This thesis describes a study of the influence of surface texturing on rolling contact fatigue of non-conformal contacts. The texture depth and density have been considered during experiments. It also presents the methodology and the procedure of creation of micro-dents on sample surfaces. Described results suggested that properly designed surface texturing should not necessarily increase the risk of fatigue failure of rubbing surfaces.

No description available.

<p dir="ltr">Rolling contact fatigue (RCF) is one of the leading causes of failure in critical tribological components such as rolling element bearings (REBs), gears, cam and followers, etc. This is especially paramount for advanced aerospace applications where REB components need to operate for billions of RCF cycles before routine maintenance or inspection is performed. The rolling motion between the rolling elements and raceway produces RCF, wherein a complex, non-proportional, alternating contract stress is applied over a small material volume. Moreover, the highly localized stress occurs on the same length scale as microstructural features such as carbides, inclusions, grain size, hardness gradients from carburization, surface roughness, thereby amplifying their effect on fatigue performance. Therefore, the objective of this dissertation is to investigate critical microstructural modifications and their effects on RCF performance via experiments and computational modeling.</p><p dir="ltr">Initially, an investigation was undertaken to investigate surface roughness effects on RCF. The surface roughness of various REBs was measured through optical surface profilometry and used to construct rough surface pressure distributions, which were then used in a continuum damage mechanics (CDM) finite element (FE) framework. The results demonstrated that life is reduced as lambda ratio decreases. It was also observed that a 2-parameter Weibull cumulative distribution function can describe the relationship between the near surface orthogonal shear stress concentration and ratio of surface failures.</p><p dir="ltr">Next, the enhancement to RCF life from grain size refinement of through hardened bearing steels was studied. To capture the effects of grain refinement, torsion stress-life data of various grain size were used in the RCF model. A predictive life equation for different grain sizes was constructed based on the exponential trend observed between grain size and life from the simulation data. The life equation was then used to calculate the quotient of RCF at two different grain sizes. This quotient was defined as the life improvement ratio and it was observed that this investigation’s ratios compared well with existing life improvement ratios from RCF experiments.</p><p dir="ltr">Hardness gradient is a common microstructural modification to improve RCF life of tribo-components. Variation of hardness gradients is prevalent in case hardened (i.e. case carburized) bearing materials. Therefore, the CDM-FE RCF model was modified to investigate the effects of various hardness gradient types and depths on fatigue life improvement. The simulation results enabled the identification of potentially optimal gradients aimed to mitigate manufacturing challenges and provided the foundation for the construction of a general fatigue life equation.</p><p dir="ltr">A fundamental study to understand the impact various common RCF failure criteria have on RCF life estimation was then conducted using computational modeling. To capture the variation of a material’s resistance to fatigue, the critical CDM damage parameters were assumed to follow a probabilistic distribution instead of a singular value. The CDM-FE model was modified to consider the shear reversal, the octahedral shear stress, the maximum shear stress, the Fatemi-Socie criteria, and the Dang Van multi-axial fatigue parameter as failure criteria. Simulation life results revealed that the CDM-FE model with shear reversal and Fatemi-Socie criteria best match empirical predictions from well-established RCF life theory. Notably, the Fatemi-Socie exhibited the best agreement over all operating conditions.</p><p dir="ltr">The next investigation focused on the cleanliness of aerospace-quality bearing steels. Torsion fatigue experiments established the stress-life (S-N) relation for three common aerospace quality bearing steels. The S-N data was later used to calibrate the RCF model’s damage equation, which considered the Fatemi-Socie criteria following conclusions from a previous investigation. Simulation results were observed to corroborate well with RCF experiments that were conducted for all three materials, while noting the simulations offered a significant time saving. As a result, a subsequent investigation focused on establishing the stress-life relationship for one of the aerospace quality bearing steels through a combined experimental and analytical approach. Good corroboration was observed between simulations and experiments at three contact pressures. This finding is particularly significant as it strengthens the reliability of computational RCF model as an efficient means to assess the RCF performance of bearing materials.</p><p dir="ltr">Furthermore, the detailed investigation on RCF performance of each critical microstructural modifications and their respective effect greatly improves the state-of-the-art. The findings emanating from the various investigations offer informed fatigue design recommendations that aid in the selection of rolling element bearings for critical tribological and aerospace applications.</p>

