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Advisor(s)
Abstract(s)
A utilização de ligações adesivas tem vindo a aumentar progressivamente no âmbito de aplicações estruturais comparativamente às ligações mecânicas convencionais. Esta mudança de padrão de ligação deve-se maioritariamente às vantagens oferecidas pelas ligações adesivas. Com a evolução da indústria, é sempre necessário estudar e melhorar o produto. Para este efeito, podem-se usar técnicas como os Modelos de Dano Coesivo (MDC), aliados ao Método de Elementos Finitos (MEF), para prever a resistência de juntas adesivas. Os MDC utilizam critérios de resistência dos materiais e conceitos da mecânica da fratura para prever a iniciação do dano e para a propagação da fenda, respetivamente. Nas simulações por MDC existem várias formas de leis coesivas que podem ser usadas para modelar a camada de adesivo, conforme o material utilizado e o seu comportamento expectável. Neste âmbito, é importante a existência de métodos de análise que permitam determinar estas leis de forma fiável e que estas sejam validadas para aplicação em projeto de estruturas. Nesta dissertação estudou-se numericamente o efeito da espessura do adesivo na sua tenacidade à tração pelo ensaio Double-Cantilever Beam (DCB). De facto, as propriedades coesivas dos adesivos variam bastante com a espessura do adesivo e dos aderentes a ligar, pelo que é de extrema importância conhecer estes efeitos. Foram utilizados dados experimentais já disponíveis para obter as leis coesivas do adesivo pelo método direto. Foi realizada uma modelação numérica por MDC, recorrendo à lei triangular, para reproduzir o comportamento do adesivo e posteriormente comparar com o comportamento verificado nos ensaios experimentais fornecidos. Por fim, foram inseridas variações nos parâmetros coesivos de forma a verificar a sua influência no comportamento do adesivo. O estudo revelou uma grande proximidade entre os dados experimentais disponibilizados e as modelações realizadas, validando as leis coesivas do adesivo e a forma de lei utilizada. Foi também possível verificar a influência dos parâmetros coesivos nos resultados da simulação, bem como a relativa importância de cada um no comportamento dos provetes à tração.
The use of adhesive joints has been progressively increasing in the scope of structural applications compared to conventional mechanical connections. This change in joint pattern is mainly due to the advantages offered by adhesive joints. With the evolution of the industry, it is always necessary to study and improve the product. For this purpose, techniques such as Cohesive Zone Models (CZM), combined with the Finite Element Method (FEM), can be used to predict the strength of adhesive joints. CZM use strength of materials criteria and concepts of fracture mechanics to predict the initiation of damage and the crack propagation, respectively. In CZM simulations, there are several cohesive law shapes that can be used to model the adhesive layer, depending on the material used and its expected behaviour. In this context, it is important to have methods of analysis that allow these laws to be reliably determined and that these are validated for application in structural design. In this thesis, the adhesive thickness effect on its tensile toughness was numerically studied by the Double-Cantilever Beam (DCB) test. In fact, the cohesive properties of adhesives widely vary with the adhesive thickness and the adherends to be bonded, so it is extremely important to know these effects. Experimental data already available were used to obtain the cohesive laws of the adhesive by the direct method. Numerical modeling by CZM was undertaken, using the triangular law, to reproduce the behavior of the adhesive and then compare it with the behavior verified in the provided experimental tests. Finally, variations in the cohesive parameters were tested in order to verify their influence on the behavior of the adhesive. The study revealed a good correspondence between the experimental data made available and the models performed, validating the cohesive laws of the adhesive and the used law shape. It was also possible to verify the influence of the cohesive parameters on the simulation results, as well as the relative importance of each one in the tensile behavior of the specimens.
The use of adhesive joints has been progressively increasing in the scope of structural applications compared to conventional mechanical connections. This change in joint pattern is mainly due to the advantages offered by adhesive joints. With the evolution of the industry, it is always necessary to study and improve the product. For this purpose, techniques such as Cohesive Zone Models (CZM), combined with the Finite Element Method (FEM), can be used to predict the strength of adhesive joints. CZM use strength of materials criteria and concepts of fracture mechanics to predict the initiation of damage and the crack propagation, respectively. In CZM simulations, there are several cohesive law shapes that can be used to model the adhesive layer, depending on the material used and its expected behaviour. In this context, it is important to have methods of analysis that allow these laws to be reliably determined and that these are validated for application in structural design. In this thesis, the adhesive thickness effect on its tensile toughness was numerically studied by the Double-Cantilever Beam (DCB) test. In fact, the cohesive properties of adhesives widely vary with the adhesive thickness and the adherends to be bonded, so it is extremely important to know these effects. Experimental data already available were used to obtain the cohesive laws of the adhesive by the direct method. Numerical modeling by CZM was undertaken, using the triangular law, to reproduce the behavior of the adhesive and then compare it with the behavior verified in the provided experimental tests. Finally, variations in the cohesive parameters were tested in order to verify their influence on the behavior of the adhesive. The study revealed a good correspondence between the experimental data made available and the models performed, validating the cohesive laws of the adhesive and the used law shape. It was also possible to verify the influence of the cohesive parameters on the simulation results, as well as the relative importance of each one in the tensile behavior of the specimens.
Description
Keywords
Junta adesiva Adesivo estrutural Ensaios de fratura Tenacidade à fratura Método de Elementos Finitos Modelos de dano coesivo Adhesive joint Structural adhesive Fracture tests Fracture toughness Finite Element Method Cohesive zone models