Neste trabalho foram soldadas 4 amostras de rodas para veículos comerciais pelo processo de soldagem Tandem-MIG. Para cada amostra foram utilizados parâmetros de soldagem diferentes de modo a se obter formatos de cordão de solda distintos. O objetivo do trabalho foi avaliar a influência desses formatos de cordão de solda no desempenho em fadiga. Para os materiais utilizados, SAE1010AA e SAE1015A, foram realizados os seguintes ensaios: análise de composição química, análise micrográfica, levantamento das curvas tensão x deformação, medidas de microdureza Vickers e levantamento das curvas SN pelo método de flexão alternada (R=-1). Através da composição química constatou-se que ambos os aços são de baixo carbono e atendem o especificado pela norma SAEJ403. Através da análise micrográfica foi verificado que as fases presentes foram ferrita e perlita, e o tamanho médio de grão foi de 11,2 ?m para os dois materiais. O aço SAE1010AA apresentou limite de escoamento médio de 265,2 MPa, limite de resistência de 384,8 MPa e alongamento médio de 43,9%. Já o aço SAE1015AA apresentou limite de escoamento médio de 260,1 MPa, limite de resistência de 407,3 MPa e alongamento de 41,9%. Para os dois materiais, o limite de resistência à fadiga (2.10-6 ciclos) foi de 225 MPa. Foi realizada a caracterização das 4 amostras soldadas. Através das curvas SN observou-se que o limite de resistência à fadiga para as amostras soldadas apresentou redução para valores entre 180 e 160 MPa. Com os mapas de microdureza Vickers levantados foi possível perceber as diferenças de microdureza obtidas de acordo com os parâmetros de soldagem utilizados e consequentemente com as microestruturas formadas. Com as imagens obtidas na análise micrográfica observou-se a formação de microestruturas com formatos e tamanhos diversos tanto nas regiões da zona termicamente afetada das amostras como na região de solidificação do cordão de solda. Finalmente, com a realização da análise via MEV, constatou-se que todas as trincas por fadiga iniciaram na superfície dos corpos de prova. Foram observadas estrias de fadiga, com maior e menor intensidade de acordo com cada amostra, ao longo da região de fratura. Além disso, foi observada a geração de múltiplas trincas por fadiga em duas amostras. / In this study four samples of commercial vehicle wheels were welded using the Tandem-MIG process. For each sample different welding parameters were used to get different kinds of weld bead shape. The aim of this study was to evaluate the influence of these weld bead shapes in fatigue performance. For the SAE1010AA and SAE1015AA materials the following tests were performed: chemical composition analysis, microstructure characterization, SN curves by alternating bending method (R=-1). Through analyzing chemical composition, it was found that both materials are low carbon steel and they meet the SAE J403 standard. In microstructure characterization, ferrite and perlite microstructure were found, with an average grain size of 11,2 ?m for both steels. The SAE1010AA showed an average yield strength of 265,2 MPa, an average tensile strength of 384,8 MPa, and alongation of 43,9%. The SAE1015AA showed an average yield strength of 260,1 MPa, an average tensile strength of 407,3 MPa, and alongation of 41,9%. For both materials, the fatigue limit (2.10-6 ciclos) was 225 MPa. The characterization of the four samples was done. In SN curves the fatigue limit for welded samples showed a reduction to values of between 180 and 160 MPa. In the Vickers microhardness test, it was possible to notice the differences between the microhardness obtained in accordance with welding parameters used and, consequently, the microstructure formed. In microstructure analysis, different kinds and sizes of microstructure were observed to have formed in both the heat-affected zone and the welded zone. Finally, through MEV analysis, all fatigue cracks were seen to have begun on the surface of the samples. Fatigue estriaton, with major and minor intensity depending on the sample, was observed along the fracture region. Furthermore, the beginning of multiple fatigue cracks were seen in two samples.

