• Journal of Korean Society of Steel Construction
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• v.18 no.6
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• pp.687-696
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• 2006

An efficient numerical method is developed to estimate the elasto-plastic post-buckling strength of space-framed structures. The inelastic ultimate strength of beam-columns and frames is evaluated by the parametric study. Applying the improved plastic hinge analysis that evaluate the gradual stiffness decrease effects due to spread of plasticity, elasto-plastic post-buckling behavior of steel frames is investigated considering the various residual stress distributions. Introducing the plastification parameter that represent pread of plasticity in the element and performing parametric study of equivalent element force and member idealization, finite-element solutions for the elasto-plastic analysis of space frames are compared with the results by plastic region analysis, shell elements and experimental results.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.10
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• pp.73-78
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• 2003

MEMS technology in the field of aerospace engineering is more important with light weight and high resolution. Therefore the investigation of thin films properties is issued and the residual stress of thin filrns is one of the important problems to solve. Ion implantation without thermal annealing is applied for the stress gradient relaxation of LPCVD polysilicon films used as the structural part in MEMS. He+ and Ar+ ion implantations reduce the stress gradient of polysilicon films. The property modification of polysilicon films by ion implantation is also investigated. The elastic modulus and hardness of polysilicon films with ion implantation is studied by CSM method which is an advanced nano-indentation method. Ion implantation decreases the elastic modulus and hardness of polysilicon films. However, they are improved with increasing ion dose.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.19 no.2
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• pp.102-108
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• 2023

A Chloride-induced stress corrosion cracking (CISCC) of austenite stainless steel in dry cask storage system (DCSS) can occur with extending service time than originally designed. Cold spray coating (CSC) not only form a very dense microstructure that can protect from corrosive environments, but also can generate compressive stress on the surface. This characteristic of CSC process is very helpful to increase the resistance for CISCC. CSC with several powders, such as 304L, 316L and Ni can be optimized to form very dense coating layer. In addition, the impact energy generated as the CSC powder collides with the surface of base metal at a speed of Mach 2 or more can remove the residual tensile stress of welding area and serve the compress stress. CSC layers include no oxidation and no contamination with under 0.2% porosity, which is enough to protect from the penetration of corrosive chloride. Therefore, the CSC coating layer can be accompanied by a function that can be disconnected from the corrosive environment and an effect of improving the residual stress that causes CISCC, so the canister's CISCC resistance can be increased.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.7 no.5
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• pp.120-126
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• 1998

Second step shot peening was applied on both smooth specimen and U-notch specimen in order to investigate the stress distribution and the improvement in fatigue strength. Various experiments and measurements such as rotary bending fatigue test and the measurement of compressive residual stress were performed. The results showed that the fatigue strength of second step shot peened specimens increased by 34 percent compared to that of unpeened ones. Compressive residual stress also considerably increased, which resulted in the increase of fatigue strength. finite element analysis showed that shot peening is effective in decreasing the bending stress by external force. The effectiveness of shot peening in reducing the compressive residual stress was anticipated by the superposition of the concentrated stress and the compressive residual stress.

• Journal of the Korean Ceramic Society
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• v.40 no.8
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• pp.770-776
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• 2003

#140 mesh and #600 mesh wheels were adopted to grind (111) and (100) oriented single crystalline silicon wafer and the grinding induced change of the surface integrity was investigated. For this purpose, microroughness, residual stress and phase transformation were analyzed for the ground surface. Microroughness was analyzed using AFM (Atomic Force Microscope) and crystal structure was analyzed using micro-Raman spectroscopy. The residual stress and phase transformation were also analyzed after thermal annealing in the air. As a result, microroughness of (111) wafer was larger than that of (100) wafer after grinding. It was observed using Raman spectrum that the silicon was transformed from diamond cubic Si-I to Si-III(body centered tetragonal) or Si-XII(rhombohedral). Residual stress relaxation was also shown in cavities which were produced after grinding. The thermal annealing was effective for the recovery of the silicon phase to the original phase and the residual stress relaxation.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.19 no.4
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• pp.539-547
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• 2010

Laser shock peening is useful to improve fatigue characteristic of multiple number of metals and alloys. This process induces a compressive residual stress on the metal surface, and when tensile load is applied, growth of crack is delayed and which changes the characteristic of the metal surface. It is an innovative surface treatment technique for strengthening metals. Specimens of SM45C are used in this study. The effect of an inertial tamping layer on the residual stress field using laser shock peening setup and Nd:YAG laser power is evaluated. Residual stress distribution measured by X-ray diffraction. As a result of this study it can be presented that following condition of Nd:YAG laser power and inertial tamping layer parameters, compressive residual stress is generated on the surface of the SM45C. Results to experimental data indicate that laser shock peening has great potential as a means of improving the mechanical performance of the metal surface.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1993.10a
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• pp.27-33
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• 1993

