• 한국추진공학회지
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• 제20권2호
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• pp.11-17
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• 2016

분말야금 니켈기 초내열합금은 항공기 터빈 엔진의 고온용 부품으로 사용되고 있다. 본 연구에서는 상온, $600^{\circ}C$ 및 $700^{\circ}C$에서 CT시편을 이용하여 피로균열진전거동이 평가되었다. ASTM E647에서 제시한 직류전위차법이 피로균열진전 동안에 균열 길이를 측정하기 위하여 사용되었다. 응력비 0.5에서 피로균열진전속도는 응력비 0.1에서와 비교하여 더 빠르게 나타났다. 피로균열진전속도는 응력비와 온도의 증가와 함께 증가하였다. 파단면 관찰은 파괴메커니즘 분석을 위해 수행하였다.

• Restorative Dentistry and Endodontics
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• 제19권1호
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• pp.27-44
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• 1994

The purpose of this study was to determine the effects of pH, temperature and time on the synthesis of hydroxyapatite(HAP) by spontaneous precipitation under relatively physiologic condition. Specimens were obtained from aqueous system with various pH, temperature and experimental time. Chemical composition, crystallographic structure and crystallinity of the synthetic HAP were evaluated by Infrared spectroscopy and powdered X-ray diffraction method. The following results were obtained. 1. No precipitate was obtained under pH 5.5. 2. All the specimens were concluded as HAP except one that was obtained under the condition of pH 6.5, $25^{\circ}C$ and 1 day. It was concluded as dicalcium phosphate dihydrate(DCPD). 3. The crystallinity of HAP was enhanced by increases in pH, temperature and time of the preparation. But, the crystallinity of the synthetic HAP was lower than that of the mineral HAP. 4. Intermediates such as DCPD and octacalcium phosphate were formed on the process of the synthesis of HAP.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2004년도 추계학술발표회 발표논문집
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• pp.1001-1002
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• 2004

• Nuclear Engineering and Technology
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• 제52권4호
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• pp.784-790
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• 2020

The accelerated corrosion by Flow Accelerated Corrosion (FAC) has caused unexpected rupture of piping, hindering the safety of nuclear power plants (NPPs) and sometimes causing personal injury. For the safety, it may be necessary to select some pipes in terms of condition monitoring and to measure the change in thickness of pipes in real time. Direct current potential drop (DCPD) method has advantages in on-line monitoring of pipe wall thinning. However, it has a disadvantage in that it is difficult to quantify thinning due to various thinning shapes and thus there is a limitation in application. The machine learning approach has advantages in that it can be easily applied because the machine can learn the signals of various thinning shapes and can identify the thinning using these. In this paper, finite element analysis (FEA) was performed by applying direct current to a carbon steel pipe and measuring the potential drop. The fundamental machine learning was carried out and the piping thinning model was developed. In this process, the features of DCPD to thinning were proposed.

• Corrosion Science and Technology
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• 제13권3호
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• pp.112-119
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• 2014

In this work, it is aimed to develop the fabrication method of axial stress corrosion cracking (SCC) defects having various sizes, on the outer diameter surface of the steam generator (SG) tubings. To control the length of the artificial SCC defect, the specific area of the SG tubing samples was exposed to an acidic solution after a sensitization heat treatment. During the exposure to an acidic solution, a direct current potential drop (DCPD) method was adopted to monitor the crack depth. The size of the SCC defect was first evaluated by an eddy current test (ECT), and then confirmed by a destructive examination. From the comparison, it was found that the actual crack length was well controlled to be similar to the length of the surface exposed to an acidic solution (5, 10, 20 or 30 mm in this work) with small standard deviation. From in-situ monitoring of the crack depth using the DCPD method, it was possible to distinguish a non-through wall crack from a through wall crack, even though the depth of the non-through wall crack was not able to be precisely controlled. The fabrication method established in this work was useful to simulate the SCC defect having similar size and ECT signals as compared to the field cracks in the SG tubings of the operating Korean PWRs.

