• Journal of Welding and Joining
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• 제26권2호
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• pp.43-49
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• 2008

Fatigue cracks due to thermal stratification or corrosion in pipelines of nuclear power plants can cause serious problems on reactor cooling system. Therefore, the development of an integrated technology including fabrication of standard specimens and their practical usage is needed to enhance the reliability of nondestructive testing. The test material was austenitic STS 304, which is used as pipelines in the Reactor Coolant System of a nuclear power plants. The best condition for fabrication of thermal fatigue cracks at the notch plate was selected using the thermal stress analysis of ANSYS. The specimen was installed from the tensile tester and underwent continuos tension loads of 51,000N. Then, after the specimen was heated to $450^{\circ}C$ for 1 minute using HF induction heater, it was cooled to $20^{\circ}C$ in 1 minute using a mixture of dry ice and water. The initial crack was generated at 17,000 cycles, 560 hours later (1cycle/2min.) and the depth of the thermal fatigue crack reached about 40% of the thickness of the specimen at 22,000 cycles. As a results of optical microscope and SEM analysis, it is confirmed that fabricated thermal fatigue cracks have the same characteristics as real fatigue cracks in nuclear power plants. The crack shape and size were identified.

• 수산해양기술연구
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• 제24권4호
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• pp.164-171
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• 1988

Recently with the rapid development in marine and shipbuilding industries such as marine structures, ships and chemical plants, it takes much interest in the study of corrosion fatigue characteristics that was close up an important role in mechanical design. In this study, characteristics of corner crack propagation on the base metal and heat affected zone of 5086 Al-Alloy was tested by using of a rotary bending fatigue tester and was investigated under the environments of specific resistance, $\rho$=25$\Omega$ cm and air. The corrosion fatigue crack initiation and corrosion fatigue life sensitivity were quantitatively inspected for 5086 Al-Alloy in the specific resistance, $\rho$=25$\Omega$ cm. Main results obtained are as follows: (1) The corrosion sensitivity of heat affected zone under specific resistance, $\rho$=25$\Omega$cm shows approximately 1.69~2.22 and corrosion sensitivity of base metal is more susceptible than that of heat affected zone. (2) The corrosion fatigue life sensitivity on heat affected zone decreases eminently than that of initial corrosion fatigue crack. (3) The characteristics of quarter elliptical corner crack propagation shows that depth crack is more grown than surface crack at crack initiation, but the surface crack is more propagated than depth crack as the crack propagation is increased. (4) The surface crack and depth crack growth on heat affected zone by softness show delayed phenomenon than that of base metal.

• 한국도로학회논문집
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• 제10권4호
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• pp.19-29
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• 2008

최근에 $135^{\circ}C^{\circ}C$ 이하의 온도에서 생산되는 중온형 아스팔트 혼합물의 새로운 생산 기술이 전세계적으로 개발되고 있다. 본 연구에서는 가열 아스팔트보다 낮은 온도에서 아스팔트를 효과적으로 골재에 분산시켜 코팅할 수 있는 폼드 아스팔트 기술을 이용하여 중온형 아스팔트 혼합물을 생산하였으며, 최근 개발된 새로운 Simple Performance Testing 장비를 이용하여 다양한 온도와 하중조건 하에서 중온형 폼드 아스팔트 혼합물의 공용성 특성을 평가하였다. 중온형 폼드 아스팔트 혼합물은 FG 64-22의 아스팔트를 거품상태로 만들어 중온으로 가열된 골재에 뿌려서 제조하였으며, 중온형 아스팔트 혼합물은 중온의 골재에 FG 64-22의 아스팔트를 액상 상태로 첨가하여 제조하였다. 중온형 폼드 아스팔트 혼합물은 중온형 아스팔트 혼합물보다 높은 동탄성계수와 Flow Number를 나타내었다. 따라서, $100^{\circ}C$로 가열된 골재를 사용하여 생산된 중온형 폼드 아스팔트 혼합물은 중온형 아스팔트 혼합물에 비하여 피로균열 및 소성변형 저항에 우수한 것으로 평가되었다.

