• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.4
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• pp.323-330
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• 2008

High cycle bending fatigue of socket welded small bore pipe was characterized, and also the fatigue crack initiation of small bore pipe was monitored in situ by the acoustic emission (AE) technique. The STS 316L stainless steel specimens were prepared by gas tungsten arc welding (GTAW) process having the artificial defect (i.e., lack of penetration) and defect free at the root. The fatigue failure was occurred at the loc for high stress and root for relatively low stress. The crack initiation cycles ($N_i$) was defined to the abrupt increase in AE counts during the fatigue test, and then the cracks were observed by the radiographic test and electron microscope before and after the fatigue crack initiation cycles. The socket welded pipe damaged by bending fatigue was studied regarding the welding defect, failure mode, and crack initiation cycles for the diagnosis and monitoring.

• Women's Health Nursing
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• v.26 no.3
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• pp.231-239
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• 2020

Purpose: The purpose of this study was to investigate factors affecting premenstrual symptoms among nursing students, focusing on depression, fatigue, and body esteem. Methods: The participants were 145 nursing students at a university located in Changwon, Korea. Data were collected from November 2 to November 30, 2019 using self-reported structured questionnaires, and analyzed using descriptive statistics, independent t-test, analysis of variance, Pearson correlation coefficients, and multiple regression analysis. Results: The mean item score for premenstrual symptoms was 2.52±0.92, indicating a low level. The mean score for depression was 16.05±7.72, and 15.2% of participants were found to be moderately depressed and 9.7% severely depressed. The mean item score for fatigue was 4.84±0.84, indicating a moderate level, and body esteem was 2.94±0.44, indicating a moderate level. The premenstrual symptoms of nursing students showed a statistically significant correlation with depression (r=-.58, p<.001), fatigue (r=.33, p<.001), and body esteem (r=-.28, p<.001). Factors impacting premenstrual symptoms of nursing students were depression (β=.47, p<.001), dysmenorrhea (β=-.18, p=.009), menstrual cycle irregularity (β=.17, p=.013), and body esteem (β=-.14, p=.038). The total explanatory power of these variables was 41.0%. Conclusion: Findings from this sample of nursing students suggest that intervention programs to relieve premenstrual symptoms should focus on depression, menstrual cycle irregularity, dysmenorrhea, and body esteem.

• Transactions of the Korean Society of Automotive Engineers
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• v.18 no.3
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• pp.133-142
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• 2010

The present work focuses on the characterization of material parameters of the Overlay(multilinear hardening) model for analyzing the non-isothermal cyclic deformation. In the previous study, all the parameters were especially based on the Overlay theories, and a simple method was suggested to find out the best material parameters for the isothermal cyclic deformation analysis. Based on the previous research this paper f dther improves the isothermal parameters and suggests how to apply the isothermal parameters to the non-isothermal conditions especially for the description of TMF(Thermo-Mechanical Fatigue) hysteresis behavior. The parameters are determined and calibrated using 400 series stainless steel test data in the reference papers. For the implementation into ABAQUS, a user subroutine is developed by means of ABAQUS/UMAT. The finite element results show good agreement with test for the case of uniaxial non-isothermal cyclic loading, signifying the proposed method can be used in the TMF analysis of the converter-inserted heavy duty muffler system and the stainless steel exhaust-manifold system which are to be done in our future research.

• Journal of KSNVE
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• v.11 no.3
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• pp.410-415
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• 2001

This paper describes an experimental analysis for improving the stability of blade failure due to the vibration resonance, which happens in the low-pressure steam turbine. Some cracks due to high cycle fatigue were found in the blades of a low-pressure turbine after long time operation. Impact test showed that such failure was mainly caused by the resonance. In other words, since one of the natural frequencies of the grouped blade is very close to the excitation frequency of the nozzle, the resonant vibration leads to a large amplitude of displacement and results in a large amount of stress that may cause fatigue failures in the blades. It is interesting that the blade failures occur only at blades neighboring with the nodal points of the natural vibration mode whose natural frequency is close to the nozzle passing frequency. The effective methods for increasing the reliability against the blade vibration are a heightening the fatigue limit of the blade using an advanced material and a removing the resonance away from the operating speed. It is well known that the removal of theresonance could be obtained by the installation of different types of shrouds, wires, and links between the blades as well as by the chance of the number of nozzles. In the present work, two kinds of modification for avoiding the resonance haute been considered; 1) slot-type finger, 2) long span cover. Full-scale mockup tests have been performed in order to confirm the verification for modification in the shop. Test results show that the use of long span cover is very useful to change the natural frequencies of the grouped blade and to avoid the resonance effectively.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.3
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• pp.521-527
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• 2002

In this paper, tensile behavior and low cycle fatigue behavior of 316L stainless steel which is currently favored structural material for several high temperature components such as the liquid metal cooled fast breeder reactor (LMFBR) were investigated. Research was performed at 55$0^{\circ}C$, $600^{\circ}C$ and $650^{\circ}C$ since working temperature of 316L stainless steel in a real field is from 40$0^{\circ}C$ to $650^{\circ}C$. From tensile tests performed by strain controls with $1{\times}10^{-3}/s,\; l{\times}10^{ -4}/s \;and\; 1{\times}10/^{ -5}/ s $ strain rates at each temperature, negative strain rate response (that is, strain hardening decreases as strain rate increases) and negative temperature response were observed. Strain rate effect was relatively small compared with temperature effect. LCF tests with a constant total strain amplitude were performed by strain control with a high temperature extensometer at R.T, 55$0^{\circ}C$, $600^{\circ}C$, $650^{\circ}C$ and total strain amplitudes of 0.3%~0.8% were used and test strain rates were $1{times}10^{-2} /s,\; 1{times}10^{-3} /s\; and\; 1{times}10^{-4} /s$. A new energy based LCF life prediction model which can explain the effects of temperature, strain amplitude and strain rate on fatigue life was proposed and its excellency was verified by comparing with currently used models.

