This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life. The work that is reported in this doctoral thesis is part results of the Nordic R&D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured. Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading. A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature. Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses. / QC 20100706

The aim of this thesis is to investigate how material surfaces are affected by various surface treatments and how this relates to the adhesion of the coating. The materials that were studied were WC-Co and Cermets and the surface treatments used were polishing, grinding with coarser and finer abrasive grains, and finally wet blasting and dry blasting. Focus was on deformations and residual stresses in the surface, surface roughness and cracks. The test methods used for examining the samples included surface roughness measurements, residual stress measurements, adhesion tests using Rockwell indentation and SEM images of the surface and the cross section.<br /><br />The results concluded that polishing gives very good adhesion. Additionally, the adhesion for ground surfaces was good for WC-Co but very poor for Cermets. Furthermore, it was observed that finer abrasive grains did not result in better adhesion. In fact, the coarser grains gave slightly better results. Finally, it was concluded that wet blasting has a clear advantage over dry blasting and results in much better adhesion, especially for the Cermets. The results for the WC-Co were a bit inconsistent and so further research is required.

Die vorliegende Dissertation befasst sich mit der ramanspektroskopischen Charakterisierung von Galliumnitrid (GaN). Der Zusammenhang zwischen Waferkrümmung und mechanischer Restspannungen wird diskutiert. Mit Hilfe konfokaler Mikro-Ramanmessungen wurden Dotierprofile nachgewiesen sowie die Ladungsträgerkonzentration und -beweglichkeit ermittelt. Sämtliche Ramantensorelemente von wz-GaN wurden erstmals durch die Anwendung verschiedener Streugeometrien bestimmt. Eine neu entwickelte Vorwärtsstreuanordnung ermöglichte die Beobachtung von Phonon-Polaritonen. Es konnte gezeigt werden, dass von der theoretischen und experimentellen Betrachtung der Ramanstreuintensitäten dieser Elementaranregungen eindeutig das Vorzeichen der Faust-Henry-Koeffizienten von wz-GaN abgeleitet werden kann. Im Rahmen dieser Arbeit wurden alle Faust-Henry-Koeffizienten für GaN experimentell bestimmt. / This thesis focuses on special aspects of the Raman spectroscopical characterization of wurtzite gallium nitride (wz-GaN). The correlation between wafer curvature and residual stress is discussed. By means of confocal micro-Raman measurements doping profiles were detected as well as the density and mobility of free charge carriers were deduced. All Raman scattering cross sections of wz-GaN were determined the first time using different scattering configurations. A novel method for near-forward scattering was developed in order to observe phonon polaritons with pure symmetry. It is shown that the theoretical and experimental consideration of the Raman scattering efficiency of these elementary excitations allow for determining the sign of the Faust-Henry coefficients of wz-GaN unambiguously. The Faust-Henry coefficients of GaN were deduced from Raman scattering efficiencies of corresponding TO and LO phonons.