Orientador: Itamar Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-12T15:59:53Z (GMT). No. of bitstreams: 1 Marcelo_AndreLuiz_M.pdf: 9740567 bytes, checksum: 965e448ceb0965ed5555dfa95cc7f5c6 (MD5) Previous issue date: 2008 / Resumo: Parafusos de alta resistência mecânica (classe ISO 10.9) fabricados com dois aços de origens diferentes são utilizados para investigar a influência da temperatura de revenido, da seqüência do processo de fabricação (laminação da rosca antes e após o tratamento térmico) e da quantidade de inclusões contidas no aço, no limite de fadiga e também qual o critério de falha por fadiga existente na literatura que melhor se aplica a parafusos de alta resistência mecânica com laminação da rosca antes do tratamento térmico. Parafusos M8 com rosca grossa foram divididos em sete grupos diferentes, três fabricados com aço nacional (AISI 4135) e tratados termicamente após laminação da rosca com temperaturas de revenido 490, 520 e 550 ºC, e quatro fabricados com aço japonês (SCM 435H), porém, desses quatro grupos, três foram idênticos aos fabricados com aço nacional e um último grupo teve a rosca laminada após tratamento térmico, com temperatura de revenido de 550 ºC. Nesses grupos foram realizadas análises metalográficas e de micro-inclusões, ensaios de tração, microdureza Vickers e fadiga e, por último, uma análise fratográfica dos parafusos que falharam durante o ensaio de fadiga, utilizando microscopia eletrônica de varredura. Os ensaios de fadiga foram realizados com pré-carga constante de 700 MPa, razão de carga por volta de 0,8, e utilizando o método estatístico "staircase" para a determinação do limite de fadiga. Além disso, foram realizados alguns ensaios de fadiga adicionais, com baixa razão de carga, 1 , 0 ??R , para um grupo específico, com a finalidade de analisar os critérios de falha por fadiga já existentes. A análise metalográfica revelou que em todos os grupos a microestrutura característica foi martensita revenida e a única diferença foi encontrada no grupo com laminação da rosca após tratamento térmico que apresentou linhas de laminação muito bem definidas nos contornos dos filetes da rosca. A análise de micro-inclusões mostrou que o aço nacional possui uma quantidade de inclusões muito superior ao aço japonês. Os ensaios de microdureza e de tração revelaram que os parafusos com laminação da rosca após tratamento térmico possuem níveis de resistência mecânica inferior somente aos grupos revenidos a 490 ºC. Todos os grupos com laminação da rosca antes do tratamento térmico apresentaram praticamente o mesmo limite de resistência à fadiga, mostrando que, neste caso, variações de até 60 ºC no tratamento de revenido não interferiram no limite de fadiga. Por outro lado, o grupo com laminação da rosca após tratamento térmico apresentou um limite de fadiga de aproximadamente 9% superior ao grupo tratado nas mesmas condições, porém com laminação da rosca antes do tratamento térmico. Esta melhora está relacionada ao alinhamento dos grãos nos contornos dos filetes da rosca e à introdução de tensões residuais de compressão na superfície da rosca. Com os ensaios de fadiga adicionais, foi possível mostrar que o critério que mais se ajusta a parafusos de alta resistência mecânica com laminação da rosca após tratamento térmico é o VDI 2230, que apresenta uma tensão alternada constante independente da razão de carga. / Abstract: High strength bolts, class ISO 10.9, were manufactured with two different steels in order to investigate the influence of tempering temperature, the sequence of manufacture process (threads rolled before and after heat treatment), and the inclusions amount contained in steels, in the bolt fatigue behavior and also what criterion of fatigue failure existent in literature is the best for application on high strength bolts with the threads rolling before heat treatment. M8 bolts with coarse threads were separated in seven different groups, three manufactured with Brazilian steel (AISI 4135) and heat treated, after the process of thread rolling, with tempering temperatures of 490, 520, and 550 ºC, and four groups were manufactured with Japanese steel (SCM 435H), however, three groups were identical of the bolts manufactured with national steel and a last one group had the threads rolled after heat treatment with tempering temperature of 550 ºC. In all groups were carried out metallographic and inclusion analyses, tension, microindentation hardness, and fatigue tests, and a fractographic analysis of bolts failed in the fatigue tests, using a scanning electron microscope. The fatigue tests were carried out with constant preload of 700 MPa, stress ratio about 0.8, and it was used the staircase statistical method for determine the fatigue limit. Besides, additional fatigue tests were carried out, with low stress ratio ( 1 . 0 ??R ), in order to analyze the existents criteria of fatigue failure. The results of metallographic analysis showed that the characteristic microstructure in all groups was tempered martensite and the only difference between the groups was the alignment of the grains around the root of the thread that occurred in the group which was heat treated before the threads rolling. The inclusion analysis revealed that the Brazilian steel has an amount of inclusions very superior than Japanese steel. The tension and microindentation hardness tests showed that the bolts with threads rolling after heat treatment have strength level lower than solely the groups tempered at 490 ºC. All the groups with threads rolling before heat treatment presented practically the same fatigue limit. It shows that variations of up to 60 ºC in the tempering temperature don't interfere in the fatigue limit. On the other hand, the group with threads rolling after heat treatment presented fatigue limit approximately 9% superior than the group treated in the same conditions, however with threads rolling before heat treatment. This improve is related with the alignment of the grains around the root of the thread and the introduction of residual compressive stress near the surface of the bolts. With the results of the additional fatigue tests, it was possible to analysis some criteria of fatigue failure and show that the best criterion of failure, existent in the literature, for high strength bolts with thread rolling after heat treatment is the VDI 2230, which presents a constant alternating stress independently of the stress ratio. / Mestrado / Materiais e Processos de Fabricação / Metre em Engenharia Mecanica

