• Transactions of Materials Processing
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• v.22 no.2
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• pp.101-107
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• 2013

The heat transfer characteristics between die and sheet and die and coolant are important parameters in hot press forming process. The determination of the quenching time that guarantees full martensitic transformation requires proper understanding of these heat transfer characteristics. The contact area changes drastically during the quenching process due to volume changes of both die and sheet by temperature drop as well as phase transformation. Several types of modeling techniques are tested in order to select the most suitable. The effect of quenching time as well as die heat conductivity on martensitic transformation is investigated and predictions are compared to experimental results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.4 s.235
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• pp.606-613
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• 2005

Effects of circumferentially local wall thinning on the fracture behavior of pipes were investigated by monotonic four-point bending. Local wall thinning was machined on the pipes in order to simulate erosion/corrosion metal loss. The configurations of the eroded area included an eroded ratio of d/t= 0.2, 0.5, 0.6, and 0.8, and an eroded length of ${\ell}\;=10mm,$ 25mm, and 120mm. Fracture type could be classified into ovalization, local buckling, and crack initiation depending on the eroded length and eroded ratio. Three-dimensional elasto-plastic analyses were also carried out using the finite element method, which is able to accurately simulate fracture behaviors excepting failure due to cracking. It was possible to predict the crack initiation point by estimating true fracture ductility under multi-axial stress conditions at the center of the thinned area.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.5 s.248
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• pp.602-608
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• 2006

Vehicle components such as lower control arm are usually affected by heat during the welding process. As a result, residual stress is generated, which has much effect on mechanical performances such as crashworthiness and durability. In this study, the residual stress in lower control arm has been measured by the x-ray diffraction method and been analyzed by finite element methods. Heat transfer during seam weld process has been calculated and used in calculating thermal deformation with temperature dependent material properties. High residual stress has been found at vertical wall both by measurement and simulation. The simulation also showed the residual stress re-distribution when the component is subjected to cyclic loading condition.

• 한국공간정보시스템학회:학술대회논문집
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• 2004.05a
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• pp.83-88
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• 2004

First author proposed a proportioning method for member sections of a single layer reticulated dome subjected to uniform and non-uniform load without any geometrical initial imperfection, and discussed the validity and effectiveness of the method which was based on linear buckling stress and a knock down factor. However, buckling of a single layer reticulated dome is strongly affected by initial imperfection. It is well known that geometrical initial imperfections reduce the nonlinear buckling capacity of a single layer raticulated dome. Thus, structural engineers may be recommended to reflect the effects of geometrical initial imperfections in proportioning member sections. In this paper, firstly, the presented proportioning method by first author is applied to dome without consideration of any imperfections and the thickness and diameter of each member are determined. Secondly, the load bearing capacities of the proportioned domes are checked with the imperfection, by the inelastic buckling analysis.

• 한국터널공학회:학술대회논문집
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• 2005.04a
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• pp.213-224
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• 2005

The safety factor of a tunnel considering the failure of supports is important because the failure of supports might cause the collapse of the tunnel. In the previous studies, shotcrete was modelled as beam elements and the failure of the shotcrete was checked according to the allowable working stress concept. In this study, shotcrete was modelled by both beam elements and continuum (elasto-plastic) elements. Safety factors of tunnels were estimated by two dimensional numerical analysis with varying rock mass class, coefficient of lateral pressure, thickness of shotcrete, rock bolt reinforcement and excavation method. Also the study suggested not only a proper amount of supports but also modelling method.

• Journal of the Earthquake Engineering Society of Korea
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• v.4 no.3
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• pp.45-53
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• 2000

강재 교각을 갖는 고가교량은 상부구조가 매우 큰 질량을 갖는 거대구조가 되고 규모가 큰 지진운동 하에서 대단히 큰 관성력을 받게 된다. 따라서 탄소성 동적응답 해석에 의해서 강재 교각의 지진거동을 파악하는 것이 필요하다 . 본 연구에서는, 탄소성 동적응답해석을 위한 합리적인 수치해석방법을 제시하고 이를 바탕으로 강재 교각에 대한 내진성 평가를 수행한다. 1995년 고베 지진 시 손상을 받은 강재 교각과 그 이후 재구축된 교각을 모델로 해서 국부좌굴 이전 소성화의 영향만을 고려한 강재 교각의 지진 거동을 파악한다. 입력지진파는 고베 지진시 관측된 Takatori 지진파이고 이를 가속도 진폭 조정하여 사용한다.

• Journal of the Korean Society for Railway
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• v.7 no.2
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• pp.142-148
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• 2004

When material properties depend much on positions in a material or it is difficult to make test specimens from a material or component, an instrumented indentation test described in ISO 14577-1, 14577-2 or KS B 0950 can be used to measure material properties and damage. In this study, first of all, the principals of the instrumented indentation test, KS B 0950 are introduced and yield strengths, tensile strengths and work hardening exponents of base materials, heat affected zones and weld materials are measured. In addition, the influence of post-weld heat treatment on the material properties is investigated. Finally the fatigue lift of butt welded specimens are evaluated by the local strain approach. To calculate local strains and stresses, elasto-plastic finite element analysis is conducted using the measured properties.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.04a
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• pp.30-34
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• 2000

In this paper we analyzed bulging condition which affect the quality of continuous casting steel by using the numerical analytic method. First solidification analyses are performed for each cooling zones. Solidification analysis are carried out by one-dimensuional finite difference method. The bulging deformation of cast slab has been calculated with a two-dimensional elasto-plastic and creep finite element model. The adequacy of the model has been checked against the experimental results. From this study the effects of the process variables such as casting speed cooling condition roll pitch are examined. The results from these analyses would be able to apply to the design of continuous casting process.

• Journal of Welding and Joining
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• v.22 no.2
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• pp.51-58
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• 2004

Residual stresses, which can be produced during the welding process, play an important role in an industrial field. Welding residual stresses are exerting negative effect on the fatigue behavior and integrity of structure. In this study, as a result of the thermal elasto-plastic finite element analysis for the welds of a nuclear component, the residual stress distributions are estimated for as-welded condition. Also, finite element techniques are developed to simulate the relaxation of the residual stresses according to the various mechanical stress relieving(MSR) loads such as hydrostatic pressure loading, tensile pipe-end loading, and mechanical stress improvement process(MSIP) loading. Finally, the results of residual stress redistributions for various loading conditions are compared and reviewed qualitatively and quantitatively to find an optimum loading condition.

• Journal of the Korean Geotechnical Society
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• v.16 no.2
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• pp.23-30
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• 2000

The use of strut-preloading method is gradually increasing in braced excavations in Korea. And it is necessary to analyze the effects of strut preloading on the wall deflection, wall bending moment and strut axial force, etc. In this study, by using the analysis method of beams on elasto-plastic foundations, measured data and calculated results of 2 sites are compared and parametric studies of correlation between preloading and earth retaining structures in sandy soils are carried out in strut preloading application. As results, about 50%~75% of design strut load is effective as preloading force in considering the displacement and member forces of earth retaining structures. And the effective stiffiness of strut should be at least 25% of th ideal value in order to restrain the excessive increase of wall deflection and bending moments. As one of some methods to prevent excessive movements in braced excavation, to preload the strut is confirmed as more effective way than to increase the stiffiness of strut in braced wall, if the excessive axial force of strut due to preloading can be avoided.