Micropitting is a form of surface contact fatigue encounteredin bearingsa nd gears, under lubricating conditions, which lead to their premature failure. All gears are susceptible to micropitting, including spur, helical and bevel. Micropitting can occur with all heatt reatmentsa ppliedt o gearsa nd with both, synthetica nd mineral lubricants. It can occur after a relatively short period of operation and, after a certain number of cycles,g earsn eedt o be replacedd ue to the increasedn oisea nd vibrations causedb y the deviations of the tooth profile. Continuing operation of affected gears can lead to a catastrophic type of failure (i. e., tooth breakage). These considerations explain the increasing current interest in micropitting. It has been reported that micropitting in bearings is associated with a specific microstructural transformation in steel, i. e. martensite decay. However, to the authoes knowledge, this transformation has not been reported in gears. In the present work, extensive metallurgical investigations have been carried out and they revealed that the same transformation occurs in gears. The aim of this project was to describe the mechanism of micropitting by taking into account the influence of several controlling factors such as, material, surface finish, lubricant, load, temperature,s peeda nd, slide-to-roll ratio. Their influence is assessed with a fractional factorial experimentadl esign.S everaln on-destructivete chniquesh ave been used in order to monitor the specimen condition during and after running, such as X-ray diffraction, optical profilometry, light microscopy. The mechanical properties of the products of martensite decay, known as dark etching regions, white etching bands and butterflies are highly relevant to the fatigue behaviour of the steel. Nanoindentation and AFM techniquesh aveb eenu sedt o determinet hesep roperties. A micropitting mechanism correlated with the mechanism of martensite decay in gears is suggestedb asedo n thesea nalyses.

No description available.

Rails play a significant role in transport of goods and passengers. In Australia railway transport industry contributes 1.6% of GDP with goods and services worth $AUD 8 billion each year which includes $ AUD 0.5 billion per year in exports (Australasian Railway Authority Inc, 2002). Rail track maintenance plays an important role in reliability and safety. The Office for Research and Experiments (ORE) of the Union International des Chemins de Fer (UIC) has noted that maintenance costs vary directly (60-65 per cent) with change in train speed and axle load. It was also found that the increase in these costs with increased speed and axle load was greater when the quality of the track was lower (ORR, 1999). Failures during operation are costly to rail players due to loss of service, property and loss of lives. Maintenance and servicing keep rail tracks in operating, reliable and safe condition. Therefore, technical and economical analysis is needed by rail players to reduce maintenance cost and improve reliability and safety of rail networks. Over the past few years, there have been major advances in terms of increased speed, axle loads, longer trains, along with increased traffic density in corridors. This has led to increased risks in rail operation due to rolling contact fatigue (RCF) and rail wear. The infrastructure providers have less incentive to maintain a given infrastructure standard if its access charges are rigid and rolling stock standard is not achieved. It has been estimated that between 40 to 50 per cent of wagon maintenance costs and 25 per cent of locomotive maintenance costs are related to wheel maintenance (Railway Gazette International, 2003). The economic analysis of Malmbanan indicates that about 50% of the total cost for maintenance and renewal were related to traffic on rails and 50% not related to traffic, such as signaling, electricity and snow-clearance. The results from the analysis have made it possible for the mining company LKAB to start up the 30 Tonnes traffic with new wagons and locomotives on the Malmbanan line in year 2001 (Åhrén et al 2003). The rail infrastructure providers have challenges to maintain infrastructure due to government control on access charges and limited control on rail operations. The aim of the research is to: · Develop a maintenance cost model for optimal rail grinding for various operating conditions; and · Develop integrated rail grinding and lubrication strategies for optimal maintenance decisions. In this research real life data has been collected, new models have been developed and analysed for managerial decisions. Simulation approach is used to look into the impact on various costs such as rail grinding, operating risk, down time, inspection, replacement, and lubrication. The results of the models for costs and the effect of rail grinding and lubrication strategies are provided in this thesis. In this research rail track degradation, rail failures and various factors that influence rail degradation are analysed. An integrated approach for modelling rail track degradation, rail wear, rail grinding and lubrication is developed. Simulation model and cost models for rail grinding are developed and analysed. It has been found through this research that rail grinding at 12 MGT interval is economic decision for enhancing rail life. It was also found that lubrication is most effective compared to stop/start and no lubrication strategies in steep curves. Rail grinding strategies developed in this research have been considered by Swedish National Rail for analysing the effectiveness of their existing policies on grinding intervals. Optimal grinding and lubrication decisions have huge potential for savings in maintenance costs, improving reliability and safety and enhancing rail life.

