Efeito do tratamento térmico no comportamento à fadiga oligocíclica extrema de aços para pipelines
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2016-11-24
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Os joelhos/curvas são elementos integrantes de redes de pipelines. Estes elementos
podem ser considerados críticos sob o ponto de vista da fadiga pois promovem a
concentração de tensões e deformações, quando sujeitos a acções cíclicas extremas (ex:
provocadas por sismos) podendo apresentar um colapso ao fim de um número reduzido
de ciclos, caracterizados por grandes amplitudes de deformação plástica.
A produção dos joelhos baseia-se em geral no processo de dobragem de tubos retos
previamente aquecidos por indução. Neste estudo pretende-se investigar o efeito do
aquecimento por indução no comportamento à fadiga dos aços X60 e X65 usados na
produção de pipelines, recorrendo-se para tal a um programa de ensaios do domínio da
fadiga oligocíclica e oligocíclica extrema. A avaliação do efeito do tratamento térmico
será baseada na comparação dos resultados obtidos com a informação experimental
disponível na literatura para os mesmos aços sem tratamento térmico.
Adicionalmente também se apresenta um estudo de simulação numérica de ensaios
cíclicos de joelhos produzidos com tubos de aço X60 e X65. Os ensaios desses joelhos
foram realizados no âmbito do projecto europeu ULCF (Ultra Low Cycle Fatigue of
Steel Under Cyclic High-Strain Loading Conditions). A simulação dos ensaios foi
realizada recorrendo a modelos de plasticidade identificados com os resultados obtidos
do programa de ensaios de caracterização experimental dos aços em causa. Neste estudo
numérico foi investigada a influência de parâmetros intervenientes no carregamento e
na geometria no comportamento à fadiga dos joelhos.
A par de todo o estudo experimental e numérico, procurou-se aferir a capacidade de
modelos de dano existentes na literatura (ex: Coffin-Manson, Xue) na descrição do
comportamento à fadiga do material e a capacidade destes para prever o comportamento
dos joelhos. Constatou-se que foi conseguida uma boa descrição do comportamento
plástico cíclico e à fadiga dos joelhos, tendo os modelos de Coffin-Manson e Xue
sobrestimado até um máximo de 28% o número de ciclos dos joelhos. Foi ainda testado
os procedimentos do código ASME VIII-Div.2, tendo resultado previsões da vida à
fadiga muito precisas para os joelhos.
Elbows are a typical pipeline component that may be considered critical from a fatigue perspective since they promote the concentration of stresses and strains when loaded under extreme cyclic actions (e.g. seismic actions). Under these circumstances characterized by large plastic strain ranges, elbows may exhibit a premature failure. Elbows are generally fabricated from bending of straight pipes previously heated by induction. In this study, the effects of the hot induction process on the fatigue behavior of the X60 and X65 pipeline steels are characterized using low- and ultra-low-cycle fatigue tests. The behavior of the referred materials subjected to the thermal process is compared with the behavior of corresponding materials without the thermal process. In addition to the small-scale testing, a numerical study is performed aiming the simulation of cyclic tests performed on full-scale elbows manufactured of X60 and X65 steels, using the hot bending process. The full-scale tests were performed in the framework of the ULCF (Ultra Low Cycle Fatigue of Steel Under Cyclic High-Strain Loading Conditions) European project. The simulation of the tests was performed using plasticity models identified using experimental data from the small-scale tests. The influence of loading and geometric parameters on cyclic elastoplastic and ultra-lowcycle fatigue behavior of elbows was investigated. In parallel with the experimental and numerical work, the performance of damage models for ultra-low-cycle fatigue available in the literature (e.g. Coffin-Manson, Xue) was assessed with two purposes: the correlation of the small-scale testing data and the simulation of the damage behaviour of the elbows. A good description of the plastic and fatigue behaviour was observed, both Coffin-Manson and Xue models producing an overestimation of the fatigue lives of elbows with a maximum overestimation of 28% in the number of cycles. The procedures of the ASME VIII-Division 2 code were also tested resulting in very accurate fatigue predictions for the simulated elbows.
Elbows are a typical pipeline component that may be considered critical from a fatigue perspective since they promote the concentration of stresses and strains when loaded under extreme cyclic actions (e.g. seismic actions). Under these circumstances characterized by large plastic strain ranges, elbows may exhibit a premature failure. Elbows are generally fabricated from bending of straight pipes previously heated by induction. In this study, the effects of the hot induction process on the fatigue behavior of the X60 and X65 pipeline steels are characterized using low- and ultra-low-cycle fatigue tests. The behavior of the referred materials subjected to the thermal process is compared with the behavior of corresponding materials without the thermal process. In addition to the small-scale testing, a numerical study is performed aiming the simulation of cyclic tests performed on full-scale elbows manufactured of X60 and X65 steels, using the hot bending process. The full-scale tests were performed in the framework of the ULCF (Ultra Low Cycle Fatigue of Steel Under Cyclic High-Strain Loading Conditions) European project. The simulation of the tests was performed using plasticity models identified using experimental data from the small-scale tests. The influence of loading and geometric parameters on cyclic elastoplastic and ultra-lowcycle fatigue behavior of elbows was investigated. In parallel with the experimental and numerical work, the performance of damage models for ultra-low-cycle fatigue available in the literature (e.g. Coffin-Manson, Xue) was assessed with two purposes: the correlation of the small-scale testing data and the simulation of the damage behaviour of the elbows. A good description of the plastic and fatigue behaviour was observed, both Coffin-Manson and Xue models producing an overestimation of the fatigue lives of elbows with a maximum overestimation of 28% in the number of cycles. The procedures of the ASME VIII-Division 2 code were also tested resulting in very accurate fatigue predictions for the simulated elbows.
Descrição
Dissertação de Mestrado em Engenharia Mecânica
Palavras-chave
Fadiga dos materiais , Aço , Tratamento térmico