• Proceedings of the KSR Conference
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• 2004.10a
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• pp.222-227
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• 2004

Under cyclic loading, the fatigue failures of welded joints occur at weld toes which induce stress concentration by weld shape. So we need to obtain the peak stress and the S-N curve to assess the fatigue strength of welded joints. However the measurement of peak stress is of high uncertainty and low reproducibility, so we use nominal stress instead in fatigue tests of welded joints. In this study, fatigue tests to obtain S-N curves and FE analyses to obtain stress concentration factors were conducted for the two types of cruciform fillet welded joints, that is, load-carrying and non load-carrying types. Then we changed the obtained S-N curves to that based on peak stress using the hot-spot stress concept. From the analyses of the S-N curves obtained, we have concluded that there is a need to develop a new method to evaluate the fatigue life.

• Structural Engineering and Mechanics
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• v.49 no.2
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• pp.237-259
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• 2014

Wind-induced failure around screwed connections has been documented in roof and wall cladding systems made with steel sheet cold-formed panels during high wind events. Previous research has found that low cycle fatigue caused by stress concentration and fluctuating wind loads is responsible for most such failures. A dynamic load protocol was employed in this work to represent fatigue under wind effects. A finite element model and fatigue criteria were implemented and compared with laboratory experiments in order to predict the fatigue failure associated with fluctuating wind loads. Results are used to develop an analytical model which can be employed for the fatigue analysis of steel cold-formed cladding systems. Existing three dimensional fatigue criteria are implemented and correlated with fatigue damage observed on steel claddings. Parametric studies are used to formulate suitable yet simple fatigue criteria. Fatigue failure is predicted in different configurations of loads, types of connections, and thicknesses of steel folded plate cladding. The analytical model, which correlated with experimental results reported in a companion paper, was validated for the fatigue life prediction and failure mechanism of different connection types and thicknesses of cold-formed steel cladding.

• Structural Engineering and Mechanics
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• v.63 no.6
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• pp.703-712
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• 2017

The composite blades of offshore wind turbines accumulate structural damage such as fatigue cracking due to harsh operation environments during their service time, leading to premature structural failures. This paper investigates various fatigue crack models for reproducing crack development in composite blades and proposes a stochastic approach to predict fatigue crack evolution and to analyse failure probability for the composite blades. Three typical fatigue models for the propagation of fatigue cracks, i.e., Miner model, Paris model and Reifsnider model, are discussed to reproduce the fatigue crack evolution in composite blades subjected to cyclical loadings. The lifetime probability of fatigue failure of the composite blades is estimated by stochastic deterioration modelling such as gamma process. Based on time-dependent reliability analysis and lifecycle cost analysis, an optimised maintenance policy is determined to make the optimal decision for the composite blades during the service time. A numerical example is employed to investigate the effectiveness of predicting fatigue crack growth, estimating the probability of fatigue failure and evaluating an optimal maintenance policy. The results from the numerical study show that the stochastic gamma process together with the proper fatigue models can provide a useful tool for remaining useful life predictions and optimum maintenance strategies of the composite blades of offshore wind turbines.

• Journal of Korean Society of Steel Construction
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• v.9 no.2 s.31
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• pp.159-169
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• 1997

Recently, due to the increasing overloaded heavy vehicles and traffic volumes fatigue failures of steel highway bridges frequently occur. Therefore, it is important to decide rational fatigue design procedure which can reflect lifetime cumulative fatigue damage reasonably. In this study, cumulative fatigue damages are simulated for various bridge systems and traffic conditions. The AASHTO LRFD fatigue design procedure is reviewed and the current fatigue design loading format, in which a single representative truck is loaded regardless of bridge width, is found to yield inconsistent safety level. Improved loading format with rational design load level for fatigue design is suggested.

• Steel and Composite Structures
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• v.39 no.4
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• pp.453-469
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• 2021

The cracking at the transverse diaphragm cutout is one of the most severe fatigue failures threatening orthotropic steel decks (OSDs), whose mechanisms and crack treatment techniques have not been fully studied. In this paper, full-scale experiments were first performed to investigate the fatigue performance of polished cutouts involving the effect of an artificial geometrical defect. Following this, comparative experimental testing for defective cutouts strengthened with carbon fiber-reinforced polymer (CFRP) was carried out. Numerical finite element analysis was also performed to verify and explain the experimental observations. Results show that the combinative effect of the wheel load and thermal residual stress constitutes the external driving force for the fatigue cracking of the cutout. Initial geometrical defects are confirmed as a critical factor affecting the fatigue cracking. The principal stress 6 mm away from the free edge of the cutout can be adopted as the nominal stress of the cutout during fatigue evaluation, and the fatigue resistance of polished cutouts is higher than Grade A in AASHTO specification. The bonded CFRP system is highly effective in extending the fatigue life of the defective cutouts. The present study provides some new insights into the fatigue evaluation and repair of OSDs.

