• 지구물리와물리탐사
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• 제7권1호
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• pp.45-50
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• 2004

This paper describes a method of detecting formation boundaries, and permeable fractures, from frequency-domain Stoneley wave logs. Field data sets were collected between the depths of 330 and 360 m in well EE-4 in the Higashi-Hachimantai geothermal field, using a monopole acoustic logging tool with a source central frequency of 15 kHz. Stoneley wave amplitude spectra were calculated by performing a fast Fourier transform on the waveforms, and the spectra were then collected into a frequency-depth distribution of Stoneley wave amplitudes. The frequency-domain Stoneley wave log shows four main characteristic peaks at frequencies 6.5, 8.8, 12, and 13.3 kHz. The magnitudes of the Stoneley wave at these four frequencies are affected by formation properties. The Stoneley wave at higher frequencies (12 and 13.3 kHz) has higher amplitudes in hard formations than in soft formations, while the wave at lower frequencies (6.5 and 8.8 kHz) has higher amplitudes in soft formations than in hard formations. The correlation of the frequency-domain Stoneley wave log with the logs of lithology, degree of welding, and P-wave velocity is excellent, with all of them showing similar discontinuities at the depths of formation boundaries. It is obvious from these facts that the frequency-domain Stoneley wave log provides useful clues for detecting formation boundaries. The frequency-domain Stoneley wave logs are also applicable to the detection of a single permeable fracture. The procedure uses the Stoneley wave spectral amplitude logs at the four frequencies, and weighting functions. The optimally weighted sum of the four Stoneley wave spectral amplitudes becomes almost constant at all depths, except at the depth of a permeable fracture. The assumptions that underlie this procedure are that the energy of the Stoneley wave is conserved in continuous media, but that attenuation of the Stoneley wave may occur at a permeable fracture. This attenuation may take place at anyone of the four characteristic Stoneley wave frequencies. We think our multispectral approach is the only reliable method for the detection of permeable fractures.

• 한국세라믹학회지
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• 제29권10호
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• pp.806-814
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• 1992

The aim of this study was to investigate the change in compressive strength, freacture behavior and degradation of piezoelectric properties with compressive cyclic loading in Pb(Zr, Ti)O3 of tetragonal, morphotropic phase boundary and rhombohedral composition. The highest compressive strength was found in rhombohedral composition. After poling treatment the strength increased by 8.4% and 6.5% in tetragonal and morphotropic phase boundary compositions respectively while changed little in rhombohedral. The increase of compressive strength after poling treatment is believed to be due to the internal stress around grain boundary by domain alginment toward electric field direction in the microstructures having tetragonality and the occurrence of domain switching to the direction perpendicular to electrical field during fracture. Fracture mode relatively change from transgranular to intergranular was observed in the large grain sized tetragonal and morphotropic phase boundary compositions before and after poling but the transgranular fracture mode always remained in the rhombohedral composition. From the X-ray diffractometer analysis the domains parallel to the electric field direction is known to undergo rearrangement during the cyclic loading into random direction that is responsible for the degradation of piezoelectric property.

• The Korean Journal of Ceramics
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• 제1권4호
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• pp.197-203
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• 1995

Nearly fully dense PZT samples both with tetragonal and with morphotropic phase boundary compositions were prepared by the conventional powder processing and sintering. A micro-indentation technique was used to evaluate the dependence of fracture toughness on remanent polarization, crack length and the direction of crack propagation. The result shows that the toughness increases with the remanent polarization along the poling direction and decreases in the transverse direction. The dependence of toughness on the remanent polarization is neither symmetric nor linear but rather shown to be saturated quickly with the increase in remanent polariztion. R-curve behaviors are observed in both poled and unpoled samples. Sequential SEM and XRD studies on annealed, poled, ground, fractured and etched samples show that domain switching is evident as a viable toughening mechanism but might depend upon the rate of crack propagation. Grain bridging is also observed as one of the active toughening mechanisms.

• 한국세라믹학회지
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• 제34권8호
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• pp.877-885
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• 1997

The dependence of strength, crack growth, fracture mode and degree of domain rearrangement of PZT ceramics on poling strength were studied. The PZT [(Pb0.94Sr0.06)(Zr0.46Ti0.54)O3+Nb(trace)] specimens were poled at 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 kv/mm, and the strength of the specimens was measured by 3 point flexure system. The bending strength of the specimen decreased in different modes according to the bending directions; xz, zx and yz plane direction with x axis of the poling direction in Cartesian coordinate system. The strength differences between the directions increased as the poling strength increased. The fracture mode transferred to intergranular fracture mode from transgranular one as the poling strength increased. The mechanical breakdown occurred when the poling strength higher than 3 kV/mm was applied to the specimen. It was observed that the crack length increased in the normal direction to the poling direction, however, decreased in the parallel direction to the poling direction when the poled PZT specimen was indented by the Vickers indenter. However, the crack produced by indentation continuously was continuously increased little by little after indentation on the specimen. The domain rearrangement occurred as the poling strength increased and the domains were rearranged more effectively when the electric field was continuously increased little by little.

• 한국정밀공학회지
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• 제22권2호
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• pp.120-125
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• 2005

Effect of transverse electric fields on fracture behavior in ferroelectric ceramics under purely electrical loading is investigated. It is shown that the shape and size of the domain switching zone depend strongly on the ratio of the transverse electric field to the coercive electric field as well as the direction of the applied electric field. Under small-scale conditions, the crack-tip mode I and II stress intensity factors induced by ferroelectric domain switching are numerically obtained. The crack kinking in ferroelectric ceramics is also discussed.

