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Advisor(s)
Abstract(s)
As ligações adesivas têm sido bastante desenvolvidas ao longo dos anos, e representam uma técnica de ligação dominante na indústria aeronáutica e automóvel, onde visam substituir os métodos tradicionais. Devido à crescente utilização das juntas adesivas, torna-se necessária a existência de modelos de previsão de resistência que sejam fiáveis e robustos. O Método de Elementos Finitos (MEF) tem ajudado ao desenvolvimento desta técnica de ligação. Uma abordagem recente consiste no uso de modelos de dano coesivo (MDC), que permitem simular o comportamento à fratura das juntas de forma bastante fiável. Esta técnica requer a definição das leis coesivas em tração e corte. Existem diversas formas de leis coesivas possíveis de aplicar em simulações por MDC, em função do comportamento expectável dos materiais que estão a ser simulados. Um método comum para melhorar as limitações dos adesivos é o reforço de juntas com partículas para melhoria das propriedades, sendo um dos reforços mais utilizados a cortiça. As partículas de cortiça podem criar obstáculos à propagação das fissuras, aumentando assim a tenacidade do adesivo. Este facto ocorre porque a cortiça apresenta uma combinação notável de propriedades. Neste trabalho, estudou-se numericamente o efeito da adição de partículas de cortiça na tenacidade à tração e ao corte de um adesivo estrutural, consistindo na análise dos resultados da força máxima (Pmáx), deslocamento de força máxima (δPmáx) e o deslocamento máximo (máx). Utilizou-se o MEF em conjunto com os MDC para modelar o comportamento do adesivo modificado. Consideram-se concentrações de cortiça de 0%, 0,25%, 0,5% e 1% de peso, à tração e ao corte. Foram realizados ensaios Double-Cantilever Beam para caracterização à tração e ensaios End-Notched Flexure para caraterização ao corte. De seguida, realizou-se um estudo da sensibilidade dos parâmetros coesivos, que consiste na averiguação da influência de tn 0 , ts 0 , GIc e GIIc no comportamento dos modelos numéricos dos ensaios DCB e ENF. Com este trabalho foi possível compreender a influência da adição de 0%, 0,25%, 0,5% e 1% de cortiça no adesivo Araldite®AV138 para os ensaios DCB e ENF. As curvas P–δ obtidas numericamente para as juntas coladas com o adesivo Araldite® AV138 com concentrações de cortiça de 0%, 0,25%, 0,5% e 1% revelaram uma boa aproximação aos resultados experimentais. Os resultados obtidos com o modelo de dano coesivo triangular apresentam uma rigidez inicial semelhante ao valor experimental e valores numéricos de Pmáx bastante próximos do valor experimental. O comportamento das curvas P–δ durante a fase de propagação justifica-se pelo facto de a propagação do dano em ensaios DCB ser governada por GIc e em ensaios ENF por GIIc.
