Hyun Suk Nam

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.

Numerical simulation and experimental validation of ductile tearing in A106 Gr. B piping system under simulated seismic loading conditions

This work presents finite element ductile tearing simulation and experimental validation of a piping system with a circumferential surface cracked (SC) A106 Gr. B pipe under simulated seismic loading condition. The damage model for simulation is based on the multiaxial fracture strain energy. The parameters in the damage model are determined from tensile and fracture toughness test results under the monotonic loading condition. For the system dynamic time history analysis, the Rayleigh damping model is employed. For cyclic constitutive equations, two models were considered to confirm its sensitivity. Predicted crack initiation and ductile tearing agree well with the experimental results.

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.

Ductile Crack Growth Modeling under Low Cycle Fatigue Loading Condition

This paper presents a numerical method using FE damage analysis to simulate ductile tearing in CT specimen under low cycle fatigue loading conditions. To define a cyclic material properties, the nonlinear kinematic hardening model is adopted. The damage model is defined based on the ductility exhaustion concept using the multi-axial fracture strain energy concept. The proposed model is then applied to simulate two cyclic fracture toughness tests with different R-ratios. Simulated results show overall good agreement with experimental results.

Ductile fracture simulation considering strain rate loading effect

This paper is based on a ductile failure simulation under dynamic loading conditions using finite element (FE) analyses. Recently a simple finite element method in a quasi-static test has been proposed to implement fracture simulation based on the well-known stress modified fracture strain model. The stress-modified fracture strain model is determined to be incremental damage in terms of stress triaxiality and fracture strain for dimple fracture from tensile test result with FE analyses technique. Since dynamic loading effect is especially important to assess pipe with crack-like defect, this work propose the integrated model which combines quasi-static with dynamic loading effect. In order to validate stress-modified fracture strain model in dynamic loading conditions, this paper compares results of FE analysis using proposed method with strain dependent smooth bar tests and notch tensile tests using Johnson-Cook equation. In conclusion, the stress-modified fracture strain model criterion can be calibrated by FE analyses with strain rate dependent fracture toughness test results.Copyright

Ductile Fracture Simulation of CRIEPI STPT 410 Pipe With Circumferential Crack

This paper provides simulation of ductile crack growth in full-scale cracked pipe tests using an element-size dependent damage model. This method is based on the stress-modified fracture strain damage model. The stress-modified fracture strain model is determined to be incremental damage in terms of stress triaxiality and fracture strain for dimple fracture from tensile test result with FE analyses technique. To validate the proposed method, this research analyses STPT 410 cracked pipes test at 300°C taken from CRIEPI (Central Research Institute of Electric Power Industry). In order to calibrate the stress-modified fractures strain model, tensile tests and fracture toughness tests were compared with simulated results using element-size dependent damage model. Tensile specimen and fracture toughness specimen were extracted from STPT 410 steel pipe. The calibrated damage model predicts ductile crack growth in 5 type circumferential cracked pipes bending test. And these results were compared with the experimental results. The results show that the proposed method can simulate ductile crack growth in full-scale cracked pipe tests.© 2014 ASME

J-Integral Estimate for Circumferential Cracked Pipes Under Primary and Secondary Stress in R6, RCC-MR A16

This paper provides a comparison of the J-integral estimation method under combined primary and secondary stress in the R6, RCC-MR A16 code. The comparisons of each code are based on finite element analysis using ABAQUS with regard to the crack shape, crack depth, and magnitude of secondary load. The estimate of the R6 code is conservative near Lr = 1, and that of the RCC-MR A16 code is conservative near Lr = 0. As a result, this paper proposes a modified method of J-integral estimation in the R6, RCC_MR A16 code. The J-integral using the modified method corresponds to the finite element analysis result.

Effect of Normal Operating Condition Analysis Method for Weld Residual Stress of CRDM Nozzle in Reactor Pressure Vessel

In pressurized water nuclear reactors (PWRs), the reactor pressure vessel (RPV) upper head contains penetration nozzles that use a control rod drive mechanism (CRDM). The penetration nozzle uses J-groove weld geometry. Recently, the occurrence of cracking in alloy 600 CRDM penetration nozzle has increased. This is attributable to primary water stress corrosion cracking (PWSCC). PWSCC is known to be susceptible to the welding residual stress and operational stress. Generally, the tensile residual stress is the main factor contributing to crack growth. Therefore, this study investigates the effect on weld residual stress through different analysis methods for normal operating conditions using finite element analysis. In addition, this study also considers the effect of repeated normal operating condition cycles on the weld residual stress. Based on the analysis result, this paper presents a normal operating condition analysis method.

Design of vertical type probe tip using finite element analysis

The design process of a micro-probe tip is very complicated and expensive. To avoid these problems, in this study, we used element (FE) analysis. To simplify design process. A new pre-probe tip (cobra-needle type) made of Ni and Co was designed by FE analysis. Experimental results were compared with those obtained by FE analysis to verify the reliability of the analysis. The contact force and over drive were respectively found to be 12.5 gf(Contact Force) and (Over drive). We propose the new designed probe tip. Material of new designed probe tip is NiCo. Values of Property are 1~2 gf(Contact Force) and (Over drive).