彈塑性破壞靭性値 J_IC 決定에 관한 硏究(Ⅳ)

崔鎔湜,梁源鎬,金永鎭,石昌星 成均館大學校 科學技術硏究所 1990 論文集 Vol.41 No.1

The objectives of this study are to determine whether the J_lC approach could be used to evaluate the ductile to brittle temperature performance of A516 Gr70 steel and to evaluate the J_lC value as a function of temperature. Fracture toughness J_lC and K_iC tests were performed on A516 Gr70 carbon steel plate and test results were analyzed according to ASTM E 813 and ASTM E 399 at temperatures ranging from -160℃ to room temperature (20℃). It may be recognized in this study that the temperature ranging from -15 to 20℃ is the upper shelf region of ductile-brittle transition temperature. In this temperature range, the elastic plastic fracture toughness J_lC values decreased with increasing temperature, however, it was insignificant. The transition temperature may be -30℃. In the transition region near -30℃, the tendency of J_lC decrease with decreasing temperature was significant. At temperatures ranging from -160℃ to -50℃, the relation between the J_lC value and the test temperature was obtained.

ΔK 減少制御疲勞試驗에 의한 ΔK_TH 決定에 관한 硏究

崔鎔湜 成均館大學校 科學技術硏究所 1984 論文集 Vol.35 No.1

In order to find out the value of fatigue crack threshold ΔK_TH, computer-on-line system of ΔK-decreasing control fatigue tests was introduced. This test system is based on LEFM principle for subcritical fatigue crack growth rate analyses. Experimental works on wide high tension steel CCT specimens were fully analyzed. Followings are conclusions: (1) Fatigue crack growth rate is effectively controled by the ΔK-control fatigue test system. (2) ΔK_TH values can be found out by the ΔK-decreasing control fatigue tests. (3) Some factors like ΔK_i and d(ΔK)/dN are seemed to affect ΔK_TH determination.

對稱 二重 表面缺陷의 疲勞龜裂成長擧動에 對한 硏究

崔鎔湜,金在元 成均館大學校 科學技術硏究所 1988 論文集 Vol.39 No.1

This paper is concerned with the fatigue crack growth behavior for twin surface cracks. PMMA was selected for the specimen material because PMMA is transparent and the crack length of thickness direction can be measured exactly. Tension specimens having twin circular arc-shaped notch were used in the testing process. The results obtained from a series of experiments are summarized as follows: 1. The fatigue crack propagation of shallow-type surfacs cracks such as the case of crack aspect ratrio c/a = 0.151 initiated at the deepest point of the crack. Therefore, stress intensity factor at this point is estimated to be maximum value. 2. The fatigue crack growth race of the width direction was larger than that of the thickness direction. 3. The interaction behavior between two surface cracks was obseved in the fatigue crack growth rates toward the thickness direction as well as in those in the width direction, particularly, in the latter was greater than in the former. The more surface crack-tips approache closely, the greater the fatigue crack growth rate increases rapidly by the crack-tip interaction effect. 4. During the stable fatigue cracks are propagating, two surface cracks keep the feature approximately semi-elliptical cracks having major axis to minor axis length ratio of 3/2.

SM55C鋼의 降伏强度가 破壞特性에 미치는 影響에 관한 實驗的 硏究

崔鎔湜,李洪吉 成均館大學校 科學技術硏究所 1986 論文集 Vol.37 No.1

In this paper, an experimental program was carried out to investigate the effect of yield strength on the fatigue crack growth behavior and fracture toughness of the heat treated SM55C steel. The fatigue tests were conducted by the standard fatigue test method described in ASTM Standard E647-81. Also, the K_IC tests Were conducted by the standard plane strain fracture toughness test method described in ASTM Standard E399-81. The following conclusions were derived from the experimental study. 1. The fracture properties (K_IC, R_sc, ΔK_th, m, C, ΔKc) of SM55C steel are influenced by the yield strength of the steel. 2. If the yield strength (σγ) increases, the K_Q increases and the R_sc decreases. 3. If the yield ratio (σγ/σ_B) increases, the K_Q and the R_sc decrease. 4. The fatigue crack growth rate decreases temporarily with the increasing of yield strength. 5. If the yield strength(σγ) increases, the ΔK_th and the ΔKc increase.

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.

複合材料의 破壞力學(Ⅱ) : 破壞力學的 觀點에서 보는 異材結合境界에서의 疲勞龜製의 問題 Fracture Mechanics Approach To the Fatigue Behavior of Interface Cracks In the Bonded Dissimilar Materials

崔鎔湜 成均館大學校 1981 論文集 Vol.29 No.-

Fatigue crack problems at and around the interface of bonded dissimilar matesials are investigated and considered in the view point of Fracture Mechanics in the present paper. Some fundamental aspects of so called interface cracks are considered and critical problems are appointed comparing with previously analyzed SIF. Followings are Summarized : (1) SIF analyses on interface cracks are not sufficiently applicable to the actual fatigue testing yet. (2) Fatigue crack behavior around the bonded interface could be approximately analyzed by the Fracture Mechanics Technigues with respect to the behaviors in the homogeneous plate materials at the present stage.

컴플라이언스法을 利用한 疲勞龜裂發生의 監視 System에 關한 硏究

崔鎔湜,石昌星,林昌鉉 成均館大學校 科學技術硏究所 1993 論文集 Vol.44 No.1

To predict the service life of many steel structures, an understanding of fatigue crack initiation in steel is required. In this paper, the monitoring system of a fatigue crack initiation by compliance technique was described and the fatigue crack initiation life by the system was compared by an ultrasonic technique. The fatigue crack initiation life of a notched specimen in SPS 5A steel was also investigated by the system.

CT型 多段 變厚部在內의 Mode I 龜裂에 대한 龜裂成長擧動 硏究

崔鎔湜,梁源鎬,金永鎭,朴鍾桂 成均館大學校 科學技術硏究所 1992 論文集 Vol.43 No.1

A plate with multi-step varying thickness is widely used in a variety of machine components. In order to provide design data required for damage tolerant design for such components, fracture mechanics analysis was performed. Compact tension specimens with thin-to-thick geometry were used for analysis models. Three-dimensional finite element analysis was performed, and stress intensity factor was obtained by crack opening displacement extrapolation method. Analysis parameters were λ (dimensionless crack length), β(thickness ratio) and ω(width ratio). Dimensionless correction factor F values decreased rapidly as λ approached 1(thickness interface). And effect of front step was more pronounced than that of rear step.

파괴인성치와 변형율 파괴기준과의 관계

崔鎔湜,金福起,石昌星 成均館大學校 科學技術硏究所 1992 論文集 Vol.43 No.1

Under large scale yielding conditions crack propagation is found to be governed by parameters based on the J-integral or on the crack opening displacement. But, initiation of crack propagation of ductile material seems to be controlled by just one parameter that is the strain, This paper describes a relation between the critical value of J-integral (J_IC) and the failure strain(ε_C) in uniaxial tensile test in the region of maximum reduction in area.