• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.7 s.166
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• pp.1120-1128
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• 1999

An elastic-viscoplastic finite element analysis is performed to investigate detailed growth behavior of creep cracks and the numerical results are compared with experimental results. In Cr-Mo steel stress fields obtained from the crack growth method by mesh translation were compared with both cases that the secondary creep rate is only used as creep material property and the primary creep rate is included. Analytical stress fields, Riedel-Rice(RR) field, Hart-Hui-Riedel(HR) field and Prime(named in here) field, and the results obtained by numerical method were evaluated in details. Time vs. stress at crack tip was showed and crack tip stress fields were plotted. These results were compared with analytical stress fields. There is no difference of stress distribution at remote region between the case of 1st creep rate+2nd creep rate and the case of 2nd creep rate only. In case of slow velocity of crack growth, the effect of 1st creep rate is larger than the one of fast crack growth rate. Stress fields at crack tip region we, in order, Prime field, HR field and RR field from crack tip.

• Nuclear Engineering and Technology
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• v.54 no.3
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• pp.917-925
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• 2022

Steam generator (SG) tubes in a nuclear power plant can undergo rapid changes in pressure and temperature during an accident; thus, an accurate model to predict short-term creep damage is essential. The theta (𝜃) projection method has been widely used for modeling creep-strain behavior under constant stress. However, many creep test data are obtained under constant load, so creep rupture behavior under a constant load cannot be accurately simulated due to the different stress conditions. This paper proposes a novel methodology to obtain the creep curve under constant stress using a modified 𝜃 projection method that considers the increase in true stress during creep deformation in a constant-load creep test. The methodology is validated using finite element analysis, and the limitations of the methodology are also discussed. The paper also proposes a creep-strain model for alloy 690 as an SG material and a novel creep hardening rule we call the damage-fraction hardening rule. The creep hardening rule is applied to evaluate the creep rupture behavior of SG tubes. The results of this study show its great potential to evaluate the rupture behavior of an SG tube governed by creep deformation.

• Journal of the Korean Society of Safety
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• v.9 no.4
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• pp.29-41
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• 1994

High temperature tensiles tests, steady state creep tests, internal stress tests and creep rupture tests using Al 7075 alloy were performed over the temperature range of 9$0^{\circ}C$~50$0^{\circ}C$ and stress range of 0.64~17.2(kgf/$\textrm{mm}^2$) in order to investigate the creep behavior and predict creep rupture life From the apparent activation energy Qc and the applied stress exponent n measured, at the temperature range of 9$0^{\circ}C$~l2$0^{\circ}C$, the creep deformation seemed to be controlled by cross slip. On the other hand at the temperature of 20$0^{\circ}C$~23$0^{\circ}C$ the creep deformation seemed to be controlled by dislocation climb but at 47$0^{\circ}C$~50$0^{\circ}C$, by diffusion creep. And the rupture life(t$_{f}$) might be represented by anthermal process attributed to the difference of the applied stress dependence of Internal stress and the ratio of the Internal stress to the applied stress, the thermal activated process attributied to the temperature dependence of the internal stress. Also the ratio between stress dependence of primary creep rate and that of minimum creep rate was measured 0.46, the minimum creep rate is expected to be appromately obtained from master creep curve including the relationship primary creep rate and minumum creep rate. Finally the relationship new rupture parameter and logarithmic stress was represented with including the ratio between the dependence of primary creep rate and that of minimum creep rate, using the new rupture parameter the rupture life predition is exactly expected.d.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.123-128
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• 2001

The creep constants which are used to the reference stress equations of creep damage were obtained to type 316LN stainless steel, and their determining methods were described in detail. Typical Kachanov and Rabotnov(K-R) creep damage model was modified into the damage equations with reference stress concepts, and the modified equations were applied practically to type 316LN stainless steel. In order to determine the reference stress value, a series of high-temperature tensile tests and creep tests were accomplished at $550^{\circ}C$ and $600^{\circ}C$. By using the experimental creep data, the creep constants used in reference stress equations could be obtained to type 316LN stainless steel, and a creep curve on rupture strain was predicted. The reference stress concept on creep damage can be utilized easily as a design tool to predict creep life because the process, which is quantified by the measurement of voids or micro cracks during creep, is omitted.

