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The aim of this work is to investigate the flutter characteristics of nanocomposite cantilever trapezoidal plates with non-uniform thickness enriched with either carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), or graphene oxide powders (GOPs) which are distributed functionally graded (FG) in the axial direction. It is assumed that the thickness of the plate and the volume fraction of the nanofillers vary in one direction from the wider clamped edge of the plate to the outer narrower free one. The modeling of the plate is done using the first-order shear deformation theory (FSDT) and the aerodynamic pressure generated by the aerodynamic pressure is modeled using the linear approximation of the piston theory. The material properties of the plate are calculated using the mixing rule (ROM) and the Halpin–Tsai model. The governing equations and boundary conditions at the clamped and free edges of the plate are derived via Hamilton's principle. An approximate solution is applied using the differential quadrature method (DQM) to calculate the natural frequencies and the damping ratios of the plate. Numerical examples show that it is possible to find an optimal thickness variation profile that provides the greatest aeroelastic stability. It is concluded that by considering the same value for the mass fractions of the nanofillers, the highest aeroelastic stability can be attained by utilizing the GNPs as the reinforcers. It is found that to attain further improvement in aeroelastic stability, most nanofillers should be distributed near the clamped edge and away from the outer free edge. [ABSTRACT FROM AUTHOR]

In this paper, an exact solution is presented for whirling analysis of rotors carrying concentrated masses. Effect of various parameters on the forward and backward frequencies are investigated including velocity of spin and quantity, translational inertia and position of the concentrated masses. It is shown that vibration characteristics of rotors can be affected by concentrated elements depended on the position of point masses and value of their translational inertia. The most advantage of the presented solution is that for rotors with any number of concentrated masses, all natural frequencies can be obtained through solution of a determinant of order 2. [ABSTRACT FROM AUTHOR]

A novel angle graded auxetic honeycomb (AGAH) core is designed for sandwich structures in the present study. The angle of the cells is varied through the thickness of the AGAH core using linear functions. Therefore, the thickness of the cell walls is kept constant along the gradation of the cell angle, and the length of the cell walls is changed through the core thickness as the result of angle variation. New analytical relations are proposed to predict the equivalent elastic properties of the AGAH core. The performance of the new proposed core is analytically assessed for the vibrational behavior of a sandwich plate. The governing equations are deduced adopting Hamilton's principle under the assumption of quasi-3D exponential plate theory. Three-dimensional finite element (3D-FE) simulation is accomplished to verify the analytical results of the vibrational response of the sandwich structure. The influence of variation of the cell wall, the cell angle and cell aspect ratio of AGAH core, and geometric parameters of the sandwich structure are investigated on the vibration response of the sandwich panel. The present graded design of the auxetic honeycomb enhances the specific stiffness (i.e., stiffness to density ratio) and consequently increases the natural frequencies of sandwich structures with this type of core. [ABSTRACT FROM AUTHOR]

In this article, free flexural vibration and supersonic flutter analyses are studied for cantilevered trapezoidal plates composed of two homogeneous isotropic face sheets and an orthotropic honeycomb core. The plate is modeled based on the first-order shear deformation theory, and aerodynamic pressure of external flow with desired flow angle is estimated via the piston theory. For this goal, first applying the Hamilton's principle, the set of governing equations and boundary conditions are derived. Then, using a transformation of coordinates, the governing equations and boundary conditions are converted from the original coordinates into new computational ones. Finally, the differential quadrature method is employed and natural frequencies, corresponding mode shapes, and critical speed are numerically achieved. Accuracy of the proposed solution is confirmed by the finite element simulations and published experimental results. After the validation, effect of various parameters on the vibration and flutter characteristics of the plate are investigated. It is concluded that geometry of hexagonal cells in the honeycomb core has a weak effect on the natural frequencies and critical speed of the sandwich plate, whereas thickness of the honeycomb core has main influence on the natural frequencies and the critical speed. Besides, it is shown that the honeycomb core thickness has optimum values that lead to the most growth in the natural frequencies or critical speed. These optimum magnitudes can be taken into account by designers to increase the natural frequencies or expand flutter boundaries and make aircrafts safer in supersonic flights. It is also concluded that geometrical parameters of the hexagonal cells and thickness of the honeycomb core have no significant effect on the value of the critical flow angle. [ABSTRACT FROM AUTHOR]

