Jin Weon Kim

PREDICTION OF RESIDUAL STRESS FOR DISSIMILAR METALS WELDING AT NUCLEAR POWER PLANTS USING FUZZY NEURAL NETWORK MODELS

A fuzzy neural network model is presented to predict residual stress for dissimilar metal welding under various welding conditions. The fuzzy neural network model, which consists of a fuzzy inference system and a neuronal training system, is optimized by a hybrid learning method that combines a genetic algorithm to optimize the membership function parameters and a least squares method to solve the consequent parameters. The data of finite element analysis are divided into four data groups, which are split according to two end-section constraints and two prediction paths. Four fuzzy neural network models were therefore applied to the numerical data obtained from the finite element analysis for the two end-section constraints and the two prediction paths. The fuzzy neural network models were trained with the aid of a data set prepared for training (training data), optimized by means of an optimization data set and verified by means of a test data set that was different (independent) from the training data and the optimization data. The accuracy of fuzzy neural network models is known to be sufficiently accurate for use in an integrity evaluation by predicting the residual stress of dissimilar metal welding zones.

UNCERTAINTY ANALYSIS OF DATA-BASED MODELS FOR ESTIMATING COLLAPSE MOMENTS OF WALL-THINNED PIPE BENDS AND ELBOWS

The development of data-based models requires uncertainty analysis to explain the accuracy of their predictions. In this paper, an uncertainty analysis of the support vector regression (SVR) model, which is a data-based model, was performed because previous research showed that the SVR method accurately estimates the collapse moments of wall-thinned pipe bends and elbows. The uncertainty analysis method used in this study was an analytic uncertainty analysis method, and estimates with a 95% confidence interval were obtained for 370 test data points. From the results, the prediction interval (PI) was very narrow, which means that the predicted values are quite accurate. Therefore, the proposed SVR method can be used effectively to assess and validate the integrity of the wall-thinned pipe bends and elbows.

Evaluation of Tensile Properties of Alloy 690TT Steam Generator Tube at Room Temperature and 343

This study conducted tensile tests on an Alloy 690TT tube at room temperature (RT) and at using tube- and ring-type specimens to investigate the stress-strain behavior and tensile properties of a steam generator (SG) tube in the axial and circumferential directions at RT and at the design temperature of a nuclear power plant (NPP). The results of the axial tensile test showed that yield point phenomena appeared at both RT and , and serrated flow in the stress-strain curve appeared at . Yield and tensile strengths for both directions were clearly lower at compared to RT; however, the elongations were approximately the same at both test temperatures. Regardless of the test temperature, the strengths in the circumferential direction were lower by approximately 5~10 % than those in the axial direction. In addition, the test data revealed that the reduction in the yield and tensile strengths of the Alloy 690TT SG tube with the test temperature was more significant than that estimated by the temperature correction factor of ASME Sec.II.

Predictions of Mechanical Properties From Small Punch Test Results Using FE Damage Analyses

This paper compares FE ductile fracture simulation results with compact tension (C(T)), small punch (SP) and notched small punch (NSP) test data for SA508 Gr. 3 material. Ductile fracture simulation method in this paper is based on the stress-modified fracture strain model, recently proposed by the authors. Tensile properties and stress-modified fracture strains are extracted from smooth and notched bar tensile tests, and SP test. To extract tensile property from SP test, tensile property constitutive equations are assumed. Using the extracted tensile properties and stress-modified fracture strains, C(T), SP and NSP tests are simulated and the results are compared with experimental data. The results using the properties from smooth and notched bar tensile tests agree well with experimental data. And the SP and NSP simulations using the properties from SP test well predict the maximum loads and fracture behaviors, However, C(T) test simulations have conservative results relatively. C(T) and NSP simulation results are sensitive to element size. However, SP simulation results are not sensitive to element size.Copyright

Measurement of Deformation on Welding Specimen Using ESPI

The residual stress was measured through the proposed method with ESPI measurement system. The proposed measurement method as a basic study combined with theory and experiments applied for residual stresses prediction and developments of safety estimation technique.

Energy-based numerical modeling of the strain rate effect on fracture toughness of SA508 Gr. 1a

This article presents an energy-based method to simulate ductile tearing under dynamic loading conditions. The strain rate–dependent material properties are characterized by the Johnson–Cook-type model. The damage model is defined based on the multi-axial fracture strain energy concept. The proposed damage model is applied to simulate the fracture toughness test of SA508 Gr. 1a under four different test speeds. Simulated results show a good overall agreement with the experimental results.

Effects of Specimen Size and Side-groove on the Results of J-R Fracture Toughness Test for LBB Evaluation

In this study, the effects of specimen size and side-groove on the results of the J-R test for leak-before-break (LBB) evaluation were investigated. A series of J-R tests were conducted at both RT and 316 o C, using three different sizes of compact tension (CT) specimens machined from SA508 Gr.1a piping material: 12.7 mm-thick 1T-CT, 25.4 mm-thick 1T-CT, and 25.4 mm-thick 2T-CT with and without side-groove. The results showed that side-grooving reduced the J-R curve for all specimens and the effect of side-grooving was more significant at 316 o C than at RT. As the thickness of the specimens decreased and the width of the specimens increased, the J-R curve slightly decreased at RT but it increased at 316 o C. However, the variation in the J-R curve of SA508 Gr.1a with the thickness and width of CT specimen was insignificant.

