시효열화시킨 1Cr-1Mo-0.25V 강의 고온화의 회전굽힘 피로강도에 관한 연구

서창민,허정훈,남승훈,Suh, Chang-Min,Huh, Jeong-Hoon,Namh, Seung-Hoon 대한기계학회 1996 大韓機械學會論文集A Vol.20 No.9

The estimation of the remaining life for the aged components in power plant as well as chemical and petroleum plants has been recently concerned. The raw materials used in this study are the 1Cr-1Mo-0.25V steel which intensified P and S compositions along with the nominal compositions of ASTM A 470 standard. Five kinds of specimens with the different degradation levels were prepared by isothermal aging heat treatment at 630.deg.C. The mechanical properties and rotated bending fatigue strength of virgin and aged 1Cr-1Mo-0.25V steel have been investigated through the hardness, tensile, fatigue test, SEM fractograph and EDS analysis at 538.deg.C and room temperature, respectively. Thus the data of aged specimens were compared with those of virgin specimen to evaluate the aging effects. The main results obtained in this study are as follows ; The decrease of the yield and tensile strength due to degradation was distinguished until 50, 000hrs simulated service time. And it was confirmed that the considerable amount of P, Mn, Cr and S was precipitated at the grain boundary of aged material through the SEM and EDS analysis. The rotated bendingd fatigue strength at 538.deg.C of virgin, 25, 000, 50, 000, 75, 000 and 100, 000 hrs aged material was decreased 44.6 %, 49.6 %, 51.5 %, 52.4% and 53.8% than that of virgin material at 10$_{7}$cycles of room temperature, respectively. The major cracks of virgin and aged materials mainly initiated at the inclusions including Si, P and Mn compositions which were located at the outer periphery of the specimen.n.

${Al_2}}O_3}-TiO_2$ 플라즈마 코팅된 유리의 입자충격에 의한 손상기구

서창민,이문환,홍대영,Suh, Chang-Min,Lee, Moon-Hwan,Hong, Dea-Yeong 대한기계학회 1998 大韓機械學會論文集A Vol.22 No.3

A quantitative study of impact damage of ${Al_2}}O_3}-TiO_2$ plasma coated soda-lime glasses was carried out and compared with that of the uncoated smooth glass specimen. The shape of cracks by the impact of steel ball was observed by stereo-microscope and the decrease of the bending strength due to the impact of steel ball was measured through the 4-point bending test. At the low velocity, cone cracks were occurred. As the impact velocity increases, initial lateral cracks were propagated on the slanting surface of a cone crack, and radial cracks were generated at the crushed site. When the impact velocity of steel ball exceeds the critical velocity, the contact site of specimen was crushed due to plastic deformation and then radial and lateral cracks were largely grown. Crack length of coated specimens was smaller than that of uncoated smooth specimen due to the effect of coating layer on the substrate surface. According to impact velocity, the bending strength of coated specimens had no significant difference, compared with that of the uncoated smooth specimen. But this represents that the bending strength of coated specimens was increased, considering the effect of sand blasting damage which was performed to increase the adhesion force of coating layer.

표면거칠기를 가진 유리의 입자충격 손상기구에 관한 실험적 연구

서창민,정성묵,이문환,Suh, Chang-Min,Chung, Seong-Muk,Lee, Mun-Hwan 대한기계학회 1996 大韓機械學會論文集A Vol.20 No.7

The damage mechanism by the impact of steel ball on the soda-lime glass having a different surface roughness was investigated. An initiation and a propagation behavior of cracks formed by each impact velocity were quantitatively studied. A 4-point bending test was carried out to evaluate the remaining bending strength of a scratched soda-lime glass which impacted by the steel ball. As the surface roughness was increased, the shape of cracks became more irregular rather than those of the smooth specimens. The phenomenon of turning up in the wing of cone cracks occurred even at the lower velocity than the critical velocity caused the crushing. The threshold velocity of cracks initiation generally became lower than those of smooth specimen. An initiation and a propagation behavior of radial cracks had no relation with the direction of scratch on the surface. The remaning benidng strength of the scratched specimen according to impact velocity had no big difference compared with those of the smooth specimen.

