• 대한기계학회논문집
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• 제19권1호
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• pp.79-86
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• 1995

The structural materials for cryogenic technology have been recently developed to support the many modern large-scale application from superconducting magnets for nuclear fusion reactor, magnetic levitation railway to LNG tankers. However it is pointed out that quenching phenomenon is one of the serious problems for the integrity of these applications, which is mainly attributed to the rapid temperature rising in the material due to some extrinsic factors of structures. From the viewpoint of fracture mechanics, it is therefore very important to clarify the mechanism of temperature rising of structural material due to cyclic loading at cryogenic temperature. From this purpose, fatigue test was carried out for high manganese steel at liquid helium temperature(4.2K) using triangular stress waveform to identify both the mechanism of temperature rising near crack tip and the effect of loading stress waveform on temperature rising near crack tip and the effect of loading stress waveforms on temperature rising. As the results, two types of temperature rising, that is, regular and burst types were observed. And a periodical temperature rising corresponding to the stress waveforms was also found. The peaks of the temperature rising were recorded near both the maximum and the minimum values of the applied stress. The sudden temperature rises, which indicated the higher values than those of periodical temperature rises under the repetition of stress, were observed at the final region of crack growth. It was shown that the peak values of the temperature rising increased with stress intensity factor range.

• 한국재료학회지
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• 제17권6호
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• pp.342-346
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• 2007

Microstructure and tensile strength of the vacuum brazed stainless steel were investigated in this study. For vacuum brazing of the stainless steel 303 and 304, the BNi-2, 3, 4 and 7 were used as filler metals. Among these filler metals, the BNi-2 showed excellent wettability at $1050^{\circ}C$. Indeed, the brazed stainless steel using the BNi-2 showed the highest tensile strength (483 MPa) among all brazed specimens. This is attributed to degree of interfacial reaction between the filler metal and stainless steel. Brazed stainless steel with BNi-2, 3 filler metals showed almost elastic deformation followed by plastic yielding and strain hardening up to a peak stress. On the other hand, it is likely that the fracture of the brazed specimens with BNi-4, 7 was occurred in elastic range without plastic yielding up to a peak stress.

• 한국농공학회지
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• 제33권2호
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• pp.112-119
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• 1991

Some experiments were carried out to investigate the effects of freezing and thawing on the strength and strain characteristics of alluvial silty clay under the different temperatures, loading and moisture conditions. The results were as follows; 1. The soil used was proved to be consisted of silty clay with honey-combed structure, and showed higher dilatancy, frost activity and lower stability in natural state. 2. Soil treated with freezing and thawing cycles showed lower compressive strength compared with the non treated, The strength decreased with incement of freezing and thawing cycles. It's shapes of stress-strain curves were flat and did not formulate a peak while the peak strength of higher moisture content soil decreased with the increment of moisture content. It's decrement ratio was most distinctly shown at the first one cycle of freezing and thawing. 3. The cohesion decreased due to freezing and thawing cycles but internal frcition angle was not changed. 4. The liquid limit decreased with increment of freezing and thawing cycles, and became almost constant after three cycles of freezing and thawing. 5. The strength under simple loading at failure mode was appeared to be higher compared with the cyclic loading after freezing and thawing but initial moisture content effect was not observed. 6. Ice lense was not observed within 50% of ice content ratio but observed over 100%. The higher the ice content ratio, the higher the peak strength. As a matter of fact, it seems that an optimum ice content ratio exists for plastic mode and the least compressive strength.

• 한국지반공학회논문집
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• 제20권5호
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• pp.145-159
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• 2004

