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Abstract(s)
Neste trabalho desenvolveram-se métodos analíticos, numéricos e experimentais para
a caracterização da madeira à fractura sob solicitações de modo misto I+II. Para tal
analisou-se a aplicabilidade de diferentes métodos de ensaio usados na caracterização
de materiais compósitos. Os ensaios End Loaded Split – Mixed Mode (ELS-MM) e Single
Leg Bending (SLB) são de fácil execução mas propiciam uma baixa amplitude de
variação dos rácios de modo misto. Foi projectada e construída uma versão do sistema
de amarras usado no ensaio Mixed-Mode Bending (MMB), tendo sidos testados numa
primeira fase dez rácios de modo misto. Para ultrapassar as dificuldades relacionadas
com a monitorização do comprimento de fenda durante os ensaios, usou-se um
procedimento de tratamento de resultados experimentais baseado no conceito de
fenda equivalente. Este método, apelidado de Compliance Based Beam Method (CBBM)
permite a obtenção de curvas de resistência e das taxas críticas de libertação de energia
evitando a medição do comprimento de fenda durante a sua propagação. Foram
realizados ensaios de fractura e simulações numéricas de provetes DCB (Double
Cantilever Beam) e ENF (End Notched Flexure) que permitiram obter as propriedades de
fractura em modos puros (I e II) e validar o método de tratamento de resultados, que
foi posteriormente aplicado ao ensaio MMB. Os resultados obtidos nos ensaios ELSMM,
SLB e com as diferentes combinações de modo misto usadas na primeira fase
de ensaios MMB permitiram obter uma lei de comportamento da madeira em modo
misto e revelaram que o critério de propagação linear é adequado para representar os
resultados experimentais no espaço GI-GII.
Na segunda fase deste trabalho, pretendeu-se identificar as leis coesivas em modo I,
modo II e modo I+II para a madeira. O método de identificação estudado baseia-se
na resposta mecânica global dos provetes e na medição da abertura da fenda na
vizinhança da sua extremidade (Crack Opening Displacement - COD) a partir do campo
dos deslocamentos medido nessa região recorrendo à correlação digital de imagem.
As leis coesivas resultam da derivação das taxas de libertação de energia (determinadas
por aplicação do método CBBM) em ordem ao COD. O método de identificação das leis coesivas foi validado numericamente recorrendo à simulação por elementos
finitos incluindo modelos coesivos. Este método revelou-se eficaz para determinar as
leis coesivas em modos puros, embora haja algumas questões a resolver em trabalho
futuro. Na segunda série de ensaios MMB foram seleccionadas quatro razões de modo
misto com vista à identificação das leis coesivas em modo misto através do método
de Högberg (Högberg 2006) e os resultados obtidos mostraram que a razão de modo
misto local, determinada pelos deslocamentos de abertura de fenda em modo I e em
modo II, não é controlada pela razão de modo misto global (GI/GII), nem é constante
ao longo de um ensaio.
Pode-se afirmar, como conclusão que o presente trabalho pode ser visto como uma
importante contribuição para a definição de procedimentos adequados para uma
correcta caracterização à fractura da madeira.
