مجله مکانیک سازه ها و شاره ها
سال پنجم شماره 1 (بهار 1394)

In this paper، an exact closed-form solution is presented for out-of-plane free vibration of thick homogeneous and isotropic multilayered rectangular plates with simply supported boundary conditions based on the linear three-dimensional elasticity theory. The solution procedure is on the basis of using of some proposed displacement fields. Proposed displacement fields satisfied simply supported boundary conditions on the edges of the plate. On the continuation of the solution، by replacing the proposed displacement fields into the three-dimensional elasticity equations of motion and simplifying the results some independent ordinary differential equations are obtained. The natural frequencies are extracted by satisfying the boundary conditions on the surfaces of the plate and surfaces between the layers. In order to establish the accuracy and stability of the proposed solution، several numerical results for one layered and two layered square and rectangular plates are presented and compared with corresponding results in the literatures and obtained results of 3-D finite element method.

In this paper، the dynamic response of a curved sandwich beam with soft flexible core، subjected to a radially low velocity foreign impact on the middleof top face sheet is studied. The dynamic responses which are composed of circumferential and radial displacement of the core، top and bottom face sheets، as well as the radial and shear stresses in the core layer are identified. The higher order shear deformation theory was used in the structural formulation. The two degrees of freedom mass-spring model with linearized stiffness was used for modeling of impact phenomena. In order to derive the governing equations of beam structure، the Hamilton principle was used. The results were validated and compared with the results obtained from finite element ABAQUS software and the latest available literature. The effects of some structural properties like the module of elasticity and the thickness of core layer on the displacement، shear stresses and strains were studied. Also، the effects of radius and initial velocity of the impactor on contact force history were investigated. it was seen that by multiplying by 2 of the elasticity modulus of core to upper layer ratio، the core deflection only reduces about 10 percent. One also sees that by increasing the core to beam thickness ratio، the core deflection reduces about 6 times.

In this paper the macromechanical behavior of dual phase steel based on actual microstructure has been predicted. In order to prepare dual phase steelsc (DP) of different percent phase combinations، a low carbon steel (C-Mn) was subjected to intercritical annealing treatment (ICT) and quenched in water. Then، the actual microstructures of dual phase steels were obtained by metallographic analysis and optical microscopy. A 2D representative volume element (RVE) was generated by finite element code Ansys on the basis of actual microstructure which was obtained by image processing code in Matlab software. The individual single-phase flow curves were obtained based on the dislocation theory and the local chemical composition of constituent. The results of 2D micromechanical RVE models under periodic boundary conditions and tension loading were compared with the experimental results. It is shown that the 2D micromechanical model can predict both strength and ductility for low volume fraction of martensite in dual phase steels. The 2D micromechanical modeling may then be used to portray the local strain evolution of the individual phases in the DP microstructures.

In an inverse problem، it is desired to construct the model of a system، using the observable characteristics of the system under consideration، so that it can predict the behavior of the system as accurately as possible. In this study، a non-uniform beam is assumed as the structure whose FE model is desired to be updated. The first three natural frequencies and mode shapes of the non-uniform beam are derived analytically and are assumed as the pseudo experimentally-obtained dynamic characteristics of the structure. Then، the mentioned dynamic characteristics are used to solve the inverse problem of identification of finite element model parameters of the non-uniform beam using genetic algorithms. Investigating the effect of the number of cross-sections of the finite element model of a non-uniform beam on the accuracy of the results، it is observed that the more cross-sections the finite element model has، the more capable it is in predicting the dynamic behavior of a non-uniform beam.

Steel and aluminum sheet metals، and also composites have vast application in the military and aerospace industries، keeping the safety of those against balls and projectiles، and prediction of damaged zone affected by ball or projectile impact is one of the challenges of engineers and researchers. In this research، first، using the finite element method، damage mechanics، and Hooputra''s ductile damage criterion existed in the ABAQUS software، effect of direct impact of projectile to an aluminum plate applied in the military industries is simulated and the results are numerically achieved. Then، the effect of oblique angle of projectile impact to the plate، on the area and depth of the damaged zone is investigated. In order to validate the results، the projectile shooting test into the aluminum plate under the corresponding angles is practically carried out and the obtained results are compared with the simulation results. Comparison of the numerical and empirical results reveals that the simulations are in good accuracy. Hence، it is concluded that the Hooputra''s ductile damage criterion can well predict the failure and fracture zone in the high strain rate deformations.