In this study, microstructural, micro hardness evaluation and residual stress distribution, of low carbon steel after single pass gas metal arc welding technique (GMAW) were investigated. The goal of this investigation were to reveal the microstructures, micro hardness, residual stress distribution and tensile strength of welded joints by using welding current as varying parameter. In order to realize this objective, welded plate of low carbon steel with thickness of 2.5mm and 250mm long by 200mm wide were welded together with different welding current of 70A, 100A through MIG process. Four different parts of samples were tried. The first part of samples was annealed in a furnace to a temperature of 750 and allowed to cool in air before welding. While the second was not heated, third and fourth trials were heated to a temperature of 200˚C and 580˚C respectively after the welding and allowed cooling in air. Nikon optic microscope and CSM micro hardness testing instrument were used to determine the microstructure of the weld bead and HAZ of the samples, and hardness tests were carried out at different zones of welded joint. Micro hardness behavior was observed by using CSM instrument using 20N load for indentation, which reveals the correlation between hardness, elasticity and plasticity on the heat affected zone (HAZ) and weld bead of the welded plates. The samples were subjected to tensile strength and the distribution of residual stress of the weld. Tensile strength test... [to full text] / Šiame darbe aprašomi mikrostruktūros, mikrokietumo nustatymo bei liekamųjų įtempimų pasiskirstymo tyrimai mažo anglingumo plienuose po lankinio suvirinimo. Tyrimo tiksla - nustatyti suvirintos jungties mikrostuktūros ir mikrokietųmo priklausomybę nuo suvirinimo srovės, kaip kintamo dydžio. Remiantis užduotimi buvo suvirintos 2.5 mm storio, 250 mm ilgio ir 200 mm pločio mažaanglio plieno plokštės, naudojant skirting dydžio (70A ir 100A) suvirininant apsauginių dujų aplinkoje (MIG) srovę. Buvo paruoštos keturios partijos bandinių. Pirmoji partija, proies suvirinant, buvo atkaitinta 750°C temperatųroje, ir ataušintas ore. Tuo tarpu antroji partija nebuvo apdorojama, o trečioji ir ketvirtoji atleista 200 C° ir 580°C temperatūroje ir atvėsinta ore. Suvirinimo vonelės ir erminio poveikio zonos mikrostruktūrai tirti bei mikrokietumui nustatyti buvo naudotas Nicon optinis mikroskopas bei CSM mikrokietumo bandymų įrenginys. Matuojant CSM įrenginiu, 20N įspaudimo apkrova, buvo pastebėtas įdomus kietuvo būvis, kuris atskleidė ryšį tarp kietumo, elastingumo ir plastiškumo esantį suvirintų plokštelių terminio poveikio zonoje bei suvirinimo vonelėje. Buvo matuojamas suvirintų bandinių stiprumas tempiant ir liekamųjų įtempimų pasiskirstymas. Tempimo stiprumo bandymų duomenys parodė ducharakteringus suirimo atvejus – plastiio-elastinio bei trapaus tipo. Liekamųjų įtempimų bandymų rezultati neparodė rėikšmingos liekamųjų įtempimų įtakos pasirinktomis sąlygomis suvirintuose bandiniuose.

No description available.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Nowadays, welded structures are widely employed in engineering design. Generally, these structures have internal stresses (residual stresses) generated by the thermal cycle to which these parts are submitted during welding process. In many cases, these stresses are not minimized by means of thermal treatment. Thus, one must take into account the residual stresses in the design of welded components. However, computation of residual stress field is not an easy task and, besides, it is important to evaluate the quality of welded components. Therefore, by using the advantage of the fact that the stress state influences the mechanical behavior of components and structures (stress stiffening effect), it was proposed to evaluate the viability of a methodology to control the quality of welded components, by means of dynamic responses. This way, it was verified experimentally the sensitivity of vibrating natural frequencies to welding residual stress in different structures. It was confirmed that thin structures are more sensitive to the stress stiffening effect. To evaluate this effect numerically, a procedure to simulate welding was validated by using experimental data from literature. This procedure was used to obtain the residual stress field. This stress field is included in a modal analysis to verify the modifications of natural frequencies. Numerical results were compared to experimental results obtained here, showing a good agreement. It was also proposed to use the electromechanic impedance technique to evaluate the stress stiffening effect. An analysis of the results shows the possibility of use the stress stiffening effect to implement a methodology for quality control of welded components. / Atualmente, estruturas soldadas encontram-se amplamente aplicadas em projetos de engenharia. Normalmente, estas estruturas apresentam tensões internas (residuais) devidas ao ciclo térmico ao qual são submetidas durante a soldagem. Em muitos casos, estas tensões não são minimizadas por meio de tratamento térmico, havendo assim a necessidade de inclusão destas em projeto. Entretanto, a determinação do campo de tensões residuais não é tarefa fácil e, no caso de uma produção em série, é importante avaliar a qualidade do componente confeccionado. Sendo assim, aproveitando o bem conhecido fato de o comportamento mecânico de componentes e estruturas sofrer influência do estado de tensão (enrijecimento por tensão), propõe-se avaliar a viabilidade de uma metodologia de controle de qualidade de peças soldadas por meio de respostas dinâmicas. Desta forma foi verificada experimentalmente a sensibilidade das freqüências naturais de vibração ao estado de tensões residuais de soldagem em diferentes estruturas. Confirmou-se que estruturas esbeltas são mais sensíveis ao enrijecimento por tensões residuais de soldagem. Com a finalidade de avaliar este efeito numericamente, um procedimento para simulação da soldagem foi validado utilizando-se dados experimentais da literatura. Na seqüência, utilizou-se tal procedimento para, após a determinação das tensões residuais de soldagem, realizar uma análise modal e avaliar as alterações nas freqüências naturais de vibração (e conseqüentemente o efeito do enrijecimento por tensão). Os resultados numéricos foram confrontados com os resultados experimentais obtidos neste trabalho, apresentando uma boa concordância. Finalizando o trabalho, propõe-se utilizar a técnica da impedância eletromecânica para avaliar o enrijecimento por tensão. Uma avaliação dos resultados mostra ser possível utilizar o enrijecimento por tensão para implementar uma metodologia para controle de qualidade de componentes soldados. / Doutor em Engenharia Mecânica

