• Advances in aircraft and spacecraft science
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• v.7 no.4
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• pp.353-369
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• 2020

This paper presents the flutter analysis and optimum design of axially functionally graded box beam cantilever wing section by considering various geometric and material parameters. The coupled dynamic equations of the continuous model of wing system in terms of material and cross-sectional properties are formulated based on extended Hamilton's principle. By expressing the lift and pitching moment in terms of plunge and pitch displacements, the resultant two continuous equations are simplified using Galerkin's reduced order model. The flutter velocity is predicted from the solution of resultant damped eigenvalue problem. Parametric studies are conducted to know the effects of geometric factors such as taper ratio, thickness, sweep angle as well as material volume fractions and functional grading index on the flutter velocity. A generalized surrogate model is constructed by training the radial basis function network with the parametric data. The optimized material and geometric parameters of the section are predicted by solving the constrained optimal problem using firefly metaheuristics algorithm that employs the developed surrogate model for the function evaluations. The trapezoidal hollow box beam section design with axial functional grading concept is illustrated with combination of aluminium alloy and aluminium with silicon carbide particulates. A good improvement in flutter velocity is noticed by the optimization.

• Structural Engineering and Mechanics
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• v.52 no.5
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• pp.1051-1067
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• 2014

The structures of concern in this study are subject to two types of forces: dead loads from the acceleration imposed on the structures as well as the installed operation machines and the additional adjustable forces. We wish to determine the critical values of the adjustable forces when buckling of the structures occurs. The mathematical statement of such a problem gives rise to a constrained eigenvalue problem (CEVP) in which the dominant eigenvalue is subject to an equality constraint. A numerical algorithm for solving the CEVP is proposed in which an iterative method is employed to identify an interval embracing the target eigenvalue. The algorithm is applied to four engineering application examples finding the critical loads of a fixed-free beam subject to its own body force, two plane structures and one wide-flange beam using shell elements when acceleration force is present. The accuracy is demonstrated using the first example whose classical solution exists. The significance of the equality constraint in the EVP is shown by comparing the solutions without the constraint on the eigenvalue. Effectiveness and accuracy of the numerical algorithm are presented.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.731-734
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• 2005

Generally, characteristics of electromechanical actuators are coupled with the mechanical and the electrical properties. Important mechanical parameters of these actuators are the achievable force and displacement in the presence of electric field. These mechanical parameters are related to the stress and strain of the materials and the actuator geometry. This paper presents how to measure the blocked force by using the micro-balance. The blocked force is defined as the force produced by the transducer under an applied voltage when the tip is constrained to zero motion. Also, a theoretical force by using the cantilever beam model is calculated under elastic domain. From the sample of 4 cm $\times$ 1 cm $\times$ 20 $\mu$m, the blocked farce measured from the equipment is 20.3 $\mu$N at 8 V$_{DC}$. By comparing it with the theoretical value, 24.9 $\mu$N, the blocked force measurement is acceptable. The furce measurement is also investigated with different AC electric fields and the frequency.

• Structural Engineering and Mechanics
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• v.29 no.3
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• pp.289-300
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• 2008

The use of metric trial functions to represent the real stress field in what is called the unsymmetric finite element formulation is an effective way to improve predictions from distorted finite elements. This approach works surprisingly well because the use of parametric functions for the test functions satisfies the continuity conditions while the use of metric (Cartesian) shape functions for the trial functions attempts to ensure that the stress representation during finite element computation can retrieve in a best-fit manner, the actual variation of stress in the metric space. However, the issue of how to handle situations where there is locking along with mesh distortion has never been addressed. In this paper, we show that the use of a consistent definition of the constrained strain field in the metric space can ensure a lock-free solution even when there is mesh distortion. The three-noded Timoshenko beam element is used to illustrate the principles. Some significant conclusions are drawn regarding the optimal strategy for finite element modelling where distortion effects and field-consistency requirements have to be reconciled simultaneously.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2005.04a
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• pp.29-32
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• 2005

