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Klavzar S.,University of Ljubljana | Klavzar S.,University of Maribor | Nadjafi-Arani M.J.,University of Kashan | Nadjafi-Arani M.J.,Golpayegan University of Technology
European Journal of Combinatorics | Year: 2014

It is proved that the Wiener index of a weighted graph (G,w) can be expressed as the sum of the Wiener indices of weighted quotient graphs with respect to an arbitrary combination of Θ *-classes. Here Θ * denotes the transitive closure of Djoković-Winkler's relation Θ. A related result for edge-weighted graphs is also given and a class of graphs studied in Yousefi-Azari etal. (2011) [25] is characterized as partial cubes. © 2013 Elsevier Ltd.

Jani S.,Golpayegan University of Technology
Journal of Enhanced Heat Transfer | Year: 2013

A heat transfer analysis of Al2o3-water nanofluid falling film over a heated horizontal circular tube used in heat exchangers and desalination systems is carried out and the results are compared to those of the base fluids. Based on extended analysis results, different correlations in terms of the film Reynolds, Prandtl, and Archimedes numbers, and on the nanoparticle volume fraction, ', for the nanofluid film and thermal boundary layer thicknesses, as well as the local and average heat transfer coefficients, have been derived. Three different values for volume fractions of the nanoparticles are considered; namely, 0, 0.03, and 0.06. It is found that the overall heat transfer coefficient over the tube generally increases when the volume fraction of the Al2o3 nanoparticles is increased. Moreover, the results show that the effect of the nanoparticle volume fraction on the heat transfer enhancement is more significant in the fully developed region compared to that of the developing region. The results of the present correlations are also shown to be in reasonably good agreement with predictions from other investigators for falling-film flow. © 2013 by Begell House, Inc.

Rahaeifard M.,Golpayegan University of Technology | Ahmadian M.T.,Sharif University of Technology
International Journal of Engineering Science | Year: 2015

In this paper the static deflection and pull-in instability of electrostatically actuated microcantilevers is investigated based on the strain gradient theory. The equation of motion and boundary conditions are derived using Hamilton's principle and solved numerically. It is shown that the strain gradient theory predicts size dependent normalized static deflection and pull-in voltage for the microbeam while according to the classical theory the normalized behavior of the microbeam is independent of its size. The results of strain gradient theory are compared with those of classical and modified couple stress theories and also experimental observations. According to this comparison, the classical theory underestimates the stiffness of the microbeam and there is a gap between the results predicted by the classical theory and those observed in experiment. It is demonstrated that this gap can be reduced when utilizing the strain gradient theory. © 2014 Elsevier Ltd.

Baghani M.,Sharif University of Technology | Naghdabadi R.,Sharif University of Technology | Arghavani J.,Sharif University of Technology | Arghavani J.,Golpayegan University of Technology | And 2 more authors.
International Journal of Plasticity | Year: 2012

The ever increasing applications of shape memory polymers have motivated the development of appropriate constitutive models for these materials. In this work, we present a 3 D constitutive model for shape memory polymers under time-dependent multiaxial thermomechanical loadings in the small strain regime. The derivation is based on an additive decomposition of the strain into six parts and satisfying the second law of thermodynamics in Clausius-Duhem inequality form. In the constitutive model, the evolution laws for internal variables are derived during both cooling and heating thermomechanical loadings. The viscous effects are also fully accounted for in the proposed model. Further, we present the time-discrete form of the evolution equations in the implicit form. The model is validated by comparing the predicted results with different experimental data reported in the literature. Finally, using the finite element method, we solve two boundary value problems e.g., a 3 D beam and a medical stent made of shape memory polymers. © 2012 Elsevier Ltd. All rights reserved.

Rahaeifard M.,Golpayegan University of Technology
Composites Part B: Engineering | Year: 2015

In this paper, size-dependent static and dynamic behavior of functionally graded microbars is investigated on the basis of the modified couple stress theory. The equation of motion and corresponding boundary conditions are derived using Hamilton's principle and presented in the dimensionless form. Equivalent mechanical properties (i.e. shear modulus, density and length scale) are extracted for the functionally graded microbar based on the mechanical properties of the material constituents. In this work, it is shown that without any simplifying assumption, two equivalent length scale parameters can be defined for functionally graded bars and the size-dependent mechanical behavior of these components can be explained using these parameters. As an example, static and dynamic behavior of a functionally graded microbar with fixed-free boundary conditions is analyzed and the effect of size-dependency on mechanical behavior of this structure is discussed. © 2015 Elsevier Ltd. All rights reserved.

