Sharif Digital Repository

Composites are made to obtain several mechanical and physical properties selectively. But joining and manufacturing of complex shapes restricts their usages. Transient liquid phase (TLP) bonding is a novel method for joining of these materials without any defects that occur in other welding methods, such as melting or solid state methods. In this research, temperture gradiant TLP bonding of AA6061/SiC/15p using gallium as interlayer was performed. To obtain the optimum conditions, joining was performed at 260, 310, 360 and 410 ˚C, and pressure of 5 and 10 Mpa with holding time of 5 and 10 minutes, the joined samples then solutionized at 530˚C for 70 minutes. Optical and scanning electron... 

In this research, the surface contact and lateral stress distribution due to the contact between a rigid flat punch and a FG coating is investigated through macro and micro mechanical modeling due to FEM. The effects of important parameters in modeling of contact mechanics, both macro mechanically and micromechanically investigated. Study of such parameters is important, especially in fracture mechanics point of view, since stress distribution of mechanical contacts can cause the crack nucleation and evolution and finally catastrophic failure. Investigating the effective parameters in FEM models and analyzing the result admittedly help to improve manufacturing methods in order to smooth the... 

Solid foams are novel materials with outstanding characteristics that have found numerous applications in the construction of light, stiff structures during the past two decades. At the micro-level, solid foams benefit from a cellular structure, which is composed of either closed or open cells. Open-cell foams, which were the concern of this study, are described as a network of inter-connected struts (or ligaments) with beam-like action. The mechanical design of foams requires their mechanical properties to meet certain criteria. These criteria can be satisfied by careful selection of the base material and the microstructural morphology (geometry). However, the selected material has to be... 

In this research Al6061-SiC composite bonding was done by transient liquid phase method using powder copper intermediate layer. In the first stage feasibility of making this junction in ambient condition was studied. In order to find process optimum temperature, experiments were performed in 560°C, 570°C, 580°C and 590°C for 2 hours. Optical microscopy, scanning electron microscopy and EDS analysis were utilized for this study. Moreover, shear strength determination test was done to assess and compare mechanical properties and also to determine the optimum bonding temperature.
In second stage, base composite and bonded specimens were homogenized in 530°C for 5 and 10 hours. Effect of... 

Through this thesis the Homogenization Theory for composite materials is studied assuming that the distribution of heterogeneities is periodic. In this theory two scales characterize the problem: microscopic and macroscopic scale. The first method that is used to solve the problem is the classical Asymptotic Expansion method where an error estimate is presented for the solution. The second method which was introduced by Luc Tartar for the first time is Oscillating Test Functions method. In the next chapter after introducing the concept of two-scale convergence, the Two-Scale Convergence method has been introduced. At last the unfolding
periodic method, which is based on the concept of... 

Stainless steels have many applications in different industries. Therefore, it was always a main concern to find an appropriate joining method for using these materials in complex structures. In the present investigation, AISI 304 stainless steel was successfully joined by transient liquid phase (TLP) bonding, using a MBF-30 interlayer. The joining process was carried out at 1150 ℃ temperature and for different bonding times. Also joints with complete isothermal solidification were homogenized at 950 ℃ for different holding times. Microstructure and compositional changes of the joints were studied using optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive... 

The present paper proposes a micromechanical damage model for two and three dimensional Representative Volume Element (RVE) with randomly distributed cohesive rough slit-like and penny-shaped micro cracks (Barenblatt-Dugdale type). First, the influence of crack roughness on the crack opening under macro hydrostatic stress state is studied and then the energy release contribution to material damage process is estimated. Considering the fractality of the crack trajectories yields to lower values of the volume crack opening. This will result in lower energy release rate in RVE and higher levels of material resistance. Based on the energy release rate of RVE, the effective material properties... 

Multiscale modeling is performed within the framework of homogenization methods for problems in which the scales are separated. The existence of representative volume element (RVE) is one of the main ingredients of homogenization methods. Due to non-existence of RVE and macroscale mesh sensitivity, the continuous homogenization method is not applicable for softening materials. Despite the non-existence of RVE for softening materials, it has been demonstrated that by performing the average over the active damage zone rather than the entire domain, objective responses with respect to RVE size could be obtained. That is why discontinuous homogenization is used instead of continuous... 

