A liga de alumínio 6082 apresenta de média para alta resistência mecânica e é de grande importância no mercado de perfis extrudados. O processo de jateamento com granalha de aço em componentes automotivos e aeronáuticos, visando induzir uma condição otimizada de tensão residual de compressão na superfície, tem se mostrado eficaz na melhoria das propriedades mecânicas em fadiga desses componentes. Neste sentido, este trabalho tem como objetivo estudar os efeitos do jateamento com granalhas de aço na tensão residual e no comportamento mecânico em fadiga da liga de alumínio 6082 solubilizada e envelhecida artificialmente. Inicialmente foram feitos tratamentos térmicos de solubilização e envelhecimento artificial visando a determinação da melhor condição em termos de resistência mecânica da liga de alumínio 6082. A seguir foram realizados experimentos com diversos tipos de granalhas esféricas de aço e em condições variadas de jateamento (velocidade de rotação e tempo), definindo-se a melhor condição de tensão residual medida por difração de raios x. Ensaios de fadiga em corpos de prova jateados com granalha de aço na melhor condição de tensão residual foram executados em máquina flexo-rotativa e comparados com os resultados do material sem jateamento. A melhor condição de tensão residual ocorreu em decorrência do jateamento com o uso da granalha de aço S170PEE, rotação da turbina à 2500 rpm e tempo de exposição de 1 minuto. A curva de tensão residual em profundidade para a condição escolhida apresentou resultados coerentes somente até 76 μm devido à presença de grãos grandes em profundidades superiores a esta. As curvas S-N mostraram que houve um ganho na vida em fadiga em tensões elevadas e baixo número de ciclos para romper devido jateamento com granalha de aço. Ao contrário, ocorreu um efeito deletério na vida em fadiga em tensões baixas e número de ciclos elevados para romper, devido à influência do aumento da rugosidade em ensaios de fadiga mais longos. / Aluminum alloy 6082 exhibits medium to high strength and is widely used as extruded products. Shot peening of automobile and aircraft components, carried out to induce optimum residual compressive stress on the surface, improves fatigue properties of these components. In this context, the aim of this investigation was to study the effects of shot peening on the residual stress and fatigue behavior of solution treated and artificially aged 6082 aluminum alloy. Initially, solution treatment and artificial ageing of this alloy were carried out to determine the best conditions in terms of alloy strength. Subsequently, experiments were carried out with different types of spherical steel shots, under a variety of peening conditions (rotation speed and duration), to define the best set of conditions to increase residual stress that was measured using x-ray diffraction. Fatigue tests were carried out using a rotating-bending machine on shot peened specimens that had the highest residual stress. These results were compared with those of specimens that were not shot peened. The highest residual stress was observed when peening was done with S170PEE steel shots, at 2500 rpm for 1 minute. The variation in residual stress as a function of depth, for the above-mentioned peening conditions, was coherent only up to a depth of 76 μm and attributable to the presence of large grains at depths higher than this. The S-N curves revealed an increase in fatigue life at high stresses and at low cycles to failure and attributed to shot peening. Contrary to this, a deleterious effect in terms of reduction in fatigue life was observed at low stresses and at high cycles to failure, due to increase in surface roughness of the specimens during the extended fatigue tests.

