• 한국결정성장학회지
• /
• 제8권4호
• /
• pp.612-618
• /
• 1998

$SiCl_4/C_3H_8/H_2$계를 사용하여 흑연기판 위에 순수한 SiC층과 SiC/C FGM층을 CVD법에 의해 증착한 후 thermal shock 시험을 통하여 두 시편의 열적 성질을 조사하였다. Thermal shock 시험시 두가지 시편 내부의 이론적인 열응력 차이를 알아보기 위해 상용프로그램을 이용하여 시편내의 온도분포, 열응력 분포를 계산하였다. SiC/C FGM층을 증착한 시편이 순수한 SiC층을 증착한 시편보다 계산상으로 경계면에서 우수한 열응력 완화효과를 나타내는 것으로 판단되었고 실험적으로로 FGM 시편의 경우 $\Delta$T=1600K의 열충격에도 견딜수 있는 것을 확인하여 이론과 실험이 일치하는 것을 입증하였다.

• International Journal of Aeronautical and Space Sciences
• /
• 제13권4호
• /
• pp.458-467
• /
• 2012

In this study, the linear viscoelastic response of a rectangular laminated plate is investigated. The viscoelastic properties, expressed by two basic spring-dashpot models, that is Kelvin and Maxwell models, is assumed in the range to investigate the influence of viscoelastic coefficients to mechanical behavior. In the present study, viscoelastic responses are performed for two popular equivalent single-layered theories, such as the first-order shear deformation theory (FSDT) and third-order shear deformation theory (TSDT). Compliance and relaxation modulus of time-dependent viscoelastic behavior are approximately determined by Prony series. The constitutive equation for linear viscoelastic material as the Boltzmann superposition integral equation is simplified by the convolution theorem of Laplace transformation to avoid direct time integration as well as to improve both accuracy and computational efficiency. The viscoelastic responses of composite laminates in the real time domain are obtained by applying the inverse Laplace transformation. The numerical results of viscoelastic phenomena such as creep, cyclic creep and recovery creep are presented.

• 폴리머
• /
• 제27권3호
• /
• pp.201-209
• /
• 2003

나일론의 종류에 따른 분쇄형 탄소 섬유/나일론 복합재료의 직류 및 교류 전도도, 그리고 전자기파 차폐 효율을 조사하였다. 탄소 섬유의 함량이 약 7 vol%에서 전도도가 급격하게 증가하는 percolation 전이가 관찰되었다. 나일론 46을 기저 수지로 하였을 경우 더욱 높은 전기 전도도를 나타냈으며, 계면 결합제의 적용 여부에 따라 전도도의 차이가 발생하였다. 온도증가에 따라 전도도가 증가하는 negative temperature coefficient 현상을 나타냈으며, percolation 전후의 탄소 섬유 함량에서의 주파수에 따른 전도기구를 완화와 공진 현상으로 각각 달리 설명할 수 있었다. 회로망 분석기를 통하여 측정한 전자기파 차폐 효율은 전도도 및 탄소 섬유의 함량에 따라 증가하였으며, 높은 전도도 영역에서의 전자기파 차폐 효율은 반사에 의한 차폐가 지배적이었다.

• Korea-Australia Rheology Journal
• /
• 제14권1호
• /
• pp.1-9
• /
• 2002

Steady and dynamic shear properties of two hydrophobically modified alkali soluble emulsions (HASE), NPJI and NPJ2, were experimentally investigated. At the same polymer concentration, NPJ1 is appreciably more viscous and elastic than NPJ2. The high hydrophobicity of NPJ1 allows hydrophobic associations and more junction sites to be created, leading to the formation of a network structure. Under shear deformation, NPJ1 exhibits shear-thinning behaviour as compared with Newtonian characteristics of NPJ2. NPJ1 and NPJ2 exhibit a very high and a low level of elasticity respectively over the frequency range tested. For NPJ1, a crossover frequency appears, which is shifted to lower frequencies and hence, longer relaxation times, as concentration increases. Three different surfactants anionic SDS, cationic CTAB, and non-ionic TX-100 were employed to examine the effects of surfactants on the rheology of HASE. Due to the different ionic behaviour of the surfactant, each type of surfactant imposed different electrostatic interactions on the two HASE polymers. In general, at low surfactant concentration, a gradual increase in viscosity is observed until a maximum is reached, beyond which a continuous reduction of viscosity ensues. Viscosity development is a combined result of HASE-surfactant interactions, accompanied by constant rearrangement of the hydrophobic associative junctions, and electrostatic interactions.

• 터널과지하공간
• /
• 제5권4호
• /
• pp.363-375
• /
• 1995

Critical literature review in this study revealed that there can be a significant influence of the residual stress on the engineering properties of rock. The review also showed that few number of research works on the quantification of the influence was attributed to the limitation of the two classical measurement techniques, viz, X-ray diffraction and mechanical relaxation method. In this study, a new way of approach was sought based on the assumption that residual stress up to the failure. A series of hoop tests conducted onthe samples from the limb of Carboniferous Limestone in Clevedon, England, revealed that (i) there is no preferential orientations of microcracks and minerals which have been widely believed as the main source of the strength anisotropy of rock; (ii) the anisotropy of the tensile strength of the limestone results from the influence of the residual stress; (iii) since jointing commenced within the fold, residual stored strain energy has been released preferentially in the direction perpendicular to the major joints(o$^{\circ}$ and 90$^{\circ}$); (ⅳ) during the hoop test making it much easier to create tensile fracture in these directons, viz 45$^{\circ}$ and 135$^{\circ}$)was released during the hoop test making it much easier to create tensile fracture in these directions, viz 45$^{\circ}$and 135$^{\circ}$;(v) the direction in which the stored strain energy may be presumed to be the least, required the greatest work to cause failure.

