• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.12
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• pp.3109-3117
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• 1994

To operate a flexible mechanism in high speed its weight must be reduced as far as the structural strength does not decrease too much, but a light-weighted mechanism causes undesirable elastodynamic responses deteriorating the system performance. Besides, clearance within the connections of mechanisms causes rapid wear, increased noise and vibration. Even if the problems described above must be considered in the initial design stage, there has been no effective design process which takes account of the correlation between dynamic characteristics of flexible mechanism and the clearance effect at the joint. In this study, the generalized elastodynamic governing equations which include dynamic characteristics and boundary conditions of flexible mechanism are derived by variational calculus and solved by using FFM theory. To take the clearance effect at joint into account a new dynamic model is presented and also the method of modified stiffness/damping matrix is proposed to activate the dynamic clearance model, which cooperates with the developed governing equation very easily. As the results of this study, the proposed method(modified stiffness/damping matrix) to calculate clearance effect was proved to be superior to the existing one(force reaction method) in solution convergency and calculation performance. Besides this method can be easily adopted to the complex shape joint without calculation of reaction force direction.

• Proceedings of the KSME Conference
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• 2001.06b
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• pp.355-360
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• 2001

By using Frenet formulation and Timoshenko beam theory, the partial differential equations of motion are derived for a helical spring having a doubly symmetrical cross section subjected to the pre-load axially. These equations of motion are solved to give the dispersion relationship and dynamic stiffness matrix is assembled. Natural frequencies are obtained from the receptance of the system. The results of the dynamic stiffness method are compared with those of the transfer matrix method from published examples and finite element method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.6
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• pp.971-980
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• 1997

This study is to investigate the stiffness degradation of a composite laminated panel including transverse matrix cracks subjected to bending and twisting moments. Micromechanics theory on the composite material is derived by introducing crack density. Iterative numerical scheme is developed to calculate the degraded composite stiffness which has nonlinear relation due to the crack density. The finite element method is used for structural analysis of the composite panel. Structural responses of the composite panel are examined for various laminated angles and crack density under the bending and twisting moments. Also, the effect of crack opening and closing is considered in the examination. It is realized that the matrix cracks may cause severe stiffness reduction and should be considered in the composite laminated panel.

• Advances in aircraft and spacecraft science
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• v.1 no.3
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• pp.291-310
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• 2014

An advanced model for the linear flutter analysis is introduced in this paper. Higher-order beam structural models are developed by using the Carrera Unified Formulation, which allows for the straightforward implementation of arbitrarily rich displacement fields without the need of a-priori kinematic assumptions. The strong form of the principle of virtual displacements is used to obtain the equations of motion and the natural boundary conditions for beams in free vibration. An exact dynamic stiffness matrix is then developed by relating the amplitudes of harmonically varying loads to those of the responses. The resulting dynamic stiffness matrix is used with particular reference to the Wittrick-Williams algorithm to carry out free vibration analyses. According to the doublet lattice method, the natural mode shapes are subsequently used as generalized motions for the generation of the unsteady aerodynamic generalized forces. Finally, the g-method is used to conduct flutter analyses of both isotropic and laminated composite lifting surfaces. The obtained results perfectly match those from 1D and 2D finite elements and those from experimental analyses. It can be stated that refined beam models are compulsory to deal with the flutter analysis of wing models whereas classical and lower-order models (up to the second-order) are not able to detect those flutter conditions that are characterized by bending-torsion couplings.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.7
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• pp.2661-2669
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• 2010

In this study, a finite element formulation based first-order shear deformation theory is developed for non-linear behaviors of laminated composite plates containing matrix cracking. The multi-directional stiffness degradation is developed for adopting the stiffness variation induced from matrix cracking, which is proposed by Duan and Yao. The matrix cracking can be expressed in terms of the variation of material properties, such as Young's modulus, shear modulus and Possion ratio of plates, and sequently it is possible to predict the variation of the local stiffness. Using the assumed natural strain method, the present shell element generates neither membrane nor shear locking behavior. Numerical examples demonstrate that the present element behaves quite satisfactorily either for the linear or geometrical nonlinear analysis of laminated composite plates. The results of laminated composite plates with matrix cracking may be the benchmark test for the non-linear analysis of damaged laminated composite plates.

