Sharif Digital Repository

The present study deals with the analysis of Atomic Microscope with crack by making use of Strain Gradient Elasticity theory. Empirical observations represent that in micro dimensions, materials show behaviors, which the classic continuum mechanics theories are not able to explain. Thus, taking advantage of non-classic theories, which are capable of explaining such phenomena or behaviors in analyzing materials in micro dimensions seems necessary and of much significance. In this direction, by applying an Euler-Bernoulli beam assumption and neglecting the shear effects, governing equations and boundary conditions of the problem were obtained via taking advantage of variations in Hamilton... 

Due to importance of micro electromechanical systems (MEMS), inability of the classical continuum theory to predict the mechanical behavior of micro-scale structures accurately, and vast applications of finite element method (FEM) in modeling of the MEMS, in this dissertation, the static and dynamic mechanical behavior of electrostatically actuated micro structures has been investigated using newly developed finite elements based on strain gradient continuum theory. The governing equations and boundary conditions have been developed for longitudinal and torsional behavior of microbars as well as flexural behavior of Euler-Bernoulli and Timoshenko microbeams based on strain gradient... 

Spinal Column is one of the most important parts of musculoskeletal system. Injuries in this region are so rampant and causing vast expense. Risk factors related to spine injuries included personal related ones such as, age, sex, muscles’ strength and career related ones such as body positions at work place and loads. Since in vivo measurements have many restrictions, developing biomechanical models that simulate the response of a person to given loads and conditions has important role in preventing injuries and improving working conditions. Inputs to current models of the spine are considered to have mean value, therefore muscles’ forces and other outputs of these models have single value... 

Wound healing by engineered scaffolds is a new step in bio-technology and medical studies in recent years. The goal of the current study is to propose a novel structure for a tissue-engineered scaffold to be used in wound healing. Influenced from the multi-layered structure of natural human skin, the fabricated scaffold consists of two layers to maximize similarity with natural skin. This product is comprised of an electrospun layer made of polycaprolactone and polyvinyl alcohol and a hydrogel layer made of chitosan and gelatin. In order to form a porous medium in the hydrogel layer, freeze-gelation was used instead of freeze drying. The evaluation of fabricated scaffolds was performed by... 

Nowadays, the usage of beam-shaped structures has widely spread in micro- and nano-electromechanical systems; consequently, scholars are extremely interested in precise modeling of static and dynamic behaviors of them. As the beams utilized in MEMS and NEMS structures have thicknesses in the order of microns and sub-microns, the experimentally validated small scale effects would be considerable in their behavior. In fact, experiments manifest that the size-dependent behavior is an intrinsic feature of materials when they are used in small-scale structures. In the classical continuum mechanics, there exists no material length scale parameter. Thus, the classical theory is unable to capture... 

Nowadays, with progresses made in manufacturing technologies, it is possible to produce micro-scale products such as micro-electromechanical systems. Micro-motors are one type of these systems that can be used in such applications as power supply of electronic devices. To achieve a better performance, these systems must rotate at high rotational speeds (about one million revolutions per minute). At such a high rotational speed, vibrational analysis is highly crucial. The results of recent studies indicate the inability of classical theories of continuum mechanics to analyze the behavior of these micro systems. Therefore, in this research, the rotor of micro-motors are modeled using the... 

Micro-rotating machinery with compact energy sources and high power density has been developed in the recent decade to run portable electronic devices. To achieve efficiency targets, these miniaturized turbo-machinery must spin at elevated rotational speeds which sometimes can reach up to one million revolutions per minute. At this relatively high spinning rate, the rotordynamic behavior of these systems plays an important role in the stage of design. On the other hand, the classical continuum theory has been proven not to be able to appropriately predict the mechanical behavior of the small-scale structures. To overcome overall mentioned weakness, in this thesis the rotor of micro-engines... 

The micropolar theory is a nonclassical continuum theory which considers a microstructure associated to each particle. This microstructure has three degrees of freedom and can rigidly rotate in the three dimensional space. Beside the classical or force stress tensor, in the micropolar theory the couple stress tensor is also considered.This research is devoted to deriving energy pair tensors for material strain measures in micropolar continuum. The subject of strains and their conjugate stresses is one of the major topics in the field of solid mechanics which has greatly been dealt with in the literature. This concept was introduced for investigating structural inequalities in solid... 

Experimental observations on multilayer graphene structures show that interlayer interactions including compressive and shear effects between layers are very important in their mechanical behavior. Existing analytical investigations have generally addressed the issue of pressure interactions between layers using interatomic potentials, while the models which consider the interlayer shear effect are rarely found in the literature. To address this shortcoming, this thesis presents a new formulation for multilayer graphene structures with desired shapes and boundaries, taking into account the interlayer shear effect according to the classical continuum mechanics theory. Next, size-dependent... 

The effects of combined task demands on the way muscles are recruited to balance external loads during posture and movement is of great importance in quantitative analysis of upper arm function. Pain and disability of shoulder joint is becoming more important as population spends more often interacting with computers in seated position and carrying repetitive tasks using upper extremities. Shoulder joint that is more dependent on the muscles for its normal function than other joints such as hip due to its small articular surface and large range motion has been used here to study its coordination during isometric exertions. Since, the numbers of tasks that could be performed are enormous. In... 

