Este trabalho investiga a possível correlação existente entre os baixos valores de CTOD e a ocorrência de zonas frágeis localizadas (ZFL) presentes na região de grãos grosseiros (RGG) da zona termicamente afetada (ZTA) de uma junta soldada de aço API 5L X80. Para isto, foi necessário obter corpos-de-prova SE(B) a partir de uma chapa de aço API 5L X80 a qual foi soldada numa junta ½V, para facilitar o posicionamento do entalhe na RGG da ZTA. As dimensões dos corpos-de-prova SE(B) utilizados foram espessura B=17mm, largura W=34mm, distância entre os apoios dos roletes S=138mm e uma relação entre o tamanho de trinca (a) e a largura, a/W=0,5. O entalhe posicionado na RGG da ZTA tem por objetivo evidenciar o efeito de ZFL e para tal os ensaios foram conduzidos nas temperaturas de -40ºC, - 50ºC e -70ºC. Os resultados de CTOD crítico obtidos neste estudo sugerem indícios de severa degradação da tenacidade na RGG da ZTA, provavelmente associada à formação de ZFL. / This work evaluates the possible correlation between low fracture toughness (critical CTOD) and local brittle zones (LBZ) occurrences in heat affected zone (HAZ) coarse grain regions. The fracture toughness of a welded API 5L X80 steel was characterized using SE(B) specimens for CTOD determination with notches located in the HAZ. The specimen dimensions are thickness B=17mm, width W=34mm, span S=138mm and crack length to width ratio, a/W=0,5. The effects of LBZ are evaluated through tests carried out in temperatures of -40ºC, -50ºC and -70ºC. The critical CTOD values obtained in this work had presented low fracture toughness and these results can be attributed to local brittle zones formed in the welding process.

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / Tensões residuais são uma das principais causas de falhas em componentes mecânicos submetidos a processos de fabricação. O objetivo do trabalho foi medir as tensões residuais presentes em um tubo quadrado soldado por resistência elétrica de alta frequência e caracterizar microestruturalmente o seu material. Para a caracterização, foram utilizadas técnicas de microscopia óptica (MO), microscopia eletrônica de varredura (MEV) e análise química quantitativa. Para a medição das tensões residuais, foi utilizado o método do furo cego, baseado na norma ASTM E837-08, onde rosetas (strain-gages) são coladas à peça para medir as deformações geradas devido à usinagem de um pequeno furo no local de medição. As deformações foram associadas às tensões residuais através de equações baseadas na Lei de Hooke. A caracterização revelou uma microestrutura composta basicamente de ferrita e perlita, típica de aços com baixo teor de carbono, corroborando com a especificação fornecida pelo fabricante. As tensões residuais encontradas foram trativas e mostraram-se elevadas, com alguns valores acima do limite de escoamento do material. / Residual stresses are one of the main failure causes in mechanical components submitted to manufacture process. The aim of this work was to measure the residual stresses present in a seame welded tube welded using a high frequency electrical resistance and to microstruturally characterize its material. Optical microscopy (OM), scanning electron microscopy (SEM) and quantitative chemical analysis techniques were used for the characterization. The hole-drilling method was applied obeying the ASTM E837-08 norm to measure the residual stresses, where strain-gages are glued to the sample to measure the deformations caused due to the machining of a small hole in the measurement point. The deformations were linked to the residual stresses using the Hookes Law equations. The characterization basically showed a composite microstructure of ferrite and perlite, tipical in lowcarbon steels, as the specification supplied by the manufacturer. The residual stresses found were tensile and high, with some values above the material yield strength.