• Journal of the Korean Society for Precision Engineering
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• v.25 no.7
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• pp.131-138
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• 2008

The microstructural effects on the ultrasonic nonlinearity were investigated in isothermally degraded ferritic 2.25Cr-1Mo steel and low cycle fatigued copper. The variation in ultrasonic nonlinearity (${\beta}/{\beta}_0$) was interpreted as resulting from microstructural changes supported by the electron microscopy and X-ray diffraction, in addition to the mechanical test (Victor's hardness and ductile-brittle transition temperature). The ultrasonic nonlinearity of 2.25Cr-1Mo steel increased abruptly in the initial 1,000 h of degradation, and then changed little due to the coarsening of carbide and precipitation of stable $M_6C$ carbide during isothermal degradation. The ultrasonic nonlinearity of copper increased with the fatigue cycles due to the evolution of dislocation cell substructure.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.3
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• pp.15-22
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• 2003

The fatigue crack growth model is derived and the retardation model is proposed. The fatigue crack growth model considers the residual plastic stretch on the crack surface which results from the plastic deformation at the tip of fatigue crack. The fatigue crack growth rate is calculated by using the cumulative fatigue damage and plastic strain energy in the material elements at the crack tip. This model gives the crack growth rate in reasonable agreement with test data for aluminum alloy AL6061-T651 and 17-4PH casting steel. The fatigue crack growth retardation model is based on the residual plastic stretch produced from a tensile overload which reduced the plastic strain range of the following load cycles. A strip-yield model of a crack tip plasticity is used for the calculation of a plastic zone size. The proposed retardation model characterized the observed features and delayed retardation of the fatigue crack growth under tensile overload.

• Journal of Advanced Marine Engineering and Technology
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• v.9 no.4
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• pp.328-338
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• 1985

When the low temperature service steels are used as materials for welded structure, some problems-brittleness and weld cracking, etc.-occur in welded part due to the change of mechanical and metallurgical characteristics resulted from the thermal cycle during the welding procedure. In this study, the experiments were conducted to investigate the change of mechanical and metallurgical characteristics of the welded part for the low temperature and high pressure service steels. Moreover, the Static and Dynamic Implant Test Method was introduced to this study in order to find out the mechnism of weld cracking. In addition, the fracture toughnesses of welded bond were inspected under the various low temperature environments. Main results obtained are as follows; 1) The effect of the hydrogen on the fatigue characteristics of the weld bond can be estimated by the new self-contrived Dynamic Implant Test equipment. 2) The fine micro-structure and low hardness in the heat affected zone can be obtained by the small heat input multi-pass welding. 3) The susceptibility of the delayed cracking is largely affected by the condition of used electrode. 4) The transition temperature of the fracture surface in weld bond appears to be higher 20 .deg. C than that in base metal.

• Journal of the Korean Society of Safety
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• v.29 no.4
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• pp.9-14
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• 2014

Self-piercing riveting(SPR) is a mechanical fastening technique which is put pressure on the rivet for joining the sheets. Unlike a spot welding, SPR joining does not make the harmful gas and $CO_2$ and needs less energy consumption. In this study, static and fatigue tests were conducted using tensile-shear specimens with Al-5052 plates for evaluation of fatigue strength of the SPR joints. During SPR joining process for the specimen, using the current sheet thickness and a rivet, the optimal applied punching force was found to be 21 kN. And, the maximum static strength of the specimen produced at the optimal punching force was 3430 N. During the fatigue tests for the specimens, interface failure mode occurred on the top substrate close to the rivet head in the most high-loading range region, but on the bottom substrate close to the rivet tail in the low -loading range region. There was a relationship between applied load amplitude $P_{amp}$ and lifetime of cycle N for the tensile-shear, $P_{amp}=3395.5{\times}N^{-0.078}$. Using the stress-strain curve of the Al-5052 from tensile test, the simulations for fatigue specimens have been carried out using the implicit finite element code ABAQUS. The relation between von-Mises equivalent stress amplitude and number of cycles was found to be ${\sigma}_{eq}=514.7{\times}N^{-0.033}$.

• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.1
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• pp.112-119
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• 2021

The failure mode of piping systems due to seismic loads is the low-cycle fatigue failure with ratcheting, and it was found that the element in which nonlinear behavior is concentrated and damage occurs is the elbow. In this study, to quantitatively express the failure criteria for a pipe elbow of SCH40 3-inch carbon steel under low-cycle fatigue, the limit state was defined as leakage, and the in-plane cyclic loading test was conducted. For the carbon steel pipe elbow, which is the vulnerable part to seismic load of piping systems, the damage index was represented using the moment-deformation angle relationship, and it was compared and analyzed with the damage index calculated using the force-displacement relationship. An attempt was made to quantitatively express the limit state of the carbon steel pipe elbow involving leakage using the damage index, which was based on the dissipated energy caused by repeated external forces.