جستجوی مقالات مرتبط با کلیدواژه "پس کمانش" در نشریات گروه "مکانیک"

In this article, the thermal buckling and post-buckling of functionally graded plates with a matrix of isotropic polymer reinforced with graphene platelets (GPLs) have been investigated. In these multilayer plates, the thickness of all layers is the same, and by changing the weight fraction of reinforcing graphene platelets in each layer, the type of layer arrangement of nanocomposite plates changes. The plates are reinforced with three functional graded distributions: X and O and a uniform U distribution. The governing equations of the plate are obtained with the help of the first-order shear deformation theory (FSDT). The thermal buckling behavior of plates has been studied by the differential quadrature method. To find the effective Young's modulus, the Halpin-Tsai modified micromechanical model is used, and the law of mixing is used to obtain the equivalent properties of composites. The results show that the distribution of graphene platelets with an X arrangement improves the resistance of the plate against buckling. Also, the effect of parameters such as the weight fraction of graphene platelets, aspect ratio, and length-to-thickness ratio has been investigated.

In the present work, the effect of smart shape-memory alloys on thermal buckling and post-buckling of monoclinic and unidirectional composite panels has been investigated. The aerodynamic pressure applied to the system was modeled using the piston theory method. The effect of thermal heating for ultrasonic flows was also estimated from the reference temperature method. The panel is modeled nonlinearly with large deformations based on Van Karmen's theory. The obtained results show that the shape-memory alloy was able to increase the critical temperature of thermal buckling. The effect of the arrangement of composite layers on increasing the thermal buckling temperature was also studied. The results show that the amount of thermal deflection is greatly reduced due to the use of this alloy. Also, in the higher temperature differences, the rate of reduction of the panel increases. In this work, the effects of thermal stress on buckling and thermal buckling in a rectangular composite panel with hinge-hinge boundary conditions were investigated. Also, the effect of shape retention alloy in controlling these two phenomena has been studied. The shape memory alloy wire was placed in martensitic mode to apply a compressive force to the panel to control the heating and aerodynamic forces after changing the phase to austenite. The governing equations of the system were extracted through the layer theory method to show the effects of in-plane displacements better. Investigations on the effect of different layers (symmetry effects and arrangement angle of composite sheets) were performed to investigate thermal buckling. According to the obtained results, the layer arrangement of plates (0.90 / 0.90), (-45.45), (30/60), and (0.90 / 90.0), respectively, had the greatest effect on raising the critical temperature of thermal buckling in dimensional ratios equal to or greater than one. This shows that the symmetry of the arrangements has a greater effect than the angle of the arrangements. Examining the buckling diagrams for the effect of a shape memory alloy, it can be concluded that by placing this alloy in the composite panel, in addition to raising the buckling temperature, this alloy has a greater effect on displacement control by increasing the temperature after the critical buckling temperature.

Due to its buckling resistance, the spherical shell is an ideal geometry for use in pressure vessels under uniform external pressure. The collapse pressure of these types of shells is much lower than its theoretical value due to the high sensitivity to imperfections and the yield stress of the material. Unlike cylindrical shells, the collapse pressure of spherical shells is not only dependent on the size of the imperfection, but is also dependent on the shape of the imperfection. Since the imperfection depends on the sheet metal forming method, it is necessary to investigate the effect of the fabrication method on the collapse pressure. This article is focused on experimental and numerical study of buckling of hemispherical shell made by spinning forming method. The most important problem of this method is the lack of control over the thickness. In this method, the imperfections are axially symmetrical, which is one of the advantages of this method. In this article, buckling analysis due to both diameter and thickness variations are done separately and simultaneously. It has been shown that thickness variations in rotational forming should be considered in the analysis of shell collapse. Also, by comparing the numerical and experimental results shown with the help of quadratic volume elements, thickness changes and boundary conditions can be applied with higher reliability compared to the shell element.

