Search

Stress distribution through the nozzle corner cross-section may be required to calculate stress intensity factor (SIF) for a nozzle corner crack in accordance with ASME Section XI Nonmandatory Appendix G. This paper proposes a simple method to predict nozzle corner cross-section stress distributions on reinforced nozzle corners of pressure vessels under internal pressure. This method includes simplified equations for predicting stresses on the inner surfaces of the nozzle corner region. These equations are expressed in terms of stress concentration factor (SCF) and geometric variables. Approximate SCF solutions for the reinforced nozzle corners are also proposed. Stress distributions using the proposed method are compared with finite element (FE) analysis results of nozzle-vessel intersections of various geometric dimensions, and agreements are quite good within postulated crack depths. Furthermore, SIFs calculated from the estimated stress distributions in accordance with ASME Section XI Nonmandatory Appendix G are compared with those from the FE results, showing good agreement.

This paper provides the through-thickness welding residual stress profile in dissimilar metal nozzle butt welds of pressurized water reactors. There have been incidents recently where cracking has been observed in the dissimilar metal weld. Weld-induced residual stress is main factor for crack growth, and thus, exact estimation of welding residual stress is important. For investigations of welding residual stress profile, 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, are systematically considered 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.

To investigate the applicability of existing methods to estimate stress intensity factors due to welding residual stresses, comparisons with finite element (FE) solutions are made for two types of generic welding residual stress profiles, generated by simulating repair welds. It is found that fitting residual stresses over the crack depth using third-order polynomials gives good estimates of stress intensity factors but fitting over the entire thickness can result in inaccurate estimates even with fourth-order polynomials. Noting that welding residual stresses are often determined from FE analyses, linearization of residual stresses to estimate stress intensity factors is proposed. Comparison with FE solutions shows good agreements.

This study provides the elastic stress intensity factors, K, for circumferential and longitudinal surface cracked straight pipes under single or combined loads of internal pressure, bending, and torsion based on three-dimensional (3D) finite element (FE) analyses. FE results are compared with two different types of defect assessment codes (API-579-1 and RCC-MR A16) to prove the accuracy of the FE results and the differences between the codes. Through the 3D FE analysis, it is found that the stress intensity factors are sensitive to the number of elements, which they were believed to not be sensitive to because of path independence. Differences were also found between the FE analysis results for crack defining methods and the results obtained by two different types of defect assessment codes.

In pressurized water nuclear reactors, dissimilar metal welds are susceptible to primary water stress corrosion cracking. To access this problem, accurate estimation of welding residual stresses is important. This paper provides general welding residual stress profiles in dissimilar metal nozzle butt welds using finite element analysis. By introducing a simplified shape for dissimilar metal nozzle butt welds, changes in the welding residual stress distribution can be seen using a geometry variable. Based on the results, a welding residual stress profile for dissimilar metal nozzle butt welds is proposed that modifies the existing welding residual stress profile for austenitic pipe butt welds.

There is a trend towards the progressive use of higher operating temperatures and stresses to achieve improved efficiencies in power-generation equipment. It is important to perform the crack assessment under hightemperature and high-pressure conditions. The C(t)-integral is a key parameter in crack assessment for transient creep states. The estimation of the C(t)-integral is complex when considering the mechanical and thermal loads simultaneously. In this paper, we study estimation of C(t)-integral under combined mechanical and thermal load depending on loading conditions.

This paper provides approximate expressions for through-wall welding residual stresses in dissimilar metal nozzle butt welds of pressurized water reactors. An idealized shape of nozzle is proposed, based on which systematic elastic-plastic thermo-mechanical finite element analyses are conducted by varying the thickness and radius of the nozzle and the length of the safe-end. Based on the results, a through-wall welding residual stress profile for dissimilar metal nozzle butt welds is proposed by modifying the existing welding residual stress profile for austenitic pipe butt welds in the R6 procedure.