The goal of this research is developing a computational framework to study mechanical fatigue and fracture at different length scales for a broad range of materials. The developed multiscale framework is utilized to study the details of fracture and fatigue for the rolling contact in rails, additively manufactured alloys, and bio-inspired hierarchical materials. Rolling contact fatigue (RCF) is a major source of failure and a dominant cause of maintenance and replacements in many railways around the world. The highly-localized stress in a relatively small contact area at the wheel-rail interface promotes micro-crack initiation and propagation near the surface of the rail. 2D and 3D microstructural-based computational frameworks are developed for studying the rolling contact fatigue in rail materials. The method can predict RCF life and simulate crack initiation sites under various conditions. The results obtained from studying RCF behavior in different conditions will help better maintenance of the railways and increase the safety of trains. The developed framework is employed to study the fracture and fatigue behavior in 3D printed metallic alloys fabricated by selective laser melting (SLM) method. SLM method as a part of metal additive manufacturing (AM) technologies is revolutionizing the manufacturing sector and is being utilized across a diverse array of industries, including biomedical, automotive, aerospace, energy, consumer goods, and many others. Since experiments on 3D printed alloys are considerably time-consuming and expensive, computational analysis is a proper alternative to reduce cost and time. In this research, a computational framework is developed to study fracture and fatigue in different scales in 3D printed alloys fabricated by the SLM method. Our method for studying the fatigue at the microstructural level of 3D printed alloys is pioneering with no similar work being available in the literature. Our studies can be used as a first step toward establishing comprehensive numerical frameworks to investigate fracture and fatigue behavior of 3D metallic devices with complex geometries, fabricated by 3D printing. Composite materials are fabricated by combining the attractive mechanical properties of materials into one system. A combination of materials with different mechanical properties, size, geometry, and order of different phases can lead to fabricating a new material with a wide range of properties. A fundamental problem in engineering is how to find the design that exhibits the best combination of these properties. Biological composites like bone, nacre, and teeth attracted much attention among the researchers. These materials are constructed from simple building blocks and show an uncommon combination of high strength and toughness. By inspiring from simple building blocks in bio-inspired materials, we have simulated fracture behavior of a pre-designed composite material consisting of soft and stiff building blocks. The results show a better performance of bio-inspired composites compared to their building blocks. Furthermore, an optimization methodology is implemented into the designing the bio-inspired composites for the first time, which enables us to perform the bio-inspired material design with the target of finding the most efficient geometries that can resist defects in their structure. This study can be used as an effective reference for creating damage-tolerant structures with improved mechanical behavior. / Doctor of Philosophy / The goal of this research is developing a multiscale framework to study the details of fracture and fatigue for the rolling contact in rails, additively manufactured alloys, and bio-inspired hierarchical materials. Rolling contact fatigue (RCF) is a major source of failure and a dominant cause of maintenance and replacements in many railways around the world. Different computational models are developed for studying rolling contact fatigue in rail materials. The method can predict RCF life and simulate crack initiation sites under various conditions and the results will help better maintenance of the railways and increase the safety of trains. The developed model is employed to study the fracture and fatigue behavior in 3D printed metals created by the selective laser melting (SLM) method. SLM method as a part of metal additive manufacturing (AM) technologies is revolutionizing industries including biomedical, automotive, aerospace, energy, and many others. Since experiments on 3D printed metals are considerably time-consuming and expensive, computational analysis is a proper alternative to reduce cost and time. Our method for studying the fatigue at the microstructural level of 3D printed alloys can help to create more fatigue and fracture resistant materials. In the last section, we have studied fracture behavior in bio-inspired materials. A fundamental problem in engineering is how to find the design that exhibits the best combination of mechanical properties. Biological materials like bone, nacre, and teeth are constructed from simple building blocks and show a surprising combination of high strength and toughness. By inspiring from these materials, we have simulated fracture behavior of a pre-designed composite material consisting of soft and stiff building blocks. The results show a better performance of bio-inspired structure compared to its building blocks. Furthermore, an optimization method is implemented into the designing the bio-inspired structures for the first time, which enables us to perform the bio-inspired material design with the target of finding the most efficient geometries that can resist defects in their structure.

