Kuk Hee Lee

Plastic limit loads for piping branch junctions under out-of-plane bending

This paper presents approximate closed-form plastic limit load solutions for branch junctions under out-of-plane bending and under combined pressure and out-of-plane bending, based on three-dimensional finite element limit analyses for an elastic-perfectly plastic material. When bending is applied to the branch pipe, the plastic limit loads for out-of-plane bending are shown to be lower than those for in-plane bending. However, for bending to the run pipe, the opposite trend is found. For combined pressure and out-of-plane bending, either the circular interaction or the parabolic interaction rule can be used, depending on the bending location and the branch geometry. Comparison with published experimental plastic limit load data shows that the predictions agree relatively well with the test data.

Limit Loads for Circular Wall-Thinned Feeder Pipes Subjected to Bending and Internal Pressure.

사고를 미연에 방지하기 위해서 감육에 대한 평가법 개발이 필요하다. 피더관 직관부에 발생한 감육에 대한 소성 붕괴 평가법은 개발되어 있으나, 현재 곡관Key Words : CANDU(캐나다형 중수로), feeder pipe(피더관), limit load(한계하중), circular wall-thinning(원형감육), bending(굽힘하중), internal pressure(내압) 초록: 캐나다형 중수로에서 피더관은 가동 중에 유동 가속 부식에 의해 감육이 발생한다. 피더관에 감육이 발생하면 배관 건전성이 떨어진다. 본 논문은 원형 감육이 발생한 피더관에서의 기기신뢰성 평가를 위한 한계 하중을 연구하였다. 유한요소 해석을 통하여 면내 굽힘 하중과 내압을 받는 경우에 대하여 연구하였다. 재료는 대변형 효과를 고려하고 탄성-완전소성 재료로 가정하였다. 원형 감육이 발생한 피더관에 대하여 내압과 닫힘 방향, 열림 방향 굽힘 하중에 대하여 한계하중해를 제시하였다. Abstract: Flow Accelerated Corrosion (FAC) occurring during in-service conditions results in localized wall-thinning in the feeder pipes of CANDU. The wall-thinning of the feeder pipes is the main degradation mechanisms affecting the integrity of piping systems. In this paper, we assess the integrity of wall-thinned feeder pipes by limit load analysis. The limit loads for wall-thinning feeder pipes subjected to in-plane bending and internal pressure were determined on the basis of finte element limit analyses. The limit loads are determined from the results of limit analyses of elastic-perfectly-plastic materials using the large geometry change. Closed-form approximations of limit load solutions for wall-thinning feeder pipes subjected to in-plane bending and pressure are proposed.

Effect of Creep Mismatch Factor on Stress Redistribution in Welded Branch Pipes

This paper describe steady state stress distribution into the weld metal of welded branch components using detailed three dimensional elastic creep finite element analyses. In order to show the effect of the loading mode, this research is carried out under various loading conditions such as internal pressure, in-plane bending to the branch pipe and out-of-plane bending to branch pipe. Also, to generalize the unique aspect, three geometries of branch components including welded large bore branch, medium bore branch, and trunnion are considered. It is a well-known fact that the creep strain rate of welds material is faster than that of parent material. Therefore, the creep exponent and constants for the parent and weld metal are systematically varied to analyze under-matching, even-matching and over-matching conditions in creep. It can be shown that mismatch effect can be quantified as mismatch factor with specific characteristics.Copyright

Reference Stress Solutions for Idealized Branch Junctions

The present work presents plastic limit load solutions for thin-walled branch junctions under internal pressure and in-plane bending, based on detailed three-dimensional (3-D) FE limit analyses using elastic-perfectly plastic materials. The proposed solutions are valid to ratios of the branch-to-run pipe radius and thickness from 0.0 to 1.0, and the mean radius-to-thickness ratio of the run pipe from 5.0 to 20.0. Comparison with FE results shows good agreement.Copyright

Method to Determine Elastic Follow-Up Factors to Predict C(t) for Elevated Temperature Structures

This paper proposes a method to determine the elastic follow-up factors for the -integral under secondary stress. The rate of creep crack growth for transient creep is correlated with the -integral. Elastic follow-up behavior, which occurs in structures under secondary loading, prevents a relaxation of stress during transient creep. Thus, both the values of and creep crack growth increase as increasing elastic follow-up. An estimation solution for was proposed by Ainsworth and Dean based on the reference stress method. To predict the value of using this solution, an independent method to determine the elastic follow-up factors for cracked bodies is needed. This paper proposed that the elastic follow-up factors for can be determined by elastic-plastic analyses using the plastic-creep analogy. Finite element analyses were performed to verify this method.

