• Proceedings of the Computational Structural Engineering Institute Conference
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• 1991.10a
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• pp.29-32
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• 1991

It is well known that the fatigue damage process in composite materials is very complicated due to complex failure mechanisms that comprise debounding, matrix cracking, delamination and fiber splitting of laminates. Therefore, the residual strength, instead of a single dominant crack length, is chosen to describe the criticality of the damage accumulated in the sublaminate. In this study, two models for residual strength degradation established by Yang-Liu and Tanimoto-Ishikawa that are capable of predicting the statistical distribution of both fatigue life and residual strength have been investigated and compared. Statistical methodologies for fatigue life prediction of composite materials have frequently been adopted. However, these are usually based on a simplified probabilistic approach considering only the variation of fatigue test data. The main object of this work is to propose a fatigue reliability analysis model which accounts for the effect of all sources of variation such as fabrication and workmanship, error in the fatigue model, load itself, etc. The proposed model is examined using the previous experimental data of GFRP and it is shown that it can be practically applied for fatigue problems in composite materials.

• Transactions of the Korean Society of Mechanical Engineers
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• v.3 no.3
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• pp.98-103
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• 1979

The fatigue limit of an ion-nitrided steel was investigated experimentally and analytically. It is found that fatigue limit can singificantly be increased by ion-nitriding, and that the case depth is the most important parameter which determines the fatigue limit. The data indicate that fatigue limit increases with the case depth as well as the surface hardness of the nitrided steel. The fracrographs of the fracture surfaces taken by a scanning electron microscope show that the fisch-eye is located at the subsurface of failed specimens. Assuming that crack propagates from the subsurface inclusions, an analytical model is proposed to predict the fatigue limit. Taking into account the stress distrbution of a nitrided specimen, fatigue limit is predicted as a function of the case depth. The proposed semiemprical formula agrees satisfactorily with the experimental data obtained from rotating beam fatigue testing.

• Journal of the Korean Society of Safety
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• v.25 no.1
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• pp.65-71
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• 2010

In this study, a series of finite element analysis using LUSAS were performed in order to assess the quantitative effects of repair and retrofit of stringer in steel plate girder railway bridge with fatigue cracks. And cutoff types of end part of upper flange were considered as right-angled type and round-angled type. Also, as a method of repair and retrofit of fatigue cracks in stringer, perforation of stop-hole and installation of bracket were considered. From the analysis result, it was possible to assess the fatigue safety and fatigue life of stringer with fatigue cracks, and to estimate the stress intensity factor range in cut-off part of stringer using J-integral method. Also, according to the method of perforation of stop-hole and installation of bracket, it was possible to calculate the crack propagation life at the cut-off part of stringer.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.11
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• pp.149-154
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• 2004

For a present study, we investigated fatigue behavior of cracked aluminum repaired by unidirectional graphite/epoxy composite material. Three different specimens were used in the fatigue tests: cracked aluminum, cracked aluminum repaired by graphite/epoxy composite patch, and plasma-treated aluminum repaired by graphite/epoxy composite patch. The surface of the aluminum was treated using a DC plasma. The results showed that the fatigue crack growth behavior of cracked aluminum was significantly improved by repairing the cracked area with a composite patch. Specifically, the specimen repaired by composite patch showed about 300％ more fatigue lift than the cracked aluminum. In particular, the plasma-treated aluminum repaired by composite patch showed almost 12 ％ more fatigue life than the cracked aluminum repaired by graphite/epoxy composite patch. The increased fatigue life of plasma-treated case was attributed to the surface roughness of aluminum by plasma treatment.

• Journal of Korean Society of Steel Construction
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• v.14 no.3
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• pp.453-461
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• 2002

This study investigated the fatigue stength of non-load carrying cruciform welded joints, which was maunfactured using the SM570 and POSTEN80 high strengh steel. Factors such as fatigue strength, fatigue crack initiation and propagation, thickness effect, and the relatioinship between the static strength and the fatigue strength were examined and compared with previous fatigue testing results. Results showed that the fatigue strength of SM570 and POSTEN80 steel are higher than the grade represented on the design specification. It is also identified the size effect and the dependence of the static stength in a few cases.

• Proceedings of the Korean Nuclear Society Conference
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• 2001.05a
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• pp.347.2-347
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• 2001

• Proceedings of the Korean Nuclear Society Conference
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• 1999.05a
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• pp.221-221
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• 1999

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.256-261
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• 2007

The purpose of this study is to investigate the effect of shot peening on the fatigue strength and fatigue life of two kinds of aluminum alloys. The fatigue strength behavior of aluminum alloys were estimated by the stress ratio and shot velocities. The fatigue life and strength increased with increasing the test shot velocity. However, at the shot velocity range between 50m/s and 70m/s, the compressive residual stress phenomena were observed in test conditions of different shot velocity. The optimal shot velocity is acquired by considering the peak values of the compressive residual stress, dislocations, brittle striation, slip, and fisheye on the fracture surface of test specimen. It was observed from the SEM observation on the deformed specimen that the brittle striation, fisheye were showed in the intergranular fracture structure boundaries at the this velocities. Therefore, fatigue strength and fatigue life would be considered that shot velocity has close relationship with the compressive residual stress.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.9
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• pp.1078-1085
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• 2011

Prediction of a minimum crack size for growth, which is defined as a crack size that grows fast enough to keep ahead of its removal by contact wear and periodic grinding, is the most demanding work to prevent rail from fatigue failure and develop cost effective railway maintenance strategy In this study, we investigated the wheel load increment due to a rail defect during a train ran over it, and its effect on the minimum crack size for growth. For this purpose, we developed simulation software based on the Fletcher and Kapoor's "2.5D" model and measured wheel load increment during a train passed over a defect. A maximum contact pressure and contact patch size were calculated by 3D FEM and crack growth analyses were performed by varying two of dominant contact contributors; surface friction coefficient(0.1, 0.2, 0.3 and 0.4) and crack aspect ratio. The minimum crack sizes for growth were calculated from 0.29 to 1.44mm depending on the contact conditions. They were decreasing with increasing surface friction coefficient and decreasing with crack aspect ratio(a/b).

• Nuclear Engineering and Technology
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• v.40 no.3
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• pp.225-232
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• 2008

To understand the effect of the cyclic strain rate on the environmentally assisted cracking behaviors of SA508 Gr.1a low alloy steel in deoxygenated water at $310^{\circ}C$, the fatigue surface and a sectioned area of specimens were observed after low cycle fatigue tests. On the fatigue surface of the specimen tested at a strain rate of 0.008 %/s, unclear ductile striations and a blunt crack tip were observed. Therefore, metal dissolution could be the main cracking mechanism of the material at this strain rate. On the other hand, on the fatigue surfaces of the specimens tested at strain rates of 0.04 and 0.4 %/s, brittle cracks and flat facets, which are evidences of the hydrogen induced cracking, were observed. In addition, a tendency of linkage between the main crack and the micro-cracks was observed on the sectioned area. Therefore, at higher strain rates, the main cracking mechanism could be hydrogen induced cracking. Additionally, evidence of the dissolved MnS inclusions was observed on the fatigue surface from energy dispersive x-ray spectrometer analyses. Thus, despite the low sulfur content of the test material, the sulfides seem to contribute to environmentally assisted cracking of SA508 Gr.1a low alloy steel in deoxygenated water at $310^{\circ}C$.