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점소성-손상모델 기반 피로균열 진전속도 전산 평가법 개발
김슬기,김정현,이치승,김명현,이제명,Kim, Seul-Kee,Kim, Jeong-Hyeon,Lee, Chi-Seung,Kim, Myung-Hyun,Lee, Jae-Myung 한국전산구조공학회 2015 한국전산구조공학회논문집 Vol.28 No.1
본 논문에서는 재료 점소성-손상모델을 기반으로 한 피로균열 진전속도(FCGR) 전산 평가법을 제안한다. 7% 니켈강 재료 거동을 모사하는 점소성-손상모델을 소개하고, 이의 유한요소해석 플랫폼에의 적용을 위해 상용 유한요소해석 프로그램인 ABAQUS에서 제공하는 사용자 정의 재료 서브루틴(UMAT)에 재료모델을 탑재하였다. 개발 UMAT의 검증을 위해 7% 니켈강 재료 인장시험 시뮬레이션을 수행하였으며, 이를 통해 재료정수를 획득하였다. 또한, 피로하중에 따른 손상해석에 있어 계산 시간 단축을 위한 jump-in-cycles 과정과 임계 손상 값 조정 및 피로 예비 균열 시뮬레이션을 수행하였고 이들 과정을 개발 UMAT에 탑재하여 해석을 수행하였다. 개발 UMAT을 활용하여 7% 니켈강의 상온 FCGR 테스트 시뮬레이션을 수행하였으며, 균열길이(a)와 주기 수(number of cycles)의 관계 및 1 cycle 당 균열성장량(da/dN)과 응력확대계수 진폭(${\Delta}K$)의 관계 등의 결과를 실험결과와 비교하여 검증하였다. In this paper, computational evaluation method for fatigue crack growth rate(FCGR) based on material viscoplastic-damage model is proposed. Viscoplastic-damage model expressing material constitutive behavior of 7% nickel steel is introduced and is implemented into commercial finite element analysis(FEA) code, ABAQUS, as a user defined material subroutine(UMAT) for application in the FEA environments. Verification of developed UMAT and material parameters of material model are carried out by uniaxial tensile test simulations of 7% nickel steel. Moreover, jump-in-cycles procedure and rearrangement of critical damage are employed and also implemented to the ABAQUS UMAT for fatigue damage analysis. Typical FCGR test results such as relationship between crack length and number of cycles and relationship between da/dN and ${\Delta}K$ could be obtained from FCGR test simulation using developed UMAT and these results are compared with experimental results in order to verify of proposed computational method.
점탄소성-손상모델 기반 전산 CTOD 테스트 기법 개발
김슬기(Seul-Kee Kim),이치승(Chi-Seung Lee),이제명(Jae-Myung Lee) 대한조선학회 2011 대한조선학회 학술대회자료집 Vol.2011 No.6
The aim of the present paper is the development of the computational CTOD test method based on viscoplastic-damage model. In general, the ductile crack characteristics such as crack tip opening displacement, fracture toughness can be obtained by CTOD test. However, it takes huge time and cost to carry out the experimental environments. Hence, as an alternative method, the simulation based CTOD test technique has been studied widely among the computational engineers. In the present study, the ductile crack characteristics of austenitic stainless steel are investigated using the viscoplastic-damage model. In order to consider the phase transformation from austenite to martensite, the Bodner-Partom model was modified. Moreover, the material degradation during the CTOD test is calculated using the Boder-Chan damage model. The proposed viscoplastic- damage model is formulated as a fully implicit form and implemented to ABAQUS user defined subroutine UMAT. The computational CTOD test method is compared to the CTOD experiments in order to validate the proposed method.
김정현 ( Jeong-hyeon Kim ),김정대 ( Jeong-dae Kim ),김태욱 ( Tae-wook Kim ),김슬기 ( Seul-kee Kim ),이제명 ( Jae-myung Lee ) 한국산업융합학회 2023 한국산업융합학회 논문집 Vol.26 No.6
Polyurethane foam insulation required for storing and transporting cryogenic liquefied gas is already widely used as a thermal insulation material for commercial LNG carriers and onshore due to its stable price and high insulation performance. These polyurethane foams are reported to have different mechanical performance depending on the density, and the density parameter is determined depending on the amount of the blowing agent. In this study, density-dependent polyurethane foam was fabricated by adjusting the amount of blowing agent. The mechanical properties of polyurethane foam were analyzed in the room temperature and cryogenic temperature range of -163℃ at 1.5 mm/min, which is a quasi-static load range, and the cells were observed through microstructure analysis. The characteristics of linear elasticity, plateau, and densification, which are quasi-static mechanical behaviors of polyurethane foam, were shown, and the correlation between density and mechanical properties in a cryogenic environment was confirmed. The correlation between mechanical behavior and cell size was also analyzed through SEM morphology analysis. Polyurethane foam with a density of 180 kg/m<sup>3</sup> had a density about twice as high as that of a polyurethane foam with a density of 96 kg/m<sup>3</sup>, but yield strength was about 51% higher and cell size was about 9.5% smaller.
