J-T에 대한 3차원 반타원 계면균열선단 응력장의 기술

이형일(Hyungyil Lee),최호승(Ho-Seung Choi) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8

Many research works have validated the J-T approach to elastic-plastic crack-tip stress fields in a variety of plane strain specimens. To generalize the validity of J-T method, further investigations are however needed for more practical 3D structures than the idealized plane strain specimens. In this work, we perform 3D finite element (FE) modeling of welded plate and straight pipe, and accompanying elastic, elastic-plastic FE analyses. Manual 3D modeling is almost prohibitive, since the models contain semi-elliptical interfacial cracks which require singular elements. To overcome this kind of barrier, we develop a program generating the meshes for semi-elliptical interfacial cracks. We then compare the detailed 3D FE stress fields to those predicted with J-T two parameters. Thereby we extend the validity of J-T application to 3D structures and infer some useful informations for the design or assessment of pipe welds.

C<SUP>*</SUP>-적분에 기초한 고온배관 수명평가

이형일(Hyungyil Lee) 한국가스학회 2000 한국가스학회지 Vol.4 No.4

완전소성하 변형경화 이종접합재의 계면균열선단 구속상태 및 J-적분

이형일(Hyungyil Lee),김용범(Yongbom Kim) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.3

Effect of the T-stress and plastic hardening mismatch on interfacial crack-tip stress and J-integral are investigated in this paper, via detailed finite element analyses. Plane strain elastic-plastic crack-tip fields have been modeled with both MBL formulation based on K-T and SEC specimen under pure bending. Modified Prandtl slip line fields are presented to illustrate the effects of T-stress in homogeneous material. As in the case of homogeneous material, compressive T-stresses reduce the interfacial crack-tip constraint substantially, but increases J<SUB>L</SUB> in bimaterial. For bimaterials consisting of two elastic-plastic materials, increasing plastic hardening mismatch increases both crack-tip stress constraint in the lower hardening material and J<SUB>L</SUB>. in bimaterial. Resulting fracture toughness for bimaterial joint will be much lower than that for lower hardening homogeneous material. Effects of the T-stress and plastic hardening mismatch on asymmetry of the J-integral for bimaterials are also discussed.

초탄성고무 물성평가용 미소압입시험법 개발 및 검증

이형일(Hyungyil Lee),이진행(Jin Haeng Lee) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4

In this work, effects of hyper-elastic rubber material properties on the indentation load-deflection curve and subindenter deformation are first examined via finite element (FE) analyses. An optimal data acquisition spot is selected, which features maximum strain energy density and negligible frictional effect. We then contrive two normalized functions, which map an indentation load vs. deflection curve into a strain energy density vs, first invariant curve. From the strain energy density vs. first invariant curve, we can extract the rubber material properties. This new spherical indentation approach produces the rubber material properties in a manner more effective than the common uniaxial tensile/compression tests. The indentation approach successfully measures the rubber material properties and the corresponding nominal stress-strain curve with an average error less than 3%.

마이크로/나노 압입시험에 의한 재료특성평가

이형일(Hyungyil Lee),이진행(Jin Haeng Lee) 대한기계학회 2008 大韓機械學會論文集A Vol.32 No.10

The present work reviews the methods to evaluate elastic-plastic material characteristics by indentation tests. Especially the representative stress and strain values used in some papers are critically analyzed. The values should not only represent the load-depth curve, but also represent the whole of deformed material around the impression. We briefly introduce other indentation techniques to evaluate residual stresses, creep properties, and fracture toughness. We also review some technical problems that are related to the accuracy issues in indentation tests.

엔진 커넥팅로드의 좌굴평가를 위한 유한요소법 및 실험방법 연구

이문규(Moon Kyu Lee),이영민(Young Min Lee),맹준수(Junsoo Maeng),이형일(Hyungyil Lee),이태수(Tae Soo Lee),신성원(Sung Won Shin),장훈(Hun Chang) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11

To reduce the weight of the connecting rod of a vehicle's engine, the accurate buckling estimation is used as a major valuation basis. This study improves the previous efforts, based on finite element analysis, to include experimental results and accurate geometrical conditions. The critical buckling stresses of connecting rods which were cut off shank portion with various slenderness ratios, were estimated using the rig device which was made in this lab. Also the finite element model was improved by addition to the geometrical contact condition on the basis of experimental results. Hence this result indicates that the buckling prediction through FEA is more effective method than the classical formula.

향상된 구형 압입 물성평가법

이진행(Jin Haeng Lee),이형일(Hyungyil Lee) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11

In this work, indentation theory of Lee et al.?¹? for 6% indentation of indenter diameter is extended to an indentation theory for 20% indentation. For shallow indentation, the effect of friction on load-depth curve is negligible, but different materials can show nearly identical load-depth curves. On the basis of this observation, a new numerical approach to deep indentation techniques is proposed by examining the finite element solutions. With this new approach, from the load-depth curve, we obtain stress-strain curve and the values of Young's modulus, yield strength and strain-hardening exponent with an average error of less than 3%.