High strength secondary hardening lath martensitic steel is a strong candidate for high performance and reliable transmission systems in aircraft and automotives. The fatigue resistance of this material depends both on intrinsic microstructure attributes, such as fine scale (M2C) precipitates, and extrinsic attributes such as nonmetallic primary inclusions. Additionally, the aforementioned attributes are affected by processing history. The objective of this research is to develop a computational framework to quantify the influence of both extrinsic (primary inclusions and residual stresses) and intrinsic (martensite laths and carbides) microstructure attributes on fatigue crack formation and the early stage of microstructurally small crack (MSC) growth that dominate high cycle fatigue (HCF) lifetime. To model the fatigue response at various microstructure scales, a hierarchical approach is adopted. A simplified scheme is developed to simulate processing effects such as shot peening that is suitable to introduce representative residual stresses prior to conducting fatigue calculations. Novel strategies are developed to couple process route (residual stresses) and microstructure scale response for comprehensive analysis of fatigue potency at critical life-limiting primary inclusions in gear steels. Relevant microstructure-scale response descriptors that permit relative assessment of fatigue resistance are identified. Fatigue crack formation and early growth is highly heterogeneous at the grain scale. Hence, a scheme for physically-based constitutive models that is suitable to investigate crack formation and early growth in martensitic steel is introduced and implemented. An extreme value statistical/probabilistic framework to assess the influence of variability of various microstructure attributes such as size and spatial distribution of primary inclusions on minimum fatigue crack formation life is devised. Understanding is sought regarding the relative role of microstructure attributes in the HCF process, thereby providing a basis to modify process route and/or composition to enhance fatigue resistance. Parametric studies are conducted to assess the effect of hot isostatic pressing and introduction of compliant coatings at debonded inclusion-matrix interface on enhancement of fatigue resistance. A comprehensive set of 3D computational tools and algorithms for hierarchical microstructure-sensitive fatigue analysis of martensitic gear steels is developed as an outcome of this research; such tools and methodologies will lend quantitative and qualitative support to designing improved, fatigue-resistant materials and accelerating insertion of new or improved materials into service.

[Truncated abstract. Please see pdf format for complete text.] Over the years much work has been done on studying sequence effects under multilevel loading. Yet, the underlying fatigue mechanisms responsible for such interactions are not fully understood. The study of fatigue under complex loading histories begins by investigating strain interaction effects arising from simple 2-step loading sequences. Fatigue for all investigations were conducted under uniaxial push-pull mode in strain-control. Fatigue is traditionally classified as either low or high cycle fatigue (LCF and HCF respectively). The boundary for LCF and HCF is not well-defined even though the fatigue life of LCF is typically dominated by crack “initiation”, while for HCF, fatigue life is usually dominated by stable crack growth. The terms LCF and HCF, apart from referring to the low and high number of fatigue cycles required for failure, also bear little physical meaning in terms of describing the state of fatigue imposed. As a result, conventional definitions of the two distinct regimes of fatigue are challenged and a new method of classifying the boundary between the two regimes of fatigue is proposed. New definitions are proposed and the terms plastically dominant fatigue (PDF) and elastically dominant fatigue (EDF) are introduced as suitable replacements for LCF and HCF respectively. PDF refers to the condition of a material undergoing significant reverse plasticity during cyclic loading, while for EDF, minimal reverse plasticity is experienced. Systematic testing of three materials, 316 L stainless steel, 6061-T6 aluminium alloy and 4340 high strength steel, was performed to fully investigate the cycle ratio trends and “damage” accumulation behaviour which resulted from a variety of loading conditions. Results from this study were carried over to investigate more complex multilevel loading sequences and possible mechanisms for interaction effects observed both under 2-step and multi-step sequences were proposed. Results showed that atypical cycle ratio trends could result from loading sequences which involve combinations of strain amplitudes from different fatigue regimes (i.e. PDF or EDF). Mean strain effects on fatigue life were also studied. The objective of this study was to identify regimes of fatigue which are significantly influenced by mean strains. Results indicated that mean strains affected EDF but not PDF. 2-step tests, similar