O jateamento com granalha (shot peening) é um processo mecânico a frio onde um jato de granalhas é impelido contra a superfície dos componentes. Cada impacto provoca deformação plástica e introduz tensões residuais de compressão na superfície, as quais aumentam a sua resistência à fadiga. Este tratamento é muito usado na indústria automotiva, particularmente no tratamento de molas automotivas devido à alta solicitação a carregamentos cíclicos. Uma variante aprimorada deste processo, exclusivo para molas automotivas planas, é o jateamento com granalha sob tensão (stress peening) onde é imposta uma pré-carga de flexão na mola que aumenta a intensidade do processo. Neste trabalho foram desenvolvidas as modelagens numéricas, usando o Método dos Elementos Finitos (MEF), do jateamento com granalha e do jateamento com granalha sob tensão, ambos aplicados a molas automotivas, para analisar o campo de tensões residuais induzido. Os modelos numéricos desenvolvidos contemplam: análise dinâmica explícita, modelagem tridimensional de múltiplos impactos de granalha numa superfície plana, avaliação da velocidade real das granalhas, atrito entre as superfícies de contato, propriedades mecânicas do aço ABNT 5160 (molas automotivas), encruamento do material e sensibilidade do material às altas taxas de deformação. A partir dos resultados da modelagem foram avaliados: o progresso da cobertura ao longo do tempo da aplicação dos múltiplos impactos de granalha, as tensões residuais, as deformações elásticas e as deformações plásticas resultantes induzidas pelo jateamento. Neste trabalho, foi realizado um programa experimental para introdução e avaliação das tensões residuais em duas molas parabólicas automotivas, uma delas submetida ao jateamento com granalha e a outra submetida ao jateamento com granalha sob tensão. A avaliação experimental das tensões residuais foi desenvolvida usando as técnicas de difração de raios-X e do furo incremental cego. Os resultados das modelagens numéricas são corroborados com as medições experimentais e com os resultados experimentais e numérico-computacionais obtidos por outros autores. Finalmente, algumas conclusões são inferidas diante da análise comparativa entre os resultados numéricos e experimentais. / The shot peening is a cold-working mechanical process where a stream of tiny small balls is impelled against the surface of components. Each impact causes plastic deformation and introduces compressive residual stresses on the surface, which consequently increases their resistance to fatigue. This process is widely used in the automotive industry, particularly in the treatment of automotive springs due to high cyclic loads required. An improved variant of the shot peening process for leaf springs is the stress peening, in which a flexion pre-load is imposed to bend the spring while a conventional shot peening process is applied, resulting in an increase in the intensity of the process. In this work, numerical models of the shot peening and the stress peening process were created to be applied to automotive leaf springs. To analyze the induced residual stress field the finite elements method was used. Numerical models include: dynamic explicit formulation, three-dimensional modeling of multiple impacts of balls on a plane target, the calculation of the real shot speed, friction between the contact areas, mechanical properties of ABNT 5160 steel (for automotive leaf springs), plastic work of the material and high strain rate sensitivity of the material. The indentation coverage progress over the analyzed area, the residual stresses, the remaining elastic and plastic strains induced by the shot peening were evaluated from the modeling. In this work an experimental programme was carried out to introduce and to evaluate residual stresses in one automotive leaf spring submitted to conventional shot peening and another submitted to stress peening. The experimental evaluation of the residual stresses was done using X-ray diffraction and incremental hole drilling techniques. Results for the numerical model are compared to the experimental measurements and the experimental measurement, as well as to numerical modeling results obtained by other authors. Finally, conclusions are drawn after comparing the numerical results to experimental ones.

<div>Magnesium alloys are regarded as the most promising structure materials in transportation and aerospace fields because of their low densities and high specific strengths. However, the unsatisfactory mechanical performance and corrosion resistance restrict their applications. Grain refinement is an effective way to improve the mechanical properties and widen the applications. Among which, ultrasonic shot peening shows a great potential in producing refined grains or even nanocrystalline. A nanocrystalline forms at the surface after ultrasonic shot peening treatment. The formed nanocrystalline has been proved to dramatically affect the mechanical properties, such as hardness, mechanical stress, wear resistance and fatigue life. </div><div><br></div><div> </div><div>In this dissertation, the microstructure evolution of AZ31 Mg alloy after the ultrasonic shot peening as well as its effect on the mechanical properties are investigated. The grain size, the twin structure, the surface roughness and the residual stress distribution after ultrasonic shot peening are characterized. A gradient nanostructure is achieved through ultrasonic shot peening and the thickness of this gradient nanostructure increases with prolonging the treated time. The grains at the top surface after 5 min treatment is refined to 45 nm and further refined to 42 nm for 10 min and 37nm for 15 min treatment from the XRD result. A lamellar nanocrystaline is below the top surface and a lot of tensile twins are found at the heavily deformed grains below the nanocrystalline layer. Below the twinned layer, a residual stress is distributed as deep as 400 $\mu$m in the matrix. A rough surface is obtained and the surface roughness of the 5 min treated sample was 5.934 $\mu$m, increased to 6.161 $\mu$m for10 min and 6.236 $\mu$m for 15 min. The nanocrystalline leads to the improvement of the microhardness, from 65 HV of the as-received to 123 HV, 127 HV and 145 HV for 5 min, 10 min and 15 min treatment, respectively. The tensile stress and compression stress are also improved remarkably. The yield stress is increased from 127.7 MPa of as-received to 198 MPa of 10 min treated sample and the compression stress is improved from 73 MPa to 100 MPa. The improved yield stress is attributed to the grain refinement and the work hardening of the nanocrystalline. </div><div><br></div><div> </div><div> </div><div>The