• 비파괴검사학회지
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• 제26권5호
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• pp.306-314
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• 2006

가동 중인 설비 및 대형 구조물은 장시간 사용, 고온 환경 및 반복되는 하중 등의 영향으로 설비재료의 교체 및 유지 보수가 요구된다. 이때 설비의 기계적 특성을 평가하는 것은 필수 불가결한 요소 이지만 대부분의 물성평가방법이 파괴적이기 때문에 가동 중인 설비에 직접 적용하는 것은 상당한 어려움이 따른다. 그러나 계장화 압입시험법은 다양한 기계적인 특성을 비파괴적으로 측정하는 최신기술로서 재료에 하중 인가 및 제거 과정 중 하중과 변위를 연속적으로 측정하여 획득된 압입하중-변위곡선의 분석을 통해 유동물성, 잔류응력, 파괴인성 등의 기계적 특성을 평가 할 수 있다. 본 연구에서는 계장화 압입시험을 이용하여 철강재료 및 용접부 유동물성과 잔류응력을 정량적으로 평가하였다. 계장화 압입시험 시 발생하는 압입자 하부의 응력 상태를 고려하여 유동응력과 변형률을 정의하고, 이를 최적화된 응력-변형률 구성방정식을 통해 유동곡선 및 항복강도, 인장강도 등의 유동물성을 평가 하였다. 계장화 압입시험을 이용하여 잔류응력을 측정하기 위해 소성변형과 직접 관련된 편차 응력 성분만으로 압입변형과 잔류응력 간의 상호작용을 분석하여 잔류응력 모델을 정의하였다. 측정된 유동물성은 일축인장시험의 결과를 통해 그 정확성을 검증하였고, 잔류응력은 홀-드릴링, 절단법 및 ED-XRD 시험과 비교하여 그 모델을 검증하였다.

• 한국전산구조공학회논문집
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• 제32권5호
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• pp.273-278
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• 2019

본 논문에서는 압입시험을 통해서 초탄성 재료 물성치를 평가하는 간단한 방법을 제시하였다. 초탄성 재료 모델 중, 3개의 물성치($C_{10}$, $C_{20}$, $C_{30}$)를 가지는 Yeoh 모델을 선택하여 주연신률로 표현되는 변형률 에너지 밀도를 적용하였다. Yeoh 물성치를 변화시키며, 구형 압입시험 유한요소해석을 수행하여 압입자 반력-변위 곡선을 획득하였다. 압입자 반력-변위 곡선을 3차 다항식으로 근사하였고, 이 다항식을 물성치($C_{10}$, $C_{20}$, $C_{30}$)의 3차 곱으로 근사된 3차 다항식으로 표현하였다. 압입자 반력-변위곡선 근사를 위해 회귀분석을 진행하여 수식들의 계수를 결정하였으며, 이 회귀식을 이용하여 초탄성 재료의 물성치를 평가하였다. 초탄성 재료 물성치 평가를 수행하고 오차를 비교하여 유효성을 보여 주었다.

• 한국세라믹학회지
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• 제44권7호
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• pp.368-374
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• 2007

CVD-SiC coating has been introduced as a protective layer in TRISO nuclear fuel particle of high temperature gas cooled reactor (HTGR) due to its excellent mechanical stability at high temperature. In order to prevent the failure of the TRISO particles, it is important to evaluate the fracture strength of the SiC coating layer. It is needed to develop a new simple characterization technique to evaluate the mechanical properties of the coating layer as a pre-irradiation step. In present work, direct strength measurement method with the specimen of hem i-spherical shell configuration was suggested. The indentation experiment on a hemisphere shell with a plate indenter was conducted. The fracture strength of the coating layer is related with the critical load for radial cracking of the shell. The finite element analysis was used to drive the semi-empirical equation for the strength measurement. The SiC hemispherical shells were successfully recovered from the section-grinding of TRISO coated particle and successive heat treatment in air. The strength of CVD-SiC coating layer was evaluated from the experimentally measured critical load during the indentation on SiC hemisphere shell. Weibull diagram of fracture strength was also constructed. This study suggested a new strength equation and experimental method to measure the fracture strength of CVD-SiC coating of TRISO coated fuel particles.