Based on the full layerwise displacement shell theory, vibration and damping characteristics of cylindrical sandwich panels are investigated. The transverse shear deformation and the normal strain are fully taken into account for structural damping modelling. Modal damping factors and frequency response functions are analyzed for various structural parameters of cylindrical sandwich beams. Present results shows that full layerwise theory can accurately predict vibration and damping characteristics of cylindrical composite panels with surface damping treatments and constrained layer damping. The viscoelastic materials depending on elevated temperature environment and exciting frequencies can be fully considered.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.133-137
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• 2001

Active control of flexural vibrations of smart laminated composite beams has been carried out using piezoceramic sensor/actuator and viscoelastic material. The beams with passive constrained-layer damping have been analyzed by formulating the equations of motion through the use of extended Hamilton's principle. The dynamic characteristics such as damping ratio and modal damping of the beam are calculated for various fiber orientations by means of iterative complex eigensolution method, This paper addresses a design strategy of laminated composite under flexural vibrations to design structure with maximum possible damping capacity.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2003.04a
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• pp.95-98
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• 2003

In this work, a closed-form analysis is performed to obtain the stiffness coefficients of thin-walled composites beams with elliptical profiles. The analytical model includes the effects of elastic couplings, shell wall thickness, torsion warping and constrained warping. Reissner's semi-complementary energy functional is used to derive the beam force-displacement relations. The theory is validated against MSC/NASTRAN results for coupled composites beams with single-cell elliptical sections. Very good correlation has been noticed for the test cases considered.

• Laser Solutions
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• v.9 no.1
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• pp.1-7
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• 2006

In order to increase the productivity of conventional microstereolithography (MSL), digital micromirror device($DMD^{TM}$) based MSL is proposed and the feasibility of 3D rnicrocomponents fabrication is demonstrated in two ways; free surface and constrained surface techniques. The clearness of optical images at the exposure plane was confirmed for the fabrication of an accurate 3D structure by controlling the dynamic viscosity of FA1260T and the shape accuracy of a structure fabricated with epoxy-based resin ($Somos^{\circledR}$ 10120) was analyzed to determine the optimum curing conditions. After finding the appropriate process variables, the feasibility of multiple microstructures is then demonstrated. Due to the high productivity, MSL using $DMD^{TM}$ showed the potential to replace the existing focused laser beam MSL.

• International Journal of Aeronautical and Space Sciences
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• v.14 no.4
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• pp.341-349
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• 2013

This paper considers the optimum design of flexbeam cross-sections for a full-scale bearingless helicopter rotor, using an efficient hybrid optimization algorithm based on particle swarm optimization, and an improved genetic algorithm, with an effective constraint handling scheme for constrained nonlinear optimization. The basic operators of the genetic algorithm, of crossover and mutation, are revisited, and a new rank-based multi-parent crossover operator is utilized. The rank-based crossover operator simultaneously enhances both the local, and the global exploration. The benchmark results demonstrate remarkable improvements, in terms of efficiency and robustness, as compared to other state-of-the-art algorithms. The developed algorithm is adopted for two baseline flexbeam section designs, and optimum cross-section configurations are obtained with less function evaluations, and less computation time.

• International Journal of Naval Architecture and Ocean Engineering
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• v.6 no.4
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• pp.763-774
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• 2014

A simple and efficient vibration analysis procedure for stiffened panels with openings and arbitrary boundary conditions based on the assumed mode method is presented. Natural frequencies and modes are determined by solving an eigenvalue problem of a multi-degree-of-freedom system matrix equation derived by using Lagrange's equations of motion, where Mindlin theory is applied for plate and Timoshenko beam theory for stiffeners. The effect of stiffeners on vibration response is taken into account by adding their strain and kinetic energies to the corresponding plate energies whereas the strain and kinetic energies of openings are subtracted from the plate energies. Different stiffened panels with various opening shapes and dispositions for several combinations of boundary conditions are analyzed and the results show good agreement with those obtained by the finite element analysis. Hence, the proposed procedure is especially appropriate for use in the preliminary design stage of stiffened panels with openings.