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초음파나노표면개질 다중충격 조건에서의 잔류응력 예측을 위한 유한요소 피닝해석 영역 결정
석태현,박승현,허남수 한국압력기기공학회 2021 한국압력기기공학회 논문집 Vol.17 No.2
Ultrasonic Nanocrystal Surface Modification (UNSM) is a peening technology that generates elastic-plastic deformation on the material surface to which a static load of a air compressor and a dynamic load of ultrasonic vibration energy are applied by striking the material surface with a strike pin. In the UNSM-treated material, the structure of the surface layer is modified into a nano-crystal structure and compressive residual stress occurs. When UNSM is applied to welds in a reactor coolant system where PWSCC can occur, it has the effect of relieving tensile residual stress in the weld and thus suppressing crack initiation and propagation. In order to quantitatively evaluate the compressive residual stress generated by UNSM, many finite element studies have been conducted. In existing studies, single-path UNSM or UNSM in a limited area has been simulated due to excessive computing time and analysis convergence problems. However, it is difficult to accurately calculate the compressive residual stress generated by the actual UNSM under these limited conditions. Therefore, in this study, a minimum finite element peening analysis area that can reliably calculate the compressive residual stress is proposed. To confirm the validity of the proposed analysis area, the compressive residual stress obtained from the experiment are compared with finite element analysis results.
초음파나노표면개질 공정변수에 따른 표면잔류응력 변화 분석
석태현,허남수 대한기계학회 2023 大韓機械學會論文集A Vol.47 No.3
Compressive residual stresses generated on the surface by ultrasonic nanocrystal surface modification vary according to the process variables. In this study, surface compressive residual stresses were analyzed according to process variables using finite element analyses. Finite element analysis methodology was introduced. The surface compressive residual stresses according to the static load, amplitude, feed rate, and line interval were calculated through 3D finite element analyses. As the static load and amplitude increased, the magnitude and depth of the surface compressive residual stress increased. Furthermore, the feed rate and line interval were analyzed in terms of the overlapping ratio. As the feed rate and line interval increased, the overlapping ratio decreased and the magnitude of the surface compressive residual stress in each direction decreased. 초음파나노표면개질에 의한 표면 압축잔류응력의 크기와 깊이는 공정변수에 따라 변화한다. 본 논문에서는 유한요소해석을 활용하여 초음파나노표면개질 공정변수에 따른 표면 압축잔류응력의 변화를 분석하였다. 초음파나노표면개질 유한요소해석을 위한 방법론을 소개하였고 3차원 유한요소해석을 수행하였다. 유한요소해석을 통해 정하중, 진폭, 이송 속도, 줄 간격에 따른 표면 압축잔류응력을 분석하였다. 정하중과 진폭이 증가함에 따라 표면 압축잔류응력의 크기와 깊이가 증가하였다. 이송 속도와 줄 간격은 중첩비 관점으로 분석하였다. 이송 속도와 줄 간격이 증가함에 따라 중첩비가 감소하였으며 각 방향으로의 표면 압축잔류응력 크기가 감소하였다.
외압이 작용하는 마모 실린더의 붕괴 압력 예측을 위한 한계하중 및 비선형 좌굴 해석
석태현(Tae-Hyeon Seok),노가람(Ga-Ram Rho),김동석(Dong-Seok Kim),강병현(Byoung-Hyun Kang),지민정(Min-Jung Ji),박승현(Seung-Hyun Park),허남수(Nam-Su Huh) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4
원자로용 SGT(Steam Generator Tubes)와 같이 외압을 받는 실린더는 접촉 표면에서의 미세 진동 운동 등에 의해 마모가 발생할 수 있다[1]. 실린더는 마모 발생 전후로 좌굴 거동에 분명한 차이가 존재하므로[2] 구조적 건전성 평가가 필요하다. 이에 본 연구에서는 외압이 작용하는 마모 실린더의 선형 좌굴(linear buckling), 비선형 좌굴(non-linear buckling), 소성 붕괴(plastic collapse) 해석을 수행하여 붕괴압력을 평가하였다. 실린더 중심부에 마모 길이 및 깊이가 다른 다양한 마모 형상을 고려하였다. 실제로는 형상의 부정(imperfection)에 의해 임계압력이 저하되므로 선형 좌굴 해석의 결과를 정확한 좌굴압력으로 보기는 어렵다[3]. 따라서 소성 붕괴, 비선형 좌굴 해석으로 얻은 한계압력, 임계좌굴압력을 비교하였다. 해석 결과 한계압력은 마모 형상이 커질수록 감소하는 경향을 보이며, 임계좌굴압력은 좌굴 형상에 따라 상이한 경향을 보였다. 형상부정계수가 0.1 인 경우 비선형 좌굴 해석으로 얻은 붕괴압력이 작았지만 마모 형상이 커짐에 따라 소성붕괴도 고려되어야 한다. 