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김민헌(Min-Heon Kim),박환성(Hwan-Seong Park),김동규(Dong-Kyu Kim),남기우(Ki-Woo Nam) 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11
Fatigue fracture is the result of repetitive cyclic short-time stress or tensile stress, or deformation well below the tensile or flexural strength of the material. In order to use the structure safely, it is necessary to predict the life of small cracks. In this study, using Ando formula considering plastic deformation, fatigue limit was evaluated by comparing the results of Nakagawa et al. Since the fatigue limit of the surface crack should be evaluated as the crack with the larger stress intensity factor, As = 1.0 should be evaluated as surface, and As = 0.4 should be evaluated as depth. At this time, evaluation by equivalent crack length showed good agreement.
김민헌(Min-Heon Kim),박환성(Hwan-Seong Park),김동규(Dong-Kyu Kim),남기우(Ki-Woo Nam) 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11
This study is to evaluate the harmless crack size according to the compressive residual stress as a basic study to evaluate the safety of hydrogen storage tank. The harmless crack dimension depends on the residual stress and the crack aspect ratio(As). As = 1.0 and 0.6 were determined as crack depth at large residual stresses and crack lengths at medium and small residual stresses. As = 0.3 was determined as the crack depth regardless of the magnitude of the residual stress. The harmless crack size by shot peening was different depending on the residual stress and the crack aspect ratio.
무한판의 관통균열과 유한판의 반타원 표면균열의 하한계응력확대계수 및 피로한도 연구
박환성(Hwan-Seong Park),김민헌(Min-Heon Kim),남기우(Ki-Woo Nam) 대한기계학회 2021 大韓機械學會論文集A Vol.45 No.2
선형파괴역학은 금속 및 세라믹의 균열문제를 정량적으로 취급할 수 있다. 특히 파괴인성(Kc)과 피로균열진전의 하한계응력확대계수(△Kth)는 균열 크기에 의존한다. 선형파괴역학은 균열선단부에 발생하는 소성 영역이나 비선형영역이 균열 크기에 비하여 매우 작은 것을 전제로 하고 있다. 그러나 작은 균열은 이 조건이 만족하지 않는다. 본 연구는 Ando 등이 제안한 피로 특유의 소성거동과 응력비를 고려한 식을 사용하여, 피로 하중에 의한 표면 작은 균열 크기의 하한계응력확대계수(△Kth) 및 피로한 도(△σω)을 평가하였다. 또한, 비교하기 위하여 무한판의 관통 균열에 대해서도 평가하였다. Linear fracture mechanics can quantitatively address the problem of cracks in metals and ceramics. In particular, the fracture toughness (Kc) and threshold stress intensity factor (△Kth) of fatigue crack propagation depend on the crack size. Linear fracture mechanics assume that the plastic zone or nonlinear zone occurring at the crack tip is very small compared to the crack size. However, short cracks do not satisfy this condition. Hence, the threshold stress intensity factor (△Kth) and the fatigue limit (△σω) of short surface crack size due to fatigue load were evaluated using an equation that considers the stress ratio and the plastic behaviors of peculiar fatigue proposed by Ando et al. In addition, the through crack of the infinite plate was evaluated for comparison.
윤서현(Seo-Hyun Yoon),김민헌(Min-Heon Kim),김병수(Byeong-Su Kim),팽재은(Jae-Eun Paeng),남기우(Ki-Woo Nam) 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11
The purpose of this study is to analyze the cause of the SCM435 high strength bolt of the accelerator shaft forging die damaged during the operating and to propose the measures. The bolts were damaged due to fatigue cracks caused by the stress of the repeated forging process. Bolts need heat treatment accuracy and precision tightening, and the compressive residual stress on the internal thread by peening.