to those performed in earlier studies were conducted to investigate the effects of mean strain on variable amplitude loading. Again, atypical cycle ratio trends were observed for loading sequences involving combinations of PDF and EDF. It is understood that fatigue crack growth interaction behaviour and mean stress effects are two dominant mechanisms which can be used to explain cycle ratio trends observed. The significance and importance of proper PDF/EDF definition and specification are also stressed. The study of fracture mechanics is an important component of any fatigue research. Fatigue crack growth in 4140 high strength steel CT specimens, under conditions of plane stress and plane strain were studied. In this investigation, the effects of R and overload ratios were also studied for both plane stress and plane strain conditions. Results indicate that differences in the point of crack “initiation” under both plane stress and plane strain conditions decrease with increasing load range, while the extent of crack retardation as a result of overloading, is greater under plane stress than plane strain conditions. The extent of crack growth retardation increases with decreasing R ratios and increasing overload ratios. The final phase of this project involves the proposal of two practical models used to predict cumulative “damage” and fatigue crack propagation in metals. The cumulative “damage” model proposed takes the form of a power law and the exponent which governs “damage” accumulation can easily be calculated by knowing the failure life, Nf, for a given strain or load level. Predictions for the “damage” model performed better when compared to other popular cumulative “damage” models. The second model proposed predicts fatigue crack growth behaviour from known monotonic and smooth specimen fatigue data. There are several benefits of having a model that can predict fatigue crack growth from monotonic and smooth specimen fatigue data: a) traditionally, engineers had to rely on expensive and time-consuming crack propagation tests to evaluate and select materials for maximum fatigue resistance, and b) monotonic and smooth specimen fatigue data are readily available. The crack propagation model is proposed to alleviate the material selection process by providing engineers a means to rapidly eliminate and narrow down selections for possible material candidates.

Devido à resistência dos fabricantes de automóveis quanto à utilização de engrenagens de FNA estudos tem sido feitos para averiguar o desempenho deste material em engrenagens. Dentro deste contexto esta dissertação tem como objetivos principais o desenvolvimento e construção de uma máquina de ensaios de fadiga de contato em engrenagens e a comparação quanto à resistência ao desgaste de engrenagens cilíndricas de dentes retos fabricadas em aço liga 8620 (cementado e temperado) e em ferro fundido nodular austêmperado (FNA). Estas engrenagens foram fornecidas em dois estados de acabamento superficial (shaving ou por fresamento). O equipamento desenvolvido funciona com o princípio de recirculação de potência e é comumente chamado de FZG. Os experimentos de desgaste têm sido feitos com dois estágios de torque: 135 N.m (running-in) e 302 N.m (steady-state) e as engrenagens são de perfil modificado. Após a realização dos ensaios, foi possível verificar que: (1) a área danificada dos flancos, devido ao fenômeno de pitting, das engrenagens de FNA foi maior do que das de aço AISI 8620; (2) a comparação entre os parâmetros de rugosidade nos estágio de fornecimento e após cada estágio do experimento mostra que as engrenagens com acabamento por shaving apresentam uma melhor distribuição da carga e um menor desgaste, (3) o regime de lubrificação na região do diâmetro primitivo durante os ensaios foi o limite (ou limítrofe) para ambos os processos de fabricação, porém o parâmetro de espessura de filme (λ) é maior para engrenagens fabricadas por shaving, fazendo com que a espessura do filme lubrificante seja maior, (4) o efeito da redução da área resistente aos esforços de contato, devido à presença dos nódulos de grafita, na resistência a fadiga foi evidenciado, porém o acabamento superficial mostrou-se mais relevante do que a distribuição e tamanho dos nódulos. / Due to the resistance of automotive manufactures on apply ADI gear studies have been made to verify the performance of this material. Inside this context this work has a main objective the construction of contact fatigue test machine in gears and the comparison, related to the wear resistance, between spur-gears made of carburized and quenched AISI 8620 low steel and austempered ductile iron (ADI). These gears were supplied in two states of finish surface (by shaving or by machining). The developed equipment functions with the principle of the power recirculation and commonly called FZG. The wear experiments were performed in two stages of torque: 135 N.m (running-in) and 302 N.m (steady-state) and the gears have modified profile. After the accomplishment of the experiments, was possible verify that: (1) The damaged area of flanks, due to the phenomenon of pitting, at ADI gears was bigger than AISI 8620 low-alloy steel gears. (2) The comparisons between the roughness parameter in the supply state and after each period of the experiment shows that gears with surface finish by shaving have better load distribution and minor wear. (3) Lubricant regime at pitch diameter was the boundary for both manufacture process, but the film thickness parameter λ is higher for shaved gears, making the lubricant film thickness has major value, (4) The reduction effect the resistive area of contact due the presence of the graphite nodules in the contact fatigue resistance was evidenced, however surface finishing was more relevant than the distribution and size nodules.

