• Journal of Korean Society of Steel Construction
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• v.20 no.6
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• pp.723-730
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• 2008

Due to the higher ratio of live load to total loads of railway bridges, the accumulated damage by cyclic fatigue is significant. Moreover, it is highly possible that the initiated crack grows faster than that of highway bridges. Therefore, it is strongly needed to assess the safety for the accumulated damage analytically. The initiation and growth of fatigue-crack are related with the stress range, number of cycles, and the stiffness of the structural system. The stiffness of the structural system includes uncertainties of the planning, design, construction and maintenance, which varies as time goes. In this study, the authors developed the design and risk assessment techniques based on the reliability theories considering the uncertainties in load and resistance. For the probabilistic risk assessment of crack growth and the remaining life of the structures by the cyclic load of railway and subway bridges, response surface method (RSM) combined with first order second moment method were used. For composing limit state function, the stress range, stress intensity factor and the remaining life were selected as input important random variables to the RSM program. The probabilities of failure and the reliability indices of fatigue life for the considered specimen under cyclic loads were evaluated and discussed.

• Composites Research
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• v.16 no.1
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• pp.13-18
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• 2003

Prediction of composite fatigue life is not a straightforward matter, depending on various failure modes and their interactions. In this paper, a methodology is presented to predict fatigue life and residual strength of composite materials based on Phenomenological Model(non-linear fatigue damage model). It is assumed that the residual strength is a monotonically decreasing function of the number of loading cycles and applied fatigue stress ratio and the model parameters(strength degradation parameter and fatigue shape parameter) are assumed as function of fatigue life. Then S-N curve is used to extract model parameters that are required to characterize the stress levels comprising a randomly-ordered load spectrum. Different stress ratios (${\sigma}_{min}/{\;}{\sigma}_{max}$) are handled with Goodman correction approach(fatigue envelope) and the residual strength after an arbitrary load cycles is represented by two parameter weibull functions.

• Journal of the Korea Convergence Society
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• v.5 no.1
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• pp.1-8
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• 2014

The purpose of this paper was to evaluate the fatigue life to apply the vibration fatigue analysis considering the stiffness change of the spot welding due to fatigue damage accumulation. For this, the mechanical and fatigue properties of base and spot welded standard specimens were obtained through the tensile and constant amplitude fatigue test. The transfer function of the spot-welded structure was obtained from the frequency response analysis and fatigue analyisis was performed under the condition of PSD=0.11. A vibration fatigue analysis that considered changes in the frequency response due to the fatigue damage that is, failure of some wleding point was conducted on spot-welded structure. The fatigue life of the spot-welded structure was determined by combining the transfer function, the S-N curve of the tensile-shear spot-welded joint and the input PSD.

• Bulletin of the Korean Institute for Industrial Safety
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• v.2 no.1
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• pp.34-40
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• 2002

화학설비나 산업설비 또는 국가 기간 설비에서 파이프(pipe)는 중요한 위치를 차지하고 있으며 파이프의 파열은 그대로 대형 사고나 설비 가동의 중단으로 연결되는 경우가 많다. 따라서 파이프의 구조건전성을 유지하는 것은 안전의 확보와 생산 설비의 효율적 운용을 위하여 긴요하다. 파이프 재료 내부에는 재료 제작 시, 또는 파이프 제작 시 발생된 결함이나 사용 중 부식이나 피로 등의 영향으로 생성된 결함이 존재한다. 이러한 결함은 파이프의 강도를 낮추게 되고, 파이프의 잔류강도가 사용압력보다 낮아지게 되면 파이프가 파열하게 된다. 따라서 본 글에서는 파이프에 균열이 존재하거나 손상된 경우의 구조안전성 평가를 위하여 필요한 해석방법에 대하여 살펴본다.(중략)

• Journal of Korean Society of Steel Construction
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• v.15 no.5 s.66
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• pp.475-487
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• 2003

This study developed an analysis model of estimating fatigue damage using the linear elastic fracture mechanics method. Stress history occurring to an element when a truck passed over a bridge was defined as block loading and crack closure theory explaining load interaction effect was applied. Stress range frequency analysis considering dead load stress and crack opening was done. Probability of stress range frequency distribution was applied and the probability distribution parameters were estimated. The Monte Carlo simulation of generating the probability various of distribution was performed. The probability distribution of failure block numbers was obtained. With this the fatigue reliability of an element not occurring in failure could be calculated. The failure block number divided by average daily truck traffic remains the life of a day. Fatigue reliability analysis model was carried out for the welding member of cross beam flange and vertical stiffener of steel box bridge using the proposed model. Consequently, a 3.8% difference was observed between the remaining life in the peak analysis method and in the proposed analysis model. The proposed analysis model considered crack closure phase and crack retard.

• Journal of Aerospace System Engineering
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• v.13 no.5
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• pp.65-71
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• 2019

In this study, as a part of the aging aircraft performance improvement project for which no design information is provided, a new type of blade antenna is installed on the main part of the aging aircraft, and the method of proving the fatigue life of the main part of the aircraft is reviewed and summarized. There are various methods to prove fatigue life according to the manufacturer and aircraft design conditions. The fatigue life prediction and damage tolerance range of the relevant site were obtained through related regulations and industry examples. From these results, the fatigue life of newly installed antennas around the main parts of the aging aircraft was evaluated and the maintenance period and criteria were set according to the damage tolerance.

• Magazine of the Korea Concrete Institute
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• v.10 no.3
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• pp.107-115
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• 1998

반복하중을 받는 무근 및 철근 콘크리트 부재는 하중의 반복작용에 의해 과도한 균열 및 처짐을 유발하고 결과적으로 전체 구조요소의 파괴를 일으킨다. 따라서 하중반복에 의한 누적손상의 진행과정을 정량적으로 평가하여 철근콘크리트 보의 사용성과 안전성을 평가하는 것이 중요하고 특히 취성적 거동 특성을 갖는 전단거동의 경우에 더욱 의미가 있다. 본 연구에서는 반복전단하중에 의한 누적손상의 정량적 분석을 위하여 평균변형도 및 평균응력개념에 기초한 수정압축장이론(modified compression field theory)에 의하여 누적손상 모델을 제안하였다. 본 연구에서 제안된 모델을 통하여 반복전단하중에 의한 처짐과 변형도의 변화를 평가하는데 유용하게 이용될 것으로 사료된다.

• Proceedings of the KWS Conference
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• 2003.11a
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• pp.188-190
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• 2003

• Proceedings of the Korean Nuclear Society Conference
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• 1998.10a
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• pp.361-361
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• 1998

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.5
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• pp.417-425
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• 2007

The ferritic 9Cr-1Mo-V-Nb heat-resisting steel was experimentally studied in order to characterize its microstructural evolution during creep-fatigue by coercivity measurement. The creep-fatigue test was conducted at $550\;^{\circ}C$ with the tensile holding time of 60s and 600s, respectively. The coercivity decreased until the failure and the hardness monotonously decreased for the whole fatigue life. As the life fraction of creep-fatigue increased, the $M_{23}C_6$ carbide coarsened following the Ostwald ripening mechanism. However, the MX carbonitrides did not grow during creep-fatigue due to so stable at $550\;^{\circ}C$. The width of martensite lath increased because of the dislocation recovery at the lath boundaries. The magnetic coercivity has an influence on the microstructural properties such as dislocation, precipitates and martensite lath boundaries, which interpreted in relation to microstructural changes. Consequently, this study proposes a magnetic coercivity to quantify the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.