O presente trabalho teve como objetivo desenvolver uma metodologia prática e confiável para previsão de vida à fadiga de juntas soldadas a cordão pelo processo MIG/MAG, quando estas estão submetidas a carregamentos cíclicos combinados. A máxima tensão linear no pé da solda, região típica de início de trinca, conhecida como hot spot foi utilizada para prever a vida através do método de Fadiga de Alto Ciclo S x N (Tensão x Vida), largamente empregado em códigos de projeto de estruturas soldadas. O Método dos Elementos Finitos foi utilizado para determinação das tensões estruturais resultantes do carregamento e das descontinuidades geométricas presentes nos cordões de solda. A análise de fadiga foi efetuada em ambiente virtual, através de um software (programa) de fadiga capaz de importar as tensões atuantes na região da solda para cada carregamento, combinando-as e obtendo assim a vida à fadiga decorrente da somatória dos diferentes tipos de carregamento ao qual a junta foi submetida. As propriedades monotônicas e cíclicas dos materiais da junta foram obtidas da literatura e de um extenso banco de dados disponível no software de fadiga. Estas propriedades foram ajustadas com base em ensaios de laboratório nas juntas investigadas. A medição ou modelagem das tensões residuais inerentes ao processo de soldagem não fazem parte do escopo deste trabalho. Contudo, os efeitos térmicos e metalúrgicos resultantes do processo de soldagem, como distorções, tensões residuais, variações microestruturais e propriedades mecânicas foram considerados de forma indireta, através da correção das curvas de fadiga nos corpos de prova investigados. Corpos de prova do tipo tubo-placa foram submetidos a carregamentos cíclicos combinados (flexão e torção) de amplitude constante. O resultado da análise virtual de durabilidade foi, portanto, calibrado com base nestes experimentos e curvas disponíveis em códigos de projeto de fadiga como BS7608 e Eurocode 3. A aplicabilidade deste método numérico-experimental e suas contribuições para a garantia da Integridade Estrutural do projeto de juntas soldadas são apresentadas. Seus desafios e melhorias são por fim discutidos. / The main purpose of this work is to develop a practical and robust methodology to evaluate the fatigue life in seam weld joints fabricated with GMAW process when subjected to combine cyclic loading. The maximum linear stress at the typical crack initiation region, better known as hot spot stress, was used to calculate the fatigue life through high cycle fatigue method S x N (Stress x Life), widely used in design codes for the life assessment of welded structures. The Finite Element Method (FEM) was used to obtain the structural stresses distribution due the external loading and geometric discontinuities very common in seam weld joints. The fatigue analysis was conducted in virtual environment. The FEM stress results from each loading were imported to fatigue code FE-Fatigue and combined to perform the fatigue life prediction. The monotonic and cyclic properties of the joint materials were obtained in the literature and from the fatigue software database. These properties were adjusted based on laboratory fatigue tests in the investigated welded joints configurations. The measurement or modeling of the residual stresses resulted from the welded process is not part of this work. However, the thermal and metallurgical effects, like distortions and residual stresses, were considered indirectly through fatigue curves corrections in the samples investigated. A tube-plate specimen was submitted to combine cyclic loading (bending and torsion) with constant amplitude. The virtual durability analysis result was calibrated based on these laboratory tests and design codes such as BS7608 and Eurocode 3. The feasibility and application of the proposed numerical-experimental methodology and contributions for the welded joints structural integrity design are presented. The challenges and improvements are finally discussed.

Bridges often use pile foundations behind MSE walls to help resist lateral loading from seismic and thermal expansion and contraction loads. Overdesign of pile spacing and sizes occur owing to a lack of design code guidance for piles behind an MSE wall. However, space constraints necessitate the installation of piles near the wall. Full scale lateral load tests were conducted on piles behind an MSE wall. This study involves the testing of four HP12X74 H-piles and four HSS12X12X5/16 square piles. The H-piles were tested with ribbed strip soil reinforcement at a wall height of 15 feet, and the square piles were tested with welded wire reinforcement at a wall height of 20 feet. The H-piles were spaced from the back face of the MSE wall at pile diameters 4.5, 3.2, 2.5, and 2.2. The square piles were spaced at pile diameters 5.7, 4.2, 3.1, and 2.1. Testing was based on a displacement control method where load increments were applied every 0.25 inches up to three inches of pile deflection. It was concluded that piles placed closer than 3.9 pile diameters have a reduction in their lateral resistance. P-multipliers were back-calculated in LPILE from the load-deflection curves obtained from the tests. The p-multipliers were found to be 1.0, 0.85, 0.60, and 0.73 for the H-piles spaced at 4.5, 3.2, 2.5, and 2.2 pile diameters, respectively. The p-multipliers for the square piles were found to be 1.0, 0.77, 0.63, and 0.57 for piles spaced at 5.7, 4.2, 3.1, and 2.1 pile diameters, respectively. An equation was developed to estimate p-multipliers versus pile distance behind the wall. These p-multipliers account for reduced soil resistance, and decrease linearly with distance for piles placed closer than 3.9 pile diameters. Measurements were also taken of the force induced in the soil reinforcement. A statistical analysis was performed to develop an equation that could predict the maximum induced reinforcement load. The main parameters that went into this equation were the lateral pile load, transverse distance from the reinforcement to the pile center normalized by the pile diameter, spacing from the pile center to the wall normalized by the pile diameter, vertical stress, and reinforcement length to height ratio where the height included the equivalent height of the surcharge. The multiple regression equations account for 76% of the variation in observed tensile force for the ribbed strip reinforcement, and 77% of the variation for the welded wire reinforcement. The tensile force was found to increase in the reinforcement as the pile spacing decreased, transverse spacing from the pile decreased, and as the lateral load increased.

