With the traditional fracture mechanics approaches, an initial crack and self-similar progression of cracks are assumed. In this treatise, theoretical and numerical tools are developed to mathematically describe non-self-similar progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in fracture mechanics as Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and lower surface connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or mixed mode. The initiation of fracture is determined by the interfacial strength and the progression of fracture is determined by the critical energy release rate. The material between two adjacent laminae of a laminated composite structure or the material between the adherend and the adhesive is idealized with an interfacial surface material to predict interfacial fracture. The interfacial surface material is positioned within the bulk material to predict discrete cohesive cracks. The proper work-conjugacy relations between the stress and deformation measures are identified for the interfacial surface theory. In the principle of virtual work, the interfacial cohesive-decohesive tractions are conjugate to the displacement jumps across the upper and lower surfaces. A finite deformation kinematics theory is developed for the description of the upper and lower surface such that the deformation measures are invariant with respect to superposed rigid body translation and rotation. Various mechanical softening constitutive laws thermodynamically consistent with damage mechanics are postulated that relate the interfacial tractions to the displacement jump. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. In addition, interfacial constitutive laws are developed to describe the contact-friction behavior. Interface elements applicable to two dimensional and three dimensional analyses are formulated for the analyses of contact, friction, and delamination problems. The consistent form of the interface element internal force vector and the tangent stiffness matrix are considered in the formulation. We investigate computational issues related to interfacial interpenetration, mesh sensitivity, the number of integrations points and the integration scheme, mathematical form of the softening constitutive law, and the convergence characteristics of the nonlinear solution procedure when cohesive-decohesive constitutive laws are used. To demonstrate the predictive capability of the interface finite element formulation, steadystate crack growth is simulated for quasi-static loading of various fracture test configurations loaded under Mode I, Mode II, Mode III, and mixed-mode loading. The finite element results are in agreement with the analytical results available in the literature and those developed in this work. A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. The methodology is validated with respect to experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts. Very good agreement between the progressive failure analysis and the experiments is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures in the postbuckling response of the panels. In addition, ideas concerning the implementation of a fatigue model incorporated with a cohesive zone model are discussed. / Ph. D.

Recentes melhorias nos processos de fabricação e nas propriedades dos materiais associadas a excelentes características mecânicas e baixo peso tornaram os materiais compósitos muito atrativos para aplicação em estruturas aeronáuticas. No entanto, mesmo novos projetos ainda são muito conservadores, pois os fenômenos de falha dos compósitos são muito complexos. Então, é estratégico entender melhor, bem como prever esses complexos mecanismos de falha, desenvolvendo modelos de materiais mais precisos que venham a diminuir o número de ensaios experimentais, gerando rapidez e economia aos projetos estruturais. Assim, este trabalho apresenta o desenvolvimento de um modelo de material baseado na Mecânica do Dano Contínuo para simular a falha progressiva de estruturas laminadas de carbono/epóxi quando submetidas a carregamentos quase estáticos e de impacto. Várias análises numéricas foram realizadas via elementos finitos, a fim de prever a falha dessas estruturas de material compósito sob essas solicitações. O modelo de dano proposto foi implementado como sub-rotinas em linguagem FORTRAN (UMAT-User Material Subroutine e, VUMAT-User Material Subroutine para simulações explícitas), que foram compiladas junto ao programa comercial de Elementos Finitos ABAQUSTM. Além disso, ensaios experimentais foram realizados, a fim de calibrar parâmetros relacionados ao modelo de material, bem como avaliar as potencialidades e as limitações do modelo de material proposto. / Recent improvements in manufacturing processes and material properties associated to excellent mechanical characteristics and low weight have become composite materials very attractive for application on civil aircraft structures. However, even new designs are still very conservative, because the composite structure failure phenomena are very complex. So, it is strategic to known better and to predict these complex failure mechanisms, developing more accuracy material models, which reduce the number of experimental tests, inducing a fast and economic structural design. Thus, this work show the development of a material model based on Continuum Damage Mechanics to simulate the progressive failure of carbon/epoxy laminate structures under quasi-static and impact loadings. Several numerical analyses were performed via Finite Element Method in order to predict the damage on composite structures under these conditions. The proposed damage model was implemented as a UMAT (User Material Subroutine) and VUMAT (User Material Subroutine for explicit simulations), which were linked to ABAQUSTM. Moreover, experiments were carried out in order to calibrate the material model parameters and to evaluate the potentialities and limitation of the proposed material model, as well.

Recentes melhorias nos processos de fabricação e nas propriedades dos materiais associadas a excelentes características mecânicas e baixo peso tornaram os materiais compósitos muito atrativos para aplicação em estruturas aeronáuticas. No entanto, mesmo novos projetos ainda são muito conservadores, pois os fenômenos de falha dos compósitos são muito complexos. Então, é estratégico entender melhor, bem como prever esses complexos mecanismos de falha, desenvolvendo modelos de materiais mais precisos que venham a diminuir o número de ensaios experimentais, gerando rapidez e economia aos projetos estruturais. Assim, este trabalho apresenta o desenvolvimento de um modelo de material baseado na Mecânica do Dano Contínuo para simular a falha progressiva de estruturas laminadas de carbono/epóxi quando submetidas a carregamentos quase estáticos e de impacto. Várias análises numéricas foram realizadas via elementos finitos, a fim de prever a falha dessas estruturas de material compósito sob essas solicitações. O modelo de dano proposto foi implementado como sub-rotinas em linguagem FORTRAN (UMAT-User Material Subroutine e, VUMAT-User Material Subroutine para simulações explícitas), que foram compiladas junto ao programa comercial de Elementos Finitos ABAQUSTM. Além disso, ensaios experimentais foram realizados, a fim de calibrar parâmetros relacionados ao modelo de material, bem como avaliar as potencialidades e as limitações do modelo de material proposto. / Recent improvements in manufacturing processes and material properties associated to excellent mechanical characteristics and low weight have become composite materials very attractive for application on civil aircraft structures. However, even new designs are still very conservative, because the composite structure failure phenomena are very complex. So, it is strategic to known better and to predict these complex failure mechanisms, developing more accuracy material models, which reduce the number of experimental tests, inducing a fast and economic structural design. Thus, this work show the development of a material model based on Continuum Damage Mechanics to simulate the progressive failure of carbon/epoxy laminate structures under quasi-static and impact loadings. Several numerical analyses were performed via Finite Element Method in order to predict the damage on composite structures under these conditions. The proposed damage model was implemented as a UMAT (User Material Subroutine) and VUMAT (User Material Subroutine for explicit simulations), which were linked to ABAQUSTM. Moreover, experiments were carried out in order to calibrate the material model parameters and to evaluate the potentialities and limitation of the proposed material model, as well.