Relação da extensão de dano interlaminar e variação da orientação de fibra com sinal acústico: carregamentos quase-estático e cíclico.
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Data
2021-11-25
Autores
Motta Júnior, Roberto Ferreira
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
Materiais compósitos de matrizes poliméricas reforçados por fibras possuem grande potencial para substituir os metais em aplicações estruturais em que o objetivo seja a obtenção de estruturas leves devido suas elevadas propriedades mecânicas específicas. Os compósitos poliméricos reforçados por fibras voltados para aplicação estrutural são compostos por camadas de fibras (laminados). Portanto, não possuem reforço na direção da espessura, o que reduz a resistência interlaminar do material, tornando-o suscetível a delaminação. Consequentemente, pesquisas têm sido realizadas para uma melhor compreensão do fenômeno e o desenvolvimento de modelos capazes de prever a delaminação em regime cíclico. Entretanto, o foco das pesquisas tem sido restrito ao desenvolvimento de modelos, enquanto o entendimento físico associado ao fenômeno se tornou um objetivo secundário. Portanto, esta pesquisa tem o foco no estudo da delaminação no curso do ciclo de carregamento em fadiga. A técnica de emissão acústica foi utilizada para avaliação da delaminação e para o desenvolvimento de uma metodologia para quantificar a liberação de energia de deformação elástica devido a propagação de dano. Além disso, o desenvolvimento de pontes de fibra durante a propagação da delaminação por interfaces com diferentes configurações foi avaliado com foco na influência desses mecanismos na liberação de energia. Os resultados mostraram que uma elevada razão de carregamento aumenta o tempo em que o ciclo permanece acima do limite de energia necessário para propagação de dano (Uth) em termos de energias totais, afetando a distribuição de dano ao longo do ciclo de carregamento. A liberação de energia de deformação ao longo do ciclo de carregamento indicou que diferentes mecanismos de dano são ativados em diferentes regiões do ciclo pois necessitam de específicos níveis de energia para ocorrerem. A presença de múltiplos limites de energia (Uth) associados a diferentes mecanismos de dano significa que o material pode apresentar diferentes resistências à delaminação. Esta variação se deve a ativação de diferentes mecanismos de acordo com o ciclo de carregamento aplicado dependendo da quantidade de Uth ultrapassados. Por exemplo, a ruptura de pontes de fibra pode afetar a liberação de energia de deformação quando as condições mínimas para a ativação desse mecanismo são excedidas. Foi observado que o aumento do ângulo (α) das fibras na interface em que a trinca se propaga eleva as tensões atuantes na fibra, aumentando o número de eventos de ruptura de pontes de fibra. Como esse mecanismo é associado a uma liberação de energia de deformação, a resistência do material é alterada.
Fiber-reinforced polymer (FRP) composites have a great potential to replace metals in applications that require lighter components and structures since these materials can reach high in-plane properties of specific strength and stiffness. The FRP composites frequently used in engineering structures are laminates comprised of continuous-fiber plies without reinforcement in the thickness direction, which reduces the material’s interlaminar strength leading to delamination susceptibility. Most of these structures operate under long-term cyclic loadings, resulting in a gradual delamination propagation. Hence, extensive research has been conducted to understand and predict fatigue delamination growth in FRPs over the past decades. However, most of the efforts were concentrated on the prediction itself rather than attributing physical explanations to the mechanisms associated with the propagation process. In order to contribute to this field, this research focuses on the assessment of delamination within a single loading cycle in FRP using double cantilever beam specimens with varying stress ratios (R). The acoustic emission (AE) technique was used to investigate damage propagation, and a new methodology was developed to quantify the strain energy release due to crack growth in fatigue. In addition, the development of fiber bridging in the crack propagation through different interface configurations was investigated with focus on the influence of bridging on the strain energy released. Results showed that under high R-ratios, the load cycle spends an increased time above the threshold energy (Uth: minimum amount of energy required to damage development) in terms of total strain energy, which affected the damage distribution within a single loading cycle. Besides that, the strain energy release behavior within the fatigue cycles indicated that different damage mechanisms are activated in different increments of the load cycle associated with different energy thresholds. The presence of multiple energy thresholds indicated that the application of different loading cycles results in distinct resistances to damage propagation (dU/dA) depending on which energy threshold is crossed. For example, the rupture of bridging fibers may impact dU/dA when the threshold energy to activate this damage mechanism is exceeded. It was observed that the angle of the fibers (α) in the interface where the crack propagates affected the stresses acting on the bridging fibers, leading to the rupture of more fibers when α was increased. In other words, α eases the activation of this specific damage mechanism. Hence, once fiber breakage releases strain energy, the material resistance to delamination growth is affected.
Fiber-reinforced polymer (FRP) composites have a great potential to replace metals in applications that require lighter components and structures since these materials can reach high in-plane properties of specific strength and stiffness. The FRP composites frequently used in engineering structures are laminates comprised of continuous-fiber plies without reinforcement in the thickness direction, which reduces the material’s interlaminar strength leading to delamination susceptibility. Most of these structures operate under long-term cyclic loadings, resulting in a gradual delamination propagation. Hence, extensive research has been conducted to understand and predict fatigue delamination growth in FRPs over the past decades. However, most of the efforts were concentrated on the prediction itself rather than attributing physical explanations to the mechanisms associated with the propagation process. In order to contribute to this field, this research focuses on the assessment of delamination within a single loading cycle in FRP using double cantilever beam specimens with varying stress ratios (R). The acoustic emission (AE) technique was used to investigate damage propagation, and a new methodology was developed to quantify the strain energy release due to crack growth in fatigue. In addition, the development of fiber bridging in the crack propagation through different interface configurations was investigated with focus on the influence of bridging on the strain energy released. Results showed that under high R-ratios, the load cycle spends an increased time above the threshold energy (Uth: minimum amount of energy required to damage development) in terms of total strain energy, which affected the damage distribution within a single loading cycle. Besides that, the strain energy release behavior within the fatigue cycles indicated that different damage mechanisms are activated in different increments of the load cycle associated with different energy thresholds. The presence of multiple energy thresholds indicated that the application of different loading cycles results in distinct resistances to damage propagation (dU/dA) depending on which energy threshold is crossed. For example, the rupture of bridging fibers may impact dU/dA when the threshold energy to activate this damage mechanism is exceeded. It was observed that the angle of the fibers (α) in the interface where the crack propagates affected the stresses acting on the bridging fibers, leading to the rupture of more fibers when α was increased. In other words, α eases the activation of this specific damage mechanism. Hence, once fiber breakage releases strain energy, the material resistance to delamination growth is affected.
Descrição
Palavras-chave
Fractografia, Mecânica da fratura, Materiais laminados, Stress ratio, Fractography, Fracture mechanics, Laminates, Razão de carregamento