Sharif Digital Repository

Todays, advances in manufacturing technologies have led to design and production of micro-scale elements, including the elements employed in micro-electro-mechanical systems (MEMS). Micro-rotating systems like the micro-turbines are that kind of systems at which the high rotational speeds and the complexity of design and analysis have led to a special attention in modeling and investigating of their dynamic-vibrational behavior. In addition, in small scales, using the non-classical continuum mechanics theories such as the couple stress theory and the strain gradient theory is required to obtain the high precise results. On the other hand, attention to the torsional deformation of rotors... 

Incapability of the classical continuum mechanics theory to justify the experimental observations of the mechanical response of the small-scale structures and parts motivated the researchers to pursue the introduction and utilization of the non-classical continuum theories for analysis and design of such structures and parts. In this paper, utilizing the modified couple stress theory and the strain gradient theory as well-known and powerful non-classical continuum theories, the mechanical response, including the displacement and stress fields, for micro-rotating disks with angular acceleration is investigated. The governing differential equations of motion and the corresponding boundary... 

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... 

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... 

Researchers have done a lot of studies about the use of CFS in blood flow modeling in order to improve the supplementary devices or find mechanical factors which cause artery to be diseased. Blood is a complex rheological fluid, blood flow is a pulastile flow, and blood flow field interacts with the deformable vessel wall. Thus, blood flow modeling like other biological phenomena has its own complexities such as anisotropy, vsicoelasticity, and nonlinearity in stress-strain relationship of vessel wall. To explore the role of hemodynamics in the initiation and progression of stenosis in carotid artery bifurcation, a 3D Computational Fluid Dynamics (CFD) technique is applied. The effect of... 

The purpose of this research is to study the behavior of conventional and retrofitted Khorjini connections. Thus the software ABAQUS 6.11 was used to fabricate the FEM models of the connections with experimental prototypes. In analytical studies, 21 specimens with different size of the connection components were analyzed. True stress–strain curves for steel and weld materials were used and damage initiation and revolution were considered in the weld material. Also cracking of the welds were considered by a micromechanical method. Behavioral characteristics of the conventional connections are specified and appropriate relations are suggested, using Stability equations of the connection... 

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... 

The stirred tank is one of the oldest device that widely used in different industrial. a special kind of stirred tank that clled bioreactor refer to any device which supplies required biological, biochemical and biomechanical engineered product. Since the aim of bioreactor is produce of desired biological product, it is essential to accurate monitoring internal and external transfer of nutrient, heat transfer, fluid velocity, Strain stress and medium PH. good design and application of bioreactor depend on product, organism, conditions for produce desired product and scale of produce. also cost of capital and function cost are important in bioreactor design. one of the most important... 

In recent decades, the importance of theoretical and experimental researches on sheet metal forming has been revealed due to its wide-spreading applications. The purpose of these researches is to optimize the forming processes and minimize the existing imperfections. Since classical theories don’t include parameters for describing the dependence of metals behavior on size effects or inhomogeneous plastic deformation effects, gradient theories are considered in many researches. In gradient theories, this dependence is considered by changing the constitutive equations and the existence of higher order of strain gradient in zones with inhomogeneous deformation. By considering the localized and... 

In this thesis, both experimental and numerical approaches are conducted to present a constitutive equation for 5052 aluminum diaphragms under quasi-static strain rate loadings. For this purpose the stress-strain curves at different strain rates are obtained using tensile tests. Brittle behavior during tensile tests is observed due to samples thin thicknesses. Employing Johnson-Cook constitutive equation no yields in reasonable agreement with these tensile tests results. Therefore, developing a more suitable constitutive equation for aluminum diaphragms is taken into consideration. This equation is then implemented into the commercial finite element software, ABAQUS, via a developed user... 

In recent years, the use of fiber reinforced polymer (FRP) jackets, as an external lateral confinement has found considerable attention in retrofitting and strengthening existing reinforced concrete (RC) columns in earthquake prone areas. The FRP composites have numerous advantages such as ease in application, high strength and stiffness-weight ratio, no corrosion and negligible changes in the member dimension. Most of previous studies have concentrated on evaluation of the effect of confinement with spiral bars or FRP composites, separately. However, in practice both types of confinements exist in strengthening RC columns.
This paper presents the results of a computational modeling to... 

Studying the behavior of materials at high strain rate is necessary for design and analysis of structures subjected to impact loading. Therefore, design and construction of apparatuses that enable us for testing various materials under impact loading is very important. The split Hopkinson pressure bar test -one of the most important impact tests- is utilized for dynamic loading at constant strain rates ranging from 100 to 10000 1/s.
In this thesis, first we explain about the behavior of materials at high strain rates and then focus on the behavior of polymers -that are widely used in different industries- under impact loading. Also, basic concepts of polymeric Hopkinson pressure bar and... 