The primary purpose of this research is to investigate the effectiveness of a scaled roller rig for accurately assessing the contact mechanics and dynamics between a profiled steel wheel and rail, as is commonly used in rail vehicles. The established creep models of Kalker and Johnson and Vermeulen are used to establish correction factors, scaling factors, and transformation factors that allow us to relate the results from a scaled rig to those of a tangent track. �Correction factors, which are defined as the ratios of a given quantity (such as creep coefficient) between a roller rig and a track, are derived and used to relate the results between a full-size rig and a full-size track. Scaling factors are derived to relate the same quantities between roller rigs of different scales. Finally, transformation factors are derived by combining scaling factors with correction factors in order to relate the results from a scaled roller rig to a full-size tangent track. Close-end formulae for creep force correction, scaling, and transformation factors are provided in the thesis, along with their full derivation and an explanation of their limitations; these formulae can be used to calculate the correction factors for any wheel-rail geometry and scaling. For Kalker's theory, it is shown that the correction factor for creep coefficients is strictly a function of wheel and rail geometry, primarily the wheel and roller diameter ratio. For Johnson and Vermeulen's theory, the effects of creepage, scale, and load on the creep force correction factor are demonstrated. �It is shown that INRETS' scaling strategy causes the normalized creep curve to be identical for both a full-size and a scaled roller rig. �It is also shown that the creep force correction factors for Johnson and Vermeulen's model increase linearly with creepage, starting with the values predicted by Kalker's theory. �Therefore, Kalker's theory provides a conservative estimate for creep force correction factors. �A case study is presented to demonstrate the creep curves, as well as the correction and transformation factors, for a typical wheel-rail configuration. �Additionally, two studies by other authors that calculate the correction factor for Kalker's creep coefficients for specific wheel-rail geometries are reviewed and show full agreement with the results that are predicted by the formulae derived in this study. �Based on a review of existing and past roller rigs, as well as the findings of this thesis, a number of recommendations are given for the design of a roller rig for the purpose of assessing the wheel-rail contact mechanics. �A scaling strategy (INRETS') is suggested, and equations for power consumption of a roller rig are derived. Recommendations for sensors and actuators necessary for such a rig are also given. Special attention is given to the resolution and accuracy of velocity sensors, which are required to properly measure and plot the creep curves. / Master of Science

During life of railway vehicles, shape irregularities develop on wheels and rails because of wear. The shape irregularities then affect forces in wheel-rail contact and cause further damage of contact surfaces, vibrations and noise and increase risk of derailment. A numerical simulation of railway vehicle motion with more details on contact surfaces geometry was created to investigate dynamic contact loads in wheel-rail contact. A variety of methods can be used to evaluate forces in rolling contact, the method chosen for this study was algorithm CONTACT based on boundary element method. Four studies are presented in this papers: contact loads from a wheel with a flat and with a wavy tread pattern, loads on wavy rail and load in a curve. The first three studies investigated effects of existing wear patterns, the last one looked for cause of common wear pattern developing on rails. Results of the studies with worn components used showed that the worst kind of shape irregularities is a flat present on wheel. This type of shape cause loss of contact and following impacts. The study of ride in curve showed that cause of high wear in curves, especially those with small radii, is caused by vibration of wheelset. This vibration is then caused by different length of inner and outer rail and wheels travelling along a different path.