Devido à resistência dos fabricantes de automóveis quanto à utilização de engrenagens de FNA estudos tem sido feitos para averiguar o desempenho deste material em engrenagens. Dentro deste contexto esta dissertação tem como objetivos principais o desenvolvimento e construção de uma máquina de ensaios de fadiga de contato em engrenagens e a comparação quanto à resistência ao desgaste de engrenagens cilíndricas de dentes retos fabricadas em aço liga 8620 (cementado e temperado) e em ferro fundido nodular austêmperado (FNA). Estas engrenagens foram fornecidas em dois estados de acabamento superficial (shaving ou por fresamento). O equipamento desenvolvido funciona com o princípio de recirculação de potência e é comumente chamado de FZG. Os experimentos de desgaste têm sido feitos com dois estágios de torque: 135 N.m (running-in) e 302 N.m (steady-state) e as engrenagens são de perfil modificado. Após a realização dos ensaios, foi possível verificar que: (1) a área danificada dos flancos, devido ao fenômeno de pitting, das engrenagens de FNA foi maior do que das de aço AISI 8620; (2) a comparação entre os parâmetros de rugosidade nos estágio de fornecimento e após cada estágio do experimento mostra que as engrenagens com acabamento por shaving apresentam uma melhor distribuição da carga e um menor desgaste, (3) o regime de lubrificação na região do diâmetro primitivo durante os ensaios foi o limite (ou limítrofe) para ambos os processos de fabricação, porém o parâmetro de espessura de filme (λ) é maior para engrenagens fabricadas por shaving, fazendo com que a espessura do filme lubrificante seja maior, (4) o efeito da redução da área resistente aos esforços de contato, devido à presença dos nódulos de grafita, na resistência a fadiga foi evidenciado, porém o acabamento superficial mostrou-se mais relevante do que a distribuição e tamanho dos nódulos. / Due to the resistance of automotive manufactures on apply ADI gear studies have been made to verify the performance of this material. Inside this context this work has a main objective the construction of contact fatigue test machine in gears and the comparison, related to the wear resistance, between spur-gears made of carburized and quenched AISI 8620 low steel and austempered ductile iron (ADI). These gears were supplied in two states of finish surface (by shaving or by machining). The developed equipment functions with the principle of the power recirculation and commonly called FZG. The wear experiments were performed in two stages of torque: 135 N.m (running-in) and 302 N.m (steady-state) and the gears have modified profile. After the accomplishment of the experiments, was possible verify that: (1) The damaged area of flanks, due to the phenomenon of pitting, at ADI gears was bigger than AISI 8620 low-alloy steel gears. (2) The comparisons between the roughness parameter in the supply state and after each period of the experiment shows that gears with surface finish by shaving have better load distribution and minor wear. (3) Lubricant regime at pitch diameter was the boundary for both manufacture process, but the film thickness parameter λ is higher for shaved gears, making the lubricant film thickness has major value, (4) The reduction effect the resistive area of contact due the presence of the graphite nodules in the contact fatigue resistance was evidenced, however surface finishing was more relevant than the distribution and size nodules.