온도변화에 대한 고분자 코팅 층에 발생하는 응력 해석

박명규(MYUNG-KYU PARK),이상순(SANG-SOON LEE),서창민(CHANG-MIN SUH) 한국해양공학회 2003 韓國海洋工學會誌 Vol.17 No.6

This paper deals with the stress singularity developed in a polymer layer that is coated to a concrete surface, due to temperature change. The boundary element method is employed to investigate the behavior of interface stresses. The polymeric layer is assumed to be a linear viscoelastic material, and is thermorheologically simple. The order of the singularity is obtained, numerically, for a given viscoelastic model. Numerical results exhibit the relaxation of interface stresses, and large gradients are observed in the vicinity of the free surface. Results show that the stress singularity factor is relaxed with time, while the order of the singularity increases with time for the viscoelastic model.

시효 열화시킨 터빈 로터강의 피로강도에 관한 연구

서창민(Chang-Min Suh),허정훈(Jeong-Hoon Huh),이해무(Hae-Moo Lee),서덕영(Duck-Young Suh) 한국해양공학회 1996 韓國海洋工學會誌 Vol.10 No.2

The estimation of the remaining life for the aged components in power plant as well as chemical and petroleum plants has been recently concerned. The raw materials used in this study are the 1Cr-1Mo-0.25V rotor steel which intensified P and S compositions along with the nominal compositions of ASTM A470 standard. Five kinds of specimens with the different degradation levels were prepared by isothermal aging heat treatment at 630℃. The mechanical properties and fatigue strength of virgin and aged 1Cr-1Mo-0.25V rotor steel have been investigated through the hardness, tensile, fatigue test, SEM fractograph and EDS analysis at room temperature. Thus, the data of aged specimens were compared with those of virgin specimen to evaluate the aging effects.<br/> The main results obtained in this study are as follows; The decrease of the hardness due to degradation was distinguished until 50,000hrs simulated service time. And it was confirmed that the considerable amount of P, Mn, Cr and S was precipitated at the grain boundary of aged material through the SEM and EDS analysis. The fatigue strength of 25,000, 50,000, 75,000 and 100,000hrs aged material was decreased 29.5%, 24.4%, 28.6%, 35.7% than that of virgin material at 10<SUP>7</SUP> cycles of room temperature.<br/> The major cracks of virgin and aged materials initiated at the inclusions including Si, P and Mn compositions which were located at the outer periphery of the specimen.

Arc Ion Plating 方式에 依한 TiCN 蒸着시 反應가스가 코팅층에 미치는 影響

서창민(Chang-Min Suh),김창근(Chang-Geun Kim),유임준(Im-Jun Yoo) 한국해양공학회 1997 韓國海洋工學會誌 Vol.11 No.3

This work was intended to study the effect of a partial pressure ratio and a total pressure of reactive gases on the properties of TiC<SUB>x</SUB>N<SUB>1-x</SUB> coated layer. In this regard, various TiC<SUB>x</SUB>N<SUB>1-x</SUB> coatings were synthesized with C₂H₂ and N₂ mixture gas of different compositions by Arc Ion Plating process which has been highlighted for an industrial purpose. It was revealed from colors and X-ray diffraction patterns that the concentration of carbon of a TiC<SUB>x</SUB>N<SUB>1-x</SUB> coating increases with a partial pressure ratio (PC₂H₂/PN₂) as well as a total pressure of C₂H₂ and N₂ mixture gas. Accordingly, the hardness of TiC<SUB>x</SUB>N<SUB>1-x</SUB> coated layer increased but the adhesion to the substrate of SKH 51 was degraded. On the other hand, the deposition rate was independent of a partial pressure ratio and a total pressure of mixture gas. It was found that a uniform gas distribution is critical for an industrial application since the composition of a coating depends strongly on the location of a substrate inside of the furnace. As a result of milling tests with different TiC<SUB>x</SUB>N<SUB>1-x</SUB> coated end mills, the one which has a low carbon concentration was better than others studied in this work.

