• Structural Engineering and Mechanics
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• v.75 no.3
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• pp.311-325
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• 2020

In applying the spectral stochastic finite element methods to the frequency response analysis, the conventional methods are known to give unstable and inaccurate results near the natural frequencies. To address this issue, a new sensitivity based stabilized formulation for stochastic frequency response analysis is proposed in this paper. The main difference over the conventional spectral methods is that the polynomials of random variables are applied to both numerator and denominator in approximating the harmonic response solution. In order to reflect the resonance behavior of the structure, the denominator polynomials is constructed by utilizing the natural frequency sensitivity and the random mode superposition. The numerator is approximated by applying a polynomial chaos expansion, and its coefficients are obtained through the Galerkin or the spectral projection method. Through various numerical studies, it is seen that the proposed method improves accuracy, especially in the vicinities of structural natural frequencies compared to conventional spectral methods.

• Elastomers and Composites
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• v.52 no.3
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• pp.211-215
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• 2017

Thermally expandable microspheres were used as post-treatment initiators of potassium persulfate, sodium bisulfite, and sodium sulfide in order to improve the foaming ability and whiteness when foaming a mixture of thermally expandable microsphers and poly(vinyl chloride). Potassium persulfate showed no significant influence on the foaming behavior, foam expansion, whiteness, and yellowing, whereas in the case of using sodium bisulfite. In particular, sodium bisulfite demonstrated the best efficiency with 2 wt% treatment. The thermally expandable microspheres prepared herein can provide excellent foamability and whiteness, and are expected to be applicable in various fields such as general coating and wallpaper.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.289-292
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• 2001

Aluminium foams, having a closed cell structure, fabricated by applying the powder compact method and an induction heating were studied. The powdered A6061 mixed with the powdered titanium hydride as a foaming agent was hot pressed into a foamable precursor. The resulting precursor was foamed by induction heating up to desired temperature. The effects of the titanium hydride content ($0.3{\~}1.5 wt.\%$), pressing pressure of the foamable precursor material (50-150kN), the forming temperature ($610{\~}690^{\circ}C$) and heating rate during foaming on the expansion behavior of the foam were investigated.

• Journal of the Korean Ceramic Society
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• v.16 no.3
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• pp.169-177
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• 1979

The effect of apparent porosity of the fired ceramics, metallizing temperature, and metallizing mixtures on the bond strength in metal-to-ceramic seals was investigated. Three different metallizing compounds were metallized on dense alumina bodies at 1300~$1500^{\circ}C$ in dry hydrogen atmosphere. Bond strength between metal and alumina body was measured by means of nstron test machine. The greater bond strength was observed as the apparent porosity and metallizing temperature was increased. This work indicated that the glassy phase in metallizing mixture, having had sufficient fluidity to migrate into the alumina body, reacted with alumina and thereby forming strong metal-ceramic interface bond. It also showed that the glassy phase having higher thermal expansion cofficient than molybdenum might contribute to the strong bond formation by providing compressive stress around the molybdenum particle.

• Journal of the Korean Ceramic Society
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• v.29 no.12
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• pp.956-962
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• 1992

The predominant sintering mechanisms of low firing temperature ceramic substrate which consists of borosilicate glass containing alumina as a filler are the rearrangement of alumina particles and the viscous flow of glass powders. In this system, sintering condition depends on the volume ratio of alumina to glass and on the particle size. When the substrate contains about 35 vol% alumina filler and the average alumina particle size is 4 $\mu\textrm{m}$, the best firing condition is obtained at the temperature range of 900∼1000$^{\circ}C$. The extensive rearrangement behavior occurs at these conditions, and the optimum sintering condition is attained by smaller size of glass particles, too. The formation of cristobalite during sintering causes the difference of thermal expansion coefficient between the substrate and Si chip. This phenomenon degradates the capacity of Si chip. Therefore, the crystallization should be prevented. In the alumina filled borosilicate glass system, the crystallization does not occur. This effect may have some relation with aluminum ions in alumina. For aluminum ions diffuse into glass matrix during sintering, functiong as network former.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.108-113
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• 2004

Korea Gas Corp. has developed the design technology of the LNG storage tank. The membrane to be applied inside of the LNG storage tank is provided with corrugations to absorb thermal contraction and expansion caused by LNG temperature changes. It is very important to measure their thermal strains under LNG temperatures by analytical and experimental stress analysis of the membrane. We have developed a stress measurement system using strain gages and measured the strain during cooldown and storing the LNG. We also analyzed the measured data by comparison with the FEM data. On the basis of these results, we could design and assure the application of the Kogas Membrane to large scale LNG storage.

• 한국가스학회:학술대회논문집
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• 2003.10a
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• pp.85-90
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• 2003

Korea Gas Corp. has developed the design technology of the LNG storage tank. The membrane to be applied inside of LNG storage tank is provided with corrugations to absorb thermal contraction and expansion caused by LNG temperature. We constructed strain measurement system by using strain gage. In this paper, some problems which should be considered when measuring strain at $-162^{\circ}C$, are discussed by presenting test results on the characteristics of strain gages, Temperature sensor, adhesive and lead wire. And presenting the procedure of the constructing strain measurement system.

• Journal of Energy Engineering
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• v.24 no.3
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• pp.55-60
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• 2015

Recent engine development has focused mainly on the improvement of engine efficiency and output emissions. The improvements in efficiency are being made by friction reduction, combustion improvement and thermodynamic cycle modification. Computer simulation has been developed to predict the performance of a spark ignition engine. The effects of various cylinder pressure, heat release, flame temperature, unburned gas temperature, flame properties, laminar burning velocity, turbulence burning velocity, etc. were simulated. The simulation and analysis show several meaningful results. The objective of the present study is to develop a combustion model for a spark ignition engine running with isooctane as a fuel and predicting its behavior.

• Structural Engineering and Mechanics
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• v.40 no.3
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• pp.335-346
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• 2011

This paper deals with the bifurcations of non-semi-simple eigenvalues at critical point of Hopf bifurcation to understand the dynamic behavior of the system. By using the Puiseux expansion, the expression of the bifurcation of non-semi-simple eigenvalues and the corresponding topological structure in the parameter space are obtained. The zero-order approximate solutions in the vicinity of the critical points at which the multiple Hopf bifurcation may occur are developed. A numerical example, the flutter problem of an airfoil in simplified model, is given to illustrate the application of the proposed method.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.22 no.7
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• pp.43-48
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• 2008

The characteristics of heat transfer of the bimetal disc for over-current protection device is specified. Bimetal consists of two metals which have a different thermal expansion coefficient. To analyze the thermal characteristics, temperature distribution when bimetal acts as a switch is calculated. As usual, heat source is applied to the bimetal and electric current is heat source in the over-current protection switch. In this paper, thermal distribution are obtained by solving a coupled electro-thermal field with 3D finite element method.