• 대한기계학회논문집A
• /
• 제27권6호
• /
• pp.890-897
• /
• 2003

This paper compares engineering estimation schemes of C＊ and creep COD for circumferential and axial through-wall cracked pipes at elevated temperatures with detailed 3-D elastic-creep finite element results. Engineering estimation schemes included the GE/EPRI method, the reference stress method where reference stress is defined based on the plastic limit load and the enhanced reference stress method where the reference stress is defined based on the optimized reference load. Systematic investigations are made not only on the effect of creep-deformation behaviour on C＊ and creep COD, but also on effects of the crack location, the pipe geometry, the crack length and the loading mode. Comparison of the FE results with engineering estimations provides that for idealized power law creep, estimated C＊ and COD rate results from the GE/EPRI method agree best with FE results. For general creep-deformation laws where either primary or tertiary creep is important and thus the GE/EPRI method is hard to apply, on the other hand, the enhanced reference stress method provides more accurate and robust estimations for C* and COD rate than the reference stress method.

• 대한기계학회논문집A
• /
• 제24권12호
• /
• pp.2891-2898
• /
• 2000

The objective of this paper is to develop a fiber-optic acoustic emission(AE) sensor applicable to on-line monitoring systems which is suitable for long-distance signal transmission. An AE sensor was developed by use of a fiber-optic cantilever and an extrinsic Fabry-Perot interferometer(EEPI). The efficiency of signal processing was improved by driving the high frequency AE signals into the low frequency ones. In order to verify the developed sensor, the tensile and the pencil lead fracture(PLF) tests were performed including the experiment showing the Kaiser effect. Form tests, AE signals were successfully detected in the elastic-plastic deformation range, especially higher signals at the crack propagation. The developed sensor was expected to be used for an on-line monitoring of crack propagation in mechanical system.

• 동력기계공학회지
• /
• 제18권5호
• /
• pp.80-87
• /
• 2014

The pipelines are apt to erosion or corrosion because of the high-speed flow of water and steam with high temperatures or high pressures. This study was carried out a finite element analysis (FEA) and an experimental for the fracture behavior of T-joint pipes with local wall thinning under internal pressure. Local wall thinning was machined on the pipes in order to simulate erosion and corrosion of the metal. The configurations of the eroded area included an eroded ratio of d/t=0.80~0.963 and an eroded length of l=25 mm, 50 mm, and 102 mm. Three-dimensional elastic-plastic analyses were also carried out using FEA, which accurately simulates failure behaviors. In regards to the relationship between pressure and eroded, the criterion that indicates what can be used safely under operating pressure and design pressure were obtained from FEA. The FEA results were in relatively good agreement with that of the experiment.

• Geomechanics and Engineering
• /
• 제14권3호
• /
• pp.225-231
• /
• 2018

In this paper, in order to explain the splitting of cylindrical rock specimen under uniaxial loading, cracks in cylindrical rock specimen are divided into two kinds, the longitudinal crack and the slanting crack. Mechanical behavior of the rock is described by elastic-brittle-plastic model and splitting is assumed to suddenly occur when the uniaxial compressive strength is reached. Expression of the stresses induced by the longitudinal crack in direction perpendicular to the major axis of the crack is deduced by using the Maxwell model. Results show that the induced stress is tensile and can be greater than the tensile strength even before the uniaxial compressive strength is reached. By using the Inglis's formula and simplifying the cracks as slender ellipse, the above conclusions that drawn by using the Maxwell model are confirmed. Compared to shearing fracture, energy consumption of splitting seems to be less, and splitting is most likely to occur when the uniaxial loading is great and quick. Besides, explaining the rock core disking occurred under the fast axial unloading by using the Maxwell model may be helpful for understanding that rock core disking is fundamentally a tensile failure phenomenon.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2001년도 춘계학술대회논문집A
• /
• pp.511-516
• /
• 2001

Recently small punch creep testing (or miniature disc bend creep test) has received much attention through European collaborative research projects. This method was considered as a substitute for the conventional creep rupture testing by which the residual creep life is measured from the specimen taken out from serviced components of high temperature plants. It would be beneficial if the material creep properties such as power law creep constants as well as the creep rupture life can be measured from the small punch creep test. In this paper a method of assessing creep constants from the small punch creep testing is proposed. Finite element analyses were performed to investigate evolution of stress and strain rate at the weakest locations of the small punch creep specimen. Elastic-plastic-secondary creep analyses were carried out. The estimation equations for creep constants by the small punch creep testing are proposed based on the finite analysis results. Small punch creep tests were also performed with 9Cr steel and the accuracy of the proposed equation was verified by the experimental results.

