Sharif Digital Repository

Subdural hemorrhage (SDH) is one of the most common traumatic brain injuries. The mortality rate of SDH is over 30 percent. In addition, patients require to visit hospital for treatment and further healthcare. According to the costs of treatment and frequency of injuries, prevention of SDH is essential. 3D-Finite element modeling of human head is an appropriate tool for evaluation of injury risk and setting up a prevention tolerance criteria. The aim of this study is to create a 3D finite element model of human head with special consideration of vessels. Vessels are modeled by truss elements at current finite element models of human head; however, geometry of vessels is regarded as simple... 

Cardiovascular diseases are the major cause of death worldwide. Atherosclerosis is one of the major types of cardiovascular diseases in which fibrous and fatty materials, called plaque, build up inside the artery and cause partial or total occlusion of the artery. Intravascular balloon angioplasty with or without stenting is the most common treatment of this disease. In 2010, approximately 954000 stent implantations were performed in the United States. The major issue associated with stenting is reclosure or renarrowing of the transverse section of the artery termed in-stent restenosis (ISR). Unfortunately, nearly one–third of the patients
who receive stent implantation require further... 

Magnetic Shape Memory Alloys (MSMAs) are a category of active materials which can be excited by magnetic field. These alloys have been used in sensor and actuator applications recently. MSMAs posses special properties such as large magnetic field-induced strains (up to %10) and high actuation frequency (about 1kHz), while ordinary Shape Memory Alloys can’t act in frequencies above 5Hz due to the time involved with heat transformation. In this thesis, MSMAs are modeled by means of incremental modelling which is a useful method in analysis of nonlinear behaviour of materials. Furthermore, stress-strain curve of MSMA is approximated using an analytical expression whose parameters can simply be... 

Using materials with better mechanical properties is always advantageous. Many sectors, including automotive and aerospace industries, are always seeking new materials with better mechanical properties and lower weight. A lighter vehicle means lower emission and higher fuel efficiency. Thus, developing new methods to investigate novel materials is very important. One of these novel materials are Metal Matrix NanoComposites (MMNCs). MMNCs are fabricated using nano scale particles embedded in metallic matrix. High strength, stiffness, and hardness are the three most important mechanical properties of MMNCs. Introducing a suitable way of modeling these materials is important since it will... 

This thesis has been done in two parts. In part I,a rupture disc wasdesigned. The rupture disc is an important part of a gas gun and is used as a fast response valve whichburst at a predefined pressure. Afterdesign,it has been used instead of an electric valve to increase the maximum velocity that can be achieved by the gas gun. Toavoid fragmentation and to have a better control on rupture pressure,a groove isusuallygraved on the disc. Cross and circumferential grooves werestudied in this thesis. It was found that in most cases the discs with a cross groovewere not fully opened because they have four independent failure paths and these paths should be opened simultaneously. However, having... 

Mechanical properties such as Young's and bulk modulus and strength of material is improved by adding Nano fillers to polymer substrates. Hence by adding Nano fillers such as graphene and carbon nanotubes to polymer substrates it is possible to achieve better properties in resultant nano composites. The aim of this study is to investigate the improvement in polymer-based composites properties by adding graphene Nano fillers for shock absorption. In this study, molecular dynamics method is used to model the molecular behavior of the material. First, a sample cell that have the properties of the whole material, is modeled, then by applying impact loading in modeling software, the response of... 

In this thesis the tow-dimensional wave propagation in a functionally graded elastic medium is considered. It is assumed that the stiffness and density of the medium vary continuously in one direction. This work presents analysis in both time and frequency domain. In time domain, a graded element is presented for static and dynamic analyses in FGMs. This element clearly simulates wave propagation in inhomogeneous materials. The most important property which an element must have to be reasonable in wave propagation analyses in FGMs is simulating reflection only from boundaries. In frequency domain a dynamic stiffness matrix is generated using stiffness and mass matrix used in time domain... 

Motorcycle has one of the most complicated geometries and mechanisms and its kinematic, according to the articles surveyed, has always been expressed with approximation and simplification in the degrees of freedom and especially in the magnitude of the angles. Analyzing the dynamic model of bodies, using methods like Kane, Lagrange, Boltzmann-Hamel, Gibbs-Apple, Newton, Hamilton and Ross, lead to mechanism’s equations of motion. While the equations of motions, obtained from alternative methods, lead to the same result, but the number of differential equations and sometimes their order, are different in different methods, and therefore their numerical solving time for the body’s motion, are... 

Split Hopkinson pressure bar (SHPB) test is one of the tests used for characterizing the mechanical behavior of materials at high strain rates between and s -1. A SHPB test consists of a gas gun, a striker bar, an incident bar, a transmission bar and a specimen which is sandwiched between the incident and transmission bars. In order to achieve constant strain rate during the test which is necessary in SHPB test, the incident pulse should obey the stress-strain curve of the specimen. For shaping the incident pulse usually a thin deformable disk -called pulse shaper- is placed between the striker and incident bars. In this thesis, first we designed, implemented and calibrated the measuring... 

Increasing the application of elastomeric structures in industries has caused an interest in the researchers for nonlinear elasticity and viscoelasticity of solids. Such materials which show large deformation and rate dependent behavior are treated by visco-hyperelastic constitutive equations. Based on visco-hyperelastic constitutive equations, total stress is divided into two parts: quasi-static and viscous stress. The method for determining the visco-hyperelastic constitutive equation is strictly dependent on the chosen rheological model. In this project, a new visco-hyperelasic constitutive equation in the integral form was introduced based on the Zener model. Also, based on the internal... 

