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A size-dependent model for functionally graded micro-plates for mechanical analyses
, Article JVC/Journal of Vibration and Control ; Volume 19, Issue 11 , 2013 , Pages 1614-1632 ; 10775463 (ISSN) ; Taati, E ; Sharif University of Technology
2013
Abstract
In this paper, a size-dependent formulation is presented for mechanical analyses of inhomogeneous micro-plates based on the modified couple stress theory. The plate properties can arbitrarily vary through the thickness. The governing differential equations of motion are derived for functionally graded (FG) plates with arbitrary shapes utilizing a variational approach. Moreover, the boundary conditions are provided at smooth parts of the plate periphery and also at the sharp corners of the periphery. Utilizing the derived formulation, the free-vibration behavior as well as the static response of a rectangular FG micro-plate is investigated
A three-dimensional elasticity solution for functionally graded rotating disks
, Article Composite Structures ; Volume 92, Issue 5 , 2010 , Pages 1092-1099 ; 02638223 (ISSN) ; Ghafoori, E ; Sharif University of Technology
2010
Abstract
A semi-analytical three-dimensional elasticity solution for rotating functionally graded disks for both of hollow and solid disks is presented. The aim is to generalize an available two-dimensional plane-stress solution to a three-dimensional one. Although for the thin disks problems the two-dimensional solution provides appropriate results, for the thick disks, a three-dimensional elasticity solution should be considered to avoid poor results. It is shown that although the plane-stress solution satisfies all the governing three-dimensional equations of motion and boundary conditions, it fails to give a compatible three-dimensional strain field. A valid three-dimensional solution has been...
Decoupled stability equation for buckling analysis of FG and multilayered cylindrical shells based on the first-order shear deformation theory
, Article Composites Part B: Engineering ; Volume 154 , 2018 , Pages 225-241 ; 13598368 (ISSN) ; Taati, E ; Asghari, M ; Sharif University of Technology
Elsevier Ltd
2018
Abstract
Based on the first-order shear deformation and Donnell's shell theory with von Karman non-linearity, one decoupled stability equation for buckling analysis of functionally graded (FG) and multilayered cylindrical shells with transversely isotropic layers subjected to various cases of combined thermo-mechanical loadings is developed. To this end, the equilibrium equations are uncoupled in terms of the transverse deflection, the force function and a new potential function. Using the adjacent equilibrium method, one decoupled stability equation which is an eighth-order differential equation in terms of transverse deflection is obtained and conveniently solved to present analytical expressions...
On the theoretical and molecular dynamic methods for natural frequencies of multilayer graphene nanosheets incorporating nonlocality and interlayer shear effects
, Article Mechanics of Advanced Materials and Structures ; 2021 ; 15376494 (ISSN) ; Taati, E ; Asghari, M ; Sharif University of Technology
Bellwether Publishing, Ltd
2021
Abstract
In this paper, a multiplate nonlocal shear model and molecular dynamic simulations are presented to investigate the effects of interlayer shear and nonlocality on the natural frequencies of multilayer graphene sheets (MLGSs). From one aspect in the optimal design of such structures, the interaction between graphene layers, which can significantly vary the static and dynamic behavior due to lack of solidity of layers stack, should be considered. On the other hand, it is requied that the nonlocality phenomenon which has an effective role in the mechanical analysis of nanostructures is taken into account. To this aim, the equation of motion along with corresponding boundary conditions is...
On the theoretical and molecular dynamic methods for natural frequencies of multilayer graphene nanosheets incorporating nonlocality and interlayer shear effects
, Article Mechanics of Advanced Materials and Structures ; Volume 29, Issue 20 , 2022 , Pages 2873-2883 ; 15376494 (ISSN) ; Taati, E ; Asghari, M ; Sharif University of Technology
Taylor and Francis Ltd
2022
Abstract
In this paper, a multiplate nonlocal shear model and molecular dynamic simulations are presented to investigate the effects of interlayer shear and nonlocality on the natural frequencies of multilayer graphene sheets (MLGSs). From one aspect in the optimal design of such structures, the interaction between graphene layers, which can significantly vary the static and dynamic behavior due to lack of solidity of layers stack, should be considered. On the other hand, it is requied that the nonlocality phenomenon which has an effective role in the mechanical analysis of nanostructures is taken into account. To this aim, the equation of motion along with corresponding boundary conditions is...
Dynamic pull-in instability of multilayer graphene NEMSs: non-classical continuum model and molecular dynamics simulations
, Article Acta Mechanica ; Volume 233, Issue 3 , 2022 , Pages 991-1018 ; 00015970 (ISSN) ; Taati, E ; Asghari, M ; Sharif University of Technology
Springer
2022
Abstract
A novel non-classical continuum model for pull-in analysis of multilayer graphene sheets (MLGSs) is developed to consider the effect of shear interaction between layers based on the nonlocal elasticity theory. The equation governing the motion and corresponding boundary conditions of electrostatically actuated MLGSs are obtained based on the nonlocal shear multiplate theory. The Galerkin method along with the first mode shapes for clamped and cantilever MLGSs together with the method of parameter expansion is used to obtain closed-form expressions of the normalized frequency and time history response. In addition, molecular dynamics (MD) simulations are carried out to validate the pull-in...
