Jae-Kon Lee

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..

A New Model to Predict Effective Elastic Constants of Composites with Spherical Fillers

In this study, a new model to predict the effective elastic constants of composites with spherical fillers is proposed. The original Eshelby model is extended to a finite filler volume fraction without using Mori-Tanaka’s mean field approach. When single filler is embedded in the matrix, the effective elastic constants of the composite are computed. The composite is in turn considered as a new matrix, where new single filler is again embedded in the matrix. The predicted results by the present model with a series of embedding procedures are compared with those by Mori-Tanaka, self-consistent, and generalized self-consistent models. It is revealed through parametric studies such as stiffness ratio of the filler to the matrix and filler volume fraction that the present model gives more accurate predictions than Mori-Tanaka model without using the complicated numerical scheme used in self-consistent and generalized self-consistent models.

An Analytical Study on Prediction of Effective Properties in Porous and Non - Porous Piezoelectric Composites

Eshelby type micromechanics model with a newly developed piezoelectric Eshelby tensor is proposed for predicting the effective electroelastic properties of the piezoelectric composite. The model is applied for piezoelectric solids containing both porosities and metal inhomogeneities. The effective electroelastic moduli of the composites such as stiffness, piezoelectric constants, and dielectric constants are predicted by the present model, which are extensively compared with the existing experimental results from the literatures. The validity of Eshelby type model for predicting the effective properties of the composite is thoroughly examined. It can be concluded from this study that a new mechanism is needed to compute correctly the dielectric constants among the effective properties of the composites.

Design and Performance Evaluation of a VANET-Based Adaptive Overtaking Assistance System

One of the most risky maneuvers in roads with both directions in the same carriageway and only one lane for each direction is overtaking another vehicle. To overtake becomes particularly dangerous at night, with bad weather conditions and in curves, due to the diminution of visibility. This paper suggests a VANET-based Adaptive Overtaking Assistance System (VAOAS). The VAOAS not only considers both the driving skill of drivers and the length of overtaking/preceding vehicles, but also supports all strategies of overtaking using VANETs. The performance of VAOAS system is evaluated as the number of V2V messages, overtaking and passing sight distances, and overtaking success rate, depending on traffic densities of both directions through a simulation.

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

Design and Evaluation of TLV-eCAST for Safety Message Dissemination in VANETs

Vehicular ad hoc networks (VANETs) will play a vital role in reducing the number of road accidents and fatalities as well as vehicular traffic optimization. In this paper, we propose TLV-eCAST, the last vehicle based early broadcasting for disseminating safety-related alert message in VANET. The proposed TLV-eCAST uses the early warning system on the basis of time to collision (TTC) as well as just broadcasting by the last vehicle in the transmission range of sending vehicle. The performance of TLV-eCAST is evaluated through simulation and compared with other message dissemination algorithms.

A Study on Prediction of Effective Thermal Conductivity of Nano-Fluids Using Generalized Self-Consistent Model and Modified Eshelby Model

Key Words: Nanofluid(나노유체), Nanolayer(나노층), Thermal Conductivity(열전도계수), Generalized Self-ConsistentModel(일반화된자기일치모델), Modified Eshelby Model(수정된에쉘비모델)초록: 복합재의물성치해석에일반적으로사용되는일반화된자기일치모델(Generalized Self-ConsistentModel)과수정된에쉘비모델(Modified Eshelby Model)을이용하여나노유체의열전도계수를예측할수있음을보였다. 이유체의열전달효과를대폭향상시키는대표적인메카니즘중하나인나노입자와기본유체사이에존재하는나노층의영향을고려하여나노유체의열전도계수를예측하였다. 본연구는나노층의열전도계수가일정한값을가질때기존대표적인모델과동일한결과를보였으며, 선형적으로변할때역시문헌에있는모델과동등한수준의예측값을보였다. 알루미나와산화구리를나노입자로물과에틸렌글리콜을기본유체로한나노유체의열전도계수에대한실험결과와본모델의예측결과를비교함으로써본모델의타당성을입증하였다.Abstract: Effective thermal conductivity of nanofluids has been predicted by using generalized self-consistentmodel and modified Eshelby model, which have been used for analysis of material properties of composites.A nanolayer between base fluid and nanoparticle, one of key factors for abrupt enhancement of thermalconductivity of nanofluids, is included in the analysis. The effective thermal conductivities of the nanofluidpredicted by the present study show good agreement with those by models in the literature for the nanolayerwith a constant or linear thermal conductivity. The predicted results by the present approach have beenconfirmed to be consistent with experiments for representative nanofluids such as base fluids of water orethyleneglycol and nanoparticles of Al

A Study on Prediction of Effective Seebeck Coefficient of Thermoelectric Composites Using Modified Eshelby Model

A coupled governing equation of thermoelectric materials can be converted into an uncoupled form to predict the effective Seebeck coefficient of thermoelectric composites, where modified Eshelby model is adopted. The predicted results by the present approach for serial- and parallel-connected composites and composite with spherical inclusions are compared with theoretical and experimental results from literatures to be justified. It is shown that the predictions by the theoretical approaches coincide exactly and show in good agreement with the experiments.