정확한 임계좌굴압력의 계산을 위해서는 형상부정계수의 정의가 중요하며, 추후 실험을 통하여 형상부정계수를 확보한다면 외압을 받는 마모 실린더의 명확한 구조건전성 평가 가이드라인 제시가 가능할 것으로 판단된다. The cylinder under external pressure, such as steam generator tubes of nuclear power plants, may wear out due to micro-oscillating movements on the contact surface[1]. Since wear defect affects buckling behavior of the cylinder[2], structural integrity assessment of wear cylinder is necessary. In this study, the collapse pressure by linear buckling, non-linear buckling and plastic collapse analyses of a wear cylinder under external pressure are investigated. Various wear shapes with different lengths and depths in the center of the cylinder are considered. It is difficult to obtain accurate collapse pressure based on the linear buckling analysis since the critical pressure decreases due to imperfection of the tube shape[3]. Therefore, we compared linear buckling pressure with both limit pressure and critical buckling pressure obtained by plastic collapse and non-linear buckling analyses. As a result, the limit pressure tends to decrease as the wear size increases as expected, and the critical buckling pressure tends to differ according to the wear shapes and imperfection scale factors. When the imperfection scale factor is 0.1, the main cause of collapse is non-linear buckling, on the other hand, plastic collapse should be considered as the wear size increases. Since the accurate determination of imperfection scale factor is crucial element in the critical buckling pressure assessment, in order to provide the clear guideline of structural integrity assessment for the wear cylinder under external pressure, the experiments on wear cylinder should be conducted.
김태형,석태현,설진운,이병호,권병기,최종윤 항공우주시스템공학회 2020 항공우주시스템공학회지 Vol.14 No.6
This study proposes a finite element (FE) model for predicting the bending strength of small gears used in electro-mechanical actuators for aircraft. First, a strain gauge was attached to the tooth root of test gear, and the strain was measured. Subsequently, the FE model was applied to calculate the strain of the test gear, and the modeled strain was compared with the experimental strain. The results confirmed that the FE strain was very close to the experimental strain and the FE model was valid. This FE model was extended to the bending strength analysis of several small gear tooth models. The bending strengths of all the tooth models were almost identical to the ISO theoretical bending strength. Finally, the FE model was validated and the reliability of the modeled bending strength was evaluated through the strain measurement experiment.
김태형,석태현,권순영,이병호,권병기,권준용,정성균 항공우주시스템공학회 2020 항공우주시스템공학회지 Vol.14 No.6
In this study, finite element fatigue analysis combined with a fatigue correlation factor is proposed to predict the bending fatigue life of a gear in an electro–mechanical aircraft actuator. First, stress–life curves are obtained for the gear material via a round bar fatigue test. Subsequently, stochastic stress–life (P–S–N) curves are derived for 50% and 1% failure probabilities, separately. The curves are applied to the fatigue analysis model of a single gear tooth, and the effect of the fatigue correction factor is analyzed. The analytical P–S–N curves reflecting the fatigue correction factor matched the experimental data. This shows that the analytical fatigue life is reliable and that the analysis technique is effective.
대면적 표면처리용 광폭 임펠러의 경량 설계 및 구조적 안정성
김태형,정준형,차준명,석태현,이세창 한국에너지학회 2020 에너지공학 Vol.29 No.3
본 연구에서는 유한요소해석을 통해 대면적 표면처리가 가능한 경량 광폭 임펠러를 설계하였으며 구조적 안정성을 확인하였다. 먼저 위상최적화를 통해 경량 브라켓 해석모델을 수립하였으며, 구조해석을 통해 최적의 모델을 선정하였다. 3차원 경량 광폭 임펠러의 굽힘변형 해석을 수행하였으며 허용변형량 범위에 포함되었다. 또한진동해석을 수행하여 1차 모드 고유진동수를 얻었으며 위험속도식에 대입하여 안전 운전속도(RPM) 기준을 제시하였다. 궁극적으로 본 연구의 해석적 기법이 경량 광폭 임펠러 설계에 유효함을 확인하였다.