Although branch plate connections with circular hollow section (CHS) members are simple to fabricate and cost-effective, they are generally very flexible under low load application resulting in the limit states design resistance being governed by an imposed deformation limit. Restricting the ultimate capacity of a branch plate connection by a deformation limit results in the inherent strength of the CHS member being under-utilized, highlighting the need to develop connection stiffening methods. Two methods to stiffen branch plate-to-CHS connections are examined: a through plate connection and a grout-filled CHS branch plate connection. Further, the current design guidelines of various plate-to-CHS connection types are reexamined including the effect of chord axial stress and chord length on connection behaviour. Finally, the behaviour of connections with non-orthogonal or skew plate orientation, which has not previously been examined, was studied in depth.The behaviour of these uniplanar connection types under quasi-static axial loading was studied through 16 large-scale laboratory experiments and 682 numerical finite element analyses, as well as an extensive review of all previous international experimental and numerical findings. The extensive study formed the basis for a complete set of proposed design guidelines and provided insight into plate-to-CHS connection behaviour. For all plate-to-CHS connection types, the plate thickness is shown to effect connection capacity, though previously this was thought not to have significant impact on connection behaviour. The existing ideology of using the same design recommendations for tension- and compression-loaded connections, which was developed from compression results, under-utilizes an inherent increase in capacity provided by a connection primarily loaded in tension. As such, the recommended design guidelines split the two load senses into separate expressions that reflect the difference in behaviour. Stiffened through plate connection behaviour was determined to be the summation of branch plate behaviour in compression and tension, leading to a significant increase in capacity and identical behaviour regardless of branch load sense. The skewed branch plate connection behaviour was found to relate directly to the established behaviour of longitudinal and transverse plate connections. A design function was developed that interpolates the capacities of intermediate angles by using the proposed design recommendations of the two extreme connection types. Finally, the examination of chord axial stress and chord length for plate-to-CHS connections yielded results similar to previous international studies on CHS-to-CHS connections. The effect of chord length, however, has wide-reaching implications as to how experimental and numerical FE research programs are developed.

Although branch plate connections with circular hollow section (CHS) members are simple to fabricate and cost-effective, they are generally very flexible under low load application resulting in the limit states design resistance being governed by an imposed deformation limit. Restricting the ultimate capacity of a branch plate connection by a deformation limit results in the inherent strength of the CHS member being under-utilized, highlighting the need to develop connection stiffening methods. Two methods to stiffen branch plate-to-CHS connections are examined: a through plate connection and a grout-filled CHS branch plate connection. Further, the current design guidelines of various plate-to-CHS connection types are reexamined including the effect of chord axial stress and chord length on connection behaviour. Finally, the behaviour of connections with non-orthogonal or skew plate orientation, which has not previously been examined, was studied in depth.The behaviour of these uniplanar connection types under quasi-static axial loading was studied through 16 large-scale laboratory experiments and 682 numerical finite element analyses, as well as an extensive review of all previous international experimental and numerical findings. The extensive study formed the basis for a complete set of proposed design guidelines and provided insight into plate-to-CHS connection behaviour. For all plate-to-CHS connection types, the plate thickness is shown to effect connection capacity, though previously this was thought not to have significant impact on connection behaviour. The existing ideology of using the same design recommendations for tension- and compression-loaded connections, which was developed from compression results, under-utilizes an inherent increase in capacity provided by a connection primarily loaded in tension. As such, the recommended design guidelines split the two load senses into separate expressions that reflect the difference in behaviour. Stiffened through plate connection behaviour was determined to be the summation of branch plate behaviour in compression and tension, leading to a significant increase in capacity and identical behaviour regardless of branch load sense. The skewed branch plate connection behaviour was found to relate directly to the established behaviour of longitudinal and transverse plate connections. A design function was developed that interpolates the capacities of intermediate angles by using the proposed design recommendations of the two extreme connection types. Finally, the examination of chord axial stress and chord length for plate-to-CHS connections yielded results similar to previous international studies on CHS-to-CHS connections. The effect of chord length, however, has wide-reaching implications as to how experimental and numerical FE research programs are developed.

Gas Metal Arc Welding (GMAW) is one of the main methods used to join structural members. One of the largest challenges involved in production of welds is ensuring the quality of the weld. One of the main factors attributing to weld quality is penetration depth. Automatic control of the welding process requires non-contact, non-destructive sensors that can operate in the presence of high temperatures and electrical noise found in the welding environment. Inspection using laser generation and electromagnetic acoustic transducer (EMAT) reception of ultrasound was found to satisfy these conditions. Using this technique, the time of flight of the ultrasonic wave is measured and used to calculate penetration depth. Previous works have shown that penetration depth measurement performance is drastically reduced when performed during welding. This work seeks to realize in-process penetration depth measurement by compensating for errors caused by elevated temperature. Neuro-fuzzy models are developed that predict penetration depth based on in-process time of flight measurements and the welding process input. Two scenarios are considered in which destructive penetration depth measurements are or are not available for model training. Results show the two scenarios are successful. When destructive measurements are unavailable, model error is comparable to that of offline ultrasonic measurements. When destructive measurements are available, measurement error is reduced by 50% compared to offline ultrasonic measurements. The two models can be effectively applied to permit in-process penetration depth measurements for the purpose of real-time monitoring and control. This will reduce material, production time, and labor costs and increase the quality of welded parts.

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

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