In this study, nitrile-butadiene rubber (NBR) based nanocomposites reinforced with PVC as a polymeric filler and NiFe2O4 as nanofllers with different weight fractions (wt%) were prepared and characterized. The experiments were developed according to response surface methodology (RSM) combined with central composite design (CCD) to optimize the effects of two variable parameters (PVC and NiFe2O4) on the mechanical properties. The NiFe2O4 nanoparticles (NPs) were synthesized by sol-gel auto-combustion method and, the NBR/PVC/NiFe2O4 nanocomposites were prepared by two-roll mill method. The elongation and tensile strength of the optimized nanocomposite were obtained to be 300 % and 18 MPa, respectively. The surface SEM images showed that the NiFe2O4 NPs were well distributed in the NBR matrix. Furthermore, cross-sectional SEM images showed that the cracks on the nanocomposite matrix decreased compared to pure NBR. Also, NBR/PVC/NiFe2O4 nanocomposite shows weak ferromagnetic behavior at room temperature than NiFe2O4NPs. The NiFe2O4 NPs and optimal sample of NBR/PVC/NiFe2O4 nanocomposites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), vibrating sample magnetometer (VSM). [ABSTRACT FROM AUTHOR]

As a useful tool for designing wings and tail fins of aircrafts, this paper presents an optimization for flutter characteristics of cantilevered functionally graded sandwich plates. The plate is composed of an isotropic homogeneous core and two functionally graded face sheets. The plate is modeled based on the first-order shear deformation theory. The aerodynamic pressure is estimated using supersonic piston theory and using Hamilton's principle, the set of governing equations and boundary conditions are then derived. Applying a transformation of coordinates, governing equations and boundary conditions are converted and solved numerically by differential quadrature method. Natural frequencies, damping ratio, corresponding mode shapes, critical aerodynamic pressure, and flutter frequency are calculated. In order to achieve an optimum design, particle swarm optimization is employed to find the best values of aspect ratio, thickness of the plate, thickness of the core, power law index, and angles of the plate which increase critical aerodynamic pressure. Some constrains on the angles of the plate and its mass and area (lift force) are also considered. [ABSTRACT FROM AUTHOR]

The free vibration of fiber–metal laminate (FML) thin circular cylindrical shells with different boundary conditions has been studied in this research. Strain–displacement relations have been obtained according to Love’s first approximation shell theory. To satisfy the governing equations of motion, a beam modal function model has been used. The effects of different FML parameters such as material properties lay-up, volume fraction of metal, fiber orientation, and axial and circumferential wavenumbers on the vibration of the shell have been studied. The frequencies of shells have been calculated for carbon/epoxy and glass/epoxy as composites and for aluminum as metal. The results demonstrate that the influences of FML lay-up and volume fraction of composite on the frequencies of the shell are remarkable. [ABSTRACT FROM AUTHOR]

In this paper, differential quadrature element method (DQEM) is used to analyze the free transverse vibration of multi-stepped rotors resting on multiple bearings. In order to increase the accuracy of the model, Timoshenko beam theory is used; also in order to generalization of the modeling of the bearings, each bearing is replaced with four springs; two translational and two rotational acting on two perpendicular directions. Governing equations, compatibility conditions at the each step and each bearing and external boundary conditions are derived and formulated by the differential quadrature rules. First, convergence and versatility of the proposed method are tested by the presented exact solutions. Then, the Campbell diagram is derived for a desired case study and variation of natural frequencies is investigated versus angular velocity of spin. The most advantage of the proposed method is being less time-consuming in comparison with the other methods, especially for cases with high number of steps and bearings which are unsolvable by exact methods. [ABSTRACT FROM AUTHOR]

In this article, analytical dynamic model derivation and robust position control of a microrobot based on fast trilayer polypyrrole-bending actuators employing quantitative feedback theory is presented. The conjugated polymer actuators based on polypyrrole can be employed to achieve microscale precision positioning, having a wide range of application including biomimetic robots and biomedical devices. There has been extensive research on modeling the electrochemical dynamics of polypyrrole bending actuators. However, the mechanical dynamics modeling of actuator remains to be unexplored. In this article, we first develop a proper mechanical dynamics model for the fast-conducting polymer actuators that matches well with the existing experimental results and then extend it to a microrobot. In the controlling part, the robust control quantitative feedback theory will be used to control the microrobot with variable tip loading. The numerical simulation results show that the quantitative feedback theory controller has consistent and robust-tracking performance. [ABSTRACT FROM PUBLISHER]

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