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Changheui Jang 대한기계학회 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.3
A simple approximation method for the stress intensity factor at the tip of the axial semi-elliptical cracks on the cylindrical vessel is developed. The approximation methods, incorporated in VINTIN (Vessel INTegrity analysis-INner flaws), utilizes the influence coefficients to calculated the stress intensity factor at the crack tip. This method has been compared with other solution methods including 3-D finite element analysis for internal pressure, cooldown, and pressurized thermal shock loading conditions. For these, 3-D finite-element analyses are performed to obtain the stress intensity factors for various surface cracks with t/R=0.1. The approximation solutions are within ±2.5% of the those of finite element analysis using symmetric model of one-forth of a vessel under pressure loading, and 1-3% higher under pressurized thermal shock condition. The analysis results confirm that the approximation method provides sufficiently accurate stress intensity factor values for the axial semi-elliptical flaws on the surface of the reactor pressure vessel.
Changheui Jang,Jeong, lll-Seok,Hong, Sung-Yull Korean Nuclear Society 2001 Nuclear Engineering and Technology Vol.33 No.2
Fracture mechanics is one of the major areas of the pressurized thermal shock (PTS) evaluation. To evaluate the reactor pressure vessel integrity associated with PTS, PFM methodology demands precise calculation of temperature, stress, and stress intensity factor for the variety of PTS transients. However, the existence of stainless steel cladding, with different thermal, physical, and mechanical property, at the inner surface of reactor pressure vessel complicates the fracture mechanics analysis. In this paper, treatment schemes to evaluate stress and resulting stress intensity factor for RPV with stainless steel clad are introduced. For a reference transient, the effects of clad thermal conductivity and thermal expansion coefficients on deterministic fracture mechanics analysis are examined.
Jang, Changheui,Cho, Pyung-Yeon,Kim, Minu,Oh, Seung-Jin,Yang, Jun-Seog Elsevier 2010 MATERIALS AND DESIGN Vol.31 No.4
<P><B>Abstract</B></P><P>The effects of weld microstructure and residual stress distribution on the fatigue crack growth rate of stainless steel narrow gap welds were investigated. Stainless steel pipes were joined by the automated narrow gap welding process typical to nuclear piping systems. The weld fusion zone showed cellular–dendritic structures with ferrite islands in an austenitic matrix. Residual stress analysis showed large tensile stress in the inner-weld region and compressive stress in the middle of the weld. Tensile properties and the fatigue crack growth rate were measured along and across the weld thickness direction. Tensile tests showed higher strength in the weld fusion zone and the heat affected zone compared to the base metal. Within the weld fusion zone, strength was greater in the inner weld than outer weld region. Fatigue crack growth rates were several times greater in the inner weld than the outer weld region. The spatial variation of the mechanical properties is discussed in view of weld microstructure, especially dendrite orientation, and in view of the residual stress variation within the weld fusion zone. It is thought that the higher crack growth rate in the inner-weld region could be related to the large tensile residual stress despite the tortuous fatigue crack growth path.</P>
Jang, Changheui,Jang, Hun,Hong, Jong-Dae,Cho, Hyunchul,Kim, Tae Soon,Lee, Jae-Gon Korean Nuclear Society 2013 Nuclear Engineering and Technology Vol.45 No.7
Environmental fatigue of the metallic components in light water reactors has been the subject of extensive research and regulatory interest in Korea and abroad. Especially, it was one of the key domestic issues for the license renewal of operating reactors and licensing of advanced reactors during the early 2000s. To deal with the environmental fatigue issue domestically, a systematic test program has been initiated and is still underway. The materials tested were SA508 Gr.1a low alloy steels, 316LN stainless steels, cast stainless steels, and an Alloy 690 and 52M weld. Through tests and subsequent analysis, the mechanisms of reduced low cycle fatigue life have been investigated for those alloys. In addition, the effects of temperature, dissolved oxygen level, and dissolved hydrogen level on low cycle fatigue behaviors have been investigated. In this paper, the test results and key analysis results are briefly summarized. Finally, an on-going test program for hot-bending of 347 stainless steel is introduced.
HUN JANG,HYUNCHUL CHO,CHANGHEUI JANG,TAE SOON KIM,CHAN KOOK MOON 한국원자력학회 2008 Nuclear Engineering and Technology Vol.40 No.3
To understand the effect of the cyclic strain rate on the environmentally assisted cracking behaviors of SA508 Gr.1a low alloy steel in deoxygenated water at 310˚C, the fatigue surface and a sectioned area of specimens were observed after low cycle fatigue tests. On the fatigue surface of the specimen tested at a strain rate of 0.008 %/s, unclear ductile striations and a blunt crack tip were observed. Therefore, metal dissolution could be the main cracking mechanism of the material at this strain rate. On the other hand, on the fatigue surfaces of the specimens tested at strain rates of 0.04 and 0.4 %/s, brittle cracks and flat facets, which are evidences of the hydrogen induced cracking, were observed. In addition, a tendency of linkage between the main crack and the micro-cracks was observed on the sectioned area. Therefore, at higher strain rates, the main cracking mechanism could be hydrogen induced cracking. Additionally, evidence of the dissolved MnS inclusions was observed on the fatigue surface from energy dispersive x-ray spectrometer analyses. Thus, despite the low sulfur content of the test material, the sulfides seem to contribute to environmentally assisted cracking of SA508 Gr.1a low alloy steel in deoxygenated water at 310˚C.
산소가 제거된 310 ℃ 순수환경에서 CF8M 주조 스테인리스강의 환경 피로거동ㆍ수소 및 미세구조의 영향
장훈(Hun Jang),조평연(Pyungyeon Cho),장창희(Changheui Jang),김태순(Tae Soon Kim) 대한기계학회 2014 大韓機械學會論文集A Vol.38 No.1
CF8M (11% ferrite) 주조 스테인리스강의 310 ℃ 순수환경에서의 저주기피로 수명에 미치는 수소 및 미세구조의 영향을 분석하였다. CF8M 의 경우, 공기환경 대비 310 ℃ 순수환경에서의 피로수명의 감소는 단조재인 316LN 에 비해 다소 작았다. 미세구조 및 파면 분석을 통해, CF8M 의 저주기피로 수명의 감소는 316LN 의 경우와 마찬가지로 수소유기균열에 의한 것으로 판단되었다. 그러나, CF8M 의 경우, 페라이트상 경계에 수소유기균열에 의한 2 차 균열이 빈번히 발생함에 따라 균열 선단에서의 응력집중이 저하되는 효과가 있었다. 이러한 응력집중의 완화로 인해 수소유기균열에 의한 피로균열진전이 둔화 되어 결과적으로 저주기피로 수명의 저하가 완화되는 것으로 판단되었다. The effects of environment and microstructure on low cycle fatigue (LCF) behaviors of CF8M stainless steels containing 11% of ferrites were investigated in a 310 ℃ deoxygenated water environment. The reduction of LCF life of CF8M in a 310 ℃ deoxygenated water was smaller than 316LN stainless steels. Based on the microstructure and fatigue surface analyses, it was confirmed that the hydrogen induced cracking contributed to the reduction in LCF life for CF8M as well as for 316LN. However, many secondary cracks were found on the boundaries of ferrite phases in CF8M, which effectively reduced the stress concentration at the crack tip. Because of the reduced stress concentration, the accelerated fatigue crack growth by hydrogen induced cracking was less significant, which resulted in the smaller environmental effects for CF8M than 316LN in a 310 ℃ deoxygenated water.