wear resistance of AZ31 Mg alloy is improved greatly after ultrasonic shot peening process. The coefficient of friction and the wear rate of the ultrasonic shot peening treated sample are both lower than that of as-received. The width of the wear track of ultrasonic shot peening treated is also narrower than that of as-received, and the worn surface has a lower surface roughness. In as-received samples, abrasion and oxidation dominate the wear mechanism at low sliding speed and low applied load. The increase of sliding speed or applied load resulted in the delamination. Severe wear such as thermal softening happens with the further increase of load value or sliding speed. In ultrasonic shot peening treated samples, oxidation, the abrasion and delamination are also existing while no severe wear is found. The improved wear resistance of the ultrasonic shot peened sample is due to the improved hardness and a higher activity of oxidation during wear process. The nanocrystalline on the top surface leads to the the transition boundary between the mild wear and severe wear to a higher sliding speed and higher applied load. </div><div><br></div><div><br></div><div>The corrosion resistance of AZ31 Mg alloy before and after ultrasonic shot peening is tested in 3.5 $\%$ NaCl solution. The corrosion resistance after ultrasonic shot peening is reduced greatly because of the Fe particles at the top surface, which was exfoliated from the shot during the treating process. After a 40 $\mu$m thick polishing, Fe particles are removed totally and the corrosion resistance is improved, compared with that of as-received. The anodic current density of the nanocrystallized surface after polishing is reduced because of the compression residual stress and a rapid formation of protective layer. Meanwhile, the grain boundary acts as a physical barrier for corrosion and reduces the corrosion rate. </div><div><br></div><div><br></div><div>The bending behavior of AZ31 Mg alloy before and after ultrasonic shot peening is studied by a V-bending test. The ultrasonic shot peening treated sample has a similar bending performance even though the ductility has been reduced after ultrasonic shot peening. A single side ultrasonic shot peening (either at the inner side or the outer side) changed the bending behavior because of its asymmetric structure. The ultrasonic shot peening at the inner side for 5 min improves the bendability and longer treated, such as 10 min and 15 min degenerates the bendability to as-received. The improved bendability of 5 min treated sample is due to the drawing back of the neutral layer. The ultrasonic shot peening at the outer side for 5 min also improves the bending performance and a longer treatment of 15 min further enhanced the bendability. The improved bendability after outer side treatment is due to the high yield stress of nanocrystalline at the convex, resulting in the smaller strain here. </div>

Dissimilar metal joining was performed with the main goal being maximization of the strength of the joined samples, but because of some potential applications of the dissimilar joints, analyzing their corrosion behavior also becomes crucial. Starting with materials that initially have suitable corrosion resistance, ensuring that the laser processing does not diminish this property is necessary. Conversely, the laser shock peening processing was implemented with a complete focus on improving the corrosion behavior of the workpiece. Thus, many commonalities occur between these two manufacturing processes, and this thesis goes on to analyze the thermal and mechanical influence of laser processing on materials’ corrosion resistances. Brittle intermetallic phases can form at the interfaces of dissimilar metal joints. A process called autogenous laser brazing has been explored as a method to minimize the brittle intermetallic formation and therefore increase the fracture strength of joints. In particular, joining of nickel titanium to stainless steel wires is performed with a cup/cone interfacial geometry. This geometry provides beneficial mechanical effects at the interface to increase the fracture strength and also enables high-speed rotation of the wires during irradiation, providing temperature uniformity throughout the depth of the wires. Energy dispersive X-ray spectroscopy, tensile testing, and a numerical thermal modelling are used for the analysis. The material pair of nickel titanium and stainless steel have many applications in implantable medical devices, owing to nickel titanium’s special properties of shape memory and superelasticity. In order for an implantable medical device to be used in the body, it must be ensured that upon exposure to body fluid it does not corrode in harmful ways. The effect that laser autogenous brazing has on the biocompatibility of dissimilar joined nickel titanium to stainless steel samples is thus explored. While initially both of these materials are considered to be biocompatible on their own, the laser treatment may change much of the behavior. Thermally induced changes in the oxide layers, grain refinement, and galvanic effects all influence the biocompatibility. Nickel release rate, polarization, hemolysis, and cytotoxicity tests are used to help quantify the changes and ascertain the biocompatibility of the joints. To directly exert a beneficial influence on materials’ corrosion properties laser shock peening (LSP) is performed, with a particular focus on the stress corrosion cracking (SCC) behavior. Resulting from the combination of an applied load on a susceptible material exposed to a corrosive environment, SCC can cause sudden material failure. Stainless steel, high strength steel, and brass are subjected to LSP and their differing corrosion responses are determined via cathodic charging, hardness, mechanical U-bend, Kelvin Probe Force Microscopy, and SEM imaging. A description accounting for the differing behavior of each material is provided as well as considerations for improving the effectiveness of the process. SCC can occur by several different physical processes, and to fully encapsulate the ways in which LSP provides mitigation, the interaction of microstructure changes induced by LSP on SCC mechanisms is determined. Hydrogen absorbed from the corrosive environment can cause phase changes to the material. Cathodic charging and subsequent X-ray diffraction is used to analyze the phase change of sample with and without LSP processing. Lattice dislocations play an important role, and transmission electron microscopy helps to aid in the analysis. A finite element model providing spatially resolved dislocation densities from LSP processing is performed.