A proposição desta tese consiste na aplicação da técnica de Sistemas Multicorpos, na dinâmica de veículos guiados em trajetória variável. Foi apresentada uma visão geral do formalismo matemático, baseado nos métodos de Lagrange e Kane, utilizados nos programas de multicorpos, para geração automática das equações de movimento. A teoria de mecânica de contato, fundamental para o cálculo das forças de contato, entre o veículo e a guia, foi exposta detalhadamente. Sua validação, foi realizada, através de ensaios de laboratório, realizados no INRETS (França). A técnica de modelagem de Sistemas Multicorpos, foi aplicada, na previsão do comportamento dinâmico de veículos guiados. Neste sentido, a elaboração do modelo, através da descrição topológica, obtenção das equações de movimentos, análise e simulação do comportamento, foram realizadas. Foram selecionados na literatura internacional, os estudos de caso propostos pela International Association of Vehicle System Dynamics (IAVSD), para avaliação de veículos guiados. Foram modelados, o rodeiro ferroviário, submetido a força lateral, truque trafegando por um desvio e um veículo completo inscrevendo uma curva. O veículo modelado com 35 graus de liberdade, produziu mais de 1100 equações algébrico/diferenciais (DAE). As simulações do comportamento dinâmico dos sistemas, foram realizadas e os resultados apresentados em gráficos temporais. Os modos de movimento foram quantificados, através da analise modal, realizada no sistema linearização em torno de um ponto de operação. Os valores obtidos, foram comparados com números publicados e algumas discrepâncias, foram encontradas e justificadas. Pode-se observar nitidamente, o movimento de lacet, a partir dos auto-vetores complexos da matriz dinâmica do sistema. De forma geral, os resultados obtidos tanto na análise temporal, como no domínio da freqüência, apresentaram boa concordância, com os resultados publicados por outros autores. Estes resultados encorajadores, promovem a confiança na aplicação da técnica de Multicorpos em veículos guiados, sendo estímulo, para aplicação no desenvolvimento de novos projetos. / The subject of this thesis is the application of multibody system modelling techniques for dynamic behaviour investigation on guided vehicle on variable track trajectories. Analysis of existing techniques for generating vehicle motion equations, using multibody systems (MBS), is also carried out. Rolling contact theory, fundamental for vehicle/track forces calculations, are extensively reviewed. Validation have been carried out with laboratorial experiments. Multibody system modelling techniques have been used to predict, guided vehicle behaviour. Topological model description, have been used to generate equation of motion for simulation purposes. It has been chosen the railway vehicle, proposed by the International Association of Vehicle System Dynamics (IAVSD), as evaluation benchmark. The solution of wheelset benchmark proposition is presented. A two wheelset bogie model in a deviation, is also solved. A complete railway vehicle, with 35 degrees of freedom model, producing more than 1100 differential/algebraic equations (DAE), is performed and analysed. Time result simulation have been presented. Modal analysis have been performed around an operation point. Results have been compare with numbers published and discrepancy founded have been justified. Hunting movement have been obtained with the complex eigen-vector from system dynamic matrix. In general, results obtained from time and frequency domain, agreed with available results published from other authors. This encouraging results, promote credibility to Multibody System technique application to guided vehicle, stimulating new developing applications.