In this paper, the strength analysis of composite sandwich structures under buckling load as well as composite polyurethane foam core in composite sandwich structures has been studied experimentally. For better performance of the shell and core in the sandwich structure as well as to increase their strength against buckling compressive load, a reinforcer with new geometry of hourglass and square with foam and without foam has been used. Having a negative Poisson's ratio and greater flexural stiffness as well as maximum load bearing capacity is the reason for using hourglass geometry. The new VARTM method has been used to create better and more uniform quality samples. The results of this study show that corrugated sandwich structures with hourglass filled with polyurethane foam withstand more load against buckling load and also have good efficiency in energy absorption. In a foamless composite sandwich structure, the core is distorted (separation of the shell from the core), and local buckling occurs due to the empty core. While in the sandwich structure with foam, due to the presence of foam in the core, the distortion of the structure is prevented. Also, despite its light weight, foam is able to withstand the compressive load due to local buckling. As a result, the structure has only suffered general buckling.

In the present study, the buckling behavior of moderately thick spherical sandwich panels with grid stiffened core and shape-memory wires (SMA) reinforced layer is studied for the first time. The core of the panel is a grid structure and its cells are tetrahedral, and the outer layers are reinforced by SMA wires with a uniform, one-way distribution. The finite element method is used to perform the simulations. The Brinson model is used for SMA super-elastic behavior definition and fuzzy transformations are applied using the UMAT subroutine in ABAQUS software. The effect of effective geometric and mechanical parameters such as the radius of curvature of the shell, the volume fraction of SMA wires, and their prestressing on the buckling loads of the shell are verified. The results show that SMA wires cause recycled stresses that are applied as a tensile force on the upper layers of the shell. This characteristic increases the stiffness of the shell and leads the buckling load growth. If α =0.1, Increasing the volume fraction of SMA wires from 0 to 0.6% leads to the buckling load growth by 325%. In addition, the buckling load per unit volume of the shell with grid core and without grid core is 0.71 and 0.79, respectively, which indicates that the presence of grid core increases the specific buckling load by 11%. This increase, along with the reduction in the weight of the structure, highlights the importance of using sandwich structures with grid cores.

In this research, the buckling of moderately thick double-curved composite sandwich panel with grid-stiffened core and carbon nanotubes reinforced surface layers under axial static loading has been studied analytically for the first time. The panel core is consisted of irregular hexagonal cells. The mechanical properties of the carbon nanotubes reinforced layers with different distributions are determined by the improved rule of mixtures. The buckling equations are obtained by using of the minimum total potential energy principle and Reddy's third-order theory and solved via the Navier method for different boundary conditions .The results show that the effect of carbon nanotubes on buckling load is in the way that the buckling performance in each case depends on the geometric parameters of the grid core. By assigning the appropriate value for grid core characteristics, the maximum buckling load can be obtained by the minimum amount of carbon nanotubes. It is observed that in the FG-V distribution mode, for the smaller amount of carbon nanotubes (0.05 volume fraction), the maximum buckling load is obtained. Also, for Rx/Ry=2 and FG-V distribution mode, the maximum buckling load gains in simply support boundary condition. For Rx/Ry=2 and UD distribution mode, the maximum buckling load obtained from Simple-Clamped boundary condition. It can be noticed that, in this shell type, the maximum buckling load doesn’t belong to fully clamped boundary conditions

Shape Memory Alloys (SMAs) are a group of intelligent materials which have distinct and superior properties compared to other alloys. The stress-strain behavior of these materials involves two specific nonlinear phenomena, namely the Shape Memory effect and the quasi-elastic effect. The behavior of the alloy whilst subject to these two phenomena is such that after it enters the plastic state, the material will regain its original form. The research is based on the modeling of these two behaviors of memory alloys in the ABAQUS software using UMAT code. In this study, the simulation of buckling and post-buckling of sandwich panels have been investigated. Preparing a UMAT memory alloy properties code, enables the designer to use any ABAQUS features to analyze the behavior of smart structures made with the shape memory alloy. Also, in this research, the first buckling critical force of the first five modes for different percentages of the alloy in the shell and core have been studied, and then the effects of volumetric percentage change of alloys, distance from neutral axis, fiber angle variations, and finally stability and post-buckling behavior of the memory alloy as the result of initial strain have been investigated. The investigations have shown large changes in the shape memory alloy due to the great recovery stress during the phase shift.