• Journal of Mechanical Science and Technology
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• v.17 no.3
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• pp.343-349
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• 2003

The fuselage-wing intersection suffers from the cyclic bending moment of variable amplitude. Therefore, the influence of cyclic bending moment on the delamination and the fatigue crack propagation behavior in AFRP/Al laminate of fuselage-wing was investigated in this study. The cyclic bending moment fatigue test in AFRP/Al laminate was performed with five levels of bending moment. The shape and size of the delamination Lone formed along the fatigue crack between aluminum sheet and aramid fiber-adhesive layer were measured by an ultrasonic C-scan. The relationships between da/dN and ΔK, between the cyclic bending moment and the delamination zone size, and between the fiber bridging behavior and the delamination zone were studied. As results, fiber failures were not observed in the delamination zone in this study, the fiber bridging modification factor increases and the fatigue crack growth rate decrease and the shape of delamination zone is semi-elliptic with the contour decreasing non-linearly toward the crack tip.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.12 no.4
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• pp.93-99
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• 2013

Parts and structures such as piston rod and knuckle joint for the use of hydraulic cylinder are often welded together in some fashion, usually due to cost and process effectiveness. Welding strongly affects the material by the process of heating and subsequent cooling as well as by the fusion process with additional filler material. Furthermore, a weld is usually far from being perfect, containing inclusions, pores, cavities, undercuts etc. As a consequence, fatigue failures appear in welded structures mostly at the welds rather than in the base metal, even if the latter contains notches. For this reason, fatigue analyses are of high practical interest for all welded structures under the action of cyclic loads. This paper describes the influence of welding parameters, material combinations and heat treatment on the fatigue behavior of welded cylinder rod. In addition, statistical characterization of stress-life response in weldment of hydraulic cylinder rod are presented.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.12 s.177
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• pp.149-155
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• 2005

The rotating components such as turbine rotors in service are generally subjected to multiaxial cyclic loading conditions. The prediction of fatigue lift for turbine rotor components under complex multiaxial loading conditions is very important to prevent the fatigue failures in service. In this paper, axial and torsional low cycle fatigue tests were preformed for 3.5NiCrMo steels serviced low pressure turbine rotor of nuclear power plant. Several methods to predict biaxial fatigue life such as Tresca, von Mises and Brown & Miller's critical plane approach were evaluated to correlate the experimental results for serviced NiCrMoV steel. The fracture mode and fatigue characteristics of NiCrMoV steel were discussed based on the results of fatigue tests performed under the axial and torsional test conditions. In particular, the Brown and Miller's critical plane approach was found to best correlate the experimental data with predictions being within a factor of 2.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1990.10a
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• pp.113-124
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• 1990

Tap lines are small branch piping generally less than two inches in diameter. They typically branch off of header piping having a much larger diameter. An example of a common tap line is a 3/4 inch size high point vent or low point drain. Most tap lines have at least one valve near the header tap connection to provide isolation. Two valves are often required for double isolation. A light water reactor(LWR) nuclear power plant will have several hundred tap lines. These lines come in many sizes and shapes and serve numerous functions. A single process piping valve may have three different tap lines associated with it (figure 1). Table 1 delineates the different categories of tap lines. Vibration failures of tap lines are a common occurrence in all industrial plants including nuclear and fossil power plants. These types of failures constitute a significant percentage of all piping related failures. An unscheduled plant shutdown or outage resulting from the failure of a tap line decreases plant reliability and may have a detrimental effect on plant safety. Most tap line vibration failures can be avoided through the use of appropriate routing and support techniques. Standardized designs can be developed for use in a myriad of applications. These designs will not only minimize failures but will also reduce the necessary analysis and installation efforts.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2001.10a
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• pp.247-252
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• 2001

Recently the steel parts used at automobiles are required to be used under high stress more than ever before in need of the weight down. To achieve this requirement of a high strength steel, it must be necessary to decrease inclusion content and surface defect as like decarburization, surface roughness etc.. In this study, the surface conditions are measured to know the influence on fatigue properties by two cases of shot peening of two-stage shot peening and single-stage shot peening. And for this study, two kinds of spring steel (SUP7, 50CrV4) are used. This study shows the outstanding improvement of fatigue properties at the case of two-stage shot peening in the rotary bending fatigue test and this is assumed to be from on low stress condition, the 1st stage shot peening is not affected by nonmetallic inclusion under metal. it is possible that the 2nd stage shot peening increases the fatigue life and the high stress but that is affected by nonmetallic inclusion under metal. so far beeasily 50CrV4 have made high stress. But, results also show fatigue failures originated at inclusion near surface, and this inclusion type is turned out to be a alumina of high hardness.