In the thesis , we present a universal model of brain tissue microstructure that dynamically links osmosis and diffusion with geometrical parameters of brain extracellular space (ECS) . In the first part , we investigate the biological aspects of the model , and in the second , we analysis the model in the mathematical framework . The first part : Our model robustly describes and predicts the nonlinear time dependency of tortuosity ($\lambda = \sqrt{D/{D^{*}}}$) changes with very high precision in various media with uniform and nonuniform osmolarity distribution , as demonstrated by previously published experimental data ($D$ = free diffusion coefficient , $D^{*}$ = effective... 

Artificial periodic mediums such as metamaterials or photonic crystals have been intensively studied in recent years due to their unique characteristics and the ability to control their effective electromagnetic parameters. Developing accurate methods for the extraction of these parameters is highly desirable, given the potential application of artificial materials in optical and microwave devices. In this thesis a new method is presented for computing the effective bulk and surface parameters of three-dimensional artificial periodic materials. The effective, bulk permeability and permittivity tensors are shown to be directly related to the eigenvectors of the generalized transfer matrix of... 

In this work, a basic model for numerically simulating matrix acidizing in carbonate formations is developed. This model employs laboratory linear acid core-flooding data and avoids any fitting parameters. Wellbore-scale numerical simulation is based on finite-volume method with Cartesian coordination and cubic meshing. Mass transfer equations are based on fluid front tracking and the momentum transfer equation is the Navier-Stokes equation for flow in porous media. The effect of fine-scale dissolution phenomena of carbonate rocks are translated into the coarse-scale by homogenization. In this work, it is tried to achieve a simple but logical approach towards a correct simulation, avoiding... 

The aim of homogenization theory is to establish the macroscopic behaviour of a system which is ‘microscopically’ heterogeneous, in order to describe some characteristics of the heterogeneous medium (for instance, its thermal or electrical conductivity). This means that the heterogeneous material is replaced by a homogeneous fictitious one (the ‘homogenized’ material), whose global (or overall) characteristics are a good approximation of the initial ones. From the mathematical point of view, this signifies mainly that the solutions of a boundary value problem, depending on a small parameter, converge to the solution of a limit boundary value problem which is explicitly described. In... 

In its most general form, analysis of macroscopic electromagnetic behaviour of a medium must include the dependence of magnetic flux density on the electric field and electric flux density on the magnetic filed. This new degree of freedom can lead to new propagation phenomena and result in novel microwave and optical devices. In recent years, numerous theoritical and exprimental studies have been carried out to fabricate these bi-anisotropic materials and understand their behaviour. A particular class of bi-anisotropic materials is electromagnetically equivalent to an anisotropic medium with gyrotropic permittivity and permeability tensors that operate on pseudo-electromagnetic fields. In... 

In this research, diffraction limit is introduced as the major challenge in wave focusing. Common wave focusing techniques have been reviewed and classified into two categories: far-field and near-field techniques. Through examples that illustrate its major inherent and practical challenges, super-oscillation has been presented as a specific idea of focusing waves using remote sources. By introducing subwavelength inhomogeneities in the region of interest, we have demonstrated sub-diffraction wave focusing in a near-field approach. Using the multipole method, the effect of injecting sub-wavelength cylindrical particles on focusing electromagnetic waves has been studied. Since when there are... 

Additive manufacturing enables fabricating lattice structures with tailored mechanical responses based on lattice materials. Full exploitation of such a possibility requires reliable and efficient mechanical analysis tools to be explored by topology optimization algorithms for designing the interior architecture of the structures to meet the desired mechanical behavior. However, detailed mechanical analysis of lattice-based structures using the conventional finite element approach is prohibitively expensive due to lattices’ complex and fine features that demand adopting very fine space discretization. As an alternative, equivalent models based on the homogenization principle have widely been...