Sharif Digital Repository

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.... 

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... 

Having unique characteristic behaviors of Shape memory alloys (SMAs) known as shape memory effect (SME), and superelasticity (SE), make them an appropriate choice for innovative engineering applications. Optimum design of SMA applications, calls for constitutive models and finite element simulations. It is evident that using structural elements (bar, beam, plate, shell) considerably reduces computational cost (CPU time) when compared to continuum elements. Therefore, structural elements are usually preferred in the early stages of design process. In this thesis, we simulate shape memory based structures using beam elements.Up to now, we have studied phenomenological constitutive models that... 

The unique characteristics of solid foams make them ideal for both natural and man-made structures. The cellular structure of these materials allows for the construction of natural structures such as bone and wood that have high strength and use minimal primary materials. Their unique structure has inspired studies in the automotive and aerospace industries, leading to a new generation of lightweight materials. The focus of this study was the effects of the microstructure of foams on the macro mechanical response of specific structures. Homogenization methods are well suited for the study of these effects due to the random nature of the microstructures. One major problem in using numerical... 

Unique characteristic behavior of shape memory alloys (SMAs), known as shape memory effect (SME) and superelasticity (SE), make them an appropriate choice for innovative engineering. Design of SMA applications calls for development of constitutive models. To accurately predict SMA behaviors, employing a suitable model with low computational costs is required. In some experimental data, SMAs have coupling in mechanical and thermal behavior. Therefore, mechanical loading changes temperature of the material and variation in temperature affects the mechanical behavior as well. This behavior is called coupled thermo-mechanical effect. In SMA applications, since coupled thermo-mechanical behavior... 

Wide applications of elastomeric (rubber-like) materials have led to significant interest of researchers to these materials. Elastomers as a great category of polymeric materials,posses specific properties such as large elastic deformations and energy absorption which make them suitable for aerospace and automotive applications as well as shock and vibration absorbers.In this thesis, an exponential strain energy function (SEF) has been proposed for elastomers which well reproduces the mechanical behavior of these materials and also the material parameters are related to the physical parameters of the material molecular network. This SEF has been concluded from a relation proposed for the... 

Porous shape memory alloys (SMAs) were fabricated by many researchers in the last decade and have been initially utilized as bone implants. Also, due to hysteresis in superelastic behavior, and the porous structure, porous SMAs can be used in energy absorbing applications. Such applications call for efficient constitutive modeling of porous SMAs. Moreover, porous SMAs with a wide range of porosity ratios and mechanical properties have been produced in the recent years; therefore, it is necessary to improve the available models and carefully study porous SMA behavior. Different approaches have been utilized for modeling porous SMA behavior. Most of the researchers used homogenization approach... 

Nowadays, surveillance cameras are an integral part of the transportation management center. On one hand, developing of surveillance cameras around the cities and on the other hand, increasing costs of driving accidents, makes it necessary to use the automatic behavior analyzing systems on video sequences. The scientists’ focuses on detecting unusual events as one of the most important usage of video analysis, and recently, the algorithms of unusual action detecting had an impressive development by using the machine learning methods. In this research, the existing unusual event detection methods and in special, traffic unusual activities will place under scrutiny, and we suggest an algorithm... 

Modern microwave and wireless communications systems are too complex today to permit the complete simulation of the nonlinear behavior at the transistor level of description. This problem presents a significant productivity bottleneck for design engineers. At the bridge between the transistor circuit and the multichip module or RF integrated circuit (RFIC) we use “behavioral models” to describe the nonlinear circuit blocks or ICs in the system. The behavioral models are simplified models of the essential nonlinear behavior of the complex sub-circuits; this simplification means that these models will execute more quickly, and use much less memory than if an entire complex subsystem was... 

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... 

In this thesis, experimental and numerical investigations are performed on dynamic behavior of fiber reinforced concrete (FRC) by split Hopkinson pressure bar (SHPB) and penetration tests. In the experimental part, effects of parameters such as quasi-static compressive strength of matrix (36 and 48 MPa), strain rate (3*10-5 up to 100 s-1), type of fibers (crimped steel, hooked-end steel, micro polypropylene and micro polypropylene) and volume fraction of the fibers (0%, 1% and 2%) are studied by employing SHPB apparatus on testing more than 150 specimens. Also, different types of FRC panels with 5 cm thickness were casted to perform 20 penetration tests with compressed gas gun apparatus. The... 

