Tribological Analysis of Bolted Joints Submitted to Vibrations

International audienceAn experimental approach was adopted to understand service damages such as crack initiation and surface degradation of bolted joints used in the junction technology of aircraft air bleed valve systems. A vibratory testing stage combined to Digital Image Correlation (DIC) focus on an experimental model device of valve body/actuator body junction. DIC coupled with in situ tribological observations and a Finite Element Model (FEM) has been used to identify more clearly local contact conditions. This approach has enlightened peeling-off and micro-slips instabilities under vibratory loadings, leading to third body formation in bolted joints contacts. The morphology of such third body is placed at the focus of damages observed in involved air bleed valve systems

Space grease lubrication modeling: A discrete element approach

International audienceThe tribological behavior of space grease is investigated with the Discrete Element Method. In the first step, the grease is described as a collection of particles of two kinds (oil and PTFE) in interaction. The development of the Grease Discrete Element Model (GDEM) used here, is based on rheometer-like characterizations. In the second step, the GDEM is subjected to tribometer-like conditions to investigate grease flow mechanisms and the role of the thickener (PTFE) in lubrication. The tribological behavior of grease seems to be controlled by the coupled influence of both granulometric (thickener particle sizes) and physico-chemical (interaction law) parameters. These results provide the starting point for identifying the parameters on which to act to reformulate greases

Numerical investigation on the electrical transmission ability of a shearing powder layer

International audienceRecent developments in powder technology gave birth to a new lubricant – powder lubricant. Compared to liquid lubricant, powder lubricant like graphite powder has several advantages, such as good electrical conductivity and good thermal resistance. Such advantages are especially appreciated in sliding electrical contacts. Thus, the study of the electrical transmission ability of a shearing powder layer under different dy-namical constraints appears to have a great interest. Recent works allowed to model the coupling of mechanical and electrical effects in a discrete medium. This algorithm was extended to study the electrical properties of a shearing powder layer with Discrete Element Method. The mechanical and electrical behaviors of the sample were studied in different dynamical regimes, characterized by the inertial number I. The results exhibit an interesting relationship between the average contact resistance and the inertial number I. An exponential increase of the sample's electrical resistance as well as the induced electrical noise are observed closed to the dense flow limit. Such observations underline the fact that to ensure the electrical transmission ability of the powder layer, one must keep the particle size and shear rate small, and a sufficiently large pressure

Role of Third Body on Bolted Joints' Self-Loosening

International audienceBolted joints are frequently subjected to self-loosening (gradual loss of clamping force) causing multiple failures, especially leaking and breaking of mechanical systems. Such physical phenomena would occur whatever the considered coating (Ag, MoS 2 , Zn-Ni and others). To enlighten this phenomenon, which remains rather misunderstood due to the confined nature of bolted joint contacts, a coupled experimental-numerical approach is adopted on a bolted joint with silver coating. Indeed, from tribological expert assessments of disassembled joints without loosening , a local view of nut/screw threads contacts is proposed, using discrete element method. This method becomes essential in tribology since it offers the ability to model the dynamic behavior of a contact interface. The model is based on a Non-Smooth Contacts Dynamics approach. The case of third body formed in contacts during tightening process, which has been ignored so far, is placed at the focus of self-loosening phenomenon

Coupling Continuous and Discontinuous Descriptions to Model First Body Deformation in Third Body Flows

International audienceThe present paper proposes an extension of the classical discrete element method used to study third body flows. Based on the concept of the tribological triplet proposed by Godet and Berthier, the aim of this work is to enrich description, by accounting for the deformation of the first body and investigating its influence on third-body rheology. To achieve this, a novel hybrid approach that combines continuous and discontinuous descriptions is used. To illustrate the advantage of such modeling, comparisons with the classical approach, which considers the first body as rigid, are performed in terms of macroscopic friction coefficient and velocity and stress profiles

Experimental and numerical modelling of the ignition of solid propellant

International audienceMixing processes of solid propellants can result in friction. Solid propellant ignitions can be observed under safety tests. Analysing a solid propellant elementary friction test from a mechanical, thermal, and physiochemical point of view, it appears that specific friction conditions allow the emergence of component flows in the solid propellant volume, readying the self-ignition. Numerical simulations of the solid propellant elementary friction test involve discrete elements to model these dynamic behaviours of the components within the contact. Indeed, comparisons between experiments and simulations are performed on mechanical parameters and on the evolution of the solid propellant components within the contact. Such comparisons exhibit qualitative and quantitative results by validating local parameters (adhesion), which make understand the solid propellant ignition scenario

Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling

International audienceSelf-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS 2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F). Precision ball bearings with these composites are manufactured since the 60's in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM)

Self-lubricating polymer composites : using numerical trbology to hightlight their design criterion

International audienceAfter the cessation of RT/Duroid 5813, manufacturing tests were performed by CNES and ESA/ESTL in order to find an alternative material. Although PGM-HT was selected as the best candidate, limitations about its tribological capabilities to replace RT/Duroid 5813 were later pointed out. Today, the predictability of the tribological behaviour of those materials is not fully overcome. The motivation to this work is to complement studies of self-lubricating materials by coupling experimental analyses with numerical modelling, in order to predict their tribological behaviour. A Discrete Element Method is chosen to construct the numerical material, because it allows to represent wear and the third body generation at the scale of the ball/retainer contact. An underlying role of the adhesion between components in controlling the tribological properties of the transfer film has been observed

Dialogues numériques entre échelles tribologiques

En tribologie, la modélisation numérique est aujourd'hui un outil indispensable pour étudier un contact afin de pallier les limites expérimentales. Pour comprendre de mieux en mieux les phénomènes mis en jeu, les modèles ne se situent plus à une seule échelle, mais en font intervenir plusieurs, rendant plus que jamais le concept de triplet tribologique incontournable. Travaillant avec cette philosophie et en se basant sur l'approche Non Smooth Contact Dynamics, dont nous rappelons les grandes lignes, nous proposons de franchir deux cas: proposer des modèles offrant des résultats quantitatifs et mettre en place les premières pièces d'une homogénéisation au niveau du contact (VER). Dans le premier cas, l'étude du couplage éléments finis/éléments discrets au sein d'une même simulation a pour but de proposer des modèles plus "réalistes". Même si l interface utilisée est déjà présente au coeur du contact et ne va pas évoluer, elle permet de mettre en évidence l utilisation d outil de mesure permettant de lier le mouvement des particules aux instabilités dynamiques et permet d avoir des résultats qualitatifs mais aussi quantitatifs puisque la comparaison avec les taux de contraintes expérimentaux sont en très bonne adéquation. Dans le second cas, le VER sous sollicitations tribologiques est étudié afin d'étendre les techniques d'homogénéisation aux problèmes de contact afin de s'affranchir de la description des interfaces aux grandes échelles en trouvant un moyen d'homogénéiser le comportement hétérogène de l'interface et de le faire dialoguer avec le comportement continu des corps en contact en faisant remonter, dans un sens, des grandeurs moyennées à l'échelle microscopique à l'échelle macroscopique des premiers corps et dans l'autre sens, se servir des données locales à l'échelle macroscopique comme conditions limites à l'échelle microscopique.In tribology, the numerical modeling has become an indispensable tool for studying a contact to overcome the experimental limitations. To have a better understanding of the phenomena involved, the models are no longer located at a single scale, but involve several ones, more than ever, making the concept of tribological triplet as a unavoidable concept. Working with this philosophy and approach based on the Non Smooth Contact Dynamics framework, which we remind some outlines, we propose to cross two steps~: model that can offer quantitative results and that implement the first ingredient to perform a homogenization at a contact level. In the first case, the study of coupling finite elements/discrete elements within the same simulation aims to propose models that are more "realistic". Even if the interface is already present in the contact and not going to evolves, it can highlight the use of measurement tool of spot particles via dynamic instabilities and allows to have not only qualitative results but also quantitative ones since the comparison with the experimental strain rates are in very good agreement. In the second case, the study of VER in tribological charges is performed to extend the homogenization techniques to contact problems in order to overcome the interface description on large scales by finding a way to homogenize the heterogeneous behavior of the interface and make a dialogue with the continue behavior of bodies in contact by send up, in a sense, average values of the microscopic scale to the macroscopic scale and in the other sense, use local data of the macroscopic scale as boundary conditions at the microscopic scale.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Modeling of Carbon/Carbon Composites under tribological Solicitations

ABSTRACT The present work proposes a methodology study of Carbon/Carbon composites under dynamical stress and conditions of rubbing contact. It is based on the use of finite elements method (FEM), and homogenization technique is applied an elementary cell of composite under contact condition. The comparison of random equivalent representative volume element underlines the importance to take into account the contact interface in such process