Jin-Gon Kim

An analytical study on prediction of effective elastic constants of perforated plate

In this study, the validity of the Eshelby-type model for predicting the effective Young’s modulus and in-plane Poisson’s ratio of the 2-dimensional perforated plate has been investigated in terms of the porosity size and its arrangement. The predicted results by the Eshelbytype model are compared with those by finite element analysis. Whenever the ratio of the porosity size to the specimen size becomes smaller than 0.07, the effective elastic constants predicted by finite element analysis are convergent regardless of the arrangement of the porosities. Under these conditions, the effective Young’s moduli of the perforated plate can be predicted within the accuracy of 5% by the Eshelby-type model, which overestimates and underestimates the effective Poisson’s ratios by 10% and 6% for the plates with periodically and non-periodically arranged porosities, respectively.

Analytical Study on Effective Thermal Conductivity of Three-Phase Composites

Effective thermal conductivity of three-phase composites, consisting of matrix and two kinds of spherical inclusions, has been derived as an explicit form by extending modified Eshelby model (MEM) for two-phase composites. The present results are compared with those by differential effective medium model (DEMM), which are also compared with the experimental results of two- and three-phase composites in the literatures to be validated. For two-phase composites, the results by MEM are better than those by DEMM for the inclusion volume fraction smaller than 0.5. Comparisons between the results by two models and experimental results have been made for three-phase composite, resulting in that MEM predicts better than DEMM for smaller volume fraction of the inclusion having larger inclusion-to-matrix thermal conductivity ratio, but DEMM predicts better as its volume fraction increases. It has been observed through parametric study that its volume fraction is the critical factor affecting the deviation of predictions by the two models. The results by them show a good agreement with the three-phase composite proposed by Molina et al..

Fault-Tolerant Gait Planning of Multi-Legged Robots

A fault-tolerant gait of multi-legged systems is defined as a gait which can maintain the gait stability and continue its walking against the occurrence of a leg failure (Yang & Kim, 1998). The notion of the fault-tolerant gait comes from the fact that legged robots with static walking have inherent fault tolerance capability against a failure in a leg, since a failed leg for itself does not cause fatal breakdown or instability to walking motions (Nagy et al., 1994). This means that for a given type of failure, the problem of finding fault-tolerant gaits can be formulated with which legged robots can continue their walking after an occurrence of a failure, maintaining static stability. As a novel field of gait study, fault-tolerant gaits are worth researching since the feature of leg failure can be involved into the frame of gait study and its adverse effect on gait planning can be analyzed with performance criteria such as stability margin, stride length, etc. Fault-tolerant gaits are classified by the kind of leg failure to be tolerated and the point of time that fault tolerance is carried out, that is, before or after a failure occurs. Several algorithms of fault-tolerant gaits developed in the past can be sorted out by these categories (Chu & Pang, 2002; Lee & Hirose, 2002, Nagy et al, 1994; Yang, 2002). Among them, Yang (Yang, 2002) proposed fault-tolerant gaits for post-failure walking of quadruped robots with a locked joint failure, in which a joint of a leg is locked in a known place (Lewis & Maciejewski, 1997). As one of common failures that can be frequently observed in dynamics of robot manipulators, the locked joint failure reduces the number of degrees of freedom of the robot manipulator by one and consequently its workspace to a certain limit. To establish the fault-tolerance scheme, the reduction of the workspace of a failed leg should be interpreted and reflected based on gait study. The objective of this article is to develop the fault-tolerant gait algorithm for a locked joint failure when the model of legged robots is hexapod. Compared with the previous results on quadruped robots (Yang, 2002, Yang, 2003), the following aspects will be considered in this article for the motion of hexapod robots. First, quadruped robots can have only one gait pattern in static walking, (4a3a4a3···), i.e., one leg is lifted off and swung while other three legs are in the support phase. But hexapod robots can have variable gait pattern, i.e., tripod, quadruped and pentaped gaits. In this article, we show that fault tolerance can be realized for walking with any gait and, especially, periodic gaits can be generated using tripod and quadruped gaits for straight-line walking on even terrain. We also present faulttolerant gaits with non-zero crab angle, or a walking motion with the direction of

Three Dimensional Stress Analysis of a Dental Implant with Central Cavity

In this study, we propose a new short dental implant and investigate its bio-mechanical characteristics by using three dimensional finite element analyses. The proposed dental implant has the central cavity which can be integrated with the core of cancellous bone remained by trepanning drill. We take the Bicon short implant as a reference model for studying the effects according to the shape of cavity. The parametric finite element model using ANSYS APDL has been built to determine which length, diameter and thread of central cavity would be effective to dissipate stress. The reduction of undesirable stress in adjacent bone which can suppress bone defects and the eventual failure of implants. The numerical results shows that the cavity of well-determined shape has the beneficial effects on reducing the bone absorption in cancellous bone.

