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    Modal Identification of Non-classically Damped Linear Systems Using Wavelet Transform

    , M.Sc. Thesis Sharif University of Technology Kamali-Asl, Arash (Author) ; Ahmadizadeh, Mahdi (Supervisor)
    Application of convolution-based transforms such as Fourier transform and wavelet transform are among the widely-used approaches in structural identification problems. These methods, however, are mostly based on a classical damping model for the structures. Hence, such methods cannot be used to accurately identify the modal properties of non-classically damped systems, such as structures equipped with energy dissipated devices, base isolated and multi-segment structures. In this study, complex-valued wavelet transform is used to identify the dynamic properties of structures with arbitrary damping models. This method is known to be able to identify the modal properties of highly damped... 

    A Criterion for Detection of Delamination in Laminated Composite Beams and Plates Based on Modal Analysis of Damaged Structure

    , Ph.D. Dissertation Sharif University of Technology Kazemi Bazardehi, Roohollah (Author) ; Kouchakzadeh, Mohammad Ali (Supervisor)
    In this study, irregularities of the mode shapes are utilized to determine the delamination situation in a composite laminated beam. Modal analysis is used to obtain the mode shapes of the beam; afterwards, the mode shapes are filtered to extract their irregularities. Presence of any delamination and its situation (number, size and location along the length and thickness of the beam) affects the extracted irregularities. Here, detection of delamination is based on these irregularities of the mode shapes. One of the most important advantages of this method is that unlike many other similar methods, in many cases, the mode shapes of damaged structure are sufficient and the exact mode shapes... 

    A new continuous model for flexural vibration analysis of a cracked beam

    , Article Polish Maritime Research ; Volume 15, Issue 2 , July , 2008 , Pages 32-39 ; 12332585 (ISSN) Behzad, M ; Ebrahimi, A ; Meghdari, A ; Sharif University of Technology
    In this paper a new continuous model for vibration analysis of a beam with an open edge crack is presented. A quasi-linear displacement filed is suggested for the beam and the strain and stress fields are calculated. The equation of motion of the beam is calculated using the Hamilton principle. The calculated equation of motion is solved with a modified weighted residual method and the natural frequencies and mode shapes are obtained. The results are compared with those obtained by finite element method and an excellent agreement has been observed. The presented model is a simple and accurate method for analysis of the cracked beam behavior near or far from the crack tip  

    Nanoscale vibrational behavior of single-layered graphene sheets

    , Article ASME International Mechanical Engineering Congress and Exposition, IMECE 2007, Seattle, WA, 11 November 2007 through 15 November 2007 ; Volume 11 PART A , 2008 , Pages 229-235 ; 079184305X (ISBN); 9780791843055 (ISBN) Sakhaee Pour, A ; Ahmadian, M. T ; Vafai, A ; Sharif University of Technology
    Molecular structural mechanics approach is implemented to investigate vibrational behavior of single-layered graphene sheets. By using the atomistic modeling, mode shapes and natural frequencies are obtained. Vibration analysis is performed under different chirality and boundary conditions. Numerical results from the finite element technique are applied to develop empirical equations via a statistical multiple non-linear regression model. With the proposed empirical equations, fundamental frequencies of single-layered graphene sheets under considered boundary conditions can be predicted within 3 percent accuracy. Copyright © 2007 by ASME  

    System Identification by Image Processing

    , M.Sc. Thesis Sharif University of Technology Raoofi Lelekaee, Hamed (Author) ; Moghaddam, Hassan (Supervisor)
    Digital image processing techniques are widely used in many areas of research and application such as astronomic monitoring, medical diagnosis, product quality control and weather forecasting. The captured film by camera, is converted into displacement data by using Matlab software and image processing methods. This study intends to provide “proof-of-concept” for the techniques, applying them to identification problems involving the simple pendulum, pendulum in-water, the car suspension system. Studies further illustrate that the digital imaging procedure and algorithms developed for the simple problems can be extended for identification of modal properties and structural characteristics.... 