• 한국해양공학회지
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• 제34권3호
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• pp.155-166
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• 2020

Ductile fracture prediction is critical for the reasonable damage extent assessment of ships and offshore structures subjected to accidental loads, such as ship collisions and groundings. A fracture model combining the Hosford-Coulomb ductile fracture model with the domain of solid-to-shell equivalence model (HC-SDDE), was used in fracture simulations based on shell elements for the punching fracture experiments of unstiffened and stiffened panels. The flow stress and ductile fracture characteristics of JIS G3131 SPHC steel were identified through tension tests for flat bar, notched tension bar, central hole tension bar, plane strain tension bar, and pure shear bar specimens. Punching fracture tests for unstiffened and stiffened panels are conducted to validate the presented HC-DSSE model. The calibrated fracture model is implemented in a user-defined material subroutine. The force-indentation curves and final damage extents obtained from the simulations are compared with experimental results. The HC-DSSE fracture model provides reasonable estimations in terms of force-indentation paths and residual damage extents.

• 방사성폐기물학회지
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• 제10권3호
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• pp.151-159
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• 2012

선진핵주기 고준위폐기물 처분시스템의 개념설계를 위하여 가상의 처분장 부지인 KURT 시설 부지의 지질조건에서 A-KRS의 입지 후보영역을 선정하였다. 부지의 모암은 한반도에 폭넓게 분포하는 중생대 화강암을 대표하는 것으로 열수변질작용을 받은 흔적이 있으며, 지표수와 지하수계는 일차적으로 지형의 영향을 받아 부지에서 남동진하여 금강으로 배출된다. 부지 내에서 확인된 단열대는 2 등급 규모로서 N-S와 E-W 주향으로 우세하게 분포한다. A-KRS 입지 후보영역을 제안하기 위하여 부지 내에서 공간적으로 -500 m 심도까지 발달되는 것으로 예상되는 단열대를 교차하지 않고 동시에 단열대로부터 50 m 이상의 충분한 이격거리를 갖는 조건에서 처분장 규모의 영역을 확보할 수 있는지를 분석하였다. 분석 결과, 본 부지의 중앙부에 우세하게 분포하는 남북 방향의 주향을 갖는 단열대의 서쪽 영역의 -200 m 이하 심도에서 충분한 영역을 확보할 수 있는 것으로 확인되었다. 단열대의 분포 특성을 감안할 때 부지의 좌하단 영역이 지질학적, 수리지질학적 측면에서 A-KRS 입지 영역으로 가장 양호한 것으로 판단된다.

• Nuclear Engineering and Technology
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• 제30권3호
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• pp.245-261
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• 1998

In order to validate the variable aperture channel model that can deal with the non-uniform How rate in flow domain, migration experiments of conservative tracer were performed in two artificial fractures, a parallel and a wedge-shaped fracture. These different fracture shapes were designed to give different flow pattern. The fractures were made from a transparent acrylic plastic plate and a granite slab with dimensions of 10 $\times$ 61 $\times$ 61 cm. Uranine (Fluorescein sodium salt) was used as a conservative tracer. The volumetric flow rates of uranine feed solution were 30 mL/ hr, giving a mean residence time in the fracture of approximately 24 hours for the parallel fracture and 34 hours for the wedge-shaped fracture. The migration plumes of uranine were photographed to obtain profiles in space and time for movement of a tracer in fractures. The photographed migration plume was greatly affected by the geometric shape of fractures. The variable aperture channel model could have predicted the experimental results for the parallel fracture with a large accuracy. It is expected that the variable aperture channel model would be effective to predict the transport of the contaminant, especially, with the flow rate variation in a fracture.

• 터널과지하공간
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• 제30권1호
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• pp.15-28
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• 2020

본 연구는 개별요소법을 이용하여 삼차원 불연속절리망 시스템의 강도 및 변형계수를 추정하기 위해 제안된 기법을 소개하였다. DFN(discrete fracture network) 시스템에서 개별 절리는 유한 길이의 정사각 평면으로 취급하였다. 해석영역은 무결암과 유사한 거동을 하도록 설정된 가상절리와 실제 개별 절리의 조합으로 형성된 다면체로 이산화하였다. 제안된 기법의 적용성을 검토하기 위하여 확정적 및 추계론적 삼차원 DFN 시스템으로 이루어진 한 변이 10m인 정육면체 해석영역에 대하여 개별요소법에 의한 강도 및 변형계수를 추정하는 수치실험이 수행되었다. 또한, 본 연구는 절리의 기하학적 속성이 DFN 시스템의 강도 및 변형 특성에 미치는 영향을 살펴보았다. 제안된 기법은 삼차원 DFN 시스템의 이방적 강도 및 변형 특성을 효과적으로 추정하는 것으로 평가되었다.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2006년도 정기 학술대회 논문집
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• pp.357-364
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• 2006

The meshfree method is extended by the local partition of unity method to model the cohesive cracks in two dimensional continuum The shape function of a particle whose domain of influence is completely cut by a crack is enriched by the step enrichment function. If the domain of influence contains a crack tip inside, it is enriched by the branch enrichment function without the stress singularity. It is found that this method is more accurate and converges faster than the meshless methods for LEFM cracks based on the visibility concept Several staic and dynamic examples are solved to verify the method.