有限板材의 前後面 對稱 表面龜裂에 대한 應力擴大係數 解析

崔鎔湜,梁源鎬,趙顯宇 成均館大學校 科學技術硏究所 1989 論文集 Vol.40 No.2

Surface crcaks are among the more common flaws in mechanical structures. The accurate stress analyses of these surface-cracked components are needed for reliable prediction of their crack growth rates and fracure strengths. In this paper, 3-D finite element method was used to obtain the stress intensity factors(SIF) along the crack front for various crack shape ratios and plate thickness. Mode I SIF was calculated by using a crack opening displacement method proposed by Ingraffea and Manu. Three crack configurations were analyzed : (1) an elliptical crack, (2) semi-elliptical surface crack, and (3) symmetric semi-elliptical surface cracks. Finite element models were composed of quarter-point singular elements around the crack front and quadratic isoparametric elements elsewhere. For the elliptical and semi-elliptical surface cracks, the SIFs were in good agreement with the Irwin's theorectical solution, and the results of 3-D finite element analysis obtained by Newman-Raju, respectively. The results of SIF analyses for symmetric surface cracks in finite thickness plate were obtained as follows: 1. SIF K of symmetric surface cracks is less than that of single surface crack in the same thickness plates; as the ratio a/t increases, the difference in SIF K increases. 2. The interaction effect was increased rapidly at d/a<1.5, however, the effect at d/a>1.5 was negligible. As the elliptic agle 2ψ/π was being increased, the interaction effect was incresed.

단독표면균열재의 피로수명 예측 해석

최용식,양원호,김영진,김재원 成均館大學校 科學技術硏究所 1990 論文集 Vol.41 No.1

Recent studies have shown that the fatigue growth behavior of semi-elliptical surface crack cannot be adequately predicted solely by the stress intensity factor analysis. The variation in constraint along the crack border, from plane stress condition at the plate surface to plane strain condition at the point of maximum crack depth, must be an important factor affecting the fatigue crack growth behavior. In this paper, an analysis on the fatigue life prediction of a semi-elliptical surface cracked body was performed, accounting for the variation in constraint along the crack border. That is, the fatigue life for a surface cracked body was predicted using linear elastic fracture mechanics concepts that were modified to account for crack closure behavior. The model for the fatigue life prediction in consideration of crack closure phenomenon was developed, and then the predicted fatigue lives using the model were compared with test data from PMMA specimens. The predicted results appeared to be conservative.

짧은 균열의 피로성장거동에 대한 연구

최용식,한지원 成均館大學校 科學技術硏究所 1990 論文集 Vol.41 No.2

The short fatigue crack growth behavior of notched specimens was studied with various stress ranges. Crack closure and crack growth experiments were performed on Al alloy 5052-H38. Computer --aided unloading elastic compliance method was employed to measure the closure of short fatigue cracks. The growth rate of short fatigue cracks decelerated with increasing crack length at first, and then accelerated or became non-propagating depending on the applied stress range. Similar deceleration was observed when the growth rate was correlated with the stress intensity factor range △K. But the growth rate of short fatigue cracks could be well represented in terms of the effective stress intensity factor range △Keff. The relation between the crack growth rate and the effective stress intensity factor range for short cracks was identical to that obtained for long cracks.

Al―합금재의 균열닫힘현상에 관한 파괴역학적 연구

최용식,한지원,김규성 成均館大學校 科學技術硏究所 1991 論文集 Vol.42 No.1

In this study, a crack closure behavior of Al-Alloy 5052-H38 was investigated. The fatigue test was performed with 25-ton capacity Instron Dynamic Test System, Model 1332, by the four-point bending test under the constant amplitude load. Stress ratio R was changed from -1.0 to 0.5 and load cycle was 20 Hz at the room temperature. To measure the fatigue crack opening point, the graph of load vs. subrtacted displacement by computer program was obtained from the X-Y plotter. In order to gain the displacement data, a strain gage was attached at the back surface against the notched side. Using the concepts of crack closure and influence of R on crack growth rate, a model for crack growth rate is developed for Al-Alloy 5052-H38. da / dN = C(ΔK_eff)^m = C(1.1 / 1.53 - R ΔK)^m : -1 ≤ R ≤ 0.5 da / dN = C(ΔK_eff)^m = C(ΔK)^m : R ≥ 0.5

프랙탈 코딩 파라미터를 이용한 디지털 영상 워터마킹

고성식,조강석,정용덕,김정화 조선대학교 전자정보통신연구소 2003 電子情報通信硏究所論文誌 Vol.6 No.1

In this paper we propose new watermarking method that a secret key is created from image contents without inserting an watermark in the image contents and the watermark is transformed to the fractal parameters Image logo watermark uses the owner photograph of the image contents and after image watermark is transformed t o the fractal parameter between domain and range of the watermark logo by using fractal theory, the fractal parameter is scrambled by the created secret key Thus, our algorithm proposed in this paper has no image distortion by an watermarking because of decoding the watermark by the created secret key Experimental results have shown that the proposed method achieves improved robustness with respect to image lossy compression, cropping attacks and various filtering attacks.