Bone is one of the most important biomechanical organs in animal bodies which comprises of a dynamic tissue that can change its internal structure through remodeling process. A comprehensive understanding of this process is of great significance in prevention and cure of many bone diseases. Due to the special conditions of hard bone tissue, empirical studies on the details of bone processes is very costly and time consuming. Thus, mathematical models and in silico simulations are key to unraveling how bone homeostasis imbalance can lead to pathological conditions. Therefore, in the recent years, there has been an increasing interest in modeling bone remodeling process in cellular level. In... 

Thin film solar cells have low efficiency compared to crystalline silicon solar cells; however, they are low-cost and flexible. In manufacturing these solar cells, thin films are deposited at high temperatures (higher than 200℃) on a thick metal, plastic, or glass substrate using sputtering and plasma enhanced chemical vapor deposition (PECVD) methods. Since the thin films and substrate have different thermal expansion coefficients, cooling the system from deposition temperature to room temperature induces thermal residual stresses in both the films and substrate. In addition, these stresses, especially those induced in the amorphous silicon layer can change the carrier mobility and band gap... 

The dual-arm cam-lock robot is a reconfigurable and adaptive robotic arm. The novel design of this robot allows it to manipulate the objects either by cooperation of two completely independent arms, or with locking proper links in each other and featuring the advantages of a parallel manipulator. Depending on the position of the cam-locked links, the robot may lose a few degrees of freedom and gain a different dynamic condition from the view point of the consumed energy, stiffness, dexterity, and etc. So the best configuration of the robot can be selected to perform the desired task, considering all possible configurations of the robot. In this dissertation, formulation and implementation of... 

In this research, penetration in a new confined ceramic-metal structure is studied experimentally and numerically. In this new structure, ceramic tile is confined on its front face by an aluminum foil. First, penetration experiment in 5 mm thick Al 1100 plate carried out. The ballistic limit velocity is obtained about 140 m/s. The difference between experiment and numerical simulation is lower than 5 percent. Then penetration analysis of 10 mm thick Alumina tiles is carried out experimentally and numerically in confined and unconfined conditions. Ceramic tiles are in square shape with 50 mm side length. The ballistic limit velocity for unconfined and confined ceramic tiles are obtained about... 

Nanocomposites are fabricated by incorporating nano-scale particles in polymer, metal, or ceramic matrices. Metal matrix nanocomposites are a new kind of materials which have shown superior mechanical features, such as high strength, stiffness, and hardness. However, they are not sensitive to high temperature, in contrast to the polymer matrix nanocomposites. Nanoporous materials, also, have wide range of applications in different fields, such as geology, biology, biomechanics, and electronic. Carbon Nanotubes (CNTs) have shown wonderful properties like high Young’s modulus, high resonance frequency and unique electrical behavior. Wonderful mechanical and electrical properties make CNTs... 

Due to the limitations on experiments in the field of cell mechanics, computational modeling of biological cells have attracted attention within two recent decades. In general, some models have been developed in two different scales, known as microstructure and continuum, both of which have their own pros and cons. Nevertheless, viscoelastic behavior of cell membrane has attracted less attention of scientists up to now. Therefore, multi-scale simulation of the viscoelastic behavior of the cell membrane has been chosen as the main goal of this thesis. Toward this goal, at first the energy of the simulation box, consisting of 128 Dipalmitoylphosphatidylcholine and 3655 water molecules, was... 

Nanostructured materials are a new kind of engineering materials which attracted researchers’ interest because of their interesting mechanical /physical properties, as well as controllable microstructural design ability for desired applications. These new materials are homogeneous at the macroscale but at the microstructural level, may have heterogeneities including common nanostructures. Because of multiscale nature of these materials, new multiscale methods should be developed and used for better understanding the behavior of them. Multiscale methods could be categorized into concurrent and hierarchical methods. In concurrent methods, the domain under study is explicitly divided into... 

Shape memory alloys (SMAs) are a type of smart materials which have unique features known as pseudo-elasticity, one-way and two-way shape memory eects. The interest in the mechanical behavior of SMAs is rapidly growing with the increasing number of potential industrial applications. The origin of SMA material features is a reversible thermo-elastic martensitic phase transformation between a high symmetry, austenitic phase and a low symmetry, martensitic phase. In most applications, SMAs experience general non-proportional thermo-mechanical loads. Thus, according to experimental observations, the so-called variant reorientation should be considered in the constitutive model development.... 

Dynamic Object Manipulation (DOM) and Dynamic Biped Locomotion (DBL) are certainly two major categories in robotics, which attract much attention of scientists. Their (hybrid) nonlinear dynamics usually make them challenging subjects to deal with. Such systems are often used as benchmarks to test new nonlinear control approaches or stability analyses. However, there is not a unified approach to both systems. Although it seems that DOM and DBL systems are completely different at first view, we deeply believe that essential similarities exist between these two robotic fields. In this dissertation, we study the correlation of DOM and DBL while addressing passive, underactuated, and fully... 

Nonholonomic constraints are a vast portion of constraints that happen in variety of dynamics applications such as robotics and control. In this project we present a new form of D’Alembert’s principle and employ it to generalize classical methods of Analytical Dynamics. It results in a new formulations which can be implemented to solve constrained dynamical systems either holonomic or nonholonomic. These formulations do not have any restrictions on the order of the nonholonomic constraints and can analyze high order nonholonomic constraints (HONCs) as well as first order constraints. In addition using these formulation one can find equations of motion of nonholonomic systems without...