Modares Mechanical Engineering
Year:2012 | Volume:12 | Issue:3|Papers Count:14

In this paper a new systematic method for deriving the equation of motion of n-rigid robotic manipulators with revolute-prismatic joints is considered. The equation of motion for this robotic system is obtained based on Gibbs- Appell formulation. All the mathematical operations are done by only 3x3 and 3x1 matrices. Also, all dynamic characteristicsof a link are expressed in the same link local coordinate system. Based on the developed formulation, an algorithm is proposed that recursively and systematically derives the equation of motion. Finally, a computational simulation for a manipulator with three revolute-prismatic joints is presented to show the ability of this algorithm in deriving and solving high degree of freedom of robotic system.

In this paper elastoplastic buckling of thin rectangular plates are analyzed with deformation theory COT) and incremental theory (IT) and the results are investigated under different loads and boundary conditions. Load is applied in plane and in uniform tension and compression form. The used material is AL7075T6 and the plate geometry is 0.01£h / a£0.05. The Generalize Differential Quadrature method is employed as numerical method to analyze the problem. The influences of loading ratio, plate thickness and various boundary conditions on buckling factor were investigated in the analysis using both incremental and deformation theories. In thin plates the results obtained from both plasticity theories are close to each other, however, with increasing the thickness of plates a considerable difference between the buckling loads obtained from two theories of plasticity is observed. The results are compared with those of others published reports. Moreover, for some different situations new results are presented. Some new consequences are achieved regarding the range of validation of two theories.

The amount of energy consumption in a building is affected not only by the components' performance, but also by the building envelope components' installation method. The negligence of the good practice methodology can have a huge impact on the thermal bridges, increasing the average thermal transmittance up to 40%.In this paper, the thermal performance corresponding to different positions of the fixed frame, without sub-frame and with different sub-frame materials including wood and steel for double-glazed windows has been analyzed. Besides, in these cases, different configurations of thermal insulation have been considered. The evaluation of thermal bridges due to different models in a sample wall, insulated externally, has been carried out using THERM program. The linear heat transfer coefficient has been calculated for each model and the impact of each parameter on the thermal performance has been evaluated. The results show that the position of the frame. the sub-frame material and the con figuration of the thermal insulation around the window frame affects considerably the thermal performance.

In this paper, stress gradient theory is used to model the static pull-in instability and size effect of electrostatic nanocantilevers in the presence of electrostatic and dispersion (Casimir/van der Waals) forces. The Differential transformation method (OTM) is employed to solve the nonlinear constitutive equation of the nanostructure as well as numerical methods. The basic engineering design parameters such as critical tip deflection and pull-in voltage of the nanostructure are computed. It is found that in the presence of dispersion forces, both pull-in voltage and deflection of the nanobeam increase with increasing the size effect. Compared to the pull-in voltage, the pullin deflection of the beam is less sensitive to the size effect at sub-micrometer scales. On the other hand, the size effect can increase the pull-in parameters of the nano-actuators only in sub-micrometer scales. The results indicate that the proposed analytical solutions are reliable for simulating nanostructures at sub-micrometer ranges.

The design of the structural supports has always been practically important in engineering applications. In addition to holding a structure properly, supports can also be utilized to improve the structural performances. In this study, by using modified finite element method (MFEM) and Imperialist Competitive Algorithm (ICA), the maximum of bending moment was minimized. In this paper both elastic and rigid supports are taken into account. As compared to other design optimization methods, ICA is robust, more efficient, and requiring fewer number of function evaluations, while leading to better quality of results. Appling the modified finite element method not only reduces computational cost and increases convergence rate, but also reach the global optimum position of supports. Three classical examples are given to demonstrate the validity and capability of the proposed optimization procedure for finding the global support positions. Results show that support position optimization by using present method, can reduce the maximal moment significantly, and deserves more investigation.

The main objective of this research is to study the nonlinear vibrations of a single walled carbon nanotube. For this purpose, the lattice structure of carbon nanotube is replaced with a continuum structure using nanoscale continuum mechanics. Firstly, each carbon-carbon bond is replaced with an equivalent beam element and then the whole discrete structure of carbon nanotube is replaced with a virtual continuum medium representing hollow cylinder. Then, governing equations for vibrations is obtained taking into account geometric nonlinearity arisen from stretching of a mid-plane due to bending. Perturbation technique is used to analyze the nonlinear vibrations of carbon nanotubes. Frequency responses of carbon nanotubes for free vibrations and force vibrations in both primary and secondary resonance cases are studied. Obtained results are in a very good agreement with numerical integration technique. The results imply on hardening behavior of carbon nanotube. Moreover, nonlinear bifurcation and nonlinear jump phenomena are observed.

In this paper the effects of the inlet fluid temperature on the electro-osmotic flow pattern in a two-dimensional microchannel with constant walls temperature is investigated with solving the governing equations by the Lattice Boltzmann method. The main objective of this research is to study the effects of temperature variations on the distribution of ions and consequently internal electric potential and velocity field. For make possible to use the Boltzmann ion distribution equation, cup mean temperature for every cross section of the microchannel is used. At the used Lattice Boltzmann method, LBCK model for modeling the Boltzmann collision function and the Zou-He boundary conditions method for velocity field has been used. Wang model for solving the Poisson-Boltzmann and He-Chen model for solving the energy equation has been used. The results show that, with increase the temperature difference between the inlet flow and the walls, the electro-osmotic flow rate increases. Also, observed that with decrease the external electric potential and the electric double layer thickness and increase the temperature difference at the inlet zone of the microchannel, a region with return flow is formed which can be used for controlling the internal flow pattern.