<p>Rolling contact fatigue (RCF) is the primary mode of failure in tribological contacts like rolling-element bearings (REBs), gears, and cam-follower systems. RCF processes have a crack initiation phase followed by a propagation and coalescence phase, resulting in spalls that lead to catastrophic failure. Crack initiation is a highly localized process that is strongly influenced by the inhomogeneity of the material microstructure. Therefore, a microstructure-sensitive model is required to simulate the damage evolution and failure due to RCF loading. This document presents the development of a microstructure-based finite element (FE) framework for RCF, which accounts for the inhomogeneity of bearing steel microstructure by using an explicit definition of polycrystal topology and material anisotropy. The granular topology of the bearing steel microstructure is described using randomly generated Voronoi tessellations. A cubic elastic material definition with a random spatial orientation is specified for each Voronoi grain to simulate the material anisotropy. The Voronoi grains generated using this approach were used to model the critically stressed microstructural volume in RCF loading. A domain size study was conducted to estimate the minimum number of grains that need to be contained by the critically stressed volume such that the macroscopic material response of the polycrystalline aggregate matches the linear elastic material properties of bearing steel. The estimated critically stressed volume was then embedded into a semi-infinite domain for the FE simulation of RCF line contact loading. The RCF domains developed were then subjected to a moving Hertzian pressure over the surface to simulate a bearing load cycle. A boundary averaging scheme was used to estimate the effective stresses along the grain boundaries of the Voronoi cells. Due to the anisotropy of the polycrystalline material, local stress concentrations occur at the grain boundaries as compared to isotropic models. The resolved grain boundary stresses were used to predict critical locations for RCF crack initiation, which closely match observations from RCF bench test data. Since RCF failures typically exhibit subsurface locations for the first crack initiation, the model uses the critical resolved shear stress (RSS) reversal along the grain boundaries and the corresponding subsurface location of the maxima as the driving parameters for RCF fatigue failures. The parameters from the model were fit into a Weibull distribution to estimate the stochasticity in initiation life. The Weibull predictions corroborate well with experimentally measured RCF life scatter. The framework was then extended using a coupled damage mechanics - cohesive element method (DM-CEM) to individually model the crack initiation and propagation phases in RCF. An explicit definition of the grain boundaries was incorporated using cohesive elements. Damage is initiated at the grain boundaries by degradation of the cohesive elements and the rate of damage/degradation is used to characterize the evolution of fatigue life. The rate of damage was calculated at each grain boundary using a fatigue damage law based on the RSS reversal parameter. The model is able to simulate the crack initiation and the propagation/ coalescence phases in RCF, with distinct life estimates for each phase. This model framework is further extended to investigate the effects of lubrication conditions in RCF by integrating an elastohydrodynamic lubrication (EHL) model to simulate the pressure load with the DM-CEM model. Further improvements to the fatigue life predictions using the DM-CEM model are made by coupling it with a crystal plasticity (CP) based submodel approach to predict the crack initiation life in RCF. CP-based metrics are used to correlate the microplasticity developed under RCF loading with the formation of fatigue micro-cracks and the corresponding initiation life estimations. The resulting final spall patterns and RCF life estimates were found to match well with experimental data available in the open literature.</p> <p><br></p>

No description available.

This master thesis deals with influence of topography of friction surfaces on rolling contact fatigue. This influence is examined by newly reconstructed R-MAT station, whose reconstruction and fully report about it, is part of this master thesis. Influence is examined on area with higher surface's roughness which it wasn't examined before. Further research is made on area of surfaces with topographical modification with higher surface's roughness.

