• Polymer(Korea)
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• v.36 no.1
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• pp.88-92
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• 2012

Long-term performance of polymer under constant sustained load has been the main research focus, which created a need for the accelerated test method providing proper lifetime assessment. Cycling fatigue loading is one of the accelerated test method and has been of great interest. Microstructure change of high density polyethylene under cyclic fatigue loading and creep was examined utilizing a tensile device specially designed for creep and fatigue test and also can be attachable to the X-ray diffractometer. In this way, the crystal morphology change of polyethylene under creep and cyclic fatigue load was successfully monitored and compared. Despite the marked differences in macroscopic deformation between the creep and cyclic fatigue tests, crystal morphology such as crystallinity, crystal size, and $d$-spacing was as nearly identical between the two test cases. Specimens pre-deformed to different strains, i.e., before yield point (BYP), at yield point (YP) and after yield point (AYP), however, showed markedly different changes in crystal morphology, especially between AYP and the other two specimens.

• Journal of Ocean Engineering and Technology
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• v.5 no.2
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• pp.207-207
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• 1991

The crack length measurements by electrical potential(EP) method for 1% Cr-Mo-V and 12%Cr steel of 0.5T-CT specimen were performed at $500^{\circ}C, 600^{\circ}C 700^{\circ}C$, and an applicability of stress intensity factor($K_I$), net section stress($\sigma_{net}$), $C^*$-ingegral and $C_t$ parameter was studied to measure creep crack growth rate(da/dt) with side groove and without side groove under static and cyclic loading condition. The experimental result could be summarized as follows: 1) Crack measurement by EP method was available and coincided with the Johnson,s analytical equation. 2) da/dt by $K_I$ and $\sigma_{net}$ was not adequate because of the wide scatter band according to load and temperature, but $C^*$-integral, except for transition region, was adequate. 3) $C_t$ parameter showed the best fitted line through total creep region without relating with both temperature and load condition. 4) Under the cyclic loading condition, $C_t$ parameter was proper to extimate da/dt. And it was shown that da/dt for 1% Cr-Mo V steel under the static condition(R=1) was 1.16 times faster than the case under cyclic loading(R=0), and for 12% Cr steel, 1.43 times.

• Journal of Ocean Engineering and Technology
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• v.5 no.2
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• pp.67-75
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• 1991

The crack length measurements by electrical potential(EP) method for 1% Cr-Mo-V and 12%Cr steel of 0.5T-CT specimen were performed at $500^{\circ}C, 600^{\circ}C 700^{\circ}C$, and an applicability of stress intensity factor($K_I$), net section stress($\sigma_{net}$), $C^*$-ingegral and $C_t$ parameter was studied to measure creep crack growth rate(da/dt) with side groove and without side groove under static and cyclic loading condition. The experimental result could be summarized as follows: 1) Crack measurement by EP method was available and coincided with the Johnson, s analytical equation. 2) da/dt by $K_I$ and $\sigma_{net}$ was not adequate because of the wide scatter band according to load and temperature, but $C^*$-integral, except for transition region, was adequate. 3) $C_t$ parameter showed the best fitted line through total creep region without relating with both temperature and load condition. 4) Under the cyclic loading condition, $C_t$ parameter was proper to extimate da/dt. And it was shown that da/dt for 1% Cr-Mo V steel under the static condition(R=1) was 1.16 times faster than the case under cyclic loading(R=0), and for 12% Cr steel, 1.43 times.

• Journal of Ocean Engineering and Technology
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• v.12 no.1
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• pp.76-84
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• 1998

In this 1st report, the relationship between pure creep properties and initial strain was studied and also its acoustic emission test was performed during creep test at 500, 600 and $700^{\circ}C$. And the applicability of the acoustic emission technique was investigated to analyze the quantitive relationship between all the pure properties (creep strength, creep repture time or creep life, steady state creep rate, total creep rate, creep strain, total creep strain, etc.) and the initial strains as well as to analyze AE properties during the pure creep loading condition.

• Proceedings of the Korean Nuclear Society Conference
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• 2003.05a
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• pp.406.1-406.1
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• 2003

• Proceedings of the Korea Concrete Institute Conference
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• 2003.05a
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• pp.555-560
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• 2003

Fatigue strength of concrete is ususlly presented by the Wohler Curve. But, new dimension T(time) from the view point of cyclic creep concept should be considerd. This paper presented four variable F-N-T-R relationship, this four variable relationship simultaneously accounts for the time effect and the effect of load rate. And analytical models are presented to predict fatigue strength of R/C beam strengthened with steel plate and carbon fiber sheet. Also, the correlation between the ratio of stress and the fatigue life was investigated.

• Computers and Concrete
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• v.12 no.6
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• pp.739-754
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• 2013

Creep and shrinkage have pronounced effects on the long-term deflection of prestressed concrete members. Under repeated loading, the rate of creep in prestressed concrete members is often accelerated. In this paper, an iterative computational procedure based on the well known Model B3 for creep and shrinkage was developed to predict the time-dependent deflection of partially prestressed concrete members. The developed model was validated using the experimental observed deflection behavior of a simply supported partially prestressed concrete beam under repeated loading. The validated model was then employed to make predictions of the long-term deflection of the prestressed beams under a variety of conditions (e.g., water cement ratio, relatively humidity and time at drying). The simulation results demonstrate that ignoring creep and shrinkage could lead to significant underestimation of the long-term deflection of a prestressed concrete member. The model will prove useful in reducing the long-term deflection of the prestressed concrete members via the optimal selection of a concrete mix and prestressing forces.

• Smart Structures and Systems
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• v.24 no.6
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• pp.759-768
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• 2019

In nuclear fusion reactors, the key structural component (i.e., the plasma-facing component) undergoes high heat flux cyclic loading. To ensure the safety of fusion reactors, an experimental study on the temperature-induced creep of stainless steel under heat flux cyclic loading was performed in the present work. The strains were measured using a stereo digital image correlation technique (3D-DIC). The influence of the heat haze was eliminated, owing to the use of a vacuum environment. The specimen underwent heat flux cycles ($500^{\circ}C-1000^{\circ}C$) with different mechanical preloads (0 kN, 10 kN, 30 kN, and 50 kN). The results revealed that, for a relatively large preload (for example, 50 kN), a single temperature cycle can induce a residual strain of up to $15000{\mu}{\varepsilon}$.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2000.04a
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• pp.373-378
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• 2000

The heat resistant material, in service, may experience static loading, cyclic loading, or a combination of two. An experimental study of crack growth behavior of STS 316L austenitic stainless steel under fatigue, and creep-fatigue loading conditions were carried out on compact tension specimens at various tensile hold times. In the crack growth experiments under hold times. In the crack growth experiments under hold time loading conditions, tensile hold times were ranged from 5 seconds to 100 seconds and its behavior was characterized using the $\Delta$K parameter. The crack growth rates generally increase with increasing hold times. However in this material, the trend of crack growth rates decreases with increasing hold times for short hold time range relatively. It is attributed to a decline in the cyclic crack growth rate as a result of blunting at the crack tip by creep deformation. The effect of grain size on the creep behavior of STS 316L was investigated. Specimens with grain size of 30, 65 and 125${\mu}{\textrm}{m}$ were prepared through various heat treatments and they were tested under various test conditions. The fracture mode of 316L changed from transgranular to intergranular with increasing grain size.

• Proceedings of the Korean Nuclear Society Conference
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• 2002.05a
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• pp.328-328
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• 2002