Elastic follow-up factors to estimate C(t) under secondary loading

This paper proposes a method to determine the elastic follow-up factors for C(t)-integral under secondary stress. The rate of creep crack growth for transient creep is correlated with C(t)-integral. The elastic follow-up behaviour, which occurs in structures under secondary loading, prevents a relaxation of stress during transient creep. Thus, both the value of C(t) and creep crack growth increase with an increasing elastic follow-up. An estimation solution for C(t) has been proposed by Ainsworth and Dean based on the reference stress method. In order to predict the value of C(t) using this solution, an independent method to determine the elastic follow-up factors for cracked bodies is required. This paper proposes that the elastic follow-up factors for C(t) can be determined by elastic-plastic analyses by using the plastic-creep analogy. Finite element analyses have been performed to verify this method.Copyright

Effect of creep mismatch factor on steady state creep stresses in welded components with heat affected zone

This paper describes steady-state stress on welded branch components using detailed three dimensional elastic creep finite element analyses. In our previous paper [1], it was found that the mismatch effect in creep on steady-state stresses within the weld metal for a various branch junction could be uniquely quantified by the mis-match factor, defined as a function of creep exponent and constant. In actual branch components, the branch junction contains the heat-affected zone (HAZ) when the branch pipe is welded. Thus, additional mismatch factor for HAZ should be presented. This paper deals with not only the mismatch effect for weld metal but also that for HAZ. The creep exponent and constant for the base and weld metal as well as HAZ were systematically varied to analyze under-matching, even-matching and over-matching conditions in creep. In order to investigate the effect of the loading mode, FEA was carried out under internal pressure and in-plane bending to the branch pipe. Two geometries such as large bore branch and medium bore branch were considered. It was found that steady-state creep stresses within HAZ can be quantified as mismatch factor with specific characteristics.© 2011 ASME

Elastic Follow-Up Factor for Cruciform Plate Under Bi-Axial Loading

This paper derives analytical solutions of the elastic follow-up factor for power-law creeping cruciform plates under bi-axial displacements to investigate the effect of multi-axial stress states on elastic follow-up behaviors. Validity of the proposed solutions is checked against the results from finite element analyses using power-law creep material. Based on proposed solutions, effects of the biaxiality, geometry, Poisson’s ratio and creep exponent on elastic follow-up factors are discussed. Present results show that the elastic follow-up factor for structure with structural discontinuity can be significantly affected by the multi-axial stress states.Copyright

Comparison of Plastic Loads for Elbows With Experimental Data

Finding plastic (limit) loads for elbows under various loading conditions such as in-plane bending and out-of-plane bending is not an easy task due to complexities involved in plastic analyses. Considering complexities involved in plastic limit analysis of elbow, deriving analytical solutions of plastic loads for elbows would be extremely difficult. So, recently the limit analysis using finite element program has been widely adopted. Based on extensive and systematic FE limit analyses using elastic-perfectly plastic materials, closed-form solutions of plastic loads for defect-free elbows under in-plane closing, in-plane opening and out-of-plane bending were presented. This paper summarizes the well-known criteria for finding plastic (limit) loads proposed by ASME BPVC Sec.III [1], Zahoor [4], Chattopadhyay et al. [17] and Kim et al. [19] The purpose of this paper is to integrate and improve the proposed solutions by Kim et al. Also, comparison results with published experimental data are presented. From these results, the pros and cons of each criterion for finding plastic (limit) loads for elbows are discussed.Copyright

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