엔진 커넥팅로드 Shank 부의 경량화를 위한 좌굴 민감도 분석

이영민(Young Min Lee),이문규(Moon Kyu Lee),문희욱(Hee Wook Moon),신성원(Sung Won Shin),장훈(Hoon Jang),이형일(Hyungyil Lee),이태수(Tae Soo Lee) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

Connecting rod under incessant dynamic motion has a major effect on the engine performance according to its lightweight degree. In engine parts design, the effect of buckling has been considered less than static load and fatigue, and there were limited studies that compared the effect of those three factors. The purpose of this study is to analyze the sensitivity of buckling by comparing the safety factors for static stress, fatigue and buckling with respected to the change of section area of the connecting rod shank. Static stress, fatigue and buckling analysis are performed by FE model which has width and thickness as design variables, and buckling sensitivity with the change of the cross sectional area in shank is analyzed by comparing the safety factor decreasing ratio. Sensitivity analysis demonstrated that the effect of buckling needs to be considered in designing the connecting rod, and reducing the width can be more profitable than reducing the thickness of the shank.

Cohesive Zone Model을 이용한 압입자 형상에 따른 균열특성분석

현홍철(Hong Chul Hyun),이진행(Jin Haeng Lee),이형일(Hyungyil Lee),김대현(Dae Hyun Kim),한준희(Jun Hee Hahn) 대한기계학회 2013 大韓機械學會論文集A Vol.37 No.12

본 연구에서는 유한요소해석과 압입시험으로 압입자 형상이 압입균열특성에 미치는 영향을 조사했다. 본 논문에서는 Lee 등(2012)이 제시한 cohesive zone 모델특성 및 균열생성, 진전을 위한 해석 조건에 기초해 다양한 균열해석을 수행했다. 우선, 사각뿔 및 삼각뿔 압입균열 시험과 해석을 비교해 해석 모델의 유효성을 검증했다. 아울러 비대칭 압입자에 의한 압입시, 압입하부에서 비대칭 균열의 발생 여부를 해석적으로 관찰했다. 최종적으로 압입발생 균열수와 균열길이 관계를 조사했다. 균열수와 균열길이 관계 및 동일 압입자형상(압입자 모서리수)에서 압입자각에 따른 균열길이 변화를 이용하면, 특정 압입자 형태의 압입시험에서 얻은 균열길이 만으로 다양한 압입자 형태의 균열길이를 예측할 수 있다. In this study, we investigated the effect of the indenter geometry on the crack characteristics by indentation cracking test and FEA. We conducted various cohesive finite element simulations based on the findings of Lee et al. (2012), who examined the effect of cohesive model parameters on crack size and formulated conditions for crack initiation and propagation. First, we verified the FE model through comparisons with experimental results that were obtained from Berkovich and Vickers indentations. We observed whether nonsymmetrical cracks formed beneath the surface during Berkovich indentation via FEA. Finally, we examined the relation between the crack size and the number of cracks. Based on this relation and the effect of the indenter angle on the crack size, we can predict from the crack size obtained with an indenter of one shape (such as Berkovich or Vickers) the crack size for an indenter of different shape.

$C^{\ast}$-적분에 기초한 고온배관 수명평가

이형일,Lee Hyungyil 한국가스학회 2000 한국가스학회지 Vol.4 No.4

In recent years, the subject of remaining life assessment has drawn considerable attention in power plants, where various structural components typically operate at high temperature and pressure. Thus a life prediction methodology accounting for high temperature creep fracture is increasingly needed for the components. Critical defects in such structures are generally found in the form of semi-elliptical surface crack, and the analysis of which is consequently an important problem in engineering fracture mechanics. On this background, we first develop an auto mesh generation program for detailed 3-D finite element analyses of axial and circumferential semi-elliptical surface cracks in a piping system. A high temperature creep fracture parameter $C^{\ast}$-integral is obtained from the finite element analyses of generated 3-D models. Post crack growth module is further appended here to calculate the amount of crack growth. Finally the remaining lives of surface cracked pipes for various analytical parameters are assessed using the developed life assessment program. 최근 들어 가혹한 온도 및 압력조건에서 운전이 요구되는 고온배관들의 잔여수명예측은 상당히 중요한 관심사로 부각되고 있다. 이에 고온손상을 정량적으로 평가하여 설비를 안전하게 사용하기 위한 유지기술 및 수명평가 기술을 확보하는 것은 매우 중요하다. 한편 이러한 고온배관내 대부분의 균열은 반타원 표면균열의 형태로 형상화되어 파괴역학의 실제응용에서 발견할 수 있는 가장 복잡한 형상과 하중특성을 갖게 된다. 이를 배경으로 본 연구에서는 먼저 축방향 및 원주방향 반타원 표면균열이 존재하는 직관의 완전 3차원 유한요소망 자동생성 프로그램을 작성하였다. 이렇게 생성된 유한요소 입력자료를 이용하여 3차원 유한요소 해석을 수행하면 반타원 표면균열선단을 따른 고온 크리프 파괴 지배변수 $C^{\ast}$-적분값을 구할 수 있다. 또한 $C^{\ast}$-적분값에 준해 균열진전량을 계산하는 후처리 균열진전 모듈을 추가하여 사용자 편의 수명평가 프로그램을 개발하였다. 최종적으로 개발된 통합 수명 평가 프로그램을 이용하여 여러 가지 해석변수들의 반타원 표면균열을 갖는 고온 직관 수명에 대한 영향을 살펴보았다.