فصلنامه مکانیک هوافضا
سال ششم شماره 4 (پیاپی 22، زمستان 1389)

In this work, polypropylene/single walled carbon nanotube nanocomposites were prepared with melt mixing method.Mechanical and damping behaviors of nanocomposites were investigated by tensile test and dynamic mechanical thermal analysis (DMTA), respectively. Experimental results showed that carbon nanotube causes tensile modulus to increase about 82% and yield strength about 27%. DMTA test showed that damping do not change with increasing temperature and amount of carbon nanotubes. Also storage modulus had the same behavior as tensile modulus. The behavior of crystals structure, spherulites growth rate, and thermal stability of nanocomposite were investigated with wide-angle X-ray diffraction, Polarize Light Microscope, and Thermal Gravitation(TG), respectively. X-ray diffraction test showed that for slow cooling specimen, size of crystal decreases, but for fast cooling specimen, it increases. Studies in stability of nanocomposite showed that temperature destruction was increased about 82◦C even with 2% weight fraction nanotubes in compare with pure polypropylene.

In this paper, path optimization for delta robot in traveling between two arbitrary points is presented. Using the dynamic model, joint trajectories of delta robot are optimized considering the required power and torque in traveling between two points. To detect optimal parameters for joint trajectories, spline functions are optimized via genetic algorithm. Results of simulation indicate good performance of suggested algorithm (specifically in the case of reducing the power usage) for robot movement from one point to another in a 3-D space. Also, solution to dynamic modeling of the robot, using Kane dynamics reveals good matching results as are compared to an ADMAS numerical simulation.

In this study, the solution of small mass impact on a moderately thick rectangular plate was derived semianalytically,based on elastic Hertzian contact law. A set of exact closed-form in-plane and transverse displacement equations are given for the first time, using the so-called Mindlin plate theory (a first-order shear deformation theory) for the plates having two opposite edges that are simply supported. The other two edges may be given any possible combination of free, simply-supported and clamped boundary conditions. The proposed analytical method is verified by observing excellent agreements to those available in the literature. Finally, the influence of boundary conditions,material properties, dimensions of the plate and initial velocity and radius of the spherical impactor on the impact parameters are examined and discussed in detail.

In this research, the critical buckling load of a FGM circular plate with actuator-actuator piezoelectric layers is calculated based on neutral-axis and using higher-order shear deformation plate theory. The plate is clamped and simply supported. Equilibrium and stability equations are derived using the Euler equations, while pre-buckling forces are obtained using the membrane theory. Then, the critical buckling loads are derived and the graphs and tables are presented and compared with other available theoretical data. The results show that for constant thickness of piezoelectric layer, critical buckling load is increased by increasing the aspect ratio (h/a) and is decreased by increasing volume fraction exponent. Also, for FGM thickness constant, critical buckling load is increased by increasing the aspect ratio (hp/h), while decreased by increasing volume fraction exponent.

This paper presents a theoretical bending analysis of rectangular composite plates with piezoelectric layers. The host plate has been chosen orthotropic and isotropic, respectively and has been analyzed under uniform and sinusoidal loads with piezoelectric property, using the extended Kantorovich iterative procedure. The solution has been developed, based on the first order shear deformation theory, and it is limited only for the solution of extension type piezoelectric mechanism. With the iterative procedure and using Galerkin method, the dependent non-linear system of differential equations of the plate have been changed to the couple linear differential equations, which is decoupled and solved by the operator method. The Result of the present analytical method for composite plate, under different boundary conditions, have been verified by comparing with classical plate theory results and with finite element and ANSYS result. The results show high accuracy, while the iterative process converges very rapidly.

In this paper, using a pulse shaper, different geometric parameters affecting the incident pulse shape and achieving a constant strain rate condition are studied. To this end, pulse shapers in different thicknesses and diameters were made from copper. Based on wave propagation analysis, proper incident pulse shape for testing brittle and hardening materials were determined. In addition, performing experiments and simulations in LS-DYNA, the effects of pulse shaper thickness, its diameter, striker velocity, and its length on incident pulse were studied. Moreover, several Hopkinson tests for cast iron (GGG-60) specimens with and without pulse shaper were carried out. The results show that using pulse shaper with proper dimensions helps to achieve constant strain rate condition which is highly desirable in split Hopkinson pressure bar testing.

In this article, a simple analytical model for a high velocity projectile penetrating into a ceramic-faced composite armor is presented. The output of the model is residual velocity, residual mass, projectile instantaneous velocity, and projectile instantaneous penetration depth. The model is based on momentum and energy balance developed by Tate,Alekseeviski, and Chocron. In this model, penetration process is investigated in two phases. The first phase is when ceramic conoid is formed and there is no penetration into yarns and the second phase is when ceramic-conoid penetrates into backing composite material along with the projectile. The governing equations are developed and the results are analyzed. The model is tested for a tungsten rod and an alumina-faced dyneema and a kevlar. The results are in good agreement with numerical simulation and analytical results of Chocron & Galvez.

In this paper, the buckling load and buckling modes of FML panel, using first order shear deformation theory, has been studied, based on the Reddy's numerical solution. Because of various applications of FML shells, GLARE panel was considered. The effects of geometry, lay-up, and MVF were determined. Boundary conditions were simply supported and the panel was axially loaded. The results for Glare panel are presented and discussed. Reducing the number of metal layers at a constant MVF increases the Buckling load. The results from the finite element model were found to be in general agreements with the other references.