• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.1572-1579
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• 2005

New procedure for evaluation of creep reduction factor using performance limit strain concept was introduced and confirmed through the creep test results. To determine the performance limit strain of the textile geogrid used in this study, the Sherby-Dorm Plots were applied and the results were compared with the results that applied existed limit strain criteria (GRI test method GG-4). The limit creep strain of the geogrid samples that determined by using the Sherby-Dorm Plots were all 11%. This value is more higher than the existed criteria as 10%. From this 11% limit strain the creep reduction factors were calculated at 100,000 hours design. It was resulted in 1.45 for all of the geogrid samples(8t/m, 10t/m). Finally, when it was compared with the creep reduction factors that using 10% criteria, there were some decrease of reduction factor values about $0.06{\sim}0.14$.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1182-1189
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• 1995

Several methods have been developed to predict the creep rupture time of the steam pipes in thermal power plant. However, existing creep life prediction methods give very conservative value at operating stress of power plant and creep rupture strain cannot be well estimated. Therefore, in this study, creep rupture time and strain prediction method accounting for material damage and grain boundary sliding is newly proposed and compared with the existing experimental data. The creep damage evolves by continuous cavity nucleation and constrained cavity growth. The results showed good correlation between the theoretically predicted creep rupture time and the experimental data. And creep rupture strain may be well estimated by using the proposed method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.10 s.253
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• pp.1219-1226
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• 2006

The ASTM test standard recommends the use of the compact tension specimen for creep crack growth rates measurement. In the creep crack growth rate test, the displacement rate due to creep is obtained by subtracting the contribution of elastic and plastic components from the total load line displacement rate based on displacement partitioning method fur determining $C^*-integral$, which involves Ramberg-Osgood (R-O) fitting procedures. This paper investigates the effect of the R-O fitting procedures on plastic displacement rate estimates in creep crack growth testing, via detailed two-dimensional and three-dimensional finite element analyses of the standard compact tension specimen. Four different R-O fitting procedures are considered; (i) fitting the entire true stress-strain data up to the ultimate tensile strength, (ii) fitting the true stress-strain data from 0.1% strain to 0.8 of the true ultimate strain, (iii) fitting the true stress-strain data only up to 5% strain, and (iv) fitting the engineering stress-strain data. It is found that the last two procedures provide reasonably accurate plastic displacement rates and thus should be recommended in creep crack growth testing. Moreover, several advantages of fitting the engineering stress-strain data over fitting the true stress-strain data only up to 5% strain are discussed.

• Journal of Ocean Engineering and Technology
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• v.10 no.1
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• pp.47-52
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• 1996

This paper deals with a study on improvement of long-time creep life prediction of steam turbine rotor steels by using initial strain method as a new approach at high temperatures of 500 to 70$0^{\circ}C$ . The main result shows that the inital strain method could be reliably utilized to predict and evaluate the long-time creep life as creep rupture strength and that the predicting equation for long-time creep life under a certain creep stress at a certain high temperature could be empirically derived out from each initial instantaneous strain measured.

• Journal of the Korean Geosynthetics Society
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• v.6 no.4
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• pp.1-6
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• 2007

New procedure for evaluation of creep reduction factor by the limit creep strain concept was introduced through the analysis of creep test results. To determine the limit creep strain of the geogrids, the Sherby-Dorm Plots were applied and the results of this procedure were compared and interpreted, respectively. From this, it is seen that the creep reduction factors were 1.45 for the geogrid samples used in this study. Through the comparison of creep reduction factors in 10% creep strain criteria, it was confirmed that the range of creep reduction factor is about 0.06~0.14 for the geogrid samples in this study.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2010.11a
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• pp.625-626
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• 2010

• Journal of Korean Tunnelling and Underground Space Association
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• v.3 no.2
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• pp.33-56
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• 2001

It is well known fact that all rocks exhibit brittle properties and time depends strain properties (creep). An understanding of the time dependent deformation behaviour of rocks is believed to be essential in the field of civil and tunnelling. The rock and concrete creep in various forms of loading conditions and physical environment are reviewed. A comparison of creep behaviour between rocks and concrete is provided, in order to bring two existing relatively independent methods of predicting creep strain closer together. It was felt that the physical process in the creep of rocks would be similar to the process in creep of concrete. Since experiments and observations have shown that non-elastic (creep) mechanical behaviour of all crystalline solids (i.e., concrete, rocks, ceramics and refractories) and single materials have a common base. Also a comparison of the results for the accepted methods of estimating creep in rocks and concrete under - multiaxial loading was attempted to extend the knowledge of deformational characteristics of these two materials.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.5
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• pp.750-757
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• 2003

In order to determine creep stress intensity limit of high-temperature components, the usefulness of the creep work and time equation, defined as W$\_$c/t$\^$p/ = B(where W$\_$c/ = $\sigma$$\varepsilon$ is the total creep work done during creep, and p and B are constants), was investigated using the experimental data. For this Purpose, the creep tests for generating 1.0% strain for commercial type i16 stainless steel were conducted with different stresses; 160 MPa, 150 MPa, 145 MPa, 140 MPa and 135 MPa at 593$^{\circ}C$. The plots of log W$\_$c/ - log t showed a good linear relation up to 10$\^$5/ hr, and the results of the creep work-time relation for p, B and stress intensity values showed good agreement to those of isochronous stress-strain curves (ISSC) presented in ASME BPV NH. The relation can be simply obtained with only several short-term 1% strain data without ISSC which can be obtained by long-term creep data. Particularly, this relation is useful in estimating stress intensity limit for new and emerging class of high-temperature creeping materials.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.2
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• pp.168-176
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• 2004

Interrupted creep tests were carried out on the Mod.9Cr-1Mo steel in order to investigate the structural degradation during creep. The ranges of creep stress and temperature were from 71 to 167MPa and 873 to 923k, respectively. The change of hardness and tempered martensitic lath width were measured in the grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, but was not stable during creep, and the structural evolution was enhanced by creep strain. The relation between the change in lath width and strain was described in the from, $\delta$W= a ($W_s-W_o$)$cdot;varepsilon$, where $\varepsilon$ is the strain, $W_o$is the initial lath width, $W_s$ is the final lath width depending solely on stress, and a is the constant of the magnitude of 0.67 $\mu$m /strain. The change in Victors hardness was expressed by a one-valued function of creep life consumption ratio. Based on the empirical relation between strain and lath width, a model was proposed to explain the relation between change in hardness and creep life consumption ratio. The model revealed that about 65$%$ of dislocations in lath structures were eliminated by the migration of subboundaries.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2015.05a
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• pp.59-60
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• 2015

In this study, stress-strain, thermal expansion strain, total strain and high temperature creep strain of ultra-high-strength concrete with compressive strengths of 80, 130, and 180MPa were experimentally evaluated considering elevated temperature and loading condition. Also, transient creep strain has been calculated by using the results of experiment. Experimental coefficient K was proposed with application of non-steady state creep model. It is considered that the experimental results of this study could be baseline data for deformation behavior analysis of ultra-high-strength concrete.