• Proceedings of the Computational Structural Engineering Institute Conference
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• 2011.04a
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• pp.493-498
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• 2011

구름접촉피로는 차륜과 레일의 반복적인 접촉으로 인하여 발생하는 표면손상현상으로 점차 증가하는 레일손상 중 하나이다. 접촉마모 및 주기적 그라인딩보다 균열의 성장속도가 더 빨라 균열진전이 시작되는 최소균열크기(minimum crack size for growth)는 레일의 파괴방지 및 유효한 유지보수전략을 수립하는데 기초자료로 활용된다. 본 연구에서는 최소균열크기를 예측하기 위하여 차륜레일의 접촉에 영향을 미치는 주요 파라미터를 변화시키면서 최소균열크기의 변화를 살펴보았다. 이를 위하여 Fletcher와 Kapoor의 "2.5D"모델을 적용한 시뮬레이션 소프트웨어를 개발하였으며, 최대접촉하중(1174MPa), 표면마찰계수(0.1, 0.2, 0.3 and 0.4), 잔류응력, 접촉에 의한 표면마모(1.0nm/cycle), 그라인딩량(0.3mm/10MGT)을 파라미터로 하여 해석을 수행하였다. 해석결과 최소균열크기는 해석조건에 따라 1.41-1.95mm로 계산되었다.

• Journal of Welding and Joining
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• v.15 no.3
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• pp.20-28
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• 1997

용접부에는 많은 취약조건들이 존재하며 파괴의 주 원인이 되고 있어 이들에 대한 많은 연구가 진행되고 있다. 따라서, 현재 용접재료, 용접 조건 및 용접방법 등 을 개선함으로써 여러 방면에서 좋은 결과를 얻고 있다. 그러나 아직도 용접시의 열소 성변형과 구속조건에 따라 분포하는 잔류응력에 의한 피로균열거동에 대한 연구는 정확한 잔류응력 측정의 어려움으로 미흡한 상태이다. 특히 잔류응력의 측정기술과 반복하중에 의한 피로균열 진전시 잔류응력의 이완 등은 이들을 해석하는데 많은 어 려움을 주고 있다. 용접시 높은 열에 의한 재료의 팽창과 냉각시의 수축변형은 용접 부재에 인장 및 압축 잔류응력을 유발시키고, 인장잔류응력은 균열 진전될 때 잔류 응력은 오히려 균열을 지연시키기도 한다. 또한 잔류응력장에서 피로 균열이 진전될 때 잔류응력은 일반적으로 작용하중의 크기와 반복 수 그리고 균열 진전 등으로 인하 여 이완되고 재분포된다. 본 해설에서는 용접재의 피로거동중에 발생하는 잔류응력의 재분포 현상을 하중의 범위, 하중 반복수, 균열 진전의 영향으로 구분하여 각각의 영향에 대해서 기술하고자 한다.

• Journal of the Korean Society of Marine Environment & Safety
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• v.27 no.4
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• pp.529-537
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• 2021

Membrane-type hull and cargo holds have been designed and built for large ship. However, there is a growing interest in applying the same technology to small and medium-sized Liquefied natural gas(LNG) carriers to meet the recent increase in demand for LNG as an ecofriendly fuel and for expanding LNG bunkering infrastructure. The purpose of this study is to apply the IMO Type-B tank to small and medium-sized LNG carriers and verify the safety and suitability of the design. Fatigue crack propagation analysis was performed to install a partial second drip tray installed at the lower part of the LNG cargo tank by calculating the amount of leaked gas in the support structure supporting the cargo tank. First, a program for fatigue crack propagation analysis was developed, in which Paris' law and British Standard 7910 (BS 79110) were applied based on the International Code for the Construction of Equipment and Ships Carrying Liquefied Gases in Bulk, an international standard for LNG carriers. In addition, a surface crack propagation analysis was performed. Next, a methodology for assuming the initial through-crack size was developed to determine the size of the partial second barrier. The analysis was performed for 15 days, which is a possible return time after cracks are detected. Finally, the safety and suitability of the IMO Type-B for LNG cargo tanks required by international regulations were verified. For the accurate analysis of fatigue crack propagation, it is necessary to develop and verify the analysis procedure based on direct analysis and international regulations.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.6
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• pp.1568-1575
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• 1990

Fatigue crack growth behavior was investigated in the center cracked plate of KS SB41 steel and the relation between the crack growth rate and various mechanical parameters was studied at small scale yielding, large scale yielding and full scale yielding. The crack opening ratio U was about 0.6-0.8 and had the larger value in the case of load control than that of strain control. Effective stress intensity factor range, .DELTA.K$_{eff}$ and J integral range, .DELTA.J were obtained from the notion of crack opening, and the crack growth rate was expressed with these values. The value of J integral range increased rapidly at stress ratio, R=0 in full scale yielding of load control test. COD value also increased rapidly with the increase of ligament net stress at large scale yielding of load control test.t.

