냉간압연률 및 두께에 따른 STS316L의 감쇠능 및 고유주파수

남기우(Ki-Woo Nam),김병수(Byeong-Su Kim),김진욱(Jin-Wook Kim),손대진(Dai-Jin Son),최병철(Byoung-Chul Choi) 한국동력기계공학회 2020 동력시스템공학회지 Vol.24 No.1

시간-주파수 해석법에 의한 5083 알루미늄의 피로균열 진전에 의한 음향방출 신호의 주파수특성

남기우(KI-WOO NAM),이건찬(KUN-CHAN LEE) 한국해양공학회 2003 韓國海洋工學會誌 Vol.17 No.3

Acoustic emission (AE) signals, emanated during local failure of aluminum alloys, have been the subject of numerous investigations. It is well known that the characteristics of AE are strongly influenced by the previous thermal and mechanical treatment of the sample. Possible sources of AE during deformation have been suggested as the avalanche motion of dislocations, fracture of brittle particles, and debonding of these particles from the alloy matrix. The goal of the present study is to determine if AE occurring as the result of fatigue crack propagation could be evaluated by the joint time-frequency analysis method, short time Fourier transform (STFT), and Wigner-Ville distribution (WVD). The time-frequency analysis methods can be used to analyze non-stationary AE more effectively than conventional techniques. STFT is more effective than WVD in analyzing AE signals. Noise and frequency characteristics of crack openings and closures could be separated using STFT. The influence of various fatigue parameters on the frequency characteristics of AE signals was investigated.

SiO₂ 나노 콜로이달 첨가량에 따른 Si₃N₄의 고온강도 특성

남기우(Ki Woo Nam),이건찬(Kun Chan Lee) 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.11

This study analyzed the characterization of high temperature strength of Si₃N₄ composite ceramics additive based on variations in the amount of nano colloidal SiO₂ added. Semi-elliptical cracks about 100 m length were obtained from a Vickers indenter using a load of 24.5 N. The results showed μ that the heat-treated smooth specimens with SiO₂ nano colloidal coating exhibited the highest bending strength at 0.0 wt% SiO₂ nano colloidal added, which is amounted to a 187 % increase over that of smooth specimen. Limiting temperature for bending strength of crack-healed zone for bending strength was about 1273 K. However, the bending strength of SSTS-3 and SSTS-4 was considerably increased while that of SSTS-1 and SSTS-2 was decreased at a temperature of 1,573K.

플랜트 승압펌프 재순환 배관 용접부의 용접비드에 의해 발생한 감육결함 분석 및 건전성 평가

남기우(Ki-Woo Nam),안석환(Seok-Hwan Ahn),도재윤(Jae-Yoon Do) 한국해양공학회 2010 韓國海洋工學會誌 Vol.24 No.4

The wall thinning phenomenon of pipes was simulated as metal loss due to erosion and corrosion. Such wall thinning defects in the pipes of power plants are a very important safety consideration, In this study, we analyzed wall thinning dejects thar occurred by weld bead of weld zone of boost pump recirculation. pipe. From the results of the analysis of pipe failures, numerical analysis was performed by Fluent v6.3.26 using the standard k-ε model of the weld bead shape, such as an elliptical or a spherical shape, on the inner wall of the pipe. Using the results obtained, we showed the overlap effect by cavitations corrosion and erosion-corrosion at the bottom of the wall-thinning defect.