• 대한기계학회논문집A
• /
• 제21권11호
• /
• pp.1965-1971
• /
• 1997

The relaxation of stress concentration in notched members can be very significant in the improvement of notched strength and fatigue life. This paper investigated the relationship of stress concentration factor, and notched strength and fatigue life. The stress concentration factors were analyzed by FEM. Uniaxial tensile and fatigue tests were carried on plain woven composite specimens which have a main hole and two defence holes. From experimental results, the notched strength and the fatigue limit increased up to about 50% and 30% respectively due to the reduction in stress concentration. The fatigue lives predicted by Juvinall's approach were underestimated than test results and this trends were remarkable as nothed strength increased. This is because of the underestimation of a coefficient. A in S-N curve (.sigma.$_{ar}$ =A $N_{f}$ $^{B}$). Therefore, considering notched strength the coefficient A was modified. The fatigue lives by this process were agreed well with the experimental results.sults.

• 한국안광학회지
• /
• 제4권2호
• /
• pp.105-111
• /
• 1999

PECVD 방법에 의하여 제작된 수소화된 비정질 탄소박막에서 측정 전압에 따른 전기전도도의 급격한 변화와 시간에 따른 전류의 이완 현상을 관측하였다. 이 현상은 Hammer 등에 의해 four-probe 측정과 샌드위치형 전극 시료에 대해서 측정한 것과 동일하였다. 본 연구에서는 "높은 전압에서 전자가 트랩에 포획될 확률이 작아진다"는 가정 하에서 정성적이지만 전기전도도의 측정 전압 의존에 관한 세 가지 현상이 비교적 잘 설명됨을 제시하였다.

• 대한의용생체공학회:학술대회논문집
• /
• 대한의용생체공학회 1995년도 추계학술대회
• /
• pp.253-256
• /
• 1995

Polymers have been one of the emerging biomedical materials in the area of biomedical research which are applicable to the human body. For human applications, noninvasive characterization of the biomedical polymers has been one of the important topics, and is valuable information. Among others, the swelling rate is one of the important measurements needed for the hydrophilic polymers. NMR imaging has been a suitable method for the noninvasive study of such a material since it is sensitive to many physical and biochemical changes of the specimens. In addition, NMR techniques possess many useful intrinsic properties such as the relaxation and diffusion effects. The present study has provided a noble and noninvasive method of measuring the process of swelling as well as volumetric changes occurred in polymers and drug delivery processes in a drug delivery system (DDS) together with changes of released drug. This gives information, relating with both water ingress process, volumetric changes of polymer specimens and the visualization of sequential drug delivery process. Also, this study provides more reliable method to ascertain the time dependent swelling process compared to the conventional method. The important aspects is that the proposed method is truly noninvasive and is able to ascertain time dependent processes.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 1993년도 추계학술대회 논문집
• /
• pp.85-89
• /
• 1993

Silicone oil exhibits the properies of both organic and inorganic substances and, thus, it has many superior properties such as higher thermal resistance and lower thermal oxidation level when compared to other dielectric liquids. In order to investigate the dielectric characteristics, dielectric liquids of viscosity 1 [cSt] is chosen as the specimen and experiment is performed in the temperature range of 20∼65 [$^{\circ}C$] and frequency range of 30∼1${\times}$10$\^$6/ [Hz] respectively. As a result, the observed linear decrease in dissipation factor at the frequency range below 3 [kHz] is due to the influence of frequency, whereas the increase in dissipation factor at higher frequency range is contributed by electrode's resistance. At a fixed frequency of 30 [kHz], increasing temperature results in higher peak value and wide width of the absorption curve. This is due to the increase in dipole and viscosity. As temperature increases, dipole moment is decreased from 0.98 to 0.64 [debye]. The activation energy which causes the relaxation and loss of dielectric is obtained about 15 [kcal/mole].

• 폴리머
• /
• 제25권3호
• /
• pp.375-384
• /
• 2001

불포화 폴리에스테르(unsaturated polyester: UP)의 희석제로 많이 사용되는 스티렌 단량체 (SM)대신에 단일 중합체의 유리전이온도가 낮은 비닐 아세테이트(VAc), 메틸 아크릴레이트(MA)를 SM과 혼합하여 사용하였을 때 혼합초성에 따른 UP의 기계적 물성 변화를 조사하였다. SM/MA 혼합계는 MA함량 증가에 따라 전체적으로 강인성의 향상이 나타난 반면, SM/VAc 혼합계의 경우에는 VAc 단량체치 함량 증가에 마른 강인성의 향상을 기대하기가 힘든 것으로 나타났다. 이러한 현상은 전체 계의 유리전이온도의 저하 효과와 상분리에 의한 응력 완화 효과에 미치는 각 단량체의 영향이 서로 다르기 때문인 것으로 해석되었다.