In this study, the forming limit stress and strain diagrams have been determined based on the strain gradient plasticity theory. Forming limit stress diagram (FLSD) has been reported as being much less path-dependent and much more favorable than the forming limit diagram (FLD) in representing forming limits in the numerical simulation of sheet metal forming processes. Therefore, the purpose of this study is to develop a methodology for prediction of the forming limits both in strain and stress spaces. All simulations are based on strain gradient theory of plasticity in conjunction with the so-called Marciniak-Kuczynski (M-K) approach. This method introduces an internal length scale into... 

One of the extensively used, yet complex, structures in the industry is pre-twisted Micro/Nano beams. Studying their mechanical behavior helps to have a broader view of them. In this present study, explores and analyzes the behavior of a pre-twisted Micro/Nano beam with a quadrangular/rectangular cross-section using the strain gradient theory and modified couple stress theory. Using the calculus of variations and the Hamiltonian principle the elastodynamics governing partial differential equations of transverse deflection of the pre-twisted Micro/Nano beam with hinged-hinged boundary conditions are derived. Then the mechanical behavior of the pre-twisted Micro/Nano beam in static mode and... 

Two methods for simulating material behavior of heart mitral valve leaflet tissue are developed in this thesis, in the finite element setting. First, a mixed pressure-displacement formulation is used to implement the constitutive material behavior with general 3D elements. Second, a shell is formulated that incorporates the 3D material behavior by use of a local plane stress iteration method. Both of these works are based on an existing invariant-based strain energy function that has been experimentally determined for the mitral valve leaflet tissue. Since this material is considered to be nearly incompressible, a mixed pressure-displacement (u/p) formulation is needed to apply the material... 

Evaluation of the seismic-induced settlement of an unsaturated soil layer depends on several variables, among which the small strain shear modulus, Gmax, and soil’s state of stress have been demonstrated to be of particular significance. Recent interpretation of trends in Gmax revealed considerable effects of the degree of saturation and hydraulic hysteresis on the shear stiffness of soils in unsaturated states. Accordingly, the soil layer is expected to experience different settlement behaviors depending on the soil saturation and seasonal weathering conditions. In this study, a semi-empirical formulation was adapted to extend an existing Gmax model to infer hysteretic effects along... 

All rotating machines (such as internal combustion engines, reciprocating compressors, propulsion systems, etc.) experience torsional vibration during Start up, continuous working conditions and blackout. Torsional vibrationis due to pressure rhythms, engine dynamic torques, etc. All components of a rotating system must convey dynamic torque in addition to the static torque.Excessive Torsional vibrations cause wear and damage such as gears, gear teeth damage and failure to the main shaft.Online monitoring of torsional vibrations of the machine can be displayed machine condition at any moment and consider the risk in early diagnosis. Shaft torsional vibration monitoring can detect cracks in... 

Damage detection and rehabiltation is one of the most economical methods of increasing safety and service life of the structures. In this research, a wave-based methodology is introduced for laminar damage location in single axial members and combined axial-flexural members within a structure. Laminar damage is a kind of local damage in which the cross section area of a member decreases within a specific length. In the proposed wave-based methodology, a structure with a laminar damage is analyzed using a Finite Element (FE) software under a high frequency loading, and the strain values in specific points of the structure is collected. Thereafter, using the wave propagation theories and the... 

Shear band is more relevant in porous medias rather than continuum media. Shear band is a narrow region of solid experiencing intense shearing. After initiation of shear band, the sliding on two sides of the band occurs. The area inside the shear band undergoes extreme plastic deformation, while experiences elastic unloading outside of the band. In porous materials, the direction of slip and discontinuity in displacement field is not same, as a result of dilation. In fact dilation requires an increase in the volume of the shear band zone. The main difficulty in modeling the shear band is that the width of shear band in contrast to the dimension of the media, is too small. Consequently, the... 

In recent decades, the study of Nano and Micro Electromechanical Systems (NEMS and MEMS) as in micro sensors and actuators or in Atomic Force Microscopes (AFM) attracted considerable attention. Since an indispensable part of such systems is a nano or micro flexible beam, the inevitable undesirable vibrations of the flexible beam necessitate the implementation of vibration control strategies. Recently, some cutting edge experiments on statics and dynamics behavior of micro beams have been conducted that reveal some deviations from the prediction of classical theories, and hence this leads to development of strain gradient elasticity theory. In addition, the often-adopted finite dimensional...