Reduzir custos com aumento na qualidade do produto, esses conceitos apesar de parecerem conflitantes são a base para o desenvolvimento industrial atualmente. Os conceitos desenvolvidos através das teorias da “engenharia de distorção”, que leva em consideração o processo de fabricação como um todo e que cada etapa tem alguma influencia no produto final, aparecem como uma possível solução para parte destes problemas. Neste ponto o projeto de pesquisa “Investigação e aperfeiçoamento de um processo de manufatura desde a trefilação a frio até o endurecimento por indução” (“Investigation and improvement of a manufacturing process chain from cold drawing processes to induction hardening”), fruto da colaboração internacional Brasil-Alemanha no projeto BRAGECRIM, tem como objetivo identificar os efeitos de diferentes etapas de fabricação na distorção de barras de aço ABNT 1045. O presente trabalho se insere dentro desta rota de fabricação ao analisar os efeitos de dois tipos diferentes de corte, cisalhante e abrasivo, com relação aos portadores de potenciais de distorção. Os resultados indicam que as influencias, dependendo do processo de corte, vão muito além do que a deformação de grão visível através de metalografias. Apresentando efeitos significativos, em termos de tensões residuais, até uma distância de aproximadamente 25 mm em relação à superfície cortada. / Reduce costs with increased product quality, although these concepts appear to be in conflict, currently they are the basis for industrial development. The concepts developed through theories of "Distortion Engineering", which takes into account the manufacturing process as a whole and that each step has some influence on the final product, appears as a probable solution to this problem. At this point the research project "Investigation and improvement of the manufacturing process chain from cold drawing processes to induction hardening", the result of an international collaboration between Brazil and Germany called BRAGECRIM, aims to identify the effects of different manufacturing steps on the distortion of ABNT 1045 steel bars. The present work fits within the analyzed manufacturing route aiming to characterize the effects of two different types of cutting, shearing and abrasive, with respect to carriers of potential distortion. The results indicate that the cutting method influences, depending on the cutting process, goes beyond the grain deformation, visible through metallographic pictures, and a possible distortion. Presenting significant effects, in terms of residual stresses, at a distance of approximately 25 mm from the cut surface.

Reduzir custos com aumento na qualidade do produto, esses conceitos apesar de parecerem conflitantes são a base para o desenvolvimento industrial atualmente. Os conceitos desenvolvidos através das teorias da “engenharia de distorção”, que leva em consideração o processo de fabricação como um todo e que cada etapa tem alguma influencia no produto final, aparecem como uma possível solução para parte destes problemas. Neste ponto o projeto de pesquisa “Investigação e aperfeiçoamento de um processo de manufatura desde a trefilação a frio até o endurecimento por indução” (“Investigation and improvement of a manufacturing process chain from cold drawing processes to induction hardening”), fruto da colaboração internacional Brasil-Alemanha no projeto BRAGECRIM, tem como objetivo identificar os efeitos de diferentes etapas de fabricação na distorção de barras de aço ABNT 1045. O presente trabalho se insere dentro desta rota de fabricação ao analisar os efeitos de dois tipos diferentes de corte, cisalhante e abrasivo, com relação aos portadores de potenciais de distorção. Os resultados indicam que as influencias, dependendo do processo de corte, vão muito além do que a deformação de grão visível através de metalografias. Apresentando efeitos significativos, em termos de tensões residuais, até uma distância de aproximadamente 25 mm em relação à superfície cortada. / Reduce costs with increased product quality, although these concepts appear to be in conflict, currently they are the basis for industrial development. The concepts developed through theories of "Distortion Engineering", which takes into account the manufacturing process as a whole and that each step has some influence on the final product, appears as a probable solution to this problem. At this point the research project "Investigation and improvement of the manufacturing process chain from cold drawing processes to induction hardening", the result of an international collaboration between Brazil and Germany called BRAGECRIM, aims to identify the effects of different manufacturing steps on the distortion of ABNT 1045 steel bars. The present work fits within the analyzed manufacturing route aiming to characterize the effects of two different types of cutting, shearing and abrasive, with respect to carriers of potential distortion. The results indicate that the cutting method influences, depending on the cutting process, goes beyond the grain deformation, visible through metallographic pictures, and a possible distortion. Presenting significant effects, in terms of residual stresses, at a distance of approximately 25 mm from the cut surface.

