• Nuclear Engineering and Technology
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• 제38권3호
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• pp.293-300
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• 2006

A liquid metal reactor (LMR) operated at high temperatures is subjected to both cyclic mechanical loading and thermal loading; thus, creep-fatigue is a major concern to be addressed with regard to maintaining structural integrity. The Korea Advanced Liquid Metal Reactor (KALIMER), which has a normal operating temperature of $545^{\circ}C$ and a total service life time of 60 years, is composed of various cylindrical structures, such as the reactor vessel and the reactor baffle. This study focuses on the creepfatigue crack initiation for a cylindrical Y-junction structure made of 316 stainless steel (SS), which is subjected to cyclic axial tensile loading and thermal loading at a high-temperature hold time of $545^{\circ}C$. The evaluation of the considered creep-fatigue crack initiation was carried out utilizing the ${\sigma}_d$ approach of the RCC-MR A16 guide, which is the high-temperature defect assessment procedure. This procedure is based on the total accumulated strain during the service time. To confirm the evaluated result, a high-temperature creep-fatigue structural test was performed. The test model had a circumferential through wall defect at the center of the model. The defect front of the test model was investigated after the $100^{th}$ cycle of the testing by utilizing a metallurgical inspection technique with an optical microscope, after which the test result was compared with the evaluation result. This study shows how creep-fatigue crack initiation for a high-temperature structure can be predicted with conservatism per the RCC-MR A16 guide.

• 한국정밀공학회지
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• 제22권6호
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• pp.152-158
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• 2005

The stress conditions acting on the practical structure are complex, and thus most cracks existing in the practical structures are under mixed-mode loading conditions. The effect of shear load component of mixed-mode loading acts more greatly in the stage of crack initiation and initial propagation than crack propagation stage. Hence, research on the behavior in the stage of crack initiation and initial propagation need to be examined in order to evaluate behavior of mixed-mode fatigue cracks. In this study, the crack tip displacement(CTD) was measured by using the direct measuring method(DMM). We examined the behavior at crack tip by determining crack opening load$(P_{op})$. From the test results, the propagation behavior of mixed-mode fatigue cracks was evaluated by considering mixed-mode crack closure. Also, we examined the characteristic of crack propagation under mixed-mode loading with crack propagation direction.

• 대한조선학회논문집
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• 제45권6호
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• pp.679-687
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• 2008

The fatigue life of ship structure under cyclic loading condition is made up of crack initiation and propagation stages. For a welding member in ship structure, the fatigue crack propagation life is more important than the fatigue crack initiation life. To calculate precisely the fatigue crack propagation life at the critical welding location, the knowledge of the residual stress sensitivity on the fatigue strength is necessary. In this study, thermo elastic-plastic analysis was conducted in order to examine the effect of residual stress on the fatigue crack propagation life. Also the fatigue crack propagation lives considering residual stress were calculated using fatigue crack growth code, AFGROW, on the basis of fracture mechanics. AFGROW is widely used for fatigue crack growth predictions under constant and variable amplitude loading. The reliability of AFGROW on the fatigue of ship structure was confirmed by the comparison of the estimated results with the fatigue propagation test results.

• Structural Engineering and Mechanics
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• 제56권2호
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• pp.257-273
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• 2015

Numerical value of correlation between effective parameters in the strength of a structure is as important as its stochastic properties in determining the safety of the structure. In this article investigation is made about the variation of coefficient of correlation between effective parameters in corrosion initiation time of reinforcement and the time of concrete cover cracking in reinforced concrete (RC) structures. Presence of many parameters and also error in measurement of these parameters results in uncertainty in determination of corrosion initiation and the time to crack initiation. In this paper, assuming diffusion process as chloride ingress mechanism in RC structures and considering random properties of effective parameters in this model, correlation between input parameters and predicted time to corrosion is calculated using the Monte Carlo (MC) random sampling. Results show the linear correlation between corrosion initiation time and effective input parameters increases with increasing uncertainty in the input parameters. Diffusion coefficient, concrete cover, surface chloride concentration and threshold chloride concentration have the highest correlation coefficient respectively. Also the uncertainty in the concrete cover has the greatest impact on the coefficient of correlation of corrosion initiation time and the time of crack initiation due to the corrosion phenomenon.

• 대한기계학회논문집
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• 제7권1호
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• pp.28-35
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• 1983

Quantitative analysis have been carried out on the micro-cracks on the surface and into the depth of unnotched smooth mild steel specimen under cyclic stains by rotating bending fatigue tests. Some of the results are; (1) Cracks initiate at the early stage of fatigue life N$_{I}$/ N$_{f}$=10 to 20%, and propagate during the rest of fatigue life. (2) Coalescence of highly crowded small fatigue cracks of random distribution seems to induce the final fracture at higher stress level. (3) The curves of crack initiation and the equal crack length on the graph of stress versus number of cycles are parallel to the S-N curve. (3) The curves of crack initiation and the equal crack length on the graph of stress versus number of cycles are parallel to the S-N curve. (4) The distributions of micro-surface crack length and depth show the composite Weibull distributions which are approximated to two straight lines separated by the value of transient region between stage I and stage II crack.k.k.

