Caracterização numérico-experimental de uma ligação roscada em tecido ósseo cortical
Ficheiros
Data
2017-05-26
Título da revista
ISSN da revista
Título do Volume
Editora
Resumo
Nesta dissertação, apresenta-se um estudo numérico-experimental do desempenho de ligações aparafusadas em tecido ósseo cortical, submetidas a esforços de arrancamento. O estudo tem como objetivo analisar o efeito de alguns parâmetros geométricos da ligação aparafusada, designadamente o passo e a configuração do filete de rosca do parafuso, para o mesmo diâmetro nominal. O efeito do passo da rosca avalia-se mediante a comparação de dois parafusos de rosca triangular de métrica normal e de rosca triangular de métrica fina. A avaliação da influência do perfil da rosca realizou-se comparando o desempenho da ligação dos dois parafusos de rosca triangular, com um parafuso, cujo perfil de rosca é adaptado ao perfil de rosca dente-de-serra. Nesta avaliação, medem-se parâmetros como a rigidez e a carga máxima, através de ensaios experimentais realizados em provetes de contraplacado marítimo (material substituto do osso), com configuração regular (quadrados), com a mesma orientação. Os resultados experimentais mostram que os dois parafusos de rosca triangular de métrica normal e de métrica fina apresentam valores muito semelhantes no que se refere à carga máxima e à rigidez, o que indica que a diminuição da altura do filete de rosca não acrescenta ganhos para a ligação. Já o parafuso de rosca adaptado ao dente-de-serra apresenta valores de carga máxima superiores, contudo não muito significativos comparativamente com os restantes parafusos, indicando no entanto que a mudança para este tipo de perfil de rosca promove uma ligação roscada mais eficiente.
O estudo da caracterização do arrancamento de ligações aparafusadas em provetes regulares, constituídos por um material pouco estudado implica a determinação das respetivas propriedades elásticas. Deste modo, importa realizar ensaios de flexão em três pontos para a determinação do módulo de elasticidade longitudinal.
Numericamente, desenvolveram-se modelos de elementos finitos tridimensionais das ligações aparafusadas em estudo, com o propósito de simular o ensaio mecânico, combinando elementos finitos de interface com superfícies de contacto. Verificou-se que o comportamento global pode ser reproduzido pela lei de dano coesivo de configuração trapezoidal. A alteração das configurações geométricas dos parafusos não influencia a dimensão da zona de processo de fratura. Relativamente à validação dos modelos numéricos verifica-se que os ensaios resultam do efeito combinado de tensões normais e de corte. As tensões normais nas diferentes ligações roscadas indicam uma região submetida à tração na extremidade próxima do ponto de aplicação da carga de arrancamento, e uma região sob compressão na extremidade oposta. Nas tensões de corte é possível visualizar uma simetria em relação ao eixo do parafuso.
A análise das tensões ao longo do comprimento de rosca indica que o início de dano acontece a meio da ligação roscada, com o contributo de tensões em modo II, e que na região próxima do ponto de aplicação da carga ocorre com o contributo de tensões em modo I. Constata-se que a propagação do dano acontece em modo misto.
This dissertation presents a numerical and experimental study of the performance of bolted connections in cortical bone, undergoing pullout efforts. The aim of this study is to analyze the effect of some geometrical parameters of bolted connection, namely the pitch and the screw thread configuration, for the same nominal diameter. The effect of the thread pitch is evaluated by comparing two triangular threaded screws, in particular normal metric thread and triangular thin metric. The evaluation of the thread profile was executed comparing the performance of the connection of the two triangular-thread screws with a screw whose thread profile is adapted to the profile of buttress thread In this evaluation, stiffness and maximum load were evaluated through experimental tests performed on specimens of plywood (bone substitute material), with a regular configuration (square) presenting the same orientation. The experimental results have shown that the two triangular threaded screws, normal and fine metric ones have very similar values in terms of maximum load and stiffness, which indicates that the decrease of the screw thread height does not lead to enhance the bolt power of the studied connection. Also the buttress thread screw shows higher values though not very much meaningful in regards to maximum load for the remaining screws. However, one can conclude that the shift to this type of thread profile promotes a more efficient threaded connection. The characterization of the pullout bolted connections on regular specimens made on a material that is not regularly studied implies the determination of the respective elastic properties. Thus, the determination of the longitudinal elastic module has been performed by means of three point bending tests. Numerically, three-dimensional finite element models of the bolted connections have been made in order to simulate the pull out test of a bolted joint, combining cohesive zone models with contact surfaces. It was found that the overall behavior was fully reproduced by a trapezoidal cohesive law. It was observed that the modification of the geometric configurations of the screws does not influence the size of the fracture process zone. Hence, the numerical analysis revealed that tensile stresses are developed in the regions closer to the applied pull out load, while compressive stresses occurred in the opposing regions. Regarding the shear stresses, a symmetric distribution relatively to the screw axis was observed. The stress analysis along the thread length revealed that damage onset occurs in the middle of the treading joint, under mode II loading. In the threading region closer to the pull out load the opening mode (mode I) was dominant in the damage onset. It was observed that damage propagation occur under mixed-mode loading.
This dissertation presents a numerical and experimental study of the performance of bolted connections in cortical bone, undergoing pullout efforts. The aim of this study is to analyze the effect of some geometrical parameters of bolted connection, namely the pitch and the screw thread configuration, for the same nominal diameter. The effect of the thread pitch is evaluated by comparing two triangular threaded screws, in particular normal metric thread and triangular thin metric. The evaluation of the thread profile was executed comparing the performance of the connection of the two triangular-thread screws with a screw whose thread profile is adapted to the profile of buttress thread In this evaluation, stiffness and maximum load were evaluated through experimental tests performed on specimens of plywood (bone substitute material), with a regular configuration (square) presenting the same orientation. The experimental results have shown that the two triangular threaded screws, normal and fine metric ones have very similar values in terms of maximum load and stiffness, which indicates that the decrease of the screw thread height does not lead to enhance the bolt power of the studied connection. Also the buttress thread screw shows higher values though not very much meaningful in regards to maximum load for the remaining screws. However, one can conclude that the shift to this type of thread profile promotes a more efficient threaded connection. The characterization of the pullout bolted connections on regular specimens made on a material that is not regularly studied implies the determination of the respective elastic properties. Thus, the determination of the longitudinal elastic module has been performed by means of three point bending tests. Numerically, three-dimensional finite element models of the bolted connections have been made in order to simulate the pull out test of a bolted joint, combining cohesive zone models with contact surfaces. It was found that the overall behavior was fully reproduced by a trapezoidal cohesive law. It was observed that the modification of the geometric configurations of the screws does not influence the size of the fracture process zone. Hence, the numerical analysis revealed that tensile stresses are developed in the regions closer to the applied pull out load, while compressive stresses occurred in the opposing regions. Regarding the shear stresses, a symmetric distribution relatively to the screw axis was observed. The stress analysis along the thread length revealed that damage onset occurs in the middle of the treading joint, under mode II loading. In the threading region closer to the pull out load the opening mode (mode I) was dominant in the damage onset. It was observed that damage propagation occur under mixed-mode loading.
Descrição
Dissertação de Mestrado em Engenharia Mecânica
Palavras-chave
Tecido ósseo cortical , Ligação roscada