• Journal of the Korean Society for Heat Treatment
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• v.11 no.4
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• pp.274-282
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• 1998

The effect of Al addition on the fatigue properties of austenitic Fe-25Mn-Al-0.5C steels was studied. When Al was not added to the Fe-25Mn 0.5C steel, the strain induced ${\varepsilon}$ martensites, deformation twins and slip bands were formed during fatigue deformation. When 2wt% of Al was added to the steel, the deformation twins and slip bands were formed during fatigue deformation. When 5wt% of Al was added, only slip bands were formed. In low cycle fatigue test, the alloys containing 0wt% and 2wt%Al showed the cyclic hardening due to ${\varepsilon}$ martensites and deformation twins, resulting in shorter fatigue lives than the alloy containing 5wt%Al. In fatigue crack propagation test, the alloy without Al showed the highest crack propagation rate. The fracture surface of the alloy without Al was flat, whereas that of the alloy with 2% or 5%Al was rough. The ${\Delta}K_{th}$, values of the alloys with 0%, 2% and 5%Al were 16, 17.5, and $20.5MPam^{1/2}$, respectively.

• Journal of the Korea institute for structural maintenance and inspection
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• v.9 no.3
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• pp.111-119
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• 2005

The objective of this study is to predict fracture movements accurately and reliably by nonlinear analysis of the response of RC shear wall or RC flange sections. Hognestad's and Vallenas's theories are used for concrete model and Ramberg-Osgood's theory is used for steel model. Non-linear analysis considering confined concrete and unconfined concrete is performed. Mander's Fiber Approach Section analysis, new strain profile considering the Gamma factor are used to this section analysis. The section analysis considering cases of precracked, uncracked, boundary warping and shear warping is performed.

• Electrical & Electronic Materials
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• v.10 no.4
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• pp.327-333
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• 1997

Experiments on mechanical fatigue were conducted using the specimens which were cut from hydrogen cooled generator(rated 22kV and 50OMW) stator windings. We have investigated the aged mechanism of mica/epoxy insulation systems under air or hydrogen by both the tensile and compressive loadings. The fracture of generator stator windings is generally affected by mechanical stress. Thus, the tensile strength test were conducted. In this case, the maximum strength and strain are quite different between sound and aged specimens. It is observed that low bonded interface parts of tapes generally have lower strength than those of normal tapes which causes stress. In order to estimate the effects of cyclic load by the electromagnetic forces while the generator starts/stops, the mechanical fatigue test was also conducted. It is confirmed that the equation of expected life depends on stress amplitude and number of cycles. Though the stress amplitude and number of cycles are very tiny, the tensile fatigue of aged specimens under hydrogen atmosphere is bigger than those under air. In the case of hydrogen atmosphere, the tensile stress gives bigger effect than the compressive one.

• International Journal of Fluid Machinery and Systems
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• v.8 no.4
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• pp.254-263
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• 2015

The failure of hydraulic turbine runners is a rare event. So in order to assess the reliability of these components one cannot rely solely on the number of observed failures in a given population. However, as there is a limited number of degradation mechanisms involved, it is possible to use physically-based reliability models. Such models are often more complicated but are able to account for physical parameters in the degradation process. They can therefore help provide solutions to improve reliability. With such models, the effect of materials properties on runner reliability can be highlighted. This paper presents a brief review of the Kitagawa-Takahashi diagram which links the damage tolerance approach, based on fracture mechanics, to the stress or strain-life approaches. Using simplified response spectra based on runner stress measurements, we will show how fatigue reliability is sensitive to materials fatigue properties, namely fatigue crack propagation behaviour and fatigue limit obtained on S-N curves. Furthermore, we will review the influence of the main microstructural features observed in 13%Cr-4%Ni stainless steels commonly used for runner manufacturing. The goal is ultimately to identify the most influential microstructural features and to quantify their effect on fatigue reliability of runners.

• Journal of Applied Reliability
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• v.13 no.2
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• pp.141-151
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• 2013

The study of environmental stress cracking (ESC) by contact with plastic rulers and PVC erasers which was artificially added to the external stress was carried out in order to accelerate the chemicals crack. To analyze a blooming plasticizer that migrates from the inside of blended eraser to the surface of the eraser, the ESC experiments were carried out at room temperature and $50^{\circ}C$ conditions. The chemicals crack shape caused by the plasticizer and the brittle fracture shape resulted from the external stress were observed in consequence with the cross-sectional surface analysis of the ruler crack by the ESC. The bending strength of the plastic rulers were fractured prior to the yield point and it had low bending flexure stress. We presented that ESC of polymer materials was affected by the polarity of the chemicals and polymer, the exposure time to chemicals, the exposure temperature and the level of strain on the polymer.

