Research On X-FEM And Its Applications In Concrete Structures

Concrete is a composite phase-material made up of aggregate, cement matrix, and so on in a certain mixing ratio. As a consequence, concrete material is prone to cracking through damage evolution in the interfacial zone between each phase under various loading conditions due to the influence of structural properties in inter-phase. The mechanism of concrete cracking and problems of concrete fracture have been the major topics of research in the field of civil and hydraulic disciplines, but so far no perfect models are available for describing the damage evolution of concrete in the serviceability state because of complexity of these problems. There are no approaches which are capable of predicting crack pattern, crack widths and development while at the same time being able to model the response of the structure all the way to failure. Thus the study on models apable of delineating and explaining the cracking and failure behavior of concrete becomes the hotspot in this field.A systematic study on the problems of fracture induced by cracks and cracks growth in concrete material is presented within the framework of the extended finite element method (X-FEM). This thesis consists of two main parts. The first part is devoted to a detailed discussion on basics of X-FEM formulation and computatipn of the stress intensity factors in two and three-dimensional crack problems. The second part focuses on the numerical simulations of crack initiation and propagation in concrete materials and structures.The components are described in more detail in the following:(1) Based on the concept of partition of unity, a displacement formulation of the X-FEM is constructed systematically. A new energy approach is proposed by coupling the virtual crack extension with the X-FEM to extract the strain energy release rates and then convert it to stress intensity factors. By means of meshes independence of the location and geometry of the crack, the proposed approach avoids the mesh perturbation around the crack tip to compute the stiffness derivatives with respect to a virtual extension of the crack. In comparison to the conventional methods, this new method is implemented more easily without the post-processing of the numerical results.(2) The full theory and method for evaluation of stress intensity factors within the X-FEM are deduced from the domain integral in three dimensions. Since the X-FEM suffers from certain drawbacks when the crack surface is expressed by the level set method for curvilinear crack front problems, the orthogonal mesh method is proposed to improve the accuracy of solution after analyzing the sensitivity of mesh to the accuracy of results.(3) The effect of the softening curve of cohesive crack model on mechanicl characteristics of concrete structure is discussed. The study shows that the initial fracture energy and the tensil strength are significant controls of the peak load of normal-size specimens (as well as most concrete structures in practice). This property can be simply used to determine the limit load without enough information about softening behavior of structures in practical engineering.(4) The X-FEM model with cohesive crack concept is developed to simulate crack growth in concrete. A nonlocal averaging method is introduced to evaluate the stress and strain fields ahead of the crack-tip, which can impove the accuracy of the computed propagation directions in the case of coarse meshes. The presented model can handle effectively the problems of not only pre-existing crack in the classical fracture mechanics but also crack initiation without any initial defects. A particularly appealing feature of the X-FEM model is that multiple mixed mode cracks propagate along an arbitrary in concrete can be modelled without the need to remesh as cracks advances.(5) The realistic concrete cracked dam is analized using the X-FEM in combination with the other structural elements to predict cracks potentially growing trajectories and evaluate the effectiveness of reinforcing repair measures as well as safety of the dam. Some important conclusions with practical significance are obtained in engineering background.