• Proceedings of the Korean Nuclear Society Conference
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• 1996.11b
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• pp.463-468
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• 1996

질소의 함량을 0.04%~0.15% 까지 변화시킨 316L 스테인레스 강으로 공기 중에서 저주기 피로시험을 수행하였다. 전체변형범위 1%, 변형속도 2$\times$$10^3$/sec, 삼각파로 상온 ~$600^{\circ}C$ 온도범위에서 시험을 수행하였다. 상온에서는 사이클이 진행됨에 따라서 연화가 계속해서 발생하지만 온도가 증가하면 초기에 경화가 발생한다. 피로시험 초기에 경화되는 정도와 saturation 응력은 온도가 증가하면 증가한다. 이러한 경화현상은 동적변형시효에 의해서 발생되는 것으로 판명되었다. 질소를 첨가하면 강도는 증가하지만 경화는 감소되었다. 질소에 의한 경화의 감소는 질소가 동적변형시효를 억제하기 때문이다.

• Journal of the Earthquake Engineering Society of Korea
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• v.7 no.5
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• pp.47-56
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• 2003

In this paper, the cyclic soil behavior model. which can accommodate the cyclic hardening, was developed by modifying the original parallel IWAN model. In order to consider the irrecoverable plastic strain of soil. the cyclic threshold strain, above which the backbone curve deviates from the original curve, was defined and the accumulated strain was determined by summation of the strains above the cyclic threshold in the stress-strain curve with applying Masing rule on unloading and reloading curves. The isotropic hardening elements are attached to the original parallel IWAN model and the slip stresses in the isotropic hardening elements are shown to increase according to the hardening functions. The hardening functions have a single parameter to account for the cyclic hardening and are defined by the symmetric limit cyclic loading test in forms of accumulated shear strain. The model development procedures are included in this paper and the verifications of developed model are discussed in the companion paper.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05c
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• pp.253-258
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• 1996

본 연구는 Zr-2.5wt%Nb합금의 고온변형특성중 특이한 유동응력의 변화거동인 어닐경화와 유동연화의 기구에 대해 고찰하였다. 연구결과에서 이러한 고온변형특성은 온도와 다단변형도중의 어닐링 시간에 영향을 받는 것으로 나타났으나 분위기 영향은 없는 것으로 나타났다. 즉, 변형 온도와 동일온도인 85$0^{\circ}C$에서 단시간인 5분동안 어닐링을 했을때 경화현상이 일어났다. 미세조직을 관찰한 결과 다단열간압연된 후의 미세조직과 집합조직은 다단열간압연된 미세조직과 집합조직과 직접적인 상관관계를 가진 것으로 나타났다. 어닐경화현상은 다단고온변형이 궁극적으로 변형시효와 유사한 효과 즉, 중간어닐링 동안에 Zr-2.5wt%Nb내의 불순물 원자와 열간압연시에 형성된 전위구조와의 강력한 인력형 상호작용하여 substructure가 안정화되기 때문이며 유동연화는 재결정으로 인한 유동응력의 감소인 것으로 사료된다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.5
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• pp.677-682
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• 1985

극치해석적인 상계정리를 기초로 하여, 평면변형 인발에서 발생하는 내부결함(central bursting)을 예측하기 위해 중심에서 공동(voids)을 가진 금속에 대해 비례흐름(proportional flow)과 내부결 함의 흐름을 비교하여 해석하였다. 이 결함을 촉진시키는 공정조건에 대한 판정식(criterion)을 변형경화 금속에 대해 유도하였다. 공동을 가진 금속은 공동들을 축소시키기 위해 정상적인 재 료(sound material)의 흐름과 동일 방법으로 흐를 수 있으며, 경우에 따라서는 내부결함을 확장 하기 위해 흐를 수도 있다. 본 연구에서는 다이의 경사각, 단면감소율 및 마찰 등의 어떤 공정 변수 영역에서 중심축 상에 많은 공동을 가진 변형경화 금속에서도 내부결함이 발생할 수 있다는 것이다.