사용자지정 재료 서브루틴을 활용한 LNG선박 단열시스템 멤브레인의 수치해석
김정현,김슬기,김명수,이제명,Kim, Jeong-Hyeon,Kim, Seul-Kee,Kim, Myung-Soo,Lee, Jae-Myung 한국전산구조공학회 2014 한국전산구조공학회논문집 Vol.27 No.4
304L 스테인리스강판은 멤브레인타입 LNG선 단열시스템의 1차 방벽으로 이용된다. 304L 스테인리스강은 변태유기소성(TRIP)강으로 복잡한 재료거동을 보이는데, 이는 소성변형이 발생하는 동안 상변태를 경험하기 때문이다. 본 연구에서는 304L 스테인리스 강의 비선형 기계적 거동분석을 위한 온도의존 일축인장시험을 수행하였으며 재료의 파단이나 비선형 거동을 예측하기 위한 점소성모델을 제안하였다. 수치해석의 결과와 시험 결과를 비교 분석하여 유효성을 검증하였으며 LNG 멤브레인에 대한 적용성을 검토하기 멤브레인 구조시편을 제작하여 구조해석 및 유한요소해석을 수행하였다. 재료모델은 개발 서브루틴을 이용하였으며 ABAQUS 사용자지정 재료 서브루틴을 탑재한 유한요소해석 결과와 극저온 구조인장시험을 수행한 결과를 비교하여 구조적용성을 검증하였다. 304L stainless steel sheets are used as a primary barrier for the insulation of membrane-type liquefied natural gas(LNG) carrier cargo containment system. 304L stainless steel is a transformation-induced-plasticity(TRIP) steel that exhibits complex material behavior, because it undergoes phase transformation during plastic deformation. Since the TRIP behavior is very important mechanical characteristics in a low-temperature environment, significant amounts of data are available in the literature. In the present study, a uniaxial tensile test for 304L stainless steel was performed to investigate nonlinear mechanical characteristics. In addition, a viscoplastic model and damage model is proposed to predict material fractures under arbitrary loads. The verification was conducted not only by a material-based comparative study involving experimental investigations, but also by a structural application to the LNG membrane of a Mark-III-type cargo containment system.
폴리우레탄 폼의 비선형 압축거동을 모사하기 위한 밀도 영향이 고려된 수정 Gurson 모델의 제안
이정호,박성보,김슬기,방창선,이제명,Lee, Jeong-Ho,Park, Seong-Bo,Kim, Seul-Kee,Bang, Chang-Seon,Lee, Jae-Myung 한국전산구조공학회 2015 한국전산구조공학회논문집 Vol.28 No.5
뛰어난 단열성능을 가지는 폴리우레탄 폼(polyurethane foam, PUF)은 다양한 구조물에서 다른 재료들과 함께 사용되고 있다. 현재 LNG 운반선의 단열시스템에는 유리섬유로 강화된 폴리우레탄 폼(reinforced-polyurethane foam, R-PUF)이 사용되고 있으며, 이는 단열재 역할뿐만 아니라 슬로싱 하중을 포함한 다양한 압축하중에 대한 구조부재 기능을 수행하고 있다. 폴리우레탄 폼은 혼합과 발포를 통해 제작되는 다공성 재료이기 때문에, 본 연구에서는 기공체적비율을 통해 재료의 거동을 모사할 수 있는 Gurson damage model을 사용하여 폴리우레탄 폼의 비선형 압축거동을 모사하였으며, 폴리우레탄 폼의 기계적 성질에 영향을 미치는 영향변수로서 기공체적비율에 의존적으로 알려져 있는 밀도를 설정하였다. Polyurethane Foam(PUF), a outstanding thermal insulation material, is used for various structures as being composed with other materials. These days, PUF composed with glass fiber, Reinforced PUF(R-PUF), is used for a insulation system of LNG Carrier and performs function of not only the thermal insulation but also a structural member for compressive loads like a sloshing load. As PUF is a porous material made by mixing and foaming, mechanical properties depend on volume fraction of voids which is a dominant parameter on density. Thus, In this study, density is considered as the effect parameter on mechanical properties of Polyurethane Foam, and mechanical behavior for compression of the material is described by using modified Gurson damage model.