Esse trabalho descreve o projeto e construção de um equipamento destinado ao estudo de fadiga de contato de rolamento e sua posterior aplicação na caracterização de ferros fundidos. O trabalho foi iniciado com a utilização de um equipamento previamente existente no Laboratório de Fenômenos de Superfície da EPUSP, a máquina I. Esse equipamento era baseado na substituição das pistas externas de um rolamento axial de esferas por corpos de prova na forma de arruelas. Ensaios preliminares mostraram que a máquina I apresentava limitações, a principal delas era não ser capaz de reproduzir integralmente o movimento de rolamento, havendo uma relação desconhecida rolamento/deslizamento no decorrer do ensaio. Com base nas dificuldades operacionais da máquina I, uma segunda máquina, também baseada em um rolamento axial, foi projetada e construída. Ensaios preliminares mostraram que o novo equipamento reproduzia mecanismos de falha por fadiga de contato de rolamento. A etapa seguinte então foi utilizar a máquina II no estudo de fadiga de contato de rolamento de dois tipos de ferro fundido com matrizes de levada dureza, um cinzento e um nodular, os quais foram testados em duas condições: com e sem pré-tensionamento e analisados segundo a estatística de Weibull. Um aço também foi ensaiado como referência. A pré-tensão foi aplicada com uma ferramenta bi-partida e modelada por elementos finitos. Foram realizados ensaios lubrificados, com a aplicação de uma pressão máxima de contato de 3,6 GPa e conduzidos até a falha dos materiais por lascamento da superfície de rolamento. Em todas as condições de ensaio o ferro fundido nodular se mostrou muito superior ao ferro cinzento. Enquanto esse último que apresentou resultados similares na condição com e sem pré-tensão, o ferro fundido nodular teve a sua curva de distribuição de falhas significativamente alterada pela aplicação de tensão externa. Uma heterogeneidade na distribuição do tamanho de nódulos de grafita fez com que as falhas ocorressem sempre na mesma região do corpo-deprova evidenciando a sensibilidade do ensaio a variáveis microestruturais. / This work presents design and construction of an equipment for rolling contact fatigue studying and its use to cast irons analysis. Firstly it was used an equipment available at Surface Phenomena Laboratory, in São Paulo University, which was called machine I. The design of the system followed the well-known architecture where races of a thrust ball bearing are substituted by washers made with the material under investigation. Preliminary tests have shown that the first machine had severe limitations, but the most important was lack of ability to reproduce rolling contact fatigue mechanisms. An unknown sliding/rolling ratio always took place in its work. Then a second machine was designed and assembled. Sets up tests have shown typical mechanisms of rolling contact fatigue. The next step was to use the new machine to study two types of cast irons, a gray iron and a ductile iron, under two test conditions: with and without pre-stress. Also a steel specimen was tested as a reference. Results were subjected to Weibull analysis. A device was developed for pre-stress application, and the specimen/device system was modeled by finite elements analysis. Tests were carried out till failure, under lubrication, and with maximum contact pressure of 3.6 GPa. Ductile cast iron behavior was superior to gray iron in all tests. Gray iron specimens with and without pre-stress presented very similar results. On the other hand, ductile iron probability failure curve was strongly affected by pre-stress. An unexpected heterogeneity on graphite nodules size distribution promoted failures on just one region, which is an evidence of test sensitivity to micro structural changes.

A proposição desta tese consiste na aplicação da técnica de Sistemas Multicorpos, na dinâmica de veículos guiados em trajetória variável. Foi apresentada uma visão geral do formalismo matemático, baseado nos métodos de Lagrange e Kane, utilizados nos programas de multicorpos, para geração automática das equações de movimento. A teoria de mecânica de contato, fundamental para o cálculo das forças de contato, entre o veículo e a guia, foi exposta detalhadamente. Sua validação, foi realizada, através de ensaios de laboratório, realizados no INRETS (França). A técnica de modelagem de Sistemas Multicorpos, foi aplicada, na previsão do comportamento dinâmico de veículos guiados. Neste sentido, a elaboração do modelo, através da descrição topológica, obtenção das equações de movimentos, análise e simulação do comportamento, foram realizadas. Foram selecionados na literatura internacional, os estudos de caso propostos pela International Association of Vehicle System Dynamics (IAVSD), para avaliação de veículos guiados. Foram modelados, o rodeiro ferroviário, submetido a força lateral, truque trafegando por um desvio e um veículo completo inscrevendo uma curva. O veículo modelado com 35 graus de