Most of the surfaces have roughness down to atomic scales. When two surfaces come into contact, the nature of the roughness determines the properties like friction and wear. Analysis of the rough surface contacts is always complicated by the interaction between the material size effects and the micro-geometry. Contact mechanics could be simplified by decoupling these two effects by magnifying the scale of roughness profile. Also, tailoring the roughness at different scale could show a way to control the friction and wear through surface micro-structure modifications. In this work, the mechanics of contact between a rigid, hard sphere and a surface with a well defined roughness profile is studied through experiments and finite element simulation. The well defined roughness profile is made up of a regular array of pyramidal asperities. This choice of this geometry was mainly dictated by the fabrication processes. The specimens were made out of an aluminium alloy (6351-T6) such that there could be a direct application of the results in controlling the tribological properties during aluminium forming. Experiments on the pyramidal aluminium surface is carried out in a 250 kN Universal Testing Machine (INSTRON 8502 system) using a depth sensing indentation setup. A strain gauge based load cell is used to measure the force of the indentation and a LVDT (Linear Variable Differential Transformer) is used to measure the penetration depth. The load and the displacement were continuously recorded using a data acquisition system. A 3-D finite element framework for studying the elastic-plastic contact of the rough surfaces has been developed with the commercial package (ABAQUS). Systematic studies of indentation were carried out in order to validate the simulations with the experimental observations. The simulation of indentation of flat surface is carried out using the implicit/standard (Backward Euler) procedure, whereas, the explicit finite element method (Forward Euler) is used for simulating rough surface indentation. It is found that the load versus displacement curves obtained from experiments match well with the finite element results (except for the error involved in determining the initial contact point). At indentation depths higher than a value that is determined mainly by the asperity height, the load-displacement characteristics are similar to that pertaining to indentation of a flat, smooth surface. From the finite element results, it is found that at this point, the elastic-plastic boundary is more or less hemispherical as in the case of smooth surface indentation. For certain geometries, it is found that there could exist an elastic island in the sub-surface surrounded by plastically deformed material. This could have interesting applications.

Recently, powders of the size d (0.1 μm < d < 10 μm) have been referred to ultrafine particles. The particle shape considered is assumed to be a sphere of the diameter d. The handling of powders is of great importance for processing of pharmaceuticals, cement, chemicals and other products. Most of these technological processes involve powder compaction, storage, transportation, mixing, etc, therefore, understanding of the fundamentals of particles interaction behaviour is very essential in the design of machines and equipment as well as in powder technology, cleaning of environment and other areas. The dynamic behaviour of particulate systems is very complicated due to the complex interactions between individual particles and their interaction with the surroundings. Understanding the underlying mechanisms can be effectively achieved via particle scale research. The problem of a normal contact may be resolved in a number of ways. In spite of huge progress in experimental techniques, direct lab tests with individual particles are still rather time-consuming and expensive. The interaction of particles as solid bodies is actually a classical problem of contact mechanics. In the case of ultrafine particles, the reduction of the particle size shifts the contact zones into the nanoscale or subnanoscale. Thus, steadily increasing contribution of adhesion has to be considered in the development of the physically correct constitutive models and numerical tools. Consequently, it may... [to full text] / Ultrasmulkios dalelės yra šiuolaikinės chemijos, farmacijos, maisto ir kitų pramonės šakų produktų sudėtinė dalis. Tiriant pramoninius technologinius procesus, neišvengiamai reikalingos teorinės žinios apie ultrasmulkių dalelių elgseną. Išsamus supratimas įmanomas tik atlikus įvairius tyrimus. Pastaruoju metu milteliai, klasifikuojami kaip ultrasmulkios (0,1 < d < 10 μm) dalelės, imti plačiai naudoti pramoniniuose procesuose, todėl suprasti ultrasmulkių dalelių elgsenos fundamentalumą miltelių technologijoje yra labai svarbu. Ultrasmulki dalelė yra itin maža, todėl su ja atlikti fizinį eksperimentą, kuris reikalauja specialios įrangos bei žinių, labai sunku. Tokiu atveju dažniausiai naudojamas skaitinis eksperimentas, kurį galima atlikti virtualiai. Skaitinio eksperimento metu yra tiriamos dinaminės ultrasmulkios dalelės savybės bei sprendžiamas dinaminis uždavinys. Taikant skaitinius modelius bei dalelės judėjimą aprašančias jėgų lygtis, naudojami sąveikos modeliai, apimantys adhezinę, klampią, tamprią bei tampriai plastinę sąveikas. Mikroskopinis adhezinės sąveikos modeliavimas – aktualus mechanikos mokslo uždavinys. Taikant sąveikos modelius, svarbu pritaikyti ir diskrečiųjų elementų metodą, kadangi, norint aprašyti dalelių elgseną, visų pirma reikia su-vokti ir aprašyti dalelės modelį. Dalelės elgsenos skaitiniam modeliavimui siūlomi teoriniai modeliai leidžia tirti dalelės sąveiką su dalele ar tampria puserdve bei sąveikos dinamiką. Šie modeliai galėtų būti pritaikyti... [toliau žr. visą tekstą]