In this paper، a novel adaptive fuzzy control design for a four degrees of freedom single-wheel robot including its electric motors is presented. Another novelty of this paper is to derive a novel model for this robot for using voltage control strategy in the control design. The dynamics of motion is described as a nonlinear multivariable system with couplings between inputs and outputs. Similar to an inverse pendulum، the single-wheeled robot is in the instable equilibrium point. In addition، number of actuators of robot are less than the degrees of freedom of robot. The control design becomes so important due to this complexity. Based on the derived model in this research، a novel feedback linearization controller by using torque control strategy is designed. Then، a novel indirect adaptive fuzzy controller in voltage control strategy is designed. The stability is guaranteed in the presence of uncertainties. The stability of equilibrium point is analyzed by direct method of Lyapunov. Simulation results show the effectiveness of controllers to keep the balance and stability of the single-wheel robot.

Study of the critical buckling load as well as post-buckling behavior of single-walled nanotubes (SWNT) under various thermal conditions is the subject of this article. For finding critical buckling loads the clamped-free and simply supported conditions are considered and for post-buckling analysis the boundary conditions are assumed to be clamped. A space-frame model is here employed for the zigzag and armchair nanotubes with different chiralities and aspect ratios. In this approach، the linkage between carbon atoms is modeled as three dimensional elastic beam. By establishing a linkage between structural mechanics and molecular mechanics، the sectional property parameters of these beam members are obtained. The obtained results indicate that، as it is expected، by increasing the aspect ratio of the nanotubes as well as chirality، the critical buckling load decreases. Also، it is noticed that the post-buckling behavior of both armchair and zigzag nanotubes are quite similar. In addition، the effect of temperature on the critical buckling load is investigated. Due to the influence of temperature over the linkage length and force field constants it is shown that when the temperature increases a decline in the critical buckling loads is observed.

Most of the studies done in low velocity impact of plates only involve plate’s response to centric impact and usually have been done in no-preload condition. In this study، nonlinear numerical elasticity analysis of eccentric low-velocity impact of rectangular sandwich plate with composite face sheets subjected to biaxial preloads، and the effect of indenter energy، the stiffness of core، the thickness of core، geometry of indenter and the existence of epoxy layers in the connection between core and face sheets on impact response are investigated. In this regard، impact 3D simulation in ABAQUS rather than approximate plate theories is utilized for extracting impact responses based on the three-dimensional theory of elasticity. Numerical results are compared with experimental data presented in other references and the numerical model is verified. The analysis results showed that the contact force increased due to the reduction of overall plate movement in the cases of eccentric impact and biaxial tension preloads، which result in augmentation of damage.

Structural failure as a result of stress concentration at the geometric discontinuities is one of the concerns of desigrs. To avoid the fatigue failure of structures، several models have been proposed in recent years to predict the fatigue life. Stress concentration factor، especially in the presence of cracks with sharp edge، is a key parameter in displaying these models. perforated composite plates can be seen in many engineering structures. Due to the complexity and breadth of engineering structures، detailed investigation of these structures seems to be needed. In this paper، try to study the effect of various parameters on the stress analysis of composite plates containing a central rectangulare cutout that are subjected to shear stress by analytical method. These parameters are: cutout bluntness، cutout orientation، fiber angle، cutout elongation and material properties. In this paper، The effect of these parameters and the optimal values of the parameters which leads to less stress concentration will be introduced. The finite element method has been used to check the accuracy of analytical results. The analytical results are in good agreement with the numerical results. The results indicated، The cutout geometry and fibr angle in composite plates can be a significant influence on the stress distribution around cutout

Nonlinear bending of a functionally graded nanocomposite plate reinforced by aligned and straight single-walled carbon nanotubes (SWCNTs) subjected to a uniform transverse load and thermal load is investigated. The material properties of the nanocomposite plate are assumed to be graded in the thickness direction، Four types of distributions of the reinforcement material are considered، that is، uniform and three kinds of functionally graded distributions of carbon nanotubes along the thickness direction of plates. The material properties of SWCNT are determined according to molecular dynamics (MDs)، and then the effective material properties at a point are estimated according to the rule of mixture. The equilibrium equations are based on first-order shear deformation plate theory (FSDT) and von Kármán strains. These system of equations are solved by Dynamic Relaxation method to determine the load-deflection and load-bending moment curves. Some results for nanocomposite plates are compared with the ones reported by the ABAQUS finite element software. Furthermore، some comparison study is carried out to compare the current solution with the results reported in the literature for isotropic and Functionally Graded Materials (FGMs) plates. Numerical results indicate that volume fraction of carbon nanotube، distribution of CNTs، plate width-to-thickness ratio، plate aspect ratio and different boundary condition have pronounced effects on the nonlinear response of nanocomposite plates.