Adhesive joints have been highly developed over the years, and are currently a dominant joining technique in the aeronautical and automotive industry, where they aim to replace traditional methods. Due to the increasing use of adhesive joints, it is necessary to have strength prediction models that are reliable and robust. The Finite Element Method (FEM) has helped to develop this joining technique. A recent approach is the use of cohesive zone models (CZM), which allow to simulate the fracture behavior of the joints in a very reliable way. This technique requires the definition of cohesive laws in traction and shear. There are several forms of cohesive laws that can be applied in CZM simulations, depending on the expected behavior of the materials being simulated. A common method to improve the limitations of adhesives is the reinforcement of joints with particles to improve properties, in this case the reinforcement used is cork. Cork particles can create obstacles to the propagation of cracks, thus increasing the toughness of the adhesive. This is because cork has a remarkable combination of properties. In this work, the effect of adding cork particles on tensile and shear strength of a structural adhesive was numerically studied, consisting of the analysis of the results of maximum load (Pmax), maximum load displacement (δPmax) and maximum displacement (δmax). The FEM was used in conjunction with the CZM to model the behavior of the modified adhesive. Cork concentrations of 0%, 0,25%, 0,5% and 1% by weight are considered, in traction and shear. Double-Cantilever Beam tests were performed to characterize traction and End-Notched Flexure tests to characterize shear. Then, a study of the sensitivity of the cohesive parameters was carried out, which consists of verifying the influence of tn 0 , ts 0 , GIc and GIIc on the behavior of the numerical models of the DCB and ENF tests. ABSTRACT XII Caracterização das propriedades mecânicas de um adesivo estrutural reforçado com partículas de cortiça expandida MIGUEL FIGUEIRAS With this work it was possible to understand the influence of the addition of 0%, 0,25%, 0,5% and 1%, of cork in the Araldite®AV138 adhesive for the DCB and ENF tests. The P-δ curves obtained numerically for joints bonded with the Araldite® AV138 adhesive with cork concentrations of 0%, 0,25%, 0,5% and 1% showed a good approximation to the experimental results. The results obtained with the triangular cohesive damage model present an initial stiffness similar to the experimental value and numerical, Pmax values quite close to the experimental value. The behavior of P–δ curves during the propagation phase is justified by the fact that the damage propagation in DCB tests is governed by GIC and in ENF tests by GIIC.
Adhesive joints have been highly developed over the years, and are currently a dominant joining technique in the aeronautical and automotive industry, where they aim to replace traditional methods. Due to the increasing use of adhesive joints, it is necessary to have strength prediction models that are reliable and robust. The Finite Element Method (FEM) has helped to develop this joining technique. A recent approach is the use of cohesive zone models (CZM), which allow to simulate the fracture behavior of the joints in a very reliable way. This technique requires the definition of cohesive laws in traction and shear. There are several forms of cohesive laws that can be applied in CZM simulations, depending on the expected behavior of the materials being simulated. A common method to improve the limitations of adhesives is the reinforcement of joints with particles to improve properties, in this case the reinforcement used is cork. Cork particles can create obstacles to the propagation of cracks, thus increasing the toughness of the adhesive. This is because cork has a remarkable combination of properties. In this work, the effect of adding cork particles on tensile and shear strength of a structural adhesive was numerically studied, consisting of the analysis of the results of maximum load (Pmax), maximum load displacement (δPmax) and maximum displacement (δmax). The FEM was used in conjunction with the CZM to model the behavior of the modified adhesive. Cork concentrations of 0%, 0,25%, 0,5% and 1% by weight are considered, in traction and shear. Double-Cantilever Beam tests were performed to characterize traction and End-Notched Flexure tests to characterize shear. Then, a study of the sensitivity of the cohesive parameters was carried out, which consists of verifying the influence of tn 0 , ts 0 , GIc and GIIc on the behavior of the numerical models of the DCB and ENF tests. ABSTRACT XII Caracterização das propriedades mecânicas de um adesivo estrutural reforçado com partículas de cortiça expandida MIGUEL FIGUEIRAS With this work it was possible to understand the influence of the addition of 0%, 0,25%, 0,5% and 1%, of cork in the Araldite®AV138 adhesive for the DCB and ENF tests. The P-δ curves obtained numerically for joints bonded with the Araldite® AV138 adhesive with cork concentrations of 0%, 0,25%, 0,5% and 1% showed a good approximation to the experimental results. The results obtained with the triangular cohesive damage model present an initial stiffness similar to the experimental value and numerical, Pmax values quite close to the experimental value. The behavior of P–δ curves during the propagation phase is justified by the fact that the damage propagation in DCB tests is governed by GIC and in ENF tests by GIIC.
Description
Keywords
Juntas adesivas Lei coesiva Modelo de Danos Coesivo Método de Elementos Finitos Parâmetros coesivos Double-Cantilever Beam End-Notched Flexure Adhesive joints Cohesive law Cohesive Damage Model Finite Element Method Cohesive parameters