نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 3 (خرداد 1392)

In this study the effect of thermocycling on the mechanical properties of a commercial dental nano-composite (Filtek Z350 XT, 3M ESPE, Germany) was investigated by using the nano-indentation experiment. For this purpose some disk specimens, each of the diameter 10 mm and the thickness 4 mm were prepared. Half of the specimens were stored at room temperature and the other half were thermocycled in distilled water for 1000 cycles between temperatures 5°C and 55°C. After the sample preparation, the nano-indentation test was applied on both non-themocycled and thermocycled specimens by Triboscope setup. Then the modulus of elasticity and hardness of the test samples were calculated from the Oliver-Pharr’s method using the data obtained from the nano-indentation experiments. Using the independent-samples t-test, the mechanical properties obtained from the non-thermocycled and thermocycled samples were compared. The results indicate that thermocycling process increases the modulus of elasticity and hardness of the dental nano-composite.

Abstract- This paper presents the results of a numerical study on the natural convection in a right triangular enclosure filled with a water- Cu nanofluid in presence of a constant magnetic field. A heat source embedded on the bottom wall of enclosure, the inclined wall is cold and the other walls are adiabatic. Discretization of the governing equations are achieved through a finite volume method and solved with SIMPLE algorithm. The effects of parameters such as the Reyleigh number, the solid volume fraction, the Hartman number, length and location of heat source on flow and temperature fields and the heat transfer rate have been examined. The results show that increasing of Hartman number caused decreasing velocity of flow and heat transfer. Also, increase in solid volume fraction causes increase in heat transfer but its change in different Reyleigh number and Hartman number is not same. Therefore, the location of heat source in bottom of enclosure affects on the rate of heat transfer from enclosure.

Aluminum foam structure is of great importance in aerospace, naval and automotive industries due to light weight and energy absorption characteristics. In this article several aluminum foam having different densities and thickness were designed and tested using light gas gun device. A series of ballistic test were defined in order to determine the effects of density, foam thickness and projectile velocity on energy absorption aluminum foam structures. The results of the experimental testes, it is shown that the amount of energy absorption of aluminum foam structures is increased as density, foam thickness and velocity of the projectile is increased.

The main object of current study is investigation of instability threshold of flow in a gradual expansion from symmetric to asymmetric situations. The expansion ratio is 1:3 and expansion angles are 30, 45, 60 and 90 degree. Discretization of governing equations is performed using finite volume method based on PISO algorithm on a staggered mesh. The CFD code is validated based on the results of sudden expansion reported in previous works. Here, the effects of expansion angle and Reynolds number on flow instability in transition from symmetric situation to two and three asymmetric vortices are investigated and the first and second critical Reynolds numbers are obtained. The bifurcation diagrams of vortices and velocity profile in centerline are plotted for each case and the effects of instability on flow field are discussed based on them. Unlike the previous studies which have been focused on the planar flow in sudden expansions, the flow instability in gradual expansions with different expansion angles is investigated which is the main innovation of current study.

In this paper, analytical solutions of low velocity transverse impact of a nanoparticle on a nanobeam are presented by using the nonlocal theory to bring out the effect of the nonlocal behavior on dynamic deflection. Impact of a mass on simply supported and clamped nanobeams are investigated by using nonlocal Euler–Bernoulli beam theory. In order to obtain an analytical result for this problem, an approximate method has been developed wherein the applied impulse is replaced by a suitable boundary condition. A number of numerical examples with analytical solutions for both nonlocal and classic beam have been presented and discussed. The dynamic deflection predicted by the classical theory is always smaller than those predicted by the nonlocal theory due to the nonlocal effects. The inclusion of the nonlocal effect increases the magnitudes of dynamic deflection and decreases frequencies. Furthermore, the mass and the velocity of the nanoparticle (striker) have significant effects on the dynamic behavior of nanobeam.

In most airbag systems, the gaseous mixture that fills up the airbag is produced by the fast combustion of a propellant in a combustion chamber called inflator. Since the process of gas production in the airbag inflator is a high-temperature combustion process, having a right understanding and precise control over the combustion in the airbag inflator has always been a challenge. In this paper, the numerical study of combustion process in a pyrotechnic inflator was carried out based on a Zero-Dimensional Multi Zones model. The parametric study show that the performance of inflator is more affected by the propellant characteristics such as mass, combustion index, and propellant temperature coefficient and is not significantly influenced by hardware elements of inflator. In order to simulate hybrid pyrotechnic inflator, the initial pressure of gas plenum was increased by 25 to 50 times. As a result, the performance both in combustion chamber and in discharge tank decreased. This lower temperature leads to a higher thermal efficiency.

There are several parameters that have significant influence in metal forming process. One of the most important of them is friction coefficient. Friction can change the pattern of metal flow and the force needed for deformation. It is necessary to determine the friction coefficient to study the effect of friction on metal forming process. In this study ring compression test was used to determine the friction coefficient in hot working of Al 6082 alloy. Variation of inner diameter of a ring during deformation is a function of friction coefficient, so it can be used to determine the friction coefficient. Graphite, PTFE and mica plate were used as lubricants. The calibration curves were developed by using FEM simulations for different friction coefficients and compared with experimental results. The friction coefficient for each deformation condition and lubricant was obtained. The results show the best lubricant for hot working of this alloy is PTFE with the friction coefficient of 0.35. Hot working without lubricant led to friction coefficient of 0.69

In this research, the aerodynamic design of a radial inflow turbine impeller is carried out using a direct design Method.This new method consists of 2 steps; one dimensional design and three dimensional design. In this design, the blade 3D geometry is obtained with new method. Moreover flow properties in various blade points can be investigated. The advantages this method in comparison with previous other method is less time & cost consuming and more accuracy. At the first step of the aerodynamic design, 1D design is done. This program’s inputs consists of; stagnation temperature, stagnation pressure, mass flow rate and pressure ratio. The goal of 1-d design is to obtain according to optimum experimental data. This procedure based on impeller efficiency convergence. At the second part of this research, by developing a novel design method, the 3D profiles of blade and impellers will be obtained. To validate of one dimensional design results, experimental results and for three dimensional designs, Computational fluid dynamic (CFD) analysis is used. In all this steps, good agreement is observed.

