• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2009년도 춘계 학술발표회
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• pp.940-951
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• 2009

A comparative study on dynamic and static measurement of initial stiffness was conducted. Because soil stiffness decreases even at very small strains, the initial stiffness has been measured by dynamic tests using shear wave velocity measurement. On the other hand, due to the advance of local strain measurement, the triaxial testing device is capable of measuring the static initial stiffness. It has been known that initial stiffness measured by static triaxial tests is generally lower than that measured by dynamic tests possibly due to the limitation of static measurement of displacement at very small strains. This study presents experimental results indicating that the elastic shear moduli could be the same both in dynamic and static measurements owing to the soil anisotropy induced by anisotropic stresses.

• 한국지반공학회논문집
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• 제24권5호
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• pp.117-127
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• 2008

본 논문에서는 불포화 상태로 존재하는 다짐 지반에 대하여 함수특성곡선의 획득뿐만 아니라 다양한 모관흡수력 변화에 따른 탄성계수 평가를 위해 종래의 압력판 추출 시험기(VPPE)를 개선하였다. 이를 위해, 비파괴 시험이 가능한 벤더엘리멘트(BE) 시험 시스템을 도입하여 전단파 속도와 최대 전단탄성계수를 평가하였다. 개발된 시험 시스템을 이용하여 시공 중인 국내 도로 현장에서 채취된 3종의 노상토에 대해 모관흡수력 및 함수비 변화에 따른 최대 전단탄성계수의 변화를 평가하였다. 또한 시험결과를 기존의 모관흡수력 조절 비듦전단시험 및 현장 탄성파 시험으로부터 획득한 결과와 비교함으로써 개발된 시험 시스템의 타당성을 검증하였다. 본 연구에서 제안된 새로운 VPPE-BE 시스템은 향후, 탄성계수의 계절적 변화 모델 개발을 위한 도구로서의 효율적인 적용이 가능함을 확인하였다

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 추계학술대회 논문집
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• pp.71-74
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• 2005

In manufacturing automotive parts, such as engine cradles, frame rails, subframes, cross-members, and other parts from circular tubes, pre-bending and pre-forming operations are often required prior to the subsequent tubular hydroforming process. During some pre-forming operations, the cross section of a bent circular tube is crushed into an oval-like shape to ensure proper geometry and sufficient clearance in the hydroforming dies. For such applications, the use of oval Instead of circular tubes could be an effective means of eliminating the pre-forming step. The oval tube could also be produced with less thinning and with less strain on the outside of the bend when controlled by a booster system without the use of mandrel. Hence, the understanding of the issues that occur in the bending of oval tubes is worthy of Investigation. This paper presents parametric studies on the bending of oval tubes without a mandrel. The finite element modeling technique is used to examine the deformation characteristics for both circular and oval tubes. In the simulations, the bending process parameters of bend radius, aspect ratio of the tube ovalness, and tube wall thickness are varied. Observations are made to obtain a hoop-buckle limit diagram in terms of a non-dimensional shape degradation factor. Suggestions based upon developed criteria are made on the acceptability of bend tubes suitable for hydroforming applications without the need ofa pre-forming step or the used of a mandrel.

• Geomechanics and Engineering
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• 제34권3호
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• pp.317-328
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• 2023

The soil behaviour can be represented by numerical modelling of element testing using diverse constitutive models. However, not all constitutive models allow the simulation of the stress-strain response at the critical state in granular soils with both contractive and dilative behaviour. Moreover, the accuracy of these models depends highly on the quality of the experimental data used for their calibration. This study addresses the modelling of the critical state behaviour of an alluvial natural soil from the Lower Tagus Valley (south of Portugal), known as TP-Lisbon sand, using the NorSand constitutive law. For this purpose, a series of numerical simulations of element testing was carried out using two algorithms performed in Visual Basic (VB) and Fast Lagrangian Analysis of Continua (FLAC). Moreover, this study presents the characterisation of of NorSand parameters from an accurate experimental programme based on triaxial and bender element testing. This experimental program allowed defining: (i) the critical state locus, (ii) the stress-dilatancy, and (iii) the soil elasticity of TP-Lisbon sand -all fundamental to calibrate the contractive and dilative behaviour of such alluvial soil. The results revealed a good agreement between experimental data and NorSand simulations using VB and FLAC. Therefore, this study showed that the quality of laboratory testing procedures and its good interpretation enables NorSand constitutive law to capture representatively the non-associated plastic strains, often expressed by the state parameter, allowing a representation of soil behaviour of alluvial soils within the critical state soil mechanics framework for different state parameters.