본 연구에서는 지진시 지반의 안정성 평가시, 진동시험에 기초하여 액상화 발생가능성 여부를 판정하는 상세평가법을 개발하였다. 개발된 평가법에서는 기존의 평가법이 지진을 단순히 정현하중화하는 등가전단응력개념에 기초한점과는 달리, 지진의 최대가속도, 유효지속시간, 지진형태, 그리고 지진규모 등 다양한 지진영향인자가 고려될 수 있도록 실지진기록 입력의 지반응답해석을 포함하도록 하였다. 지반의 고유한 저항특성을 응력-변형률 시험 결과로부터 액상화 전환시점까지의 누적 소성 전단변형률로 하였으며 이와 연계하여 지진의 액상화 발생특성을 지반응답해석을 통해 획득 가능한 전단변형률 시간이력곡선에 기초하도록 하였다. 이때, 액상화를 유발시키는 실지진기록의 특성분석을 위해 실지진하중 재하의 진동삼축시험을 수행하였다. 시험결과, 충격형 지진인 경우, 지진기록의 최대하중이 재하된 직후, 과잉간극수압이 급진적으로 발전하며 액상화가 발생하는 것으로 나타났으며 진동형 지진의 경우에는 최대하중이 재하된 경우, 눈에 띄는 과잉간극수압의 변화가 관찰되었으며 이후, 일정수준 이상의 큰 하중재하시 액상화가 발생하였다. 이로부터 액상화 발생에 가장 큰 영향인자는 최대하중인 것을 알 수 있었으며 진동형 지진형태의 경우, 일정수준 이상의 후속하중에 대한 고려가 필요함을 알 수 있었다. 이상의 결과로부터 본 평가법에서는 우선적으로 충격형 지진에 한하여 사용할 것을 제안하며 이때, 최대 전단변형률까지의 시간이력곡선으로부터 소성 전단변형률을 누적계산하여 이를 해당입력지진의 액상화 발생특성치로 정하였다. 기존의 등가응력개념에 기초한 상세평가법과의 비교를 통한 타당성 분석결과, 본 평가법은 기존의 상세평가법보다 유효응력경로 및 응력-변형률 상관곡선 등 실제적인 지반거동변화에 관한 진동시험결과에 기초하여 지반의 고유특성을 결정하고 지반응답해석을 통해 증폭현상을 포함한 지반 내 지진거동변화와 지진시간이력이 보유하고 있는 지진특성을 충분히 반영하고 있으므로 신뢰성 높은 액상화 상세평가가 가능할 것으로 판단된다.

• 한국안전학회지
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• 제22권6호
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• pp.27-34
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• 2007

Static or dynamic elastomeric O-ring seals are installed between joining parts, and play key roles of high pressure-tightening. Sealing performance and structural safety of the O-ring are dependent on groove design, plain diameter, squeeze and applications such as pressure and temperature. In this study, to solve O-ring problem squeezed and highly pressurized under laterally one-sided constrained condition, hyperelastic FE analyses are performed, and FE results are compared with measured ones by computer-aided tomography, deformed shape and extrusion depth of the O-ring. Through the comparisons, FE analysis technique was verified. In order to evaluate design sensitivity, Taguchi method was used to select FE analysis cases. Adjustment parameters are clearance gap, groove comer radius, plain diameter and squeeze. By means of verified FE analysis technique, it has been analysed how the parameters have effects on contact stress fields, internal stress fields, and extrusion depths. Sealing performance has been evaluated based on contact stress fields and contact widths, and structural safety on internal stress and strain, extrusion lengths.

• 한국지반공학회지:지반
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• 제12권1호
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• pp.21-34
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• 1996

이 논문은 일련의 실내시험을 통해 얻어진 삼척 유기질토의 공학적 특성을 제시한 것이다. 액성한계는 시료의 조제방법에 따라 그 값이 현저히 달라진다는 사실을 알게 되었는데 자연상태로 시험한 값이 노건조시켜 얻어진 값보다 1.5내지 4배나 컸다. 유기질토의 응력변형특성은 변형의 증가에 따라 응력이 점진적으로 증가하는 경향을 보였다. 즉 변형율 20%의 강도가 한국산업규격에 규정된 변형을 15%의 강도보다 더 크다는 것을 알 수 있었다. 또한 유기질토의 강도증가율(0.43~0.65)은 점토에 비해 현저히 크다는 사실도 알 수 있었다. 압밀시험 결과 연직 및 수평방향의 투수계수는 거의 같으나 교란시에는 연직방향의 1/2~1/6로 감소된다는 사실도 알게 되었다. 또한 2차 압밀계수는 유기물함량과 함수비가 많을수록 커지며 유기물함량 15%를 기점으로 하여 그 이하에서는 변화폭이 완만하나 그 이상에서는 큰 변화폭을 보인다.