• 자원환경지질
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• 제30권4호
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• pp.353-362
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• 1997

P wave velocities of core samples from the Pocheon granite were measured before and after applications of cyclic loading. Then. distribution of the pre-existing microcracks and microcracks developed due to the cyclic loading was investigated by analyzing P wave velocity anisotropies and microscopic observations from thin sections. Anisotropy constants were calculated with three different ways: (1) $C_A$ between the maximum and the minimum velocities, (2) $C_AI$ between velocities measured along the axial direction and the average of six velocities measured in the planes perpendicular to the loading axis (rift plane) and (3) $C_AII$ between the maximum and the minimum velocities measured in the plane perpendicular to the loading axis. Among anisotropy constants. $C_AI$ was the most effective anisotropy constant to identify the rift plane whose orientation is parallel to the pre-existing microcracks as well as the distribution of stress induced microcracks. $C_AI$ decreased after cyclic loading and the relationship between $C_AI$ and number of cycles shows comparatively coherent negative trends. indicating that stress induced microcracks are aligned perpendicular to the orientation of pre-existing microcracks and that the amounts are proportional to the number of loading cycles. The difference of anisotropy constants before and after cyclic loading was effective in delineating the level of cracks and we called it Induced Crack Index. Velocity measurements and microscopic observations show that anisotropy was caused mainly due to microcracks aligned to a particular direction.

• 열처리공학회지
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• 제9권1호
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• pp.69-77
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• 1996

The fatigue crack initiation behavior of plasma ion nitrided SACM645 steel was investigated through the rotary bending fatigue test and residual stress measurement by XRD. It was shown by XRD and EPMA that the plasma ion nitrided surface was composed of ${\gamma}^{\prime}(Fe_4N)$phase and ${\varepsilon}(Fe_{2-3}N)$phase, and that the nitrogen atoms existed in Fe matrix in diffusion layer. The OM, SEM and Auger spectroscopy showed that the depth of compound layer, mixed compound and diffusion layer, and diffusion layer was $8{\mu}m$, $30{\mu}m$ and $300{\mu}m$, respectively. However, the microhardness test showed that the depth of hardened layer was $500{\mu}m$. The tensile strength of the ion nitrided SACM645 was lower than that of the unnitrided SACM645, and the ion nitrided specimen was fractured without plastic deformation. The nitrided SACM645 showed much poorer low cycle fatigue properties than the unnitrided one. In rotary bending fatigue, the fatigue strength of the ion nitrided SACM645 was higher than that of the unnitrided specimen, and the fatigue crack initiation sites changed by applied fatigue stress levels. The XRD result showed that the ion nitrided SACM645 has the compressive residual stress from surface to $600{\mu}m$ deep and the tensile residual stress from $600{\mu}m$ to deeper site. It is thought that crack initiation takes place at the point where the total stress of residual stress and applied stress is maximum.

• 한국전산구조공학회논문집
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• 제29권4호
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• pp.347-354
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• 2016

가스 터빈은 기동 및 정지 횟수가 많기 때문에 열피로나 취화 현상으로 인한 가스터빈 케이싱의 균열 또는 케이싱의 플랜지면에서 고온고압 가스의 누설이 발생할 가능성이 높다. 따라서 가스터빈 케이싱의 구조안전성 및 플랜지면에서의 누설평가는 반드시 수행되어야 하는 부분이다. 본 논문에서는 유한요소해석을 바탕으로 터빈 케이싱의 ASME B&PVC VIII-2 구조안전성 평가 및 접촉압력을 통한 누설 평가 그리고 볼트의 구조안전성 평가를 진행하였다. 또한 가스터빈 케이싱의 유한요소모델링 및 해석/평가 방법을 제안하여 가스터빈 개발에 활용할 수 있게 하였다.

