• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.868-880
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• 2020

One of the main concerns in the structural integrity of offshore pipelines is mechanical damage from external loads. Pipelines are exposed to fatigue failure in welded joints due to geometric discontinuity. In addition, fatigue loads such as currents, waves, and platform motions may cause significant plastic deformation and fracture or leakage within a relatively low-cycle regime. The 2007 ASME Div. 2 Code adopts the master S―N curve for the fatigue evaluation of welded joints based on the mesh-insensitive structural stress. An extension to the master S―N curve was introduced to evaluate the low-cycle fatigue strength. This structural strain method uses the tensile properties of the material. However, the monotonic tensile properties have limitations in describing the material behavior above the elastic range because most engineering materials exhibit hardening or softening behavior under cyclic loads. The goal of this study is to extend the cyclic stress-strain behavior to the structural strain method. To this end, structural strain-based procedure was established while considering the cyclic stress-strain behavior and compared to the structural strain method with monotonic tensile properties. Finally, the improved prediction method was validated using fatigue test data from full-scale girth-welded pipes.

• KEPCO Journal on Electric Power and Energy
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• v.7 no.2
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• pp.241-248
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• 2021

High cycle fatigue life for the cables with two different types of clamps is estimated comparatively through acceleration testing. The high cycle fatigue fracture of overhead lines is caused mainly by the aeolian vibration which is induced by vortex shedding. It is necessary to manage the integrity of cables continuedly considering that the aeolian vibration is unavoidable since it occurs in steady and relatively low wind velocity. Two types of clamps which are largely used for overhead lines of the distribution grids are selected and failure data are obtained by step stress testing with a electrodynamic shaker with them. The inverse power law is assumed to describe the stress-life relationship and the fatigue limit at any specified life is supposed to follow Weibull distribution. The life of the cable is defined as the number of cycles to the time that one of strands is completely broken. Finally, the fatigue limits of the cables with two clamp types are estimated at the reference life of 500 Mcycles and compared each other based on a bending vibration amplitude.

• Journal of Mechanical Science and Technology
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• v.20 no.5
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• pp.675-680
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• 2006

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 mm thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et al., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter-or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.24 no.6
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• pp.589-593
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• 2015

The demand of high cycle fatigue behavior on plate material is increasing because of its various applications. However, the high-cycle fatigue life data of the plate material is very rare compared to the rod material. Thus, in this study, a plate specimen is designed for the ultrasonic fatigue test because it is time efficient as compared to the conventional fatigue test. To apply the ultrasonic fatigue test, the specimen design is required to resonate at 20 kHz. Therefore, the dynamic elastic modulus was determined by measuring the resonance frequency with a piezoelectric element and laser doppler vibrometer (LDV). As a result, the plate specimen is designed and demonstrated using the ultrasonic fatigue testing machine. The ultrasonic fatigue test results were compared with the hydraulic fatigue test results.

• Journal of Mechanical Science and Technology
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• v.19 no.7
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• pp.1393-1404
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• 2005

Fatigue crack growth and life have been estimated based on established empirical equations. In this paper, an alternative method using artificial neural network (ANN) -based model developed to predict fatigue damages simultaneously. To learn and generalize the ANN, fatigue crack growth rate and life data were built up using in-plane bending fatigue test results. Single fracture mechanical parameter or nondestructive parameter can't predict fatigue damage accurately but multiple fracture mechanical parameters or nondestructive parameters can. Existing fatigue damage modeling used this merit but limited real-time damage monitoring. Therefore, this study shows fatigue damage model using backpropagation neural networks on the basis of X -ray half breadth ratio B / $B_o$, fractal dimension $D_f$ and fracture mechanical parameters can estimate fatigue crack growth rate da/ dN and cycle ratio N / $N_f$ at the same time within engineering limit error ($5\%$).

