Sharif Digital Repository

Various theoretical studies have been developed to obtain a deeper understanding of concrete behavior lead to the concrete constitutive models in the macroscale. In these models, however, the microstructure of the concrete and its effects on the concrete behavior has not been taken into account.Several numerical approaches have been incorporated to determine the effects of concrete mesostructure on the overall behavior of concrete. They may be classified at least in three main groups. In the first group, Continuum finite element methods (FEM) equipped with interface elements is incorporated. Second group is to incorporate more efficient elements, such as lattice or truss elements, instead of... 

In a classical finite element approach, interfaces are tracked and meshed. This mesh strategy introduces mesh distortions and difficulties to mesh the possibly complex interface shape. The Partition of Unity Method (PUM) alleviates these difficulties by allowing the discontinuities to be mesh-independent. This thesis concentrates on the blending elements in the eXtended Finite Element Method (XFEM).
The XFEM method enables local enrichments of approximation spaces. The standard finite elements are used in the major part of the domain and enriched elements are employed where special solution properties such as discontinuities and singularities shall be captured. In the extended finite... 

In this thesis a nonlinear thermomechanical model based on Lagrangian-extended finite element method is proposed to simulate discontinuities under thermal and mechanical loads. At first, a geometrically nonlinear model is presented for large deformation problems and then the model is completed by considering thermomechanical aspects. In extended finite element method, different enrichment functions are stated and also using analytical methods, a new set of functions are introduced to enrich the temperature filed around biomaterial crack tips. For numerical simulations, an object oriented code is created in C++. The results of numerical simulations are verified by the use of ABAQUS  

Every year, fracture imposes high economic costs and casualties to all societies. Since the beginning of the twentieth century, scientific approach to this issue has lead to invention of a new branch in mechanics, called fracture mechanics. In general, fracture problems fall into two categories. Brittle fracture, like what happens in glass, in which, few plastic deformations and energy absorption occurs and ductile fracture, which is preceded by large plastic deformations and energy absorption. This kind of fracture is usual in ductile metals like low carbon steel. Finite Element which is considered as the most important numerical method in the mechanics of materials, is also, widely used in... 

Geotechnical problems behavior depends on their interaction with existed fluid phases in their voids. In this research three phase porous media is introduced and it’s governing equations is presented. For solving this set of fully coupled dynamic equations finite element method is applied using elements with displacement and pressure degrees of freedom. Essential drawback of FEM method is in discontinuities modeling. In solid mechanic problems, discontinuity may occur in displacement field, such as crack or contact problems, or in their derivatives, such as multi-material problems. Major soil structures include some internal regions that there are some meshing problems during their FEM... 

The probability of crack appearance in soil structures and porous media is not avoidable, which could be the reason of structures collapse. According to the important affects, which they play in the vulnerability of the structures, they should be taking into account. The cracks have different effects on various materials. The most properties that cracks have, is their ability of conveying the fluid flow. For the most accurate analysis of discontinues domains, their governing equations should be taken and solved. Finite Element Method is one of the best solutions of differential governing equations. However, the appearance of some problems in the modeling of discontinues domain, was the... 

In many mechanical components, contact loading has a major role in stress field analysis. In this thesis, a combination of crack growth simulation and unilateral contact problem was carried out numerically. The Finite Element Method provides a powerful numerical tool to analyze contact problems. In the traditional FEM discontinuities should not cross through elements, so crack will grow through boundaries of elements, therefore, the model should remesh in each step of crack growth in order to align the mesh along the boundaries of discontinuities. This limitation causes major errors and computational costs. To overcome this limitation, the extended Finite Element Method (XFEM) will be used... 

Isogeometric Analysis method is a newly introduced method for the analysis of problems governed by partial differential equations. The method has some features in common with the finite element method and some in common with the mesh-less methods. This method uses the Non-Uniform Rational B-Splines (NURBS) functions as basis function for analysis. With this basis functions, the refinement procedure is much easier than the classical finite element method by eliminating the need to communicate with the CAD model. Modeling cracks in classical finite element method requires very fine mesh near the crack tip. One can model crack propagation by means of classical finite element, using an updating... 

In this study, Interfacial crack is modeled by the extended finite element method (XFRM) in order to evaluate the stress intensity factors for interface crack problems. These problems have crack edge discontinuity, material interfaces and singularity at the crack tip, which is inappropriate to finite element (FEM) mesh. Furthermore, around crack tips and interface edge, extremely fine discretization is required to achieve reasonable accuracy. All these factors increase the time and cost of the FEM solution. Recently the extended finite element method is developed and applied to interface fracture problems. In the XFEM, special enrichment functions are added to the finite element displacement... 