L'apparition récente de nouveaux matériaux dans les structures de chaussée associée à une diminution de l'épaisseur des couches de surface et une augmentation du chargement des poids lourds et de leur fréquence de passage a entrainé de nouvelles pathologies de dégradation. Outre les problèmes d'orniérage bien connus, apparaissent désormais des fissures descendantes (top down cracking) ainsi que des problèmes de décohésion aux interfaces. Ces nouvelles pathologies entrainent des dépenses considérables sur l'ensemble du réseau (environ 15 milliards d'euros par an), particulièrement en zones urbanisées plus sujettes aux dégradations de surface et ne permettent pas d'estimer convenablement les durées de vie de la chaussée, le plus souvent surestimée dans les méthodes de dimensionnement actuelles. Ce travail de doctorat propose une nouvelle approche du contact pneu-chaussée permettant de mieux appréhender les contraintes principales et résiduelles dans une structure de chaussée bitumineuse. A l'aide d'un outil numérique rapide de calcul basé sur une approche semi-analytique (« Semi-Analytical Methods » (SAM)), la géométrie précise du pneumatique est intégrée afin d'obtenir une répartition de pression de contact ainsi qu'un cisaillement surfacique réelle sur la chaussée. Dans un premier temps, un modèle de contact roulant tractif élastique est implémenté pour des cas théoriques simples et validé par des résultats analytiques et numériques de la littérature. Ensuite, ce modèle est étendu pour prendre en compte le comportement élasto-plastique des corps en contact. Ce dernier est comparé à un résultat numérique basé sur la méthode des éléments finis issu de la littérature. Les résultats, pour une application contact pneu-chaussée, montrent une répartition non homogène des contraintes dans la structure et principalement dans les premiers centimètres sous la surface avec des niveaux beaucoup plus importants que peuvent le prédire les modèles actuels qui utilisent une charge uniformément répartie. La pression de contact est comparée aux mesures effectuées par un système nommé TekScan et les champs mécaniques en sous couches sont comparés à ceux d'Alizé-LCPC dans le cas d'une structure simple. Les cisaillements surfaciques sont déterminés dans le cas du roulement tractif. Une application est effectuée sur la modélisation des dégradations des chaussées en surface. Dans un premier temps, des analyses sur le comportement de la chaussée en surface sont effectuées pour une couche de béton bitumineux semi grenu (BBSG) semi-infinie supposée élastique, homogène sous conditions d'accélération, de freinage et de virage. Pour des études sur le top down cracking, des déformations et directions principales sont déterminées et analysées. Ensuite, le modèle de contact élasto-plastique est appliqué sur une couche semi-infinie de grave bitume GB3. Des déformations et contraintes résiduelles générées dans la structure sont déterminées en vue d'une analyse sur les ornières d'instabilité. Une fois validés, ces résultats permettront d'estimer plus fidèlement la durée de vie résiduelle des chaussées mais également de comprendre et d'éviter les mécanismes de dégradation en surface ou proche de la surface. / The recent appearance of new materials in road structures associated with surface layers thickness decreasing and the increasing of trucks loading and their passage frequency has led to new pathologies of degradation. In addition to the well-known rutting problems, top down cracking is now appearing as well as problems of decohesion at the interfaces. These new pathologies led to considerable expenditure on the entire network (around 15 billion euros per year), particularly in urbanized areas that are more prone to surface damage and do not make it possible to adequately estimate the lifetimes of the roadway, most often overestimated in current design methods. This doctoral work proposes a new approach of the tire-road contact allowing for better apprehend of the main and residual stresses in a bituminous pavement structure. Using a fast numerical tool based on a semi-analytical approach ("Semi-Analytical Methods" (SAM)), the precise geometry of the tire is integrated in order to obtain a real contact pressure distribution as well as surface shear on the pavement surface. Initially, an elastic tractive rolling contact model is implemented for simple theoretical cases and validated by analytical and numerical results from the literature.Then, this model is extended to take into account the elastoplastic behavior of the bodies in contact. This is compared to a numerical result based on the nite element method from the literature. The application for tire-pavement contact results, show a non-uniform distribution of stresses in the structure and mainly in the rst centimeters below the surface with much higher levels than can be predicted by current models that use a uniformly distributed load. The contact pressure is compared to the measurements made by a system called TekScan and the mechanical elds in sublayers are compared to those of Alizé-LCPC in the case of a simple structure. The surface shears are determined in the case of tractive rolling. An application is carried out on the modeling of surface pavement damage. Firstly, analyzes of the behavior of the surface pavement are carried out for a semi-innite semi-grit asphalt concrete layer supposed to be elastic, homogeneous under conditions of acceleration, braking and turning. For studies on top down cracking, principals deformations and directions are determined and analyzed. Then, the elastoplastic contact model is applied on a semi-innite asphalt agragate layer. Deformations and residuals stresses generated in the structure are determined for an analysis on the instability ruts. Once validated, these results will make it possible to more accurately estimate the residual life of pavements but also to understand and avoid surface or near surfacedegradation mechanisms.