천마 연소관 및 Bracket는 고강도 특수강의 일종인 Maraging steel C-250 Grade로서 유동성형공정 (flow forming process: F/F) 및 용접공정을 거쳐 시효경화후의 경도(HRC 48-52)가 높아 절삭가공 및 Tapping에 어려움이 있다고 판단되어 브라켓 밀링가공(Tapping포함)을 시효경화공정 이전에 완성하는 것으로 공정 FLOW를 설정하였으나, 시효경화 시 유동성형공정 잔류응력 및 재질특성에 의한 수축, 변형 등으로 도면상 요구된 품질(형상 및 위치공차) 만족이 미흡하였을 뿐만 아니라 오히려 전체 공정 수만 증가하였음. 따라서 연소관 및 Bracket 완성가공을 시효경화 후에 실시하는 것으로 공정 개선 하고자 시험 작업한 결과, 선삭, 밀링작업등 다른 기계가공 공정의 문제점은 대부분 해결할 수 있었으나, Tapping공정만은 해결할 수 없어서 진정한 공정개선을 기할 수 없었음. 그러나, 제품의 품질 및 생산성을 고려 시효경화 후 Tapping 공정실시 필요성이 강력히 대두되어 Maraging steel 재질특성에 적합한 공구 및 작업조건을 검토, 설정 시험작업 함으로써 공정개선을 이룰 수 있었음.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.2 no.4
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• pp.67-72
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• 2003

In this study, to investigate the effects of 2-step shot peening at the surface of spring steel, crack growth tests are conducted on spring steel and shot peened specimens. And then the residual stresses and fractographs are examined. The crack growth equation that can describe the whole crack growth behavior is used to evaluate the experiment results. The results show that fatigue crack glows slowly in the shot peened specimen than in the unpeened. And in the case of the 2-step shot peened specimen the initial stress intensity factor range and the fracture toughness is higher than the unpeened specimen due to the compressive residual stress. Fractographs show that the compressive residual stress of the surface suppress the fatigue crack opening and consequently slow crack growth rates.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.7 no.4
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• pp.1-11
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• 2011

This paper presents the effects of preventive maintenance schemes on the residual stress distributions in dissimilar metal welds. Dissimilar metal weld is known susceptible to PWSCC and thus, effective maintenance schemes to prevent PWSCC are needed. Three preventive maintenances schemes, i.e. weld overlay, MSIP and inlay weld which are widely used in nuclear power plants, are selected and their effects on welding residual stresses are investigated via finite element analyses. As results, weld overlay and MSIP were proved effective method to mitigate residual stresses and inlay weld, on the other hand, produces strong tensile residual stresses in the inner surface. Although Alloy 690 known to be resistant to PWSCC are used in inlay weld, continuous careful observation are needed since tensile welding residual stresses are key parameter for PWSCC.

• Bulletin of the Society of Naval Architects of Korea
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• v.30 no.3
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• pp.23-27
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• 1993

현재까지 SFCP 해석법은 기초단계에 있으며, 아직도 정립된 방법이 없는 것이 사실이다. 따라서, 이 분야에는 많은 개선을 필요로 하고 있다. 이와 같은 개선은 이론적인 연구뿐만이 아니라, 실험적인 연구가 바탕이 되어야 라며, 동시에SFCP에 영향을 주는 초기균열의 변동성, 하중의 변동성 등을 정확한 모델링 방법에 대한 연구가 병행되어야 한다. 그리고 더 나아가서 실제 구 조물에서 피로파괴에 영향을 주는 중요한 요소인 잔류응력, 부식 등의 고려하는 방법에 관한 연구가 수행 되어져야 할 것이다. 또한, 지금까지의 연구가 주로 구조부재에 하나의 균열이 존 재한다는 가정을 내포하고 있는데, 실구조물에 적용하기 위해서는 여러개의 균열이 동시에 존 재하는 경우에 대한 연구와 균열이 성장하면서 합체(coalescence)하는 경우에 대한 연구도 수행 되어야 한다. 이와 같은 연구가 꾸준히 진행되어 소기의 성과를 거둠으로써, 구조물의 피로파괴 확률을 정확하게 추정할 수 있을 것이며, 이에 따라 합리적인 설계가 가능해질 것이다.