• 대한기계학회논문집A
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• 제24권6호
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• pp.1446-1455
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• 2000

Fatigue crack initiation and propagation behavior under cyclic biaxial loading has been investigated using thin-walled tubular specimen with a hole. Two types of biaxial loading system, i.e. cyclic tensile loading with super-imposed static torsional load and cyclic torsional loading with superimposed static tensile load, with various values of the biaxial loading ratio, $\tau$ s/ $\sigma$ max (or $\tau$ max/ $\sigma$s) were employed. Fatigue tests show that fatigue crack near the hole initiates and propagates at 900 and 450 direction to the longitudinal direction of the specimen under cyclic tensile and torsion loading with static biaxial stress, respectively, and the static biaxial stress doesn't have any great influence on fatigue crack initiation and growth direction. Stress analysis near the hole of the specimen shows that the crack around the hole initiates along the plane of maximum tangential stress range. Fatigue crack growth rates were evaluated as functions of equivalent stress intensity factor range, strain energy density factor range and crack tip opening displacement vector, respectively. It is shown that the biaxial mode fatigue crack growth rates can be relatively consistently predicted with these cyclic parameters.

• 대한기계학회논문집A
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• 제41권9호
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• pp.853-860
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• 2017

본 연구는 균열 발생 및 전파단계에서 하중모드 II의 영향을 평가하기 위해 피로균열 하한계 영역의 혼합모드 I+II 하중을 통해 실험적으로 평가하였다. 균열 발생단계(Stage I)에서는 혼합모드상태에서 하중작용 각도(${\theta}$)가 증가할수록 모드 II 영향으로 인하여 낮은 하중에서 균열이 발생하고, 균열 전파단계 (Stage II)에서는 균열전파 속도는 감소하였다. 다단계 하중작용 각도변화에 따른 하중모드 II영향은 균열전파단계 실험을 통해 평가하였다. 혼합모드 I+II 하중 작용 시 작용각도 ($0^{\circ}{\rightarrow}{\theta}{\rightarrow}60^{\circ}$) 증가에 따라 피로균열전파속도는 감소하였으며 늦게 발생한 균열에서도 마찬가지로 감소하였다. 작용각도가 ${\theta}{\geq}75^{\circ}$ 범위에서는 하중작용각도 증가에 따라 피로균열전파속도가 증가하고 피로수명이 감소하는 것을 확인하였다.

• Composites Research
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• 제21권6호
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• pp.15-22
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• 2008

본 연구는 SiC입자를 20% 강화된 알루미늄기 복합재료를 이용해서 평면굽힘피로시험을 향했다. 표면미소피로균열의 발생 및 진전거동은 레프리카법으로 연속관찰을 했고 파괴원인과 파괴기구를 규명하기 위해서 주사전자현미경을 이용했다. da/dn-$K_{max}$ 관계에서 저응력 레벨에서는 여러 개의 균열이 진전하고, 합체 등이 일어나는 것으로부터 진전속도는 꽤 분산(흐트러짐)이 심하고, 고응력 레벨에서는 비교적 흐트러짐은 적게 나타나는 것을 알 수 있었다.

• Tribology and Lubricants
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• 제20권5호
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• pp.252-258
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• 2004

In this study, using high cycle fatigue (HCF) criteria, the simulation of rolling contact fatigue is conducted under elliptical contact. The HCF criteria fall into three categories: the critical plane approach, the stress invariant approach and the approach based on the mesoscopic scale. The accurate calculation of contact stresses and subsurface stresses is essential to the prediction of crack initiation life. Contact stresses are obtained by contact analysis of a semi-infinite solid based on the use of influence functions and the subsurface stress field is obtained using rectangular patch solutions. The simulation results show that the critical load is decreasing rapidly and the site of crack initiation also moves rapidly to the surface from the subsurface when the friction coefficient exceeds a specific value for all of three fatigue criteria.

• 열처리공학회지
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• 제12권1호
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• pp.40-46
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• 1999

Rotary bending fatigue tests were carried out to investigate the improvement of fatigue limit in annealed and austempered spheroidal graphite cast iron. Main results obtained are as follows. (1) The tensile strength(hardness) of Series C is higher than that of Series B, and fatigue limits are 245MPa in Series C, 230MPa in Series B and 195MPa in Series A, respectively. (2) The fatigue limits of Series B and Series A are mainly governed by the resistance to fatigue crack initiation. Whereas, the fatigue limit of Series C is governed by the resistance to fatigue crack initiation and growth. The defect size and the resistance to crack initiation and growth should be considered to clarify the fatigue properties in spheroidal graphite cast iron. (3) Improvement of fatigue limit by half-austempering is more reasonable than that of full-austempering treatment in multi defective materials as spheroidal graphite cast iron.