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Derivation of Q* Parameter for Evaluating Creep Crack Growth Rate of Modified 9Cr-1Mo Steel
I.M.W. Ekaputra,Woo-Gon Kim(김우곤),Jae-Young Park(박재영),Seon-Jin Kim(김선진),Min-Hwan Kim(김민환),Yong-Wan Kim(김용완) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
The concept of the Q* parameter was first proposed by Yokobori et al. The Q* parameter is defined as the exponent of the exponential function in the thermal activation process equation. In this study, the creep crack growth (CCG) behavior of the modified 9Cr-1Mo steel is evaluated using the Q* parameter. The CCG tests were carried out under various loads at 550oC and 600oC. The K and C* parameters have been used to characterize the CCGR of modified 9Cr-1Mo steel. The results show that the K parameter exhibits the largest scatter data, and there is no systematic trend in each series of tests, while C* showed a narrower scattering of data of CCGR than the K parameter. However, C* decreased during the early stage of crack growth and subsequently increased, i.e., a dual value due to the nose appearance, and it did not distinguish among the series of data under various loads at 550oC and 600oC clearly in a one linear line. The Q* parameter was able to evaluate the CCG by a simple monotical linear function without a dual value owing to nose existing in the early stage, and it exhibited an increase or decrease regardless of the testing conditions. The Q* was regarded as an independent parameter that is not depending on stress and temperature, whereas the C* was regarded as a dependent parameter depending on the creep stress and temperature.
Evaluation of Work Hardening Parameters in Modelling the Plastic Flow Behavior for Alloy 617
I.M.W. Ekaputra,Woo-Gon Kim(김우곤),Jae-Young Park(박재영),Seon-Jin Kim(김선진),Seon-Eung Kim(김응선) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
This paper attemps to investigate the work hardening parameters used in modelling the plastic flow of true stress and true plastic strain behavior for alloy 617. The tensile tests were conducted at the strain rate of 5.55×10<SUP>-4</SUP> s<SUP>-1</SUP> under wide range of temperatures (RT ~ 950 ℃). Several equations or flow relationships such as Hollomon, Ludwik, Swift, Ludwigson, and Voce were proposed to investigate the plastic flow behavior. The result showed that the work hardening parameters of Ludwigson can describe the plastic flow behavior adequately, since these parameters yielded the smallest error quantification in fitting line under all temperature ranges as low, intermediate, and high. At the intermediate temperatures, an anomalous work hardening behavior appeared like a plateaus. It was considered as a dynamic strain aging (DSA) phenomenon, where the serrated plastic-flow phenomenon occured due to the interaction between solute atoms and mobile dislocation. In addition, the anomalous behavior was also observed from the graph of yield strength and/or ultimate tensile strength under various temperature ranges.
Creep Crack Growth Behavior at High Temperatures of Alloy 617
Woo-Gon Kim(김우곤),Jae-Young Park(박재영),I.M.W. Ekaputra,Seon-Jin Kim(김선진),Eung-Seon Kim(김응선) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Alloy 617 is a major candidate material for use of the intermediate heat exchanger (IHX) of a very high temperature reactor (VHTR) system, which is designed to be used for a 60 year lifetime at a high temperature. Creep crack growth (CCG) behavior as well as creep deformation, due to creep damage during the long service life at elevated temperatures, is very important. However, the CCG design data to meet the needs of the conceptual designers and/or to develop ASME design code of VHTR system are not available elsewhere. In this study, a series of experimental CCG data was obtained from the CCG tests performed under different applied loads at 800 and 850oC for Alloy 617. The CCG behavior was evaluated in terms of the C<SUP>*</SUP> fracture parameter, and the CCG laws for the two temperatures were proposed and fracture microstructures were observed and discussed.
Tensile Behavior and Full-range Stress-Strain Curves for Gr. 91 Steel
Woo-Gon Kim(김우곤),Jae-Young Park(박재영),I.M.W. Ekaputra,Hyeong-Yeon Lee(이형연),Seon-Jin Kim(김선진) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
High-temperature tensile behavior for Grade 91 (Gr. 91) steel for use of a sodium-cooled fast reactor (SFR) structure was investigated, and full-range stress-strain curves were described in terms of RCC-MRx procedures (French SFR code) and four constitutive equations: power-law form, logarithmic form, exponential form, and modified form (powerlaw + exponential). To do this, a series of high-temperature tensile data was obtained from the tensile tests performed with a strain rate of 6.67 x10-4 (1/s) at R.T to 650oC. On the basis of experimental tensile data, full-range stress-strain curves were designed by RCC-MRx procedures. In addition, a modified constitutive equation in terms of a combination of power-law form and exponential form were proposed to well describe the full-range stress-strain curves, and the suitable equation in the four equations was investigated by fitting to the experimental curves. Results showed that the stress-strain curves designed by RCC-MRx were in accordance with the experimental curves at all of the temperature ranges, and a proposed modified equation was found to be superior in modeling of the stress-strain curves to the other equations. It can be conveniently used in modeling full-range stress-strain curves without calculation procedures in RCC-MRx code.
인장 및 크리프 재료상수가 크리프 균열성장에 미치는 영향
박재영(Jae-Young Park),김우곤(Woo-Gon Kim),I.M.W.EKAPUTRA,김선진(Seon-Jin Kim),김응선(Eung-Seon Kim) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Time-dependent fracture mechanics parameter C<SUP>*</SUP> is well used to evaluate the creep crack growth (CCG) rate for creep-ductility material. Calculation of C<SUP>*</SUP> includes fully-plastic contribution to J-integrals (J<SUB>p</SUB>) and load-line displacement rate at creep condition (V<SUB>c</SUB>). These values are influenced by the material constants of D, m and n obtained in the tensile and creep tests. This study investigated the influence of the material constants to the CCG rate. The values for J<SUB>p</SUB>, V<SUB>c</SUB> and C* were calculated with the variations of D, m and n values using an experimental CCG data. Various plots on the influence of the material constants to the CCG rate were presented and the results were discussed.