A maximum likelihood approach to estimate the change point of multistage Poisson count processes
, Article International Journal of Advanced Manufacturing Technology ; Volume 77, Issue 5-8 , March , 2015 , Pages 1443-1464 ; 02683768 (ISSN) ; Akhavan Niaki, S. T ; Asghari Torkamani, E ; Sharif University of Technology
Springer-Verlag London Ltd
2015
Abstract
The difference between the signaling time and the real change point of a process is an important monitoring issue. If the exact time at which the change manifests itself into the process is known, then process engineers can identify and eliminate the root causes of process disturbance efficiently and quickly, resulting in considerable amount of time and cost savings. Multistage count processes that are often observed in production environments must be monitored to assure quality products. In this study, multistage Poisson count processes are first introduced. Then, the process is modeled using a first-order integer-valued autoregressive time series (INAR(1)). For out-of-control signals...
Standard and boundary layer perturbation approaches to predict nonlinear axisymmetric behavior of cylindrical shells
, Article Composite Structures ; Volume 204 , 2018 , Pages 855-881 ; 02638223 (ISSN) ; Taati, E ; Asghari, M ; Reddy, J. N ; Sharif University of Technology
Elsevier Ltd
2018
Abstract
The feasibility and performance of standard and boundary layer perturbation techniques in nonlinear analyses of cylindrical shells are investigated. To this end, the nonlinear axisymmetric behavior of short and long functionally graded (FG) cylindrical shells is considered. The nonlinear governing equations of shell theory with first-order approximation and the von Karman nonlinearity are decoupled. This uncoupling makes it possible to present an analytical solution. A new boundary layer perturbation solution is presented by reducing the governing equations to a normalized form of boundary-layer type. Also, the uncoupled governing equations are solved using standard one-, two-, and...
Geometrically nonlinear micro-plate formulation based on the modified couple stress theory
, Article International Journal of Engineering Science ; Volume 51 , 2012 , Pages 292-309 ; 00207225 (ISSN) ; Sharif University of Technology
2012
Abstract
The couple stress theory is a non-classical continuum theory which is capable to capture size effects in small-scale structures. This property makes it appropriate for modeling the structures in micron and sub-micron scales. The purpose of this paper is the derivation of the governing motion equations and boundary conditions for the geometrically nonlinear micro-plates with arbitrary shapes based on the modified version of the couple stress theory. The consistent boundary conditions are provided at smooth parts of the plate periphery and also at the sharp corners of the periphery using variational approach
Basis free expressions for the stress rate of isotropic elastic materials in the cases of coalescent principal stretches
, Article International Journal of Solids and Structures ; Volume 47, Issue 5 , 2010 , Pages 611-613 ; 00207683 (ISSN) ; Sharif University of Technology
2010
Abstract
In this paper, some basis-free expressions for the material time derivative of Lagrangian stress tensors are presented which are generally valid in all cases of coalescent principal stretches. The material is assumed to be elastic and isotropic
Effect of cement type on the mechanical behavior of a gravely sand
, Article Geotechnical and Geological Engineering ; Volume 24, Issue 2 , 2006 , Pages 335-360 ; 09603182 (ISSN) ; Hamidi, A ; Hosseini, S. M ; Asghari, E ; Toll, D. G ; Sharif University of Technology
2006
Abstract
The behavior of a cemented gravely sand was studied using triaxial compression tests. Gypsum, Portland cement and lime were used as the cementing agents in sample preparation. The samples with different cement types were compared in equal cement contents. Three cement contents of 1.5%, 3.0% and 4.5% were selected for sample preparation. Drained and undrained triaxial compression tests were conducted in a range of confining pressures from 25 kPa to 500 kPa. Failure modes, shear strength, stress-strain behavior, volume and pore pressure changes were considered. The gypsum cement induced the highest brittleness in soil among three cement types while the Portland cement was found to be the most...
Variational principles for stability analysis of multi-walled carbon nanotubes based on a nonlocal elastic shell model
, Article ASME 2010 10th Biennial Conference on Engineering Systems Design and Analysis, ESDA2010, Istanbul, 12 July 2010 through 14 July 2010 ; Volume 5 , 2010 , Pages 591-598 ; 9780791849194 (ISBN) ; Rafati, J ; Sharif University of Technology
2010
Abstract
The nonlocal continuum theories are capable to reflect the small length characteristic of nanostructures. In this work, variational principles are presented for the stability analysis of multi-walled carbon nanotubes under various mechanical loadings based on the nonlocal elastic Donnell's shell by the semi-inverse method. In this manner, a set of proper essential and natural boundary conditions for each layer of the multi-walled nanotube is derived
Flexural vibration characteristics of micro-rotors based on the strain gradient theory
, Article International Journal of Applied Mechanics ; Volume 7, Issue 5 , October , 2015 ; 17588251 (ISSN) ; Hashemi, M ; Sharif University of Technology
World Scientific Publishing Co. Pte Ltd
2015
Abstract
In this paper, the coupled three-dimensional flexural vibration of micro-rotors is investigated by taking into account the small-scale effects utilizing the strain gradient theory, which is a powerful nonclassical continuum theory in capturing small-scale effects. A micro-rotor consists mainly of a flexible micro-rotating shaft and a disk. With the aid of Hamilton's principle, governing equations of motion are derived and then transformed to the complex form. By implementing the Galerkin's method, a coupled ordinary differential equation is attained for the system. Expressions for the first two natural frequencies of the spinning micro-rotors are obtained with truncated two-term equation....