Keratoconus is one of the most common optical (visual, eye) diseases that usually occurs in youth or early third decade of life. In this disease, cornea gets thin and loses it initial shape and becomes conical. Doctors use different methods for its treatment including use of glasses, contact lenses (hard or soft), cornea transplantation and also cross linking of cornea. Each of these methods have their limitations, on the other hand, considering that this disease progresses and intensifies, ophthalmologists use corneal ring implant surgery for the treatment of this disease in its advanced levels. A good ring implant in corneal causes increased strength and also cause corneal to get back to... 

In this study, the linear dynamic response of an inclined FGM Timoshenko beam on linear viscoelastic foundation subjected to a traveling mass with constant and variable velocity is investigated. Timoshenko beam theory and von-karman strain-displacement relations are utilized to model the problem. the FGM beam is made of metal and ceramic and it is assumed that material properties of the beam vary continuously in the thickness direction according to the exponential law and the power-law form. The kelvin-voigt damping model is applied to model the internal damping of the FGM beam. The partial differential equations of motion for the bending rotation of cross-section, longitudinal and... 

In this thesis, the formulation of beams and plates in the micro- and nano scales is presented based on the second strain gradient theory (SSGT) introduced by Mindlin. The SSGT is a non-classical continuum theory that includes the second-order gradients of strain tensor in its formulation and this leads to more accurate modeling for mechanical behavior of materials than the strain gradient theory. On the other hand, due to the ever-increasing use of micro- and nano-electromechanical systems, analysis of the mechanical behavior of their deformable components for design and construction is of particular importance. For this purpose, analysis of Timoshenko beams and Euler-bernoulli beams made... 

In modelling of large deformations in deformable solids, rate type constitutive equations are of great importance. In order to fulfill the material objectivity conditions it is necessary to choose an objective rate of stress tensor. On the other hand, using corotational rates, because of their special properties (which are resulted of the fact that varaition of parameters are computed from an observer connected to a rotated triad), in rate type non-linear elastic constitutive equations (called hypo-elastic equations) are notable. In this thesis, the necessary conditions for a linear rate type isotropic tensor-valued function of a tensor argument and its rate to be integrable are driven. In... 

In this study, an elastic-plastic analysis of fiber reinforced metal matrix composite structures loaded by uniaxial uniform tension in the direction perpendicular to the fiber is carried out by assuming elastic-plastic matrix with kinematic hardening. Representative volume element (RVE) is used for this analysis. The element consists of a combined square field composed of a solid circular cylinder fiber and matrix in plane strain condition. Elastic analysis is carried out using Airy stress function and Michell solution. Unknown coefficients of the Airy stress function are determined by satisfying boundary conditions as well as continuity conditions. The governing equation of plastic zone is... 

Pumps are widely used in oil and gas industry. Therefore condition monitoring for this type of equipment is very important. The motor current signature analysis (MCSA) technique as one of the condition monitoring techniques is very effective in the industry, but it is less used in industry. All conventional methods require additional sensors to be installed in the system; current signature analysis as a sensorless method can reduce the cost of condition monitoring. In this study, motor current signature analysis technique was used to diagnosis a centrifugal pump with a capacity of 187 kW from the refinery industry. The impeller wearing ring clearance problem has been investigated. This... 

The aim of this work is numerical analysis of wave propagation in earth by using spectral element method for single and multi phase with application in oil industry. In this method, numerical solution has high accuracy due to using high order polynomials and appropriate integration points. Also, the mass matrix is diagonal by using this method which results in considerable decrease in computational cost with respect to finite element method.
The wave propagation in the mentioned models is studied by many researchers. The main problem in petroleum industry is large dimension of the model which results in high computational costs. Thus, using the proper numerical approach for these... 

In recent years, the world has seen a great progress in micro electro-mechanical systems (MEMS). Small size, low weight, high accuracy and low energy consumption have made these devices applicable in a variety of usages. In MEMS devices, mechanical components are used for specific purposes among which one of the most widely used are micro plates. Microplates are used in the structure of many devices such as microswitchs and atomic force microscopes. Therefore, studying of static and dynamic behavior of microplates is important. As the object gets smaller (to the scale of micro and nano meters), the classic theory of mechanics of continuous media cannot predict the behavior due to its... 

Polymeric microspheres which can load biomolecules, proteins and growth factors play an important role in tissue engineering and drug delivery systems. The size of the microspheres and encapsulation efficiency exert a considerable impact on the usability of them. In this study double emulsion method is used for fabrication of microspheres. Effect of different parameters namely the speed of homogenization, time of homogenization, the amount of polymer in oil phase, the surfactant concentration in different phases on the size and surface morphology of the microspheres are investigated. Moreover, a release test for BSA loaded microspheres is conducted. Results indicate that by increasing the... 

The main purpose in this thesis is the finite element simulation of the stress-strain behavior in an aluminum-silicon-magnesium alloy, subjected to low cycle fatigue and thermo-mechanical fatigue loadings. This finite element modeling is performed in the ABAQUS software. Two time-dependent and time-independent models are used to simulate the behavior of the material. The time-independent model is the combined nonlinear isotropic/kinematic hardening model, which predicts the mean stress relaxation behavior, the cyclic softening or hardening and the translation of the yield surface, while it ignores the effect of the strain rate. Time-dependent models include Johnson-Cook and the two-layer...