liberdade, produziu mais de 1100 equações algébrico/diferenciais (DAE). As simulações do comportamento dinâmico dos sistemas, foram realizadas e os resultados apresentados em gráficos temporais. Os modos de movimento foram quantificados, através da analise modal, realizada no sistema linearização em torno de um ponto de operação. Os valores obtidos, foram comparados com números publicados e algumas discrepâncias, foram encontradas e justificadas. Pode-se observar nitidamente, o movimento de lacet, a partir dos auto-vetores complexos da matriz dinâmica do sistema. De forma geral, os resultados obtidos tanto na análise temporal, como no domínio da freqüência, apresentaram boa concordância, com os resultados publicados por outros autores. Estes resultados encorajadores, promovem a confiança na aplicação da técnica de Multicorpos em veículos guiados, sendo estímulo, para aplicação no desenvolvimento de novos projetos. / The subject of this thesis is the application of multibody system modelling techniques for dynamic behaviour investigation on guided vehicle on variable track trajectories. Analysis of existing techniques for generating vehicle motion equations, using multibody systems (MBS), is also carried out. Rolling contact theory, fundamental for vehicle/track forces calculations, are extensively reviewed. Validation have been carried out with laboratorial experiments. Multibody system modelling techniques have been used to predict, guided vehicle behaviour. Topological model description, have been used to generate equation of motion for simulation purposes. It has been chosen the railway vehicle, proposed by the International Association of Vehicle System Dynamics (IAVSD), as evaluation benchmark. The solution of wheelset benchmark proposition is presented. A two wheelset bogie model in a deviation, is also solved. A complete railway vehicle, with 35 degrees of freedom model, producing more than 1100 differential/algebraic equations (DAE), is performed and analysed. Time result simulation have been presented. Modal analysis have been performed around an operation point. Results have been compare with numbers published and discrepancy founded have been justified. Hunting movement have been obtained with the complex eigen-vector from system dynamic matrix. In general, results obtained from time and frequency domain, agreed with available results published from other authors. This encouraging results, promote credibility to Multibody System technique application to guided vehicle, stimulating new developing applications.

In this thesis, the behavior of pearlitic steel was characterized under controlled wear conditions in the laboratory and service conditions in two ore mining stages, comminution and transportation. The thesis consists in three experimental chapters, divided according to the tribosystems analyzed. On all the chapters Electro Microscopy techniques for the microstructural analysis were employed. Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB-SEM), Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM) were used. The first experimental chapter shows the analysis of the pearlite under abrasive wear with loose abrasive particles in multi-events conditions. The sample was taken from Semi-Autogenous Grinding mills (SAG) and experimental simulation was carried out in laboratory using the Dry Sand Rubber Wheel Abrasion Test (DSRW). The results show a polycrystalline layer formation in both cases, characterized by ultra-fine grains of ferrite in the layer closer to the surface. It was also concluded that the DSRW can simulate the wear produced on field (superficial and microstructural features) in conditions of higher normal load than recommended by the ASTM Standard G65. The second experimental chapter explores the characterization of the microstructure after the indenter pass in scratch test using two conditions of normal load applied and five sequences of scratch. The microstructural analysis shows the formation of two subsuperficial layers identified by the level of the microstructural alterations. In the subsuperficial layer (close to the surface), the formation of new ultra-fine grains of ferrite was observed. A second layer was observed deeper in the sample and denominated as layer of the microstructure transition, characterized by the combination of deformed (reduction of the interlamellar spacing) and pearlite colonies not affected plastically by the mechanical loading. On this layer, the crystallographic texture in RD // in samples tested at 4 N (normal load) and one-pass scratch was determined. Later, on this chapter, the microstructure in a ground rail (industrial procedure characterized as a multi-event scratch test) was analyzed. Two grinding conditions were used for the analysis with variation of the grinding linear speed and load on the grinding stones (discs). The combination of low grinding speed and high load promotes a higher deformed layer formation beneath the patch zone and low randomized