| In this thesis, a comprehensive approach to the modeling of nonlinear behavior of concrete structures using the discrete crack model in the finite element method is proposed.; It is generally accepted that the discrete crack approach does not exhibit the strong mesh sensitivity as most of the smeared crack models. In addition, the properties used by the discrete crack model can be directly determined by well established experimental methods. The main disadvantage of this model is the need to modify the finite element mesh at each crack increment. Whereas, various remeshing techniques have been proposed in the literature to address this problem, they all have been restricted to two-dimensional problems, and their extension to three-dimensions is part of the proposed model.; In this work, a fundamentally different approach to the remeshing problem is adopted. The finite element model is used only as a tool to solve the differential equations of elasticity, and a boundary representation is used for problem definition. The cracks are considered being part of this boundary definition. Thus, each time a violation of the crack propagation criterion is detected in the finite element model, a finite element mesh is adaptively regenerated. From this approach, the classical partial remeshing can be easily recovered as a special case. In addition, this method can be readily extended to error based adaptivity.; Therefore, in this approach, three levels can be identified: (1) boundary representation of the structure with cracks, (2) finite element model and (3) fracture mechanics based model. In this work, both linear and nonlinear fracture mechanics theories are considered. A new implementation of the domain integral method is proposed for the computation of stress intensity factors along a general three-dimensional crack front. For the nonlinear theory, an interface crack model is used to model the fracture process zone in cementitious materials. This model accounts for the shear effects in the crack, and reduces to classical Hillerborg's fictitious crack model in the case of pure mode I (i.e. opening) fracture. An unconditionally stable constitutive algorithm is developed for the finite element implementation of this model, and on large scale examples, the nonlinear model is shown to approach the linear elastic fracture mechanics solution.; An algorithm for fully automated mesh generation for structures with cracks is developed based on the properties of Delaunay's triangulation.; The mixed iterative method is used to improve the behavior of triangular and tetrahedral elements generated by the aforementioned algorithm and to compute continuous stress and strain fields. The continuity of these fields is exploited in the implementation of the various fracture models.; The proposed methodology is verified on numerous problems with known experimental or analytical results, and is also applied to both two- and three-dimensional non-linear analyses of concrete dams. Fracture mechanics analyses of a buttressed concrete dam provide significantly different results, from those obtained by classical hand calculation, and they clearly demonstrate the importance of mode II fracture for dam sliding stability. |
