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Abstract(s)
Ao longo dos últimos anos a utilização das juntas adesivas tem vindo a verificar um aumento acentuado em aplicações estruturais em detrimento dos métodos tradicionais de união. Em aplicações reais, as juntas adesivas são submetidas a solicitações de modo misto I+II, o que aumenta significativamente a complexidade da previsão de resistência das juntas adesivas. De facto, os ensaios desenvolvidos para o estudo de juntas adesivas, sob carregamento em modo misto, são bastante complexos e considerados realmente relevantes para a caracterização à fratura. Existem vários ensaios de modo misto, dos quais se destaca o ensaio Mixed-Mode Bending (MMB) pela sua capacidade de permitir a variação do rácio de modo misto praticamente sem limitações e, desta forma, permitir analisar a influência do rácio de modo misto na resistência à fratura da junta. Contudo, apesar de existirem várias propostas de configuração MMB, o ensaio ainda é considerado complexo, sendo que algumas das soluções existentes não permitem o ensaio em materiais mais rígidos, pelo que este foi um dos principais aspetos considerados no desenvolvimento da dissertação. Neste sentido, o desafio proposto para esta dissertação consistiu em projetar, modelar numericamente e validar experimentalmente um equipamento de ensaio MMB, para a realização de ensaios de adesivos estruturais. No trabalho experimental, foi desenvolvido um equipamento com uma configuração mais simples e versátil, que permite medir a tenacidade de juntas adesivas em vários rácios de modo misto, baseado unicamente na curva carga-deslocamento (P-δ) e sem a necessidade de medição do comprimento de fenda (a). Este equipamento foi projetado e fabricado para realização de ensaios e tratamento de dados conducente à validação do funcionamento do equipamento para ensaio de adesivos, que consistiu na obtenção dos envelopes de fratura de adesivos e comparação com resultados da literatura. No trabalho numérico, a geometria do equipamento foi criada em software e foram realizadas as respetivas simulações, de modo a otimizar a geometria do equipamento projetado. Nomeadamente, foi efetuada uma comparação entre o equipamento de base de estudo, desenvolvido por Chaves [1], e as soluções apresentadas durante a evolução do projeto. O equipamento desenvolvido apresenta um fácil manuseamento durante a realização de ensaios e características melhoradas ao nível da sua geometria, apesar de robusta. Este equipamento é adequado para provetes de ensaio de elevada rigidez, pelo que permite uma carga máxima de 20 kN.
Over the past few years, the use of adhesive joints has seen a marked increase in structural applications at the expense of traditional joining methods. In real applications, adhesive joints are subjected to mixed mode I+II loading, which significantly increases the strength prediction complexity of adhesive joints. Actually, tests developed for the study of adhesive joints under mixed-mode loading are quite complex and considered really relevant for fracture characterization. There are several mixed-mode tests, of which the Mixed-Mode Bending (MMB) test stands out for its capacity to allow the variation of the mixed-mode ratio practically without limitations and, thus, to allow the analysis of the influence of the mixed-mode ratio on the fracture toughness of the joint. However, although there are several proposals for the MMB configuration, this test is still considered complex, and some of the current solutions do not allow performing the test on stiffer materials, so this was one of the main aspects considered in the development of the dissertation. In this sense, the challenge proposed for this dissertation was to design, numerically model and experimentally validate a MMB test equipment for structural adhesives’ testing. In the experimental work, an equipment with a simpler and more versatile configuration was developed, which allows measuring the toughness of adhesive joints in several mixed-mode ratios, based solely on the load-displacement (P-δ) curve and without the need of measuring the crack length (a). This equipment was designed and manufactured to perform tests and data processing leading to the validation of the operation of the equipment for adhesive testing, which consisted in obtaining the fracture envelopes of adhesives and comparison with literature results. In the numerical work, the geometry of the equipment was created in software and the respective simulations were performed in order to optimize the geometry of the designed equipment. In particular, a comparison was made between the basic study equipment, developed by Chaves [1], and the solutions presented during the evolution of the project. The developed equipment presents easy handling during testing and improved characteristics in terms of its geometry, although robust. This equipment is suitable for test specimens with high stiffness, allowing a maximum load of 20 kN.
Over the past few years, the use of adhesive joints has seen a marked increase in structural applications at the expense of traditional joining methods. In real applications, adhesive joints are subjected to mixed mode I+II loading, which significantly increases the strength prediction complexity of adhesive joints. Actually, tests developed for the study of adhesive joints under mixed-mode loading are quite complex and considered really relevant for fracture characterization. There are several mixed-mode tests, of which the Mixed-Mode Bending (MMB) test stands out for its capacity to allow the variation of the mixed-mode ratio practically without limitations and, thus, to allow the analysis of the influence of the mixed-mode ratio on the fracture toughness of the joint. However, although there are several proposals for the MMB configuration, this test is still considered complex, and some of the current solutions do not allow performing the test on stiffer materials, so this was one of the main aspects considered in the development of the dissertation. In this sense, the challenge proposed for this dissertation was to design, numerically model and experimentally validate a MMB test equipment for structural adhesives’ testing. In the experimental work, an equipment with a simpler and more versatile configuration was developed, which allows measuring the toughness of adhesive joints in several mixed-mode ratios, based solely on the load-displacement (P-δ) curve and without the need of measuring the crack length (a). This equipment was designed and manufactured to perform tests and data processing leading to the validation of the operation of the equipment for adhesive testing, which consisted in obtaining the fracture envelopes of adhesives and comparison with literature results. In the numerical work, the geometry of the equipment was created in software and the respective simulations were performed in order to optimize the geometry of the designed equipment. In particular, a comparison was made between the basic study equipment, developed by Chaves [1], and the solutions presented during the evolution of the project. The developed equipment presents easy handling during testing and improved characteristics in terms of its geometry, although robust. This equipment is suitable for test specimens with high stiffness, allowing a maximum load of 20 kN.
Description
Keywords
Mixed-Mode Bending Juntas adesivas Adesivos estruturais Mecânica da fratura Envelope de fratura Tenacidade à fratura Mixed-Mode Bending Adhesive joints Structural adhesives Fracture mechanics Fracture envelope Fracture toughness