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Effect of vacancy defects on the fundamental frequency of carbon nanotubes
, Article 3rd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, NEMS 2008, Sanya, 6 January 2008 through 9 January 2008 ; 2008 , Pages 1000-1004 ; 9781424419081 (ISBN) ; Ahmadian, M. T ; Asempour, A ; Tajalli, S. A
2008
Abstract
Carbon nanotubes are widely used in the design of nanosensors and actuators. Any defect in the manufactured nanotube plays an important role in the natural frequencies of these structures. In this paper, the effect of vacancy defects on the vibration of carbon nanotubes is investigated by using an atomistic modeling technique, called the molecular structural mechanics method. Vibration analysis is performed for armchair and zigzag nanotubes with cantilever boundary condition. The shift of the principal frequency of the nanotube with vacancy defect at different locations on the length is plotted. The results indicate that the frequency of the defective nanotube can be larger or smaller or...
Finite element model of SWCNT under hydrostatic pressure
, Article AIP Conference Proceedings, 10 April 2007 through 12 April 2007, Sharjah ; Volume 929 , 2007 , Pages 82-88 ; 0094243X (ISSN) ; 0735404399 (ISBN); 9780735404397 (ISBN) ; Ahmadian, M. T ; Sharif University of Technology
2007
Abstract
A finite element technique is used to mimic radial deformation of single-walled carbon nanotubes under hydrostatic pressure. The elastic deformation of nanotubes is modeled via elastic beams. Properties of the beam element are evaluated by considering characteristics of the covalent bonds between the carbon atoms in a hexagonal lattice. Applying the beam model in a three dimensional space, the elastic properties of the nanotube in the transverse direction are evaluated. The effects of diameter and wall thickness on the radial and circumferential elastic moduli of zigzag and armchair nanotubes are considered. Results are in good agreement with molecular structural mechanics data in the...
Effect of centrifugal force on frequency characteristic of rotating hollow cylinders using a newly designed cylindrical super element
, Article 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006, Chicago, IL, 5 November 2006 through 10 November 2006 ; 2006 ; 10716947 (ISSN); 0791837904 (ISBN); 9780791837900 (ISBN) ; Ahmadian, M. T ; Sharif University of Technology
American Society of Mechanical Engineers (ASME)
2006
Abstract
A sixteen node cylindrical super element is presented for evaluating the free vibration characteristics of a rotating laminated cylinder with conventional boundary conditions. It is shown that the natural frequencies are affected considerably when the centrifugal force is also taken into account. The vibration frequencies of rotating finite cylinder, obtained by conventional finite element are used to evaluate the accuracy of this approach. The special case of a stationary cylinder with zero spinning velocity is also considered as a check on this method. Results indicate only few number of cylindrical super elements are capable of predicting the natural frequency of the rotating cylinder...
Closed form solutions for the motion of electrically excited micro-cantilever beams
, Article 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006, Chicago, IL, 5 November 2006 through 10 November 2006 ; 2006 ; 1096665X (ISSN); 0791837904 (ISBN); 9780791837900 (ISBN) ; Ahmadian, M. T ; Sharif University of Technology
American Society of Mechanical Engineers (ASME)
2006
Abstract
The differential equation governing the motion of an electrically excited capacitive microcantilever beam is a nonlinear PDE [1]. Accurate analysis about its motion is of great importance in MEMS' dynamical response. In this paper first the nonlinear 4th order 2 point boundary value problem (ODE) governing the static deflection of the system is solved using three methods. 1. The nonlinear part is linearized and its exact solution is obtained. 2. For low applied DC voltages (not near pull-in) the solutin is found using the direct straight forward perturbation analysis. 3. Numerical computer solutions which are used for the previous solution's verifications. The next parts are devoted to the...
Effects of the electric field configuration's variation due to micro-cantilever beam's curvature on pull-in phenomenon
, Article 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006, Chicago, IL, 5 November 2006 through 10 November 2006 ; 2006 ; 1096665X (ISSN); 0791837904 (ISBN); 9780791837900 (ISBN) ; Ahmadian, M. T ; Sharif University of Technology
American Society of Mechanical Engineers (ASME)
2006
Abstract
Micro-electromechanical systems (MEMS) have wide application in the development of sensors for the detection of magnitudes in almost any domain [1]. Resonant mode operation of micro and nano-scale oscillators have gained wide interest for applications including filters, amplifiers, non-linear mixers, atomic scale imaging, biological and chemical sensors. The device that we propose is an electrically actuated microcantilever beam. More precisely, in our design the microcantilever constitutes the movable plate of a micro-capacitor and its displacement is controlled by the voltage applied across the plates. Review of literature shows that the electric field pattern between the beam and the...
