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Authors
Abstract(s)
As propriedades de fadiga são, normalmente, obtidas utilizando provetes normalizados,
embora muitas vezes seja preferível recorrer a geometrias específicas que melhor
reproduzam as reais condições de serviço do componente em análise. Os problemas
devido à fadiga têm sido objecto de fortes estudos nos últimos anos, dado que este
fenómeno continua a ser uma das principais causas de inutilização de componentes e
estruturas na indústria aeronáutica.
O presente trabalho centra-se no estudo numérico de propagação de fendas na liga de
alumínio 6082-T6. Sendo este um material tradicionalmente usado em componentes
aeronáuticos, observa-se, normalmente, deformação em pequena escala na extremidade
da fenda, de modo que o factor de intensidade de tensão (K) pode ser utilizado para
estudar a influência da gama de tensão e do comprimento de fenda no comportamento à
fadiga, pretendendo-se definir provetes relativamente finos com estado plano de
deformação. Para o efeito, foram construídos modelos computacionais de provetes com
entalhes laterais aguçados ou com raio de concordância, tenso sido estudadas diferentes
espessuras, diferentes ângulos de entalhe e diferentes raios de concordância, obtendo-se
para as melhores geometrias as formas estáveis de fenda, as soluções de K e a
triaxialidade de tensões tanto ao longo da espessura do provete, como ao longo de
frentes de fenda. Através dos parâmetros de triaxialidade, Θ e h, foi possível averiguar
se o provete se encontrava ou não em estado de deformação plana. Alguns dos
resultados aqui obtidos foram comparados com as soluções teóricas existentes na
literatura, nomeadamente de Roark [27], Murakami [60] e Antunes [52], havendo
diferenças aceitáveis e justificáveis dado as condições da malha imposta pelo Zencrack
e a geometria do provete MT. Os provetes obtidos serão de grande utilidade para
estudar fenómenos com grande influência do estado de tensão, tais como propagação de
fendas por fadiga a alta temperatura, ou fecho de fenda induzido por deformação
plástica.
Fatigue properties of common aerospace components are usually obtained from standard specimens used for experimental testing, but there are some cases where the use of particular specimen geometries is more suitable in order to reproduce the real operational conditions of components. This investigation is focused in the computational simulation of fatigue cracks in the 6082-T6 aluminium alloy. This material is used in certain types of aeronautical components which will eventually face out some fatigue cracking process characterized by a small scale yielding deformation in the crack tip region. Therefore, the use of the stress intensity factor K is adequate in order to evaluate the influence of both stress fields and crack length in the fatigue behaviour of the material. At the same time, plane strain conditions is a requirement for certain type of applications and specific specimens geometries must be considered in these cases. This work tried to evaluate the possibility of using a traditional MT specimen with lateral notches for this purpose, which could be more advantageous than the standard geometries since it allows the use of moderate component thicknesses with triaxial stress conditions in the most part of the crack propagation region. Several computational simulations were carried out by using FEM softwares and considering the influence of different types of variables, namely, the geometry of the lateral notches (circular or V shaped) and the thickness of the specimen. The stress intensity factor K was determined for several crack front positions as to evaluate the effect of these geometrical features. Also, two stress triaxility parameters were used in order to confirm the stress state condition in the crack propagation region. Finally, an automatic crack propagation procedure was implemented and results have been compared with other authors. The conclusions of this investigation are optimistic concerning the possibility of using a MT specimen with two lateral notches and low thickness with plane strain capabilities, which is a valuable element when considering the investigation of particular crack propagation mechanisms, such as those related with high temperature conditions.
Fatigue properties of common aerospace components are usually obtained from standard specimens used for experimental testing, but there are some cases where the use of particular specimen geometries is more suitable in order to reproduce the real operational conditions of components. This investigation is focused in the computational simulation of fatigue cracks in the 6082-T6 aluminium alloy. This material is used in certain types of aeronautical components which will eventually face out some fatigue cracking process characterized by a small scale yielding deformation in the crack tip region. Therefore, the use of the stress intensity factor K is adequate in order to evaluate the influence of both stress fields and crack length in the fatigue behaviour of the material. At the same time, plane strain conditions is a requirement for certain type of applications and specific specimens geometries must be considered in these cases. This work tried to evaluate the possibility of using a traditional MT specimen with lateral notches for this purpose, which could be more advantageous than the standard geometries since it allows the use of moderate component thicknesses with triaxial stress conditions in the most part of the crack propagation region. Several computational simulations were carried out by using FEM softwares and considering the influence of different types of variables, namely, the geometry of the lateral notches (circular or V shaped) and the thickness of the specimen. The stress intensity factor K was determined for several crack front positions as to evaluate the effect of these geometrical features. Also, two stress triaxility parameters were used in order to confirm the stress state condition in the crack propagation region. Finally, an automatic crack propagation procedure was implemented and results have been compared with other authors. The conclusions of this investigation are optimistic concerning the possibility of using a MT specimen with two lateral notches and low thickness with plane strain capabilities, which is a valuable element when considering the investigation of particular crack propagation mechanisms, such as those related with high temperature conditions.
Description
Keywords
Indústria aeronáutica Provete MT Propagação de fendas por fadiga Métodos dos elementos finitos Zencrack Entalhes laterais Intensidade de tensões Triaxilidade