• The Journal of the Acoustical Society of Korea
• /
• v.12 no.4
• /
• pp.21-31
• /
• 1993

불규칙한 기하구조 및 기하학적 특이점들을 갖는 방사체에 의해 형성되는 음장을 예측하는 작업은 매우 어려운 일이다. 이러한 종류의 문제를 해결하기 위하여는 Seybert에 의해 제창된 내, 외부를 연성하여 해석하는 경계요소법에 의한 해석이 유용하다고 여겨지고 있다. 본 연구에서는 연성경계요소법을 재 구성하여 예제로서 얇은 벽면을 갖는 개방된 관에서 방사되는 음장을 선택한 후, 이 방법의 신뢰성, 적용성 및 오차에 대한 해석을 해?ㄴ다. 외부 방사 문제에 있어서의 비유일성문제는 소외 CHIEF 기법을 도입하여 해결하였다. 두 개의 마이크로폰을 사용하여 신호처리를 통한 실험 결과와 본 경계요소법에 의한 결과는 서로 잘 일치하였다. 한편 경계면에 몹시 가까운 지점에서의 음장을 예측할 때의 오차 해석을 수행한 결과, 예측 오차가 10% 이내에서 유지되려면 경계요소법의 가장 짧은 변의 길이가 예측점과 벽면 사이의 거리보다 최소한 10배 이상은 커야함을 알아내었다. 이 기법은 기하학적인 특이점을 포함하는 각종 음향 문제에 매우 유효 적절한 방법으로 생각된다.

• Journal of the Korean Society for Precision Engineering
• /
• v.12 no.10
• /
• pp.89-101
• /
• 1995

선형 탄성 등방성 물체 내에 있는 일반적인 복합모드 크랙 문제들을 해석하기 위한 이중 경계적분방정식의 일반식과 계산해법이 제시되었다. 크랙면이 포함된 물체 해석에 있어서 유일한 해를 얻기 위하여, 한 면상의 점에는 변위 경계적분방정식이 적용되었고 마주하고 있는 상대면 상의 점에는 인력 경계적분방정식이 적용되었다. 인력 및 변위 경계적분방정식의 강특이해 및 초특이해 적분항들은 수치해법을 적용하기 전에 정상화되었다. 정상화과정 중 보정되는 강특이적분항이 상대 크랙면 상의 특이해 요소를 따라 직접 적분되는 것을 격리시키기 위하여, 특이해 적분 경로를 완만한 곡면으로 우회시킨 가상의 비특이해 보조경계로 대치하여 적분값을 계산하였다. 제시된 해법의 정확성과 효율성을 예시하기 위하여, 2차원 및 3차원 크랙 문제의 변형 후 모습과 응력강도계수 계산 결과를 보였다.

• Computational Structural Engineering
• /
• v.9 no.4
• /
• pp.181-187
• /
• 1996

This paper concerns the singular thermal stresses at the interface corner between the elastic fiber and the viscoelastic matrix of a two-dimensional unidirectional laminate model induced during cooling from cure temperature down to room temperature. Time-domain boundary element method is employed to investigate the nature of residual thermal stresses at the interface. Numerical results show that very large stress gradients are present at the interface corner and such stress singularity might lead to local yielding or fiber-matrix debonding.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.24 no.3 s.174
• /
• pp.685-690
• /
• 2000

This paper deals with the stress singularity induced at the interface corner between the viscoelastic thin film and the rigid substrate as the film absorbs moisture from the ambient environment. Th e time domain boundary element method is employed to investigate the behavior of interface stresses. The order of the free-edge singularity is obtained numerically for a given viscoelastic model. It is shown that the stress singularity factor is relaxed with time, while the order of the singularity increases with time for the viscoelastic model considered.

• Proceedings of The KACE
• /
• 2017.08a
• /
• pp.51-54
• /
• 2017

본 연구의 목적은 오픈 러닝과 소셜 러닝 플랫폼의 핵심요소 비교분석 결과를 바탕으로 학습자와 학습 환경 개선을 위해 고려해야 할 핵심 요소와 방향에 대한 시사점을 고찰하는 데 있다. MOOC로 대변되는 오픈 러닝 사례들의 핵심요소를 분석하여 각각의 공통점과 차이점은 무엇인지 알아보고자 했다. 소셜 러닝 플랫폼을 활용한 사례들을 살펴봄으로써 소셜 러닝 플랫폼의 정의와 특이사항을 살펴보았다. 오픈 러닝과 소셜 러닝 플랫폼의 특이사항은 서로 어떤 상호작용을 가질 수 있을지 비교분석 내용을 바탕으로 오픈 러닝에서 발견 된 장단점과 소셜 러닝 플랫폼의 장단점으로 대치하여 서로 보완해줄 수 있는 진화한 러닝 플랫폼을 구상해보는 것은 유의미 할 것이다.