In this paper the effects of position and shape of cutout on the buckling load of composite cylindrical panel under compressive load is investigated. The laminated cylindrical panel with an arbitrary cutout shape is simulated by the spline finite strip method. The first order shear deformation theory and Sanders shell nonlinear theory are considered in this study. Using a linear buckling analysis, an eigenvalue problem is solved to obtain the buckling load of the panel under the compressive axial load. A comparison between the results obtained from the finite strip, finite element and analytical methods were made to show the validity of the results obtained in this study. Several case studies are presented to investigate the effects of some parameters such as shape and position of cutout, central angle of panel, ply sequence of the composite layers and boundary condition of the panel on its buckling load. The results show that the position and shape of cutout have considerable effects on the buckling load of the panel. The buckling load of the panel reaches its minimum when the cutout center has eccentricity from the loading direction. The results also show that the buckling load with quasi-isotropic configuration is greater than those of cross-ply and angle-ply.

The optimal design of multilayer substrates containing the cutout under compression is very important to achieve maximum buckling resistance in comparison with structural weight, especially in aerospace structures. In this study, buckling and post-buckling behavior of composite laminated plates with orthogonal and symmetrical layup containing the cutout with different diameters has been investigated experimentally, semi-analytically, and numerically. To study the buckling of the composite plate with cutout semi-analytically, a finite strip method is developed. A finite element method was used for numerical analysis. The required material parameters for modeling were obtained from standard tests. The results of the current study show that the size of diameter of cutout does not have considerable effect on elastic rigidity of plate, but the buckling load significantly decreases by increasing cutout diameter. Also, buckling load and elastic rigidity of plate are considerably increased by increasing the number of composite layers. The thickness of plate has more effect on buckling load than the diameter of hole. Studies show that there is a good match between the results of buckling behavior derived from semi-analytical and finite element methods with experimental results.

In this study, the buckling behavior of laminated composite plates with and without cutouts subjected to in-plane loading has been investigated. The composite plate with different cutout shapes is subjected to various types of in-plane compressive loads. A numerical study using finite element method (FEM) has been carried out to study the effect of various cutout shapes including circular, rectangular and combination of these two types of cutout shapes. Furthermore, the effects of plate aspect ratio, plate length/thickness (a/t), ply orientation and boundary conditions on the buckling behavior of laminated rectangular composite plates are all considered in the analysis. In order to have a basic reference in our study two analytical solutions based on CLT and FSDTmethods are also have been used to predict the buckling behavior of laminated composite plates without cutouts. An experimental method has been used to investigate the behavior of buckling loads of the composite rectangular composite plates with and without cutouts. Finally, the outcome results of the FE and experimental methods are compared.

Effects of initial geometric imperfection and pre- & post-buckling deformations on vibration of isotropic rectangular plates under uniaxial compressive in-plane load have been studied. Vibration equations of a plate, using Mindlin theory and Von-Karman stressstrain relations for large deformations, have been extracted. The response of nonlinear differential equations was assumed as the summation of dynamic and static responses. Due to the large static deflection of a plate in comparison with its vibration amplitude, the differential equations have been solved in two steps. First, the static equations have been solved using the differential quadrature method and the Arc-length strategy. Next, considering small vibration amplitude about the deformed plate (the first step result) and eliminating nonlinear terms, natural frequencies have been calculated using the differential quadrature method and solving the obtained eigenvalue problem. The results for different initial geometric imperfection and different boundary conditions reflect the impact of the mentioned factors on the natural frequencies of plates.