This paper presents finite element simulation results of residual stresses in dissimilar metal welds of a PWR pressurizer safety/relief nozzle. The present results are believed to be significant in two aspects. The first one is to consider the effect of the presence of similar metal welds on resulting residual stresses. The second one is the mitigation effect of the overlay welding thickness on residual stresses. After dissimilar metal welding, tensile residual stresses are present both at the inner surface and at the outer surface of dissimilar metal welds. Adjacent similar metal welding, however, decreases residual stresses to compressive ones at the inner surface of dissimilar metal welds, possibly due to the bending mechanism caused radial contraction of the weld. At the outer surface of dissimilar metal welds, similar metal welding increases residual stresses. Overlay welding further decreases residual stresses at the inner surface of dissimilar and similar metal welds, but increases slightly residual stresses at the outer surface.

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

일반적으로 압력용기는 설계, 제작 그리고 용접을 거친 뒤 사용하게 된다. 입력용기의 설계, 제작 및 용접은 반드시 관련 기관 혹은 산업체가 허용하는 코드에 따라 수행되어야 하며, 미국의 기계학회 보일러 및 압력용기 코드(ASME BPV; American Society of Mechanical Engineers Boiler and Pressure Vessel Code) 등 다양한 코드가 있다. 이러한 Construction코드들은 주로 압력용기의 설계와 용접 및 제작에 관련된 기술기준을 제공하며, 미국 ASME BPV 코드의 Sec XI과 같이 가동 중 압력용기의 검사와 관련된 기준을 때로는 제시하기도 한다. 하지만 가동 중인 압력용기의 적절한 관리(Maintenance)는 설계와 제작만큼 중요한 문제이고 최근 대두되는 설비수명연장과 효율적인 관리 측면에서 상당히 중요한 문제로 취급되고 있다. 따라서 가동 중 압력용기의 효율적인 관리를 위해 다양한 기술과 기법이 제시되었고 이를 토대로 최근 다양한 Post-construction코드가 제시되고 있다. 설비의 효율적인 관리와 설비 수명연장을 통해 비용을 절감하고 산업의 경쟁력을 극대화할 수 있는 Post-construction코드는 가동 중인 압력용기의 검사 및 진단, 결함 및 구조 건전성 평가, 운전 및 운영과 관련된 방대한 기술을 취급해야 하므로 다양한 형태의 코드로 제시된다. 주로 가동 중인 압력용기의 위험도를 기반으로 관리하는 기법을 제시하는 Risk-Based Inspection(RBI), Risk-Based Maintenance(RBM), Risk-Centered Maintenance(RCM)등이 있으며, 가동 중인 압력용기에 발견된 결함에 대해 결함이 발견될 시 구조 건전성 평가를 수행하여 계속 운전이 적절한 가를 판단하고 나아가서 잔여수명을 평가할 수 있는 사용적합성(Fitness-For-Service; FFS)코드가 제시되어 있다. 일반적으로 가동 중 압력용기에서 결함이 발생하면 설계코드의 개념에서는 해당요소를 일괄적으로 교체하거나 보수하여야 한다. 그러나 사용적합성 평가의 개념에서는 균열과 같은 결함이 구조물의 건전성에 미치는 영향을 파악하여 계속 사용 여부 또는 보수 및 교체를 판단할 있으며, 이를 동해 보수적으로 설계된 설비의 수명을 연장하고 효율적으로 설비를 관리할 수 있을 것이다. 따라서 사용적합성 평가의 가장 큰 특징은 결함을 반영하여 설비의 건전성 및 수명을 평가한다는 점이다. 압력용기의 설계 시에는 용기에 요구되는 성능과 관련된 해석과 더불어 사용 시 예상되는 파손기구들(failure mechanisms, 예 침/부식, 피로, 파괴)을 고려하여 역학적 구조해석을 통해 용기의 재료, 형상 및 두께 그리고 수명을 결정한다. 특별한 경우 설계 시 가상결함을 고려하여 건전성평가를 수행하는 경우도 있으나 기본적으로는 설계 시에는 결함을 반영하지 않는다. 결함평가를 수행해야 하는 사용적합성평가에서는 비선형 해석 및 파괴역학 등과 같은 기법을 사용하지만 기본적으로는 설계와 같은 개념을 가지나, 사용목적이 상이함에 따라 설계코드와 사용적합성코드는 적용 시 차별성이 존재한다. 본 고에서는 사용적합성평가를 위한 기존 코드들을 간략히 소개한 뒤 적용 시 설계코드와의 차별성과 관련된 몇 가지 사항을 정리하고자 한다.