SiO₂ 나노 콜로이드 첨가량에 따른 질화규소의 트라이볼러지

남기우,정영규,황석환,김종순,문창권,Nam, Ki-Woo,Chung, Young-Kyu,Hwang, Seok-Hwan,Kim, Jong-Soon,Moon, Chang-Kwon 대한기계학회 2011 大韓機械學會論文集A Vol.35 No.3

본 연구는 $SiO_2$ 나노 콜로이드 첨가량에 따르는 $Si_3N_4$ 세라믹스의 마멸 특성을 평가하였다. 시험 편은 35 MPa, 2123 K의 질소 가스 분위기 1시간동안 소결하였다. 마멸 시험은 링 블록 시험기를 사용 하였으며, 시험 조건은 직경 35 mm인 링의 회전속도50 rpm, 하중 9.8 N, 실험중의 대기 온도 293 K이 었다. 세라믹스의 마멸계수는 약 1.0, 마멸손실은 약 0.02 mm이었다. 본 연구에 사용된 시험편 중에서 1.3 wt% $SiO_2$ 나노 콜로이드가 첨가된 시험편은 가장 낮은 마멸계수와 마멸 손실을 나타내어, 최상의 마멸 저항성을 나타내었다. 이것은 가장 높은 비커스 경도와 굽힘 강도를 나타내었다. 마멸계수는 경도 와 굽힘 강도에 역비례 관계를 보였다. We analyzed the wear characterization of $Si_3N_4$ ceramics according to the amount of added $SiO_2$ nanocolloid. The test specimen was prepared by hot-press sintering at 35 MPa and 2123 K in an $N_2$ gas atmosphere for 1 h. A wear test was performed with a block-on-ring tester, and the test conditions were as follows: (1) the ring with a diameter of 35 mm had a rotational speed of 50 rpm; (2) the load was 9.8 N; and (3) the temperature was $25^{\circ}C$. The test results show that $Si_3N_4$ ceramics have a friction coefficient of about 1.0 and a wear loss of about 0.02 mm. Of the specimens used this study, the test specimen with 1.3 wt% of added $SiO_2$ nanocolloid has the best wear resistance because it has the lowest friction coefficient and the smallest wear loss. This specimen also has the highest Vickers hardness and bending strength. In this study, the friction coefficient is inversely proportional to the hardness and bending strength.

SiC/SiC 복합재료 세라믹스 표면균열 탐지를 위한 초음파법 적용에 관한 기초연구

남기우(Ki Woo Nam),이건찬(Kun Chan Lee),香山晃(Akira Kohyama) 한국비파괴검사학회 2009 한국비파괴검사학회지 Vol.29 No.5

세라믹스의 비파괴평가 기술은 산업분야에 응용하기 위한 세라믹스 신뢰성 개발에 있어서 필수적인 기술이다. 본 연구는 초음파 C-Scan 방식을 이용하여 SiC 세라믹스의 표면균열을 탐상하기 위한 실험적 연구결과를 제시하고자 한다. 이를 위해 SDS-win과 μ-SDS 두 종류의 초음파 장치와 25, 50 및 125 ㎒의 초음파센서를 이용하여 세라믹스의 표면균열 탐상 가능성에 대해 실험적인 연구를 수행하였다. 본 연구 결과, 세라믹스의 표면미소균열은 결국 25 및 50 ㎒ 센스로 정밀하게 검출할 수 없었으나, 125 ㎒ 센서에 의한 집속법 탐상 결과 희미한 형상 정도를 검출할 수 있었으며, 비집속법의 경우는 비커스 압입자의 형상 검출이 가능함을 알 수 있었다. 따라서 본 연구를 통하여, 초음파 C-Scan 집속 및 비집속 방법은 미세균열의 탐상방식으로 어느 정도 접근 가능함을 실험적으로 확인하였다. Nondestructive evaluation(NDE) of ceramic matrix composites is essential for developing reliable ceramics for industrial applications. In the work, C-Scan image analysis has been used to characterize surface crack of SiC ceramics nondestructively. The possibility of detection of surface crack were carried out experimentally by two types of ultrasonic equipment of SDS-win and μ-SDS, and three types of transducer of 25, 50 and 125 ㎒. A surface micro-crack of ceramics was not detected by transducer of 25 ㎒ and 50 ㎒. Though the focus method was detected dimly the crack by transducer of 125 ㎒, the defocus method could detect the shape of diamond indenter. As a whole, the focus method and the defocus method came to the conclusion that micro crack have a good possibility for detection.