Shock Simulation and Experimental Verification of HDD

Abstract This study deals with the shock response analysis of HDD subjected to a half-sine shock pulse and its experimental verification. Comparatively, accurate computer simulation allows designers to determine complete mechanical information during the product impact time period, compared with only segmental messages by sensors in a test, to predict potential failures. But, impact/shock simulation technology is rather sensitive to various factors to predict the shock behavior without validation. In our shock simulation, the methodology of analysis with LS-DYNA3D and test validation is adopted to predict the shock behavior of HDD. We can confirm the soundness of the present shock simulation through the comparison with electromagnetic shock test(200G/1ms) and linear drop test(300G/2ms).Key Words : Hard Disk Drive, Shock Simulation, Shock Test, Finite Element Analysis, LS-DYNA * 교신저자 : 김진곤(kimjg1@cu.ac.kr)접수일 09년 07월 28일 수정일 09년 09월 30일 게재확정일 09년 10월 14일 1. 서론 정보화 산업에서 가장 중요한 요소 중의 하나인 고성능 컴퓨터는 대용량의 정보저장과 고속의 정보 입출력이 가능한 정보저장기기를 필요로 한다. 현재 사용되고 있는 다양한 정보저장기기들은 SRAM, DRAM, flash memory 등과 같은 반도체 메모리와 자기를 이용한 하드 디스크, 테이프 그리고 광자기 디스크 등이 있다. 이 중에서도 가격, 품질, 성능과 신뢰성 측면에서 HDD(Hard Disk Drive)를 능가하는 기억장치는 아직까지 없다. 하지만 HDD의 급격한 기록 밀도 증가 및 이에 따른 높은 속도의 디스크 회전으로 인해 진동 및 충격에 대한 신뢰성 수준은 계속 높아지고 있어 설계자들에게 큰 어려움을 주고 있다.최근 높은 성능의 HDD가 비작동 중에 받는 치명적인 충격 입력은 대략 0.2~0.5 ms 정도의 짧은 시간동안의 충격에 의해 일어나는 것으로 알려져 있다. 이때 HDD가 손상을 입는 파괴 메커니즘은 헤드-슬랩(head-slap)과 관련되어 있으며, 이는 충격을 받은 서스펜션 끝단의 헤드가 디스크로부터 lift-off하고 나서 디스크와 충돌하는 현상을 말한다. 이러한 충격에 의해서 헤드의 균열(crack) 및 파편 조각으로 인한 기록/재생 시에 HDD에 치명적인 오동작이 발생할 수 있다. 따라서 대부분의 업체들은 150-400G의 충격치와 0.5-2ms의 짧은 충격전달시간동안 견딜 수 있는 제품 개발을 위해 충격현상을 규명하고, 이를 예측할 수 있는 해석모델을 개발하는 연구를 활발히

Harmonic axisymmetric thick shell element for static and vibration analyses

In this study, a new harmonic axisymmetric thick shell element for static and dynamic analyses is proposed. The newly proposed element considering shear strain is based on a modified Hellinger-Reissner variational principle, and introduces additional nodeless degrees for displacement field interpolation in order to enhance numerical performance. The stress parameters selected via the field-consistency concept are very important in formulating a trouble-free hybrid-mixed elements. For computational efficiency, the stress parameters are eliminated by the stationary condition and then the nodeless degrees are condensed out by the dynamic reduction. Several numerical examples confirm that the present element shows improved efficiency and yields very accurate results for static and vibration analyses.

Prediction of Effective Thermal Conductivity of Composites with Coated Short Fibers of Different Aspect Ratios Using Hybrid Model

Abstract A hybrid model is proposed to easily predict the effective thermal conductivity of composites with aligned- and coated-short fibers, whose aspect ratio is not constant. The thermal conductivities of coated fillers are computed by using the generalized self-consistent model, resulting in that composites are simply simulated by the matrix with the equivalent short fibers. Finally, the thermal conductivity of the composites is predicted using the modified Eshelby model. The predicted results by the representative models and hybrid model are compared for the composite with aligned- and coated-short fibers of single aspect ratio. It is demonstrated that the hybrid model can be applied to the composite with aligned- and short-fibers of aspect ratios, 2 and 10, without any difficulties. Key Words : Aspect Ratio Distribution, Coated Short Fiber, Generalized-Self Consistent Model, Modified Eshelby Model, Thermal Conductivity 이 논문은 2012년도 대구가톨릭대학교 교내연구비 지원에 의한 것임. * Corresponding Author : Jae-Kon Lee(Catholic Univ. of Daegu)Tel: +82-53-850-2720 email: leejk@cu.ac.krReceived March 27, 2013 Revised May 6, 2013 Accepted June 7, 2013