Crack propagation problem is one of the most important problems that are being investigated for a long time. Plenty of various approaches have been utilized to simulate the crack propagation phenomenon. Continuum based methods like Finite element (FE), Extended Finite element (XFEM), have been successfully applied, and the obtained results are valid in macro scale. However, the stress filed near crack tip in FEM modeling of crack, is not exquisite enough due to inability of continuum based approaches in revealing atomistic aspects of the material.
In order to gather efficiency of the continuum based domain and the accuracy of the atomistic based domain, Multiscale methods are employed.... 

Investigation about Hydraulic Fracturing phenomenon in fractured porous medium which was occurred by in-situ fracture pressure upon the crack wings, owes the fact that creating enormous damages. However, it might include advantages such as increasing the rate of crude oil production from deep and high pressure/ high temperature reservoirs. On account of the fact that, existence of cracks and natural discontinuities and heat sources such as boundary of geo thermal reservoirs in porous mediums it is undeniable fact. Also, cross sectioning hydraulic fracturing cracks with natural cracks it is an obvious impact. Actually, investigation and analyzing the break throw of HF crack with natural... 

Nowadays in high-tech industries there is a serious demand for using advanced materials. Functionally graded materials (FGMs) are in the last generations of these group of materials. FGMs have shown good behavior in special conditions. According to sensitive applications of FGMs , there is a large amount of effort to understand it’s behavior in the presence of crack. Finite element method and other numerical methods, in recent years are widely used in modeling fracture problems.Remeshing requirements and mesh sensitivity are among the disadvantages of analyzing crack growth using the conventional FEM. Recent finite element methods such as extended finite element method, are proposed to model... 

In recent years, more attention has been paid to the development of lightweight concrete (LWC). Study of such this material had been marked due to more importance of use of it.It is now well known that in order to model cracks the finite element model is more suitable.The fracture of quasi-brittle material such as concreteincludesthe fracture process zone (FPZ).Cohesive zone model is considered the most common model used for FPZ modeling.Therefore, in this article the propagation of cohesive cracks inLWC is modeled using the extended finite element method (XFEM). In this study, modeling showed fastgrowth and propagation of cracks inLWC. Due to its cavities and pores, LWC shows to be more... 

Heat transfer phonomenon is very important in solids and fluids, specially in fractured medium which has a major role in response of domain.In fact the fracture in special conditions can prevent the heat flow and in different conditions can aid the heat flow and conducts it.Propagation of hydraulic fracture by non-isothermal injection in fractured domain is a problem that has many physical phenomena like modeling of porous medium, heat transfer process in medium, discontinuities presence, concevtion of heat process by hydraulic fracture, fracture propagation, interaction of hydraulic fracture with existing fractures, junctions of cracks, contact phenomenon, etc. Therfore, solving this... 

Crack propagation in materials is an attractive problem in engineering because of the impact on the safety as well as economic issues. Much research studies have been done on the crack initiation, crack propagation criteria and path in the materials with different characteristics and conditions. Crack modeling depending on the material properties in brittle and quasi-brittle materials is done as Linear Elastic Fracture Mechanics (LEFM) and cohesive crack, respectively. The aim of this thesis is the modeling of crack propagation using energy method and comparing it with the cohesive crack. In order to model this problem, it is necessary to solve the governing equilibrium equation of the... 

In the present research, a numerical model is developed based on extended finite element method to simulate single phase multicomponent fluid flow through naturally fractured porous media containing explicit fractures. Each of fracture and porous matrix are separate continuous media that interact with each other due to fluid and solute exchange. The governing equations involve continuity equation of fluid phase and mass conservation equation of one component. The extended finite element method allows for explicit and accurate representation of cracks to capture the mass transfer of fluid components between fracture and matrix. Existence of fracture in the domain results in discontinuity of... 

The problem of fretting fatigue due to the inherent complexity and high probability of its occurrence in many engineering systems, is very important for engineering design. Fretting Fatigue is a significant failure mode in many contacting mechanical components, in which two contact surfaces experience low amplitude oscillatory movement due to cyclic loading. There are many significant contact fatigue problems in structural design which fretting fatigue at lap joints in aging aircraft and fretting fatigue of dovetail joints in gas turbine engine are the most important of them. Numerical simulations provide an appropriate and powerful tool for parametric study of fretting fatigue behavior of...