Various safety systems have been designed and developed in recent decades, but one of the most important issues is the improvement of safety of such systems and that is the goal of this research. In this thesis, while introducing different methods of designing an electronic system, we designed one of these safety systems: the Airbag system in accordance with ISO 26262 by considering standard requirements, prediction of potential hazards of the system, and its safe design at the system level. The designed system is capable of detecting the car crash in a precise and timely maner. The airbag will deploy within 45 milliseconds protecting the health and safety of occupants of the car and in... 

Nowadays many of us spend a big part of our daily times on social media.One of the most important research problems in social media analysis is how to engage users. The trace of user activity over these websites is a valuable resource for user understanding and engagement, but this data is very huge and unstructured. An approach to deal with this problem is user behavior modeling. In this process, first a behavioral model is considered for users, then using the activity data and the behavioral model, some parameters are learned. Finally, using the learned parameters, a user profile is constructed for each user. This profile can be used for user engagement and many other applications.... 

Magnesium alloys have been used in the production of light weight components in automobile industries (for example: the ladder frame and the valve cover) due to a good combination of low density (1740 kg/m3), the tensile strength (160-365 MPa) and the elastic modulus (45 GPa). For the design and the optimization of these components under various loadings and different conditions, it is necessary to determine fatigue properties of the material by using thermo-mechanical and low cycle fatigue tests at different temperatures. In thermo-mechanical fatigue tests, the temperature varies in a specific range with the mechanical strain. The goal of this research is to investigate low cycle fatigue... 

Today, everyone is looking for ways to achieve comfort and safety in the workplace and, in so doing, appeals to various sciences. One of these sciences is "ergonomics", which examines the human relationship with the work. One of the areas that could be of great interest is the driving ergonomics. After all nowadays, many people use personal vehicles. The increasing use of personal vehicles has increased the number of accidents and deaths. In recent decades, many researches have been done in the field of driver fatigue detection. To avoid road accidents, researchers have focused on monitoring driver and vehicle behavior and tried to analyse status of the driver. Using computer vision, we can... 

The low female labor force participation rate in the Iranian labor market relative to the global average and even to countries that do not differ significantly from the socio-economic characteristics of our country is one of the important questions of the Iranian labor market. In this study, we seek to answer the question of whether the economic participation rate of married women will increase if the gender gaps in wages, job findings and job losses among women and men are eliminated in the Iranian labor market. To answer this question using household expenditure and income data and labor force data, we show that between the two categories of occupations, in terms of the share of women's... 

A new types of architected cellular materials are those with negative poisoon’s ratio named auxetic. This materials indicate unormal behavior under different loads as if they were stretched in one direction, they also expanded in other directions and if pressure is exerted in one direction, they are compressed in all directions. Auxetic materials can improve mechanical properties such as shear strength, strength to weight ratio, thoghness, energy and vibration dissipation and crack expansion due to fatigue.The useful features of this material have been taken into account in a variety of industries, including the automotive industry, in the manufacture of parts such as body and bumper,... 

The deposits in many regions in Iran, particularly eastern zones like Golestan, Semnan, Yazd, Kerman, Hormozgan, Sistan and Baluchestan provinces consist of deposits of loess, which are categorized as collapsible soils. This kind of soil show sufficient shear strength and low deformability in dry or natural condition. However, if a structure is built on the soil and water seeps or infiltrates into the soil due to several reasons such as sewer leakage, water seepage or heavy precipitations in a way that its moisture reaches to a critical value, the interparticle bonds may break or the present suction in the soil may decrease in a way that the soil experiences sudden collapse, which may result... 

In this thesis, a deformation measure is introduced which leads to a class of strain measures in the Lagrangian and Eulerian descriptions. In order to develop a constitutive equation, a second-order constitutive relation based on these strain measures is considered for modeling the mechanical behavior of solids at finite deformation. For this purpose and performance evaluation of the proposed strains, a Hookean-type constitutive equation is considered and the uniaxial loading as well as simple shear and pure shear tests are examined and the results are compared with the test data. Also, in order to characterize the mechanical behavior of elastic continua, constitutive equations through a...