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Under certain loading conditions, surfaces topography coupled with materials degree of brittleness can significantly compromise the mechanical performance of structures. The foregoing remains valid even if roughness is intentionally introduced for engineering reasons. In either case, stress can concentrate. The case of the stress concentration in surfaces having randomly distributed pits is a problem that, although being very practical, yet it remains unsolved. The complexity of a random configuration renders difficult the problem of analytically finding relationships between surface parameters and markers indicative of mechanical failure. Another difficulty is the reproducibility of replicates of specimens possessing random rough surfaces, for destructive testing followed by statistical analysis. An experimental technique to produce highly controlled replicates of random rough surfaces (including modeling of degradation growth) was developed. This method was used to experimentally and statistically study the effects on fracture of early randomly degraded surfaces of poly methyl methacrylate (PMMA) versus topographical parameters. Growth of degradation was assumed to go from an engineering surface to one whose heights are normally distributed. (Early stage of degradation is meant to be that level of roughness which is in the neighborhood of the critical flaw size for a given material). Among other findings, it was found that neither stress nor strain alone can be used to predict fracture at this early stage of degradation. However, fracture location was found to be strongly correlated to the ratio of the root-mean square roughness (RMS) to auto correlation length (ACL), above some RMS threshold. This correlation decreases as the material becomes less brittle (i.e., decrease of Young’s modulus or increase of percent of elongation). Simultaneously, a boundary value problem involving traction-free random rough surfaces was solved using a perturbation method, assuming elastic and isotropic conditions. For small RMS/ACL ratio, the solution for the RMS stress concentration factor, kt was found to be: kt = 1 + 2*SQRT(2)*(RMS/ACL), which agrees very well with the experimental work. Finally, a generalization of stress concentration factor formulas for several geometrical configurations and loading conditions into the Modified Inglis Formula was proposed. Finite element analysis was carried out and comparison was made with both experimental and analytical results. Applications of these results are broad. In surface engineering, for example, our analytical solution can be coupled with Fick’s Law to find critical conditions under which a film could become unstable to random roughness. Additionally, in design and maintenance of surfaces in service, it can be used to preliminarily assess how stress concentrates in surfaces where well defined notches cannot be used as an approximation.

The observed multiscale phenomenon of rough surfaces, i.e. the smaller mountains mount on the bigger ones successively, renders the hierarchical structures which are described by the fractal geometry. In this situation, when two rough surfaces are loaded together with a higher load, the smaller asperities will undergo plastic flow and immerge into the bigger asperities below them. In other words, the higher load needs to be supported by the bigger asperities. However, when the GW model was proposed in 1966, its analytical method considered that the length-scale of asperities is fixed, which is independent of load (or surface separation). In such condition, the analytical results for a specific asperity length-scale can only suit the situation of a certain narrow range of load. In this research, a new model, called the multiscale GW model, has been developed, which takes into account the relationship between the load and the asperity length-scale. At first, based on the Nayak¡¦s model the multiscale asperity properties with different surface parameters have been derived, and based on the material yielding theory a criterion for determining the optimal asperity length-scale, which functions as supporting the load, is developed. Then both of the above are integrated into the GW model to build the multiscale GW model. The new model is compared with traditional one qualitatively and quantitatively and show their essential differences. The effects of surface parameters and material parameters are discussed in this model. Finally a comparison with the experiment is made, and reveal the good coincidence.