• Archives of Plastic Surgery
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• v.35 no.1
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• pp.7-12
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• 2008

Purpose: Since the report of artificial dermis manufacturing method using collagen by Yannas in 1980, collagen has been effectively used as dermal substitute with its merits such as, lower antigeneicity, controllable biodegradation rate, and minimal inflammatory cytotoxic properties in the dermal tissue engineering field. However, weak mechanical durability was the main drawback of collagen dermal substitute. To improve its stability, mechanical or chemical cross-linking was used. Despite of such process, its clinical use was restricted due to weak durability. To improve the durability of collagen matrix, we designed elastin-incorporated collagen matrix and compared its durability with conventional collagen matrix. Methods: 15mm diameter with 4mm thick collagen dermal matrix was made according to Yannas protocol by mixing 0.5% bovine collagen and chondroitin-6-sulfate followed by degassing, freeze drying, dehydrodermal cross-linking and chemical cross-linking procedure. In elastin incorporated collagen matrix, same procedure was performed by mixing elastin to previous collagen matrix in 4:1 ratio(collagen 80% elastin 20%). In comparison of the two dermal matrix in vitro tests, matrix contracture rate, strain, tensile strength, was measured and stiffness was calculated from comparative analysis. Results: In terms of matrix contracture, the elastin-incorperated added collagen dermis matrix showed 1.2 times more contraction compared to conventional collagen matrix. However, tensile strength showed 1.6 times and stiffness showed 1.6 times increase in elastin-incorporated matrix. Conclusion: Elastin incorperated collagen matrix manufactured by our team showed increased durability due to improvement in tensile strength and stiffness compared to previous collagen matrix($Integra^{(R)}$).

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.04a
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• pp.544-549
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• 2011

Simplified formulae for axial and bending natural frequencies of bellows are developed using an equivalent thin-walled pipe model. The axial and bending stiffness of bellows is determined using lumped transfer matrix method. Accordingly, the Expansion Joint Manufacturers Association (EJMA) formula for axial and bending stiffness calculation is modified using two different equivalent radii. The results from the simplified formulae are verified by those from a experiment result and a finite element (FE) model and good agreement is shown between the each other.

• Structural Engineering and Mechanics
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• v.40 no.2
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• pp.279-295
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• 2011

A solution of space curved bars with generalized Winkler soil found by means of Transfer Matrix Method is presented. Distributed, concentrated loads and imposed strains are applied to the beam as well as rigid or elastic boundaries are considered at the ends. The proposed approach gives the analytical and numerical exact solution for circular beams and rings, loaded in the plane or perpendicular to it. A well-approximated solution can be found for general space curved bars with complex geometry. Elastic foundation is characterized by six parameters of stiffness in different directions: three for rectilinear springs and three for rotational springs. The beam has axial, shear, bending and torsional stiffness. Numerical examples are given in order to solve practical cases of straight and curved foundations. The presented method can be applied to a wide range of problems, including the study of tanks, shells and complex foundation systems. The particular case of box girder distortion can also be studied through the beam on elastic foundation (BEF) analogy.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.2
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• pp.409-422
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• 2013

For stability design and P-${\Delta}$ analysis of steel frames with semi-rigid connections, the explicit form of the exact tangential stiffness matrix of a generalized semi-rigid frame element having rotational and translational connections is firstly derived using the stability functions. And its elastic and geometric stiffness matrix is consistently obtained by Taylor series expansion. Next depending on connection types of semi-rigidity, the corresponding tangential stiffness matrices are degenerated based on penalty method and static condensation technique. And then numerical procedures for determination of effective buckling lengths of generalized semi-rigid frames members and P-${\Delta}$ and shortly addressed. Finally three numerical examples are presented to demonstrate the validity and accuracy of the proposed method. Particularly the minimum braced frames and coupled buckling modes of the corresponding frames are investigated.

• Geomechanics and Engineering
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• v.1 no.2
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• pp.121-142
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• 2009

The paper deals with the applications of spectral finite element method to the dynamic analysis of framed foundations supporting high speed machines. Comparative performance of approximate dynamic stiffness methods formulated using static stiffness and lumped or consistent or average mass matrices with the exact spectral finite element for a three dimensional Euler-Bernoulli beam element is presented. The convergence of response computed using mode superposition method with the appropriate dynamic stiffness method as the number of modes increase is illustrated. Frequency proportional discretisation level required for mode superposition and approximate dynamic stiffness methods is outlined. It is reiterated that the results of exact dynamic stiffness method are invariant with reference to the discretisation level. The Eigen-frequencies of the system are evaluated using William-Wittrick algorithm and Sturm number generation in the $LDL^T$ decomposition of the real part of the dynamic stiffness matrix, as they cannot be explicitly evaluated. Major's method for dynamic analysis of machine supporting structures is modified and the plane frames are replaced with springs of exact dynamic stiffness and dynamically flexible longitudinal frames. Results of the analysis are compared with exact values. The possible simplifications that could be introduced for a typical machine induced excitation on a framed structure are illustrated and the developed program is modified to account for dynamic constraint equations with a master slave degree of freedom (DOF) option.