مجله مهندسی مکانیک امیرکبیر
سال چهل و دوم شماره 1 (تابستان 1389)

Understanding the influences of cutout in load bearing capacity and buckling behavior of cylindrical shells is a fundamental concern in the design of structural components used in automobiles, aircrafts, and marine structures.
In this article, simulation and analysis of steel cylindrical shells with various lengths, including quasi elliptical cutout, subjected to axial compression load were systematically carried out using finite element method and the investigation examined the influence of the cutout location and the shell aspect ratio (L/D) on the buckling, and the postbuckling responses of the cylindrical shells. For several specimens an experimental investigation was also carried out via an INSTRON 8802 servo hydraulic machine. The results obtained from the experiments were compared with numerical results. A very good agreement was observed between the mentioned results. Finally, corresponding to experimental and numerical results, an expression was derived for finding the buckling load of such structures

In the present paper, nonlinear transient heat transfer and thermoelastic analyses of a thick hollow FGM cylinder is accomplished using the finite element method and taking the temperature-dependency of the material properties into consideration. Due to incorporation of the effect of the temperature-dependency of the material properties, the resulted governing FEM equations of both transient heat transfer and thermoelastic stress analyses are nonlinear. In this regard, various thermal, geometrical, and stress boundary conditions are incorporated. An efficient numerical algorithm based on successive updating and time integration is used to derive the results. Finally, results obtained considering the temperature-dependency of the material properties are compared with those derived based on temperature independency assumption. Furthermore, influences of various boundary conditions on the temperature distribution and the radial and circumferential stresses are investigated. Results reveal that the temperature-dependency effect is significant.

Holes or cutouts present in many practical structures. Cutouts are needed to reduce the weight of the system or provide access to other parts of the structure. These cutouts create highly localized stresses or stress concentration at the vicinity of the cutout. The understanding of the effects of cutouts on stress concentration of such structures is very important in design. In this study, a simple analytical method for stress analysis of perforated plates is presented. The Lekhnitiskii’s solution for circular and elliptical cutout is expanded to triangular cutout shape using complex variable mapping. The solution is capable of considering large variety of cutouts with different shapes, bluntness, fiber angle, load angle and rotation angle. The results obtained clearly demonstrate the effect of these parameters on maximum stresses in perforated plates subjected to uni-axial tensile load.

Closed multi cell sections under torsion are used as structural members to investigate how the shear stresses vary over the cross section and to find the magnitude and location of the maximum stress. The boundary of cells in one type of sections, are circles. Sections with different geometrical ratios are considered. The shear stress is calculated by finite element method, and the results are plotted. The maximum of these stresses are determined and classified according to the area and polar moment of inertia ratios and plotted in separate diagrams to simplify interpolation. Rectangular boxes with the same and different wall thickness are also investigated as another type of closed multi cell sections. Stresses along the inner and outer edges near the corners and the mean shear stress along the middle line of all section walls are determined. Diagrams show considerable increase in the magnitude of stresses. Some findings are compared with similar quantities found in other references and differences are discussed.

This paper presents weight optimization of laminated sandwich panels with simply supported edges, subjected to out-of-plane load. Face wrinkling and intracell buckling are two famous local buckling of sandwich panels that are comprised important constraints of this optimization. A nonlinear analytical method has been exhibited using shear deformation effect and midplane displacement. Because of problem complexity and discontinuity of variables, genetic algorithm is preferred for this optimization. In this algorithm, a new method has been considered to stacking sequence and selection of facing and core materials. Finally, comparative results for various loads are presented.

Understanding the role and significance of the different components of the trunk neuro-musculo-skeletal system is of great importance for development of proper preventive and curative measures for low back pain. The experimental investigations, using lumbar moment measurement devices, are considered the most useful sources of information for detail understanding of the biomechanics of trunk and its components. This paper describes the conceptual design of a new lumbar moment measurement system, based on minimum deflection, anthropometric characteristics, and measurement considerations. The system is capable to measure the 3-D trunk moments using uni-directional torque sensors within a proper configuration of links and joints. Some other features of the system include convergence of its joints, high stability, and ability to adapt to a wide range of anatomical postures is standing and sitting positions.

In this paper the free vibration of a cantilever beam containing a breathing crack is investigated. Generally, the researchers have applied numerical methods to study the vibrational behavior of a cracked beam. In this research the governing equation of the motion for free vibration of the cracked beam is derived, and by applying suitable modifications and variable changes is expressed as the multiplication of a decaying exponential function by the Mathieu equation. By analyzing the parameters of the governing equation, this equation is solved by employing perturbation method. Presented method makes it possible to estimate damping characteristic of the system. The result shows that the system damping depends on the crack parameters, geometric dimensions, mechanical properties of the beam, and an increase in the crack depth results in an increase in the system damping. In order to validate the results, a comparison is made between the response of the cracked beam with a given crack depth and location obtained by the proposed analytical solution and that of the numerical method. Also, changes in fundamental frequency ratios versus crack severities are compared with those of the experimental results available in the literature. The obtained results from the proposed method agree well with the experimental results presented in the literature.

In this paper, the plugging failure of metallic plates under normal penetration of deformable projectiles has been analyzed by considering plastic stress wave theory. The material behaviors of plate material and projectile are considered rigid- plastic linear work hardening. In this model based on propagation of plastic stress waves in projectile, plug and outer region of plug, the penetration process is divided in six steps and governing equations at each step are derived. The results of analytical model are compared with experimental results and modeling by LS-Dyna software. The residual velocity, flattening area of projectile, penetration time and ballistic limit velocity predicted by new analytical model are good agreement with experimental results.

In this paper, deformation of blunt projectiles is studied for normal penetration into a deformable medium metallic target, by using of plastic wave propagation in the projectile and plug. The new analytical approach presented in this investigation, is established based on the concept of the conservation energy across the plastic wavefront in the projectile and plug, during penetration process. This model assumes that the projectile and plate materials are rigid-linearly strain-hardening and rigid-perfectly plastic respectively. In this method, by merging the equation of energy equilibrium in the projectile and plug with the equation of mass conservation of the projectile, an equation is obtained that cross section of the deformed part of the projectile are calculated by solving it with numerical methods ,such as Muller method. The theoretical results have compared to the experimental ones and found to be in good agreement.

Chatter is a particular case of self-excited vibrations, which arise in rolling operations because of the interaction between the structural dynamics of the mill stand and dynamics of the rolling process. The model presented here has two sub models one for structure of the mill and the other for the rolling process. The structure of a mill stand modeled as a system of linear spring and lumped masses in order to simulation the interaction between the rolls.
In this paper, to analyze the chatter vibration a dynamic model of cold rolling process using finite element method (FEM) is considered. The influence of rolling parameters such as rolling speed, reduction and friction coefficient on chatter vibration is investigated and it was showed that the possibility of chatter onset increased by raising the rolling velocity and decrease friction coefficient. Predicted values of the model are in good agreement with that of the experiments as well as the values obtained by other researchers.