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Influence of data size on the reliability assessment of creep life of grade 91 steel
김우곤,박재영,B. K. Choudhary,김선진,김민환,장진성 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.11
Influence of data size on a reliability assessment of the long-term creep life of Grade 91 steel was investigated for two different sets oflarge body of creep-rupture data comprising 690 (Set-1) and 1072 (Set-2) data points. The Z-parameter on the Larson-Miller parameter ofcreep rupture data supported a normal distribution well. Based on the normal distribution, the reliability of the predicted creep life for thetwo data sets was assessed to obtain variations in reliability owing to fluctuations in the service conditions and the dispersion of the creepdata using a service condition-creep rupture property interference (SCRI) model. A large number of random variables for Z-parameterwere generated using Monte-Carlo simulation. A reliability assessment for the two data sizes under specific service conditions has beencomparatively demonstrated. It was found that the size of the creep-rupture data influenced the reliability of the creep life prediction.
김우곤,윤송남,구경회 대한금속·재료학회 2009 METALS AND MATERIALS International Vol.15 No.5
This study deals with the generation of isochronous stress-strain curves for modified 9Cr-1Mo steel. Material property data (sy, D, and m) from tensile tests and strain-time curves from creep tests were obtained at 550 °C. On the basis of the experimental data, the creep curves were characterized by Garofalo’s creep model. The three parameters of P1, P2 and P3 in Garofalo’s model could be optimized properly by a nonlinear least square fitting analysis. The three parameters showed a good stress dependency with a linear relation. However, the P3 parameter, which represents the steady state creep rate, exhibited two-slope behavior with different stress exponents at a transient stress of about 250 MPa. The results verify that the long-term creep curves for G91 steel can be modeled by Garofalo’s model with only several short-term creep data. When the modeled creep curves are used, long-term isochronous curves of up to 105 h can be successfully constructed.
참조응력을 이용한 316LN 스테인리스강의 크리프 해석
김우곤,류우석,Kim, Woo-Gon,Ryu, Woo-Seog 대한기계학회 2002 大韓機械學會論文集A Vol.26 No.10
Creep damage using a reference stress(RS) was analyzed for type 316LN stainless steel. The generalized K-R equation was reconstructed into the RS equation using a critical stress value $\sigma$. The RS equation was derived from the critical stress in failure time $t_f$ instead of material damage parameter $\omega$, which indicates the critical condition of collapse or approach to gross instability of materials during creep. For obtaining the reference stress, a series of creep tests and tensile tests were conducted with at 55$0^{\circ}C$ and $600^{\circ}C$. The stress-time data obtained from creep tests were applied to the RS equations to characterize the creep damage of type 316LN stainless steel. The value of creep constant r with stress levels was about 18 at 55$0^{\circ}C$ and 21 at $600^{\circ}C$. This value was almost similar with r = 24 in the K-R equation, which was obtained by using damage parameter $\omega$. Relationship plots of creep failure strain and life fraction $(t_f /t_r)$ were also obtained with different λ values. The RS equation was therefore more convenient than the generalized K-R equation, because the measuring process to quantify the damage parameter $\omega$ such as voids or micro cracks in crept materials was omitted. The RS method can be easily used by designers and plant operator as a creep design tool.