873~1273 K에서 열화된 강화흑연강의 기계적 특성 연구

남기우(Ki Woo Nam),이수철(Soo Cheol Lee) 한국비파괴검사학회 2013 한국비파괴검사학회지 Vol.33 No.2

벌레 모양 흑연철로 알려진 강화흑연강(CGI)은 주철보다 더 강하고 더 가벼운 것이 요구되는 응용제품에서 인기를 얻고 있는 금속이다. 최근 강화흑연강 디젤엔진 블록에 사용되고 있다. 본 연구에서는 디젤엔진의 배기 매니폴드에 사용되는 것을 고려하여, 873~1273 K에서 1~96 시간 열처리를 실시하여 CGI340의 기계적 특성을 평가하였다. 열처리를 실시한 시험편의 인장강도는 모재에 비하여 전체적으로 낮은 인장강도를 나타내었다. 열처리 시간이 증가할수록 인장강도가 감소하였으며, 열처리 온도가 높고 시간이 길어질수록 더 많이 감소하였다. 초음파 피로시험에 의한 피로한도는 모재는 약 130 ㎫, 1173 K(96 hrs) 시험편은 약 100 ㎫을 나타내었다. 경도는 열처리 시간이 증가할수록 감소하였으며, 열처리 온도가 높을수록 경도의 분포가 낮아졌다. 그리고 CGI340에 분포하는 구상흑연의 평균경도는 120 Hv, 벌레 모양 흑연의 평균경도는 114 Hv로 기지조직인 페라이트보다 낮은 경도를 나타내었다. 열처리 온도 및 시간에 따라서 구상흑연 및 벌레 모양 흑연에 대한 변화는 일정하지 않지만, 열처리 시간이 길어짐에 따라 기지조직의 결정립이 커지는 것을 확인하였다. Compacted graphite iron(CGI), also known as vermicular graphite iron, is a metal which is gaining popularity in applications that require either greater strength, or lower weight than cast iron. Recently compacted graphite iron has been used for diesel engine blocks. Considering that using in exhaust manifold of the diesel engine, CGI340 was conducted the heat treatment during 1 hour to 96 hours from 873 to 1273 K. Mechanical characteristics were evaluated. The obtained results are as follows; The tensile strength of the heat treated specimens showed overall lower tensile strength than that of the base metal. Tensile strength decreases with increasing of heat treatment time, and the higher heat treatment temperature and the longer time, were more reduced. The fatigue limit by the ultrasonic fatigue test was approximately 130 ㎫ of base metal, 100 ㎫ of 1173 K (96 hrs) specimen, respectively. The hardness decreases with increasing heat treatment time, and the higher the heat treatment temperature was lowered hardness distribution. In CGI340, average hardness of nodular graphite was 120 Hv, average hardness of vermicular graphite was 114 Hv. This showed lower hardness than the base structure ferrite. The nodular graphite and vermicular graphite according to the heat treatment temperature and time didn’t have a consistent change. However, the grain size of base structure grew with increasing of heat treatment time.

리벳구멍을 갖는 2024 알루미늄 합금의 피로 균열 진전 동안에 발생한 음향방출 파형 특성

남기우(KI-WOO NAM),안석환(SEOK-HWAN AHN) 한국해양공학회 2001 韓國海洋工學會誌 Vol.15 No.1

This study were looks at the effect of the initial cut length or stress concentration level, on the wave forms produced by crack propagation. The signals were collected, then classified visually for each type of sample. They were put into three classes according to their shapes in the time and frequency domain. Each class should contain signals which could be correlated to a certain micro-failure mechanism that occurs during the fatigue process. Classes of these signals compared, with each sample. To see if there were any classes common to the three samples. The fatigue test attempted to determine if the initial cut length has any influence on the type of signals.

알루미늄 5083의 피로균열 진전에 따른 수명예측 및 비파괴평가

남기우(KI-WOO NAM) 한국해양공학회 2001 韓國海洋工學會誌 Vol.15 No.2

Fatigue life and nondestructive evaluation were examined experimentally using surface crack specimen and compact tension specimen of 5083 aluminium alloy. Acoustic emission signals emanated during local failure of aluminum alloys has been the subject of numerous investigations. Possible sources of AE during deformation have been suggested as the dislocations, fracture of brittle particles and debonding of these particles from the alloy matrix. Fatigue life and penetration behavior of long surface crack can be evaluated quantitatively using K values proposed by authors. The influence of stress ratio on the frequency characteristics of AE signals were investigated.

CFRP적층 형태에 따른 파괴시 음향방출 신호특성

남기우(KI-WOO NAM),문창권(CHANG-KWON MOON) 한국해양공학회 2002 韓國海洋工學會誌 Vol.16 No.2

Damage process of CFRP laminates was characterized by Acoustic Emission(AE). The main objective of this study is to determine if the sources of AE in CFRP laminates could be identified from the characteristics of the waveform signals recorded during monotonic tensile test. The time history and power spectrum of each individual wave signal recorded during test were examined and classfied according to their special characteristics. The waveform and frequency of AE signal from a specimens is an aid to the determination of the extent of the different fracture mechanism such as matrix crack, debonding, fiber pull-out and fiber fracture as load is increased. Four distinct types of signals were observed regardless of specimen condition. The result showed that the AE method could be effectively used for analysis of fracture mechanism in CFRP laminates.

