Sharif Digital Repository

Vibrating ring gyroscopes (VRG) as MEMS devices are employed to micro scale systems for determination of the rotation rate and rotation speed of them. It is experimentally approved that micro scale structures behaves differently in comparison to macro scale systems, Therefore higher order continuum theories are required for modeling and analysis of these systems. In addition to that there is no comprehensive investigation on the dynamic performance of ring gyroscopes in the literature, In view of this in the present research the static and dynamic analysis of vibrating ring gyroscopes based on the strain gradient theory and the proposed finite element model of the gyroscope is performed. The... 

In this work a three-dimensional thermo-mechanical finite element analysis has been utilized to predict the large deformation of workpiece during the friction stir welding process using Arbitrary Lagrangian-Eulerian formulation in commercially finite element code, ABAQUS. The model can consider various process parameters including rotational and longitude speeds of the tool, backing plate and delay time. Ultimately, with optimizing the process parameters by presented model, experimental works carry out to improvement appropriate mechanical parameters of the weld. Comparison between the predicted and the experimental data displays a good consistency between distributions of the equivalent... 

In the limit case of incompressibility, the displacement-based finite element methods are not capable of finding the solutions with adequate accuracy. Moreover, discontinuities in displacement field or strain field which exist in the interior of the elements should be dealt with appropriately. The u/p mixed formulation provides a suitable context for modeling the incompressible problems. It is capable of solving general problems in which there exist geometrical or material nonlinearities. In the case of employing the eXtended Finite Element Method (XFEM), uniform meshes can be used for problems with discontinuities and in fact the discontinuities can be decoupled from the mesh. In this... 

In this thesis the effects of some casting parameters on residual stress and strain in steel casting parts have been studied. An H shape model has been chosen in order to investigate effects of casting parameters such as mold and cast constraints and inhomogeneous cooling on residual stress formation during casting in sand mold. These aforementioned factors have been simulated and studied in this research. Cooling of casting parts in sand mold and coupled thermo mechanical analysis has been simulated with the help of commercial finite element package ABAQUS. The results of this study show that, the complex samples with different section area, because of mold and cast constraints and... 

In this research study, two constitutive models of improved Microplane are presented. The microplane model M1 is capable of simulation of a brittle aggregate material that undergoes progressive tensile fracturing or damage. It is assumed that the normal stress on a plane of any orientation within the material is a function of only the normal strain on the same microplane. This strain is further assumed to be equal to the resolved component of the macroscopic strain tensor, while the stress on the microplane is not equal to the resolved component of the macroscopic stress tensor. This model also is capable of simulating realistic tensile stress-strain curve with strain-softening and reduction... 

Morphology and alignment of endothelial cells (ECs) have great influence on plaque formation. These alignments are based on the cyclic strains and wall shear stress (WSS) on the arterial wall. In the present study, cyclic strains and WSS are investigated to predict the secondary plaque locations downstream of the primary stenosis. Therefore, left coronary artery (LCA) with its branches to left anterior descending (LAD) and left circumflex (LCX) are 3D located on a virtual sphere to consider the cardiac motion. Stenoses are placed at the beginning of the bifurcation with varying degrees from 40% to 70% based on diameter reduction. Healthy coronary is also constructed to compare with those of... 

Sheet Metals are widely used in different industries such as ship building and pressure vessels. One important subject in these industries is to create the desired sheets through line heating process. Designing the heating lines in order to get the desired shape with the sufficient accuracy has always been a concern for the engineers in this process. In this thesis, the Strain-Based Method in designing the heat lines for creating special surfaces is discussed. Two sets of surfaces were considered. The first one is 2D surfaces set such as cylindrical surfaces. The second set is 3D surfaces set. The examples of such surfaces are double-curved surfaces such as a pillow and a saddle shape. The... 

There has been a rising trend towards robotic tele-surgery operations in recent years. A major concern, however, is the lack of direct contact between the surgeon and patient’s body. Several researchers have proposed various designs of tactile sensors for surgical instruments to improve the dexterity of surgeons. Previously designed sensors, however, are mostly suitable for instruments with small contact areas. In this thesis, a novel tactile sensor is introduced in integration with the teeth of a surgical grasper for large organs. It includes strain gauges embedded underneath a toothed plate, which also acts as grasping teeth of the jaws of the instrument. The thickness of the plate and the... 

In many problems, the traditional elasticity cannot predict phenomenon such as nano-scale defects, surface effects, and stress concentration correctly. For example, analysis near the crack tip, dislocation and inhomogeneity shows inconsistent results because in this case values of stress go to infinity. Therefore for solving these kinds of problems, higher order continuum theories were introduced. The appearance of additional constants in the equations of motion can represent the atomic nature of materials. Furthermore, they can be utilized for determining properties of materials vibration with high frequency, granular materials, and polymers.First strain gradient theory introduced by... 

