• 대한기계학회논문집B
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• 제40권11호
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• pp.689-695
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• 2016

연료전지의 성능에 가장 큰 영향을 주는 요소 중에 하나가 가스확산층과 촉매층에서 물의 거동이다. 따라서 가스확산층의 특성에 따른 유체의 거동의 변화를 이해하는 것은 연료전지의 성능개선과 가스확산층의 설계를 위한 필수적인 요소이다. 이 연구에서는 가스확산층의 설계요소인 기공도, 굴곡도와 유효확산계수를 수치적으로 계산할 수 있는 방법을 제안한다. 제안한 방법의 검증을 위하여 지름이 일정한 구형입자를 이용하여 기공도가 다른 다공체를 만들고 구형입자에 Bounceback 조건을 적용한 격자 볼쯔만법 유동해석을 수행하였다. 다공체 내의 유동효과를 나타내는 투과도는 다공체에 의한 압력강하와 평균유속으로 계산하고, 질량이 없는 입자의 평균 다공체 통과 거리로부터 계산한 굴곡도와 기공도를 이용하여 계산한 유효확산계수를 Neale의 이론식과 비교하여 정확하게 일치하는 것을 확인하였다. 이 방법은 실제 다공체의 이미지를 이용한 계산에도 수정없이 이용할 수 있어 연료전지의 성능향상과 설계를 위한 가스확산층의 특성분석에 활용될 수 있다.

• 소성∙가공
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• 제18권4호
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• pp.296-303
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• 2009

The effect of porosity on the high-cycle fatigue properties of Al-Si-Mg casting aluminum alloys was investigated in this study. Microstructure examination, tensile and high-cycle fatigue test were conducted on both Al-Si-Mg casted (F) and heat-treated (T6) conditions. Porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The microstructure observation results showed that eutectic Si particles were homogeneously dispersed in the matrix of the Al-Si-Mg casting alloys, but there were porosities formed as cast defects. The high-cycle fatigue results indicated that the fatigue strength of the 356-T6 alloy was higher than that of the 356-F alloys because of the significant reduction in volume fraction of pores by heat treatment. The SEM fractography results showed that porosity affected detrimental effect on the fatigue life: 80% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. It was found that fatigue life decreased as the size of the surface pore increased. A comparison was made between surface pore and inner pore for its effect on the fatigue behavior. The results showed that the fatigue strength with the inner pores was higher than that of the surface pore.

• Steel and Composite Structures
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• 제51권2호
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• pp.185-202
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• 2024

In this research paper, and for the first time, wave propagations in sigmoidal imperfect functionally graded material plates are investigated using a simplified quasi-three-dimensionally higher shear deformation theory (Quasi-3D HSDTs). By employing an indeterminate integral for the transverse displacement in the shear components, the number of unknowns and governing equations in the current theory is reduced, thereby simplifying its application. Consequently, the present theories exhibit five fewer unknown variables compared to other Quasi-3D theories documented in the literature, eliminating the need for any correction coefficients as seen in the first shear deformation theory. The material properties of the functionally graded plates smoothly vary across the cross-section according to a sigmoid power law. The plates are considered imperfect, indicating a pore distribution throughout their thickness. The distribution of porosities is categorized into two types: even or uneven, with linear (L)-Type, exponential (E)-Type, logarithmic (Log)-Type, and Sinus (S)-Type distributions. The current quasi-3D shear deformation theories are applied to formulate governing equations for determining wave frequencies, and phase velocities are derived using Hamilton's principle. Dispersion relations are assumed as an analytical solution, and they are applied to obtain wave frequencies and phase velocities. A comprehensive parametric study is conducted to elucidate the influences of wavenumber, volume fraction, thickness ratio, and types of porosity distributions on wave propagation and phase velocities of the S-FGM plate. The findings of this investigation hold potential utility for studying and designing techniques for ultrasonic inspection and structural health monitoring.