• 콘크리트학회지
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• 제3권4호
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• pp.97-106
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• 1991

본 연구의 목적은 휨거동을 하는 철근콘크리트 부재의 변위를 해석적으로 정확하게 구하기 위해 평균 휨균열간격 및 휨모멘트-등가곡률 관계(M-$\Phi_eg$)의 해석법을 제안한 것이다. 제안식은 비균열 구간에서의 철근과 콘크리트 간의 부착특성 및 재료의 소성영역을 고려하여 정확한 곡률분포를 계산함으로써 구할 수 있다. 제안된 해석법의 타당성을 검증하기 위해 34개의 철근콘크리트 보 부재를 제작, 휨재하 실험을 실시하였으며 해석치와 비교검토하였다. 그 결과 실험치와 해석치는 매우 잘 일치하여 본 해석법의 실용성 및 정확성이 입증되었다.

• 대한기계학회논문집A
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• 제26권8호
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• pp.1672-1679
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• 2002

This paper provides an engineering J estimation equation for cylinders with finite internal axial surfacecracks under internal pressure. The proposed equation is the J estimation equation based on deformation plasticity using Ramberg-Osgood (R-O) materials. Based on detailed 3-D FE results using deformation plasticity, plastic influence functions for fully plastic J components are tabulated for practically interesting ranges of the mean radius-to-thickness ratio, the crack depth-to-length ratio, the crack depth-to-thickness ratio. the strain hardening index for the R-O material, and the location along the semi-elliptical crack front. Based on tabilated plastic influence functions, the J estimation equation along the crack front is proposed and validated for R-O materials. Good agreements between the FE results and the proposed J estimation provide confidence in the use of the proposed method to elastic-plastic fracture mechanics of pressurized piping.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.365-371
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• 2004

Generally, fracture of a material is influenced by plastic zone size developed near the crack tip. Hence, according to the relative size of plastic zone in the material, the mechanics as a tool for analyzing the fracture process are classified into three kinds, that is, Linear Elastic Fracture Mechanics, Elastic Plastic Fracture Mechanics, Large Deformation Fracture Mechanics. Even though the plastic zone size is such an important parameter, the practical measurement techniques are very limited and the one for in-situ measurement is not virtually available. Therefore, elastic-plastic FEA has been performed to estimate the plastic zone size. In this study, it is noticed that side necking at the surface is a consequence of plastic deformation and lateral contraction and the relation between the plastic zone and side necking is investigated. FEA for modified boundary layer models with finite thickness, various mode mixities $0^{\circ}$, $30^{\circ}$, $60^{\circ}$, $90^{\circ}$ and strain hardening exponent n=3, 10 are performed. The results are presented and the implication regarding to application to experiment is discussed.

• 한국정밀공학회지
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• 제21권1호
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• pp.71-79
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• 2004

The deformed layers generated in face milling works were comparatively investigated to type 316L and nitrogen(N)-added type 316LN stainless steels. In order to characterize mechanical properties between type 316L and type 316LN, high-temperature tensile tests were conducted with different temperatures: R.T to $700^{\circ}C$. The cutting forces of three components, Fx, Fy and Fz were measured using a tool dynamometer through the face milling cutting tests. The deformed layers were measured by micro-hardness tests along deformed layers. The results of mechanical properties showed that type 316LN was superior to type 316L. The deformed layers of two steels were generated in the 1501m-3001m ranges, and type 316L was higher than type 316LN. The reason for this is due to the high strength properties by nitrogen effect. It was found that deformed structures were well observed for type 316L, but were minutely observed for type 316LN in this cutting conditions.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2004년도 춘계 학술발표회 제16권1호
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• pp.734-737
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• 2004

The synthetic fibers such as polypropylene(PP) and polyvilyl-alcohol(PVA) fiber are poised as a low cost alternative for reinforcement in structural applications. It has been reported that synthetic fiber in cement composites can control restrained tensile stresses and cracks and increase toughness, resistance to impact, corrosion, fatigue and durability. High performance fiber reinforced cementitious composite(HPFRCCs) shows ultra high ductile behavior in the hardened state, because of the fiber bridging properties. Therefore, a variety of experiments have being performed to access the performance of HPFRCCs recently. The research emphasis is on the flexural behavior of HPFRCCs made in synthetic fibers, and how this affects the composite property, and ultimately its strain-hardening performance. Three-point bending tests on HPFECCs are carried out. As the result of the bending tests, HPFRCCs showed high flexural strength and ductility. HPFRCCs made in PVA or Hybrid fiber were, also, superior to PP of singleness. On the other hand, effect of sand volume fraction on HPFRCCs made in PP was insignificant.

