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The presence of residual stresses in a structure influences his behaviour. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses, of flaws and notch sharpness at the weld toe. Plasma dressing at the weld toe is frequently used to remove flaws and to lessen the notch sharpness at the weld toe. With this re-melt, the stress field in the weld toe is also profoundly modified. The Finite Element Method (FEM) proves to be effective to simulate the stress fields generated by welding. The simplified two-dimensional models (2D) are efficient in this estimation. This paper presents a 2D finite element model to predict the residual stresses generated by Plasma dressing at the weld toe of a T-joint. The welded T-joints are made in St 52-3 steel, are obtained by covered electrode process and improved with plasma dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress results obtained using X-ray diffraction

Abstract. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses due to welding, of flaws and notch sharpness at the weld toe. Compressive residual stresses enhance the fatigue behavior of the structure, while tensile ones impair it. TIG dressing at the weld toe is frequently used to remove flaws to lessen the notch sharpness at the weld toe and to introduce compressive residual stresses. This paper presents a 2D finite element model to predict the residual stresses generated in a TIG dressing at the weld toe of a T joint. The welded T joints are made in St 52-3 steel and are obtained by covered electrode process. The analysis was developed with the Marc finite element code. The modeled stress field is compared with some measurements of residual stresses obtained using X-ray diffraction and the strain gauges technique.

Resumo. As técnicas de refusão do pé do cordão de soldadura por TIG e plasma são frequentemente usadas para aumentar a resistência e a fiabilidade de peças solicitadas à fadiga. Neste trabalho apresentam-se resultados de um estudo de investigação sobre a utilização destas técnicas na recuperação da resistência à fadiga de juntas soldadas em T solicitadas por flexão em três pontos. As juntas são fabricadas em aço St 52 – 3 e obtidas por soldadura com eléctrodo revestido. São estudadas quatro situações: juntas não tratadas, juntas tratadas por refusão TIG, juntas pré-fissuradas e posteriormente tratadas por refusão TIG e juntas pré-fissuradas e posteriormente tratadas por refusão plasma. A eficiência dos tratamentos de refusão TIG e plasma na reabilitação da resistência à fadiga é analisada e discutida com base nos resultados de fadiga e em elementos complementares determinados durante a investigação, tais como: o perfil do pé dos cordões, a microestrutura e os perfis de dureza nas regiões soldada e afectada térmicamente e dos defeitos detectados no cordão. A eficiência obtida pelo processo de refusão por plasma é bastante satisfatória e muito superior à da refusão por TIG.

Due to economic and environmental constrains, the currently trend is to use the welded structures beyond their design lives. The predominant cause of in service failure of these aged structures is the fatigue of the welded joints. The use of improvement techniques in welded joints, as a repair technique, has been suggested by several authors. TIG dressing is one of the most promising of these repair techniques. However, the effectiveness of TIG remelting is closely linked to the depth of the repaired crack. The use of strain gauges can be effective to detect the presence of fatigue cracks in their initial phase of propagation, however their effectiveness in inspection programs on jobsite needs to be proven. Some TIG variants associated to recent technological innovations of fusion arc welding, are appointed to improve the penetration and the sustainability of the remelting process. In this article are presented some results of the work developed by the authors in the last years, relevant to assess the efficiency of sustainable repair, by TIG and plasma dressing, of welded structures. Relevance is given to the repair, complete or defective, of deep cracks, as well as the possible advantage of using TIG variants.

In this article, a three-dimensional finite element model (FEM) is used to predict the crack growth at the weld toe of a pre-fatigued T-joint that was repaired with a remelting technique. The numerical models were developed using the MSC.Marc software. Fatigue life is estimated by integrating the Paris-Erdogan law. The stress intensity factors are obtained by the virtual crack closure technique (VCCT). The T-welded joints, made of S355 steel, are obtained by covered electrode process and pre-cracked by fatigue. These welded joints were repaired by TIG dressing. The stress field generated by this dressing technique was estimated using a FEM model, presented in authors’ previous works. For the crack growth was used the VCCT three-dimensional model recently presented by the authors to predict the effect of overloads. The pre-existence of an elliptical crack at the weld toe, with a depth of 0.5 mm was considered. It is also studied the growth of pre-existing cracks which have been poorly repaired. It was observed that the TIG dressing produce residual compression stress fields on the weld toe that causes a delay in crack growth. The obtained results are compared with experimental ones. The fatigue’s lives obtained by simulations with the numerical model presented in this paper allows to evaluate the application conditions of TIG remelting technique in the repair of pre-cracked welded joints.

In this article, a three-dimensional finite element model is used to predict the growth of cracks at the weld toe of a T-joint. The model is developed using the MSC Marc software. Fatigue life is estimated by integrating the Paris-Erdogan law and the stress intensity factors are obtained by the virtual crack closure technique. The influence of residual stresses generated by plastic deformation at the weld toe on the crack propagation speed is analyzed. The existence of residual compression stress fields causes a delay in crack growth. The obtained results are compared with the integration solutions of the Paris-Erdogan law using the stress intensity factor computed through the Mk factor proposed by Bowness and Lee, included in BS 7910 standard.

No presente trabalho pretende-se efetuar a caracterização mecânica do aço S 355 AR, EN 10025-4, a alta temperatura. A avaliação rigorosa da variação da tensão de cedência, do módulo de Young, do módulo de encruamento, do calor específico e do coeficiente de dilatação térmica, com a temperatura é essêncial na simulação numérica da distorção e das tensões residuais induzidas por campos térmicos. Embora esta informação esteja disponível em alguns códigos estruturais, como o Eurocódigo 3 parte 1.2, ela não inclui o efeito da composição química e da microestrutura. O aço S 355 AR é um material bastante utilizado em estruturas soldadas, onde a avaliação de distorções e tensões residuais é um problema usual. A caracterização mecânica do aço S 355 AR, EN 10025-4, foi obtida através de ensaios de tração isotérmicos realizados a temperaturas até 600 °C, por Calorimetria Exploratória Diferencial (DSC) realizada até à temperatura de 950 °C e por Análise Termomecânica (TMA) conduzida até aos 1350 °C. Os resultados obtidos diferem significativamente dos valores apresentados na parte 1.2 do Eurocódigo 3, para um aço ferrítico genérico.