    Crack Detection in a Rotor using Traditional Mode Shape Based Modal Analysis Methods and Experimental Evaluation

    , M.Sc. Thesis Sharif University of Technology Ashtiani, Mohammad Reza (Author) ; Mohammad Navazi, Hossein (Supervisor)
    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... 

    Application of Existing Damage Detection Measures for Long-Span Cable Stayed Bridges under Multiple Support Excitations

    , M.Sc. Thesis Sharif University of Technology Bakhshizadeh, Arezoo (Author) ; Rahimzadeh Rofooei, Fayyaz (Supervisor)
    Damage and collapse in structural systems due to unexpected structural loading that seems to be inevitable sometimes leads to financial and human losses, despite of all safety measures considered for the system. Therefore, using structural health monitoring methods in order to estimate structural damages and preventing structural collapse, have drawn a lot of attention. In this study, the performance of different system identification methods such as mode shape curvature method, energy method, etc., to extract the dynamic properties of a cable-stayed bridge is examined. One of the main features of this study is to consider the seismic response of the bridge under multiple support excitations... 

    Using Vibration Behavior Sensitivity of Beams to Damage for Detection of Damage Location and Extent

    , Ph.D. Dissertation Sharif University of Technology Banihashem, Alireza (Author) ; Joghataie, Abdolreza (Supervisor)
    This research is on damage detection in beams using changes in their vibration behavior with respect to damage. The aim of this research is to investigate the sensitivity of strain mode shapes to damage and the feasibility of their using in damage detection. Comparison with displacement mode shapes is another main subject of this research. For this aim, at first an experimental study has been conducted at the structures lab of Sharif University of Technology, to investigate the sensitivity of modal parameters of an Aluminum beam to progressive damage. Then a beam has been modeled numerically sing shell elements. Also a new frequency response function (FRF), here called the strain frequency... 

    Experimental Identification of Repeated Modes for a Plate Under Ambient Loading Using Time Domain Methods of Operational Modal Analysis

    , M.Sc. Thesis Sharif University of Technology Hajirezayi, Sajjad (Author) ; Hosseini Kordkhili, Ali (Supervisor)
    In this research, the reliability of time domain modal identification methods is studied. First three different MIMO time domain method are studied which are EITD, PRCE and ERA. The effectiveness and accuracy of the methods are evaluated for simulated mass-spring system and numerical cantilever beam in EMA (using time history data of impulse response directly) and OMA (using correlation function of random response of the structure) cases. Based on the results achived in this section, the most qualified method is identified (which was ERA) to continue the research. In order to evaluate the reliability of time domain OMA technique in case of repeated modes, first a numerical plate is studied... 

    Analysis & Vibration Control of a Rotating Beam with Variable Properties of the Cross Section

    , M.Sc. Thesis Sharif University of Technology Sahlolbeye, Zahra (Author) ; Moradi, Hamed (Supervisor)
    The purpose of this project is to model a rotating beam with variable properties of the cross section and also to present an active control method for suppressing the undesired vibrations due to external loading. The beam is modeled as a cantilever Euler-Bernoulli beam with nonuniform physical and geometrical properties. For a practical example, the characteristics of a wind turbine blade was used to solve the problem. The analyzed blade in this study is NREL 5-MW wind turbine blade. But the method can be extended to be used for other blades as well . First the natural frequencies and modeshapes were computed by MATLAB. Then by applying pole placement method, a control strategy was designed... 

    Detection of Damage Location in Two-way Slabs Using Structural Vibration Modes with Wavelet Transform

    , M.Sc. Thesis Sharif University of Technology Kianipour, Ali (Author) ; Bakhshi, Ali (Supervisor)
    This study aimed to develop a method for locating damages in the early stages of damage expansion in plate components of the structure using a one-dimensional discrete wavelet function. Continuous monitoring of the health of structures to preserve and repair them to protect the lives and property of individuals and national capitals and to lower the direct and indirect economic costs imposed on society is vital and inevitable for society's decision-makers. Because the damage to the structure caused by earthquakes occurring during the lifetime of the structure, exhaustion of structural components, improper implementation of the structure at the time of construction, and other events that... 

