Thesis (M.S.)--University of Wisconsin--Madison, 1969. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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La prédiction de l’amortissement des structures complexes se heurte actuellement à un certain nombre de difficultés. L'utilisation de matériaux métalliques dans les assemblages aéronautiques ne permet pas d'exploiter les faibles capacités d'amortissement interne des pièces de structure. Le potentiel d'amortissement de ces assemblages se trouve donc dans les interfaces de contact au niveau des liaisons fixes (vissées, rivetées) ou mobiles (rotules, pivots). La conception de jonctions fixes amortissantes demande de favoriser les micro-glissements, ce qui entraine l'apparition d'endommagements par fretting. Pour tenter de résoudre les problèmes de durée de vie liés au fretting, les traitements et revêtements de surface prennent de plus en plus d'importance. Ces travaux de thèse mettent en place des moyens expérimentaux (développement d’un banc haute température) et des méthodologies d'essais permettant de sélectionner des solutions palliatives au fretting avec un objectif de dissipation d'énergie maximale. L’approche énergétique mise en place permet de distinguer les différents régimes de glissement et d’identifier les configurations les plus favorables pour l’amortissement. Deux cas d'application (un à l'ambiante, un à 600°C) ont servi de base expérimentale, mettant en jeux différents alliages métalliques (aciers inoxydables 17.4-PH et XD15NW, Ti6Al4V, Inconel 718) protégés par des revêtements DLC, ou de l'implantation ionique de WS2 avec et sans traitement de sous-couche (CrN et CrC). / Predicting the damping behavior of complex mechanical assemblies faces many technical and scientific challenges. The internal damping of metallic materials is insufficient for aeronautical applications. The energy dissipation potential is located at the contacting surfaces of fixed (bolted, riveted) or mobile joints (pivot, ball-joint). Ensuring micro-slip is required to design dampening fixed joints, but it promotes fretting damage. Coatings and surface treatments are extensively used in order to tackle the ensuing durability issues. This work presents experimental means (newly designed high temperature test rig) and test methods for fretting palliatives selection with a maximum dissipated energy criterion. Energy based approaches enable the distinction between different sliding regimes and the identification of the most favorable ones. The experimental study was based on two industrial cases (at ambient temperature and 600°C) involving various metallic alloys (17.4-PH and XD15NW stainless steels, TI6Al4V and Inconel 718) protected with DLCs or WS2 ionic implantation (on raw substrate or CrC/CrN underlay).

La sollicitation de fretting apparait comme un processus de dégradation très pénalisant dans les multiples systèmes industriels (aéronautique, industrie automobile, industriels du nucléaire, transports ferroviaires etc. …). Ce processus d’usure engendre dans la plupart des cas soit une réparation (rechargement) des surfaces soit le remplacement des composants (situation la plus souvent observée). La sollicitation de fretting est associée à des micro-déplacements alternés entre deux surfaces en contact. On observe généralement ce type de sollicitation dans tous les contacts soumis à des vibrations ou à des cycles thermiques. La sollicitation de fretting usure dans les contacts électriques est particulièrement pénalisante. Outre l’endommagement des surfaces, ce processus induit une augmentation significative de la résistance électrique des contacts et une perte de fonctionnalité du connecteur en particulier dans les connecteurs basses tensions très sensibles aux fluctuations de la résistance électrique. Pour palier ce problème les industriels de la connectique et leurs clients (industriels de l’automobile) cherchent à mettre en place de nouveaux dépôts palliatifs pour résoudre ces problèmes tout en limitant les surcouts liés à l’utilisation de dépôts nobles tels que l’or.Le sujet de cette thèse porte sur cette problématique avec pour objectif de mettre en place une méthodologie de choix des dépôts pour optimiser la durée de vie des connecteurs. L’étude aborde essentiellement l’influence des conditions de chargements mécaniques et plus particulièrement l’amplitude de débattement / The degradation by fretting appears as a very troublesome process in many industrial systems (spatial, automotive, nuclear industry, railway systems etc. …). This wear process makes it necessary to repair or to replace the destroyed component. The process of degradation by fretting is associated with the micro-displacement between two surfaces in contact. This type of degradation is observed in all types of contacts subjected to the vibrations or thermal variations. The degradation by fretting in electrical contact is particularly dangerous. The wear of the contacting surfaces causes a significant increase of the electrical resistance of the contact and the decay of the functional properties of the connector. This situation concerns especially the low tension connectors very sensible to the fluctuations of the electrical resistance. In order to avoid this situation the automotive industry and its suppliers, the manufacturers of the electrical connectors, are looking for new materials and coatings.The objective of this thesis consists to develop the methodology of choice of the coating materials in order to optimise the lifetime of the electrical contact. The study focuses mainly on the influence of the mechanical conditions, the displacement amplitude in particular, on the performance of the electrical connector.