• Journal of Ocean Engineering and Technology
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• v.22 no.6
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• pp.58-63
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• 2008

The short-term high temperature creep rupture behavior of Ni-based Alloy718 steels jointed by friction welding wasinvestigated at the elevated temperatures of 550 to $700^{\circ}C$ under constant stress conditions. The creep rupture characteristics such as creep stress, rupture time, steady state creep rate, and initial strain were evaluated. Creep stress has a quantitative correlation between creep rupture time and steady state creep rate. The stress exponents (n, m) of the experimental data at 550, 600, 650 and $700^{\circ}C$ were derived as 26.1, -22.4, 22.5, -18.5, 17.4, -14.3 and 6.9, -8.1, respectively. The stress exponents decreased with increasing creep temperature. The creep life prediction was derived by the Larson-Miller parameter (LMP) method and the result equation obtained is as follows: T(logtr+20)=-0.00148${\sigma}^2$-3.089${\sigma}$+23232. Finally, the results were compared with those of the base metal for Alloy718.

• Nuclear Engineering and Technology
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• v.52 no.4
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• pp.802-810
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• 2020

Three-dimensional finite element simulations are implemented for the in-pile thermo-mechanical behavior in U-Mo/Al monolithic fuel plates with different thermal creep rates of cladding involved. The numerical results indicate that the thickness increment of fuel foil rises with the thermal creep coefficient of cladding. The maximum Mises stress of cladding is reduced by ~85% from 344 MPa on the 98.0^th day when the creep coefficient of cladding increases from 0.01 to 10.0, due to its equivalent thermal creep strain enlarged by 3.5 times. When the thermal creep coefficient of Aluminum cladding increases from 0 to 1.0, the maximum mesoscale stress of fuel foil varies slightly. At the same time, the peak mesoscale normal stress of fuel foil can reach 51 MPa on the 98.0^th day for the thermal creep coefficient of 10, which increases by 60.3% of that with the thermal creep un-occurred in the cladding. The maximum through-thickness creep strain components of fuel foil differ slightly for different thermal creep coefficients of cladding. The dangerous region of fuel foil becomes much closer to the heavily irradiated side when the creep coefficient of cladding becomes 10.0. The creep performance of Aluminum cladding should be optimized for the integrity of monolithic fuel plates.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.8 s.251
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• pp.873-879
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• 2006

To investigate applicability of the reference stress approach as simplified inelastic stress analysis to estimate local creep rupture, detailed finite element stress analyses of a T-piece pipe with different inner pressure and system loading levels are performed. The reference stresses are obtained from the finite element (FE) limit analysis based on elastic-perfectly-plastic materials, from which the local reference stress for creep rupture is determined from R5. The resulting inelastic stresses are compared with elastic stresses resulting from linear elastic FE calculations. Furthermore they are also compared with the stresses from full elastic-creep FE analyses. It shows that the stresses estimated from the reference stress approach compare well with those from full elastic-creep FE analysis, which are significantly lower than the elastic stress results. Considering time and efforts for full inelastic creep analysis of structures, the reference stress approach is shown to be a powerful tool for creep rupture estimates and also to reduce conservatism of elastic stress analysis significantly.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1994.03a
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• pp.167-174
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• 1994

The creep behavior and creep fracture of alumina at high temperature were investigated under four point flexural test. The steady-state creep behavior was observed at low bending stress and the primary creep until fracture was observed at high bending stress. The loading history of bending stress did not affect on the steady-stated creep rate. Intergranular fracture was dominant for fracture of alumina at room and high temperature. However, transgranular fracture was dominant on creep fracture of alumina under high temperature by nuclueation and growth of microcracks due to residual flaws or cavities in the material.

• Journal of the Korean Ceramic Society
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• v.31 no.5
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• pp.543-551
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• 1994

The creep behavior and creep fracture of sintered alumina at high temperature were investigated under four point flexural test. Steady-state creep behavior was observed at low bending stress and primary creep until fracture was observed at hish bending stress. The loading history of bending stress did not affect on steady-state creep rate. Intergranular fracture was dominant for fracture of alumina at room and high temperature. However, transgranular fracture was dominant on creep of alumina under high temperature by nucleation and growth of microcracks due to residual flaws or cavities in the material.

• Journal of the Korean Ceramic Society
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• v.32 no.11
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• pp.1219-1226
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• 1995

Creep behaviors of Cr3C2 composites containing 90 wt% Cr3C2 and 10 wt% Ni were studied at high temperature. Compression tests at 100$0^{\circ}C$ and bending tests at 100$0^{\circ}C$ and 105$0^{\circ}C$ were done in argon environment. In all test conditions primary and steady-state creep behaviors were observed. Stress exponent and activatiion energy were determined from the experimental data. By microstructural analysis of Cr3C2 composites after creep test, the separate agglomerations of Ni phase were observed. Numerical analysis was also studied to analyze bending creep behaviors of Cr3C2 by assumming different tensile and compressive creep behavior in a bending sample. From the analysis, it was found that the stress state at the compressive region as applied stress increased. The observed creep rates were compared with the predicted creep rates by estimating power-law creep parameters from bending test data.