• Nuclear Engineering and Technology
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• v.45 no.1
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• pp.67-72
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• 2013

Stress corrosion cracking (SCC) behaviors of Alloy 690 were studied in lead-containing aqueous alkaline solutions using the slow strain rate tension (SSRT) tests in 0.1M and 2.5M NaOH with and without PbO at $315^{\circ}C$. The side and fracture surfaces of the alloy were then examined using scanning electron microscopy after the SSRT test. Microstructure and composition of the surface oxide layer were analyzed by using a field emission transmission electron microscopy, equipped with an energy dispersive X-ray spectroscopy. Even though Alloy 690 was almost immune to SCC in 0.1M NaOH solution, irrespective of PbO addition, the SCC resistance of Alloy 690 decreased in a 2.5M NaOH solution and further decreased by the addition of PbO. Based on thermodynamic stability and solubility of oxide, high Cr of 30wt% in the Alloy 690 is favorable to SCC in mild alkaline and acidic solutions whereas the SCC resistance of high Cr Alloy 690 is weakened drastically in the strong alkaline solution where the oxide is not stable any longer and solubility is too high to form a passive oxide locally.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.207-212
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• 2009

In order to optimize the press formability of incremental sheet forming for complex shape (e.g human face), a combination of both CAM and FEM simulation, is implemented and evaluated from the histories of stress and strain value by means of finite element analysis. Here, the results, using ABAQUS/Explicit finite element code, are compared with fracture limit curve (FLC) in order to predict and optimize the press formability by changing parameters of tool radius and tool down-step according to the orthogonal array of Taguchi's method. Firstly, The CAM simulation is used to create cutter location data (CL data). This data are then calculated, modified and exported to the input file format required by ABAQUS through using MATLAB programming. The FEM results are implemented for negative incremental sheet forming and then investigate by experiment.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.364-367
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• 2007

In nano-imprint lithography (NIL) process, which has shown to be a good method to fabricate polymeric patterns, several kinds of pattern defects due to thermal effects during polymer flow and mold release operation have been reported. A typical defect in NIL process with high aspect ratio and low resist thickness pattern is a resist fracture during the mold release operation. It seems due to interfacial adhesion between polymer and mold. However, in the present investigation, FEM simulation of NIL molding process was carried out to predict the defects of the polymer pattern and to optimize the process by FEA. The embossing operation in NIL process was investigated in detail by FEM. From the analytical results, it was found that the lateral flow of polymer resin and the applied pressure in the embossing operation induce the weld line and the drastic lateral strain at the edge of pattern. It was also shown that the low polymer-thickness result in the delamination of polymer from the substrate. It seems that the above phenomena cause the defects of the final polymer pattern. To reduce the defect, it is important to check the initial resin thickness.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.9
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• pp.1493-1500
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• 2001

Recently small punch creep testing (or miniature disc bend creep test) has received much attention through European collaborative research projects. This method was considered as a substitute for the conventional creep rupture testing by which the residual creep life is measured from the specimen taken out from serviced components of high temperature plants. It would be beneficial if the material creep properties such as power law creep constants as well as the creep rupture life can be measured from the small punch creep test. In this paper a method of assessing creep constants from the small punch creep testing is proposed. Finite element analyses were performed to investigate evolution of stress and strain rate at the weakest locations of the small punch creep specimen. Elastic-plastic-secondary creep analyses were carried out. The estimation equations for creep constants by the small punch creep testing are proposed based on the finite analysis results. Small punch creep tests were also performed with 9Cr steel and the accuracy of the proposed equation was verified by the experimental results.

• Journal of the Earthquake Engineering Society of Korea
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• v.16 no.6
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• pp.21-28
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• 2012

Welded Unreinforced Flange-Welded Web (WUF-W) connection is one of Special Moment Frame (SMF) specified in ANSI/AISC-358. From the experimental test of WUF-W connection specimens conducted by the previous study, fracture occurred in the beam flange before achieving total inter-story drift angle of 0.04radian required for Special Moment Frames (SMF) system even though the specimens satisfied the design and detailing requirement specified in ANSI/AISC-358. These results are estimated as problem of the access hole geometry. In this study, a full-scale WUF-W connection specimen was made with a modified access hole geometry, and tested with the same test setting and loading as the previous test. From test results, the deformation capacity of the tested WUF-W connection specimen exceeded 4%, which is required for connections in SMF system. Comparing with the WUF-W specimens of the previous study, the strain demand of the beam flange in the tested specimen was decreased and energy dissipation capacity of the specimen was improved.