• 한국연소학회지
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• 제17권3호
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• pp.17-29
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• 2012

The effects of strain rate and preferential diffusion of $H_2$ on flame extinction are numerically studied in interacting premixed syngas-air flames with fuel compositions of 50% $H_2$ + 50% CO and 30% $H_2$ + 70% CO. Flame stability diagrams mapping lower and upper limit fuel concentrations at flame extinction as a function of strain rate are examined. Increasing strain rate reduces the boundaries of both flammable lean and rich fuel concentrations and produces a flammable island and subsequently even a point, implying that there exists a limit strain rate over which interacting flame cannot be sustained anymore. Even if effective Lewis numbers are slightly larger than unity on extinction boundaries, the shape of the lean extinction boundary is slanted even at low strain rate, i.e. $a_g=30s^{-1}$ and is more slanted in further increase of strain rate, implying that flame interaction on lean extinction boundary is strong and thus hydrogen (as a deficient reactant) Lewis number much less than unity plays an important role of flame interaction. It is also shown that effects of preferential diffusion of $H_2$ cause flame interaction to be stronger on lean extinction boundaries and weaker on rich extinction boundaries. Detailed analyses are made through the comparison between flame structures with and without the restriction of the diffusivities of $H_2$ and H in symmetric and asymmetric fuel compositions. The reduction of flammable fuel compositions in increase of strain rate suggests that the mechanism of flame extinction is significant conductive heat loss from the stronger flame to ambience.

• 비파괴검사학회지
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• 제26권5호
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• pp.329-335
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• 2006

기계구조물 설계시 파손이나 갑작스런 붕괴를 방지하기 위하여 응력집중을 실험적으로 측정해야 할 필요가 있다. 본 연구에서는 중앙에 원형구멍이 있는 외팔보 시편의 응력집중계수를 스트레인 게이지와 광탄성법으로 측정하였다. 스트레인 게이지법에서는 구멍 주위에 3개의 스트레인 게이지를 부착하여 측정된 값으로부터 외삽법을 사용하여 구멍에 인접한 지점에서 최대 변형률을 추정하였다. 광탄성법에서는 두 가지 측정법, 즉 배비넷-솔레일 보상법과 위상이동법을 이용하여 응력분포를 측정하였다. 스트레인 게이지와 광탄성법에 의한 측정값들을 서로 실험오차 이내로 근접하였다.

• 소성∙가공
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• 제16권2호
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• pp.101-106
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• 2007

Uniaxial displacement controlled tests were performed on annealed Type 304 stainless steel at room temperature. A servo-controlled testing machine and strain measurement on the gage length were employed to measure the response to a given input. The test results exhibit that the flow stress increases nonlinearly with the strain rate and the relaxed stress at the end of the relaxation periods depends strongly on the strain rate preceding the relaxation test. The rate-dependent inelastic deformation behavior is simulated using a new unified viscoplasticity model that has the rate-dependent format of nonlinear kinematic hardening rule, which plays a key role in modeling the rate dependence of relaxation behavior. The model does not employ yield or loading/unloading criteria and consists of a flow law and the evolution laws of two tensor and one scalar-valued state variables.

• 한국산학기술학회:학술대회논문집
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• 한국산학기술학회 2009년도 추계학술발표논문집
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• pp.498-500
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• 2009

본 연구에서는 한국인 아동의 우식치아로부터 분리한 S. mutans K7으로부터 HtrA gene을 동정하고 HtrA expression이 산성환경하에서 S. mutans의 생육에 미치는 영향을 알아보았다. S. mutans K7의 HtrA mutant strain은 산성환경에서 parental strain과 비교하였을 때, 생육에 있어서 상당한 차이를 나타내었다. 또한 Biofilm formation에 관여하는 GtfB, 와 GtfC의 발현량도 현저히 줄어들었다. 그리고 HtrA mutant strain에 HtrA gene을 삽입하여 HtrA의 발현량을 회복하였을 경우에는 acid stress하에서 control과 같은 nomal growth phenotype을 회복하였다. 이러한 결과들은 S. mutans K7에서 HtrA가 acid stress동안에 중요한 역할을 담당함을 제시하고 있다.