O presente trabalho destina-se à discussão sobre o cálculo de vida útil de rolamentos. Mostrando o avanço do processo de cálculo ao longo das décadas até o mais alto grau de desenvolvimento atual. A preocupação do texto é demonstrar de forma simples e objetiva as divergências que existem entre a formulação teórica e a real vida dos rolamentos, no que tange a fadiga de contato. Neste contexto foram realizados testes, em máquina especialmente destinada ao ensaio da fadiga de rolamentos. Variando-se para as duas séries de ensaios, apenas, a temperatura (aproximadamente 85°C e 110°C). Os resultados obtidos indicam que a vida real dos rolamentos apresenta grande divergência se comparada com a vida útil calculada, principalmente, no regime com maior temperatura. Atribui-se a esta disparidade, a ausência de cálculos precisos quanto à correlação da vida útil com o fator l, que é uma forma de se calcular o espaçamento entre as superfícies de contato, e o não emprego do cálculo do fator de carga, na formulação da vida útil de rolamentos. / The present work has the purpose of discussing the life of rolling bearings, describing the evolution of bearing life calculation until its current state of the art. Our focus is to demonstrate, simply and objectively, the inconsistencies occurring between the actual life of rolling bearings and their theoretical fatigue life estimation. For such purpose, tests were developed in a special bearing test rig to assess bearing fatigue. Two test sets were carried out with temperature being the only variation (approximately 85°C and 110°C). Results obtained from these tests suggest that the real life of rolling bearings is indeed very different from calculated bearing life, especially under higher temperature. Such disparity can be attributed to the lack of a precise computation of the relationship between bearing real life and the l factor - which determines the thickness of lubricant separating raceways and balls - as well as to the failure to compute the load factor in bearing life estimation.

Os processos de manufatura, bem como os parâmetros de processamento utilizados produzem diferentes resultados com relação à integridade de uma superfície. Estas alterações se refletem em mudança das propriedades da superfície, as quais podem influenciar o desempenho dos componentes. O presente trabalho estuda o efeito da integridade superficial gerada pelos processos de torneamento e retificação na resistência à fadiga de contato do aço ferramenta AISI H13. A influência do processo de usinagem e dos parâmetros de corte no estado da superfície e subsuperfície, e como estas características afetam o desempenho são determinados. Foram geradas quatro condições diferentes para as superfícies torneadas mediante a combinação de parâmetros de corte. As superfícies retificadas foram produzidas empregando as mesmas condições de retificação, com isso, obteve-se a mesma condição para todos os corpos de prova ensaiados. As características medidas da integridade superficial foram: os parâmetros de rugosidade, tensões residuais, microdureza Vickers da seção transversal e ruído magnético de Barkhausen. Foi determinado o desempenho das superfícies por fadiga de contato de rolamento, num equipamento na configuração esfera-plano. Os corpos de prova foram anéis de AISI H13 temperados e revenidos. A totalidade dos ensaios foi realizada até a ocorrência de lascamento (Spalling). Os ensaios foram realizados com lubrificação e com uma máxima pressão de contato de Hertz de 3,6 GPa. Foi observado que dependendo do nível de integridade superficial induzido pelo processo de usinagem e dos parâmetros selecionados, o desempenho da superfície em fadiga de contato de rolamento foi afetado. Contudo, as melhores condições de desempenho foram dos materiais retificados, e para os materiais torneados o melhor desempenho foi obtido com maiores velocidades de corte. / The manufacturing process and the machining parameters selected lead to different integrity of the surface, and these characteristics will influence the functional performance of the components. This study evaluated and analyzed the effect of surface integrity on the contact fatigue resistance of the H13 hot work tool steel. The surfaces were manufactured by hard turning and grinding process. Therefore, the influences of the machining process and the cutting parameters on the surface and on the subsurface layer were studied. Four different turned surfaces were obtained as a combination of cutting parameters selected for the tests. On the other hand, grinding surfaces were generated using the same machining conditions. The characteristics of surface integrity evaluated were: surface roughness parameters, residual stresses, Barkhausen noise, and microhardness of surface and sub-layers in transverse section. It was also evaluated the performance of the samples by rolling contact fatigue tests conducted in a ball on washer machine. The specimens were rings made of AISI H13 steel, which were quenched and tempered. All tests were performed until the occurrence of spalling. The tests were carried out under lubrication and with a maximum Hertz contact pressure of 3.6 GPa. Monitoring and failure detection were carried out by analyzing the changes in the vibration signal of the test machine sensors. It was observed that the performance of the surface in rolling contact fatigue resistance was affected by the level of the surface integrity, related to the machining process and the machining parameters.

