Hyeong-Yeon Lee

Green's function approach for crack propagation problem subjected to high cycle thermal fatigue loading

An efficient numerical approach using Greens function for the analysis of crack propagation under thermal transient load has been presented. The present approach based on multi-Greens functions pre-determined for each stage of the incremental crack growth substantially shortens the calculation time of the stress intensity factor (SIF) ranges. It was shown that the Greens function method (GFM) can be efficiently used to evaluate not only thermal stresses for fatigue analysis but also the SIF for crack propagation analysis. The crack propagation analysis results have been compared with those of the actual observation for the piping structure subjected to thermal striping load in a liquid metal fast breeder reactor. It was shown that the function determined at a fixed temperature can be applied to a relatively wide range of temperatures because of the compensation effect of the material properties, that is, some properties increase while the others decrease as the temperature increases.

Numerical evaluation of stress intensity factor for vessel and pipe subjected to thermal shock

Abstract The thermal weight function method and the finite element method were employed in the numerical computation of the stress intensity factor for a cracked vessel and the cracked pipe subjected to thermal shock. In the weight function technique, the necessary information on the fundamental loading was computed from the finite element analysis. The shape function used for the calculation of the fundamental loading was incorporated into the numerical evaluation of the stress intensity factor in the weight function technique. The stress intensity factor was also computed by the finite element method for the methodological comparison with the weight function method. A wall subjected to thermal shock was analyzed, and it has been shown that the effect of thermal shock on the stress intensity factor is dominant for the crack with small crack length to thickness ratio. Convection at the crack face had an influence on the stress intensity factor in the early stage of thermal shock.

Creep-fatigue crack growth behavior of a structure with crack like defects at the welds

A study on a creep-fatigue crack growth behavior has been carried out for a cylindrical structure with weldments by using a structural test and an evaluation according to the assessment procedures. The creep-fatigue crack growth behavior following the creep-fatigue crack initiation has been assessed by using the French A16 procedure and the conservatism for the present structural test has been examined. The structural specimen is a welded cylindrical shell made of 316 L stainless steel (SS) for one half of the cylinder and 304 SS for the other half. In the creep-fatigue test, the hold time under a tensile load which produces the primary nominal stress of 45 MPa was one hour at 600°C and creep-fatigue loads of 600 cycles were applied. The evaluation results for the creep-fatigue crack propagation were compared with those of the observed images from the structural test. The assessment results for the creep-fatigue crack behavior according to the French A16 procedure showed that the A16 is overly conservative for the creep-fatigue crack propagation in the present case with a short hold time of one hour.

Comprehensive Residual Stress Distributions in a Range of Plate and Pipe Components

A comprehensive review of through thickness transverse residual stress distributions in a range of as-welded and mechanically bent components made up of a range of steels has been carried out, and simplified generic transverse residual stress profiles for a plate and pipe components have been proposed. The geometries consisted of welded pipe butt joints, T-plate joints, tubular T-joints, tubular Y-joints and a pipe on plate joints as well as cold bent tubes and pipes. The collected data covered a range of engineering steels including ferritic, austenitic, C-Mn and Cr-Mo steels. Measured residual stress data, normalised with respect to the parent material yield stress, has shown a good linear correlation versus the normalised depth of the region containing the residual stress resulting from the welding or cold-bending process. The proposed simplified generic residual stress profiles based on the mean statistical linear fit of all the data provides a reasonably conservative prediction of the stress intensity factors. Whereas the profiles for the assessment procedures are fixed and case specific, the simple bilinear profiles for the residual stresses obtained by shifting the mean and bending stress from the mean regression line have been proposed and validated.