A liga de alumínio 6082 apresenta de média para alta resistência mecânica e é de grande importância no mercado de perfis extrudados. O processo de jateamento com granalha de aço em componentes automotivos e aeronáuticos, visando induzir uma condição otimizada de tensão residual de compressão na superfície, tem se mostrado eficaz na melhoria das propriedades mecânicas em fadiga desses componentes. Neste sentido, este trabalho tem como objetivo estudar os efeitos do jateamento com granalhas de aço na tensão residual e no comportamento mecânico em fadiga da liga de alumínio 6082 solubilizada e envelhecida artificialmente. Inicialmente foram feitos tratamentos térmicos de solubilização e envelhecimento artificial visando a determinação da melhor condição em termos de resistência mecânica da liga de alumínio 6082. A seguir foram realizados experimentos com diversos tipos de granalhas esféricas de aço e em condições variadas de jateamento (velocidade de rotação e tempo), definindo-se a melhor condição de tensão residual medida por difração de raios x. Ensaios de fadiga em corpos de prova jateados com granalha de aço na melhor condição de tensão residual foram executados em máquina flexo-rotativa e comparados com os resultados do material sem jateamento. A melhor condição de tensão residual ocorreu em decorrência do jateamento com o uso da granalha de aço S170PEE, rotação da turbina à 2500 rpm e tempo de exposição de 1 minuto. A curva de tensão residual em profundidade para a condição escolhida apresentou resultados coerentes somente até 76 &mu;m devido à presença de grãos grandes em profundidades superiores a esta. As curvas S-N mostraram que houve um ganho na vida em fadiga em tensões elevadas e baixo número de ciclos para romper devido jateamento com granalha de aço. Ao contrário, ocorreu um efeito deletério na vida em fadiga em tensões baixas e número de ciclos elevados para romper, devido à influência do aumento da rugosidade em ensaios de fadiga mais longos. / Aluminum alloy 6082 exhibits medium to high strength and is widely used as extruded products. Shot peening of automobile and aircraft components, carried out to induce optimum residual compressive stress on the surface, improves fatigue properties of these components. In this context, the aim of this investigation was to study the effects of shot peening on the residual stress and fatigue behavior of solution treated and artificially aged 6082 aluminum alloy. Initially, solution treatment and artificial ageing of this alloy were carried out to determine the best conditions in terms of alloy strength. Subsequently, experiments were carried out with different types of spherical steel shots, under a variety of peening conditions (rotation speed and duration), to define the best set of conditions to increase residual stress that was measured using x-ray diffraction. Fatigue tests were carried out using a rotating-bending machine on shot peened specimens that had the highest residual stress. These results were compared with those of specimens that were not shot peened. The highest residual stress was observed when peening was done with S170PEE steel shots, at 2500 rpm for 1 minute. The variation in residual stress as a function of depth, for the above-mentioned peening conditions, was coherent only up to a depth of 76 &mu;m and attributable to the presence of large grains at depths higher than this. The S-N curves revealed an increase in fatigue life at high stresses and at low cycles to failure and attributed to shot peening. Contrary to this, a deleterious effect in terms of reduction in fatigue life was observed at low stresses and at high cycles to failure, due to increase in surface roughness of the specimens during the extended fatigue tests.