Ces travaux de thèse concernent l’étude des effets des oscillations de faibles amplitudes sur la durée de vie en fatigue de contact pour des composants d’actionneurs de vol. Des outils théoriques et expérimentaux ont pour cela été employés.Un modèle semi-analytique de contact rugueux élasto-plastique a été codé puis utilisé. Celui-ci, couplé au modèle de Dang Van, a permis de réaliser une étude théorique de l’influence du sens de passage de contacts sur la durée de vie en fatigue. Une comparaison en fatigue entre mouvements continus et mouvements oscillants a ainsi pu être faite.Un banc bi-disque a été spécifiquement développé chez UTAS pour pouvoir réaliser des essais de fatigue de roulement aussi bien en mouvements continus qu’en mouvements oscillants. Des essais de lubrification ont été réalisés sur des éprouvettes en XD15NW et en CX13VD afin de caractériser le phénomène de faux effet Brinell. Des essais de fatigue en lubrification optimale ont finalement été réalisés sur les éprouvettes en XD15NW en mouvements continus et en mouvements oscillants, les résultats ont été comparés et mis en vis-à-vis avec ceux de l’analyse théorique. / This PhD thesis is a study of small reciprocating motions effects on contact fatigue life of flight actuator components. Theoretical and practical tools are used for this purpose.A semi-analytical elasto-plastic rough contact code has been implemented and, then, used. This code, associated with Dang Van model, allowed us to perform a theoretical study of rolling direction effect on fatigue life. A comparison has been made between continuous motions and reciprocating motions.A twin-disc test bench has been specifically design at UTAS to allow us to perform rolling fatigue tests with continuous motions or with reciprocating motions. Lubrications tests have been done on XD15NW and CX13VD samples in order to study false brinelling phenomenon. Finally, fatigue tests in ideal lubrication conditions have been done on XD15NW samples with continuous motions and with reciprocating motions, results have been cross-checked each other and cross-checked with theoretical results.

Diversos componentes mecânicos como engrenagens, rolamentos, cilindros de laminação, trilhos e rodas de trem sofrem uma determinada solicitação conhecida por fadiga de contato, que consiste em uma solicitação causada pela tensão gerada entre o contato de dois corpos sob a ação de uma carga cíclica. Este trabalho teve como objetivo avaliar a resistência a esta solicitação para um aço ferramenta com microestruturas martensíticas e bainíticas. Para isto, foi utilizado um equipamento na configuração esfera contra plano, onde as esferas foram confeccionadas em material comercial ABNT 52100 e os discos em aço ferramenta tratados termicamente para obtenção de microestruturas martensíticas e bainíticas. Os ensaios foram realizados com a aplicação de uma pressão máxima de contato de 4,8 GPa até que ocorresse a falha por microlascamento da superfície do disco. Os resultados de vida em fadiga destes materiais foram apresentados por uma distribuição de Weibull que demonstraram uma maior resistência do material bainítico. Os resultados apresentados para ambas as microestruturas demonstraram que as falhas se iniciaram predominantemente na sub-superfície, atendendo as premissas iniciais do trabalho que buscava uma baixa ocorrência de falhas superficiais prematuras ocasionadas por possíveis inclusões, defeitos ou poros superficiais que agiriam como um concentrador de tensões, levando a uma falha antes que efetivamente as características físicas e metalúrgicas das microestruturas sejam testadas. / Several mechanical components such as gears, bearings, rolling mill rolls, rails and train wheels suffer a particular request known to contact fatigue, which consists of a request caused by the tension generated between the contact of two bodies subjected to a cyclic loading. This study aimed to evaluate the resistance to this request for tool steel with bainitic and martensitic microstructures. For this, was used a machine in the configuration ball against flat washer, where balls were made of commercial material ABNT 52100 and flat washer in tool steel heat-treated to obtain martensitic and bainitic microstructures. The tests were performed by applying a maximum contact pressure of 4.8 GPa until the failure by micro-spalling on the disk surface or subsurface. The results of the fatigue life of those materials were presented by a Weibull distribution that showed a better resistance to the bainitic material to this request. The results presented for both microstructures showed that the failures were initiated predominantly sub-surface, given the initial goals of the work, as evidenced by the low occurrence of premature superficial failures possibly caused by inclusions, pores or surface defects that would act as a stress concentrator leading to a premature failure before the physical and metallurgical microstructures characteristics were really tested.