In tube hydroforming process، due to friction condition، uniform wall thickness، as well as sharp corners may not be achieved. Use of ultrasonic vibration can improve the contact conditions at the tube-die interface. The current work studies the effect of applying ultrasonic vibration on wall thickness and corner filling of hydroformed tubes. By comparing the FEM models of tubes in two cases of with vibration and without vibration، it is possible to investigate the effects of vibration on wall thickness and corner filling. In addition، the finite element analysis، as the modal and harmonic analysis، are used to design a set of ultrasonic tube hydroforming. Annealed copper tubes are used for the experimental tests. The simple mechanism introduced in this paper is used to form the tube، by which the final piece is produced with lower costs and without any need to hydroforming machine. The results indicate superimposing ultrasonic vibrations to the process will increase corner filling ratio of the tube significantly، and more uniform tube wall thickness will be achieved.

In this paper، magnetic field effect on the free vibration of cylindrical panel made of functionally graded materials resting on pasternak elastic foundation using first order shear deformation theory for simply supported edges is investigated. The governing equations of motion are obtained by using principle of the Hamilton and energy method. These equations are solved by the navier method. The effect of geometrical parameters such as the radius to length ratio، thickness to length ratio، sector angle of cylindrical panel and Pasternak elastic foundation on the natural frequencies are studied. It is observed that the natural frequencies of cylindrical panel made of functionally graded materials with increasing the radius to length ratio، thickness to length ratio، and sector angle of cylindrical panel decreases، while its stability increases by considering the effect of pasternak elastic foundation. Also the natural frequencies of cylindrical panel made of functionally graded materials increases by applied magnetic field and influence of magnetic field on the higher natural frequencies is higher than that of the lower frequencies.

Minarets are valuable historical structures from ancient times in Iran. In this paper، the effect of wind on a number of historical brick masonry minarets in Isfahan، built in the eleventh and twelfth centuries A. D.، has been studied. Different wind velocities including the maximum wind velocity of Isfahan and Iran have been used. The non-linear finite element method and the Willam-Warnke failure criterion have been used. In order to investigate the effect of central column and spiral staircase on structural behaviour، analyses have been performed for two cases: 1) the whole minaret (including the outer shell، central column and spiral staircase)، and 2) only the outer shell. Results indicate that under maximum wind speed recorded for Isfahan، minarets do not fail. In higher speeds، the amount damage depends on the diameter of central column، height، thickness and diameter of shell.

In this paper، single-flash geothermal power cycle with Kalina and ORC combined cycles are examined from energy، exergy and exergoeconomic viewpoints. Isobutane، isopentane، n-butane، n-pentane and R123 are used as working fluids in ORC. Exergoeconomic analysis is performed using the SPECO method. The results show that geothermal power cycle-ORC with R123 and n-pentane have the highest value of first law efficiency among considered combined cycles whereas geothermal power cycle-Kalina has the lowest unit cost of power produced by turbines. According to optimization of the combined cycles، the minimum unit cost of power produced for the geothermal power cycle-Kalina is 8. 63% lower than the unit cost of power produced in the maximum first law efficiency. Also the geothermal power cycle-Kalina with different ammonia concentration combined cycle has the lowest cost rate associated with exergy destruction. Finally parametric study is done and the effects of various parameters on exergoeconomic parameters of combined cycles are investigated.

During the course of expansion of steam in LP steam turbines، the vapor becomes supersaturated، and subsequently nucleates. Then condensation occurs، that causes some damages. Exact prediction of wetness terms، and formed droplets radius has extensive effects on accurate estimation of the flow properties، and damages due to vapor condensation. Nucleation rate is one of the governing equations of liquid phase which finally determines number of generated droplets. Considering exchange of mass and energy between two phases، different models of radius averaging at each cross-section of nozzle have been used. In the present investigation، various nucleation models have been combined with different droplet radius averaging methods in a semi-analytical Eulerian-Lagrangian model. The pressure distribution and droplets radius، in several cases، have been compared with experimental data. According to the results، using Girshik-Chiu’s refinement on Hale’s nucleation model with simultaneous application of Sauter averaging approach، provide the best prediction of the flow properties. In addition، classical nucleation equation with Girshick-Chiu’s refinement and without any refinement in combination with Sauter averaging approach; are standing on the second and third ranks and specially focusing on the pressure distributions.

Sudden change in discharge brings about significant pressure oscillations in a piping system which is known as waterhammer. Unsteady flow of a non-Newtonian fluid due to instantaneous valve closure is studied. The Cross model is used to model non-Newtonian effects. Firstly، the appropriate governing equations are derived and then، they are solved by a numerical approach. A fourth-order Runge Kutta scheme is used for time integration and a central difference scheme is employed for spatial derivatives discretization. To verify the proposed mathematical model and numerical solution، a comparison with corresponding experimental results are made. The results reveal a remarkable deviation in pressure history and velocity profile with respect to conventional waterhammer models in Newtonian fluids. The significance of the fluid behavior is manifested in drag reduction and line packing effect observed in the pressure history results. A detailed discussion regarding the fluid viscosity and its shear-stress diagrams are also included.