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균열 진전에 의한 Al₂O₃ 복합 세라믹스의 비파괴 평가
김진욱(J.W. Kim),김해숙(H.S. Kim),안석환(S.H. Ahn),손창석(C.S. Son),김종순(J.S. Kim),남기우(K.W. Nam) 한국동력기계공학회 2007 한국동력기계공학회 학술대회 논문집 Vol.- No.-
The elastic waves released by inducing vickers indentation were detected by fracture wave detector. From the result of wavelet analysis of elastic wave signal, the smooth specimen and heat-treated crack healing specimen of 0.3 ㎛ Al₂O₃ monolithic and Al₂O₃/SiC composite ceramics had characteristics of frequency about 58 ㎑. The dominant frequencies increased with increasing the amount of Y₂O₃. It is known that wave frequency of homogeneous materials is converged on around dominant frequency. The dominant frequency has direct connection with the bending strength.
남기우(K. W. Nam),안석환(S. H. Ahn),김진욱(J. W. Kim),이시윤(S. Y. Lee) 한국동력기계공학회 2004 한국동력기계공학회 학술대회 논문집 Vol.- No.-
We studied on the nondestructive evaluation of the elastic wave signals of straight pipe with local wall thinning, Wavelet transform was applied for the time-frequency analysis of wave forms obtained by fracture wave detector due to the dropping steel ball. The time-frequency analysis provides time variation of each frequency component involved in a waveform, which makes it possible to evaluate the shape of local wall thinning at each frequency. In this study, comparison by wavelet transform of the AE signals and monotonic bending tests without internal pressure are conducted on 1.91 inch diameter full-scale carbon steel pipe specimens. As the results of tests, fracture behaviors could be shown the characteristic of mechanical strength of locally wall thinned pipes and the wave forms could be evaluated for the integrity insurance of the piping system according to the length and depth range between the defected shape pipes in the real field.
섬유기기용 Al₂O₃계 세라믹스의 균열치유거동과 균열치유재의 강도특성
안병건(B. G. Ahn),김미경(M. K. Kim),김현수(H. S. Kim),안석환(S. H. Ahn),김진욱(J. W. Kim),남기우(K. W. Nam) 한국동력기계공학회 2005 한국동력기계공학회 학술대회 논문집 Vol.- No.-
Alumina ceramic for textile machinery was sintered and subjected to three-point bending. A semicircular surface crack was made on each sample. Crack-healing behavior was systematically studied, as a function of crack-healing temperature, crack size. The bending strength and fracture toughness of the crack-healed sample from 1200℃ to 1400℃ was investigated. A statistical approach based on Weibull distribution was applied to the test data to evaluate the dispersion in the fracture toughness. Alumina ceramic for textile machinery have the ability to heal after cracking, from over 1300℃. The material can completely heal a 65 ㎛ diameter semielliptical crack. The fracture toughness could be explained by 2-parameter Weibull distribution.
안석환(S. H. Ahn),김태일(T. I. Kim),이문용(M. Y. Kim),김동규(D. K. Kim),남기우(K. W. Nam) 한국동력기계공학회 2008 한국동력기계공학회 학술대회 논문집 Vol.2008 No.11
The surface hardening treatment such as high-frequency induction hardening method is widely used to increase fatigue life and to prevent failure of materials by increasing locally the surface hardness. This method, specially, brings an improvement of static strength by the compressive residual stress of surface due to the hardening. In this study, mechanical properties of high-frequency induction hardened SCM440 steel were investigated. These results, also, were compared with those of base metal and Q/T (tempering after quenching) treatment specimen. From test results, we showed that the partially high-frequency induction hardened SCM440 steel were more improved at static strength, surface hardness, fatigue limit and anti-wear than base metal and Q/T treatment specimen. Specially, fatigue limit of the high-frequency induction hardened SCM440 steel increased more about 52% than that of base metal and about 25% than that of Q/T specimen.
안석환(S. H. Ahn),이수식(S. S. Lee),남기우(K. W. Nam) 한국동력기계공학회 2004 한국동력기계공학회 학술대회 논문집 Vol.- No.-
In pipes used at energy plants, those had exposed to various degradations such as fatigue, creep and corrosion etc. in-service periods. Of them, sometimes, the local wall thinning may result from severe erosion-corrosion damage. Therefore, fracture behaviors of pipes with local wall thinning are very important for the integrity of nuclear power plant. However, studies on the effects of local wall thinning on strength and fracture behaviors of piping system are rarely. In this paper, so, the monotonic bending test was performed of full-scale carbon steel pipes with local wall thinning. A monotonic bending load was applied to wall thinned straight pipe specimens by four-point loading at ambient temperature without internal pressure. From the tests, fracture behaviors and plastic collapse strength of locally thinned pipes were manifested systematically. The observed failure modes were divided into four types; ovalization, crack initiation after ovalization, local buckling and crack initiation after local buckling. Also, the strength and the allowable limit of piping system with local wall thinning were evaluated.
Al₂O₃ 복합 세라믹스의 굽힘강도와 Mapping 특성 평가
김해숙(H.S. Kim),안석환(S.H Ahn),손창석(C.S. Son),김종순(J.S. Kim),남기우(K.W Nam) 한국동력기계공학회 2007 한국동력기계공학회 학술대회 논문집 Vol.- No.-
Al₂O₃/SiC composite ceramics were sintered to evaluate the bending strength and elastic wave characteristics. The three-point bending test was carried out under room temperature. The elastic wave was detected by fracture wave detector. The crack healing behavior was investigated from 1373 K to 1723 K The bending strength of Al₂O₃/SiC composite by nanocomposite is higher than that of Al₂O₃ monolithic. Crack-healing behavior depended on an amount of additive powder Y₂O₃ In Al2O3/SiC composite ceramics with 3 wt. % Y₂O₃ for additive powder, the bending strength at 1573 K is about 100% increase than that of the smooth specimens. The strength of Al₂O₃/SiC composite ceramics was a little higher than those of Al₂O₃ monolithic. In proposition to an amount of additive, a large amount of Si and O, and little C were detected.