Modified projection model-based constant-stress creep curve for alloy 690 steam generator tube material

문성인,김종민,권준엽,이봉상,최권재,김민철,한상배 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.3

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

초고온가스로 중간 열교환기용 Alloy 617의 장시간 크리프 변형률-시간 곡선 모델링

김우곤 ( Woo Gon Kim ),윤송남 ( Song Nan Yin ),김용완 ( Yong Wan Kim ) 대한금속·재료학회 2009 대한금속·재료학회지 Vol.47 No.10

The Kachanov-Rabotnov (K-R) creep model was proposed to accurately model the long-term creep curves above 10(5) hours of Alloy 617. To this end, a series of creep data was obtained from creep tests conducted under different stress levels at 950℃. Using these data, the creep constants used in the K-R model and the modified K-R model were determined by a nonlinear least square fitting (NLSF) method, respectively. The K-R model yielded poor correspondence with the experimental curves, but the modified K-R model provided good agreement with the curves. Log-log plots of ε*-stress and ε*-time to rupture showed good linear relationships. Constants in the modified K-R model were obtained as λ=2.78, and κ=1.24, and they showed behavior close to stress independency. Using these constants, long-term creep curves above 10(5) hours obtained from short-term creep data can be modeled by implementing the modified K-R model.

θ 투영법을 이용한 터빈 블레이드의 크리프 특성 분석

이무형(Mu-Hyoung Lee),한원재(Won-Jae Han),장병욱(Byung-Wook Jang),이복원(Bok-Won Lee),박정선(Jung-Sun Park) 한국항공우주학회 2011 韓國航空宇宙學會誌 Vol.39 No.4

본 연구는 theta 투영법을 이용하여 터보제트 터빈 블레이드 크리프 특성을 파악하는데 목적이 있다. Theta 투영법은 우수한 정확성을 지니고 있어 폭넓게 사용되고 있다. 크리프 특성을 파악하기 위해 고온 크리프 시험을 수행하였으며, 시험 조건은 터보제트 엔진 운용조건과 소재의 비선형 특성을 고려하여 선정하였으며, four theta 모델의 적합성을 평가하기 위해 크리프 시험 결과와 비교하였다. 크리프 시험결과를 활용하여 크리프 곡선은 four theta model을 이용하여 생생하였다. 반복적인 계산을 통하여 비선형 최소자승법을 이용하여 시험 결과에 대해 최적의 theta 값을 도출하였다. Theta 투영법을 이용하여 크리프 곡선을 생성한 결과 R²값이 0.95이상의 우수한 정확성을 지니는 것을 확인하였다. 또한 four theta 모델의 검정을 위해 수행한 시험 결과와 비교하여 예측된 theta 값이 90.0%의 정확도를 가지어, theta 투영법은 크리프 거동을 예측하여 설계목적에 이용하기 위해 유용하게 사용될 수 있다. The present work is aimed to analyze the creep characteristics of a turbojet engine turbine blade using the theta projection method. The theta projection method has been widely used due to its advantages and flexibility. For the creep characteristic analysis of the turbine blade, tests are performed considering the operating conditions and the non-linear material properties. Results from the creep test are fitted using the four theta model. The predicted proprieties using the four theta model are compared with the prediction model and creep test results. To obtain an optimum value of the four theta parameters in non-linear square method, a number of computing processes in the non-linear least square method were carried out to obtain full creep curves. Results using the theta model has more than 0.95 value of R². The results between the experimental values and predicted four theta model has about 90.0% accuracy. The theta projection method can be utilized for a design purpose to predict the creep behavior.

Model-based Methodology to Develop the Isochronous Stress-Strain Curves for Modified 9Cr Steels

Woo-Gon Kim,Song-Nan Yin,Ik-Hee Jung,Sung Ho Kim,Chan-Bock Lee 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

Since high temperature materials are designed with a target life based on a specified amount of allowable strain and stress, their isochronous stress-strain curves (ISSC) are needed to avoid an excessive deformation during an intended service life. In this paper, a model-based methodology to develop the isochronous curves for a G91 steel is described. Creep strain-time curves were reviewed for typical high-temperature materials, and Garofalo's model which conforms well to the primary and secondary creep stages was proper for the G91 steel. Procedures to obtain an instantaneous elastic-plastic strain, e, were given in detail. Also, to accurately determine the Ph P2 and P3 parameters in the Garofalo's model, a nonlinear least square fitting (NLSF) method was adopted and useful. The long-term creep curves for the G91 steel can be modeled by the Garofalo's model, and the long-term ISSCs can be developed using the modeled creep curves.

변형 에너지를 이용한 크리프 한계응력강도(S<SUB>t</SUB>)의 결정

김우곤(Woo Gon Kim),이경용(Kyung Yong Lee),윤기봉(Kee Bong Yoon),류우석(Woo Seog Ryu) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8

Creep stress intensity limit(S₁) value for 1% strain in 10? hr was determined using strain energy method based on the work done during creep. For this purpose, 1% creep strain tests for commercial type 316 (J316) and type 316LN (K316LN) stainless steels were conducted with various stress conditions at 550℃ or 593℃. The S₁ value of the K316LN stainless steel was 135 MPa at 550℃ and creep work was 87 KJ/m³. In order to identify reasonability of the strain energy method, the method of isochronus stress-strain curves(ISSC) for the J316 stainless steel was used comparatively. The S₁values for it showed good agreement between two methods. The strain energy method can be simply obtained using only three or four short-term 1% strain data without ISSC obtained by long-term creep data. Particularly, this method is useful in estimation of the stress intensity for new and emerging class of high-temperature materials where creep and tensile data for empirical extrapolation to design life is lacking.

