Sharif Digital Repository

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... 

Soil structures have important roles in civil engineering applications. The most important types of these structures are soil dams and foundations which their damages may cause huge loss. Thus, appropriate analysis of these structures under possible loading conditions is unavoidable. The real behavior of this media can be achieved from the solution of its coupled governing differential equations. In order to solve this set of equations in practical applications, appropriate numerical solutions should be used. The finite element method can be called as one of the most important numerical solutions of differential equations which have been used in analyzing different types of engineering... 

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... 

One of the most important issues in modeling natural phenomena that always takes into consideration is the problem of contact and collision between objects. Finite element method is known as the most basic and commonly used in numerical modeling techniques to simulate the above phenomenon.This method has specific problems such as dependency of this approach to the domain mesh especially near the contact region. Due to the dynamic and complex nature of impact problems in which drastic changes occur in short duration that often deals with large deformations, one of the perennial concerns, is the weaknesses of methods applied to enforce contact constrains, and so it is essential to use an... 

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 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 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... 

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.... 

Nowadays, regarding to knowledge progresses in various fields,advanced materials for which have acceptable performance in sensitive and special conditions must be used. Functionally graded materials are advanced multiphase composite materials that are characterized by continues spatial variations in mechanical and thermal properties. Clearly, studying on fracture mechanics of FGMs is important because they are used in igh-tech and sensitive applications.In this research, numerical modeling of fracture mechanics in isotropic/orthotropic/ transversely isotropic FGMs via interaction integral method is performed. Both incompatible and non-equilibrium formulations of interaction integral are... 

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... 

In this thesis, based on the combination of Extended Finite Element Method (X-FEM) and appropriate material constitutive models, an efficient method is proposed to model ductile fracture under quasi-static, cyclic and dynamic conditions. X-FEM is a novel method in which with the use of appropriate enrichment functions, discontinuity geometry is separated from the computational mesh and hence the need for a remeshing step is alleviated. The non-linear material behavior in the crack tip process zone can best be modeled with methods based on continuum damage mechanics. These methods model the ductile fracture mechanisms, phenomenologically. In this thesis, in order to model the ductile... 

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... 

When a crack is subjected to a compression field, it will close and its edges will get into contact with each other. Depending on the direction and magnitude of the loads and also the coefficient of friction, ‘stick’ or ‘slip’ situationsbetween the edges will occur. This type of crack is known as ‘frictional crack.’ In this project, first these cracks are studied analytically and the order of singularity is derived using asymptotic analysis and also the analytical fields are determined for both ‘isotropic’ and ‘orthotropic’ materials. Then, numerical simulations are carried out using extended finite element method which is considered as the most powerful means for analyzing the problems... 

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... 

Usage of FRP material is developed in recent year, because advantages of this material researcher studied the effect of this strengthening method on beam column and masonry structures, but few research’s related to RC arches that strengthen with FRP. This studies showed performance of structure properties but many parameters influence. In this research, a model developed that can model RC arch with FRP till complete failure, after verifying model with experimental result parametric study done. This parameter are effect of strengthening scheme, influence of shape of arch and geometry of arches. Concrete damage plasticity used for modeling behavior of concrete, also for modeling interface... 

In this research, multi-scale modeling of mixed-mode failure mechanism of quassi-brittle materials is presented. For modeling a realistic crack growth in heterogeneious media, crack initiation criterion, crack growth orientation and the macroscopic cohesive law are derived from a microscopic sample. As a microscopic crack initiation criterion, acoustic tensor is investigated and scaled acoustic tensor has been proposed by comparison its results with maximum principal tensile stress. For crack growth direction based on micro-scale, acoustic tensor and multiscale aggregating discontinuities has been investigated and multiscale aggregating discontinuities by comparison results of these methods... 

In practice, structures made of concrete are full of cracks. The strength of concrete is mainly determined by the tensile strength, which is about 10% of the compressive strength. As long as cracking in concrete is unavoidable, we have to try to minimize their detrimental effects. This objective can be achieved by resisting (or limiting) propagation of existing cracks. Because of this, reinforcement (mostly steel) is used to increase the carrying capacity of the material and to control the development of cracks. Concrete structures that fail, already shows a large number of large and small cracks before their maximum carrying capacity is reached. The failure of concrete can be characterized...