In this work, analytical, numerical and experimental methods were developed for the fracture characterization of wood under mixed mode I+II loading. The applicability of different test methods used in the characterization of composite materials was analysed. The End Loaded Split - Mixed Mode (ELS-MM) and the Single Leg Bending (SLB) tests are easy to execute but provide a low amplitude variation of mixed-mode ratios. A version of the Mixed-Mode Bending (MMB) test apparatus was designed and built, and ten mixed-mode ratios were tested in a first testing campaign. To overcome the difficulties related to crack length monitoring during the tests, a data reduction scheme based on the concept of equivalent crack was used. This method, called the Compliance Based Beam Method (CBBM), provides Resistance-curves and critical energy release rates using exclusively the specimen compliance and avoiding the measurement of crack length during propagation. Fracture tests and numerical simulations of DCB (Double Cantilever Beam) and ENF (End Notched Flexure) specimens were carried out to determine the fracture properties in pure modes (I and II) and to validate the data reduction method, which was later applied to the MMB test. The results obtained in the ELS-MM, SLB and MMB tests allowed to obtain the wood fracture criterion under mixed-mode I+II loading. It was concluded that the linear propagation criterion is adequate to represent the experimental results in the GI-GII space. The second part of this work was dedicated to wood cohesive laws identification under mode I, mode II and mode I + II loading. The followed method is based on the global mechanical response of the tested specimens and on the measurement of the Crack Opening Displacement (COD) from the field of displacements measured at the crack tip using digital image correlation. Cohesive laws result from the differentiation of energy release rates (determined by applying the CBBM method) in order to COD. The method of identification of cohesive laws was numerically validated using finite element simulation including cohesive models. This method proved to be effective in determining cohesive laws in pure modes, although there are some issues to be solved in future work. In the second series of MMB tests four mixed mode ratios were selected for the identification of mixed mode cohesive laws by the Högberg method (Högberg 2006). The results obtained showed that the local mixed mode ratio, determined by the crack opening displacement for mode I and mode II, is not controlled by the overall mixed mode ratio (GI/GII), neither is it constant during an experimental test. As a final remark, it can be affirmed that the present work can be seen as an important contribution in the definition of suitable procedures to perform proper fracture characterization of wood.
In this work, analytical, numerical and experimental methods were developed for the fracture characterization of wood under mixed mode I+II loading. The applicability of different test methods used in the characterization of composite materials was analysed. The End Loaded Split - Mixed Mode (ELS-MM) and the Single Leg Bending (SLB) tests are easy to execute but provide a low amplitude variation of mixed-mode ratios. A version of the Mixed-Mode Bending (MMB) test apparatus was designed and built, and ten mixed-mode ratios were tested in a first testing campaign. To overcome the difficulties related to crack length monitoring during the tests, a data reduction scheme based on the concept of equivalent crack was used. This method, called the Compliance Based Beam Method (CBBM), provides Resistance-curves and critical energy release rates using exclusively the specimen compliance and avoiding the measurement of crack length during propagation. Fracture tests and numerical simulations of DCB (Double Cantilever Beam) and ENF (End Notched Flexure) specimens were carried out to determine the fracture properties in pure modes (I and II) and to validate the data reduction method, which was later applied to the MMB test. The results obtained in the ELS-MM, SLB and MMB tests allowed to obtain the wood fracture criterion under mixed-mode I+II loading. It was concluded that the linear propagation criterion is adequate to represent the experimental results in the GI-GII space. The second part of this work was dedicated to wood cohesive laws identification under mode I, mode II and mode I + II loading. The followed method is based on the global mechanical response of the tested specimens and on the measurement of the Crack Opening Displacement (COD) from the field of displacements measured at the crack tip using digital image correlation. Cohesive laws result from the differentiation of energy release rates (determined by applying the CBBM method) in order to COD. The method of identification of cohesive laws was numerically validated using finite element simulation including cohesive models. This method proved to be effective in determining cohesive laws in pure modes, although there are some issues to be solved in future work. In the second series of MMB tests four mixed mode ratios were selected for the identification of mixed mode cohesive laws by the Högberg method (Högberg 2006). The results obtained showed that the local mixed mode ratio, determined by the crack opening displacement for mode I and mode II, is not controlled by the overall mixed mode ratio (GI/GII), neither is it constant during an experimental test. As a final remark, it can be affirmed that the present work can be seen as an important contribution in the definition of suitable procedures to perform proper fracture characterization of wood.
Description
Keywords
Materiais compósitos - Métodos de ensaio Mecânica de fractura - Madeira - Ensaios Madeira - Lei coesiva - Modo I Madeira - Lei coesiva - Modo II Madeira - Lei coesiva - Modo I e II