlights the nonlinear behavior of the joint with a hysteresis effect. This phenomenon includes the geometric shape defects, clearances and local deformations of the parts. The extraction of a surface response of the rotating joint stiffness overhang can be exploited in an approach like "Virtual Testing"

Optimal catalogue selection and custom design of belleville spring arrangements

International audienceA Belleville spring is usually used in mechanical systems to obtain large loads for small distances of travel. Such a spring has a non-linear load-length curve. Moreover, Belleville springs are commonly stacked together in series or in parallel or both. Thus, defining a design that uses Belleville springs is not an easy matter. The existing assistance tool only checks whether a proposed design is acceptable or not. To improve design assistance, we show how optimization processes can be used in order to build a tool that interactively proposes an optimal design directly from the designer’s requirements. The tool presents both an optimal design using a Belleville spring taken from a catalogue and an optimal custom design. An illustrative example is presented to highlight the benefits of using such an assistance tool

Riveted assembly modelling: Study and numerical characterisation of a riveting process

International audienceIn the field of structural stress analysis and especially in transitory dynamics (crash and impact simulations), study/design accuracy requires increasingly predictive models. A compromise between cost and precision entails modelling and simplification of all the link elements. Riveting is a particularly sensitive case. The characteristics needed to ensure effective modelling are indeed difficult to measure. This paper presents adjustment of a numerical model simulating a riveted link using a number of different approaches. The results analysis considerably improve knowledge about the riveting process and behaviour of riveted links. This study will pave the way for resistance tests to be conducted in order to numerically characterise riveted links under load. The aim is to develop an approach that reduces the model size and calculation time without adversely affecting the validity of the simulation results, and to show the effect of strains and residual stresses on the link in post-riveting

Optimization of a high-pressure pore water extraction device

International audienceHigh-pressure squeezing is a technique used for the extraction of the pore water of porous materials such as sediments, soils, rocks, and concrete. The efficiency of extraction depends on the maximum pressures on the materials. This article presents the design of a high-pressure device reaching an axial pressure of 1000MPa which has been developed to improve the efficiency of extraction. The increase in squeezing pressure implies high stresses inside the chamber, so specialized expertise was required to design a safe, functional device that could withstand pressures significantly higher than common laboratory equipment. The design includes finite element calculations, selection of appropriate materials, and descriptive construction details for the apparatus. It also includes an experimental study of the performance of the apparatus in terms of extraction efficiency