    Blind modal identification of non-classically damped systems from free or ambient vibration records

    , Article Earthquake Spectra ; Volume 29, Issue 4 , 2013 , Pages 1137-1157 ; 87552930 (ISSN) Abazarsa, F ; Nateghi, F ; Ghahari, S. F ; Taciroglu, E ; Sharif University of Technology
    A significant segment of system identification literature on civil structures is devoted to response-only identification, simply because lack of measurements of input excitations for civil structures is a fairly common scenario. In recent years, several researchers have successfully adapted a second-order blind identification (SOBI) technique-a method originally developed for "blind source separation" of audio signals-to response-only identification of mechanical and civil structures. However, this development had been confined to fully instrumented classically damped systems. While several approaches have been proposed recently for extending SOBI to non-classically damped systems, they all... 

    Resonance frequencies and stability of two flexible permanent magnetic beams facing each other

    , Article Journal of Sound and Vibration ; Volume 331, Issue 26 , 2012 , Pages 5745-5754 ; 0022460X (ISSN) Firouz Abadi, R. D ; Mohammadkhani, H ; Sharif University of Technology
    This paper aims at investigating the interaction of two flexible permanent magnet beams facing each other. The governing equations of motion are obtained based on the Euler-Bernoulli beam model along with Hamiltons principle. Assuming that the beams tips are far enough, each magnet beam is considered as a series of dipole segments and the external force and moment distributions over each beam due to the magnetic field of the other one is calculated in the deformed configuration. The transverse deflections of the beams are written as series expansions of the mode shapes of an unloaded cantilever beam and the Galerkin method is applied to determine the stability and resonance frequencies.... 

    Forced vibration of delaminated Timoshenko beams subjected to a moving load

    , Article Science and Engineering of Composite Materials ; Volume 19, Issue 2 , June , 2012 , Pages 145-157 ; 0334181X (ISSN) Kargarnovin, M. H ; Ahmadian, M. T ; Jafari-Talookolaeia, R. A ; Sharif University of Technology
    A composite beam with single delamination under the action of moving load has been modeled accounting for the Poisson's effect, shear deformation, and rotary inertia. The existence of the delamination changes the stiffness of the structure, and this affects the dynamic response of the structure. We have used a constrained mode to simulate the behavior between the delaminated surfaces. Based on this mode, eigensolution technique is used to obtain the natural frequencies and their corresponding mode shapes for the delaminated beam. Then, the Ritz method is adopted to derive the dynamic response of the beam subjected to a moving load. The obtained results for the free and forced vibrations of... 

    An exact Analytical solution to exponentially tapered piezoelectric energy harvester

    , Article Shock and Vibration ; Volume 2015 , 2015 ; 10709622 (ISSN) Salmani, H ; Rahimi, G. H ; Hosseini Kordkheili, S. A ; Sharif University of Technology
    Hindawi Publishing Corporation  2015
    It has been proven that tapering the piezoelectric beam through its length optimizes the power extracted from vibration based energy harvesting. This phenomenon has been investigated by some researchers using semianalytical, finite element and experimental methods. In this paper, an exact analytical solution is presented to calculate the power generated from vibration of exponentially tapered unimorph and bimorph with series and parallel connections. The mass normalized mode shapes of the exponentially tapered piezoelectric beam with tip mass are implemented to transfer the proposed electromechanical coupled equations into modal coordinates. The steady states harmonic solution results are... 