The couple stress-based nonlinear coupled three-dimensional vibration analysis of microspinning Rayleigh beams
, Article Nonlinear Dynamics ; Volume 87, Issue 2 , 2017 , Pages 1315-1334 ; 0924090X (ISSN) ; Hashemi, M ; Sharif University of Technology
Springer Netherlands
2017
Abstract
The nonlinear coupled three-dimensional vibrations of microspinning Rayleigh beams are analytically studied utilizing the modified couple stress theory to take into account the small-scale effects. The considered nonlinearity is of geometrical type due to the mid-plane stretching. The rotary inertia and gyroscopic effects are both included in the formulation. Governing equations of motion are derived with the aid of the Hamilton Principle and then transformed into complex form. Then, the Galerkin and multiple scales methods are utilized to solve the nonlinear partial differential equation. Approximate analytical expressions for nonlinear natural frequencies of the spinning beams in forward...
On the objective corotational rates of eulerian strain measures
, Article Journal of Elasticity ; Volume 90, Issue 2 , 2008 , Pages 175-207 ; 03743535 (ISSN) ; Naghdabadi, R ; Sharif University of Technology
2008
Abstract
In the present paper, some new basis-free expressions for an arbitrary objective corotational rate of the general Eulerian strain measures are provided which are in compact form. Moreover, a complete discussion on the requirements for the continuity of the objective corotational rates are presented. © 2008 Springer Science+Business Media B.V
Unified basis-free relation between two stress tensors conjugate to arbitrary Hill's strain measures
, Article ASME PVP2006/ICPVT-11 Conference, Vancouver, BC, 23 July 2006 through 27 July 2006 ; Volume 2006 , 2006 ; 0277027X (ISSN); 0791837823 (ISBN); 9780791837825 (ISBN) ; Naghdabadi, R ; Sharif University of Technology
American Society of Mechanical Engineers(ASME)
2006
Abstract
The concept of energy conjugacy for stress and strain measures states that a stress tensor T is conjugate to a strain measure E if T: Ė provides the rate of change of the internal energy per unit reference volume of the body in an adiabatic process. The applications of the conjugate stress and strain measures are in the development of the basic relations in nonlinear analysis of solids. In this paper using eigenprojection method, unified explicit basis-free relation between two arbitrary stress tensors T(f) and T (g), respectively conjugate to two measures of Hill's strains is determined. The result is valid for arbitrary dimension of the Euclidean inner product space and for all cases of...
Conjugate stresses to two-point deformation tensors
, Article International Journal of Solids and Structures ; Volume 44, Issue 22-23 , 2007 , Pages 7457-7467 ; 00207683 (ISSN) ; Naghdabadi, R ; Sharif University of Technology
2007
Abstract
In this paper some expressions for stresses conjugate to two-point deformation tensors are derived. These expressions are offered in both the component and basis-free forms. Although, the material time rate of a two-point deformation tensor is not an objective quantity, the stress tensor conjugate to it may be determined. The component-form expressions are obtained by using the notion of conjugacy together with the objectivity of the stress power. The component-form expressions are then extended to a unified basis-free form, using a theorem established for this purpose. The specific results are provided for all different cases of distinct and coalescent principal stretches in a...
Introducing a Set of Material Strain Measures in Non-Linear Kinematics of Micropolar Continuum Mechanics and Determining their Time Rates
, M.Sc. Thesis Sharif University of Technology ; Asghari, Mohsen (Supervisor)
Abstract
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...
Formulation for Analyzing of the Functionally Graded Kirchhoff Plate Based on the Modified Couple Stress Theory
, M.Sc. Thesis Sharif University of Technology ; Asghari, Mohsen (Supervisor)
Abstract
In this project, a size-dependent formulation is presented for mechanical analyses of inhomogeneous micro-plates based on the modified couple stress theory. The modified couple stress theory as a non-classical continuum theory has the ability to consider the small size effects in the mechanical behavior of the structures.The material properties are supposed arbitrarily to vary through the thickness of the plate. The governing differential equations of motion are derived for functionally graded plates utilizing variational approach. Based on the derived formulation, the static and free-vibration behaviors as well as buckling analysis of a rectangular functionally graded micro-plate are...