• Journal of Korean Society of Steel Construction
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• v.29 no.1
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• pp.1-12
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• 2017

This paper proposes nominal flexural strength considering strain-hardening effect of HSB600 high performance steel for compact composite I-girders in positive bending. Unlike conventional steels, HSB600 undergoes strain-hardening just after yielding without going through yield plateau. However, because the nominal flexural strength specified in domestic and foreign bridge design specifications has been developed for the conventional steel composite girders, the nominal flexural strength does not appropriately consider the strain-hardening of HSB600. Therefore, plastic moment considering a strain-hardening is proposed so as to consider effect of the strain-hardening of HSB600 on flexural strength and then moment-curvature analysis is performed to a wide range of cross-sections. From results of the analysis, a parameter representing the effect of the strain-hardening on the flexural strength of HSB600 composite girders is proposed. Furthermore, by using this parameter, the nominal flexural strength considering the strain-hardening effect for HSB600 composite I-girders in positive bending is proposed and then evaluated by comparing with the current AASHTO LRFD bridge design specifications.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.11
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• pp.6464-6471
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• 2014

The UV curing method is a popular process for lens molding on a unit wafer. This process, however, has several drawbacks including wafer adhesion during the ejection process after curing, errors in lens shape and wafer warpage due to material shrinkage during the curing process, and lens centering errors on both sides of a wafer. Among these, the lens shape error and warpage are influenced directly by the UV curing process due to factors including the UV radiation uniformity, the degree of cure according to UV intensity, and the shrinkage characteristics of the material. Therefore, a theory is needed not only to understand the change in the material characteristics, such as the shrinkage rate due to the curing reaction, but also to establish a model. In addition, an analysis system is needed to realize the model. This study proposes a new analysis method for the wafer lens molding process by Comsol modeling. This method was verified by comparing the results with those of the actual process.

• Bulletin of the Society of Naval Architects of Korea
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• v.26 no.2
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• pp.41-48
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• 1989

This paper investigates the relationship between strain hardening exponents(n) of various marine materials and fatigue crack retardation effect after applying single overload. Using the four different sheet materials, following results are obtained. (1) The fatigue crack propagation rate after applying single overload was retarded and the effects of this retardation were closely related to the strain hardening exponent. (2) The larger the strain hardening exponents were, the more were the fatigue crack retardation effects after applying single overload. (3) The considerable crack closure with the applying of a overload was observed in matrals with large strain hardening exponent. When n is smaller than 0.1, the fatigue crack retardation effects are negligible. On the contrary, when n is larger than 0.2 the fatigue crack retardation effects are significant.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.9
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• pp.5769-5777
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• 2015

According to the forming or bending deformation in the press die, the thin plate occurs a work-hardening, the sheet hardening and cam unit's deformation causes incomplete forming during the cam molding process by the reacting spring forces. This study treated the input parameters of the stress and strain as given properties and also used Cam forming pressure considering the sheet hardening in the forming process of the aluminum sheet. The Hyperstudy are operated be linked with the Abaqus of the finite element analysis tool and the shape of Cam were carried out with non-linear shape optimization analysis. As a result removing the deformation of plate, the cam shape were optimized under conditions reduced deformation, having a minimum stress range and the minimum deformation. Therefore, a stress-strain curve and a normal distribution of stress-thickness can be obtained and optimization could be obtained for the shape of the stress and strain on the die plate hardened cam considering the thickness and reaction force of gas spring as iteration process.

• Journal of the Korea Concrete Institute
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• v.25 no.4
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• pp.371-379
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• 2013

This paper investigates the cracking and tension-stiffening behavior of 100 MPa shrinkage-compensated strain-hardening cement composite (SHCC) and conventional concrete tie elements in monotonic and cyclic tension. Strain and surface crack formation of tension ties were monitored with two strain displacement transducers and a photo microscope with a lens of magnification 50 times. Three different cement composites such as conventional concrete, shrinkage-compensated SHCC, and normal SHCC were used in the tie specimens to investigate the influence of the cement composite type on the tension stiffening and cracking behavior. Test results indicated that initial shrinkage of the ultra high-strength cement composites is greatly reduced as the 10% replacement of cement by the shrinkage-compensating admixture based on calcium sulfo-aluminate (CSA). The test results on the SHCC tension ties showed that the first cracking load decreases proportionally to the initial shrinkage strain. Reinforced ultra high-strength SHCC ties with the initial shrinkage compensation exhibited improved tension stiffening and smaller crack spacings, i.e. the reduction in crack width. Cyclic loading did not have a significant effect on tension stiffening and cracking behavior of tension ties with normal concrete and SHCC materials.

• Journal of Korean Society of Steel Construction
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• v.19 no.3
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• pp.291-301
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• 2007

The overlay model is a certain kinds of numerical analysis method to present the material non-lineariy which is represented the baushinger effect and the strain hardening. This model simulates the complex behavior of material by controlling the properties of the layers which like the hardening ratio, the section area and the yield stress. In this paper, the constitutive equation and plastic flow rule of each layer which are laid in the plane stress field are obtained by using the thermodynamics. Two numerical examples were tested for the validity of proposed method in uniaxial stress and plane stress field with comparable experimental results. The only parameter for the test is the yield stress distribution of each layers.