    Free longitudinal vibration of tapered nanowires in the context of nonlocal continuum theory via a perturbation technique

    , Article Physica E: Low-Dimensional Systems and Nanostructures ; Volume 43, Issue 1 , November , 2010 , Pages 387-397 ; 13869477 (ISSN) Kiani, K ; Sharif University of Technology
    The free longitudinal vibration of tapered nanowires is investigated in the context of nonlocal continuum theory. The problem is studied for the nanowires with linearly varied radii under fixedfixed and fixedfree boundary conditions. In order to assess the problem in a more general form, a perturbation technique is proposed based on the Fredholm alternative theorem. The natural frequencies, corresponding mode shapes, and phase velocities of the tapered nanowires are derived analytically up to the second-order perturbation for different boundary conditions. The predicted results by the perturbation technique are successfully verified with those of the exact solution. The obtained results... 

    Blind identification of soil-structure systems

    , Article Soil Dynamics and Earthquake Engineering ; Volume 45 , February , 2013 , Pages 56-69 ; 02677261 (ISSN) Ghahari, S. F ; Ghannad, M. A ; Taciroglu, E ; Sharif University of Technology
    Surrounding soil can drastically influence the dynamic response of buildings during strong ground shaking. Soil's flexibility decreases the natural frequencies of the system; and in most cases, soil provides additional damping due to material hysteresis and radiation. The additional damping forces, which are in non-classical form, render the mode shapes of the soil-structure system complex-valued. The response of a soil-foundation system can be compactly represented through impedance functions that have real and imaginary parts representing the stiffness and damping of the system, respectively. These impedance functions are frequency-dependent, and their determination for different... 

    Acoustic characteristics of a rocket combustion chamber: radial baffle effects

    , Article Applied Acoustics ; Volume 70, Issue 8 , 2009 , Pages 1051-1060 ; 0003682X (ISSN) Farshchi, M ; Mehrjou, H ; Salehi, M. M ; Sharif University of Technology
    This paper describes methods used for determining the characteristic acoustic modes and frequencies of a liquid-propellant rocket-motor combustion chamber and effects of radial baffles on the chamber's acoustic field. A multi-point sensing experimental setup, including stationary and moving sensors, was used to measure characteristic frequencies and mode shapes of a combustion chamber. A new technique based on the comparison of signal phase angles from stationary sensors to that of a moving sensor was used to map complex characteristic mode shapes of a combustor. A three-dimensional Helmholtz acoustic solver was also developed using an efficient finite volume approach for complex geometries... 

    Damage Detection in Offshore Jacket Structures Using Invers Vibration Problem (IVP)Technique with a Look on Probabilistic Bayesian Network Modeling

    , M.Sc. Thesis Sharif University of Technology Ahmadi, Ali (Author) ; Abbaspour, Madjid (Supervisor) ; Tabeshpour, Mohammad Reza (Supervisor)
    Damage detection in all kinds of human-made structures has been given serious attention as one of the main branches of engineering sciences, and significant progress has been made in this field, especially in the field of civil engineering, aerospace and heavy industries. However, although this topic is discussed academically in the field of offshore structures for two decades, not much development has happened in practice and it is still in the stage of knowledge development. One of the most important characteristics of practical online damage detection methods is the ability to be used knowing little information about the modal characteristics of the structure. In this research, an attempt... 

    Forced vibration of delaminated timoshenko beams under the action of moving oscillatory mass

    , Article Shock and Vibration ; Volume 20, Issue 1 , 2013 , Pages 79-96 ; 10709622 (ISSN) Kargarnovin, M. H ; Ahmadian, M. T ; Jafari Talookolaei, R. A ; Sharif University of Technology
    This paper presents the dynamic response of a delaminated composite beam under the action of a moving oscillating mass. In this analysis the Poisson's effect is considered for the first time. Moreover, the effects of rotary inertia and shear deformation are incorporated. In our modeling linear springs are used between delaminated surfaces to simulate the dynamic interaction between sub-beams. To solve the governing differential equations of motion using modal expansion series, eigen-solution technique is used to obtain the natural frequencies and their corresponding mode shapes necessary for forced vibration analysis. The obtained results for the free and forced vibrations of beams are...