Publication: Criterios de fallo aplicables a polímeros de uso estructural sometidos a cargas impulsivas
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2017-03
Defense date
2017-03-24
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Abstract
El uso de materiales poliméricos para el diseño de componentes mecánicos
que durante su vida en servicio pueden ser susceptibles de estar sometidos
a cargas dinámicas y de impacto, ha sufrido un incremento en las últimas
décadas debido, entre otros, a la resistencia al impacto que presentan este tipo
de materiales, frente a otros de mayor peso y coste. En esta Tesis Doctoral se
presenta un modelo de daño capaz de predecir y reproducir la transición en el
modo de fallo que se produce en algunos de estos materiales.
El modelo de daño propuesto es independiente del modelo constitutivo, y
considera la competencia entre dos modos de fallo que actúan simultáneamente:
uno frágil y otro dúctil. El criterio de fallo frágil se ha formulado de forma local
y está basado en la tensión principal máxima que se alcanza en el material,
estableciéndose la fractura del mismo en el instante en que dicha tensión principal
máxima alcanza un valor crítico, el cual se considera dependiente de la
velocidad de deformación. Por otro lado, el criterio de fallo dúctil establece la
degradación del material en función de su densidad de energía de deformación,
permitiendo considerar un daño dúctil progresivo en el material. Este modelo
de daño se ha implementado mediante una subrutina de usuario en un código
comercial de Elementos Finitos.
La verificación del modelo de daño propuesto se ha realizado utilizando
como polímero de referencia el Policarbonato, ya que recoge la transición en
el modo de fallo de forma natural en condiciones normales de presión y temperatura.
Para ello, se ha llevado a cabo una amplia campaña experimental
que incluye ensayos de caracterización y ensayos de validación que permiten
contrastar las predicciones numéricas con su correspondiente resultado
experimental, obteniéndose una buena correlación entre ambos. A raíz de esta
comparación de resultados, se ha realizado un análisis detallado del proceso
de transición en el modo de fallo, destacando la necesidad de implementar
simultáneamente ambos criterios de daño.
The use of polymeric materials in the design of mechanical components subjected to impact and dynamic loads, has increased in the last decades due to their impact resistance behavior, compared to other materials with greater weight and cost. In this PhD Thesis is presented a damage model capable of predicting and reproducing the failure mode transition that may occurs in some of these materials. The proposed damage model is independent of the constitutive model, and considers the competition between two modes of failure acting simultaneously, one brittle and the other ductile. The criterion of brittle fracture has been formulated locally and is based on the maximum main stress on the material, establishing the fracture when that maximum principal stress reaches a critical value, which is dependent on the strain rate. On the other hand, the ductile failure criterion establishes the degradation of the material as a function of its strain energy density, allowing a progressive ductile damage to be considered in the material. This damage model has been implemented by means of a user subroutine in a Finite Element code. The verification of the proposed damage model was carried out using Polycarbonate as reference polymer, since it naturally exhibits failure mode transition under normal conditions of pressure and temperature. To this end, an extensive experimental campaign has been carried out, which includes characterization tests and validation tests that allow the comparison of numerical predictions with their corresponding experimental results, obtaining a good correlation between both. As a result of this comparison, a detailed analysis of the transition process in the failure mode has been performed, highlighting the need to simultaneously implement both failure criteria
The use of polymeric materials in the design of mechanical components subjected to impact and dynamic loads, has increased in the last decades due to their impact resistance behavior, compared to other materials with greater weight and cost. In this PhD Thesis is presented a damage model capable of predicting and reproducing the failure mode transition that may occurs in some of these materials. The proposed damage model is independent of the constitutive model, and considers the competition between two modes of failure acting simultaneously, one brittle and the other ductile. The criterion of brittle fracture has been formulated locally and is based on the maximum main stress on the material, establishing the fracture when that maximum principal stress reaches a critical value, which is dependent on the strain rate. On the other hand, the ductile failure criterion establishes the degradation of the material as a function of its strain energy density, allowing a progressive ductile damage to be considered in the material. This damage model has been implemented by means of a user subroutine in a Finite Element code. The verification of the proposed damage model was carried out using Polycarbonate as reference polymer, since it naturally exhibits failure mode transition under normal conditions of pressure and temperature. To this end, an extensive experimental campaign has been carried out, which includes characterization tests and validation tests that allow the comparison of numerical predictions with their corresponding experimental results, obtaining a good correlation between both. As a result of this comparison, a detailed analysis of the transition process in the failure mode has been performed, highlighting the need to simultaneously implement both failure criteria
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Keywords
Polímeros, Policarbonato, Ensayo de materiales, Fractura, Mecánica de sólidos, Modelo de daño, Análisis numérico