The problem of rough surface scattering and propagation over rough terrain in ducting environments has been receiving considerable attention in the literature. One popular method of modeling this problem is the parabolic wave equation (PWE) method. In this method, the Helmholtz wave equation is replaced by a PWE under the assumption of predominant forward propagation and scattering. The resulting PWE subjected to the appropriate boundary condition(s) is then solved, given an initial field distribution, using marching techniques such as the split-step Fourier algorithm. As is obvious from the assumption on which it is based, the accuracy of the PWE approximation deteriorates in situations involving appreciable scattering away from the near-forward direction, i.e. when the terrain under consideration is considerably rough. The backscattered field is neglected in all PWE-based models. An alternative and more rigorous method for modeling the problem under consideration is the boundary integral equation (BIE) method, which is formulated in two steps. The first step involves setting up an integral equation (the magnetic field integral equation, MFIE, or the electric field integral equation EFIE) governing currents induced on the rough surface by the incident field and solving for these currents numerically. The resulting currents are then used in the appropriate radiation integrals to calculate the field scattered by the surface everywhere in space. The BIE method accounts for all orders of multiple scattering on the rough surface and predicts the scattered field in all directions in space (including the backscattering direction) in an exact manner. In homogeneous media, the implementation of the BIE approach is straightforward since the kernel (Green's function or its normal derivative) which appears in the integral equation and the radiation integrals is well known. This is not the case, however, in inhomogeneous media (ducting environments) where the Green's function is not readily known. Due to this fact, there has been no attempt, up to our knowledge, at using the BIE (except under the parabolic approximation) to model the problem under consideration prior to the work presented in this thesis. In this thesis, a closed-form approximation of the Green's function for a two-dimensional ducting environment formed by the presence of a linear-square refractivity profile is derived using the asymptotic methods of stationary phase and steepest descents. This Green's function is then modified to more closely model the one associated with a physical ducting medium, in which the refractivity profile decreases up to a certain height, beyond which it becomes constant. This modified Green's function is then used in the BIE approach to study low grazing angle (LGA) propagation over rough surfaces in the aforementioned ducting environment. The numerical method used to solve the MFIE governing the surface currents is MOMI, which is a very robust and efficient method that does not require matrix storage or inversion. The proposed method is meant as a benchmark for people studying forward propagation over rough surfaces using the parabolic wave equation (PWE). Rough surface scattering results obtained via the PWE/split-step approach are compared to those obtained via the BIE/MOMI approach in ducting environments. These comparisons clearly show the shortcomings of the PWE/split-step approach. / Ph. D.

Interface roughness can have a significant effect on the scattering of sound energy, and therefore an understanding of the effects of roughness is essential to making predictions of sound propagation and transmission underwater. Many models of roughness scattering currently in use are two dimensional (2D) in nature; three dimensional (3D) modeling requires significantly more time and computational resources. In this work, an effort is made to quantify the effects of 3D scattering in order to assess whether or under what conditions 3D modeling is necessary. To that end, an exact 3D roughness scattering model is developed based on a commercially available finite element package. The finite element results are compared with two approximate scattering models (the Kirchhoff approximation and first order perturbation theory) to establish the validity and regimes of applicability of each. The rough surfaces are realizations generated from power spectra measured from the sea floor. However, the surfaces are assumed to be pressure release (as on an air-water interface). Such a formulation is nonphysical, but allows the assessment of the validity of the various modeling techniques which is the focus of this work. The comparison between the models is made by calculating the ensemble average of the scattering from realizations of randomly rough surfaces. It is shown that a combination of the Kirchhoff approximation and perturbation theory models recovers the 3D finite element solution. / text