O jateamento com granalha (shot peening) é um processo mecânico a frio onde um jato de granalhas é impelido contra a superfície dos componentes. Cada impacto provoca deformação plástica e introduz tensões residuais de compressão na superfície, as quais aumentam a sua resistência à fadiga. Este tratamento é muito usado na indústria automotiva, particularmente no tratamento de molas automotivas devido à alta solicitação a carregamentos cíclicos. Uma variante aprimorada deste processo, exclusivo para molas automotivas planas, é o jateamento com granalha sob tensão (stress peening) onde é imposta uma pré-carga de flexão na mola que aumenta a intensidade do processo. Neste trabalho foram desenvolvidas as modelagens numéricas, usando o Método dos Elementos Finitos (MEF), do jateamento com granalha e do jateamento com granalha sob tensão, ambos aplicados a molas automotivas, para analisar o campo de tensões residuais induzido. Os modelos numéricos desenvolvidos contemplam: análise dinâmica explícita, modelagem tridimensional de múltiplos impactos de granalha numa superfície plana, avaliação da velocidade real das granalhas, atrito entre as superfícies de contato, propriedades mecânicas do aço ABNT 5160 (molas automotivas), encruamento do material e sensibilidade do material às altas taxas de deformação. A partir dos resultados da modelagem foram avaliados: o progresso da cobertura ao longo do tempo da aplicação dos múltiplos impactos de granalha, as tensões residuais, as deformações elásticas e as deformações plásticas resultantes induzidas pelo jateamento. Neste trabalho, foi realizado um programa experimental para introdução e avaliação das tensões residuais em duas molas parabólicas automotivas, uma delas submetida ao jateamento com granalha e a outra submetida ao jateamento com granalha sob tensão. A avaliação experimental das tensões residuais foi desenvolvida usando as técnicas de difração de raios-X e do furo incremental cego. Os resultados das modelagens numéricas são corroborados com as medições experimentais e com os resultados experimentais e numérico-computacionais obtidos por outros autores. Finalmente, algumas conclusões são inferidas diante da análise comparativa entre os resultados numéricos e experimentais. / The shot peening is a cold-working mechanical process where a stream of tiny small balls is impelled against the surface of components. Each impact causes plastic deformation and introduces compressive residual stresses on the surface, which consequently increases their resistance to fatigue. This process is widely used in the automotive industry, particularly in the treatment of automotive springs due to high cyclic loads required. An improved variant of the shot peening process for leaf springs is the stress peening, in which a flexion pre-load is imposed to bend the spring while a conventional shot peening process is applied, resulting in an increase in the intensity of the process. In this work, numerical models of the shot peening and the stress peening process were created to be applied to automotive leaf springs. To analyze the induced residual stress field the finite elements method was used. Numerical models include: dynamic explicit formulation, three-dimensional modeling of multiple impacts of balls on a plane target, the calculation of the real shot speed, friction between the contact areas, mechanical properties of ABNT 5160 steel (for automotive leaf springs), plastic work of the material and high strain rate sensitivity of the material. The indentation coverage progress over the analyzed area, the residual stresses, the remaining elastic and plastic strains induced by the shot peening were evaluated from the modeling. In this work an experimental programme was carried out to introduce and to evaluate residual stresses in one automotive leaf spring submitted to conventional shot peening and another submitted to stress peening. The experimental evaluation of the residual stresses was done using X-ray diffraction and incremental hole drilling techniques. Results for the numerical model are compared to the experimental measurements and the experimental measurement, as well as to numerical modeling results obtained by other authors. Finally, conclusions are drawn after comparing the numerical results to experimental ones.