O comportamento do aço API 5L X65 foi estudado em solução sintética NS4. Estudou-se o efeito da hidrogenação ao aplicar um potencial catódico nas amostras de tração e de impacto Charpy. Estas amostras foram imersas em solução saturada de sulfato de cobre para evitar a saída de hidrogênio, sendo posteriormente foram testadas à tração e ao impacto. Algumas amostras foram tratadas superficialmente por shot peening antes de serem hidrogenadas. Após todos esses tratamentos foi medida a tensão residual nas amostras por tensometria de raios X. Adicionalmente, foram levantadas curvas de polarização anódica e catódica do aço na solução NS4 desaerada com nitrogênio gasoso e simulado o equilíbrio químico desta solução. As superfícies de fratura das amostras foram analisadas por microscopia eletrônica de varredura. / The behavior of API 5L X-65 steel immersed in synthetic solution NS4 was studied. The changes caused on the effect of the hydrogen when applying a cathodic electrical potential on tensile and impact samples were also studied. The samples were immersed into saturate solution of copper sulfate to avoid the hydrogen desorption and after that, were performed tensile and impact tests. Some samples were submitted to shot peening process before suffering the hydrogen action. After these treatments the residual stress was determined by X ray tensometry. Additionally, cathodic and anodic polarization curves were obtained in NS4 solution deaerated with nitrogen gas. Moreover, the chemical balance of NS4 solution was performed. The fracture surfaces of the samples were analyzed using scanning electron microscopy.

Made available in DSpace on 2016-12-08T17:19:14Z (GMT). No. of bitstreams: 1 Introducao.pdf: 448655 bytes, checksum: 598f8294a6f16ff80e9151c41d1a27f9 (MD5) Previous issue date: 2011-12-15 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Residual stresses in polymeric parts are a great source of uncertainties for the design of industrial components. They are originated mainly during the injection molding process. Some of the problems related to this phenomenon are reduction to the mechanic strength, cracks propagation and warpage. Many efforts are employed to minimize these residual stresses and, to achieve that, an essential step is its measuring. There are methods and devices already available to measure residual stress in metallic materials, but in the field of polymeric material, many aspects remain unexplored. In the present work, a device to measure strain gage signals was built, which was employed to determine residual stresses according to the most established techniques nowadays. Injection simulation techniques were utilized to estimate the build-up of residual stresses in polymeric test specimens under different manufacturing setups. Statistical analysis was performed to select the most favorable parameters for parts tensioning. Test specimens were fabricated in the selected process conditions. Samples weight measurements confirmed statistically that the process parameters selected have significantly altered the parts final mass. Residual stress measurements were carried out by employing the hole drilling technique in the manufactured specimens, thus being determined tensile residual stresses from 2.7 MPa by 6.2 MPa, in the layers closest to the parts surface. The typical profile for residual stress distribution, as described in literature, was observed in the experiments performed. / Tensões residuais em peças poliméricas são grande fonte de incertezas para o projeto de componentes industriais. São causadas principalmente durante o processo de injeção. Alguns dos problemas relacionados a esse fenômeno são a diminuição da resistência mecânica, surgimento de trincas e empenamento das peças. Muitos esforços são empregados na minimização dessas tensões residuais e, para isso, uma etapa indispensável é a sua medição. Existem métodos e equipamentos já disponíveis para medição de tensões residuais em materiais metálicos, porém, no campo dos materiais poliméricos ainda há muitos aspectos a serem melhor explorados. Neste trabalho foi construído um dispositivo para medição de sinais de extensiometria, o qual foi utilizado para determinação de tensões residuais segundo as técnicas mais utilizadas atualmente para metais. Técnicas de simulação de injeção foram utilizadas para estimar o surgimento de tensões residuais em corpos-de-prova poliméricos sob diversas condições de fabricação. Análise estatística foi realizada para selecionar as condições de fabricação mais propícias ao tensionamento das peças. Corpos-de-prova foram fabricados nas condições de processo selecionadas. Medições de peso das amostras confirmaram estatisticamente que os parâmetros de processo utilizados alteraram significativamente a massa final das peças. Medições de tensões residuais foram realizadas aplicando-se a técnica do furo cego nas amostras fabricadas, determinando-se tensões residuais de tração entre 2,7 MPa e 6,2 MPa, nas camadas próximas às superfície das amostras. O perfil típico de tensões residuais previsto em literatura foi observado nos experimentos realizados.