Modares Mechanical Engineering
Year:2013 | Volume:13 | Issue:5|Papers Count:16

Sandwich panels (structures) of metal surface with aluminum foam core are of great importance in aerospace, naval and automotive industries due to high strength to weigh ratio and high energy absorption characteristics. In this article several aluminum sandwich panels with aluminum foam core 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 and ballistic limit velocity of sandwich structures. The material model used for metal foam was Deshpande-Fleck-Foam and coefficients were determined experimentally using foam and Matlab capabilities. Also, numerical simulation using LS-DYNA software were performed. The results of the experiment and numerical simulation were compared and there was a good agreement between experimental investigation and numerical results. Using experimental testes and parametric studies, it is shown that the amount of energy absorption of sandwich structures is increased as density, foam thickness and velocity of the projectile is increased.

In this study, 2D Asymmetric heat transfer in multi-layer composite cylinder is investigated, analytically. The boundary conditions are the most linear general boundary conditions which can cover all the heat transfer mechanisms consists of convection, conduction and radiation. The fibers are wounded around the cylinder. The angle of fibers and composite materials can be changed layer by layer. The governing equation of orthotropic conduction has been extracted and temperature distribution series have been obtained using the separation of variables method. In order to find the unknown series' coefficients, three independent set of equations have been constructed by applying the boundary conditions inside/outside of cylinder and temperature/heat flux continuity between the layers. These set of equations have been solved using the orthogonal function relations and Thomas algorithm to find the recursive relation for unknown coefficients. The effect of design parameter, fiber angle and layers' materials, have been investigated via two functional examples.

In this paper, the free vibration of a two-dimensional functionally graded circular cylindrical shell is analyzed. To describe the material properties of the two-phased FGM material Mori-Tanaka micromechanical model is used. The spatial derivatives of the equations of motion and boundary conditions are discretized using the methods of generalized differential-Integral quadrature (GDIQ). To validate the results, comparisons are made with the solutions for FG cylindrical shells available in the literature. The results of this study show that the values of natural frequency of 2D FGMs are higher than those of ID FGMs in parallel conditions. Furthermore, application of a confining elastic foundation increases the value of natural frequencies.

The aim of this paper is to develop a numerical procedure for simulating underwater explosion phenomena with a simplified mathematical and two fluid model. The two fluid Kapila five equation model is selected as the governing equations and the ideal gas and Stiffened gas equations of state (SG-EOS) are used to obtain pressure in the gas bubble and the surrounding water zone, respectively. The modified Schmidt EOS is used to simulate the cavitation regions with low pressure. A Godunov numerical method and HLLC Reiman solver is extended for Kapila two fluid model. The numerical results of the present method and comparing them with available experimental results, verify that the proposed method has good capability of predicting complex physics involved in a spherical underwater explosion and its interaction with free surface. The method also shows a very good performance with no spurious oscillation in cavitation zone simulation in two-dimensional problems.

In this paper, optimal control of vortex shedding behind square cylinder in laminar flow regime (Re=200) has been investigated. When Navier-Stokes equations are used as state equations, the discretization of the optimality system leads to large scale optimization problems that represent a tremendous computational task. In order to reduce the number of state variables during the optimization process, a Reduced-Order Model (ROM) based on POD modes is derived to be used as state equations. Then, these equations are modified by introducing a Control Function to ROM in order to gain equations which are suitable for optimal control. Optimal trajectory for control input has been found by employment of Quasi-linearization which is one of the numerical methods for solving optimal control.  

Dynamic analysis of the civil structures such as bridges, towers and buildings is required for their design and maintenance. Modal analysis is a powerful tool to conduct some part of dynamic analysis in determination of the modal parameters in terms of natural frequencies, damping factors and mode shapes. However, excitation of these structures is usually difficult and sometimes impossible. As these structures are usually excited by ambient forces such as wind, this idea is suggested that the structure is modeled considering the natural forces as the inputs. However, the ambient forces are unknown and have a complicated nature to be measured. An alternative approach is using the operational modal analysis concepts in which only the responses are measured and the modal parameters are extracted. In this article Frequency Domain Decomposition (FDD) method is used for identification of modal parameters of a clamped-clamped beam and the results are compared with those of the FEM. The operational modal analysis is conducted on a type of a bridge under ambient forces in a real test and the results are compared with those of the conventional Modal testing. The results confirm that the method is suitable for engineering applications.

Three-point bend (TPB) specimen is an important test sample in fracture study of notched components Made from brittle materials like rocks and ceramics. On the other hand, the notch stress intensity factors (NSIFs) are vital parameters in brittle fracture assessment of V-notched structures. Therefore, computation of SIFs in TPB specimens is of practical interest to engineers and researchers. Since the available methods for calculating NSIFs are often cumbersome and need complicated calculations, it is preferred to show them as a set of dimensionless parameters. In this research, by using a finite element approach called FEOD method, the stress intensity factors are determined numerically for notched TPB specimens having different geometry and loading conditions. The obtained values of NSIF are then converted to dimensionless parameters called notch shape factors YIv and are illustrated in a number of discrete figures. It is shown that each of the three parameters of: the notch opening angle, the notch length and the distance between the two supports has direct relation with YIv The results presented in this paper can be used by designers and engineers for performing the necessary fracture experiments through the TPB specimen without needing complicated and time-consuming calculations.