Les normes environnementales toujours plus sévères associées à une constante augmentation de la demande énergétique rendent nécessaires les actions à entreprendre en vue d’améliorer l’efficacité énergétique. Dans le domaine du transport, responsable à lui seul de plus de la moitié de la consommation des ressources pétrolières, les actions visant à optimiser la consommation énergétique se traduisent par la mise en place de nouvelles technologies hybrides, par un travail sur l’aérodynamisme des véhicules, la conception de pneumatiques plus performants ou bien encore la réduction de la taille des moteurs et des boites de vitesse afin de réduire le poids des véhicules. Cependant, selon le VTT (Centre de recherche technique finlandais), dans les voitures particulières, les pertes par frottement représentent un tiers de la consommation totale de carburant. Plusieurs millions de litres de carburant sont ainsi utilisés dans le monde chaque année pour vaincre les forces de frottement. Pour cette raison la recherche en tribologie dans le domaine de l’automobile s’est plus particulièrement focalisée ces dernières années sur le développement de matériaux à faible coefficient de frottement ainsi que lubrifiants plus performants. Ce travail de thèse a pour objectif d’étudier le potentiel des nanoparticules de dichalcogénures métalliques en tant qu’additifs de lubrification pour applications automobiles dans le but de développer de nouveaux lubrifiants hautement performants. Ces nanoparticules, synthétisées pour la première fois dans les années 90, ont déjà montré des propriétés tribologiques intéressantes lors de tests effectués en laboratoire, en régime de lubrification limite. Toutefois, leur utilisation dans des systèmes mécaniques réels nécessite une meilleure compréhension de leurs performances dans des conditions plus proches de la réalité, à savoir en présence de surfaces rugueuses ainsi qu’en présence d’additifs couramment utilisés dans les lubrifiants industriels. Au cours de ce travail, nous nous sommes focalisés sur des nanoparticules de bisulfures de tungstène produites industriellement. Le contexte industriel de ce travail de thèse ainsi que les bases de la tribologie seront exposés dans un premier chapitre consacré à l’état de l’art. Dans ce même chapitre, une revue bibliographique des propriétés lubrifiantes des nanoparticules de bisulfures métalliques (MoS2, WS2) observées lors d’essais tribologiques effectués en laboratoire sera également présentée. L’influence de certains paramètres tels que les conditions d’essais, l’effet de la température, de la concentration, de la cristallinité sera également présentée dans ce premier chapitre.Les résultats de mon travail de thèse seront présentés à travers deux grandes parties. Dans une première, les caractéristiques morphologiques et chimiques des nanoparticules étudiées seront présentées. Leurs propriétés tribologiques dans l’huile de base en présence de surfaces en acier de différentes rugosités seront discutées. Enfin, leurs performances en présence d’additifs couramment utilisés dans les applications industrielles ont également été étudiées. Tous ces essais ont été réalisés en régime de lubrification limite et à 100°C. Dans une seconde partie, le potentiel des nanoparticules pour une application boîte de vitesses a été exploré. Des essais tribologiques ont été réalisés à la fois dans une huile de base ainsi qu’en présence d’un cocktail d’additifs, tout d’abord à l’échelle du laboratoire puis lors de tests réels effectués avec des boites de vitesses utilisées dans l’automobile. Les résultats montrent que les nanoparticules peuvent être utilisées pour augmenter la durée de vie des engrenages. / The growing environmental concerns, along with the continuous increase of energy demand, have encouraged research to improve energy efficiency in every technological field. In the transport industry, responsible of more than half of the world’s oil consumption, manufacturers have bet on hybrid fuel technologies, more aerodynamic car profiles, innovative tires and even downsizing of engines and gearboxes to reduce the weight of the vehicles to face the problem. However, according to VTT Technical Research Centre of Finland, in passenger cars one third of fuel consumption is due to friction loss. This means that several millions of liters of fuel are used every year to overcome friction around the world. As a consequence, reduction on the friction losses would have a direct impact in oil consumption. For this reason, research in the tribology field has specially focused in the development of low friction materials and more efficient lubricants. This work investigates the potential of metal dichalcogenide nanoparticles as lubricant additives for automobile applications with the aim of formulating more efficient lubricants. These nanoparticles which were synthetized for the first time in the 90’s have shown interesting tribological properties when added to base oil under specific laboratory test conditions. However, their future use in real-life mechanical systems needs a better comprehension of their behavior on rough surfaces and in the presence of additives commonly used in industrial lubricants.Industrially produced tungsten disulfide nanoparticles were used in this work. First of all, the industrial context of this work and the basis of tribology science in general and of tribology in the automotive industry in particular are exposed in the state of the art part. In this section, a literature review of the lubricating properties of laboratory scale produced metal dichalcogenides nanoparticles of tungsten and molybdenum disulfide is exposed. The effect of different conditions (temperature, concentration in oil, contact pressure, among others) is also presented in this first section. The research work done for this thesis is divided in two main parts. In the first one, the nanoparticles were first morphologically and chemically characterized and their tribological potential in base oil was investigated on smooth and rough surfaces under different test conditions. Then, their tribological behavior in the presence of additives that are commonly used in industrial applications, in the boundary lubrication regime and at 100°C was studied.In the second part, the use of nanoparticles for a gearbox application was explored. The potential of the nanoparticles in base oil and in the presence of a commercial package of additives for this application was studied, first at the laboratory scale, and then in scaled-up tests with gearboxes used in cars. The results suggest that nanoparticles can be used to increase life span of the mechanical parts of gears.

