Sharif Digital Repository

Micro electromechanical systems (MEMS) such as sensors and actuators are gaining more popularity in recent years. These systems have different application in automation, medicine and other industries. Mechanical structures are governed by electrical systems in MEMS devices and this is one of the challenges of MEMS studies. Usually these systems made by one deformable beam or plate over a fixed substrate. Due to applied voltage between substrate and deformable plate or beam, the deformable plate deflects toward the fixed substrate. The voltage at which the system becomes unstableis called pull-in voltage. Pull-in voltage is the most important characteristics of MEMS beyond which pull-in takes... 

Machining chatter is unstable and destructive vibrations that leads to damages to the surface of workpiece and reduces the tool life. These damages have significant effects on surface quality and material removal rate. Many studies have been done on chatter modeling and control in various machining processes. On the other hand, recently, ultrasonic vibrations have been used in many manufacturing processes including cutting to enhance the quality of products. Therefore, it is important to investigate the effect of ultrasonic vibrations on the stability of cutting process. Although a number of advantages such as improved surface roughness of workpiece and reduced machining forces have been... 

With the recent trend to use thin shell structures in severe operational conditions, it is not sufficient to employ the classical linear theory to analyze their dynamic behavior, especially with large amplitudes. When the transverse deflection of a shell raise to the order of its thickness, the nonlinear effects grow significantly, leading to a variety of complex responses, such as the variable frequencies depending on the amplitude and the jump phenomenon. This dynamic behavior should be analyzed by the nonlinear theory of shells. In this research, the nonlinear vibration of composite shallow circular cylindrical shells is considered. The geometric nonlinear strains are of the von Karman... 

Process of casting with evaporative patterns and unbonded sand is a new method that due to different economic, environmental and technological benefits is considered to large and small industrial parts manufacturing in recent years. Different parameters are effective on the surface and metallurgical quality in this process, such as foam density, sand type and size, level and mode of vibration, sand compaction, coating and pouring time and temperature. If coating permeability that is function of type, thickness and concentration of coating is chosen improper, pyrolysis vapor of foam don’t exit completely and in this condition carbon of vapor deposit on the metal surface and produces defect... 

Tool Condition Monitoring (TCM) is a vital demand of advanced manufacturing in order to develop automated unmanned production. Tool condition has an essential influence on machined surface quality and dimension of manufactured parts. Continuing machining operation with a worn or damaged tool will result in damages to workpiece and even the machine tool itself. This problem becomes more important in supplementary machining processes like drilling in which the workpiece is usually at the final stages of production and any damage to workpiece at this stage is irreparable and results in high production losses. In this thesis, sound and vibrations signals are analyzed for drill wear detection.... 

This thesis presents several methods time-frequency based approach for classifying the vibration signals of rotatery machine. It uses the features extracted from the time-frequency distribution (TFD) of the vibration signal segments. Results of applying the method to a database of real signals reveal that, for the given classification task, the selected features consistently exhibit a high degree of discrimination between the vibration signals collected from healthy and fault machine. A comparison between the performances of the features extracted from several TFDs shows that the STFT slightly outperforms other reduced interference TFDs.


 

Static and dynamic instability of truncated conical shells made up of functionally graded materials (FGMs) conveying hot fluid is presented in this paper. Temperature-dependent material properties are considered and Material properties of the truncated FGM conical shells are functionally graded in the thickness direction according to a volume fraction power-law distribution. Governing equation of FGM conical shells is written based on First-Order Shear Deformation Theory (FSDT). The treatment is developed within the theory of linear elasticity, when materials are assumed to be isotropic and inhomogeneous through the thickness direction. The two-constituent functionally graded shell consists... 

Structural vibrations can be reduced using an opposite force generated from an active controller. Nowadays implementation of smart materials for active controlling of vibration is growing increasingly specially in aircrafts, spacecrafts and other state of art manmade machines. In aerospace structures as the weight is extremely a limiting factor, the lightweight structural control strategies are more demanded. Considering these motivations, in recent decades, engineers have done many researches to implement active structural damping methods in industrials applications. Among all active vibration approaches, using piezoelectric materials is one of the most attractive ways because of their... 

The current study is dedicated to free vibration and buckling analysis of thick FGM conical shells conveying hot flows with the consideration of initial geometric imperfections. To this end, the higher order governing equations of motion and the corresponding boundary conditions are derived using Hamiltonian formulations. Due to the solution procedure of Frobenius series expansion, the differential form of equations is obtained by applying by part integration to the integral form of equations of motion. Radial and longitudinal temperature distributions are considered while pressure distribution and geometric imperfection variations are found to be in longitudinal direction, only. The final... 

Electrical connectors that used in high speed systems interrupt the connection at the effect of Inertia. Also connectors should be able to work in high vibration conditions without any problems. Vibration of the connector pins can reduce the contact area or even remove it at very short time intervals, which lead to increase connector resistance or disconnection of instantaneous current. These current fluctuations will result in unpredictable performance disturbanceof the electrical system in very sensitive systems and therefore it should be avoided. In this Project, impacts of shock and vibration loads on the connectors is modeled by using FEM and with consideration of this model, connection... 

