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저강도 접합 인장시편의 한계해석과 알루미늄 용접부에의 적용
오창식(Chang Sik Oh),이아미(Amy Lee),김윤재(Yun Jae Kim) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5
The fusion zone of aluminium weldment typically shows lower strength than base metal. Due to such under-matching nature, tensile test of aluminium weldment specimen does not provide “intrinsic” tensile properties of the fusion zone, due to deformation constraint imposed by the stronger base material. A problem arises as such constraint effect depends on the size and geometry of the specimen, and the loading mode. To analyze deformation and failure of structures consisting of aluminium welds, intrinsic tensile properties of the fusion zone is definitely needed. The present paper presents limit analyses of undermatched tensile specimens to quantify the effect of strength mismatch on resulting tensile curves. For a given strength mismatch, the weld width is also systematically varied to see its effect. Furthermore, to get insight on the effect of deformation constraint on fracture, detailed fully plastic stress fields from FE limit analysis are reported.
Inconel 617 노치시편의 상온 및 고온 인장실험 해석
오창식(Chang-Sik Oh),마영화(Young-Wha Ma),윤기봉(Kee-Bong Yoon),김윤재(Yun-Jae Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
In this paper, notched bar tensile tests of Inconel 617 were performed at room (20℃) and elevated (800℃) temperature. Finite element analyses are also performed. It is found that, at the room temperature, smooth bar tensile test results could be used to simulate notched bar tensile tests. However, at the elevated temperature, notched bar tensile test results can not be simulated from smooth bar tensile test results. Metallurgical examination reveals that strength weakening results from many cavities over the specimens for smooth bar test at the elevated temperature. “True” tensile properties at the elevated temperature is found using FE simulations. It also suggests that cautious should be taken to determine tensile properties of Inconel 617 at elevated temperatures using smooth bar tests.
오창식(Chang-Sik Oh),김윤재(Yun-Jae Kim) 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.6
In this paper, the coupled model with hydrogen transport and elasto-plasticity behavior is introduced. This model is implemented to the general-purpose FE code, ABAQUS, via the user-defined subroutine UMAT and UMATHT. In UMAT, the spatial gradients of hydrostatic stress and hydrogen induced deformation are calculated, and then are passed into UMATHT. Heat transfer equation within UMATHT is substituted by hydrogen transport equation including the effects of stress states and strain hardening. To validate this model, the finite element analyses coupled with hydrogen transport and mechanical loading are performed for the boundary layer specimens with low and high strength steel properties. The FE results are compared with the previous studies by Taha and Sofronis (2001).
대변형 효과를 고려한 90 도 엘보우 감육 한계 모멘트
오창식(Chang-Sik Oh),김윤재(Yun-Jae Kim),박치용(Chi-Yong Park),이성호(Sung-Ho Lee),김태룡(Tae-Ryong Kim) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11
Based on (3-D) FE limit analyses using large geometry change option, this paper provide plastic limit load solutions for pipe bends with local wall-thinning under in-plane bending. Plastic limit loads are determined from load-displacement curves using TES (Twice-elastic slope) method. For the bending mode, both closing and opening bending are considered. Wall-thinning both in extrados and in intrados is considered. Combinations of bending modes and wall-thinning locations lead to different stress states (tension or compression) over wall-thinning regions. These are significantly affecting plastic limit loads. Other relevant variables affecting plastic limit loads are varied, related to pipe bend geometry (mean radius, thickness and bend curvature) and defect geometry (length, depth and width of wall-thinning). Based on FE results, closed-form approximations of plastic limit loads are proposed for pipe bends with local wall-thinning under in-plane bending.
오창식(Chang-Sik Oh),김낙현(Nak-Hyun Kim),김윤재(Yun-Jae Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
This paper introduces structural integrity assessment by using Finite Element analysis based on damage mechanics. Several FE damage methods as like GTN model have been proposed up to the present. These damage models have their own advantages and disadvantages. It is important to select the proper damage model for the integrity assessment of the structure in interest. In this paper, selected several damage models are apply to simulate fracture behaviours of structures with various geometries, and the FE results are compared with the experimental results. These models are implemented to general purpose FE program, ABAQUS, via user-defined subroutines.
소규모 천이 크리프 조건에서 C<SUB>t</SUB>와 유한요소 영역적분 C와의 관계
오창식(Chang-Sik Oh),김윤재(Yun-Jae Kim),윤기봉(Kee-Bong Yoon) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
This paper proposes the relationship between Ct and contour integral C under small-scale, via elastic-creep finite element analyses. Like J for elastic-plastic fracture, C<SUB>t</SUB> and C(t) characterize the singular crack tip stress and strain rate field. C<SUB>t</SUB> is experimentally evaluated based on the expansion of a creep zone at the crack tip, whereas C(t) can be numerically evaluated using a contour integral definition at the vicinity of the crack tip. C(t) values are typically lower than C<SUB>t</SUB> values, and contour integral can shows significant path dependence under transient creep conditions. However, FE results shows that the contour integral value at the certain amount of distance from crack tip is identical to C<SUB>t</SUB> value. In this paper, relationship between C<SUB>t</SUB> and contour integral C is investigated by using creep zone size.
유한요소 간략기법을 이용한 곡관의 shakedown 하중 예측
오창식(Chang-Sik Oh),이국희(Kuk-Hee Lee),김윤재(Yun-Jae Kim) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6
This paper provides shakedown limit loads for pipe bends subject to constant internal pressure and cyclic bending moment, via simplified finite element analysis technique. The simplified technique requires two FE analyses; one elastic analysis, and the other elastic-perfectly plastic analysis. The technique is verified through the Bree cylinder problem, by comparing with the results from full inelastic analyses. Then the technique is applied to pipe bends to derive shakedown interaction diagrams for constant internal pressure and cyclic bending moment loading.
Code of Practice를 이용한 Inconel617 재료의 다축응력 크리프 실험
오창식(Chang-Sik Oh),김희진(Hee-Jin Kim),마영화(Young-Wha Ma),윤기봉(Kee-Bong Yoon),김윤재(Yun-Jae Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10
In this paper, notched bar creep tests of Inconel 617 were performed at elevated (800℃) temperature. The notch depth ratio, a diameter of parallel portion to diameter at notch plane, is fixed with 2. This is different from that of code of practice, 1.41, but new skeletal stresses are obtained using the stress analysis procedure in COP. This shows that the skeletal stresses are still valid. In this paper, the multi-axial stress rupture criterion of Inconel 617 was provided.
축방향 표면균열이 존재하고 내압을 받는 직관의 J 예측 방법
오창식(Chang-Sik Oh),송태광(Tae-Kwang Song),김윤재(Yun-Jae Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
In the present paper, limit pressures for axial surface cracked pipe are proposed, and a reference stress based J estimation method is also provided based on the proposed limit pressure solutions. Employed cracks are assumed to be constant-depth, internal surface cracks, and wide ranges of variables are considered.