Um dos possíveis modos de falha em engrenagens é relacionado com o carregamento cíclico e, por consequência, a ocorrência de fadiga. O início da falha por fadiga ocorre devido à intensificadores de tensão e é geralmente observado no pé do dente (fadiga de flexão) ou na região de contato na superfície do dente (fadiga de contato). Experimentos para avaliação da fadiga de contato em engrenagens são geralmente conduzidos em bancadas de teste e tem custos elevados. Um método alternativo de experimentação de fadiga de contato em engrenagens é avaliado neste trabalho, consistindo no rolamento de esferas sobre um plano (corpo de prova). Dois grupos de corpos de prova foram fabricados com características e propriedades superficiais semelhantes às da superfície do dente de engrenagens nas condições: a) sem shot-peening e b) com shot-peening. As diferentes características dos corpos de prova foram testadas utilizando um equipamento desenvolvido em trabalho anterior para avaliar a fadiga de contato de rolamento, que ocorre no diâmetro primitivo das engrenagens. Resultados indicaram a ocorrência de dois mecanismos de falha comumente encontrados em engrenagens: pitting e spalling, entretanto, o equipamento se mostrou incapaz de monitorar a ocorrência de pitting. Ademais, os corpos de prova com shot-peening apresentaram menor resistência à fadiga para as condições testadas, em desacordo com ensaios de bancada anteriores. Algumas hipóteses foram discutidas para prover justificativas relacionadas aos resultados obtidos. / One of the possible mechanisms of gear failure is related to the cyclic loading and, consequently, the fatigue occurrence. The onset of fatigue failure occurs due to stress intensifiers and it is generally observed on the fillet (bending fatigue) or on the tooth surface contact region (contact fatigue). Gear fatigue evaluation is usually carried out by test rigs, which are expensive. An alternative method of contact fatigue test of gears is evaluated in this work, which consists of balls rolling on a plane (specimen). Two groups of specimens were manufactured to obtain surface characteristics and properties like gears tooth\'s surface: a) without shot-peening and b) with shot-peening. The different features of specimens were tested by using an in house developed equipment to evaluate the rolling contact fatigue, which occurs on gears on its diametral pitch. Results indicate the occurrence of two failure modes: pitting and spalling, however, the equipment showed to be unable to monitor the occurrence of pitting. Also, the shot-peened group specimens presented lower fatigue life resistance for the analyzed conditions, which are not in agreement with previous rig tests. Some hypotheses were discussed to provide justifications related to the obtained results.

[pt] Esta pesquisa teve como objetivo avaliar o efeito do jateamento de partículas (shot-peening) sobre a vida-fadiga do aço estrutural R4, largamente adotado na fabricação de componentes estruturais para sistemas de ancoragem de unidades flutuantes. Inicialmente, amostras cilíndricas do material foram retiradas das seções retas de elos tipo Kenter. Em seqüência, algumas amostras foram temperadas e revenidas em diferentes tempos de tratamento, com o objetivo de se promover uma homogeneidade microestrutural, bem como propriedades mecânicas similares àquelas dos elos tratados industrialmente. As amostras foram submetidas a ensaios de dureza e tração e, com base nos resultados, providenciou-se o tratamento térmico de todas as amostras. Dando continuidade a etapa experimental, corpos de prova para ensaios de fadiga foram usinados das amostras tratadas e submetidos a jateamento com microesferas de aço temperado sob pressões de 20 psi, 30 psi e 40 psi. Finalmente, corpos de prova nas condições sem jateamento e após jateamento foram submetidos a ensaios de fadiga por flexão rotativa, com o objetivo de se levantar a curva tensão versus número de ciclos para a falha (curva S-N) do aço estrutural R4 nas diferentes condições de superfície. A extensão da vida em fadiga do aço estrutural R4 foi relacionada com o jateamento de microesferas. De acordo com os resultados, houve uma influência benéfica do tratamento de superfície sobre a vida-fadiga do material. Tal influência foi caracterizada por um maior número de ciclos para a falha e aumento do limite de resistência à fadiga. Os resultados mostraram que a extensão da vida útil esteve diretamente relacionada com a intensidade do jateamento, significando que uma maior intensidade de jateamento provocou uma maior extensão da vida-fadiga do material. / [en] The objective of the present work was to evaluate the effect of shot peening on the fatigue life of a grade R4 structural steel, largely adopted for fabricating offshore mooring chains. Initially, cylindrical samples of the material were cut of from the straight sections of Kenter links. In the sequence, a number of specimens were austenitized ed and tempered making use of different treatment times, in order to promote a microstructural homogeneity as well as mechanical properties similar to those associated with industrial heat treatments. The specimens were subjected to hardness and tensile tests and on the basis of the results, all samples were heat treated. After that, fatigue specimens were machined from the treated samples and shot peened with quenched steel shots under pressures of 20 psi, 30 psi and 40 psi. Finally, rotating bend fatigue tests were performed with unpeened and shot peened specimens aiming to establish the stress-life curves of the grade R4 structural steel. The fatigue life extension of the grade R4 structural steel was related to the shot peening. Regarding the results, the surface treatments affected the fatigue life of the material beneficially. This influence was characterized by a longer fatigue cycles and a fatigue life improvement. The results showed that the fatigue life extension was directly related to the shot peening intensity, i.e., the fatigue life extension has increased when increasing the shot peening intensity.