orientation of the pearlite colonies. Finally, in the third experimental chapter, the pearlitic characterization was concluded with the study of samples of railway wheel and rail under wear in service and Rolling Contact Fatigue (RCF) in laboratory. The laboratorial simulation was carried out using the twin-disc rolling contact tribometer with a variation of number of cycles. The characterization of railway wheel shows that the WEL is characterized by levels of breaking and aligned cementite and zones with dissolution of the carbon atom in the ferrite to form the supersaturated carbon ferrite. The polycrystalline ferrite formation (ultra-fine grains) in the sub-superficial layer and it was identified a preferential orientation of RD // in the layer of microstructural transition. The results of the laboratory test show surface crack nucleation and propagation at low angle in the more severe deformed layer. The microstructure of the layer consists in polycrystalline ferrite and the cementite dissolution. / Nesta tese foi caracterizado o comportamento do aço perlítico em condições controladas de desgaste em laboratório e em serviço em dois estágios do processo de mineração de minério, cominução e transporte ferroviário. A tese consiste em três capítulos experimentais divididos segundo o tribosistema analisado. Em todos os capítulos do trabalho foi utilizada a técnica de microscopia eletrônica para análise microestrutural. Foi utilizado Microscopia eletrônica de varredura (MEV), Focused Ion Beam (FIB-SEM), Electron Backscatter Diffraction (EBSD) e Microscopia eletrônica de transmissão (MET). O primeiro capítulo experimental mostra a análise da perlita in condições de desgaste abrasivo com partículas soltas em eventos múltiplos. As amostras foram tiradas de um moinho semi-autógeno (SAG) e realizada uma simulação experimental do desgaste em condições controladas usando o tribômetro de roda de borracha (RWAT). Os resultados mostraram a formação de camada branca em ambas as condições de análise, consistindo em uma camada poli cristalina caracterizada pela formação de grãos ultrafinos na camada mais próxima da superfície de desgaste. Também foi concluído que a roda de borracha pode simular o desgaste produzido nos moinhos SAG tanto nas características superficiais quanto microestruturais em condições de maior severidade as comumente utilizadas na norma ASTM G65 (procedimento B). O Segundo capítulo experimental explora a caracterização da microestrutura depois da passagem do endentador no ensaio de riscamento (scratch test) utilizando duas condições de carga normal aplicada e 5 sequências de riscamento. A análise microestrutural mostrou a formação de duas camadas subsuperficiais identificadas pelo nível de alteração microestrutural. Na camada mais próxima da superfície de desgaste foi observada a formação de grãos ultrafinos de ferrita. A segunda camada identificada mais profundamente na amostra, denominada como camada de transição, é caracterizada pela combinação de colônias deformadas (redução do espaçamento interlamelar) e camadas não afetadas pelos esforços produzidos no contato. Nesta camada foi determinada a texturização em direção RD // nas amostras testadas a 4 N (carga normal aplicada) e uma passada. Posteriormente à análise de riscamento foi caracterizada a microestrutura de uma amostra tirada de um trilho esmerilhado (processo industrial que pode ser considerado como aplicação do ensaio de riscamento). Foram consideradas duas condições de esmerilhamento com variação de velocidade de esmerilhamento (deslocamento linear do veículo esmerilhador) e potência dos motores dos rebolos usada no procedimento. A combinação de baixa velocidade de esmerilhamento e alta potência nos motores controladores dos rebolos promoveu uma grande deformação nas camadas subsuperficiais na região de contato e uma baixa aleatoriedade das orientações cristalográficas das colônias de perlita. Finalmente, no capítulo três, a caracterização da microestrutura perlitica foi finalizada com o estudo de amostras de roda e trilho em condições de desgaste em campo e de Rolling Contact Fatigue (RCF) em ensaios de laboratório. A simulação experimental foi realizada utilizando o tribômetro twin-disc rolling (configuração disco-disco) com variação do número de ciclos. A caracterização da roda ferroviária mostrou a formação da camada branca caracterizada por níveis de cementita fraturada e alinhada em direção do movimento de rolamento/deslizamento com áreas de dissolução do átomo de carbono na ferrita formando uma ferrita supersaturada. Foi identificado a formação de policristais de ferrita (grãos ultrafinos) na camada mais superficial e uma orientação preferencial RD // na camada de transição. Os resultados dos ensaios de laboratório mostraram a nucleação de trincas superficiais se propagando a baixo ângulo na camada branca. A transformação microestrutural dessa camada após ensaios de laboratório consiste em policristais de ferrita e dissolução da cementita.