피로균열의 지연거동에 따른 수명예측 및 비파괴평가

남기우(Ki-Woo Nam),김선진(Seon-Jin Kim) 한국해양공학회 1999 韓國海洋工學會誌 Vol.13 No.3(Ⅰ)

Fatigue life and crack retardation behavior after penetration were experimentally examined using surface pre-cracked specimens of aluminium alloy 5083. The Wheeler model retardation parameter was used successfully to predict crack growth behavior after penetration. By using a crack propagation rule, the change in crack shape after penetration can be evaluated quantitatively. Advanced, waveform-based acoustic emission (AE) techniques have been successfully used to evaluate signal characteristics obtained from fatigue crack propagation and penetration behavior in 6061 aluminum plate with surface crack under fatigue stress. Surface defects in the structural members are apt to be origins of fatigue crack growth, which may cause serious failure of the whole structure. The nondestructive analysis on the crack growth and penetration from these defects may, therefore, be one of the most important subjects on the reliability of the leak before break (LBB) design. The goal of the present study is to determine if different sources of the AE could be identified by characteristics of the waveforms produced from the crack growth and penetration. AE signals detected in four stages were found to have different signal per stage. With analysis of waveform and power spectrum in 6061 aluminum alloys with a surface crack, it is found to be capabilities on real-time monitoring for the crack propagation and penetration behavior of various damages and defects in structural members.

레이저 TWB된 이종접합강의 기계적 특성

남기우(Ki Woo Nam),박상현(Sang Hyun Park),이규현(Kyu Hyun Lee),이문용(Mun Yong Lee) 대한기계학회 2012 大韓機械學會論文集A Vol.36 No.8

본 연구에서는 승객의 안전성을 확보하기 위하여 상부 고강도강 SABC1470, 하부 냉간 압연강 SPFH590과 SPFC980으로 경량화를 위하여 레이저 TWB 용접하였다. TWB 이종재료 시험편은 열처리에 의하여 SABC1470 재료에 고강도화 하였다. 수냉까지 대기 시간에 따르는 인장강도와 센터필라와 같은 부정 조건에 의한 고속 굽힘의 변형 거동을 평가하였다. 동일 온도에서 열처리 된 TWB 시험편은 수냉까지 대기 시간에 따라서 인장강도와 굽힘강도가 감소하였다. 850 ℃에서 열처리한 SABC1470 + SPFH590 TWB 시험편은 추돌에 의해 차체 파손시 파괴거동을 유도하여 승객을 보호할 수 있는 적절한 조건이었다. 사고에 의한 승객의 안전을 고려하면, 센터필라는 상부재로서 SABC1470, 하부재료로 SPFH590을 사용하는 것이 안전하다. In this study, tailor-welded blanks(TWB) were formed between high-strength steel(SABC1470) and cold rolled steels(SPFH590 and SPFC980) to improve passenger safety and reduce the weight of cars. Multi-material TWB specimens were highly strengthened through the heat treatment of SABC1470. The change in tensile strength caused by the stand-by time until water cooling after stamping and the deformation behavior of high-speed bending in a statically indeterminate condition such as in the center-pillar were evaluated. Multi-material TWB specimens that were heat-treated at the same temperature tended to show a decrease in tensile and yield strength, depending on the stand-by time until water cooling. On the other hand, Multi material TWB specimens(SABC1470+SPFH590) that were heat treated at 850°C showed good ? properties that were suitable for ensuring passenger safety in car accidents. From the viewpoint of passenger safety, it is best to use SABC1470 and SPFH590 in the upper and lower area of the center-pillar, respectively.

