• 소성∙가공
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• 제5권2호
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• pp.122-129
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• 1996

The main objective of the study is to offer some basic information in relation to optimal shape and dimensions of the rotating band through the development of three-dimensional finite element method for metal forming analysis of the rotating band whose primary function is to impart spin to the projectile. The three-dimensional metal forming analysis of the rotating band has perfor-med by using recurrent boundary conditions. Such design factors as the outside diameter the total length and the profile of the rotating band must be considered carefully in order to design an optimal rotating band. Above design factors can be determined from such available analysis results as the deformed shape and the deformation load. of the rotating band and the normal pressure of the rotating band on a projectile shell. The remeshings are needed to carry out plastic deformation analysis with severe deformation through which the complete process analysis gets possible. The results can be utilized effectively in determining the optimal shape and size of the rotating band.

• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2008년도 International Symposium on Remote Sensing
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• pp.422-425
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• 2008

TerraSAR-X is new radar satellite operated at X-band, multi polarization, and multi beam mode. Compared with C-band or L-band SAR, the X-band system inherently suffers from more temporal decorrelation, but is more sensitive to surface deformation monitoring due to short wavelength (3.1 cm) and high spatial resolution (1m-3m). It is generally expected that sensitivity to estimate surface movement using TerraSAR-X will be increased by the factor of 10, compared to current C-band system with low spatial resolution such as ERS-2, Envisat. Many urban areas are experiencing land subsidence due to water, oil and natural gas withdrawal, underground excavation, sediment compaction, and so on. Monitoring of surface deformation is valuable for effectively limiting damage areas. In addition high accuracy and spatially dense subsidence map can be achieved by X-band InSAR observation, promoting identification and separation of various subsidence processes and leading to enhanced understanding via mechanical modeling. In this study we will introduce some initial InSAR results using new TerraSAR-X SAR data for surface deformation monitoring.

• 대한기계학회논문집
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• 제18권5호
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• pp.1134-1143
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• 1994

A study of adiabatic shear band formation and propagation of 4340 steel was done using the stepped speciment which was subjected to high velocity impact. The high velocity impact was performed on compression Hopkinson bar impact machine. After the controlled impact, the specimen was prepared for visual inspection. Numerical simulation was also performed with same geometrical dimension using explicit time integration finite element code. Experimental results were then compared with the numerical prediction. It was found that the numerical prediction is quite accurate, average thickness of adiabatic shear band is about $10{\mu}m$, the macro crack around shoulder is due to folding, and the deformation control ring is effective to freeze the propagation of adiabatic shear band.

• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2007년도 Proceedings of ISRS 2007
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• pp.138-141
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• 2007

The permanent scatterer SAR interferometry (PSInSAR) technique has been developed more recently and has been applied to monitor slow but consistent ground subsidence. Since PSInSAR has the advantages in terms of baseline and temporal decorrelation, PSInSAR technique using X-band may also provide useful information about a ground deformation in detail. We developed our codes for a persistent scatterer analysis, and then apply to ERS-1/2 C-band data over Las Vegas in order to validate our new developed algorithm. Based on this test, PS technique using X-band observation such as TerraSAR-X or KOMSAT 5 will be developed.

• 대한기계학회논문집
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• 제17권7호
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• pp.1700-1709
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• 1993

The stepped specimen which is subjected to step loading is modeled to study the initiation and growth of adiabatic shear band using explicit time integration finite element method. Three different clearance sizes are tested. The material model for the stepped specimen includes effects of strain hardening, strain rate hardening and thermal softening. It is found that the material inside the fully grown adiabatic shear band experiences three phase of deformation, (1) homogeneous deformation phase, (2) initiation/incubation phase, and (3) fast growth phase. The second phase of deformation is initiated after sudden shear stress drop which occurs at the same time regardless of the clearance size. The incubation time prior to fast growth phase increases, as the clearance size of the stepped specimen increases. Whereas, after incubation period, the growth rate of the adiabatic shear band decreases, as the clearance size decreases. It is also found that two adiabatic shear band may develop instead of one for the smaller clearance size.

• Geomechanics and Engineering
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• 제5권6호
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• pp.499-517
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• 2013

In order to investigate shear behavior of granular materials due to excavation and associated unloading actions, load-controlled plane strain compression tests under decreasing confining pressure were performed under drained conditions and the results were compared with the conventional plane strain compression tests. Four types of granular material consisting of two quartz sands and two glass beads were used to investigate particle shape effects. It is clarified that macro stress-strain behavior is more easily influenced by stress level and stress path in sands than in glass beads. Development of localized deformation was analyzed using photogrammetry method. It was found that shear bands are generated before peak strength and shear band patterns vary during the whole shearing process. Under the same test condition, shear band thickness in the two sands was smaller than that in one type of glass beads even if the materials have almost the same mean particle size. Shear band thickness also decreased with increase of confining pressure regardless of particle shape or size. Local maximum shear strain inside shear band grew approximately linearly with global axial strain from onset of shear band to the end of softening. The growth rate is found related to shear band thickness. The wider shear band, the relatively lower the growth rate. Finally, observed shear band inclination angles were compared with classical Coulomb and Roscoe solutions and different results were found for sands and glass beads.

• Coupled systems mechanics
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• 제2권2호
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• pp.147-157
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• 2013

We numerically investigate the electronic band structure of carbon nanotubes (CNTs) under radial corrugation. Hydrostatic pressure application to CNTs leads to a circumferential wave-like deformation of their initially circular cross-sections, called radial corrugations. Tight-binding calculation was performed to determine the band gap energy as a function of the amplitude of the radial corrugation. We found that the band gap increased with increasing radial corrugation amplitude; then, the gap started to decline at a critical amplitude and finally vanished. This non-monotonic gap variation indicated the metal-semiconductor-metal transition of CNTs with increasing corrugation amplitude. Our results provide a better insight into the structure-property relation of CNTs, thus advancing the CNT-based device development.

• 한국지반환경공학회 논문집
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• 제14권9호
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• pp.39-47
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• 2013

Local deformations in back filling materials of two sands and one glass bead with different particle shapes behind a rigid retaining wall were studied. Two kinds of boundary conditions were compared: active wall translation and active rotation of the wall about its toe. Effect of the speed of active wall translation was also investigated. The digital image correlation method was used to analyze local deformation developments inside the materials. Test results showed that particle shape and density mainly influence the inclination angle and width of the shear band. The general shear band pattern is strongly dependent on the wall movement mode, while it was little influenced by particle shape. Within a limited range of wall speed in this study, shear band became wider and local deformation became larger with increase of wall speed.

• 한국추진공학회지
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• 제4권1호
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• pp.1-12
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• 2000

유도탄의 성능 향상을 위해 추진기관의 경량화가 요구되고 있으며 연소관 모타 마개의 경우, Ti 합금 (Ti-6Al-4V)이 사용되고 있다. 제작 과정 중에 열간 형단조 공정이 이용되나 이 과정에서 Ti alloy의 좁은 성형 온도 구간 관계로 단조시 국부적 소성 변형 집중에 의한 결함이 단조품의 다수 생성되며, 심한 경우 전단 파단까지 이르고 있다. 성형 과정에서 금형과의 접촉에 의한 Ti 합금 온도 하강이 전단 파단 발생과 밀접한 관계를 갖고 있으며 따라서 각 단조 조건에 따른 Ti 합금의 내부 및 표면에 대한 변형 및 온도의 분포가 관찰되었다. 아울러 전단 밴드 형성에 대한 별도의 분석도 포함되었다.

• 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.