In this paper the meshless local Petrov-Galerkin (MLPG) method is implemented to study the vibration of a Functionally Graded Material (FGM) cylindrical shell. Displacement field equations, based on Donnell and first order shear deformation theory, are taken into consideration. Material properties are assumed to be temperature-dependent and graded in the thickness direction according to different volume fraction functions. A FGM cylindrical shell made up of a mixture of ceramic and metal is considered herein. The set of governing equations of motion are numerically solved by the Meshless method in which a new variational trial-functional is constructed to derive the stiffness and mass matrices so the natural frequencies are obtained in various boundary conditions by using discretization procedure and solving the general eigenvalue problem. The influences of some commonly used boundary conditions, variations of volume fractions and effects of shell geometrical parameters are studied. The results show the convergence characteristics and accuracy of the mentioned method.

The closely spaced modes exist in symmetric structures such as circular plates, gears or disks. Theoretically, closely spaced modes are known as two separated modes with the same amount but, these modes are often detected as only one mode in the classical modal methods. In this article, at the first, some classical modal method is introduced and the most important of their difficulties is considered then, Operational Modal Analysis (OMA) is applied to abate these problems. The Stochastic Subspace Identification based on covariance (SSI-COV) driven is used for this purpose. Different conditions for closely spaced modes is considered and simulated on a free-free steel plate using Matlab software. In order to consider the SSI-COV method in identification of closely spaced modes experimentally, classical and operational modal testing are done on a steel ring. The numerical and experimental results from simulations demonstrate the effectiveness of OMA for identifying and separating the close modes.

In this study, the results of a direct numerical simulation (DNS) of turbulent drag reduction by microfibers in a plane channel flow at a shear Reynolds number of Re = 950 are reported. For this purpose, we make use of a numerical solution of three-dimensional, time-dependent Navier-Stokes equations for the incompressible turbulent flow of a non-Newtonian fluid. The non-Newtonian stress tensor which is required to solve the problem depends on the orientation distribution of the suspended fibers, which is computed by a recently-proposed algebraic closure model. It is shown that the use of this algebraic closure, due to the great reduction in computational efforts, enables us to perform a DNS at high Reynolds numbers. Ultimately, statistical quantities of turbulence (in particular, the mean velocity profile, Reynolds stresses, etc.) are presented and discussed. Variations in the isotropy of the Reynolds stress tensor are explained by the aid of Lumley anisotropy map.

In this paper, a novel transducer called Magnetostrictive Torsional Resonant Transducer (MTRT) is introduced. The transducer is composed of a magnetostrictive horn, a stainless steel backing and housing. In this transducer a spiral magnetic field, made up of longitudinal and circumferential components, is applied to the magnetostrictive horn. As a result, the magnetostrictive horn oscillates torsionally according to the Wiedemann effect. The magnetostrictive horn is made of "2V permendur", which has isotropic magnetic properties. The differential equations of the torsional vibration of the transducer are derived, and the transducer is designed for a resonant frequency of 12075 Hz. Natural frequency and mode shape of the transducer are considered theoretically, numerically, and experimentally. The effects of important parameters such as axial and circumferential magnetic fields, and torsional prestress on the torsional displacement of the MTRT are considered, and the optimum working point is determined. These are promising features for industrial applications.

In this study random vibration of a cantilever tapered beam under distributed stationary stochastic excitation with Gaussian probability density function is investigated. early free vibration analysis is performed to obtain the mode shapes of beam in form of Bessel functions, then the response is described in summation of mode shapes, and auto correlation of response is shaped by considering the mode shapes of tapered beam, also spectral density matrix of excitation is derived with cooperation of mode shapes and two dummy variables. in next step by means of frequency response and taking Fourier integral of autocorrelation of response, spectral density of displacement is computed and by using spectral density of displacement, variance of random displacements for various positions along the beam are achieved. Finally elasticity equation is applied to derive random strain and stress of beam. Comparing the variance of random stress with yield stress of beam leads to obtain probability of beam failure.

Friction stir welding as a joining process in solid state welding various alloys widely used metal, particularly aluminum alloys. Although the low heat generated during the process does not melt the base metal, but the thermal cycle applied to the sample, which reduces the mechanical properties of the junction. Recently, this method of welding process is used in the cooling methods. In this study the microstructure and mechanical properties of 5050 aluminum alloy weld in two conditions: with friction stir welding in the air and on underwater friction stir welding was studied. The results of underwater friction stir welding were compared with samples of friction stir welding in the air. Results showed that the structure of the underwater welding was 36% more finely than welded structures in air and its tensile strength was improved about 6%. Also, the SZ zone in underwater friction stir welding has a higher hardness than friction stir welding in air.