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Obtenção das leis coesivas em modo misto de adesivos estruturais em função da espessura de adesivo
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Abstract(s)
A utilização dos adesivos em construções industriais tem vindo nos últimos anos a aumentar em larga escala, em substituição dos métodos tradicionais como a soldadura, brasagem, e ligações mecânicas como aparafusamentos e rebitagens. Esta tendência verifica-se devido às elevadas vantagens que este tipo de ligação oferece, dos quais se destacam a leveza, o bom comportamento sob cargas cíclicas ou fadiga, e sem dúvida a maior de todos, permitem a ligação de diferentes materiais e menores concentrações de tensões. No sentido de se aumentar a confiança na projecção de juntas adesivas, é importante conseguir prever com a máxima precisão a sua resistência mecânica e respectivas propriedades de fractura (taxa critica de libertação de energia de deformação à tracção e ao corte, GIC GIIC respectivamente). Estas duas propriedades relacionam-se com a mecânica da fractura e são estimadas através de uma análise energética. Para esse efeito existem três modelos distintos, sendo os que necessitam de medição do comprimento de fenda durante a propagação do dano, o modelo que utiliza o comprimento de fenda equivalente e o modelo baseado no integral J. Na maioria dos casos as solicitações ocorrem em modo misto (combinação de tracção e corte em simultâneo), a importância da fractura deve ser entendida nestas condições, nomeadamente das taxas de libertação de energia relativamente a diferentes critérios e envelopes de fractura. Esta comparação possibilita averiguar qual o critério energético de ruptura mais preciso a utilizar em modelos numéricos baseados em Modelos do Dano Coesivo. No presente trabalho são apresentados os dois estudos efectuados, experimental e numérico utilizando o ensaio Single-Leg Bending (SLB) em provetes colados com o adesivo Sikaforce®-7752, com diferentes espessuras. Para isso, são aplicados alguns modelos de redução da taxa de libertação de energia de deformação à tracção GI, e corte, GII, enquadrados nos modelos que necessitam da medição do comprimento de fenda e nos modelos de que utilizam um comprimento de fenda equivalente (CBBM). Na fase seguinte, procedeu-se à análise dos valores obtidos experimentalmente GI e GII de cada espessura de adesivo. Na discussão de resultados foi efectuada uma análise dos valores em diversos envelopes de fractura, no sentido de se conseguir averiguar qual o critério de ruptura mais adequado a considerar para ta de 1 mm. Na análise dos dados experimentais obtidos através dos vários modelos, conseguiu-se uma concordância bastante boa entre modelos de determinação de GI e GII. Nos ensaios numéricos foi possível retractar o comportamento verificado nos ensaios experimentais, apresentando estes, critérios de propagação aproximados relativamente aos resultados experimentais.
The use of adhesives in industrial construction has come in the last years growing up, in replacement of traditional bonding methods which welded, brazed, riveted and bolted joints. This trends is due to the many advantages of adhesive joints, like as allowing the construction of lighter structures, more resistance to fatigue, lower stress concentrations, and the most important, the possibility to joint different materials. With the point of increase the trust in adhesive joints, it’s important to prevent with precise the maximum strength and their mechanical properties to fracture (GIC, GIIC). This two properties are related with mechanical of fracture and can be calculated through energetic analyses. For this purpose, several fracture characterization tests were developed in the various fracture modes, which are based on the concepts of linear elastic fracture mechanics to determine the rate of strain energy release. In the most of the cases occurs mixed mode (combination of traction and lap shear simultaneously), a fracture propagation have to be understood in this conditions, such an energy release rates for different fracture criteria or envelopes. This allows the understanding of fracture energetic criteria more precise and the utilization of numeric models based on CZM. In this work are presented numeric and experimental analyzes using Single-Leg Bending (SLB) tests in bonding specimens with Sikaforce®-7752 using different thickness. For that, was applied some reduction models of rate of strain energy release deformation to traction GI, and shear GII, related with models that need of fracture measure length and aeq for the CBBM. In the next step was made in the experimental results an analyze value of GI e GII of each thickness. After this analysis a verification was made of the values in the fracture envelopes for 1 mm of thickness, because is the fracture propagation that could be compered. In the results discussion was made an envelope fracture values analyze, to can get conclude the most appropriate fracture criteria to ta of 1 mm. Overall, it was possible to obtained an agreement between methods for the determination of GI and GII. In the numerical simulations it was possible to reproduce the observed behavior of the experimental tests, they show an equal propagation criteria in compered to the experimental results obtained.
The use of adhesives in industrial construction has come in the last years growing up, in replacement of traditional bonding methods which welded, brazed, riveted and bolted joints. This trends is due to the many advantages of adhesive joints, like as allowing the construction of lighter structures, more resistance to fatigue, lower stress concentrations, and the most important, the possibility to joint different materials. With the point of increase the trust in adhesive joints, it’s important to prevent with precise the maximum strength and their mechanical properties to fracture (GIC, GIIC). This two properties are related with mechanical of fracture and can be calculated through energetic analyses. For this purpose, several fracture characterization tests were developed in the various fracture modes, which are based on the concepts of linear elastic fracture mechanics to determine the rate of strain energy release. In the most of the cases occurs mixed mode (combination of traction and lap shear simultaneously), a fracture propagation have to be understood in this conditions, such an energy release rates for different fracture criteria or envelopes. This allows the understanding of fracture energetic criteria more precise and the utilization of numeric models based on CZM. In this work are presented numeric and experimental analyzes using Single-Leg Bending (SLB) tests in bonding specimens with Sikaforce®-7752 using different thickness. For that, was applied some reduction models of rate of strain energy release deformation to traction GI, and shear GII, related with models that need of fracture measure length and aeq for the CBBM. In the next step was made in the experimental results an analyze value of GI e GII of each thickness. After this analysis a verification was made of the values in the fracture envelopes for 1 mm of thickness, because is the fracture propagation that could be compered. In the results discussion was made an envelope fracture values analyze, to can get conclude the most appropriate fracture criteria to ta of 1 mm. Overall, it was possible to obtained an agreement between methods for the determination of GI and GII. In the numerical simulations it was possible to reproduce the observed behavior of the experimental tests, they show an equal propagation criteria in compered to the experimental results obtained.
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Keywords
Single-leg bending Ligação adesiva Mecânica da fratura Espessura de adesivo Ductilidade Envelope de fratura Método dos elementos finitos Modelo de dano coesivo Adhesive joint Fracture mechanics Ductile Adhesive thickness Fracture envelope Finite element method Cohesive zone models