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[論文] 표면균열의 피로성장거동 연구-인장 반복 하중하에서의 균열형상비 예측-
최용식(Y.S.Choy),양원호(W.H.Yang),김영진(Y.J.Kim),김재원(J.W.Kim) 한국자동차공학회 1990 오토저널 Vol.12 No.2
The fatigue growth behavior of surface cracks cannot be adequately predicted solely by stress intensity factor analysis. This is caused by different plastic deformation due to variations in the stress field triaxiality along the crack tip. Therefore, a new model which accounts for the crack closure phenomenon is proposed in this paper to predict the fatigue crack growth patterns for surface cracks. Fatigue tests were performed to develop the new model for the prediction and to assess the accuracy of the analysis. The predicted crack growth behavior for PMMA and Aluminum alloy 7075-T6 materials agreed well with the experimental data.
최용식(Y.S.Choy),김재원(J.W.Kim) 한국자동차공학회 1992 오토저널 Vol.14 No.2
An analytical scheme for predicting the crack aspect pattern of materials which contain twin surface cracks was developed. Fatigue tests were performed on twin surface cracked PMMA plate specimens to obtain the interaction factor accounting for the interference effect of adjacent cracks. Here, the interaction factor is defined as the ratio of the stress intensity factor for twin surface cracks to that for a single surface crack. From the analysis of the fatigue test results, the interaction factor was presented as the ninth-order polynomial expression having a function of dimensionless crack spacing ratio. Then the polynomial expression was incorporated into the prediction program of the crack aspect pattern for twin surface cracked materials. And, the interaction effect and the coalescence condition of adjacent cracks were simplified in the newly developed prediction scheme of the crack aspect pattern. The predicted crack growth pattern using the prediction scheme was compared with test data from PMMA specimen. The predicted pattern agreed well with the test data.
이진호,최용식,김영진,Lee, J.H.,Choy, Y.S.,Kim, Y.J. 대한기계학회 1993 대한기계학회논문집 Vol.17 No.7
The objective of this paper is to develop a computational model for predicting the fatigue propagation of collinear multiple surface cracks under constant amplitude and variable amplitude loadings. After examining fatigue crack growth behavior for CT specimens and single surface crack specimens, empirical equations of(11) and(12) are proposed for the prediction of fatigue life in a multiple surface crack geometry. The accuracy of the proposed model is verified using a life prediction computer program. Several case studies were performed to check the accuracy of the proposed model and to verify the usefulness of the developed program. Good agreement is observed between the numerical results based on the proposed model and the published experimental data.
[論文] 두께가 변화하는 부재 내의 혼합모드 I/Ⅲ 균열의 응력확대 계수해석-3차원 유한요소해석 중심으로-
양원호(W.H.Yang),최용식(Y.S.Choy),조명래(M.R.Cho) 한국자동차공학회 1993 오토저널 Vol.15 No.2
Variable thickness plates are commonly encountered in the majority of mechanical/structural components of industrial applications. And, as a result of the unsymmetry of the structure or the load and the anisoptropy of the materials, the cracks in engineering structures are generally subjected to combined stresses. In spite of considerable practical interest, however, a few fracture mechanics study on combined mode crack in a variable thickness plate have carried out. In this respect, combined mode<br/> I/Ⅲ stress intensity factors K₁ and K/_Ⅲ at the crack tip for a variable thickness plate were obtained by 3-dimensional finite element analysis. Variable thickness plates containing a central slant crack were chosen. the parameters used in this study were dimensionless crack length λ, crack slant angle a, thickness ratio βand width ratio ω. Stress intensity factors were calculated by crack opening displacement(COD) and crack tearing displacement(CTD) method proposed by Ingraffea and Manu. The effect of thickness ratio β on K₁ is relatively great in comparison to KⅢ.
[論文] 직교이방성 재료에 대한 경계요소법 (BEM)의 기본해에 관한 연구
이갑래(K.R.Lee),조상봉(S.B.Cho),최용식(Y.S.Choy) 한국자동차공학회 1990 오토저널 Vol.12 No.2
According to the developments of various composite materials, it seems to be very important to evaluate the strength and fracture behavior of composite materials. When the composite material is considered as orthotropic material, the characteristic equation of orthotropic mater.<br/> ial have complex roots. If characteristic roots are equal, the fundamental solutions of BEM become singular ones. This paper analyse the fundamental solutions of the singular problem of orthotropic material using the analogous method to isotropic material.
최용식,석창성,조용근,박종주,임창현 한국산업안전학회 1994 한국안전학회지 Vol.9 No.1
This paper has described the analysis of the Stress Intensity Factor behaviour of a short crack initiated from notch tip. The model for finite element analysis is a double edge notched specimen. The parameters used in this study are crack length and notch root curvature radius.
최용식,한지원,김규성 한국산업안전학회 1991 한국안전학회지 Vol.6 No.3
In this study, a crack closure behavior of Al-alloy 5052-H38 was investigated. The fatigue test was performed by the four-point bending test under the contast amplitude load. Stress ratio R was changed from-1.0 to 0.5. To measure the fatigue crack opening point, the graph of load vs. subtracted displacement by computer program was obtained from the K-Y polotter. In order to gain the displacement data, a strain gage was attached at the back surface against the notched side. Using the conception of crack closure and influence of on crack growth rate a model for crack growth rate is developed for Al-alloy 5052-H38.
최용식,한지원,우흥식 한국산업안전학회 1993 한국안전학회지 Vol.8 No.4
The growth of short cracks can be well described in terms of the effective stress intensity factor range, which is calculated on the base of crack closure. The relation between the crack opening SIF and crack length is determined from the experimental results. The crack opening SIF of short cracks, K_(op) can be predicted from the crack opening SIF at threshold of long crack, K_(op.L). The growth rate of short cracks at notch root can be predicted from the crack opening SIF of short cracks, K_(op), and the growth equation of long cracks in region Ⅱ.
SA - 508 압력용기용강 변후재의 피로균열 Arrest 거동에 대한 변후 형상비의 영향
최용식,이갑래,이환우 한국산업안전학회 1991 한국안전학회지 Vol.6 No.4
The purpose of a fatigue crack arrest desing is to prvent a fatigue fracture of machine and structure resulted from unstable crack growth. In all cases of load transfer to second elements such as stringers, doublers or flangers, crack arrest is possible; arrest occuring when the fatigue crack reaches the second element. In the present work, the possibility of crack arrest and the design criterion of fatigue crack arrest in the variable thickness plates are examined numericaiiy by using fatigue crack arrest thresthod △K_(th)of SA-508 reactor vessel steel and stress intensity factor which was obtained in the previous work as a result of 3-dimensional finite element analysis for CT type variable thickness plates having discontinuous interface.