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... 

The concept of generalized strains is a well-established subject in the classical continuum mechanics. The characteristics and various applications of the generalized strains have been the point of interest of many researchers in the classical continuum mechanics. The aim of this thesis is the introduction of generalized strains in the micropolar theory, as a non-classical continuum theory. A set of generalized strains is suggested for the deformation of macro-elements. Moreover, a set of generalized strains is proposed for the micro-structures. Since the rate of strains possesses a significant importance in the constitutive equations of nonlinear analysis of solids, expressions for the rate... 

First stage blades tempreture in the turbine rotor section for the advanced turbines has increased up to 1100oc. The blades should be under mentioned condition for the long times up to 70000 hours and accordingly creep happens. Fatigue–creep interaction is the main reason for the failures of many engineering components under high temperature and cyclic loading. In this thesis, a model for the life prediction of fatigue–creep interaction which was developed by other investigators has been modified to predict fatigue–creep interaction on the nickel-based superalloys. In this model, the law of energy conservation and the momentum conservation principle are used to describe the process of... 

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... 

One of the methods of forming the metal parts is to apply a localized strain field to the workpiece. This field can be generated by creating a deformation in an actuator connected to the workpiece, or directly by a localized imposing heat (thermal localized strain) to the workpiece. For example in laser forming process, the workpieces in the forms of sheet and tube can be deformed by creating localized strain field induced from thermal field caused by laser irradiation. The main mechanisms in laser forming are temperature gradient mechanism, upsetting mechanism and buckling mechanism. It is worth mentioning that until now, the localized strain field induced by laser irrediation is... 

Nowadays the combination of mechanical and electronic systems in small scales is gaining attention. Micro electro-mechanical systems are widely used in various industries such as car manufacturing and electronic chips. In these devices, the most important and useful mechanical structures are beams and plates. Therefore, investigating the mechanical properties of these structures in very small scales (micro and nano) is of great importance. Decreasing the size to these scales leads to the dependence of the behavior of these structures to size. Accordingly, some theories for prediction of these behaviors have been presented, one of which is the strain gradient theory. This theory is used in... 

Evaluating the magnitude and distribution of ground movements adjacent to an excavation wall is an important part of the design process when excavating in an urban environment. In urban environment, this movement controlled by monitoring and the result of monitoring need to compared with analysis result. 2D analysis can’t explain Three dimesional effects in excavation. This effects caused by the relatively high stiffness at the corners of an excavation lead to smaller ground movements near the corners and larger ground movement towards the middle of the excavation excavation's wall. Plane strain Ratio (PSR) quantify the magnitude of 3D corner effect in excavation defined as the ratio of the... 

Due to the features like small dimensions, low manufacturing cost and low power consumption, micro-electromechanical systems (MEMS) are widely utilized in engineering applications. Many experimental investigations have indicated that the mechanical behavior of constructive microelements of these systems isn’t predictable by classical continuum theory. Therefore, to analyze the behavior of microelements, the non-classical continuum theories which can capture size effects should be utilized. On the other hand, various experimental observations have confirmed that thermoelastic damping (TED) is a dominant source of energy dissipation in microelements, in contrast to the non-small parts and... 

In this study, a novel multi-scale hierarchical method has been employed to explore the role of edge dislocation on Nano-plates with hexagonal atomic structure in large deformation. multiscale hierarchical atomistic/molecular dynamics (MD) finite element (FE) coupling methods are proposed to demonstrate the impact of dislocation on mechanical properties of Magnesium in large deformation. The atomic nonlinear elastic parameters are attained via computing first-order derivation of stress with respect to strain of Representative Volume Element (RVE). To associate between atomistic and continuum level, the mechanical characteristics are captured in the atomistic scale and transferred to the... 

Here the differential equations of a Graphene sheet deformation is determined by making use of the most general form of first Strain Gradient Elasticity (SGE) theory. In order to derive the equations of in-plane and out-of-plane deformations, the matrix representation of Strain Gradient constitutive law is used. Vibration of the nano-plate is studied by modeling the plate based on Kirchhoff theory and application of variation of energy, therefor the in-plane and out-of-plane vibration of the nano-plate is found through Navier analytical solution. After all natural frequencies are obtained based on SGE parameters. SGE parameters are determined by calculating the potential of a representative... 

The analysis of nonlinear forced vibration of micro-rotors (rotating micro-shafts with disks) is carried out under loads of mass eccentricity distribution and in the presence of internal damping. For the analysis, the non-classical continuum theories of couple stress and strain gradient are employed. Vibrational behavior of micro-rotors is extremely sensitive due to the very high rotational speed, which is at the order of several millions per minute speed. The considered nonlinearity is of geometrical type due to the mid-plane stretching. First, the governing equations of motion of micro-rotors are derived by utilizing the Hamilton principle. In the next step, the Galerkin and multiple scale...