جستجوی مقالات مرتبط با کلیدواژه « ارتعاشات آزاد » در نشریات گروه « مواد و متالورژی »

In this paper, the free vibrations of multi-layered composite cylindrical shells with rectangular cutout are analyzed analytically. Cylindrical shell equilibrium equations are derived based on classical shell theory (CST) and using Newton's method. Boundary conditions of the two-headed shell are considered simply supported. According to the boundary conditions, the displacement components are written as double Fourier series expansions. Relationships of the strain-displacement and curvature displacement are considered based on Love’s approximation theory. Modeling of the cutout by distribution function (Heaviside) is considered based on the equilibrium equations of the composite cylindrical shell. In order to obtain the natural frequency of the rectangular cutout composite cylindrical shell, the equations of motion have been solved using the Galerkin method. For validation, firstly, the results of the composite cylindrical shell have been compared with the Abaqus finite element software and the Previous literature in this field, that there is a good match between them. Finally, the effect of the geometrical parameters of the composite cylindrical shell with cutout, such as the length, radius, thickness and layering of the cylindrical shell, as well as the dimensions, direction and location of the cutout, on the natural frequency of the structure was investigated, that the results show that with increasing The dimensions of the cutout increase the reduction of the natural frequency of the structure.

In this study, an analytical model was presented for investigating the free vibrations of sandwich conical shells with lattice cores. A novel smeared method was employed to determine the stiffness contribution of the stiffeners. In this approach, the lattice core was approximated with an equivalent composite conical shell. The stiffeners were considered as a beam which support the shear loads and bending moments in addition to the axial loads. For this purpose, the stiffness contribution due to the stiffeners was firstly determined through the forces and moments analysis of a unit cell and then superimposed with those of the inner and outer skins in order to obtain the stiffness parameters of the whole structure. The governing equations were deduced using the classical shell theory of Donnell type and Galerkin method. In order to validate the analytical results, a 3-D finite element model was also created using ABAQUS software. Comparison of the results, revealed good agreements between the two approaches and demonstrated that the proposed analytical model is qualified enough to investigate the free vibrational behavior of sandwich conical shells with lattice cores. Results given are novel and can be used as a benchmark for future studies.

This paper investigates free vibration of a smart sandwich composite polymeric micro-panel blend of polyvinylidene fluoride reinforced with boron nitride nanotubes under an electric field resting on an elastic substrate using first order shear deformation theory. The distribution of nanotubes in the polymeric matrix is assumed uniformly. The Winkler springs and Pasternak shear layer are used for modeling the elastomeric substrate and the higher-order modified strain gradient theory is implemented to investigate the effects of size. First, using the microstructural modeling technique, the constitutive equations of the nanocomposite are extracted for a representative volume element, and then the stress-strain relations are obtained in terms of mechanical and electrical terms. Also, the equations of motion are derived using the Hamilton principle, and finally using the method of variational calculus and extracting the mass and stiffness matrices, the natural frequency of the micro-panel is obtained. The results of this paper show that by increasing the aspect ratio and reducing the volume fraction of nanotubes, the panel's hardness decreases and the natural frequency decreases. Further, various parameters such as the stiffness of elastic medium, the effect of electric field, different modes, aspect ratio and other factors are investigated. A comparison is also made between the classical, modified coupled stress, and higher-order modified strain gradient theories.

Due to unique properties, lattice composite shells are used extensively in aviation, marine and automotive industry. The aim of this research is experimental and numerical free vibration analysis of composite sandwich cylindrical shells with lozenge cores. For the fabrication of this shells, silicone mold, filament winding, and hand lay-up method were used. Stiffened shells and simple shells are fabricated, separately. Then, composite sandwich cylindrical shells with lozenge cores were created by attaching the two parts together. The modal test is done on the shells and natural frequencies have been extracted. The comparison of experimental results and, numerical results obtained from Abaqus showed that there is a good agreement between them. By using Taguchi method, a parametric study was performed on the vibrational behavior of sandwich shells with lozenge cores via six parameters that such as stiffeners’ pair number, stiffener thickness, unit cell number, skin thickness, layers sequence and boundary condition. The results show that the natural frequency has a most sensitive to the boundary condition, skin thickness and least sensitive to stiffener thickness, layers sequence. To evaluate the efficiency of a sandwich shell, the natural frequency of sandwich shell are compared with simple shell in the different boundary condition. The results show that the natural frequency of sandwich shell with lozenge core is 176% and 34% higher than an equivalent simple shell at free and clamp boundary condition, respectively.