Effect of actuation frequency on the performance of diffuser micropumps
, Article 4th International Conference on Nanochannels, Microchannels and Minichannels, ICNMM2006, Limerick, 19 June 2006 through 21 June 2006 ; Volume 2006 B , 2006 , Pages 725-731 ; 0791847608 (ISBN); 9780791847602 (ISBN) ; Ahmadian, M. T ; Sharif University of Technology
2006
Abstract
Valveless piezoelectric micropumps are in wide practical use due to their ability to conduct particles with absence of interior moving mechanical parts. The objective of this paper is to obtain the fluid flow response to actuation frequency of a passive diffuser valve under harmonic pressures. In this regards a 2D model of a micropump valves and chambers is analyzed. The analysis is performed for 10Kpa back pressure on micropump chamber and actuation frequencies within the range of 1 Hz to 10 KHz. Results show the highest velocity in the direction of diffuser axis occurs at the narrow diffuser neck while flow direction reverses every half period. For low frequencies, a parabolic velocity...
Dynamic modeling of geometrically nonlinear electrostatically actuated microbeams (Corotational Finite Element formulation and analysis)
, Article Journal of Physics: Conference Series ; Volume 34, Issue 1 , 2006 , Pages 606-613 ; 17426588 (ISSN) ; Ahmadian, M. T ; Sharif University of Technology
Institute of Physics Publishing
2006
Abstract
In this paper, a complete nonlinear finite element model for coupled-domain MEMS devices with electrostatic actuation and squeeze film effect is developed. For this purpose, a corotational finite element formulation for the dynamic analysis of planer Euler beams is employed. In this method, the internal nodal forces due to deformation and intrinsic residual stresses, the inertial nodal forces, and the damping effect of squeezed air film are systematically derived by consistent linearization of the fully geometrically nonlinear beam theory using d′Alamber and virtual work principles. An incremental-iterative method based on the Newmark direct integration procedure and the Newton-Raphson...
Dynamic and vibration analysis of a 3-serial-link micro/nano-manipulator with piezoelectric actuation
, Article Microsystem Technologies ; Volume 27, Issue 3 , 2021 , Pages 703-721 ; 09467076 (ISSN) ; Ahmadian, M. T ; Sharif University of Technology
Springer Science and Business Media Deutschland GmbH
2021
Abstract
In this work, the dynamic and vibrational behavior of a micro/nano-manipulator has been studied. The manipulator comprises three links attached serially. While the connection of the links is rigid, which makes the whole body a monolithic structure, the movement of the manipulator is derived from the links deflection. Piezoelectric layers were employed as an actuator to impose deflection on the links, thereby moving the manipulator. After defining the dynamic behavior of each link, manipulator equations of motion were developed. To evaluate the validity of the mathematical model, two FEM models were constructed. The observed results showed a good agreement between the models, and they...
Tire modeling with nonlinear behavior for vehicle dynamic studies
, Article Scientia Iranica ; Volume 8, Issue 2 , 2001 , Pages 145-148 ; 10263098 (ISSN) ; Ahmadian, M. T ; Sharif University of Technology
Sharif University of Technology
2001
Abstract
In this paper, a nonlinear tire model based on the elliptic concept and tire Calspan data has been developed. The effect of cornering force and aligning moment as a function of slip and camber (inclination) angles, normal load, tire adhesion characteristics and skid number are studied. Furthermore, in this model, the rolling resistant effect has been considered. Through this simple tire model, the tire behavior as well as the experimental results are represented
Energy Optimization of an Arm with Seven Degrees of Freedom Using Imperialistic Competitive Algorithm (ICA) and Designing an Artificial Neural Network
, M.Sc. Thesis Sharif University of Technology ; Ahmadian, M.T (Supervisor) ; Asghari, M (Supervisor)
Abstract
In recent years, great attention has been devoted to the design of artificial arms. The most crucial problem in such a design is the trajectory of movement. In this paper, a seven degree of freedom arm is modeled and simulated. Also, the optimization method named “Imperialistic Competitive Algorithm” has been modified and better performance of the new version is presented. Energy optimization is performed based on trajectory of the arm with angular velocity, angular acceleration and joint angles using modified imperialistic competitive algorithm (ICA). Considering ICA as a fast optimization algorithm, it would be reasonable to use this algorithm for robotic purposes for online answering. The...