Orientador: Itamar Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-11T17:15:29Z (GMT). No. of bitstreams: 1 Alves_CleverGama_M.pdf: 6160446 bytes, checksum: 5afa2cd3848388247de70ceee21a3cf8 (MD5) Previous issue date: 2008 / Resumo: O presente trabalho descreve um procedimento de aprovação de um braço de controle de suspensão automotiva sujeito a fadiga de alto ciclo, ao mesmo tempo em que propõe uma sistemática alternativa de validação baseada em conceitos e teorias estatísticas de confiabilidade. Nesse aspecto, a pesquisa não só avalia o procedimento seguido pelo fabricante, como também executa comparações gráfico-analíticas de distribuições probabilísticas (normal, lognormal e Weibull) a fim de caracterizar a massa de dados completos e suspensos obtidos em ensaios acelerados de bancada. Um espaço amostral constituído por quatro observações completas da configuração final da peça e oito da inicial, complementado por doze dados suspensos, foi usado para determinar os parâmetros dos modelos. Essa análise levou à escolha do modelo de Weibull bi-paramétrico para o tempo até a falha para as duas configurações em foco. A estimação final dos parâmetros foi feita pelo método da máxima verossimilhança, o qual superou um método alternativo específico para Weibull na comparação com a distribuição referencial de categoria. Dessa forma, calculou-se o ganho efetivo em confiabilidade conseguido com o esforço adicional de desenvolvimento da peça. O teste de hipóteses de Kruskal-Wallis permitiu concluir que as duas configurações realmente possuem performances de durabilidade diferentes. É notável o ganho em confiabilidade obtido por meio das mudanças que levaram à configuração final: em um universo de um milhão de peças, o número de falhas esperadas aos 30.000 ciclos caiu de 96.384 para 5 partes por milhão / Abstract: This dissertation aims at describing a procedure for approval of a suspension control arm subjected to high-cycle fatigue and, simultaneously, at proposing a validation alternative method based on reliability concepts and statistical theories. In that manner, the research provides not only an assessment of the procedure followed by the manufacturer, but also analytical and graphical comparisons of probabilistic distributions (normal, lognormal, Weibull) in order to characterize the set of complete and suspended data from bench accelerated tests. A sample space comprised by four complete final configuration observations and eight complete primary configuration ones, in addition to twelve suspended figures, was the basis for determining the model parameters. Such an analysis led to choose the bi-parametric Weibull for both focused configurations¿ time to failure. The ultimate estimation of the parameters was performed through the maximum likelihood method, which beat a specific alternative method for Weibull when compared with the referential category distribution. Thus, the effective gain in reliability resulting from the product development additional effort was calculated. The Kruskal-Wallis hypothesis testing guided the conclusion that the two configurations actually have different durability performances. It is impressive the gain in reliability brought by the changes that led towards the final configuration: taking an amount of one million parts into consideration, the expected number of failures at 30,000 cycles dropped from 96,384 to 5 parts per million / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Fretting is a serious concern in many industrial components, specifically, in nuclear industry for the safe and reliable operation of the component/system. Till date, a lot of efforts has already been made to understand the basic mechanics and mechanism involved in fretting, but still limited understanding on the following domains exists, (i) No standard experimental set up and procedures exists which could quantify the entire fretting domain. (ii) Limited data available for the designer under controlled environment conditions. (iii) Limited work in correlating fretting damage with the mechanical responses, specifically, for the materials with good adhesion properties. (iv) Limited work to understand the nucleation/initiation of cracks under fretting condition and, the effect of loading on crack propagation. (v) Displacement and shear force distribution at the contact interface accounting the failure mechanism. The whole efforts in this thesis are focused on the above points and, are investigated in detail. Further, studies are focused to simulate fretting conditions in a Fast Breeder Reactor (FBR). Reactor core components are exposed to sodium environment, which is a low oxygen environment. Experiments under liquid sodium are difficult and as a first step, the tests were done under vacuum condition to simulate condition in sodium environment. Stainless steel (SS316L) is a reactor core component material used in FBRs. Chromium carbide coatings are already qualified based on the performance criteria for friction coefficients, wear rates and galling resistance, but are not evaluated under fretting conditions. Thus, stainless steel and chromium carbide coatings are investigated in detail. In this thesis, mechanics and mechanisms involved in the degradation processes for self-mated stainless steel under fretting conditions are examined in detail. Further, chromium carbide with 25% nickel chrome binder coatings using plasma spray and high-velocity oxy-fuel (HVOF) processes on stainless steel are also investigated. The choices of the coating processes have been made such that the substrate must be maintained in a particular metallurgical condition. The effect of normal load, displacement amplitude, environment conditions, surface roughness, and stress field are critically examined. Stainless steel (SS) is often used in the nuclear industry because of its excellent mechanical properties under high temperature and irradiation environment, but on the other hand, SS is characterized as having relatively poor wear and galling resistance. In nuclear power plants (NPPs), different components move relative to each other, due to differential thermal expansion or flow-induced vibration or during loading and unloading events, and such conditions can be categorized under fretting. The objective of the present work is to understand the mechanics and mechanisms of nuclear grade material (NGM), specifically for sodium-cooled NPPs, under fretting conditions. The first-of-a-kind fretting machine has been designed and developed to simulate fretting condition in both, air and vacuum. The test in vacuum simulates conditions under sodium environment. The major challenge in the design of a fretting machine is to achieve low displacement amplitude, as low as 1µm, between the contact surfaces under constant normal load. The hydraulic actuated machine works under displacement controlled mode, for any frequency between 4Hz and 120Hz, under high vacuum of 10–5mbar and for temperatures up to 873K. A unique feature of the machine is the design of flexural member which provides not only high axial stiffness but also flexibility in the lateral direction. A robust control system with an efficient data acquisition system adds to the reliability of the system. Contact conditions prevailing at the interface were identified on the basis of variation of coefficient of friction (COF) with number of cycles, running condition fretting loops, and total energy dissipation