연마판의 거칠기에 따르는 SiC 세라믹스의 굽힘강도 특성

남기우(Ki Woo Nam),김은선(Eun Sun Kim) 대한기계학회 2011 大韓機械學會論文集A Vol.35 No.7

본 논문은 첨가제 SiO₂를 가지는 3종류의 SiC 세라믹스의 연마판 거칠기에 따르는 균열 치유 연구를 실시하였다. 혼합물은 질소가스 분위기에서 2053 K, 35 ㎫, 1시간 동안 소결하였다. SiC 세라믹스 시험편의 최적 균열 치유 조건은 1373 K에서 1시간이다. 균열부의 치유 물질은 SiC와 O₂의 산화반응에 의한 유리상 SiO₂ 이다. 최적 치유 조건에서 미연마 SiC 세라믹스의 굽힘 강도는 완전하게 회복되지 않았다. 그러나 SAY, SAYS-1 및 SAYS-2 시험편에서 경제적인 면을 고려하면 1시간 열처리에서 상온 굽힘 강도가 가장 높게 나타난 SAY 시험편이 우수하다. 125 ㎛ 연마판의 연마 시험편은 결함의 수나 크기가 줄어들었으나, 열처리 후에도 표면 미세 결함이 완전하게 치유되지 못하였다. 40 ㎛ 연마판의 연마 시험편은 1시간 열처리 후 표면 결함이나 기공은 거의 없었으며, 굽힘 강도는 6 ㎛ 연마판 경면 시험편의 강도만큼 회복하였다. This study was carried out on the crack healing of three types of SiC ceramics based on a SiO₂ additive, taking into account the roughness of the polishing plate used for polishing the specimens. The mixtures were subsequently hot-pressed in N₂ gas for one hour under 35 ㎫ at 2053 K. In these specimens, the optimized crack-healing condition was 1373 K for one hour in air. The crack-healing material of the cracked part was the glassy phase of SiO₂ that was formed by the oxidation of SiC. In the optimum healing condition, the bending strength of non-polished SiC ceramics was not completely recovered. However, the bending strength of the SAY specimen was excellent, considering the economic aspects of SAY, SAYS-1, and SAYS-2. The SAY specimen is definitely superior to the others after an hour of heat treatment. There was a decrease in the number and size of defects in the specimen polished by using a 125-㎛ polishing plate; however, the micro-surface defects were not completely repaired. The specimen polished by using a 40-㎛ polishing plate showed little voids or surface defects after an hour of heat treatment. The bending strength of the specimen mirror-polished by using a 6-㎛ polishing plate was completely recovered.

Si₃N₄ 세라믹스의 균열 치유와 굽힘 강도 특성

남기우,박승원,도재윤,안석환,Nam, Ki-Woo,Park, Seung-Won,Do, Jae-Youn,Ahn, Seok-Hwan 대한기계학회 2008 大韓機械學會論文集A Vol.32 No.11

Crack-healing behavior of $Si_3N_4$ composite ceramics has been studied as functions of heat-treatment temperature and amount of additive $SiO_2$ colloidal. Results showed that optimum amount of additive $SiO_2$ colloidal and coating of $SiO_2$ colloidal on crack could significantly increase the bending strength. The heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature depends on the amount of additive $SiO_2$ colloidal. Crack healing strength was far the better cracked specimen with $SiO_2$ colloidal coating on crack surface. After heat treatment at the temperature 1,273 K in air, the crack morphology almost entirely disappeared by scanning prob microscope. At optimum healing temperature 1,273 K, the bending strength with additive $SiO_2$ colloidal 0.0 wt.% without $SiO_2$ colloidal coating recovered to the value of the smooth specimens at room temperature for the investigated crack sizes $100\;{\mu}m$. But that with $SiO_2$ colloidal coating increase up to 140 %. The amount of optimum additive $SiO_2$ colloidal was 1.3 wt.% and crack healed bending strength with $SiO_2$ colloidal coating increase up to 160 % to smooth specimen of additive $SiO_2$ colloidal 0.0 wt.%. Crack closure and rebonding of the crack due to oxidation of cracked surfaces were suggested as a dominant healing mechanism operating in $Si_3N_4$ composite ceramics.