• Geomechanics and Engineering
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• 제24권3호
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• pp.215-226
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• 2021

To study the evolution mechanism of cracks in rocks with multiple defects, rock-like samples with multiple defects, such as strip-shaped through-going cracks and cavity groups, are used, and the crack propagation law and changes in AE (acoustic emission) and strain of cavity groups under different inclination angles are studied. According to the test results, an increase in the cavity group inclination angle can facilitate the initial damage degree of the rock and weaken the crack initiation stress; the initial crack initiation direction is approximately 90°, and the extension angle is approximately 75~90° from the strip-shaped through-going cracks; thus, the relationship between crack development and cavity group initiation strengthens. The specific performance is as follows: when the initiation angle is 30°, the cracks between the cavities in the cavity group develop relatively independently along the parallel direction of the external load; when the angle is 75°, the cracks between the cavities in the cavity group can interpenetrate, and slip can occur along the inclination of the cavity group under the action of the shear mechanism rupture. With the increase in the inclination angle of the cavity group, the AE energy fluctuation frequency at the peak stress increases, and the stress drop is obvious. The larger the cavity group inclination angle is, the more obvious the energy accumulation and the more severe the rock damage; when the cavity group angle is 30° or 75°, the peak strain of the local area below the strip-shaped through-going fracture plane is approximately three times that when the cavity group angle is 45° and 60°, indicating that cracks are easily generated in the local area monitored by the strain gauge at this angle, and the further development of the cracks weakens the strength of the rock, thereby increasing the probability of major engineering quality damage. The research results will have important reference value for hazard prevention in underground engineering projects through rock with natural and artificial defects, including tunnels and air-raid shelters.

• Journal of Mechanical Science and Technology
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• 제18권6호
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• pp.895-903
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• 2004

In this paper are presented the blade vibration characteristics at the starting conditions of the low pressure multistage axial compressor of heavy-duty 100 MW gas turbine. Vibration data have been collected through strain gauges during aerodynamic tests of the model compressor. The influences of operating modes at the starting conditions are investigated upon the compressor blade vibrations. The exciting mechanisms and features of blade vibrations are investigated at the surge, rotating stall, and buffeting flutter. The influences of operating modes upon blade dynamic stresses are investigated for the first and second stages. It is shown that a high dynamic stress peak of 120 MPa can occur in the first stage blades due to resonances with stall cell excitations or with inlet strut wake excitations at the stalled conditions.

• Geomechanics and Engineering
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• 제10권2호
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• pp.207-224
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• 2016

Granulated coal ash (GCA), a mixture of the by-product from milling processes with a small amount of cement added, has recently come to be used as a new form of geomaterial. The shear strength and deformation behaviours of GCA are greatly determined by its relative density or void ratio. A series of drained triaxial compression tests were performed on cylindrical specimens of GCA at confining pressures of between 50 kPa and 400 kPa at initial relative densities of 50%, 70% and 80%. Experimental results show that a rise in relative density increases the peak shear strength and intensifies the dilation behaviour. The initial tangent modulus and secant modulus of the stress-strain curve increase with increasing initial relative density, whereas the axial and volumetric strains at failure decrease with level of initial relative density. The stress-dilatancy relationships of GCA at different relative densities and confining pressures display similar tendency. The dilatancy behaviour of GCA is modelled by the Nova rule and the material property N in Nova rule of GCA is much larger than that of natural sand.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2008년도 춘계 학술발표회 제20권1호
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• pp.761-764
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• 2008

본 연구는 40, 60, 80MPa급 고강도콘크리트의 변형특성에 있어서 $20{\sim}700^{\circ}C$ 범위로 상승되는 온도의 영향을 연구하는데 그 목적이 있다. 본 연구에서 시험의 종류는 설계하중 사전재하 및 잔존 강도 시험방법으로서 시험체를 가열하기 전에 극한강도의 25%하중을 사전재하한 후 가열을 실시하고, 가열하는 동안 하중을 유지하며, 목표온도에 도달한 후 고온상태 및 상온에서 24시간 냉각상태에서 시험체가 파괴될 때까지 재하를 실시했다. 시험은 W/B 46%, 32% 및 25%로 이루어진 콘크리트 시험체에 대하여 $20{\sim}700^{\circ}C$의 다양한 온도하에서 실시하였다. 시험결과 콘크리트 강도가 증가할수록 고온에서의 상대적인 탄성계수는 감소하였으며, 최대하중에서의 축방향 변형은 설계하중 사전 재하와 상관성이 높은 것으로 나타났다. 또한 온도상승에 따른 콘크리트의 열팽창변형은 압축강도뿐만 아니라 초기사전재하의 영향을 받는 것으로 나타났다.