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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. Source

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. Source

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. Source

Heydari M.,Sharif University of Technology | Jani S.,Golpayegan University of Technology
Acta of Bioengineering and Biomechanics | Year: 2010

The objective of this study was to propose a new analytical model for studying response of head impacts. Head is modeled by fluidfilled ellipsoidal shell of inconstant thickness impacted by a solid elastic sphere. Modeling the head as an ellipsoid is more realistic than modeling it as a sphere, the previous model existing in the literature [3]-[8]. In this model, the effect of Hertzian contact stiffness and local shell stiffness are combined to derive explicit equations for impact duration, the peak force transmitted to head, and the head injury criterion. One of the advantages of the model presented is sensitivity to the site of impact. A comparison between the present analytical results with the analytical data from spherical model [8] has been done to verify the validation of the present model. Source

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. Source

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