High-Temperature Design of Sodium-to-Air Heat Exchanger in Sodium Test Loop

KAERI가 개발 중인 소듐냉각 원형로의 붕괴열 제거를 위한 잔열제거계통(Decay heat Removal Circuit: DRC)은 안전등급으로 분류되며, 이는 설계의 다양성 확보를 위해 Fig. 1 에서와 같이 피동형 잔열제거계통(Passive DRC : PDRC)과 능동형 잔열제거계통(Active DRC : ADRC)으로 구성된다. 소듐대 공기 열교환기(sodium-to-air heat exchanger) 에는 ADRC 에 설치되는 핀형(finned) 소듐대 공기 열교환기인 FHX(Finned-tube Sodium-to-Air Heat Exchanger)와 PDRC에 설치되는 헬리컬형 소듐대 공기 열교환기인 AHX(Helical-coil Air Heat Exchanger)가 있다. 여기서는 이미Fig. 2 의 STELLA-1 소듐 시험루프 내에 설치된 AHX 와 KAERI 부지 내에 설치 예정인 Fig. 3 의 강제통풍형 소듐대 공기열교환기 Key Words: Sodium Test Loop(소듐 시험루프), Sodium-to-Air Heat Exchanger(소듐-공기열교환기), High Temperature Design(고온 설계), Creep-Fatigue(크리프-피로) 초록: 제4세대 소듐냉각 고속로에는 중간열교환기(IHX), 붕괴열제거 열교환기(DHX), 공기 열교환기(AHX), 핀형 소듐-공기 열교환기(FHX) 및 증기발생기(SG)를 포함한 다양한 열교환기들이 설치된다. 본 연구에서는 STELLA-1 시험루프에 설치된 소듐-공기 열교환기인 AHX와 SELFA 시험루프에 설치될 핀형(finned) 소듐-공기 열교환기인 FHX 등 2 기의 열교환기 설계에 대해 3D 상세 유한요소해석을 수행하고, 동 결과에 기초하여 고온설계 기술기준을 따라 크리프-피로 손상평가를 수행하였다. 손상 평가결과 AHX 와 FHX 는 의도하는 크리프 피로 손상 하중 하에서 구조 건전성을 유지하는 것으로 확인되었다. Abstract: In a Korean Generation IV prototype sodium-cooled fast reactor (SFR), various types of high-temperature heat exchangers such as IHX (intermediate heat exchanger), DHX (decay heat exchanger), AHX (air heat exchanger), FHX (finned-tube sodium-to-air heat exchanger), and SG (steam generator) are to be designed and installed. In this study, the high-temperature design and integrity evaluation of the sodium-to-air heat exchanger AHX in the STELLA-1 (sodium integral effect test loop for safety simulation and assessment) test loop already installed at KAERI (Korea Atomic Energy Research Institute) and FHX in the SEFLA (sodium thermal-hydraulic experiment loop for finned-tube sodium-to-air heat exchanger) test loop to be installed at KAERI have been performed. Evaluations of creep-fatigue damage based on full 3D finite element analyses were conducted for the two heat exchangers according to the high-temperature design codes, and the integrity of the high-temperature design of the two heat exchangers was confirmed.

Stress intensity factor solution for radial and circumferential cracks in hollow cylinders using indirect boundary integral

An approximate weight function technique using the indirect boundary integral equation method has been presented for the analysis of a hollow cylinder with circumferential or radial cracks. A one-term boundary integral, single-layer potential was introduced to represent the crack surface displacement field for the displacement-based weight function method. An explicit closed-form stress intensity factor (SIF) solution applicable to both radial and circumferential cracks of a circular body was derived. For a practical application, the analysis was extended to solve the radial and circumferential crack problems of a hollow cylinder. The SIF solution of the present method can be easily applied to a wide range of similar crack problems without any modification, and the calculation procedure of SIFs for these cracked cylinders using one closed-form solution is very simple and straightforward. The effect of the reference load cases for the present weight function approach on the solution behaviour was investigated. The analysis results using the present solution were in good agreement with those available in the literature.

