Tae Kwang Song

Assessment of Round Robin Analyses Results on Welding Residual Stress Prediction in a Nuclear Power Plant Nozzle

This paper provides simulational round robin test results for welding residual stress prediction of safety/relief nozzle. To quantify the welding variables and define the recommendation for prediction and determination of welding residual stress, 6 partners in 5 institutes participated in round robin test. It is concluded that compressive axial and hoop residual stress occurs in dissimilar metal weld and pre-existing residual stress distribution in dissimilar metal weld was affected by similar metal weld due to short length of safe end. Although the reason for the deviation among the results was not pursued further, the effect of several key elements of FE analyses on welding residual stress was investigated in this paper.

Effects of Similar Metal Weld on Residual Stress in Dissimilar Metal Weld According to Safe End Length

Nozzle in nuclear power plant is connected to pipe using safe end. Dissimilar metal weld between nozzle and safe end is followed by similar metal weld between safe end and pipe. And thus residual stress in dissimilar metal weld can be affected by similar metal weld. Similar metal weld impose bending stress on dissimilar metal weld, which is according to the length of safe end. In this study, simple nozzle model which covers various radius to thickness ratios was proposed to quantify residual stress in dissimilar metal weld based on finite element analyses. As a result, short length of safe end was proved to be more effective to mitigate residual stress in dissimilar metal weld and critical effective length of safe end is provided according to the radius to thickness ratio.

Mis-match limit load analyses and approximate J-integral estimates for similar metal weld with weld-center crack under tension load

In this work, the effect of strength mismatch on plastic limit loads is quantified for similar metal weld plates with cracks under tension load, via three-dimensional, small strain elastic-perfectly plastic finite element analyses. Relevant variables related to plate geometry and crack length are systematically varied, in addition to the weld width. An important finding is that mis-match limit loads can be uniquely quantified through strength mis-match ratio and one geometry-related parameter. Based on the proposed limit load solutions, reference stress based J-integral estimates is also investigated. When the reference stress is defined by the mis-match limit load, predicted J-integral values agree overall well with FE results.

V-Factor Estimation Under Thermal and Mechanical Stress for Circumferentially Cracked Cylinder

Abstract This paper provides V-factor estimation under combined mechanical and thermal load for circumferential cracks. Results are based on finite element analyses and effect of types and magnitudes of the thermal stress, crack geometry, the loading mode and plastic strain hardening on variations of the V-factor are investigated. The results of finite element analyses are compared with R6 values. As a result, it is shown that R6 gives generally conservative results. The conservatism is especially increased for the combination of large mechanical and thermal load. As a result, new estimation method which uses failure assessment line in R6 is proposed for V -factor and gives less conservative results.기호설명 K IP : 1차 하중에 대한 응력확대계수 K JP : 1차 하중에에 대한 유효응력확대계수 K IP+S : 1, 2차 복합하중에 대한 응력확대계수 K JP+S : 1,2차 복합하중에 대한 유효응력확대계수 L r : 구조물에 작용하는 하중 비[식 (2)] K r : 파괴인성 비 [식 (1) or (8)] 1. 서 론 일반적인 원전 배관은 인장, 굽힘 등의 기계하중(1 차 하중)과 온도 구배 등에 의한 열 하중(2 차 하중)을 동시에 받고 있다. 기계하중 및 열 하중은 대표적인 하중 제어 및 변위 제어 조건으로서 그 특성이 다르며 또한 기계 및 열하중이 복합적으로 작용할 때에는 상호작용(interaction)에 의한 열응력 완화효과가 나타난다. 따라서 1, 2차 복합하중이 가해지는 균열 배관에 대한 탄소성 파괴역학 해석을 수행하는 경우 정확한 평가를 위해 열 및 기계하중의 상호 작용을 고려할 필요가 있다.

Through-thickness welding residual stress profile in dissimilar metal nozzle butt weld

This paper provides the through-thickness welding residual stress profile in dissimilar metal nozzle butt welds of pressurized water reactors. For systematic investigations of the effects of geometric variables, i.e. the thickness and the radius of the nozzle and the length of the safe end, on welding residual stresses, idealized shape of nozzle is proposed and elastic-plastic thermo-mechanical finite element analyses are conducted. Through-wall welding residual stress profiles for dissimilar metal nozzle butt welds are proposed, which take a modified form of existing welding residual stress profiles developed for austenitic pipe butt weld in R6 code.Copyright

Estimations of the C(t)-Integral in Transient Creep Condition for Pipe with Crack Under Combined Mechanical and Thermal Stress (I) - Elastic-Creep -

The C(t)-integral describes amplitude of stress and strain rate field near a tip of stationary crack under transient creep condition. Thus the C(t)-integral is a key parameter for the high-temperature crack assessment. Estimation formulae for C(t)-integral of the cracked component operating under mechanical load alone have been provided for decades. However, high temperature structures usually work under combined mechanical and thermal load. And no investigation has provided quantitative estimates for the C(t)-integral under combined mechanical and thermal load. In this study, 3-dimensional finite element analyses were conducted to calculate the C(t)-integral of elastic-creep material under combined mechanical and thermal load. As a result, redistribution time for the crack under combined mechanical and thermal load is re-defined through FE analyses to quantify the C(t)-integral. Estimates of C(t)-integral using this proposed redistribution time agree well with FE analyses results.