In this study, nanocomposites based on polypropylene and polyethylene containing 0 to 7phr of nanoclay and 3phr of maleated polypropylene (PP-g-MA) and maleated polyethylene (PE-g-MA) as compatibilizer were prepared by melt compounding followed by injection molding. Morphology of the mixtures was carried out through Field Emission Scanning Electron Microscopy (FESEM). Mechanical properties of different samples were examined empirically by tension and izod impact tests. Test results show that the addition of clay particles to the PP and LLDPE increases the tensile strength, yield strength, tensile modulus and decreases impact strength and elongation at break compared to pure polymer. The mixture design method and Minitab 16 software were utilized for designing the tests, statistical analyses, and optimization the mechanical properties of the mixtures. Results showed that the in compounds based on polypropylene and polyethylene, respectively, compounds includes 3.96 and 2.12 percent clay has the best mechanical properties. Blends base on PP have better tensile modulus and LLDPE blends have better impact strength.

Since there are struggles with CNTs dispersion in the resin and production costs, synthesis and test of epoxy/carbon nanotube (CNT) nanocomposites is not economical. For this reason, simulation methods are proper techniques to predict mechanical properties of these nanocomposites. But the actual dimensions of CNTs and their length to diameter (aspect) ratio is a cause for concern in nano and micro scale finite element modeling. In this paper, different arrangements of CNTs in epoxy matrix have been presented using a beam element as a CNT and creating representative volume element of nanocornposite in micro scale. Effects of volume fraction, aspect ratio and wave effects of CNTs on nanocrnposite effective elastic moduli have been investigated. The results show that this method eliminates the limitation of both micro and nano molding and simulates the real conditions of nanocomposites and can be used to examine the effects of geometric parameters in the effective moduli. On the other hand, the simulation results have a good agreement with experimental results.

Exoskeleton is a machine composed of a wearable anthropomorphic structure which noticeably magnifies user's might via its actuators. In this research, dynamic modeling and control system design for a lower limb type of this robot were done. In the literature at most a I DOF part of the robot is modeled and controlled which doesn't give a good insight on how all of the robot parts are controlled simultaneously. First, a suitable structure was chosen similar to that of UC Berkeley's BLEEX project. Then dynamic equations were derived in sagittal plane using the Newton-Euler method. By an experiment using Xsens system, gate kinematics data were measured and the inverse dynamics was simulated both in Sim Mechanics and on the model in MATLAB that proved accuracy of the derived model. Impedance control was investigated and some corrective remarks were included in that algorithm. Using this method the robot was controlled. It stabilized the system and the robot followed user's movement exactly. While a load of 50 kilograms was carried, mostly moments of less than1 (Nm) were applied at each interface among man and robot.

In this paper, steady incompressible flow patterns inside two-dimensional triangle, trapezoidal, semicircular and arc-square cavities with moving boundaries are studied via lattice Boltzmann method. The effects of geometry of the cavities on flow pattern are also discussed. The effects of Reynolds number on the flow patterns in triangular, semi-circular and arc-square cavities are studied. Also, for arc-square cavity, it can be observed that by changing the size of top and bottom walls which means a change in the size of cut arcs from the circular, different flow patterns are formed. The influences of the side angles at constant Reynolds number on the flow patterns in the trapezoidal cavities are investigated. It is found out that the vortex near the bottom wall of trapezoidal cavity breaks up into two smaller vortices as side angles increase. The obtained results of the lattice Boltzmann method and the presented boundary condition are compared with those presented in the literature. It can be seen that the lattice Boltzmann method is a suitable method for flow simulation in the mentioned cavities.

Flexible members such as solar panels of robotic systems during a maneuver may get stimulated and vibrate. Therefore, such vibrations will cause some oscillatory disturbance forces on the moving base and manipulated object, which in turn produces error in the position and speed of the manipulating end-effectors, which should be prevented. In this paper, a new control algorithm for an object manipulation task by a wheeled mobile robotic system with flexible members is proposed. To this end, a new dynamics modelling approach for control implementations on compounded rigid-flexible multi-body systems is introduced. Then, based on a designed path/trajectory for a wheeled mobile robotic system, an Adaptive Hybrid Suppression Control (AHSC) is proposed to perform an object manipulation task by such complicated rigid-flexible multi-body systems. Finally, a wheeled mobile robotic system is simulated which contains two manipulators, and a rotating antenna and a camera as its third and fourth arms, appended with two solar panels as has been proposed for space explorations. Obtained results reveal the merits of the proposed AHSC algorithm which will be discussed.

The present study proposes an analytical solution for the axisymmetric/asymmetric buckling analysis of thin circular/annular nanoplates under uniform radial compressive in-plane load. In order to consider small scale effects, nonlocal elasticity theory of Eringen is employed. To ensure the efficiency and stability of the present methodology, the results are compared with other presented in literature. Material properties including Young's modulus, density, Poison's ratio are assumed to be constant throughout the body of the nanoplate. In addition, the effect of small scales on critical buckling loads for different parameters such as radius of the nanoplate, boundary condition, mode number and geometry parameters are investigated. In order to obtain the critical buckling load, the asymmetric modes as well as axisymmetric modes are considered. The thin nanoplate is modeled using Kirchhoff plate theory.

Ultrasonic flow meters are contactless so they don't cause pressure drop. In this article an ultrasonic flow meter based on transit time measurement by methodology of Heterodyne technic is designed and fabricated. Firstly, Functional principles of these kinds of flow meters, selection procedure of transmitter and receiver transducers and the excitation frequency of those are presented. A flow cycle has been designed in order to test the flow meter. The results present that the flow measurement in range of 22 up to 37 Liter per second contain errors in range of -2.7 up to +4.2 present of actual flow.