فصلنامه مکانیک هوافضا
سال سیزدهم شماره 1 (پیاپی 47، بهار 1396)

Different steps of design process in aerospace structures, interaction of design parameters in the complete design process and the necessity of the preliminary prediction of structural geometrydictate the presence of a general parametric design method for preliminary sizing of different structural components. The design method of structural index concept is a simultaneous design and analysis process that can provide complete details of panel geometry by having the structural index and material properties of a stiffened panel. In this paper, the design and analysis process of compressed stiffened panel is described by structural index concept with general configuration and the results are extracted with two different approaches. Then the results are modified to confirm the common iterative methods. Finally, the results are compared with the results of current methods. In addition to its application as a design method, structural index concept also can be used to determine optimal region of design space and choose initial design points which required for numerical optimization methods based on analytical or F.E.M methods.

The main purpose of this study is to investigate the nonlinear bending and buckling analysis of radially functionally graded sector plates subjected to uniform in-plane compressive loads. The mechanical properties of plates assumed to vary continuously along the radial direction by the Mori–Tanaka distribution. The incremental form of nonlinear formulations are derived based on first order shear deformation theory (FSDT) and large deflection von Karman equations. The dynamic relaxation (DR) method combined with the finite difference discretization technique is employed to solve the equilibrium equations. Also, due to the lack of similar research for the buckling of functionally graded sector plates with material variation in the radial direction, some results are compared with the ones reported by the ABAQUS finite element software. The achieved good agreements between the results indicate the accuracy of the present numerical method. Finally, numerical results for the maximum displacement and critical buckling load for various boundary conditions, effects of grading index, thickness-to-radius ratio, sector angle and inner radius-to-outer radius ratio are presented.

In this article, the interfacial stresses in single and double lap joints are studied. A quadratic displacement field is assumed in the adhesive layer. For Analyzing stress in joint the free body diagram element of fiber in adhesive and unadhesive part of join has been drawn. By writing the static equation for this elements, the differential equation for joint has been derived. Then by solving this differential equation and using the boundary condition equation, displacement in fibers and so the shear stress in adhesive and axial force in fibers has been derived. The effect of thermal and hygrothermal stresses are also included. It is observed that the peak shear stress developed within the adhesive layer is function of mechanical and physical parameters as Young’s module, Poisson’s ratio, thickness, thermal and hygrothermal coefficients of expansion. The results show that the double lap joint appears to reduce the peak shear stress within the joint significantly. For example maximum shear stress in double lap joints are nearly the half of single lap joints. The results are validated by FEM solution. An excellent agreement is observed between analytical and FE methods.

In this paper thermal buckling of a thin cylindrical panel made of piezo-magnetic two dimensional functionally graded materials (2D-PFGM) subjected to magnetic field have been investigated. The material properties of structure assumed varying by exponential functions of volume fraction in longitudinal and circumferential directions. In order to solving the problem at the first, the equilibrium equations have been derived by the first order shear deformation theory by concerning the nonlinear terms from the strain-displacement relations. In the next step, by giving the incremental changes on the displacement components and calculating the resultant forces and moments, using Lagrang,s equations on the second functional of potential energy the stability equations have been derived. After solution the mentioned equations by generalized differential quadrature method based on the simply supported boundary conditions the critical buckling temperature has been determined. By solving the numerical example in special cases and comparing the obtained results we confident from the method of analysis. At the end, effect of geometrical features, applied voltage on the external surface of panel, and changing the material properties on the critical buckling load have been investigated.

Nano-beam is one of the most important nano-structures with applications in different fields such as NEMS (Nano electromechanical systems) considering their low weight, low energy consumption and high sensitivity for use in medical, computer, bio-sensors and etc. Since the behavior of material in nano scale is different from usual scales, new methods are innovated to study the mechanical behavior of material in nano scale which one of them is nonlocal model. In this paper, nonlocal elasticity theory is used to describe the behavior of beam on the nonlinear elastic foundation. The Euler-Bernoulli beam theory is used to modeling the beam. The governing equations are obtained using principle of virtual displacements and analytical solution is used to solve governing equations. Maximum deflection, critical buckling load and natural frequencies for simply supported boundary condition and different spring constant, and nonlocal parameter is presented. The obtained results show that deflection is increased and critical buckling load and natural frequencies are decreased by considering nonlocal effects. Also elastic foundation decreases deflection and increases critical buckling load and natural frequencies.

In this paper, the complex variable method of Lekhnitskii has been applied for solving stress concentration problems in symmetric laminates with triangular hole. Lekhnitskii solution is limited to circular and elliptical holes in anisotropic plates. In order to use this approach to triangular hole, by means of conformal mapping, the area external to the hole can be represented by the area outside the unit circle. The effect of rotation angle of hole, stacking sequence, hole curvature and load angle on stress distribution around triangular hole are considered. The results based on analytical solution are compared with the results obtained using finite element methods. Good agreement is observed and provides confidence in the accuracy of the present results. The results presented in this paper, Indicated that the presented method can be used to determine accurately the stress concentration in symmetric laminates with special shape holes.

In the present paper the optimization process of a thin-walled collapsible shock absorber geometry is performed using the well-known design of experiments method. The optimizing target parameters are the maximum energy absorption and the low impact shock. A number of four geometrical control parameters have been chosen in two different levels making a totally 16 different geometries. The collapse behavior of the absorbent geometries then has been simulated using a full 3D shell element LS-Dyna numerical model with an elastic-plastic material behavior and some main absorbing characteristics extracted. The collapse behavior of the geometries is simulated under axial impact of a traveling mass with a presumed kinetic energy. A comparison between the different geometries can be performed to obtain the effectiveness of the control parameters and decide about the most effective ones. It has been shown that while the thickness of the crushable structure is an important factor, an increase in the radius of curvature at the end of the presumed geometry is the most effective parameter in absorber efficiency. A study also is performed to validate the numerical simulation process with conical absorber geometry under axial quasi-static loading. The comparison exhibited a very good agreement between the numerical finite element results and the data acquired from the experimental test that showed the validity of the simulation procedure.

Nowadays due to increasing industrial utilization of shock waves, it became popular to use analytical, numerical modeling and experimental tests to evaluate effect of shock waves on metal deformation. To explain this effect on metal properties, current paper considers both experimental and numerical plastic deformation of clamped rectangular steel and aluminum plates, under hydrodynamic loading of drop hammer system. To validate the explicit finite element simulation by solver ABAQUS, experimental test on rectangular plates with various thickness and energy levels have been performed. Obtained results by the numerical simulation are in a very good agreement with those in the experimental tests. Hence, it is desirable to utilize the numerical modeling to predict mid-point deflection of rectangular plate under hydrodynamic loading.

The purpose of this study is to estimate the fatigue life of laminated composites using a novel model based on continuum damage mechanics. For this purpose, a closed form criterion based on the energy method has been presented in the context of damage mechanics such that considers the difference of damage in three directions and moreover is not limited to a special layup. The proposed model has been implemented in the ANSYS finite element software using the material coded by the user (Usermat). The method of characterization of the model constants has been presented in this paper and the constants have been determined for AS4/3501-6 Carbon-Epoxy composite and finally, validation of the model for unidirectional and multidirectional layered composites has been evaluated. For multidirectional laminates, results of fatigue life for cross-ply laminates with hole with the layups [04/904]s and [904/04]s compared with experimental results. The results show that the developed model can predict the fatigue life of laminated composites only by using the material constants which is obtained from experiments of unidirectional laminates in two stress levels.