Welding Residual Stress Determination and Crack Analysis in Butt-Welded Thin-Walled Plate by Finite-Element Method

현대 사회는 다양한 구조물과 다양한 운송 수단에 묻혀서 삶을 살아가고 있다. 이러한 다양성 속에서 금속 구조물은 현대 사회를 이루는 근간이 됨은 의심할 나위가 없으며, 이러한 금속 구조물들은 다양한 방식으로 접합함 으로서 그 형태를 이루게 된다. 그 다양한 접합 방법 중 하나가 용접으로 이는 금속과 금속을 서로 맞붙여 금속 사이를 용접재를 이용하여 강한 열로 접합시키는 맞대기(butt weld) 접합이 대표적인 경우이다. 다양한 구조물에서 이러한 용접방식이 사용되어지고 있는데, 이러한 접합 구조물의 안전성을 위해서는 용접부에 대한 안전성이 우선 확보되어야 하며, 이를 위해서 다양한 방법으로 용접부에 대한 신뢰성 검증이 진행되어 지고 있다. 이러한 상황에서 최근에 선박과 같은 구조물에서 연료의 효율성 및 속도 증가를 위해서 선박의 전체적인 무게를 감소시키려고 하고 있으며, 이러한 요구는 사용되어지는 철판(panel)의 두께를 점점 얇아지도록 만들고 있는 상황이다. 이러한 현실에서 잘못된 설계로 인해서 얇은 철판(panel)들의 맞대기 용접을 진행하여 접합함에 있어서 용접 진행간 발생하는 열 응력에 의해서 과도한 변형이 일어나며, 이를 보정하기 위해서 막대한 수선비용을 초래하는 경우가 발생하고 있다. 이러한 문제들을 사전에 예방하기 위해서는 다양한 용접재, 용접방법에 대한 실험을 통해서 올바른 설계가 진행되어야만 한다. 그러나, 이러한 실험을 진행하기 위해서는 많은 시간과 비용이 소모되는 바 이러한 단점을 보완하기 위해서 다양한 방법의 시뮬레이션이 진행되고 있다. 본 연구에서는 실제 실험을 진행하지 않고도 실제 용접에서 발생할 수 있는 잔류응력을 예측할 수 있는 박판용접(thin section weld)을 모사할 수 있는 모델링 기법을 제시하고자 한다. 이를 위해서 2D 와 3D

Mis-Match Limit Load Analyses and Fracture Mechanics Assessment for Welded Pipe with Circumferential Crack at the Center of Weldment

초록: 본 논문에서는 유한요소 해석을 통해 용접부 중앙에 원주방향균열이 있는 배관에 대한 강도불일치 한계하중 해석 및 파괴역학 해석을 수행하였다. 강도불일치 한계하중식을 제시하기 위해 강도불일치 비, 용접부 폭, 균열 길이 및 배관 반경비에 대한 체계적인 변수 해석을 수행하였으며 참조응력법을 바탕으로 스테인리스강 및 페리틱 강에 대한 J-적분 계산을 수행하였다. 본 연구에서 제시한 강도불일치 한계하중을 사용하여 참조응력을 정의할 때, 보다 정확한 J-적분 결과를 얻을 수 있었다. Abstract: In this paper, limit load analyses and fracture mechanics analyses were conducted via finite element analyses for the welded pipe with circumferential crack at the center of the weldment. Systematic changes for strength mis-match ratio, width of weldment, crack shape and thickness ratio of the pipe were considered to provide strength mis-match limit load. And J-integral calculations based on reference stress method were conducted for two materials, stainless steel and ferritic steel. Reference stress defined by provided strength mis-match limit load gives much more accurate J-integral.

A Prediction of V-Factor for the Cracked Pipe Under Combined Mechanical and Thermal Stresses With Elastic Follow-Up

In this paper, a prediction of V-factor, the correction factor for the plasticity interaction between the primary stress and secondary stress, for the cracked pipe under combined mechanical and thermal stresses with significant elastic follow-up is presented. Recently, present authors suggested enhanced V-factor solution using failure assessment curve, which is less conservative than R6 code. However, this solution is limited to weak elastic follow-up condition. The degree of elastic follow-up is related to the variations of Vo with β according to this study. Based on this observation, a prediction solution for the V-factor which can be applied to the crack assessment ranging from weak elastic follow-up condition to strong elastic follow-up condition is proposed.Copyright

Estimation of V Factor Under Combined Thermal and Mechanical Stress for Circumferential Cracked Pipes

This paper provides V-factor estimation under combined mechanical and thermal load for circumferential cracked pipes. In this study, the effects of types and magnitudes of the thermal stress, mechanical loading mode, crack geometry, and plastic strain hardening on variations of V-factor are estimated. The results of finite element analyses are compared with R6 values. As a result, it is shown that R6 gives generally conservative results. The conservatism is especially increased for the combination of large mechanical and thermal load. As a result, new estimation equation which uses failure assessment curve in R6 is proposed for V-factor and gives less conservative results.© 2009 ASME