• Composites Research
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• 제24권2호
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• pp.9-13
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• 2011

본 논문에서는 zinc blende계열의 결정구조를 갖는 실리콘 내부의 결함을 분석할 수 있는 대칭축 파라메터 (Symmetric axis parameter)방법을 이용하여, 탄성 및 소성 변형을 구별하는 방법을 제시하였다. 분자 동력학 해석프로그램인 LAMMPS를 사용하여, 실리콘에 대한 나노인덴테이션 해석을 수행하였다. 구형 인덴터 아래에 발생한 실리콘내부의 결함은 ring crack에서의 threefold 무늬와 전위발생경로를 보여주였다. 또한, 해석결과는 기존의 이론이나, 실험결과와도 일치하는 것을 확인하였다.

• Tribology and Lubricants
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• 제30권4호
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• pp.199-204
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• 2014

Various heat-treated and surface coating methods are used to mitigate abrasion in sliding machine parts. The most cost effective of these methods involves hard coatings such as diamond-like carbon (DLC). DLC has various advantages, including a high level of hardness, low coefficient of friction, and low wear rate. In practice, a supporting layer is generally inserted between the DLC layer and the steel substrate to improve the load carrying capacity. In this study, an indentation and sliding contact problem involving a small, hard, spherical particle and a DLC-coated steel surface is modeled and analyzed using a nonlinear finite element code, MARC, to investigate the influence of the supporting layer thickness on the coating characteristics and the related coating failure mechanisms. The results show that the amount of plastic deformation and the maximum principal stress decrease with an increase in the supporting layer thickness. However, the probability of the high tensile stress within the coating layer causing a crack is greatly increased. Therefore, in the case of DLC coating with a supporting layer, fatigue wear can be another important cause of coating layer failure, together with the generally well-known abrasive wear.

• 한국분말재료학회지
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• 제5권1호
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• pp.15-21
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• 1998

The effect of $\alpha$/$\beta$ phase on the mechanical properties and contact damage of silicon nitrides $Si_3N_4$) was investigated. Silicon nitride materials were prepared from two starting powders, at selective increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse $\alpha$-phase powder, essentially equiaxed $\alpha$-$Si_3N_4$ grains, with limited, slow transformation to $\beta$-$Si_3N_4$ grain; (ii) from relatively fine $\alpha$-phase powder, a more rapid transformation to $\beta$-$Si_3N_4$, with attendant grain elongation. The resulting micro-structure thereby provided a spectrum of $\alpha$/$\beta$ phase ratios, grain sizes, and grain shapes. Fracture strength, hardness, and toughness were measured, and contact damage and strength degradation after indentation were investigated by Hertzian indentation using spherical indenter. A brittle to ductile transition in $Si_3N_4$ depended on $\alpha$/$\beta$ phase ratio as well as grain size. Silicon nitride with elongated $\beta$ grains showed a superior, contact damage resistance.

• Macromolecular Research
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• 제17권8호
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• pp.568-574
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• 2009

We investigated the sintering and consolidation phenomena of silver nanoparticles under various thermal treatment conditions when they were patterned by a contact printing technique on polyimide substrate films. The sintering of metastable silver nanoparticles commenced at 180 $^{\circ}C$, where the point necks were formed at the contact points of the nanoparticles to reduce the overall surface area and the overall surface energy. As the temperature was increased up to 250 $^{\circ}C$, silver atoms diffused from the grain boundaries at the intersections and continued to deposit on the interior surface of the pores, thereby filling up the remaining space. When the consolidation temperature exceeded 270 $^{\circ}C$, the capillary force between the spherical silver particles and polyimide flat surface induced the permanent deformation of the polyimide films, leaving crater-shaped indentation marks. The bonding force between the patterned silver metal and polyimide substrate was greatly increased by the heat treatment temperature and the mechanical interlocking by the metal particle indentation.