High energy pulse laser peening (HEPLP) is a manufacturing process, in which a strong shock wave is produced and induces high pressures on the surface of the target material. Generally, this process is used to improve material properties such as the hardness and fatigue life. First a 2D multi-physics model for the process was investigated, which simulates the pressure induced on the surface of the target material. The model can be coupled with commercial finite element software, such as ABAQUS, to more accurately simulate the HEPLP process to find stresses and deformations on the surface. Next two novel applications using the HEPLP process were investigated. The first, laser shock bending is a sheet metal micro-forming process using HEPLP to accurately bend, shape, precision align, or repair micro-components with bending angles less than 10°. Negative bending angle (away from laser beam) can be achieved with the high-energy pulsed laser, in addition to the conventional positive laser bending mechanism. In this thesis, various experimental and numerical studies on aluminum sheets were conducted to investigate the different deformation mechanisms, positive and negative. The experiments were conducted with the sheet thickness varying from 0.25 to 1.75 mm and laser pulse energy of 0.2 to 0.5 J. A critical thickness threshold of 0.7-0.88 mm was found that the transition of positive negative bending mechanism occurs. A statistic regression analysis was also developed to determine the bending angle as a function of laser process parameters for positive bending cases. The second application studied used HEPLP to imprint complex two-dimensional (2D) patterns dental implant material of cpTi. Pure titanium (commercial pure cpTi) is an ideal dental implant material, without the leeching of toxic alloy elements. Evidence has shown that unsmooth implant surface topologies may contribute to the osteoblast differentiation in human mesenchymal pre-osteoblastic cells, which is helpful to avoid long-term peri-abutment inflammation issues for the dental implant therapy with transcutaneous devices. Studies have been conducted on the grit blasted, acid etched, or uni-directional grooved Ti surface, however, for these existing approaches the surface quality is difficult to control or may even damage the implant. The strong shock wave generated by HEPLP is used to press a stainless steel grid, used as a stamp, on Ti foils to imprint a 2D pattern. In this study, the multiple grid patterns and grid sizes were applied to test for cell-attachment improvements. Then, the cell culture tests were conducted with the patterned surface to investigate the contribution of these 2D patterns, with the control tests of the other existing implant surface topography forming approaches. The micro-patterns proved successful in increasing the cell-attachment, increasing the number of cells attaching to the material and also contributing to the cell-growth within the grooved areas.

Fatigue assessments have been carried out predominantly with quasi-deterministic approaches, such as the use of SN curves. However, both the loading and the resistance of fatigue prone components are subjected to significant uncertainties. Consequently, a prediction of the remaining fatigue life based on deterministic load and resistance models can lead to unreliable results. This work presents a general reliability-based approach to predict fatigue life of steel components. The approach incorporates prediction of fatigue crack initiation, modeled with a strain-based correlation approach, and propagation, modeled using a linear elastic fracture mechanics approach, and is applicable to new, cracked or repaired structural components. Based on the analysis of existing test results and additional crack initiation and propagation tests on weld metal, the relevant probabilistic fatigue material properties of grade 350WT steel and a matching weld metal were established. An experimental program was carried out on welded details tested either in the as-welded, stress-relieved, conventionally peened, or ultrasonically peened condition. It was demonstrated that ultrasonic peening is superior to the other investigated post weld treatment methods. Using finite element analyses, the results of the tests were deterministically predicted for several different initial conditions, including initial flaw and crack sizes and locations, as well as different levels of residual stresses. A model incorporating an initial flaw and accounting for crack closure and the threshold stress intensity factor range was retained. A probabilistic analysis using Monte Carlo Simulation was carried out to calibrate the relevant parameters. A general reliability-based approach, which includes both the loading and resistance sides of the limit state function was proposed and applied to three practical examples: prediction of test results from two test programs and the prediction of the remaining fatigue life of a cracked component as a function of the safety index. These three applications demonstrated that accurate fatigue life predictions targeting a predefined safety index are achieved. / Structural Engineering