• Biomaterials and Biomechanics in Bioengineering
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• 제4권1호
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• pp.21-32
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• 2019

Eggshell is a waste material after the usage of egg. In this work, biowaste chicken eggshells were used for preparing carbonated hydroxyapatite (HA) nanoparticles of high purity through aqueous precipitation method at room temperature. The eggshell-derived HA will be a cost-effective bioceramics for biomedical applications and an effective material-recycling technology. Additionally, mulberry leaf extract was used as a template to regulate the morphology, size and crystallinity of HA, and the effects of pH value were also examined. Characterization of the samples was performed by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. Scanning electron microscopy (SEM) was used to determine the size, shape and morphology of HA. The results indicate that only one phase of HA were synthesized in the both absence and presence of mulberry leaf extract at pH of 7 and above, while DCPD or DCPA/DCPD phase was observed at pH 4 condition. The crystallite sizes of the HA samples obviously decreased when adding mulberry leaf extract as a template, while they decreased gradually as the solution pH levels increased. With increasing pH level from 7 to 14, the rod-like HA nanoparticles gradually changed to spherical shape at pH 14. Note that, the obtained product is Mg and Sr containing A- and B-type carbonate HA at alkaline pH and it can be a potential material for biomedical applications.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.129-136
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• 2001

The remaining life estimation for the aged components in power plants as well as chemical plants are very important beacuse mechanical properties of the components are degraded with time of service exposure in high temperature. Since it is difficult to take specimens from the operating components to evaluate mechanical properties of components, nondestructive techniques are needed to estimate the degradation. In this study, test materials with 4 different degradation levels were prepared by isothermal aging heat treatment at $630^{\circ}C$. And the DC potential drop method and destructive methods such as tensile, $K_{IC}$ and hardness tests were used in order to evaluate the degradation of 1Cr-1Mo-0.25V steels. The objective of this study is to investigate the possibility of the application of DCPD method to estimate the material degradation, and to analyse the relationship between the electrical resistivity and the degree of material degradation.

• 대한기계학회논문집A
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• 제24권4호
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• pp.874-880
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• 2000

In this paper, a DC potential drop measurement system was used to find the position of the flaw on a simple thin plate. Four-point probe test was evaluated and used for this study. In the four-point probe test, the more distance between current pins provides the more measurable scope, the less voltage difference, and the more voltage difference rate. In the other hand, the more distance between voltage pins provides the less voltage difference and the less voltage difference rate. An optimized four-point probe was applied to measure the relation between voltage and the relative position of flaw to the probe. The Maxwell 21) simulator was used to analyze the electromagnetic field, and it showed that the analytical result was similar to the experimental result within 11.4% maximum error.

• 대한기계학회논문집A
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• 제30권4호
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• pp.420-427
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• 2006

The structural and leakage integrity of steam generator tubes should be sustained against all postulated loads even if a crack is present. During the past three decades, most of the efforts with respect to integrity evaluation of steam generator tubes have been focused on limit load solutions but, recently, the applicability of elastic-plastic fracture mechanics was examined cautiously due to its effectiveness. The purpose of this paper is to introduce a testing method to estimate fracture resistance characteristics of steam generator tubes with a through-wall crack. Due to limited thickness and diameter, inevitably, the steam generator tubes themselves were tested instead of standard specimen or alternative ones. Also, a series of three dimensional elastic-plastic finite element analyses were carried out to derive closed-form estimation equations with respect to J-integral and crack extension for direct current potential drop method. Since the effectiveness of $J_{IC}$ as well as J-R curves was proven through comparison with those of standard specimens taken from pipes, it is believed that the proposed scheme can be utilized as an efficient tool for integrity evaluation of cracked steam generator tubes.

• 대한기계학회논문집A
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• 제27권1호
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• pp.119-125
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• 2003

A way to measure the second parameter $A_2$of CT specimens is described. The displacement $\delta$$_{5}$ which is measured continuously from visual images of the lateral surface during crack growth is used to calculate the A, as a function of crack growth. The crack length is measured by DCPD(Direct Current Potential Drop) method and the J-resistance curve is determined according to ASTM standard E1737-96. To prove the validity of this method, three dimensional finite element analyses were performed, and variations of the displacements $\delta$$_{5}$ and $A_2$along the thickness were explored. As the result, it has been shown that the $\delta$$_{5}$ measured from the visual images of the lateral surface and the corresponding $A_2$can be regarded as the average through the thickness for 1T and 1/2T specimens of SA106Gr.C steel.steel.