• 동력기계공학회지
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• 제15권4호
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• pp.19-24
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• 2011

The operational efficiency of domestic gas turbine is about 25% and it is now in the trend of the gradual growth in spite of the severe temperature, frequent starting and shutdown according to the environmental management and the energy-efficient use. Rotor bolts of gas turbine in power plants have been the cause of defects because these gas turbines have been operated for a long time under the high pressure and temperature environment experiencing the aging change and stress concentration of the bonded part. The connection parts of the bolt revealed various failure shape and these parts were elongated under very low pressure when operated in the relaxed condition. The cause is in the lack of the metal distribution in the bottle lack area and the cap screw of the bolt is broken totally in case that the nut is fastened in most cases. Gas turbine rotor bolts are connected to the rotor wheel and these bolts caused the vibration, the bulk accident of the rotor in the event that the coupling power among these bolts was relaxed. Therefore, we would like to evaluate the soundness of the main part of the gas turbine rotor bolt through the measurement of the inner condition change along with the mechanic deterioration and temperature, stress in the gas turbine rotor material.

• 대한기계학회논문집A
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• 제20권7호
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• pp.2246-2256
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• 1996

To investigate the effect of material and precess variables on stamping formability of sheet materials, simulations for the cup drawing and the Yoshida buckling test were carried out using ABAQUS, commercial nonlinear finite element analysis code. The various factor effects on stamping formability of sheet materials were analyzed by the designed process according to Taguch's orthogonal array experiment. Cup drawing simulation showed that local neckling was very sensitive to plastic anisotropy parameter of sheet material and friction coefficient between sheet and tool interface. Simulations for the Yoshida buckling test have clarified that buckling behaviour of sheet material was mostly susceptible to yield stress and sheet thickness mostly. However, plastic anisotropy parameter and strain hardening coefficient affect moderately buckling behaviour of steel sheets after the buckling initiation.

• 한국정밀공학회지
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• 제29권10호
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• pp.1137-1143
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• 2012

Creep characteristic is an important failure mechanism when evaluating engineering materials that are soft material as polymers or used as mechanical elements at high temperatures. One of the popular thermo-plastic polymers, Acrylonitrile Butadiene Styrene (ABS) which is used broadly for machine elements material, as it has excellent mechanical properties such as impact resistance, toughness and stiffness compared to other polymers, was studied for creep characteristic at different levels of stress and temperatures. From the experimental results, the creep limit of ABS at room temperature is 80 % of tensile strength which is higher than PE and lower than PC or PMMA. Also the creep limits decreased to linearly as the temperatures increased, up to $80^{\circ}C$ which is the softening temperature of Butadiene ($82^{\circ}C$). Also the secondary stage of creep among the three creep stages for different levels of stress and temperature was non-existent which occurred for many metals by strain hardening effect.

• 소성∙가공
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• 제14권1호
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• pp.71-77
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• 2005

Thermal stability and dry sliding wear behavior of ultra-fine grained 6061 Al alloy fabricated by an accumulative roll-bonding (ARB) process have been investigated. After 4 ARB cycles, an ultra-fine grained microstructure of the 6061 Al alloy composed of grains with average size of 500nm, and separated mostly by high-angle boundaries was obtained. Though hardness and tensile strength of the ARB processed Al alloy increased with ARB cycles up to 4 cycles, the processed alloy exhibited decreased ductility and little strain hardening. Thermal stability of the ARB-processed microstructure was studied by annealing of the severely deformed alloy at $423K{\sim}573K$. The refined microstructure of the alloy remained stable up to 473K, and the peak aging treatment of the alloy at 450K for 8 hrs increased the thermal stability of the alloy. Sliding-wear rates of the alloy increased with the number of ARB cycles in spite of the increased hardness with the cycles. Wear mechanisms of the ultra-fine grained alloy were investigated by examining worn surfaces, wear debris, and cross-sections by a scanning electron microscopy (SEM).

• 열처리공학회지
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• 제3권4호
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• pp.1-9
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• 1990

In this study, the ausformed martensite cooled to $-196^{\circ}C$ with various deformation degrees in Fe-30%Ni-0.24%C alloy was transformed to reversed austenite at $500^{\circ}C$ by cyclic reverse martensitic transformation. The effects of prior deformation and the number of cyclic reverse transformation on the microstructure and the mechanical properities of reversed anstensite were investigated. Experimental results showed that the strength of reversed austenite was higher than that of original austenite. This is due to higher dislocation density and grain refining. The reversed austenite formed from ausformed martensite was highly strengthened by prior deformation. This strengthening effect of reversed austenite is attributed to higher dislocation density than grain fefining. The yield strength of reversed austenite below 30% prior deformation, but above 30% prior deformation the strength of reversed austenite is lower than that of deformed austenite. This is due to partly disappearance of strain hardening effect at higher deformation degree by reverse transformation. The strength of reversed austenite is increased with the number of cyclic transformation. Especially, it is principally strengthened by the first cyclic transformation and shows higher increase in yield strength than that of ultimate tensile strength.