This paper reports finite element analyses for an axially loaded cylinder with three different types of thermal stress. The magnitudes of the thermal and mechanical loadings are varied and creep crack tip parameters are evaluated for part-through-wall and fully penetrating defects with a range of sizes. The relaxation rate for the creep crack tip parameter is examined in detail and is found to depend on the magnitudes of the mechanical and thermal loads and on whether or not there is plasticity on initial loading. Simplified formulae are developed for describing the rate of relaxation by a modified redistribution time.

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.

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.

This paper presents finite element solutions for elastic–plastic J for circumferentially cracked pipes under combined mechanical and thermal loads in terms of the V/Vo factor used within the failure assessment diagram approach. Systematic analyses suggest that the two major variables affecting V/Vo are the relative magnitude of the secondary stress and the primary load magnitude. It has been found that the variations with these parameters obtained from the FE results agree well with current R6 predictions for modest thermal loads and low primary loads. For larger thermal stresses, the R6 predictions are overly conservative and more accurate predictions are suggested.

This paper firstly presents net-section limit moments for circumferential through-wall and part-through surface cracks at the interface between elbows and attached straight pipes under in-plane bending. Closed-form solutions are proposed based on fitting results from small strain FE limit analyses using elastic–perfectly plastic materials. Net-section limit moments for circumferential cracks at the interface between elbows and attached straight pipes are found to be close to those for cracks in the centre of elbows, implying that the location of the circumferential crack within an elbow has a minimal effect on the net-section limit moment. Accordingly it is also found that the assumption that the crack locates in a straight pipe could significantly overestimate the net-section limit load (and thus maximum load-carrying capacity) of the cracked component. Based on the proposed net-section limit moment, a method to estimate elastic–plastic J based on the reference stress approach is proposed for circumferential cracks at the interface between elbows and attached straight pipes under in-plane bending.

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.

The present work provides mis-match limit loads for welded pipes with constant-depth, circumferential part-through surface cracks in the centre of the weld metal. As limiting cases, circumferential through-wall and fully circumferential cracks are also considered. Based on systematic finite element limit analyses, mis-match limit loads are quantified by the strength mis-match ratio and only one geometry-related slenderness parameter, not requiring additional geometry-related parameters. This implies that mis-match limit loads for surface cracked pipes can be found from those for idealized axi-symmetric problems, which offers significant benefit in assessment of surface cracks in welds of pipes.

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.

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.

Weld overlay is one of the residual stress mitigation methods which arrest crack initiation and crack growth. Therefore weld overlay can be applied to the region where cracking is likely to be. An overlay weld used in this manner is termed a preemptive weld overlay(PWOL). In pressurized water reactor(PWR) dissimilar metal weld is susceptible region for primary water stress corrosion cracking(PWSCC). In order to examine the effect of PWOL on residual stress mitigation, PWOL was applied to a specific dissimilar metal weld of Kori nuclear power plant by finite element analysis method. As a result, strong compressive residual stress was made in PWSCC susceptible region and PWOL was proved effective preemptive repair method for weldment.

In nuclear power plants, ferritic low alloy steel components were connected with austenitic stainless steel piping system through alloy 82/182 butt weld. There have been incidents recently where cracking has been observed in the dissimilar metal weld. Alloy 82/182 is susceptible to primary water stress corrosion cracking. Weld-induced residual stress is main factor for crack growth. Therefore exact estimation of residual stress is important for reliable operating. This paper presents residual stress computation performed by 6” safety & relief nozzle. Based on 2 dimensional and 3 dimensional finite element analyses, effect of welding variables on residual stress variation is estimated for sensitivity analysis. 1. 서 론 최근 V.C Summer 원전 등에서 이종금속용접부 (Dissimilar Metal Weld) 균열이 관찰되었다. (1~3) 균열은 Inconel 계열 용접 금속에서 발생하였으며 일차수응력부식균열(PWSCC)이 균열의 주요 발생 원인이었다. 일차수응력부식균열은 재료의 민감도, 용접부 인장 잔류 응력 및 사용 중 하중(In-service load), 부식 환경과 같은 3가지 인자의 상호작용에 의해 발생한다. (2,4,5)