No / A methodology, involving fretting tests, to develop wear and crack resistant materials for tribological applications for automotive valve train parts (e.g. cams, tappets) has been recently reported for high speed steels. Modifications to one of these sintered steels, M3 Class 2, were effected by additions, singly and in combination, of 5 wt.% of wear resistant titanium carbide and of solid lubricant manganese sulphide. In our fretting tests alternate displacements were imposed between the test material (plane) and a chromium steel or alumina ball. Running conditions fretting and material response fretting maps were constructed for the four materials. Two types of fretting damage were detected and analysed: cracking or particle detachment and wear through the tribologicaly transformed structure (TTS). Crack initiation, associated with porosity and interfaces, was detected when the maximum tensile stress in the contact reached 1.2 GPa. Cracking analyses were also carried out using static and fatigue mechanical tests and replica scanning electron microscopy. Crack growth and propagation were influenced by details of the microstructure, e.g. TiC was observed to arrest crack growth, whereas MnS made it easier. Wear analysis included the determination after each test of the wear volume, which could be related to the coefficient of friction and the cumulative dissipation energy during the fretting test.

Prediction of fatigue damage due to fretting is complex due to the number of influential factors and the competitive interaction between wear and fatigue. The majority of current fretting damage modeling approaches are limited to narrow ranges of conditions where little competition between damage mechanisms occurs. Recent models which account for damage interaction are largely phenomenological in nature and are still limited to a narrow range of applicability. A method to characterize and model the level of fatigue damage due to fretting was developed in this work to address the shortcomings of the current methods available by extending the range of conditions captured and enhancing the physical basis of the damage model. Baseline material properties for thin sheets of AISI 301 stainless steel in the full hard condition were determined as a function of temperature through tensile tests, fatigue tests, and metallography. Fretting experiments were performed for contact between 301 stainless steel and each ANSI A356 aluminum and AISI 52100 steel. Fretting experiments were performed over a range of material combinations, normal forces, displacement amplitudes, atmospheres, and temperatures. Subsequent characterization of the damage due to fretting was performed to determine the level of wear and fatigue damage incurred for each condition tested. A finite element model of the experiment was created to determine the cyclic stress-strain behavior and local frictional energy dissipation for each condition. Fatigue damage metrics were evaluated to determine the effects of the contact conditions on the driver for fatigue damage. A new model for fatigue damage due to fretting was developed which incorporates the wear behavior to describe the effect of wear on the level of fatigue damage caused by fretting. The level of fatigue damage is influenced using a function of frictional energy dissipation and wear rate to account for differences in wear mechanisms and changes in the severity of wear caused by changes in oxidation behavior and mechanical properties which result from changes in temperature or contacting materials.

Nucleation and propagation of cracks under fretting conditions has been a subject of study for many years. An extensive experimental investigation to study these cracks was undertaken by Royal Aerospace Establishment (RAE Farnborough). Of particular interest to RAE was an Aluminium alloy (L65) developed for aerospace applications. Many researchers have studied fretting damage and fatigue cracks. Some have examined damage development due to wear, whilst others have analysed cracks under linear elastic fracture mechanics (LEFM) domain. To date, no attempt has been made to develop an integrated numerical model which incorporates all aspects of fretting fatigue i.e. nucleation, initial (or early) crack growth, and long crack propagation. The development of such a model is the principal aim of this work. It is expected that the integrated approach will provide the basis for a standard fretting fatigue analysis of other materials, components, and structures using the finite element method (FEM).This study uses the earlier experimental results with RAE as the reference for comparison. The approach followed is to implement the various stages of fretting in a commercial finite element code, ABAQUS. Unlike previously used simple FE models, both specimen (Aluminium alloy) and the fretting pad (steel) are modelled to simulate the real contact conditions including slip. Various predictive models for crack nucleation (based on damage) and propagation (based on fracture mechanics) are developed, tested, and implemented in ABAQUS. Results clearly show that these models together provide a good estimation tool for predicting total life in complete contact fretting fatigue. It is envisaged that the integrated model will be easily utilised for other materials, components, and structures subjected to fretting fatigue conditions with minimum experimental testing required.

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