A strain gradient Timoshenko beam element: Application to MEMS
, Article Acta Mechanica ; Vol. 226, issue. 2 , Jul , 2014 , pp. 505-525 ; ISSN: 00015970 ; Asghari, M ; Ahmadian, M. T ; Sharif University of Technology
Abstract
The classical continuum theory not only underestimates the stiffness of microscale structures such as microbeams but is also unable to capture the size dependency, a phenomenon observed in these structures. Hence, the non-classical continuum theories such as the strain gradient elasticity have been developed. In this paper, a Timoshenko beam finite element is developed based on the strain gradient theory and employed to evaluate the mechanical behavior of microbeams used in microelectromechanical systems. The new beam element is a comprehensive beam element that recovers the formulations of strain gradient Euler–Bernoulli beam element, modified couple stress (another non-classical theory)...
A strain gradient based yield criterion
, Article International Journal of Engineering Science ; Vol. 77 , 2014 , pp. 45-54 ; ISSN: 00207225 ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
Abstract
The classical yield criteria do not describe the yield size-dependency that has been observed in micro-scale structures (Fleck, Muller, Ashby, & Hutchinson, 1994; Stolken and Evans 1998; Moreau et al. 2005), and may also significantly underestimate the yield loads of structures at micron and sub-micron scales (Son, Jeong, & Kwon, 2003; Liu et al., 2013). The present paper suggests a specific size-dependent yield criterion based on the strain gradient theory that is aimed at addressing this inadequacy. In order to develop the strain gradient based yield criterion, the deviatoric part of the strain energy is calculated based on this theory and equated to the deviatoric strain energy of a...
Effect of thermal wave propagation on thermoelastic behavior of functionally graded materials in a slab symmetrically surface heated using analytical modeling
, Article Composites Part B: Engineering ; Vol. 60 , 2014 , pp. 413-422 ; ISSN: 13598368 ; Ahmadian, M. T ; Taati, E ; Sharif University of Technology
Abstract
Design and development of FGMs as the heat treatable materials for high-temperature environments with thermal protection require understanding of exact temperature and thermal stress distribution in the transient state. This information is primary tool in the design and optimization of the devices for failure prevention. Frequently FGMs are used in many applications that presumably produce thermal energy transport via wave propagation. In this study, transient non-Fourier temperature and associated thermal stresses in a functionally graded slab symmetrically heated on both sides are determined. Hyperbolic heat conduction equation in terms of heat flux is used for obtaining temperature...
Effects of casimir and van der waals forces on the pull-in instability of the nonlinear micro and nano-bridge gyroscopes
, Article International Journal of Structural Stability and Dynamics ; Vol. 14, issue. 2 , 2014 ; ISSN: 02194554 ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
Abstract
The influence of Casimir and van der Waals forces on the instability of vibratory micro and nano-bridge gyroscopes with proof mass attached to its midpoint is studied. The gyroscope subjected to the base rotation, Casimir and van der Waals attractions is actuated and detected by electrostatic methods. The system has two coupled bending motions actuated by the electrostatic and Coriolis forces. First a system of nonlinear equations for the flexural-flexural deflection of beam gyroscopes is derived using the extended Hamilton's principle. In modeling, the nonlinearities due to mid-plane stretching, electrostatic forces, including fringing field, Casimir and van der Waals attractions, are...