• 한국해안·해양공학회논문집
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• 제23권4호
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• pp.285-291
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• 2011

이 연구에서는 신형소파블록을 대상으로 실험 및 수치해석을 수행하여 역학적 성능을 평가하고 적절한 보강재의 배근에 따른 구조성능 향상 효과를 검토하였다. 수치해석을 통하여 신형소파블록에 인장응력이 발생하는 위치와 크기를 예측하여 보강재를 설계하였다. 보강재로는 일반 철근 및 해양환경에 적합하도록 부식과 피로에 장점을 지닌 섬유보강재(FRP)를 사용하였다. 실험을 통하여 보강재가 없는 무근 신형소파블록의 파괴하중은 350 kN으로 소파블록의 자중에 비하여 6.2배로 나타났으며, 철근이나 FRP보로 보강한 실험체는 모두 실험의 최고하중인 900 kN 이상의 강도를 보였다. 위험단면을 통과하는 보강재의 개수는 시공의 편의를 위해서는 굵은 지름의 단일 보강재를 사용하는 것이 유리하지만 가는 지름의 보강재 여러 개를 사용하여 균열 폭을 감소시키는 것이 바람직하다는 결과를 얻었다.

• Journal of Welding and Joining
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• 제23권6호
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• pp.37-42
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• 2005

Most of the welding steel plate structures have complicated mechanical problems such as rolling directional characteristics and residual stresses caused by manufacturing process. For the enhancement of reliability and safety in those structures, therefore, a systematic investigation is required. SS400 steel plate used for common structures was selected and welded by FCAW butt-welding process for this study, and then it was studied experimently about characteristics of fatigue crack propagations with respect to rolling direction and welding residual stress of welded steel plates. When the angles between rolling direction and tensile loading direction in base material are increased, their ultimate strength not show a significant difference, but yielding strength are increased and elongations are decreased uniformly. It is also shown that fatigue crack growth rate can be increased from those results. When the angles between welding bead direction and loading direction in welded material are increase, fatigue crack growth rate of them are decreased and influenced uniformly according to the conditions of residual stress distribution. In these results, it is shown that the welded steel plate structures are needed to harmonize distributed welding residual stress, rolling direction and loading direction fur the improvement of safety and endurance in manufacture of their structures.

• 한국추진공학회지
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• 제21권1호
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• pp.26-35
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• 2017

원심압축기 운전 중에 발생하는 고주기 피로균열이나 공진 등은 임펠러 파손의 주된 원인이다. 이러한 파손을 회피하기 위해 공진영역 운전에서도 견딜 수 있는 충분한 강도의 임펠러를 설계하거나, 공진이 발생하지 않도록 임펠러를 튜닝 한다. 이러한 회피설계는 임펠러 내부유동 및 성능특성 변화를 야기하게 된다. 본 연구에서는 밀폐형 임펠러에 대해 블레이드 두께를 증가시킨 모델과 스켈럽을 적용한 모델에 대한 유동 및 성능특성을 파악하기 위해 전산해석을 수행하였다. 전산해석 결과, 블레이드 두께 증가 모델 경우는 기본모델 대비 압력계수가 0.5% 감소하였으며, 전효율은 0.1% 감소하였다. 스켈럽 적용모델은 압력계수가 0.4% 증가하였으며, 전효율은 1.6% 감소하였다.

• 대한금속재료학회지
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• 제50권4호
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• pp.271-279
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• 2012

9-12% Cr steels have been used in thermal power plants which repeat start and stop operations. Major factors of fatigue life are temperature, frequency, stress ratio, holding time, microstructure, and environment. Normally, fatigue life decreases at high temperature, low frequency, high stress ratio, and long holding time conditions. A Mod.9Cr-1Mo steel, called G91, was developed at ORNL (Oak Ridge National Laboratory, USA) and was adopted as a high-temperature structural material in the ASME Code in 2004. However, its low-cycle fatigue and fatigue crack growth characteristics have been rarely studied. In this work, we have investigated the low-cycle fatigue crack growth behaviors of G91 steel under various test conditions in terms of temperature and stress ratio. As temperature and stress ratio increase, the crack growth rate becomes faster and striation distance also increases. On the other hand, the number of branch cracks decreases.