Assessment and Test of the Creep-Fatigue Crack Behavior for a High Temperature Component

An assessment of creep-fatigue crack initiation and growth is of key importance for the evaluation of a high temperature structural integrity of a reactor structure such as the liquid metal reactor KALIMER Korea advanced liquid metal reactor1, operating in a creep regime. The high temperature design codes, ASME Section III Subsection NH 2 and RCC-MR 3, provide design guidelines for a defect-free body while the RCC-MR A16 Technical Appendix4 provides assessment procedures for creepfatigue crack initiation and propagation. The A16 guide provides an assessment procedure for creepfatigue crack initiation and propagation only for austenitic stainless steel because it does not provide the essential coefficients and material properties for other materials. The current A16 guide does not provide a procedure for Mod. 9Cr‐Mo ASME Grade 91 steel. In this study, the assessment of creep-fatigue crack initiation with an extended d approach has been carried out as a case study for a Grade 91 structure based on a proposed d value, a material specific characteristic distance from a defect tip, as d=36 m 5 and on a d value of d=50 m. The assessment results with the two different d values were compared with the observed images. The creep-fatigue crack propagation behavior of the austenitic stainless steel part of a specimen was examined with the results by an assessment according to the A16 guide and a test. Comparison of the two results should be useful for quantifying the conservatism of the A16 guide. Similar research on creep-fatigue damage and creep-fatigue crack behavior has been carried out for an austenitic stainless steel structure with weldments 6‐8.

Evaluation of Creep-Fatigue Integrity for High Temperature Pressure Vessel in a Sodium Test Loop

In this study, high temperature integrity evaluation on a pressure vessel of the expansion tank operating at elevated temperature of 510°C in the sodium test facility of the SEFLA(Sodium Thermal-hydraulic Experiment Loop for Finned-tube Sodium-to-Air heat exchanger) to be constructed at KAERI has been performed. Evaluations of creep- fatigue damage based on a full 3D finite element analyses were conducted for the expansion tank according to the recent elevated temperature design codes of ASME Section III Subsection NH and French RCC-MRx. It was shown that the expansion tank maintains its integrity under the intended creep-fatigue loads. Quantitative code comparisons were conducted for the pressure vessel of austenitic stainless steel 316L.

High Temperature Design and Damage Evaluation of Mod.9Cr-1Mo Steel Heat Exchanger

High temperature design and evaluation of creep-fatigue damage for sodium-sodium heat exchanger, DHX (Decay heat exchanger) in a sodium test loop have been conducted. The DHX is a shell- and tube-type heat exchanger with outer diameter of 21.7mm, thickness of 1.65mm and effective length of 1.73m. The DHX shell and tube materials were Mod.9Cr-1Mo steel. The temperatures of shell inlet and shell outlet in the DHX are 510°C and 308°C, respectively, while the temperatures of tube inlet and outlet are 254°C and 475°C, respectively. Three dimensional finite element analysis was conducted for the DHX and evaluation of creep-fatigue damage at several critical locations of the heat exchanger was carried out according to the elevated temperature design codes of the ASME Section III Subsection NH and RCC-MR. Evaluations on the integrity of the DHX and code comparisons were carried out for the critical locations of the DHX.Copyright

Creep-Fatigue Damage and Crack Initiation for a Mod 9Cr-1Mo Structure With Weldments

A structural test and evaluation on creep-fatigue damage, and creep-fatigue crack initiation have been carried out for a Mod. 9Cr-1Mo steel structural specimen with weldments. The conservatisms of the design codes of ASME Section III subsection and NH and RCC-MR codes were quantified at the welded joints of Mod.9Cr-1Mo steel and 316L stainless steel with the observed images from the structural test. In creep damage evaluation using the RCC-MR code, isochronous curve has been used rather than directly using the creep law as the RCC-MR specifies. A y-shaped steel specimen of a diameter 500mm, height 440mm and thickness 6.35mm is subjected to creep-fatigue loads with two hours of a hold time at 600°C and a primary nominal stress of 30MPa. The defect assessment procedures of RCC-MR A16 guide do not provide a procedure for Mod.9Cr-1Mo steel yet. In this study application of σd method for the assessment of creep-fatigue crack initiation has been examined for a Mod. 9Cr-1Mo steel structure.© 2007 ASME