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.1
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• pp.129-136
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• 2018

The estimation of the fatigue design life for large welded structures is usually performed using the liner cumulative damage method such as Palmgren-Miner rule or the equivalent damage method. When a fatigue crack is detected in a welded steel structure, the residual service life has to be estimated base on S-N curve method and liner elastic fracture mechanics. In this study, to examine the 3D fatigue crack behavior and estimate the fatigue life of out-of-plane gusset fillet welded joint, the fatigue tests were carried out on the model specimens. Investigations of three-dimensional fatigue crack propagation on gusset welded joint was used the finite element analysis of FEMAP with NX NASTRAN and FRANC3D. Fatigue crack growth analysis was carried out to demonstrate the effects of aspect ratio, initial crack length and stress ratio on out-of-plane gusset welded joints. In addition, the crack behaviors of fatigue tests were compared with those of the 3D crack propagation analysis in terms of changes in crack length and aspect ratio. From this analysis result, SIFs behaviors and crack propagation rate of gusset welded joint were shown to be similar fatigue test results and the fatigue life can also be predicted.

• Journal of Korean Society of Steel Construction
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• v.22 no.6
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• pp.589-597
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• 2010

Steel structures have a higher probability of being damaged by fatigue than by other causes of deterioration. As such, their maintenance to prevent fatigue damage is essential to sustain their safety and performance during their service period. In their maintenance, the current state of their fatigue cracks must be assessed to determine appropriate reinforcement methods and the suitable time intervals of periodic inspections when fatigue cracks are detected. Determining the crack growth rate is a successful method of predicting fractures, but it requires technical knowledge on fracture mechanics and experience in numerical methods and software for finite element analysis. In this study, a fatigue crack growth test on through-thickness cracked steel plates was conducted to assess the crack growth rate without superior technical knowledge and experience. The relationship between the Crack Opening Displacement (COD) and the crack growth rate was found in relatively long fatigue cracks.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.3
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• pp.121-125
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• 2018

In this study, finite element analyses of crack propagation along a sinusoidal interface are performed by using cohesive elements. BK law is used for cohesive zone to consider mixed mode traction-separation relation at the crack tip on a sinusoidal interface of a double cantilever beam specimen. The shape of a sinusoidal interface crack and the cohesive strength and the cohesive energies in mixed mode cohesive laws are varied in numerical experiments, and load-displacement curves at the ends of a double cantilever beam specimen are obtained to investigate the crack propagation behavior along a sinusoidal interface.

• Journal of Korean Society of Steel Construction
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• v.11 no.1 s.38
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• pp.33-42
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• 1999

In the present study, crack initiation criteria, static failure and tensile mode fatigue behavior for a rail steel are evaluated to assure the railway vehicle's safety. The transverse fissure, which is the most critical damage in the rail, is initiated by the maximum shear stress and its location is subsurface. In addition, the possibility of transition from the shear mode to the mixed mode increases with increasing the length of subsurface crack. Because of the brittleness by the welding, the fracture toughness of the welded part is lower than of the base metal. For low ${\Delta}K$, the stage II fatigue crack growth rates of the welded part is slower than of the base metal but, for high ${\Delta}K$, this different behavior for fatigue crack growth rate is nearly diminished. These trends are more remarkable for low stress ratio, R=0.1. It is believed that this behavior is caused by the change of the microstructure which that of the welded part is coarser than of base metal.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.3
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• pp.916-923
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• 1991

본 연구에서는 표면 균열이 있는 평판의 탄소성 피로하중 상태에서 성장하는 균열 형태의 변화와, 작용하는 응력의 크기에 따른 균열 개페구 특성의 변화를 연구하 였다.또, 유효 응력 확대계수 범위, .DELTA.K$_{eff}$와 J적분범위, .DELTA.J가 탄소성 응력 상태에서의 표면 피로균열 진전속도를 나타내는 역학양으로 사용되는데 따른 적합성등 을 검토하였다.

• Journal of the Society of Naval Architects of Korea
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• v.28 no.2
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• pp.228-240
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• 1991

Many experimental data of fatigue crack propagation show that the fatigue crack propagation process is stochastic. Therefore, the study on the crack propagation must be based on the probabilistic approach. In the present paper, fatigue crack propagation process is assumed to be a discrete Markov process and the method is developed, which can evaluate the reliability of the structural component by using Markov chain model(Unit step B-model) suggested by Bogdanoff. In this method, leak failure, plastic collapse and brittle fracture of the critical component are taken as failure modes, and the effects of initial crack distribution, periodic and non-periodic inspection on the probability of failure are considered. In this method, an equivalent load value for random loading such as wave load is used to facilitate the analysis. Finally some calculations are carried out in order to show the usefulness and the applicability of this method. And then some remarks on this method are mentioned.