In this study, the buckling and postbuckling behavior of composite laminates with piezoelectric layers subjected to compressive in-plane loading have been investigated. The effects of coupled electro-mechanical field on the postbuckling and bifurcation point in cross-ply and general lay-up sequences have been studied using layerwise theory (LWT). The LWT used in this study for analyzing the piezo-composite laminate is based on the assumptions of the first order shear deformation theory (FSDT). In order to obtain the equilibrium equations, the principle of minimum potential energy has been employed. The obtained nonlinear equilibrium equations have been solved using Newton-Raphson iterative algorithm. Furthermore, the three dimensional finite element analysis has been performed to examine the accuracy of the results obtained using the proposed method. The obtained analytical results are in good agreement with those achieved through the finite element analysis. Obtained results showed that, location of the piezoelectric layers have significant effect on the buckling and postbuckling behavior of the composite plates. Moreover, number of degrees of freedom which is used in proposed method are less than finite element method which, decreased the computational time cost.

Exact and numerical solution of eccentrically stiffened panels in the industry is a major step forward in the design of these structures. This paper presents an analytical approach to investigate the nonlinear stability analysis of eccentrically stiffened thin FG cylindrical panels on elastic foundations subjected to hygro-thermo-mechanical loads. The stiffeners are assumed to be spiral-type. The panel has the initial geometrical imperfection. The material properties are assumed to be temperature-dependent and graded in the thickness direction according to a simple power law distribution. The elastic foundation is considered based on Winkler and Pasternak proposed model. Governing equations are derived basing on the Lekhnitsky smeared stiffeners technique and classical shell theory incorporating Von Karman-Donnell geometrical type nonlinearity. Explicit relations of load–deflection curves for FG cylindrical panels are determined by applying stress function and Galerkin method. The effects of angel of stiffener, different dimensional parameters, volume fraction index, initial geometrical imperfection, the stiffness of elastic foundation and moisture concentration on the postbuckling of FG panel are investigated. Also effects of temperature gradient through the thickness and effects of different boundary conditions are investigated for thermo-mechanical loading. The obtained results are validated by comparing with those in the literature.

In this paper, the nonlocal buckling behavior of a biaxially loaded graphene sheet with piezoelectric layers based on an isotropic smart nanoplate model is studied. The equilibrium equations are derived with the von Karman-type geometrical nonlinearity by considering the small scale effect. The buckling of multilayer smart nanoplate made of graphene and piezoelectric materials in open circuit conditions is investigated. Based on the nonlocal elasticity and shear and normal deformation theories, the governing equilibrium equations are obtained using the principle of minimum total potential energy and Maxwell’s equation.
Using an analytical approach, the governing stability equations of smart nanoplate have been presented in terms of displacement components and electrical potential. In order to obtain the stability equations, the adjacent equilibrium criterion is used. The stability equations are then solved analytically, assuming simply supported boundary condition along all edges. To validate the results, the critical buckling load values have been compared with available resources. Finally, following validation of the results, numerical results for intelligent nanoplate are presented.
Also, the effects of different parameters such as nanoplate length, different nonlocal parameter, piezoelectric layers thickness, the graphene thickness to length ratio, the piezoelectric layer thickness to graphene thickness ratio and type of Piezoelectric material on the critical buckling loads of intelligent nanoplate are studied in detail. Furthermore, the effect of the mentioned parameters on the critical buckling loads have been presented in some figures.

In this article the effect of circular cutout size، fiber orientation، and reinforcer edge size on buckling behavior of composite plates is investigated by experimental and numerical. Samples have clamped boundary conditions on L-shaped edges and free on other two sides. In the experimental method، the E-glass/epoxy L-shaped samples with and without circular cutout is applied in three different lay-up under uni-axial compression. Then the buckling and failure loads will be recognized. In the second method، according to numerical method (Ansys 10)، the effect of reinforcer edge size changing and size of circular cutout on composite plate buckling in four different lay-ups with the same experimental materials properties is investigated. The results show that there is the optimum height for reinforcer edge during different conditions that increasing this size just causes the extra weight and cost and does not have a good effect on buckling load.

