The application of advanced composite materials, such as graphite/epoxy, has been on the rise for the last four decades. The mechanical advantages, such as their higher specific stiffness and strength as compared to monolithic materials, make them attractive for aerospace and automotive applications. Despite these advantages, composites with brittle fibers have lower ductility and fracture toughness than monolithic materials. One way to increase the fracture toughness of composites is to have a weak fiber-matrix interface that would blunt crack tips by crack deflection into the interface and hence enhance fracture toughness. However, this also reduces the transverse properties of the composite. Therefore, an optimum fiber-matrix interface would be the one that is just weak enough to cause crack deflection into interface. This study investigates the effect of fiber-to-matrix moduli ratio, fiber-volume fraction, fiber orthotropy, and thermal stresses on the possibility of crack deflection. A finite element model is used to analyze a 2-D axisymmetric representative volume element- a three-phase composite cylinder made of fiber, matrix, and composite. A penny shaped crack is assumed in the fiber. To determine whether the crack would deflect into the interface or propagate into the matrix, maximum stresses at the fiber-matrix interface and in the matrix are compared to the interface and matrix strengths. As opposed to most studies in the literature, this study found that fiber-volume fractions do have an impact on crack deflection and this impact increases with large fiber-to-matrix moduli ratios. The presence of orthotropic fiber in the composite increases the possibility of crack deflection with increasing fibervolume fraction in the early and middle stages of the fiber crack growth. The thermal stresses decrease the likelihood of crack deflection when the thermal expansion coefficient of the matrix is larger than that of the fiber.

The use of composite materials brought a tremendous breakthrough in the scientific world of aerospace engineering. The lack of understanding of the failure of composite materials can be disastrous. Composite laminated structures need to be thoroughly studied and investigated in the design stage. In this thesis, formed-hat skin stringer made of composite laminates is investigated. Delamination is the most common failure of laminated composites, which has two stages delamination onset and delamination propagation. In the preliminary design phase, firstly the structures need to be investigated for low-velocity impact to check the formation of damage onset due to the impact that may arise during manufacturing. In the detailed design phase, the structure is investigated to study the evolution of delamination growth under loading conditions. The structure is modeled using 3 D elements because of the presence of Interlaminar stresses in the width and thickness direction and anisotropic nature. In this thesis, more emphasis is given on the interface between the skin and the stringer. The debonding effect of the interface is studied using cohesive zone model(CZM).

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Este trabalho apresenta um estudo sobre o fenômeno do deslizamento na interface entre concreto e polímero reforçado com fibra (PRF) de carbono por meio da realização e análise de ensaios experimentais. O objetivo geral é investigar o efeito de escala no descolamento entre concreto e PRF. Buscou-se analisar a influência da variação da dimensão do corpo de prova de concreto no colapso de interface ao manter a mesma geometria de PRF aderida. A metodologia adotada consiste em sobrepor dois corpos de prova cilíndricos sobre o plano de seção transversal e ligados por três tiras de PRF de forma simétrica. A parte inferior do conjunto é fixa e aplica-se uma força de tração na parte superior, induzindo o surgimento de tensões de cisalhamento na interface. Foram utilizados corpos de prova moldados e extraídos de vigas de concreto, com diferentes dimensões: 5cm x 10cm, 10cm x 20cm e 15cm x 30cm. Treze geometrias diferentes do material compósito aderido foram usadas. Em alguns experimentos observou-se a ruptura no concreto, pois alcançou-se a tensão normal limite que o corpo de prova resiste antes de atingir a tensão de cisalhamento necessária para acontecer o descolamento. Isso foi notado principalmente nos corpos de prova menores, que resistem a uma força menor de tração. Não se notou uma influência da dimensão do corpo de prova na força máxima até o descolamento, tensão de cisalhamento e rigidez. Apesar de experimentos com corpos de prova menores levarem a bons resultados na análise do descolamento, verificou-se que o que limita seu uso é que muitas vezes é atingida a tensão normal limite que o concreto resiste, causando sua ruptura. Além disso, nos casos de deslizamento verificados com corpos de prova menores foi necessário utilizar uma geometria de PRF muito pequena, o que conduz a uma elevada tensão de cisalhamento máxima, exigindo o uso de um alto fator de correção. O processo de extração pode causar danos no concreto, como microfissuração, principalmente nos corpos de prova menores. Não foi possível concluir sobre a influência do uso de testemunhos no colapso de interface, mas foi notada uma alteração nas curvas de força versus deslocamento ao se usar corpos de prova extraídos, e em cada ensaio a influência foi de uma forma diferente. Não pareceu adequado analisar o deslizamento em testes com testemunhos. / This research is a study about the debonding phenomenon at interface between concrete and carbon fiber reinforced polymer (FRP) by way of performance and analysis of experimental tests. The objective of this study is to investigate the size effect in concrete and FRP debonding. The influence of variation of size concrete specimen on interface failure was analyzed when the same FRP geometry was adhered. Two concrete cylinders specimens are superimposed on plane of cross section, and the specimens are connected by three FRP pieces symmetrically adhered in the longitudinal perimeter. The lower part is fixed and the top is pulled. This induces appearance of shear stresses at interface. Different cylinders specimens made and core samples were used: 5cm x 10cm, 10cm x 20cm and 15cm x 30cm. Different thirteen geometries of the composite material were adhered. The failure happened in concrete in some experiments because concrete reached the normal strength before the shear stress required for the debonding, especially in smaller specimens because the tensile load carrying capacity for these cases is lower. An influence of specimen size was not noticed in the maximum load and shear stresses for debonding, and stiffness. Although experiments with smaller specimens generate good results, the reason that limits their use is the fact that the concrete often reaches the normal stress capacity. In addition, a very small FRP geometry was used in tests of debonding with smaller specimens. The use of a small FRP geometry resulted in a high shear stress, and with this use, it is necessary to use a high correction factor. The extraction process can cause damage to concrete such as micro cracking, especially in smaller specimens. It was not conclude about the influence of use of core samples in interface failures, but a change of the load versus displacement graph was noted. The influence of use of core samples happened differently in each test. An debonding analysis in tests with core samples may not be appropriate. / Dissertação (Mestrado)