Carbon nanotube-based nano-fluidic devices
, Article Journal of Physics D: Applied Physics ; Vol. 47, issue. 8 , 2014 ; ISSN: 00223727 ; Rastgoo, A ; Ahmadian, M. T ; Sharif University of Technology
Abstract
The paper investigates the influences of fluid flow on static and dynamic behaviours of electrostatically actuated carbon nanotubes (CNTs) using strain gradient theory. This nonclassical elasticity theory is applied in order to obtain more accurate results possessing higher agreement with the experimental data. The effects of various fluid parameters such as the fluid viscosity, velocity, mass and temperature on the pull-in properties of the CNTs with two cantilever and doubly clamped boundary conditions are studied. The results reveal the applicability of the proposed nano-system as nano-valves or nano-fluidic sensors
A timoshenko beam element based on the modified couple stress theory
, Article International Journal of Mechanical Sciences ; Vol. 79, issue , 2014 , pp. 75-83 ; ISSN: 00207403 ; Asghari, M ; Ahmadian, M. T ; Sharif University of Technology
Abstract
Since the classical continuum theory is neither able to evaluate the accurate stiffness nor able to justify the size-dependency of micro-scale structures, the non-classical continuum theories such as the modified couple stress theory have been developed. In this paper, a new comprehensive Timoshenko beam element has been developed on the basis of the modified couple stress theory. The shape functions of the new element are derived by solving the governing equations of modified couple stress Timoshenko beams. Subsequently, the mass and stiffness matrices are developed using energy approach and Hamilton's principle. The formulations of the modified couple stress Euler-Bernoulli beam element...
Vibration and buckling analysis of functionally graded beams using reproducing kernel particle method
, Article Scientia Iranica ; Vol. 21, Issue 6 , 2014 , pp. 1896-1906 ; e-ISSN : 23453605 ; Ahmadian, M. T ; Farrahi, G. H ; Sharif University of Technology
Abstract
This paper presents vibration and buckling analysis of functionally graded beams with different boundary conditions, using reproducing kernel particle method (RKPM). Vibration of simple Euler-Bernoulli beam using RKPM is already developed and reported in the literature. Modeling of FGM beams using theoretical method or finite element technique is not evolved with accurate results for power law form of FGM with large power of "n" value so far. Accuracy of the RKPM results is very good and is not sensitive to n value. System of equations of motion is derived using Lagrange's method under the assumption of Euler-Bernoulli beam theory. Boundary conditions of the beam are taken into account using...
Size-dependent dynamic behavior of microcantilevers under suddenly applied DC voltage
, Article Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science ; Vol. 228, Issue. 5 , May , 2014 , pp. 896-906 ; ISSN: 09544062 ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
Abstract
This paper investigates the dynamic behavior of microcantilevers under suddenly applied DC voltage based on the modified couple stress theory. The cantilever is modeled based on the Euler-Bernoulli beam theory and equation of motion is derived using Hamilton's principle. Both analytical and numerical methods are utilized to predict the dynamic behavior of the microbeam. Multiple scales method is used for analytical analysis and the numerical approach is based on a hybrid finite element/finite difference method. The results of the modified couple stress theory are compared with those from the literature as well as the results predicted by the classical theory. It is shown that the modified...
Pull-in behaviors of carbon nanotubes with vacancy defects and residual stresses
, Article Journal of Computational and Theoretical Nanoscience ; Vol. 11, issue. 1 , 2014 , pp. 153-159 ; ISSN: 15461955 ; Rastgoo, A ; Ahmadian, M. T ; Sharif University of Technology
Abstract
The paper deals with investigation of the effects of vacancy defects on the pull-in behaviors of the carbon nanotubes. The influences of single, double and triple vacancies in different peripheral and longitudinal positions are studied. The results reveal that the vacancy defects drastically reduce the pull-in charge because they weaken the nanostructure. Moreover, the effects of residual stresses on the pull-in and vibrational properties of the carbon nanotubes are scrutinized. The influences of the angular deviations from the parallel positions of the CNT and ground plate during the fabrication process are also reported. Copyright
The influence of the intermolecular surface forces on the static deflection and pull-in instability of the micro/nano cantilever gyroscopes
, Article Composites Part B: Engineering ; Vol. 56, issue , January , 2014 , p. 336-343 ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
Abstract
In this paper, the effects of van der Waals and Casimir forces on the static deflection and pull-in instability of a micro/nano cantilever gyroscope with proof mass at its end are investigated. The micro/nano gyroscope is subjected to coupled bending motions which are related by base rotation and nonlinearities due to the geometry and the inertial terms. It is actuated and detected by capacitance plates which are placed on the proof mass. The extended Hamilton principle is used to find the equations governing the static behavior of the clamp-free micro/nano gyroscopes under electrostatic, Casimir and van der Waals forces. The equations of static motion are discritized by Galerkin's...