Tadi Beni Y.,Shahrekord University | Mehralian F.,Shahrekord University | Razavi H.,Golpayegan University of Technology
Composite Structures | Year: 2015

In this paper, size-dependent equations of motion for functionally graded cylindrical shell were developed using shear deformation model and rotation inertia. Material properties of the shell were assumed as continuously variable along thickness, and consistent with the variation in the component's volume fraction based on power law distribution. To consider the size effect, modified couple stress theory in conjunction with first order shear deformation shell model were used, and general equations of motion and classical and non-classical boundary conditions were derived based on Hamilton's principle. Finally, in the special case, using the Navier procedure, the free vibrations of simply supported functionally graded cylindrical nanoshell were obtained, and the effects of parameters such as dimensionless length scale parameter, distribution of FG properties, thickness, and length on the natural frequency were identified and was compared with the classical theory. Results obtained through the modified couple stress theory are indicative of the considerable effect of the size parameter, particularly in bigger thicknesses and shorter lengths of nanotubes, on the natural frequency. © 2014 Elsevier Ltd.

Baghani M.,Sharif University of Technology | Naghdabadi R.,Sharif University of Technology | Arghavani J.,Sharif University of Technology | Arghavani J.,Golpayegan University of Technology
Smart Materials and Structures | Year: 2012

In this paper, the responses of shape memory polymer (SMP) helical springs under axial force are studied both analytically and numerically. In the analytical solution, we first derive the response of a cylindrical tube under torsional loadings. This solution can be used for helical springs in which both the curvature and pitch effects are negligible. This is the case for helical springs with large ratios of the mean coil radius to the cross sectional radius (spring index) and also small pitch angles. Making use of this solution simplifies the analysis of the helical springs to that of the torsion of a straight bar with circular cross section. The 3D phenomenological constitutive model recently proposed for SMPs is also reduced to the 1D shear case. Thus, an analytical solution for the torsional response of SMP tubes in a full cycle of stress-free strain recovery is derived. In addition, the curvature effect is added to the formulation and the SMP helical spring is analyzed using the exact solution presented for torsion of curved SMP tubes. In this modified solution, the effect of the direct shear force is also considered. In the numerical analysis, the 3D constitutive equations are implemented in a finite element program and a full cycle of stress-free strain recovery of an SMP (extension or compression) helical spring is simulated. Analytical and numerical results are compared and it is shown that the analytical solution gives accurate stress distributions in the cross section of the helical SMP spring besides the global loaddeflection response. Some case studies are presented to show the validity of the presented analytical method. © 2012 IOP Publishing Ltd.

Rahaeifard M.,Golpayegan University of Technology
International Journal of Engineering Science | Year: 2016

This paper investigates the size-dependent static behavior of micro scale thermal actuators. The actuator is considered as a bilayer microcantilever and the modified couple stress theory is utilized to capture the size-dependency of the mechanical behavior of the device. Considering Euler–Bernoulli beam theory, the equations governing axial and lateral deformations of the microbeam are derived. Equivalent material properties including length scale parameter are defined for the bilayer microbeam and deflection of the actuator due to temperature change is calculated using these equivalent quantities. For numerical analysis, a bilayer microbeam made of aluminum and polysilicon is considered. For this microbeam, the lateral deflection due to temperature change is obtained based on the modified couple stress theory and compared to the results of the classical model and the influence of size-dependency on mechanical response of the microbeam is discussed. Finally, the effect of the thickness of layers on the lateral deflection of the device is investigated. © 2016 Elsevier Ltd

Rafiaei S.M.,Golpayegan University of Technology
Transactions of the Indian Institute of Metals | Year: 2016

In this paper, YVO4:Eu3+ nanophosphors were synthesized via the simple combustion method by urea and aniline, individually. The particle size of produced powders and the combustion flame temperature were explained in thermodynamic terms. These phosphors were characterized by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscope and photoluminescence (PL) analyses. It was concluded that the amounts of released gas, adiabatic flame temperature, remaining compounds after combustion reaction, size of powders and optical properties depended strongly on the used combustion fuel. Also, the PL intensity of the phosphors synthesized by aniline was remarkably higher than that of phosphors synthesized by urea. © 2016 The Indian Institute of Metals - IIM

Mojahedi M.,Golpayegan University of Technology | Rahaeifard M.,Golpayegan University of Technology
International Journal of Engineering Science | Year: 2016

This paper presents a new nonlinear model for coupled 3D displacements of the doubly-clamp microbeams based on the modified couple stress theory. The microbeam is assumed to deflect in axial and lateral directions and the nonlinearities caused by the mid-plane stretching are considered. Utilizing modified couple stress theory and applying extended Hamilton's principle, nonlinear equations of the coupled three-dimensional motion of the microbridge are derived. As case studies, the static deflection and nonlinear free vibration of the doubly-clamp microbeams are investigated using the presented model. For vibration analysis, the method of multiple scales is utilized to calculate the frequency of the microbridge versus the amplitudes of vibration in two lateral directions. The effects of the applied loads and amplitudes of vibration in each direction are investigated on the static deflection and frequencies of vibration respectively. Comparing the results obtained by classical and nonclassical theories shows that the classical theory underestimates the stiffness and natural frequency of the structures especially for microbeams that have thicknesses in order of the length scale parameter. Furthermore, findings shows that the mid-plane stretching causes a coupling between lateral directions so that applying load in one direction leads to change in deflection of the other direction. © 2016 Elsevier Ltd. All rights reserved.

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