at the contact interface. Gross sliding conditions have been observed under normal load of 70N and 250N and displacement amplitude in the range of 50µm to 200µm, except for normal load of 250N and displacement amplitude of 50µm. The tests were conducted under both ambient and vacuum environment. Higher value of COF observed for self-mated SS, compared to SS versus coated surface, has been attributed to the existence of strong adhesion prevailing at the contact interface. Running condition fretting loops were correlated with damage observed from the scar profile and the micrographs. In addition to elliptical and quadratic loops, triple loops were also identified. The existence of strong adhesion results in an increase of shear force, whereas subsequent drop in shear force is due to third body formation at the contact interface. Higher magnification micrograph reveals fatigue striations at the contact edge, while the fracture features were observed in the central region. The surface morphological features of the material under seizure conditions, which have been observed under a normal load of 250N and displacement amplitude of 50µm, shows large scale cracking on one side of the pin and the flat. Micrographs at higher magnification of the cracked surface just adjacent to the contact interface shows formation of slip bands within the grains, whereas the central region reveals shear fracture. Coated surfaces shows major surface degradation mainly in the form of fracture and spalling of the coatings. Energy dispersive spectroscopy (EDS) shows the occurrences of material transfer between the contacting surfaces. Frictionally constrained conditions have also been investigated at high normal load of 600N and for displacement amplitude in the range of 25µm to 200µm. Constant shear force with number of cycles and dependence of friction force on displacement amplitude were observed as the typical characteristics of frictionally constrained bodies. Two distinct regions, viz. center stick region and annular micro slip region, indicate the existence of partial slip regimes. Junction growth due to plastic flow of the material resulted in an increase of real area of contact at the contact interface. It is believed that the cyclic variation in the contact area, under cyclic tangential loading, may have given rise to micro slip in the annular region, and finally resulted in two distinguishable regions. It has been observed that the occurrence of micro slip in the annular region resulted in the material transfer from flat to pin surface, as evident from EDS responses. Damage in the form of circumferential cracks has also been observed in the annular region, whereas the center region shows features of shear fracture. Detail micro structural studies have been carried out for two extreme conditions, viz., gross sliding and seizure conditions. The conditions were identified mainly based on shear force variation with number of cycles and running condition fretting loops. Subsurface damage under both conditions has been compared based on the severity of plastic deformation and the orientation of subsurface cracks. Severity of plastic deformation has been quantified based on hardness variation along depth. A steep gradient of hardness indicates that the damage is very much confined in the region just beneath the contact interface. Gross sliding condition at the contact interface resulted in the propagation of subsurface cracks parallel to the surface, whereas under seizure condition the cracks were found inclined at an angle between 450 and 540 to the surface. Further, severe plastic deformations under seizure condition have resulted in the formation of shear bands and were found oriented in the direction of macroscopically imposed plastic flow. Influence of initial surface roughness on wear damage has also been quantified based on energy wear coefficient. Higher energy wear coefficient has been found for machined pin under sliding condition, whereas, under seizure condition polished pin shows higher energy wear coefficient. A computer code has been developed for the evaluation of surface and subsurface stress field, under both partial slip and gross sliding condition. Cattaneo and Mindlin approach has been adopted to model the partial slip condition. Energy based approach has been adopted for the quantification of damage observed under both contact conditions. Shear strain energy density and normalized strain energy release rate have been evaluated at the surface and in the subsurface region. Effect of contact conditions and the influence of coefficient of friction on stress field have been studied in detail. Analysis results shows that gross sliding results in higher tensile stress at the trailing edge, as compared to the stress induced under partial slip condition. Further, it has been observed that higher shear strain energy density at the surface and in subsurface region controls the nucleation of damage under both partial slip and gross sliding conditions. A criterion for the no growth of subsurface cracks has been discussed based on the distribution of stress intensity factor and normalized strain energy release rate as a function of crack size. It has been observed that subsurface cracks can grow up to significant depth depending on the crack propagator energy. The availability of crack propagator energy depends on coefficient of friction and contact conditions prevailing at the contact interface. The analytical results were found in good agreement with experimental observations. Non-linear analyses have been carried out using finite element analysis to evaluate stress and strain fields, assuming the existence of fully stick condition at the contact interface. Fully stick condition simulates the contact condition under strong adhesion. The analysis investigates the effect of shear or tangential loading on pressure distribution, contact radius, energy dissipation, and damage mechanism involved under elastic-plastic deformation. The accumulation of equivalent plastic strain in each cycle is believed to be responsible for ductile fracture. It has been observed that both cyclic plasticity and ratcheting are involved in the damage mechanism. Ratcheting has been observed as the governing damage mode under cyclic tangential loading condition. In contrast to this, due to limited ductility or brittle nature of coated surfaces, stress based criteria governs the damage. Continuous micro slip model has been developed to evaluate the displacement field and shear force distribution for partial slip and gross sliding condition. Further, the studies have been carried out to study the influence of relative tangential modulus of the contacting bodies on displacement field and shear force distribution. Plane strain and axisymmetric elastic elements are considered in the modeling while the interfacial layer has been modeled as an elastic-plastic layer. The model gives the shear force distribution at the contact interface and subsequently subsurface stress field can be estimated, once the tangential stiffness of the contact interface layer is known. The value of tangential modulus can be estimated either from numerical analysis or from experiments. Further, the study shows that as the relative tangential modulus of an interfacial layer increases, the shear force becomes more intense in the stick-slip transition region making this location more prone for damage nucleation.