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.6
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• pp.707-712
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• 2012

Very high cycle fatigue (VHCF) behavior of aerospace components has emerged much attention due to their long service life. In this study, a piezoelectric ultrasonic fatigue testing (UFT) system has been developed by Mbrosiatec Co., Ltd. to study the high cycle fatigue (HCF) strength of Ti-6Al-4V alloy. Hourglass-shaped specimens have been investigated in the range from $10^6$ to $10^9$ cycles at room temperature under completely reversed R = -1 loading conditions,. Scanning electron microscopy (SEM) analysis revealed that failures occurred in the entire range up to the gigacycle regime, and the fractures have beenfound to be initiated from the surface, unlike in steels. However, it was found from the SEM microgprahs that microcracks transformed into intergranular fractures. Thus, it can be concluded from according to the results that this test method can be applicable to commercialized automotive and railroad parts that require high cycle fatigue strength.

• Physical Therapy Korea
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• v.12 no.2
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• pp.90-97
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• 2005

By measuring changes in blood lactate and plasma enzyme (CPK, GOT, GPT) with electrical stimulation applied at two duty cycles, this study is intended to look into which type of duty cycle may have more effects on blood lactate and plasma enzyme constituents through animal experiment so as to determine any duty cycle appropriate for electrical treatment. In this study, electrical stimulation was applied to total 20 Korean house rabbits (weight: 3~3.5 kg) by means of an electrical therapeutic apparatus called TS6000 (made in Netherlands) at duty cycle of 50% and 20% respectively for 30 minutes. Here, 5 cc of blood was collected from their carotid artery before stimulation and in 30 minutes after stimulation respectively to carry out biochemical experiment and analysis. As determined through the above experiment, blood lactate rate was increased to 333.07% at 50% duty cycle after experiment and 185.71% at 20% duty cycle after experiment respectively. In both cases, blood lactate rate was significantly increased to higher level after electrical stimulation than before. Moreover, the rate of change in the average of blood lactate rate at both duty cycles also showed significant differences. CPK rate was boosted to 301.82% at 50% duty cycle after experiment and 321.35% at 20% duty cycle after experiment respectively. In both cases, CPK rate was remarkably boosted to higher level after stimulation than before (p<.05). However, there was not any significant difference in the rate of change in average CPK at both duty cycles (p<.05). GOT rate was significantly boosted up to 38.97% at 50% duty cycle after experiment (p<.05), while it was slightly increased to 1.68% at 20% duty cycle after experiment without any significant difference. Rather, GPT rate dropped slightly at both duty cycles after experiment, but there was not any significant difference. Although blood lactate and GOT were relatively less generated at 20% duty cycle after electrical stimulation than at 50% duty cycle, the change of duty cycle didn't have any significant influence on CPK rate. In this regard, this study failed to come any consistent conclusion about the association between change of duty cycle and muscle fatigue. Therefore, it is advisable that follow-up studies seek various ways to a little more effectively apply electrical stimulation to laboratory animals by avoiding their muscle fatigue. GOT rate was significantly boosted up to 38.97% at 50% duty cycle after experiment (p<.05), while it was slightly increased to 1.68% at 20% duty cycle after experiment without any significant difference. Rather, GPT rate dropped slightly at both duty cycles after experiment, but there was not any significant difference. Although blood lactate and GOT were relatively less generated at 20% duty cycle after electrical stimulation than at 50% duty cycle, the change of duty cycle didn't have any significant influence on CPK rate. In this regard, this study failed to come any consistent conclusion about the association between change of duty cycle and muscle fatigue. Therefore, it is advisable that follow-up studies seek various ways to a little more effectively apply electrical stimulation to laboratory animals by avoiding their muscle fatigue.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2008.06a
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• pp.755-756
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• 2008

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2010.11a
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• pp.757-758
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• 2010

• Tribology and Lubricants
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• v.22 no.6
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• pp.320-328
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• 2006

In this study, the simulation of rolling contact fatigue based on stress analysis is conducted under Elastohydrodynamic Lubrication state. To predict a crack initiation life accurately, it is necessary to calculate contact stress and subsurface stresses accurately. Contact stresses are obtained by contact analysis of a semi-infinile solid based on the use of influence functions and the subsurface stress field is obtained using rectangular patch solutions. And a numerical algorithm using Newton-Rapson method was constructed to calculate the Elastohydrodynamic lubrication pressure. Based on these stress values, several multiaxial high-cycle fatigue criteria are used and the critical loads corresponding to fatigue limits are calculated.