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Avaliação do comportamento de compósitos à fluência e à relaxação
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A presente dissertação avalia o comportamento à fluência e à relaxação de polímeros e
compósitos de matriz termoplástica reforçados com fibra de carbono, com o objetivo de
compreender a resposta viscoelástica destes materiais e a adequação de modelos lineares à
sua representação. Para o efeito, foi realizada uma revisão bibliográfica sobre materiais
compósitos, mecanismos de deformação dependentes do tempo, processamento de matrizes
termoplásticas e modelos viscoelásticos clássicos. Foram também desenvolvidos e analisados
modelos baseados em associações de molas e amortecedores, recorrendo a transformadas de
Laplace para a sua aplicação aos ensaios experimentais. A componente experimental incluiu a
caracterização mecânica de polietileno teraftalato, policarbonato e compósitos reforçados com
fibra de carbono, obtidos por pultrusão e por compressão, seguida da realização de ensaios de
fluência e relaxação em tração e flexão. Os resultados mostraram que o modelo generalizado
de Maxwell permite aproximar adequadamente a resposta experimental, sobretudo para
pequenas deformações. Verificou-se ainda que o reforço reduz a evolução viscoelástica quando
contribui diretamente para a rigidez do provete, enquanto orientações menos favoráveis
tornam o comportamento mais dependente da matriz e da qualidade de consolidação.
This dissertation assesses the creep and stress relaxation behaviour of polymers and carbonfibre- reinforced thermoplastic matrix composites, with the aim of understanding the viscoelastic response of these materials and the suitability of linear models for its representation. For this purpose, a bibliographic review was carried out on composite materials, time-dependent deformation mechanisms, thermoplastic matrix processing and classical viscoelastic models. Models based on combinations of springs and dashpots were also developed and analysed, using Laplace transforms for their application to the experimental tests. The experimental component included the mechanical characterisation of polyethylene terephthalate, polycarbonate and carbon-fibre-reinforced composites obtained by pultrusion and compression, followed by creep and stress relaxation tests under tensile and flexural loading. The results showed that the generalized Maxwell model provides a suitable approximation to the experimental response, particularly for small deformations. It was also found that the reinforcement reduces the viscoelastic evolution when it contributes directly to the stiffness of the specimen, whereas less favourable orientations make the behaviour more dependent on the matrix and on the quality of consolidation.
This dissertation assesses the creep and stress relaxation behaviour of polymers and carbonfibre- reinforced thermoplastic matrix composites, with the aim of understanding the viscoelastic response of these materials and the suitability of linear models for its representation. For this purpose, a bibliographic review was carried out on composite materials, time-dependent deformation mechanisms, thermoplastic matrix processing and classical viscoelastic models. Models based on combinations of springs and dashpots were also developed and analysed, using Laplace transforms for their application to the experimental tests. The experimental component included the mechanical characterisation of polyethylene terephthalate, polycarbonate and carbon-fibre-reinforced composites obtained by pultrusion and compression, followed by creep and stress relaxation tests under tensile and flexural loading. The results showed that the generalized Maxwell model provides a suitable approximation to the experimental response, particularly for small deformations. It was also found that the reinforcement reduces the viscoelastic evolution when it contributes directly to the stiffness of the specimen, whereas less favourable orientations make the behaviour more dependent on the matrix and on the quality of consolidation.
Descrição
Palavras-chave
Viscoelasticity Creep Stress relaxation Thermoplastic composites Carbon fibre Viscoelasticidade Fluência Relaxação Compósitos termoplásticos Fibra de carbono