Fatigue Failure Behavior of Pipe Bends with Local Wall-Thinning Under Cyclic Bending Condition

In this study, fatigue tests were carried out using real-scale pipe bend specimens with wall-thinning defects under a cyclic bending load together with a constant internal pressure of 10 MPa. The wall-thinning defect was located at the extrados and the intrados of the pipe bend specimens. A fully reversed cyclic in-plane bending displacement was applied to the specimens. For the pipe bends with wall thinning at the extrados, an axial crack occurred at the crown of the pipe bend rather than at the extrados where the defect was located. In addition, the fatigue life was longer than that of a sound pipe bend predicted from the design fatigue curve in ASME Sec.III, and it was less dependent on the axial length of the wall-thinning defect. For the pipe bends with wall thinning at the intrados, a circumferential crack occurred at the intrados. In this case, the fatigue life was much shorter than that of a sound pipe bend predicted from the design fatigue curve, and it clearly decreased with decreasing axial length of the wall-thinning defect.

Development of finite element ductile tearing simulation model considering strain rate effect

This paper proposes ductile failure simulation under high strain rate conditions using finite element (FE) analyses. In order to simulate a cracked component under a high strain rate condition, this paper applies the stressmodified fracture strain model combined with the Johnson/Cook model. The stress-modified fracture strain model determines the incremental damage in terms of stress triaxiality (σm/σe) and fracture strain (ef) for a dimple fracture using the tensile test results. To validate the stress-modified fracture strain model under dynamic loading conditions, the parameters are calibrated using the tensile test results under various strain rates and the fracture toughness test results under quasi-static conditions. The calibrated damage model predicts the CT test results under a high strain rate. The simulated results were then compared with the experimental data. † Corresponding Author, kimy0308@korea.ac.kr C 2016 The Korean Society of Mechanical Engineers 남현석 · 김지수 · 김진원 · 김윤재 168 하중도 실험속도에 따라 다른 경향을 보인다고 보고되었다. 하지만 현재 적용되고 있는 지진하중 조건의 배관 건전성 평가 절차를 보면, 선형탄성해석을 통해 배관에 작용되는 지진하중의 크기를 계산하고 정적 하중 조건에서 실험된 재료 물성치를 이용하여 결함 안정성을 평가한다. 그러나 지진하중 조건에서 실제 배관에 작용하는 하중은 동적이고 반복적인 특징을 가지고 있다. 따라서 지진 하중 조건에서 배관 계통에 대한 건전성 평가를 수행하기 위해서 다양한 실험속도에 따른 재료 물성치 및 파손 거동 모사에 대한 연구가 요구되고 있다. 그러나 다양한 실험속도에 따른 재료에 대한 물성 실험을 수행하기 위해서는 많은 비용 및 시간과 노력이 요구 된다. 이러한 실기실험의 대안으로서 유한요소 손상해석을 이용한 가상 실험법은 하중과 시편 형상에 변경이 용이하고 경제적으로도 효율적이다. 이러한 이유로 본 논문에서는 다양한 실험 속도의 인장 물성 및 정적 하중조건에서의 파괴인성 실험결과를 이용하여 동적 하중조건이 작용하는 구조물의 파괴거동을 예측할 수 있는 파손 모델에 대한 연구를 수행 하였으며 이와 같은 기법을 토대로 미국 바텔 연구소(Battelle Memorial Institute)에서 수행한 동적 하중이 작용하는 파괴인성 시편(CT 시편) 실험결과를 모사하고 해석을 통한 예측 결과와 실험결과를 비교하여 변형률 속도에 따른 연성파손 해석기법의 신뢰성을 검증하였다.

Burst Pressure Prediction of Cracked Steam Generator Tube Using FE Damage Analysis

This paper predicts burst pressures of the steam generator tubes with multiple cracks using finite element (FE) damage analysis. Alloy 690(TT) tube (the outer diameter of 19.05 mm and the thickness of 1.07 mm) widely used in the nuclear power plant is considered in this study. Ductile failure at each element is predicted with the damage model known as ‘multi-axial fracture strain model’ as well as crack propagation is simulated by stress reduction technique in the FE analysis. Simplified ductile damage model for Alloy 690(TT) are determined using tube tensile test data and elastic-plastic FE analysis. FE damage analysis results are sensitive to the element size used in the crack propagation region. Using notched tube fracture test data, proper element size for Alloy 690(TT) is found. Single, collinear, parallel, non-aligned axial-cracks are considered in the simulations and the predicted burst pressures are compared with burst test data.Copyright