In nuclear power plants, ferritic low alloy steel nozzle was connected with austenitic stainless steel piping system through alloy 82/182 butt weld. Accurate estimation of residual stress for weldment is important in the sense that alloy 82/182 is susceptible to stress corrosion cracking. There are many results which predict residual stress distribution for alloy 82/182 weld between nozzle and pipe. However, nozzle and piping system usually connected through safe end which has short length. In this paper, residual stress distribution for pressurizer nozzle of Kori nuclear power plant was predicted using FE analysis, which consideded safe end. As a result, existing residual stress profile was redistributed and residual stress of inner surface was decreased specially. It means that safe end should be considered to reduce conservatism when estimating the piping system. 1. 서 론 최근 들어 V.C Summer, Ringhals 및 Tsuruga 2 등의 원전 이종 금속 용접부(DMW, Dissimilar Metal Weld)에서 (1,2)균열이 발생되었다. 균열의 위치는 상대적으로 운전 압력 및 운전 온도가 높은 원자로(reactor) 및 가압기(pressurizer)의 노즐 용접부로서 균열의 원인은 용접부 금속인 니켈 합금(Alloy 82/182) 재료의 일차수 응력부식균열(PWSCC)로 판명되었다. Alloy 82/182 재료는 응력 부식 균열에 민감하다고 알려져 있으며 주로 이종 금속 용접부 용접 재료로 사용되었다. 응력부식균열은 재료의 민감도, 수화학 환경 및 용접 잔류응력(welding residual stress)의 상호 작용에 의해 발생한다.

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.

The present work proposes a method for elastic–plastic fracture mechanics analysis of the circumferential through-wall crack in weldment joining elbows and attached straight pipes, subject to in-plane bending. Heterogeneous nature of weldment is not explicitly considered and thus, the proposed method assumes cracks in homogeneous materials. Based on small strain finite element limit analyses using elastic–perfectly plastic materials, closed-form limit loads for circumferential through-wall cracks between elbows and straight pipes under bending are given. Then applicability of the reference stress-based method to approximately estimate J and crack opening displacement (COD) is evaluated. It was found that the limit moments for circumferential cracks between elbows and attached straight pipes can be much lower than those for cracks in straight pipes, particularly for a crack length of less than 30% of the circumference; this result is of great interest in practical cases. This result implies that, if one assumes that the crack locates in the straight pipe, limit moments could be overestimated significantly, and accordingly, reference stress-based J and COD could be significantly overestimated. For the leak-before-break analysis, accurate J and COD estimation equations based on the reference stress approach are proposed.

This paper provides net-section limit pressures and a reference stress based J estimation method for pipes with constant depth, internal axial surface cracks under internal pressure. Based on systematic small strain finite element (FE) limit analyses using elastic perfectly plastic materials, net-section limit pressures are firstly determined, and based on FE results, a closed-form limit pressure solution is proposed. Furthermore, based on the proposed limit pressure solution, a method to estimate elastic-plastic J is proposed based on the reference stress approach. When the reference stress is defined by the proposed (global) limit pressure, estimated J values based on the reference stress approach are overall slightly lower than FE results, implying that the method is non-conservative. By re-defining the reference using optimised reference loads, resulting J estimates agree well with FE results.

The present work provides mismatch limit loads and approximate J estimates for tensile plates with constant-depth, part-through surface cracks in the center of the weld metal. Based on systematic three-dimensional FE limit analyses, effects of strength mismatch related variables on limit loads are firstly quantified by the strength mismatch ratio and one geometry-related parameter. Mismatch limit loads for part-through surface cracks are then correlated to those for two-dimensional, through-wall crack problems. Based on the proposed limit load solutions, the applicability of the reference stress based J estimates is also investigated. When the reference stress is defined by the mismatch limit load, predicted J values agree overall well with FE results.