• 한국해양공학회지
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• 제12권3호통권29호
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• pp.42-48
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• 1998

구형 관입시험에 의한 얼음의 균열을 연구 하였다. $-10^{circ}C$에서 S2 기둥얼음의 시편(152mm X 152mm X 152mm)에 stainless 강으로 된 구(지름 25.4mm)로 하중을 가하였다. 구형indentor는 얼음 시편의 장축인 기둥방향에 수직으로 하중을 가하였으며 이때 변위율은 0.038mm/s로 하여 단조증가 하중 시험을 하였다. 하중을 가하기 시작하면 indentor 하부에서 crushing 이 발생하고, 하중이 증가함에 따라서 방사선 균열 또는 횡균열이 성장하여 splitting 또는 spallation이 발생하였다. 단조증가 하중 때와 동일한 indentor를 사용하여 하중 및 비하중율 0.5KN/s로 맥박하중을 가할 때 이들 방사선 균열 및 횡 균열이 발생 성장하였다. 첫 맥박 하중의 크기는 1KN으로 하고 그 뒤 계속 이어지는 시험은 맥박 하중의 크기를 증가시킨 뒤 행하였으며 균열 길이는 맥박과 맥박 사이에서 계측 하였다. 기타 취성고체에서 관찰 되었던 것과 같이 방사선 균열 및 측면균열의 길이는 impression 반지름과 하나의 지수법칙이 성립함을 보여주었다.

• 한국세라믹학회지
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• 제42권8호
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• pp.554-560
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• 2005

The engineering ceramic needs the properties of high strength, hardness, corrosion-resistance and heat-resistance in order to withstand thermal shock or applied nonuniform stresses without failure. The densely coated porous ceramics can be used for machine component, electromagnetic component, bio-system component and energy-system component by their high-performances from superior coating properties and light-weight characteristics due to the structure including pore by itself. In this study we controlled the porosity of silica and alumina, $8.2\~25.4\%$ and $23.4\~36.0\%$, respectively, by the control of sintering temperature and starting powder size. We made bilayer structures, consisting of a transparent glass coating layer bonded to a thick substrate of different porous ceramics by a thin layer of epoxy adhesive, facilitated observations of crack initiation and propagation. The elastic modulus mismatch could be controlled using different porous ceramics as the substrate layer. Then we applied 150 N force using WC sphere with a radius of 3.18 mm by Hertzian indentation. As a result, the crack initiation in the coating layer was delayed at lower porosity in the substrate layer, and the damage in the coating layer was relatively smaller at the bilayer structure coated on higher elastic substrate.

• Tribology and Lubricants
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• 제31권6호
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• pp.264-271
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• 2015

Adhesion force of nanomaterials such as nanoparticle, nanowire, and nanorods should be significantly considered for its mechanical applications. However, examination of the adhesion force is limited since it is technically challenging to carry out experiments with such small objects. Therefore, in this work, molecular dynamics simulation (MDS) was conducted to determine the adhesion force between a nanowire and a flat surface, which could not be readily assessed through experiments. The adhesion force of a cylindrical-shaped nanowire was assessed by performing MDS and applying an equation of Van der Waals interaction. Simulation was conducted in two steps: indentation of a spherical tip on the flat surface and indentation of a cylinder on the flat surface, because the purpose of the simulation was comparing the results of the simulation and calculation of the Van der Waals interaction equation. From the simulation, Hamaker constant used for the equation of Van der Waals interaction was determined to be 2.93 °ø 10?18 J. Using this constant, the adhesion force of the nanowire on the flat surface was readily estimated by calculating Van der Waals equation to be approximately 65~89 nN with respect to the diameter of the nanowire. Moreover, the adhesion force of the nanowire was determined to be 52~77 nN from the simulation It was observed that there was a slight discrepancy (approximately 15~25%) between the results of the simulation and the theoretical calculation. Thus, it was confirmed that the calculation of Van der Waals interaction could be utilized to assess the adhesion force of the nanowire.