In der vorliegenden Arbeit wurden Untersuchungen zum Initiierungs- und Ausbreitungsverhalten kurzer Ermüdungsrisse in einem austenitischen sowie einem austenitisch-ferritischen Edelstahl durchgeführt. Dazu erfolgten zyklische Verformungsexperimente sowohl ex situ als auch in situ im Rasterelektronenmikroskop. Die Auswertung der Experimente erfolgte im Rasterelektronenmikroskop sowohl abbildend in verschiedenen Modi als auch über Rückstreuelektronenbeugungsmessungen. Bezüglich der Rissinitiierung wurde eine Häufigkeitsverteilung der Rissinitiierungsorte für beide Stähle erstellt. Die dabei ermittelte stark unterschiedliche Häufigkeit für die transkristalline Rissinitiierung in der austenitischen Phase konnte mit der unterschiedlichen Textur und Mikrostruktur der beiden Stähle in Zusammenwirken mit den elastischen Eigenschaften der beiden Phasen erklärt werden. Für die Rissausbreitung wurde gezeigt, dass eine Korrelation zwischen der Risslänge und der Rissausbreitungsrate besteht, aus der hervorgeht, dass die Übergangsrisslänge zwischen den Bereichen der mikrostrukturell und mechanisch kurzen Risse etwa einen Korndurchmesser beträgt. Anhand der in situ Messung der (plastischen) Rissöffnung und -scherung wurden die Unterschiede im Rissausbreitungsverhalten in den verschiedenen Phasen herausgearbeitet. Für die austenitische Phase ergibt sich dabei ein öffnungsdominierter und für die ferritische Phase ein scherungsdominierter Mechanismus. Die im Ferrit auftretenden zwei unterschiedlichen Ausprägungen der Oberflächenrisspfade („rau“ und „glatt“) konnten mit der Orientierung der jeweils risstragenden Körner korreliert werden. In Zusammenwirken mit der beobachteten Systematik der Anordnung der Gleitspuren um kurze Risse im Ferrit sowie einer Analyse der Spannungsverteilung um die Rissspitze wurde ein Modell des Rissausbreitungsmechanismus\' erstellt sowie die bisher in der Literatur vorherrschende These des Einfachgleitens widerlegt. Schließlich konnte die Barrierenwirkung von Korn- und Phasengrenzen sowohl anhand der Messung der Rissausbreitungsrate als auch der plastischen Rissöffnung bzw. -scherung gezeigt und daraus Rückschlüsse auf die Ausdehnung der plastischen Zone vor der Rissspitze gezogen werden. Gleichzeitig wurde dabei die Korrelation zwischen Rissausbreitungsrate und plastischer Rissöffnung bzw. -scherung nachgewiesen.

In der Dissertation werden Konzepte vorgestellt, mit denen die Abschätzung der Dauerfestigkeit von Bauteilen auf Grundlage von Werkstoffkennwerten möglich ist. Aus den sich dabei ergebenden Problemen sowie den speziellen Anforderungen der Werkstoffgruppe Sinterstahl wird ein neues Konzept aus bruchmechanischen Überlegungen abgeleitet. Damit ist eine Anwendung ohne prinzipielle Einschränkung bezüglich der Bauteilgeometrie und unter Berücksichtigung der individuellen Kerbempfi ndlichkeit des Werkstoffs möglich. Die Validierung und statistische Auswertung anhand einer breiten Datenbasis belegt eine gute Treffsicherheit im Vergleich zu anderen Verfahren. Empfehlungen zur praktischen Anwendung und den Grenzen der verschiedenen Konzepte werden gegeben.