The present work proposes a method for elastic-plastic fracture mechanics analysis of the circumferential through-wall crack in between elbows and attached straight pipes, subject to in-plane bending. Based on small strain finite element limit analyses, closed-form limit load solutions are given first. Then applicability of the reference stress based method to approximately estimate J is proposed. One interesting finding is that a popular approach to assume that the crack locates in the straight pipe could lead to significantly different assessment results.

This paper presents plastic limit loads and approximate J-integral estimates for circumferential part-through surface crack at the interface between elbows and pipes. Based on finite element limit analyses using elastic-perfectly plastic materials, plastic limit moments under in-plane bending are obtained and it is found that they are similar those for circumferential part-through surface cracks in the center of elbow. Based on present FE results, closed-form limit load solutions are proposed. Welds are not explicitly considered and all materials are assumed to be homogeneous. And the method to estimate the elastic-plastic J-integral for circumferential part-through surface cracks at the interface between elbows and straight pipes is proposed based on the reference stress approach, which was compared with corresponding solutions fur straight pipes.

This paper proposes plastic loads (limit load and twice-elastic-slope (TES) plastic load) for pipe bends with circumferential through-wall and part-through surface cracks under in-plane bending, based on three-dimensional FE limit analyses. The material is assumed to be elastic–perfectly plastic, and both the geometrically linear (small strain) and nonlinear (large geometry change) effects are considered. Regarding a crack location, both extrados and intrados cracks are considered. Based on the FE results, closed-form approximations of limit and TES plastic loads are proposed for practical applications, and compared with corresponding solutions for straight pipes.

This paper discusses possibilities to extract elastic-plastic properties of nano-scale materials using combined nano-indentation tests with FE simulations. One interesting finding is that FE simulations of nano-indentation with a number of different plastic properties give same load-displacement response, which suggests that plastic properties cannot be determined from simulating load-depth curves from nano-indentation tests. However, careful examination of possible plastic properties suggests a concept of the reference strain, which makes it possible to effectively determine plastic properties from nano-indentation tests with FE simulations.

In this paper, a prediction of C(t), a crack tip parameter for the transient creep condition, for the circumferential cracked pipe under combined mechanical and thermal stresses are presented. The estimation formulae for C(t)-integral of the cracked component operating under mechanical load alone have been provided for decades [1–6]. However, high temperature structures usually work under combined mechanical and thermal load. And the interactions between mechanical and thermal loads make the relaxation rate different from those produced under mechanical load alone. In this study, 3-dimensional finite element analyses are conducted to calculate the C(t)-integral under combined mechanical and thermal load. As a result, redistribution time for the crack under combined mechanical and thermal load is re-defined to quantify the C(t)-integral. The estimation of C(t)-integral using this proposed redistribution time agrees well with FE results.

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 © 2010 by ASME

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 © 2010 by ASME

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.

This paper presents a micro-mechanical model of ductile fracture for the API X65 steel using the Gurson-Tvergaard-Needleman (GTN) model. Experimental tests and FE damage simulations using the GTN model are performed for smooth and notched tensile bars, from which the parameters in the GTN model are calibrated. As application, the developed GTN model is applied to simulate small-sized, single-edge-cracked tensile and bend bars, via three-dimensional FE damage analyses. Comparison with experimental test data shows overall good agreements.Copyright © 2007 by ASME

This paper provides net-section limit pressures and a reference stress based J estimation method for pipes with internal axial surface cracks under internal pressure. Based on systematic small strain FE limit analyses using elastic-perfectly plastic materials, closed-form approximations of net-section limit pressures are presented. Then, based on proposed net-section limit moments, a method to estimate elastic-plastic J is proposed based on the reference stress approach. Comparison with extensive FE results shows overall good agreement.Copyright © 2007 by ASME