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Abstract(s)
A utilização de juntas adesivas em aplicações industriais tem vindo a aumentar, em detrimento dos
métodos tradicionais tais como a soldadura, brasagem e ligações aparafusadas e rebitadas. Este facto deve-se às vantagens que estas oferecem, como o facto de serem mais leves, comportarem-se bem sob cargas cíclicas ou de fadiga, a ligação de materiais diferentes e menores concentrações de tensões. Para aumentar a confiança no projeto de estruturas adesivas, é importante conseguir prever com precisão a sua resistência mecânica e respetivas propriedades de fratura (taxa crítica de libertação de energia de deformação à tração, GIC, e corte, GIIC). Estas propriedades estão diretamente relacionadas com a Mecânica da Fratura e são estimadas através de uma análise energética. Para este efeito, distinguem-se três tipos de modelos: modelos que necessitam da medição do comprimento de fenda durante a propagação do dano, modelos que utilizam um comprimento de fenda equivalente e métodos baseados no integral J. Como na maioria dos casos as solicitações ocorrem em modo misto (combinação de tração com corte), é de grande importância a perceção da fratura nesta condições, nomeadamente das taxas de libertação de energia relativamente a diferentes critérios ou envelopes de fratura. Esta comparação permite, por exemplo, averiguar qual o melhor critério energético de rotura a utilizar em modelos numéricos baseados em Modelos de Dano Coesivo.
Neste trabalho é realizado um estudo experimental utilizando o ensaio Single-Leg Bending (SLB) em provetes colados com três tipos de adesivos, de forma a estudar e comparar as suas propriedades de fratura. Para tal, são aplicados alguns modelos de redução da taxa de libertação de energia de deformação à tração, GI, e corte, GII, enquadrados nos modelos que necessitam da medição do comprimento de fenda e nos modelos que utilizam um comprimento de fenda equivalente. Numa fase posterior, procedeu-se à análise e comparação dos resultados adquiridos durante a fase experimental de GI e GII de cada adesivo. A discussão de resultados foi também feita através da análise dos valores obtidos em diversos envelopes de fratura, no sentido de averiguar qual o critério de rotura mais adequado a considerar para cada adesivo.
Foi obtida uma concordância bastante boa entre métodos de determinação de GI e GII, com exceção
do adesivo mais dúctil, para o qual o método baseado no comprimento de fenda equivalente apresentou resultados ligeiramente superiores.
The industrial applications of bonded joints have been increasing, instead of more traditional methods like welding, brazing, and bolted and riveted connections. This is due to the advantages offered by bonded joints, opposed to the other joining methods, such as less weight, better behavior under cyclical or fatigue loads, enabling joining different materials and smaller stress concentrations. In order to increase the confidence in the design of adhesive structures, it is important to accurately predict their strength and respective fracture properties (critical strain energy release rate in tension, GIC, and shear, GIIC). These properties are directly related with Fracture Mechanics and are estimated by an energetic analysis. Three types of data reduction models are available: models that require the measurement of crack length, models that use an equivalent crack length and models based on the J integral. Since bonded joints are typically loaded in mixed-mode (combination of tensile and shear loads), it is highly important the perception of the fracture behavior under these conditions, namely the strain energy release rates with respect to different criteria or fracture envelopes. This comparison allows, for example, the investigation of the most suited energetic criterion to use in numeric models based on cohesive damage models. This work consists of an experimental study using Single-Leg Bending (SLB) tests in specimens bonded by three kinds of adhesives, to compare their fracture properties. With this purpose, different models are applied to obtain strain energy release rate in tension, GI, and shear, GII, either requiring the measurement of the crack length or based on the use of an equivalent crack length. Afterwards, a comparison of the obtained GI and GII results of each adhesive is performed. The results were also plotted against several fracture envelopes to understand which is the most adequate criterion to be used for each adhesive. A very good agreement was found between GI and GII estimation methods, apart from the most ductile adhesive, for which the method based on the equivalent crack presented slightly higher results.
The industrial applications of bonded joints have been increasing, instead of more traditional methods like welding, brazing, and bolted and riveted connections. This is due to the advantages offered by bonded joints, opposed to the other joining methods, such as less weight, better behavior under cyclical or fatigue loads, enabling joining different materials and smaller stress concentrations. In order to increase the confidence in the design of adhesive structures, it is important to accurately predict their strength and respective fracture properties (critical strain energy release rate in tension, GIC, and shear, GIIC). These properties are directly related with Fracture Mechanics and are estimated by an energetic analysis. Three types of data reduction models are available: models that require the measurement of crack length, models that use an equivalent crack length and models based on the J integral. Since bonded joints are typically loaded in mixed-mode (combination of tensile and shear loads), it is highly important the perception of the fracture behavior under these conditions, namely the strain energy release rates with respect to different criteria or fracture envelopes. This comparison allows, for example, the investigation of the most suited energetic criterion to use in numeric models based on cohesive damage models. This work consists of an experimental study using Single-Leg Bending (SLB) tests in specimens bonded by three kinds of adhesives, to compare their fracture properties. With this purpose, different models are applied to obtain strain energy release rate in tension, GI, and shear, GII, either requiring the measurement of the crack length or based on the use of an equivalent crack length. Afterwards, a comparison of the obtained GI and GII results of each adhesive is performed. The results were also plotted against several fracture envelopes to understand which is the most adequate criterion to be used for each adhesive. A very good agreement was found between GI and GII estimation methods, apart from the most ductile adhesive, for which the method based on the equivalent crack presented slightly higher results.
Description
Keywords
Single-Leg Bending Ligação adesiva Adesivo estrutural Mecânica da fratura Tenacidade à fratura Envelope de fratura Adhesive joint Structural adhesive Fracture mechanics Fracture toughness Fracture envelope