중국경제성장이 중국 소수민족의 민족주의에 미치는 영향

신주식,최용호 慶北大學校 經濟經營硏究所 2003 經商論集 Vol.31 No.1

Al-합금 용접재의 피로균열 성장거동 추정과 재분포 잔류응력의 영향

최용식,정영석 成均館大學校 科學技術硏究所 1989 論文集 Vol.40 No.2

본 연구는 초기용접잔류응력과 모재의 C, m값을 기초 Data로 하여, Al-합금 용접재의 피로균열 성장거동의 추정을 위한 이론적인 Model을 제시하였고, 또한 피로균열 성장거동에 미치는 재분포 잔류응력의 영향에 관하여 고찰하였다. 이론적인 Model에서 잔류응력 확대계수(K_res)는 Weight Function으로 구하였으며, 중첩법과 유효응력비(Effective Stress Ratio ; Reff)를 결정하였다. 그 결과 본 추정법으로 구한 Al-합금 용접재의 피로균열 성장거동은 실험결과와 잘 일치하였다. 피로균열 성장거동에 미치는 재분포 잔류응력의 영향은, 작용하중(Applied Load)의 크기에 의존하며, 즉, 작용하중이 작을수록 균열성장거동에 미치는 잔류응력의 영향이 지배적으로 나타났다. The objectives of this study are to develop a theoretical model for predicting fatigue crack growth behavior based on initial welding residual stress distribution and fatigue properties of parent metal specimen(ie, C,m values of Paris power law), and to investigate the effect of redistributed residual stress on fatigue crack growth behavior_ In this model, calculation of the residual stress intensity factors were effectively achieved by using a weight function, and effective stress intensity factor range(ΔK_eff) near the crack tip and effective stress ratio(R_eff) were determined by superposing the less intensity factors for the residual stresses and for the applied stresses. Predicted results shored goad agreement mill those by experiments. The effect of redistributed residual stresses on fatigue crack growth rate depends on the magnitude of applied load. The lower the applied load, the higher the effect of residual stress. Higher applied loads cause greater residual stress relaxation.

Plane Stress 상태와 Plane Strain 상태에서의 피로균열 성장 거동에 관한 연구

최용식,이주섭 成均館大學校 科學技術硏究所 1987 論文集 Vol.38 No.1

This paper is concerned with the fatigue crack growth rates for both the plane stress and the plane strain conditions. In general, the specimen surface may be assumed to satisfy the plane stress condition and inner part of specimen the plane strain condition. As the specimen material, PMMA was selected. Crack length was measured both at the inner part of specimen and on the specimen surface. Since PMMA is transparent, the inner part crack length could be measured exactly. Center cracked tension (CCT) specimens which were recommened by ASTM Standard E647-85 were used in the testing proceses. In order to analyze da/dN-ΔG relation, G-K relation was applied to both test conditions, the plane stress and the plane strain. From the experimental studies, following conclusions were obtained. 1) Concerning with the relation of da/dN-ΔK the value of m for plane strain condition is appeared greater than that of plane stress condition. 2) As the thickness increases, the difference between the value of m in the plane strain condition and in the plane stress condition becomes rather greater. 3) The power of the exponential relation between da/dN and ΔG; da/dN=C(ΔG)^m appears similar to that of the relation; da/dN=C'(ΔK)^m' ; m??m'. 4) The value of m is constant for plane stress condition. The value of m for PMMA is approximately 3.5.

혈청 neuron specific enolase를 이용한 심정지후 신경학적 예후의 예측

강용선,정성필,박기일,김승환,김태승,이한식 大韓應急醫學會 1999 대한응급의학회지 Vol.10 No.2

Background: The purpose of this study was to determine that the assessment of serum neuron specific enolase(NSE) could provide a reliable early predictor of neurologic outcome after cardiac arrest. Methods: Prospective, observational study was performed from April 1996 to March 1998 at a university teaching hospital ED. Serum NSE concentrations were analysed twice at 24 and 48 hours after return of spontaneous circulation(ROSC). Neurologic outcome was categorized using cerebral performance category(CPC). Results: Twenty-nine patients(16 were men) were enrolled during the study period. The mean age was 50.8 years. Nine(31%) of them showed good outcome defined as CPC 1-3, and 20(69%) patients showed bad outcome defined as CPC 4-5. In the good outcome group, the serum NSE was revealed 33.8±9.3 ng/ml at 24 hours, 34.0±4.73 ng/ml at 48 hours. While in the bad outcome group, it was 99.5±11.7 ng/ml and 114.6±15.8 ng/ml. The NSE at 48hr after ROSC was more prescise than that of 24hr. When the cutoff value of 50 ng/ml at 48hr, the sensitivity was 82%, and specificity was 93%. Conclusion: This study suggest that the serum NSE may represent a valuable, noninvasive, and useful clinical tool for prediction of neurologic outcome after cardiac arrest.

노치材의 疲勞壽命評價에 關한 硏究

崔鎔湜,石昌星,朴鐘柱,林昌鉉 成均館大學校 科學技術硏究所 1995 論文集 Vol.46 No.2

This paper mainly deals with fatigue life estimation and prediction in notched structures. The fatigue crack initiation life and the fatigue crack growth behavior in the DEN specimens were predicted using S.I.F. K solution derived in this study and the Paris' crack growth equation. Predicted results showed good agreement with experimental crack growth behaviors under constant-load-amplitude.