Reducing Curve Squeal Noise Using Composite Materials Based on Experimental Investigation

Yang Soo Yun,Jae-Chul Kim 한국정밀공학회 2021 International Journal of Precision Engineering and Vol.22 No.9

Curve squeal noise is one of the greatest noise sources among railway noise when a vehicle is passing through the sharp radius curve. Furthermore, its high frequency and pure tone component cause a great deal of pain to residents around the track. Squeal noise is caused by lateral creep of wheel/rail contact point in curve. When railway vehicle passes through a sharp curve, negative friction under conditions of lateral creepage induces stick–slip oscillations, and these in turn induce resonance of the wheel web. Thus, wheel squeal noise is generated. In this study, laser cladding rail with composite material on the rail head was tested to prevent decrease of adhesion coefficient versus creepage. To make a composite rail specimen for test rig, composite material was cladded using high power diode laser. Four distinct types of composite material were processed to prepare rail disc specimens. To determine their effect on squeal noise reduction, adhesion coefficient and noise test were performed using a wheel / rail scale test rig. Lateral adhesion coefficient and squeal noise were also measured. Our results showed that steel rail disc had negative slope of adhesion coefficient and generated squeal noise in 2 kHz and 4 kHz. However, there was less or no negative slope when composite material rail specimen was used. Some cladded rail specimens had negative friction at higher degree of yaw angle than steel rail. Particularly, the specific test specimen exhibited excellent friction characteristic without decreasing the adhesion coefficient even when the yaw angle increased, and that specimen did not generate the squeal noise. Experimental results showed that it is possible to suppress the wheel squeal noise by changing the friction characteristics of the contact point by applying the composite material.

Proposed Survey Steps for Investigation of Land-Creeping Susceptibility Areas: A Focus on Geophysical Mapping of the Yongheung-dong, Pohang, Korea

( Jeong-in Kim ),( Sun-joong Lee ),( Kwan-soo Kim ),( Jae-eun Lee ),( Jin-hyun Sa ),( Ji-soo Kim ) 대한지질공학회 2021 지질공학 Vol.31 No.3

Land creeping is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Because creep-related failures occur frequently on a large scale without notice, they can be hazardous to both property and human life. Korea Forest Service has operated the prevention and response system from land creeping which has been on the rise since 2018. We categorized and proposed three survey steps (e.g., preliminary, regional, detailed) for investigation of creeping susceptibility site with a focus on geophysical mapping of a selected test site, Yongheung-dong, Pohang, Korea. The combination of geophysical (dipole-dipole electrical resistivity tomography and reciprocal seismic refraction technique, well-logging), geotechnical studies (standard penetrating test, laboratory tests), field mapping (tension cracks, uplift, fault), and comprehensive interpretation of their results provided the reliable information of the subsurface structures including the failure surface. To further investigate the subsurface structure including the sliding zone, we performed high-resolution geophysical mapping in addition to the regional survey. High-resolution seismic velocity structures are employed for stability analysis because they provided more simplified layers of weathering rock, soft rock, and hard rock. Curved slip plane of the land creeping is effectively delineated with a shape of downslope sliding and upward pushing at the apex of high resistive bedrock in high-resolution electrical resistivity model with clay-mineral contents taken into account. Proposed survey steps and comprehensive interpretation schemes of the results from geological, geophysical, and geotechnical data should be effective for data sets collected in a similar environment to land-creeping susceptibility area.

지오그리드의 크리프 한계 변형률에 관한 해석학적 고찰

전한용,정연인,Jeon Han-Yong,Chung Youn-In 한국섬유공학회 2006 한국섬유공학회지 Vol.43 No.3

New procedure for the 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-Dorn Plots were applied and the results were compared with the results that applied existed limit strain criteria (GRI test method GG-4). The creep-limited strain of the geogrid samples that was deter-mined by using the Sherby-Dorm Plots was all 11%. This value is higher than the existing creep-limited stran value of 10%. From this 11% limit strain, the creep reduction factors were calculated at 100,000 hours design. It resulted in 1.45 for all of the geogrid samples (8 t/m, 10 t/m). Finally, when it was compared with the creep reduction factors using 10% criteria, there was slight decrease in reduction factor values of about $0.06{\sim}0.14$.

지수 함수를 이용한 크리프 곡선 모델링과 수명예측

이원(W. Yi),윤송남(S. N. Yin),김우곤(W. G. Kim),류우석(W. S. Ryu),김용완(Y. W. Kim) 한국정밀공학회 2006 한국정밀공학회 학술발표대회 논문집 Vol.2006 No.10월

변형률 에너지를 이용한 크리프 속도 및 수명 예측

윤송남(S. N. Yin),김우곤(W. G. Kim),박재영(J. Y. Park),장진성(J. Jang) 한국정밀공학회 2010 한국정밀공학회 학술발표대회 논문집 Vol.2010 No.11월