2¼ Cr-1 Mo강의 평활재상의 미소한 표면피로균열의 성장특성

徐昌敏(Chang-Min Suh),禹炳哲(Byung-Chul Woo),張喜洛(Hee-Rak Jang) 한국해양공학회 1990 韓國海洋工學會誌 Vol.4 No.2

In this paper, fatigue tests were carried out at stress test levels of 461 MPa, 441 MPa, and 431 MPa by using smooth specimen of 2¼ Cr-1 Mo steel with the stress ratio(R) of 0.05. The initiation, growth and coalescense process of the major cracks and sub-cracks among the fatigue cracks on the smooth specimen are investigated and measured under each stress level at a constant cycle ratio by the replica technique with optical microscope. Some of the important results are as follows :<br/> In spite of the difference of stress levels, the major crack data gather into a small band in the curve of surface crack length and crack depth against cycle ratio N/Nf. The sub-crack data, however, deviate from the band of the major crack.<br/> The growth rates, da/dN, of major and sub-crack plotted against the stress intensity factor range, ΔK, have the tendency to be compressed on a relatively small band. But it is more effective to predict fatigue life through major cracks.<br/> The propagation behavior of surface microcracks on the smooth specimens coincides with that of the specimen having an artificial small surface defect or through crack.

작은 표면균열의 성장특성에 의한 수명예측

서창민(Chang-Min Suh),임창순(Chang-Soon Lim),강용구(Young-Goo Kang) 한국해양공학회 1989 韓國海洋工學會誌 Vol.3 No.2

This paper deals with a fatigue life prediction of a surface crack based on the experimentally obtained relationship between surface crack length ratio a/af and cycle ratio N/Nf using micro computer.<br/> Firstly a/af-N/Nf curves obtained from experimental tests, were assumed as three curves UC (the upper limit curve), LC (the lower limit curve) and MC (the middle curve), and these were utilized to predict the fatigue life.<br/> Comparing the calculated values which represent the characteristics of crack growth behaviors from the three assumed curves with the experimental ones, it has been found that in the stable crack growth region, they coincide reasonably well each other. And the differences between the fatigue lives obtained from the assumed curves and the experimental fatigue life did not exceed 20%. Using the characteristics of a/af-N/Nf curves, it is possible to predict the da/dN-Kmax curves and the S-Nf curves.

2 1/4 Cr-1 Mo鋼의 작은 表面균열의 成長에 관한 基礎的 硏究

徐昌敏(Chang Min Suh),姜用求(Yong Goo Kang) 한국해양공학회 1987 韓國海洋工學會誌 Vol.1 No.1

Fatigue tests by axial loading (R=0.05) were carried out to investigate fatigue crack growth characteristics of small surface cracks in 2 1/4 Cr-1 Mo steel at room temperature by using flat specimens with a small artificial pit.<br/> All the data of the fatigue crack growth rate obtained in the present test are determined as a function of the stress intensity factor range about a semi-elliptical crack, so that the application of linear fracture mechanics to the surface fatigue crack growth and to the fatigue crack growth into depth, and all the data obtained from tests were discussed in comparison with the data of Type 304 stainless steel and two type of mild steel under the same test conditions.<br/> The obtained results are as follows :<br/> 1) When the cycle ratios are same, surface fatigue crack length and its depth are almost same and fall within a narrow scatter band in spite of different stress levels.<br/> 2) Relations of the surface fatigue crack growth rate (da/dN) and fatigue crack growth rate into depth (db/dN) to its stress intensity factor range (ΔKIa, ΔKIb) can be plotted as a straight line at log-log diagram without dependence of stress level and coincide with the data of part-through crack in various steels.

2024-T3 및 황동의 작은 표면결함재의 피로균열 성장특성에 관한 연구

서창민(Chang-Min Suh),오명석(Myung-Suck Oh) 한국해양공학회 1996 韓國海洋工學會誌 Vol.10 No.1

In this paper, rotating bending fatigue tests have been carried out to investigate the growth behaviors of surface fatigue crack initiated from a small artificial surface defect, that might exist in real structures, on 2024-T3 and 6:4 brass. The test results are analysed in the viewpoints of both strength of materials and fracture mechanics, it can be concluded as follows.<br/> The effect of a small artificial surface defect upon the fatigue strength is very large. The sensitivity of 2024-T3 on the defect is higher than that of 6:4 brass.<br/> The growth behavior of the surface fatigue crack of 2024-T3 is different from that of 6:4 brass. The growth rate of the surface fatigue crack of 2024-T3 is considerably rapid in the early stage of the fatigue life and apt to decrease in the later stage.<br/> It was impossible to establish a unifying approach in the analysis of crack growth behavior of 2024-T3 and 6:4 brass using the maximum stress intensity factor because of their dependence on stress level. But if the elastic strain and cyclic total strain intensity factor range were applied to obtain the growth rate of surface fatigue cracks of the materials, the data were found to be nearly coincided.