• 전산구조공학
• /
• 제2권1호
• /
• pp.71-78
• /
• 1989

보의 비선형 해석프로그램의 개발에 있어서 변형연화현상을 고려해 주기 위하여 한점의 변위를 점증시키면서 구조물의 나머지 변위와 가해지는 하중들을 구하는 변위제어법을 사용하였으며, 신속한 결과의 도출을 위하여 단면의 성질인 모멘트-곡률 곡선을 이용하였다. 이때 변형연화현상으로 인하여 요소길이에 따라 같은 구조물일지라도 해석결과가 다르게 나타나는데 이 점을 보완해 주기 위해서 파괴에너지 개념을 도입하여 모멘트-곡률 곡선을 보정하였으며, 비선형 해석을 보다 단순화시키기 위하여 과다철근보에 대해서는 탄성-연화, 과소철근보에 대해서는 탄성-소성-연화로 선형화된 모델을 사용하였다. 이러한 본 해석프로그램을 이용하여 실험된 철근콘크리트 보들을 해석한 결과 보의 하중-처짐 곡선은 실험결과와 거의 일치함을 보였다.

• 대한기계학회논문집A
• /
• 제25권9호
• /
• pp.1493-1500
• /
• 2001

Recently small punch creep testing (or miniature disc bend creep test) has received much attention through European collaborative research projects. This method was considered as a substitute for the conventional creep rupture testing by which the residual creep life is measured from the specimen taken out from serviced components of high temperature plants. It would be beneficial if the material creep properties such as power law creep constants as well as the creep rupture life can be measured from the small punch creep test. In this paper a method of assessing creep constants from the small punch creep testing is proposed. Finite element analyses were performed to investigate evolution of stress and strain rate at the weakest locations of the small punch creep specimen. Elastic-plastic-secondary creep analyses were carried out. The estimation equations for creep constants by the small punch creep testing are proposed based on the finite analysis results. Small punch creep tests were also performed with 9Cr steel and the accuracy of the proposed equation was verified by the experimental results.

• 한국재료학회지
• /
• 제19권7호
• /
• pp.397-402
• /
• 2009

In this study, Ti powder was fabricated from Ti scrap by the Hydrogenation-Dehydrogenation (HDH) method. Hydrogenation reactions of Ti scrap occurred at near 450 $^{\circ}C$ with a sudden increase in the reaction temperature and the decreasing pressure of hydrogen gas during the hydrogenation process in the furnace. The dehydrogenation process was also carried out at 750 $^{\circ}C$ for 2hrs in a vacuum of $10^{-4}$ torr. After the HDH process, a deoxidation treatment was carried out with the Ca(purity: 99.5) at 700 $^{\circ}C$ for 2hrs in the vacuum system. It was found that the oxidation content of Ti powder that was deoxidized with Ca showed noticeably lower values, compared to the content obtained by HDH process. In order to fabricate Ti compacts, Ti powder was sintered at $1100\sim1400^{\circ}C$ for 2hrs under a vacuum of $10^{-4}$ torr. The relative density of compact was 94.9% at 1300 $^{\circ}C$. After sintering, all of the Ti compacts showed brittle fracture behavior, which occurred in an elastic range with short plastic yielding up to a peak stress.

• 대한기계학회논문집A
• /
• 제29권1호
• /
• pp.156-162
• /
• 2005

Stouts are frequently used throughout the human body, but the most critical areas are in coronary arteries. They open pathways in vessels and supply blood directly to the heart muscle. To simulate behavior of expansion for the coronary stent by balloon, the commercial finite element code LS-DYNA and ANSYS were used in the analysis. The explicit method is used to analyze the expansion of the stent and the implicit method is performed to simulate the springback that developed in a stent after the balloon pressure has been removed. Finally the experimental results for the expansion of the PS153 stents were compared with the FEM results. The springback was measured with the stents subjected to no external pressure to which stents are subjected in vivo. The simulated results were in good agreement with experimental results. Standard mechanical characteristics such as stress, plastic strains, and springback can be derived from the numerical results. These data can be used to determine maximum expansion diameter without fracture and expansion pressure considering elastic recoil.

• 한국식품과학회지
• /
• 제18권3호
• /
• pp.215-220
• /
• 1986

리오메타 또는 회전점도계를 사용하여 $9{\sim}30%$ 쌀보리와 겉보리전분 젤의 역학적 성질을 조사하였다. 20 및 30% 보리전분젤의 압축파괴특성은 $0{\sim}2\;kg$의 하중하에서 탄성, 소성 및 파괴의 3영역을 포함한 변형을 나타내었다. 압축응력완화 곡선은 시료전분젤의 점탄성이 2개의 Maxwell 모형을 병렬로 연결한 4요소역학 모형으로 설명할 수 있음을 보였다. 또 9% 전분젤에 대한 정상전단하의 응력완화는 선형점탄성 이론으로 설명할 수 있었으며 이들 응력완화과정은 전분젤의 구조에 미치는 온도 또는 첨가제영향의 검토에 유효한 것으로 시사되었다.