Thin sheets stiffened with lattice structures are used widely in many engineering industries. Investigation of stability behavior for the grid structures and determination of the buckling load under compressive loads is an issue that has attracted the attention of many researchers; and extensive studies have been done in this field. In this paper, a new grid called Diacube is introduced and its buckling load is examined. For this aim, first, the buckling behavior of 5 common types of stiffened flat lattice panels containing hexagonal, triangular, square, diamond and kagome grid are investigated under compressive axial load; and the results are compared with Diacube grid. The effect of network density used in each structure on the buckling of these structures will be studied under different boundary conditions. In addition to common grids,. Regarding to the mass difference of samples, specific critical load parameter (the buckling load to mass ratio) is used for comparison between the structures. Using the finite element modeling and numerical analysis, the grid that has the highest buckling load in each boundary condition is determined It is found that if unloaded edges in lattice panels are simply supported, this new Diacube grid will have the highest buckling load among all structures. Finally, validity of the numerical result obtained for two samples of the structures including hexagonal and Diacube grid is evaluated experimentally; and the numerical results are confirmed.

Due to their light weights and high load carrying capacities, composite structures are widely used in various industrial applications especially in aerospace industry. Stiffening ribs are the main features of lattice type composite structures. Strength to weight ratio is known to be as one of the most critical design parameter in these structures. In this study, the effect of some parameters such as the physical characteristics of the material, shell thickness, angle, thickness and number of ribs on distribution of stresses and buckling loads of shell are investigated. For this purpose, 3D finite element analysis using ANSYS software explicitly was done and then compared with experimental test results. Increasing the thickness of the outer shell causes the structural strength will rise to 50 percent. The next effective parameter is reduction of rib angle which provides an increase of 30 percent in specific load. Although Stiffenerrs (ribs) have a major role in load carrying, but increasing the rib number causes the structural weight rises, thus compared with the two previous parameters do not have a significant effect on the strength of structures.

Careful and numerical analysis eccentrically stiffened shells in the industry is a major step forward in the design of these shells. In this paper, a careful analysis of post-buckling behavior of eccentrically stiffened FGM thin circular cylindrical shells is surrounded by an elastic foundation and external pressure is presented. The two parameter elastic foundation based on Winkler and Pasternak elastic model is assumed. Stringer and ring stiffeners are internal. Shell properties and eccentrically stiffened are FGM. Fundamental relations and equilibrium equations are derived based on the smeared stiffeners technique and the classical theory of shells and according to von- Karman nonlinear equations. The three-term approximation for the deflection shape, including the pre-buckling, linear buckling shape and nonlinear buckling shape was chosen that using the Galerkin method, the critical load and post-buckling pressure-deflection curves is calculated. The effects of different dimensional parameters, buckling modes, volume fraction index and number of stiffeners are investigated. Numerical results show that stiffeners and elastic foundation enhance the stability of the shells. Increasing the shell thickness, reducing the volume fraction index, raising the number of Stringer and ring stiffeners and applying foundation elastic, causes the critical buckling load is increased, too.

The instability behavior of stiffened cylindrical shells and determination of the corresponding buckling loads under axial compression, according to the extended range of structural applications of them in various fields of engineering, has been paid a lot of attention from researchers and extensive amount of studies have been performed on it so far. Because of a lack of the general closed form responses due to complexity of the governing equations and analyses process, using the FE software codes as the main technique of the stiffened shell's buckling load determination is inevitable. Accordingly the present paper has been studied the reinforcement effects of ring and stringer and also compared the buckling loads which are evaluated by analysis of the FE numerical modeling in ABAQUS software with instability results that obtained from a general analytical equation derived by other references via applying the simplifying assumptions to the governing equations. Furthermore an attempt has been performed for extraction of the finite element instability load vs. structure reinforcement correspondence that enables the designers to accurately determine the instability load of structure for other values of structure's stiffening volume without performing additional FE analyses which are much more expensive in term of computer time.