In this paper, a linear model for symmetrical vibrations of electrostatically actuated annular microplate with thermoelastic damping is considered for calculating the quality factor of this damping. The Kirchhoff-Love plate theory is used to model the microplate which is coupled with thermal conduction equation one dimensionally. For calculating the Q-factors in each mode, two methods are compared with respect to linearization of frequency equation. Also the dependency of thermoelastic damping to electrostatic load and geometry of annular microplate is investigated with clamped-clamped and clamped-free boundaries. A silicon annular microplate is considered as an example. The results show that, there are a critical radius and thickness which make the thermoelastic damping to be maximal. Also the results show that the effect of electrostatic load on thermoelastic damping depends on the type of boundary conditions. The effect of electrostatic load on thermoelastic damping for clamped-free boundaries is more than for clamped-clamped boundaries.

This paper reports the finite element modeling of axial crack growth in a thin aluminum tube under gaseous detonation loading. The finite element method was used to handle the moving load and also the nonlinear characteristics of the problem. The simulation results were compared with the experimental results reported in the literature and also with the results obtained from an analytical model. Moreover, the cohesive element with traction-separation law was used for the crack growth modeling. The final part of the paper is devoted to comparisons between the numerical crack growth simulations obtained from the current work and the numerical results based on the CTOA criteria that were previously reported in the literature. The very good agreement between the two methods was indicative of the robustness of the implemented procedures.

Drying process is influenced by a variety of parameters including the geometry of part being dried. To evaluate the effect of part geometry on drying process, and resultant defects, the process is analyzed and studied. Based on the assumption related to the porous media the governing equation of the mass transfer and static equilibrium are presented. The mechanical stresses generated by the drying strains are expressed according to the linear-elastic model. Dependence of physical and mechanical properties such as Young's modulus and diffusion coefficient as a function of moisture are considered in simulation for a chemically known ceramic material. it's Assumed that Extended thin film evaporation is the mechanism of evaporation in constant rate period has been studied. The Von Misses criterion is used for crack anticipation in 2D and 3D drying. A significant difference was observed in possibility of crack initiation for the two different configurations. Yield stress in hygroscopic moisture has been determined experimentally. Developed model made it possible to predict the time and the place of crack initiation. Different part thicknesses were studied to examine the effect of thicknesses variations on cracking. It is observed that the danger of cracking is highest at the beginning of the drying, since the yield stress is low.

The aim of this article is to compare a direct hybrid system of gas turbine and solid oxide fuel cell with an indirect system from thermodynamic and exergy viewpoints. According to importance of fuel cell role in hybrid cycles and providing further proportion of produced power, discrete and complete thermodynamic, electrochemical and thermal analyses have been done. Calculation of working temperature which has an impact on system performance is one of the most significant works that is done in this article. In addition, by parametric study of this hybrid system, the influence of inlet air rate and compression ratio on efficiency, power and exergy destruction rate has been perused and eventually an optimized state for system will be offered. Results indicate that a direct hybrid system is more efficient in comparison with an indirect system. Higher efficiency, less irreversibility, higher power, and less pollution are the most important advantages of direct hybrid systems.

Electrostatic motors are presented special advantages compare to electromagnetic motors such as lightweight, compactness and simple to fabricating. Due to these capabilities, recently, many researchers are working on electrostatic motors to make them applicable in industries. Accordingly, in this paper a new design idea for these motors is investigated theoretically and experimentally. This motor has driving electrodes on both rotor and stator, however, no wire is attached to rotor and signals are transferred to rotor using the induction electrodes. Modeling is implemented to study the effective parameters on performance. Then, using the modeling results, the design parameters are optimized using numerical method to improve the torque and minimize the ripple. Optimization findings are identified an optimum value for ratio of width to gap for driving electrodes and an optimum value for ratio of induction electrodes surface to total surface. Finally, the motor performance is evaluated using experimental setup and several experiments.

In this research, the effects of different parameters on simulation of Young's modulus of a Graphene sheet are studied. In simulation of Young's modulus of Graphene sheet, different parameters such as the thickness of a single layer of Graphene, type of loading and boundary conditions, effects of interactions non-neighbor atoms, type of element for carbon-carbon bond, mechanical properties of carbon-carbon bond and the size of the Graphene sheet influence the results. It was found that the thickness of a single layer Graphene and the type of element are effective parameters. Moreover, the type of loading and boundary conditions did not influence the Young's modulus of the Graphene sheet. Therefore, the Graphene sheet can be considered as an isotropic material. Considering the effects of interactions of non-neighbor atoms increases the run-time and improves the accuracy of calculations. Mechanical properties of carbon-carbon bond are important parameters and must be chosen carefully. Also, it has been observed that when the length and width of the Graphene sheet are smaller than one nanometer, the size of Graphene sheet has a great influence on the Young's modulus.

Misalignment and unbalancing are the most common factors that are causing energy loss and reduce the life of rotating machinery. Several methods for modeling and detecting these faults have been presented up to now, but very rare cases concentrated on the energy loss rate due to these faults, especially the theoretical one. In the present paper, based on equations of motion for a system with misalignment and unbalance faults, a new idea is presented for energy loss calculation and the governing equations are solved numerically for this target. The amounts of energy loss, for both cases of with and without the faults and also for various speeds of shaft rotation, are computed and compared. The results of this theoretical investigation show that rate of energy loss strongly depend on the amount of shaft misalignment, unbalance and speed of shaft rotation.