• Computational Structural Engineering
• /
• v.10 no.2
• /
• pp.131-138
• /
• 1997

This paper concerns the stress singularity at the interface corner of the viscoelastic adhesive layer bonded between rigid adherends, subjected to a uniform transverse tensile strain. The characteristic equation is derived in the Laplace transformed space, following Williams, and the transformed characteristic equation is inverted analytically into real time space for the viscoelastic model considered here. The order of the singularity is obtained numerically. The time-domain boundary element method is employed to investigate the nature of stresses along the interface. Numerical results show that the order of the singularity increases with time while the free-edge stress intensity factors are relaxed with time.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.22 no.1
• /
• pp.206-214
• /
• 1998

When a crack meets bimaterial interface stress singularity depends on the elastic constants of the adjacent materials. In the present study we are going to describe the finite element formulation for problems with a crack to be embedded in the stiffer material$({\mu}_2/{\mu}_1)$. The cubic isoparametric singular element, represented by adequately shifting the mid-side nodes adjacent to the crack tip is constructed to enclose the crack tip. An alternative method to obtain the optimal position of the mid-side nodes of cubic isoparametric elements is presented. In addition, a proper definition for the stress intensity factors of a crack normal to bimaterial interface is provided. It is based upon near a tip displacement solutions. Models for numerical analysis are two dimensional elastic bodies with a through crack under plain strain. The results obtained are compared with the previous solutions.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.19 no.3 s.73
• /
• pp.259-269
• /
• 2006

In this paper, the stress intensity factor, energy release rate and crack opening displacement are computed using the finite element method for axisymmetric viscoelastic cylinders with the penny-shaped and circumferential cracks. The triangular elements with quarter point nodes are used to describe the stress singularity around the crack edge. The analytical solutions are also derived by using the elastic-viscoelastic correspondence principle and compared with the numerical results to show the validity and accuracy of the presented method. Viscoelastic materials are assumed to behave elastically in dilatation and like a three-parameter standard linear solid.

• Clinical and Experimental Pediatrics
• /
• v.45 no.4
• /
• pp.482-488
• /
• 2002

Purpose : We performed this study to evaluate the diagnostic usefulness of endoscopic finding of nodular gastritis, CLO and HpKit test for H. pylori infection in children. Methods : Gastroduodenal endoscopy and mucosal biopsy were performed on 212 children who visited our hospital between Jul. 1999 and May 2000 due to abdominal pain. We performed CLO and HpKit test for H. pylori with the time interval of 15, 30 minutes, 1, 2, 3, 24, 48, 72, 96, 120 and 144 hours. Histological examination of H. pylori was made by H-E or Alcian yellow stain with biopsy specimens. Sensitivity, specificity, positive predictive and negative predictive value of nodular gastritis, CLO and HpKit test were calculated from the analysis of above data. Results : Sensitivity and specificity of 3 hour-CLO test was 68.4% and 100% respectively. Sensitivity and specificity of 3 hour-HpKit test was 65.8% and 100% respectively. No significant difference in sensitivity and specificity was found between in 3 hour-CLO and HpKit test(P>0.05). Sensitivity of CLO test increased as time lapsed, but corresponding specificity did not decrease as time lapsed(sensitivity and specificity at 144 hours : 89.5% and 94.8% respectively). However, sensitivity of HpKit test increased as time lapsed, but specificity markedly decreased. Sensitivity and specificity of the nodular gastritis was 78.9% and 93.7% respectively. Conclusion : Both CLO and HpKit test have relatively low sensitivity and specificity for the detection of H. pylori in 3 hours of testing in children. The endoscopic finding of nodular gastritis is another good standard in the diagnosis of H. pylori infection in children.

• Computational Structural Engineering
• /
• v.9 no.4
• /
• pp.165-172
• /
• 1996

For thin elastic structures such as beams, arches, plates and shells, there may exist the boundary layer in the narrow thin region neighborhood of boundaries, where the solution displays the singular behavior exponentially decaying in the normal direction to the boundary. In the finite element analysis of these structures, finite element mesh patterns have a significant role to capture this singularity. This paper introduces the analytic study of this problem and provides a guideline to construct optimal mesh patterns together with numerical experiments.