This article for the first time is concerned with effects of parameters as local smart stiffness and smart attached mass dimensions of dynamic behavior of thick laminated plates with attached mass. In deriving governing equations, Gark-Kant’s higher order theory is used and effects of SMA and attached mass`s stiffness taken into account and finally vibration equations of thick plate carrying attached mass contain SMA is reached by using Hamilton`s principal. Then by means of Galerkin method, mass and stiffness matrices are extracted for achieving to a standard eigenvalue vibration problem. In this problem simply supported boundary condition is used for each four edges. Some important parameters such as ratio of local stiffness to stiffness of host structure and smart attached mass dimension on main frequencies of laminated thick plate is studied. A Comparative result with those in published references is presented. These numerical results show that local stiffness due to smart attached mass has a great role on dynamic behavior of thick laminated plates that if SMA didn`t active properly or ignore the effect of attached mass`s stiffness the dynamic response of system will have a lot of changes.

Laminated composite conical shells are used as components of aerospace, marine industries and civil engineering structures. In this research free vibration of grid stiffened composite conical shell with simply support boundary condition is studied. No study has yet been done on vibration analysis of these structures. Smeared method is employed to superimpose the stiffness contribution of the stiffeners with those of shell in order to obtain the equivalent stiffness parameters of the whole structure. The stiffeners are considered as a beam and support shear loads and bending moments in addition to the axial loads. Geodesic path is applied to the stiffeners. Equations were derived using classical shell theory of Donnell type and were solved using energy functional with the Rayleigh-Ritz method. A 3D finite element model was built using ABAQUS software. Results were compared and validated for grid stiffened structure with ABAQUS software. Comparisons and validations revealed good agreements. The effects of shell geometrical parameters and variations in the cross stiffeners angle on the natural frequencies were investigated.Results given are novel and can be used as a benchmark for further studies.

In this paper, free vibration of an embedded double-layered nano-plate reinforced by functionally graded carbon nanotubes (FG-CNT) is analytically investigated. Carbon nanotubes are distributed through the thickness in two ways: uniform distribution and symmetrically linear distribution (or decreasing-increasing layup). To accurately model this nanocomposite behavior, the elastic medium around the nano-plate is modeled by Pasternak elastic foundation and the Van der waals forces between two nano-plates are taken into account. Governing equations of motions are obtained using energy method in association with Eringen nonlocal theory and solved by Navier method for a simply-supported rectangular plate. Finally, the effect of elastic foundation parameters, different distributions of CNT and nonlocal parameters are investigated on the vibration behavior of orthotropic double-layer nano-plate. Results show that natural frequencies of a double-layer nano-plate increase by increasing the Winkler elastic constants while Pasternak elastic constant has less effect on the results. Also, increasing the nonlocal parameter at a constant length decreases the natural frequencies. By increasing the length to thickness ratio (L/h) of nano-plate, the nonlocal frequencies reduce and natural frequency of symmetrically linear distribution is more than those of uniform distribution for constant value of L/h.

In this study, free vibration and static bending analysis of curved sandwich panel with magneto-rheological (MR) fluid layer in sheets have been studied. Sandwich panel that is studied is double curved with simply support boundary condition and it is under bending load. In order to derive the governing equations of motion, an improved high order sandwich panel theory and Hamilton's principle are used for the first time. After comparing with similar results in the domain of this issue and ensure that the accuracy of the derived equations, the effect of magnetic field on the frequency of the panel has been investigated. The effects of magnetic field intensity and changing the geometric parameters such as aspect ratio, thickness of MR layer, radius of curvature and thickness ratio on the characteristics of vibration and deflection have been studied. In the free vibration analysis section, the obtained results showed that the natural frequency of panel increases by increasing the magnetic field, also increases by increasing the panel aspect ratio, and decreases by increasing the core thickness to panel thickness ratio. Likewise the obtained results showed that the natural frequenciy for the case of double curved panel is more than single curved and flat panel. In the static bending analysis section, obtained results showed that the natural deflection of the panel decreases by increasing the magnetic field, increases by increasing the radiuses of curvatures ratio, and increases by increasing the core thickness to panel thickness ratio. Therefore, by changing these parameters, the natural frequency and deflection of the system, can be changed in the desired range, and also by having a controllable magnetic field in the system, the natural frequency and deflection of the system can be controlled.