This thesis deals with experimental measurement of angular distribution of electromagnetic radiation scattered from solar cells surfaces. The measuring apparatus is the second genaration of a scattermeter. This apparatus has been put into operation within the framework of this thesis. Results of the thesis are materials for numerical simulations aforementioned scattering and quantitative evaluation of scattering of electromagnetic waves from given surfaces.

A numerical study of electromagnetic scattering by one-dimensional perfectly conducting randomly rough surfaces with an ocean-like Pierson-Moskowitz spectrum is presented. Simulations are based on solving the Magnetic Field Integral Equation (MFIE) using the numerical technique called the Method of Ordered Multiple Interactions (MOMI). The study focuses on the application and validation of this integral equation-based technique to scattering at low grazing angles and considers other aspects of numerical simulations crucial to obtaining correct results in the demanding low grazing angle regime. It was found that when the MFIE propagator matrix is used with zeros on its diagonal (as has often been the practice) the results appear to show an unexpected sensitivity to the sampling interval. This sensitivity is especially pronounced in the case of horizontal polarization and at low grazing angles. We show - both numerically and analytically - that the problem lies not with the particular numerical technique used (MOMI) but rather with how the MFIE is discretized. It is demonstrated that the inclusion of so-called "curvature terms" (terms that arise from a correct discretization procedure and are proportional to the second surface derivative) in the diagonal of the propagator matrix eliminates the problem completely. A criterion for the choice of the sampling interval used in discretizing the MFIE based on both electromagnetic wavelength and the surface spectral cutoff is established. The influence of the surface spectral cutoff value on the results of scattering simulations is investigated and a recommendation for the choice of this spectral cutoff for numerical simulation purposes is developed. Also studied is the applicability of the tapered incident field at low grazing incidence angles. It is found that when a Gaussian-like taper with fixed beam waist is used there is a characteristic pattern (anomalous jump) in the calculated average backscattered cross section at incidence angles close to grazing that indicates a failure of this approximate (non-Maxwellian) taper. This effect is very pronounced for the horizontal polarization and is not observed for vertical polarization and the differences are explained. Some distinctive features associated with the taper failure are visible in the surface current (solution to the MFIE) as well. Based on these findings we are able to refine one of the previously proposed criteria that relate the taper waist to the angle of incidence and demonstrate its robustness. / Ph. D.

La diffusion électromagnétique par des surfaces rugueuses concerne un ensemble très vaste de problèmes actuels en optronique (maîtrise des signatures/cloaking, analyse des signatures laser et infrarouge, imagerie active, localisation de la lumière, imagerie optique haute résolution, modélisation des interactions lumière matière et des signatures optiques, applications photovoltaïques et détecteurs infrarouges, biotechnologie). Les travaux réalisés dans le cadre de la thèse "Contrôle de la lumière par des éléments de surface désordonnés'' consistent à réaliser expérimentalement des surfaces rugueuses dont les propriétés statistiques sont contrôlées. Pour cela, un banc expérimental de photofabrication a été développé, utilisant sles propriétés statistiques des figures de speckle issues d'un faisceau laser mis en forme spatialement.Les surfaces réalisées présentent ainsi des propriétés statistiques qui n'existent pas à l'état naturel (fonction d'autocorrélation non gaussienne). Au delà de ces surfaces photofabriquées, le travaux de cette thèse s'intéressent également aux propriétés de surfaces de silicium fortement rugueuses caractérisées de Black Silicon. / Scattering of electromagnetic waves from rough surfaces is involved in a wide area of research in optronics (cloaking, laser and infrared signature analysis, active imaging, light localisation, high resolution optical imaging, laser-matter interaction and optical signature modelling, photovoltaics, infrared sensors, biotechnologies). Studies performed during the thesis ``Ligth control by random surface elements'' consist in the experimental fabrication of rough surfaces which statistical properties are controled. An experimental setup has been implemented, using the properties of a spatially shaped laser speckle pattern. The photofabricated surfaces show statistical properties that do not exist in nature such as non Gaussian autocorrelation function. Beyond these photofabricated surfaces, we also studied very rough surfaces of semi-conductor known as Black Silicon.