حسین مهبادی

In this research, stress intensity factor of a small radial crack in a rotating thick cylinder made of viscoelastic materials subjected to internal pressure and radial temperature distribution is investigated. The radial crack is assumed to be located at the outer edge of the cylinder. The Zener model (i.e., standard three parameter solid) is applied to simulate the viscoelastic behavior of the cylinder. To obtain the stress intensity factor of the viscoelastic cylinder, the problem of uncracked viscoelastic cylinder subjected to internal pressure and radial temperature distribution is analyzed with plane strain assumptions. Then, the hoop stress distribution is determined. Finally, by assuming the small crack length and applying the superposition method, the problem of cracked viscoelastic cylinder is replaced by the problem of cracked viscoelastic cylinder subjected to determined average stress in the previous step. By employing the proper geometry factor, the stress intensity factor of the viscoelastic cylinder is determined. A parameter study is performed to investigate the effects of various parameters on the stress intensity factor of the cylinder. To validate the results, the problem of viscoelastic cracked cylinder is simplified to an elastic cracked cylinder by neglecting the viscoelastic terms, and the data obtained in the present work are compared with those given in the literature search. The results show that the proposed method is quite capable of estimating the stress intensity factor of small cracks in structures made of viscoelastic materials. A parameter study is performed to investigate effect of various parameters on stress intensity factor of the cylinder. The parameter study shows that a) the stress intensity factor of the viscoelastic cylinder in early moments of loading is higher than its equivalent elastic cylinder, b) increasing the elastic properties of the viscoelastic material results in higher stress intensity factor, and increasing the viscous properties of the material decreases the stress intensity factor.

The length-scale parameter as a primary factor has an important role in approximation of natural frequencies of micro-structures. Applying the exact length scale parameters which are recently determined by researchers, natural frequencies of porous microstructures are determined. In view of the fact that assumption of constant length-scale parameter leads to deviation of natural frequencies from their exact value, this research applies a length scale parameter which is function of plate thickness and material. To model the porous structures of the plate, the proposed porous models including evenly porosity mode, unevenly symmetric porosity model, and unevenly asymmetric porosity model are employed. The micro-plate is assumed to be thin, and classical plate theory is applied to displacement field of the micro-plate. The modified stress couple theory is employed to approximate the behavior of the plate in micro-scale. The trial functions which satisfy the boundary conditions are taken as the polynomial form. Evaluating the energy values of the system, the Rayleigh-Ritz method is imposed to solve the governing equations of the system. Validated the solution procedure with data given in the literature search, effects of various parameters on natural frequency of the plate are studied in the form of tables and curves.

In this paper, free vibration analysis of two-dimensional functionally graded annular sector plates with two piezoelectric layers on elastic foundation using the Rayleigh-Ritz method is investigated applying the third order shear deformation theory of plates. Despite the existence of more complex equations, the third-order shear deformation theory of plates leads to more accurate results, this is due due to the effects of transverse shear deformation and inertial rotation,.The material properties of the annular plate is graded two-dimensionally in both of the circumferential and thickness directions. The volume fraction power law distribution is used to model the two dimensional functionally graded plate properties. The sinusoidal function is used to determine the electrical potential of the piezoelectric layers. The assumed potential function satisfies the Maxwell electrostatic equations. To obtain the natural frequencies of the plate, the Lagrangian function is defined by subtracting the kinetic and potential energies of the plate and then the Hamilton’s principle and the Ritz method is applied to obtain the natural frequencies of the plate. Finally, the results are validated by various references and the effect of various parameters including the sector angle, ratio of thickness to radius of the plate and foundation coefficients on the natural frequency of the structure has been discussed.

In this paper the laminar Bingham fluid flow through a converging conical duct is analyzed. The constitutive law of Bingham fluids is applied to model the yield dependent properties behavior of the fluid. An analytical solution is obtained for the pressure gradient assuming small conical angle of the duct. According to the yield dependent properties of the fluid, a region is developed in the fluid wherein the rate of deformation tensor is zero. This region is called plug region. According to obtained pressure gradient, the plug region is determined. Then, assuming the cylindrical symmetry, the governing equations are solved in cylindrical coordinate system and velocity field is obtained across the duct section. The results obtained in the paper is compared with the experimental data given in the literature search and Fluent numerical solution. The comparison of these two methods shows that the result of current paper is well compared with those given in the literature search. Finally, the effect of parameters such as yield strength and conical angle on velocity field and plug region is investigated.

Viscoelasticity is a property of materials that exhibit both viscous and elastic characteristics. In linear viscoelasticity, the stress is linearly related to the history function of strain. This paper discusses vibration analysis of functionally graded viscoelastic rectangular plate. The viscoelastic behavior of the plate is modeled using the Zener three-parameter model. Also, the material properties of the plate are graded through the thickness according to the volume fraction model. The maximum stress and strain are calculated based on the linear first-order shear deformation theory and the simply support boundary conditions is assumed at all four edges of the plate. A code is prepared using the Mathematica software to obtain the frequency values and effect of inherent and geometric characteristics of the sheet on natural frequency of the plate. These effects are studied using the tables and graphs represented in the results and discussion section of the paper. The results obtained in this paper are simplified to a functionally elastic plate to compare with those given in the literature search. The comparison of results shows good agreement against data given in literature for both cases.

In this study, the effect of resonator length and working fluid on operation of heat-driven thermoacoustic refrigerator is investigated. The working fluid is considered to be Helium, Nitrogen and Argon. The simulation is carried out via a numerical method and the results are compared with DELTA-EC software. The results of these two methods are in good agreement with each other. The simulations show that increase in the resonator length results to increase in pressure amplitude for all mentioned working fluids and the frequency reduces by applying the Helium, Nitrogen and Argon respectively. Increasing the resonator length from 0.5m through 1.4m leads to reduction of the coefficient of performance for Nitrogen and Argon while this coefficient increases for Helium. The best coefficient of performance is gained for Helium gas and resonator length equal to 1.4m.