Drilling in oil industry, is an expensive process in produce and extract oil from well. Any mistake in drilling may cause many drawbacks for industry owners. Phisic’s quantities information at downhole like tempreture, pressure and force on bit synchronously can inform user from torrid conditions and prevent undesired events by actions like evoke bit from downhole or change at rotation velocity in drilling and finally stop operations. With attention to a lot of problems in data transport from long distance more than one kilometer from downhole, scientists believe that one of the methods to do this process is converting data gathered at the bottom hole to acoustic waves and transmit them to... 

Diagnostic and prognostic of spall defect in the rolling element bearings is investigated in this thesis. A new numerical model is proposed which considers defect size, defect clearance and defect roughness. The results of this model based on surface roughness show superior similarity to experimental data than classic model. To improve diagnostic ability in the rolling element bearings, four new methods are proposed in this thesis. These methods are short-time statistical features, local curve roughness, level crossing and wavelet packet decomposition. These methods are applied in outer ring, inner ring and ball defects and it is showed that these methods have better detection ability... 

The purpose of this thesis parametric study the natural frequencies and the criti-cal buckling multilayer composites truncated cone with the effect of lateral shear deformation. For this purpose five-bending tensile deformation equation of motion of the truncated cone shell in a suitable coordinate system have been studied. Then solve the five-coordinate movement for power series-are consid-ered. The natural frequencies and critical buckling force for the various truncat-ed cones with four different boundary conditions are calculated and the results with the results of similar conical shells with the same boundary conditions, tak-ing into account the effect of lateral shear deformation and... 

In this project, the equilibrium structure of clamped-free silicon nano-beams is investigated using molecular dynamics (MD) simulations. Four main interatomic potentials for silicon nanostructures, i.e. Tersoff, MEAM, Stillinger-Weber and EDIP, are implemented in the current study. Using aforementioned potentials, the structural stability of Si nano-beams are studied and effects of thickness, width, length, size, lattice direction, facets of cross section, and temperature on the equilibrium structure of nano-beams are investigated. The conducted simulations demonstrate that equilibrium structure of nano-beam is completely dependent on the type of atomic potential used for modeling.... 

Nanotechnology with the tools, materials and their applications in areas such as engineering mechanics, materials, electronics, computers, and machines at the nanoscale is dealing. The Fnarvy, atoms and molecules as building materials, engineering bricks counts. In mechanical engineering branch, two aspects are discussed in view of nanotechnology, nano-scale structures and analyzing the behavior of phenomena that occur at the nanoscale. In this project, methods to analyze the behavior of nanoscale materials has been studied.
The present thesis examines the natural frequencies of nano graphene sheets on elastic foundation deals. The elastic foundation modeled with two springs.At first The... 

Chatter in milling causes problems like damage to the device, losing precision or sleekness, increase in cannibalization, damage to the spindle, higher level of noise, and problem in thin objects. Current theories in regenerative chatter in filing, has a high relation with a work time (ρ~1), but in less deep filing (ρ≪1), these theories have errors (ρ is time per rotation period). Some of the usages of less deep filing are: 1- surfaces that are hard to file 2- creating curve surfaces 3- sleeking surfaces.Formulating these chatters and solving them using nonlinear methods help using the correct filing depth to reduce them. Goal of this project is to solve chatter equations in low depth filing... 

Fatigue crack growth can have harmful effects on the reliability rotary machine. While an early warning to crack, can be used to extend the life of these expensive machines and increase reliability.Structural Vibration Condition Monitoring as a primary fault detection method, achieved to a lot amazing result and for this reason studies vibration behavior of cracked rotational shaft is a big issue for engineers of dynamic machine.In this research, first check the fault detection methods based on structural vibration condition monitoring, mode shape methods were extracted. Then by modeling rotational shaft in computer and creation of crack on it, fault detection quality of these methods have... 

Suspension systems play a significant role in vehicles. They are responsible for providing ride comfort to the passengers and maintain car handling. Vibrations induced by the road can cause harmful effects on human body and this reveals the importance of designing and controlling the suspension system. Semi-active control is a reliable and safe method that has attracted a great deal of attention in the suppressing the car vibration. MR damper is a sample of semi-active control method. In most of the studies that have been done in literature the human body model has not been considered in designing the control strategy. In this study, a complete model of suspension system integrated with the... 

By new advancements in vehicle manufacturing; vehicle quality evaluation and assurance has become a more critical issue. Today noise and vibration generated inside and outside a vehicle are more important factors than before. In present work, the vibration transfer path analysis and vibration path ranking of a car interior has been performed. The method is similar to classical multilevel TPA methods but has distinct differences. The method is named VIVS which stands for Vehicle Interior Vibration Simulation. Performance of some tests like chassis dyno test, virtual mass function test and body transfer function test are required in this approach. The accelerations on both sides of the engine... 

Reinforced concrete shells are widely used to cover the small to large area with more aesthetics at minimum cost. Shell structures carries load through their shape rather than material strength. Funicular shells are special type of shells that their shape is obtained so that stresses be compressive under a special load (for shell, this load is its dead weight). This study deals with forced vibration of funicular shells on a rectangular ground plan under impulse loads using shallow shells theory. Two boundary conditions, simply supported and clamped, are considered. Displacement components are product of position and time functions. The analysis is based on the expansion of displacement...