STS316L용접재의 표면파에 의한 잔류응력 측정과 균열진전시의 음향방출특성

남기우(KI-WOO NAM),박소순(SO-SOON PARK),안석환(SEOK-WHAN AHN) 한국해양공학회 2003 韓國海洋工學會誌 Vol.17 No.1

In this study, the residual stress and the acoustic emission characteristics from fatigue crack propagation were investigated, based on the welded material of STS316L. The residual stress of welding locations could be evaluated by ultrasonic parameters, such as LCR wave velocity and LCR wave frequency; the residual stress between base metal and weld metal was evaluated. In the fatigue tests, three types of signals were observed, regardless of specimen condition, base metal, and weld metal. Based on NDE analysis of AE signals by the time-frequency analysis method, it should also be possible to evaluate, in real-time, the crack propagation and final fracture process, resulting from various damages and defects in welded structural members.

TWB보론강의 기계적 특성 및 성형성

남기우(Ki Woo Nam),황석환(Seok Hwan Hwang),김대용(Dae Yong Kim),이문용(Moon Yong Lee) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.2

오스테나이트계 내열강의 크리프 유효응력 해석

남기우,박인덕,Nam, Ki-Woo,Park, In-Duck 대한기계학회 2002 大韓機械學會論文集A Vol.26 No.7

This paper describes the comparison of calculated effective stress with experimental one in austenitic heat resistant steels, STS310J1TB and STS310S with and without a small amount of Nb and N. Based on a solute atoms diffusion model, contribution from soluble nitrogen to the high-temperature strength was numerically examined for austenitic heat-resisting Fe-Cr-Ni-N(STS310J1TB) and Fe-Cr-Ni (STS310S) alloys. The solute atmosphere dragging stress of dislocation was calculated in optional dislocation velocity of STS310J1TB and STS310S at $650^{\circ}C$, $675^{\circ}C$ and $700^{\circ}C$. As a result of the numerical calculation, the solute atmosphere dragging stress of STS310J1TB was about 50 times larger than that of STS310S. When the temperature became high, the maximum value of solute atmosphere dragging stress was small and the velocity of moving dislocation was fast. From the relationship between the dislocation rate and the solute atmosphere dragging stress, the relation of both was proportional and the inclination is about 1 in the level with low velocity of moving dislocation. From above results, the mechanism of dislocation movement in STS310J1TB was the solute atmosphere dragging stress. The solute atmosphere dragging stress, which was calculated from the numerical calculation was close to the effect stress in stress relaxation tests.

Si₃N₄ 세라믹스의 균열 치유와 굽힘 강도 특성

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

Crack-healing behavior of Si₃N₄ composite ceramics has been studied as functions of heat-treatment temperature and amount of additive SiO₂ colloidal. Results showed that optimum amount of additive SiO₂ colloidal and coating of SiO₂ 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 SiO₂ colloidal. Crack healing strength was far the better cracked specimen with SiO₂ 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 SiO₂ colloidal 0.0 wt.% without SiO₂ colloidal coating recovered to the value of the smooth specimens at room temperature for the investigated crack sizes 100 ㎛. But that with SiO₂ colloidal coating increase up to 140 %. The amount of optimum additive SiO₂ colloidal was 1.3 wt.% and crack healed bending strength with SiO₂ colloidal coating increase up to 160 % to smooth specimen of additive SiO₂ 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 Si₃N₄ composite ceramics.

흡수된 CFRP의 AE에 의한 파괴거동과 신뢰성 평가

남기우(Ki-Woo Nam),김선진(Seon-Jin Kim) 한국해양공학회 1996 韓國海洋工學會誌 Vol.10 No.4

Recently carbon fiber reinforced plastic (CFRP) has been used structural materials in corrosive environment such as for water, chemical tank and pipes. However, mechanical properties of such materials may be change when CFRP are exposed to corrosive environment for long periods of time.<br/> The degradation behavior of carbon fiber/epoxy resin composite material in distilled water is investigated using acoustic emission (AE) technique. Fracture toughness tests are performed on the compact tension specimens that are pilled by two types of [0₂/90₂]₃s and [0/90]6s. During the testes, AE test was carried out to monitor the damage of CFRP by moisture absorption. The data was treated by 2-parameter Weibull distribution and the fracture surface was observed by scanning electron microscope.

SiC 첨가한 ZrO₂의 기계적 특성에 대한 와이블 통계 해석

남기우(Ki Woo Nam),김선진(Seon Jin Kim),김대식(Dae Sik Kim) 대한기계학회 2015 大韓機械學會論文集A Vol.39 No.9

비커스 경도 실험은 세라믹스 재료의 경도를 특성화하는데 사용되는 일반적인 실험법이다. 그러나 경도도 하나의 확률변수로 취급하는 것이 일반적이다. 본 연구의 목적은 단상 ZrO₂ 와 SiC 첨가한 ZrO₂/SiC 복합 세라믹스의 굽힘강도와 비커스 경도의 통계적 성질을 조사하는 것이다. 본 연구에서는 와이블 통계 해석에 기초하여 그들의 결과를 특성치와 변동을 비교 고찰하였다. 굽힘강도 및 비커스 경도는 모두 와이블 분포에 비교적 잘 적합할 수 있음을 알았다. 또한 단상 ZrO₂ 와 SiC 첨가한 ZrO₂/SiC 복합 세라믹스와 그들의 열처리재에 대한 비커스 경도의 확률분포에 대한 척도 및 형상 파라메터 값을 평가하였다. The Vickers hardness test is a common method used to characterize the hardness of ceramic materials. However, the hardness is not a deterministic value, but is a random variable. The objective of this paper is to investigate the statistical properties of the bending strength and a set of Vickers hardness values in single ZrO₂ and composite ZrO₂/SiC with a SiC additive. In this work, we compare the characteristic value and variation with the results based on Weibull statistical analysis. The probability distributions of the bending strength and Vickers hardness agreed relatively well with the Weibull distribution. We evaluate the scale parameter and shape parameter in asreceived ZrO₂ and ZrO₂/SiC composite ceramics,as well as in their heat treated ceramics.