• Applied Microscopy
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• 제45권2호
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• pp.44-51
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• 2015

At room temperature, metallic glasses deform inhomogeneously by strain localization into narrow bands as a result of yielding due to an external force. When shear bands are generated during deformation, often nanocrystals form at the shear bands. Experimental results on the deformation of bulk metallic glass in the current study suggest that the occurrence of nanocrystallization at a shear band implies the loading condition that induces deformation is more triaxial in nature than uniaxial. Under a compressive stress state, the geometrical constraint strain imposed by the stress triaxiality plays a crucial role in the deformation-induced nanocrystallization at the shear bands.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 춘계학술대회 논문집
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• pp.205-208
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• 2001

Localized shear band is investigated through the analysis of one-dimensional model for simple shearing deformation of thermally rate dependent material. Generally mesh size or interval of nodes play an important role in determining the overall flow behavior of the material. In order to observe these size effects we adapted non-local theory by including higher order strain gradients of the equivalent strain into the constitutive equation for the flow stress. for the ease of convergence and numerical stability the inplicit finite difference scheme is employed.

• Structural Engineering and Mechanics
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• 제5권6호
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• pp.785-801
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• 1997

Reinforced Concrete beams with tension and compression softening material constitutive laws are studied. Energy-based and non-local regularisation techniques are presented and applied to a R.C. element. The element characteristics (sectional tangent stiffness matrix, element tangent stiffness matrix restoring forces) are directly derived from their symbolic expressions through numerical integration. In this way the same spatial grid allows us to obtain a non-local strain estimate and also to sample the contributions to the element stiffness matrix. Three examples show the spurious behaviors due to the strain localization and the stabilization effects given by the regularisation techniques, both in the case of tension and compression softening. The possibility to overestimate the ultimate load level when the non-local strain measure is applied to a non softening material is shown.

• Structural Engineering and Mechanics
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• 제5권6호
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• pp.755-765
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• 1997

Within the framework of anisotropic combined viscoplastic hardening formulation, accounting macroscopically for residual stress as well as texture development at finite deformations of metals, simple shear analyses for the simulation of fixed-end torsion experiments for ${\alpha}$-Fe, Al and Cu at different strain rates are reviewed with an emphasis on the role of constitutive spins. Complicated responses of the axial stresses with monotonically increasing shear deformations can be successfully described by the capacity of orthotropic hardening part, featuring tensile axial stresses either smooth or oscillatory. Temperature effect on the responses of axial stresses for Cu is investigated in relation to the distortion and orientation of yield surface. The flexibility of this combined hardening model in the simulation of finite inelastic strains is discussed with reference to the variations of constitutive spins depending upon strain rates and temperatures.

• 설비공학논문집
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• 제28권9호
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• pp.367-372
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• 2016

Gas diffusion layer (GDL) of PEMFCs plays a role that it diffuses the reactant gases to the catalyst layer on the membrane and discharge water from the catalyst layer to the channel. Physical parameters related to the mass transport of GDL are mostly from the uncompressed GDLs while actual GDLs in the assembled stacks are compressed. In this study, the relation of compression and strain of GDLs with various Polytetrafluoroethylene (PTFE) loading is measured experimentally and In-plane gas permeability is measured at the condition that the GDLs are in compressive strain. The gas permeability decreased with the loading of PTFE and the presentation of gas permeability under compressive stain is expected to improve the accuracy of modeling work of mass transport in the GDL.

• 열처리공학회지
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• 제33권6호
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• pp.296-302
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• 2020

Magnesium alloys have been rapidly attracting as lightweight structural material in various industry fields because of having high specific strength and low density. It is well known that the crystallographic texture plays an important role in improvement of poor room temperature ductility of magnesium alloys. In this study, high-temperature plane strain compression deformation was conducted on extruded AZ80 magnesium alloy at 723K by varying the strain rates ranging from 5.0×10^-3s^-1 to 5.0×10^-2s^-1 in order to investigate the behaviors of texture formation. It was found that texture formation behaviors in three kinds of specimens were affected by continuous and discontiuous deformation mechanism.