• Geomechanics and Engineering
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• 제12권5호
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• pp.739-752
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• 2017

This study was intended to evaluate the improved strength of the ground by applying the bio grouting method to a loose sandy ground. The injection material was prepared in the form of cement-like powder, with the bio injection material produced by microbial reactions. The grouting test was conducted under the conditions similar to the field where the bio injection material can be applied. In addition, the injection materials (cement and sodium silicate No. 3) used for Labile Waterglass (LW) method and the conventional grouting methodwere prepared through a two-solution one-step process. The injection into the specimens was done at a pressure of 150 kPa and then, with a bender element, their moduliof elasticity were measured on the 7th, 14th, 21st and 28th curingdays to analyze their strengths according to the duration of curing. It was confirmed that in all injection materials the moduli of elasticity increased over time. In particular, when 30% of the bio injection material was added to 100% cement, the modulus of elasticity tended to increase by about 15%. This confirmed that the applicability became higher when the bio injection material was used in place of the conventional sodium silicate.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 추계 학술발표회
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• pp.192-200
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• 2010

Korea is part of a region of low or moderate seismic zone in which few earthquakes have been monitored, so it is difficult to approve design ground motions and seismic responses on structures from response spectrum. In this study, a series of dynamic centrifuge model tests for demonstrating seismic amplification characteristics in soil-foundation-structure system were performed using electro-hydraulic shaking table mounted on the KOCED 5.0 m radius beam centrifuge at KAIST in Korea. The soil model were prepared by raining dry sand and $V_S$ profiles were determined by performing bender element tests before shaking. The foundation types used in this study are shallow embedded foundation and deep basement fixed on the bottom. Total 7 building structures were used and the response of building structures were compared with response spectrum from the acceleration records on surface.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2008년도 춘계 학술발표회 초청강연 및 논문집
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• pp.442-451
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• 2008

The evaluation of field degree of consolidation on soft clays has been an important problem in geotechnical areas. Monitoring either settlements or pore water pressures has been widely applied in the filed, but occasionally they have some problems. This study addresses the suggestion and application of another method for evaluating the degree of consolidation using shear wave velocities. A research site where soft clay layers were consolidated by surcharging loads was chosen. Laboratory tests were performed to determine the relation between shear wave velocity and effective stress. Field seismic tests were conducted several times during the consolidation of the clay layers. The tests results show that the shear wave velocity increased significantly as clays consolidated. The shear wave velocities at each field stress states were derived from the laboratory results and the degree of consolidation was evaluated by comparing the shear wave velocities obtained by laboratory and field seismic methods. In most stress states, the degree of consolidation evaluated using the shear wave velocity matched well with that obtained from field settlement record, showing the potential of applying the method using shear waves in the evaluation of field degree of consolidation on soft clay deposits.

• 한국기계가공학회지
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• 제16권1호
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• pp.70-76
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• 2017

The pollution problem of fossil energy sources has caused the development of green energy harvesting systems. Piezoelectric energy harvesting technology has been developed under those external environmental factors. A piezoelectric energy harvester can be defined as a device which transforms mechanical vibration or impact energy into electrical energy. Most researches have focused on bender structures. However, these have a limitation on energy efficiency because of the small effective electromechanical coupling factor, around 10%. Therefore, we should look for a new design for energy harvesting. A cymbal energy harvester can be a good candidate for the high-power energy harvester because it uses a high amplification mechanism using endcaps while keeping a higher electromechanical coupling factor. In this research, we focused on energy efficiency improvements of the cymbal energy harvester by changing the polarization direction, because the electromechanical coupling factor of the k33 mode and the k15 mode is larger than that of the k31 mode. Theoretically, we checked the cymbal harvester with radial polarization and it could obtain 6 times larger energy than that with the k31 direction polarization. Furthermore, we verified the theoretical expectation using the finite element method program. Consequently, we could expect a more efficient cymbal harvester with the radial polarization by comparing two polarization directions.

• 한국정밀공학회지
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• 제22권7호
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• pp.164-169
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• 2005

The micro-gripper system is one of the systems that should be improved in the respect of performance for practical usage. In the previous works, the important issues are considered and presented using axiomatic design approach. In this paper, the functional requirements and design parameters are evaluated in order to improve the performance and efficiency of the system. The evaluation is a very difficult task since many variables are related to the outcomes. To provide a basis for correct design decisions, axiomatic design principles have been advanced. Since the framework of axiomatic design makes design issues easier to understand when they are analyzed, we used those as an evaluation tool. The object of the system is to handle micro-size parts. Main device is a micro-gripper using two bender-typed and one stack-typed PZTs as actuators. And it has three tips made of tungsten wires fur holding function. Also the system must satisfy other functional requirements for appropriate handling performance. The results of this study show design improvements of micro-gripper system such as structural change of gripper, additional element, and integration of physical parts. Axiomatic design guides presented suitable design parameters corresponding to functional requirements and made the design elements improve through diagrams of whole system.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2009년도 춘계 학술발표회
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• pp.331-340
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• 2009

This study evaluates the relationship between cone tip resistance ($q_c$) and small-strain shear modulus ($G_{max}$) of cemented sand. For this purpose, a series of miniature cone penetration and bender element tests are performed in calibration chamber specimens with various gypsum contents. Experimental results show that both $q_c$ and $G_{max}$ of sand increase with increasing cementation level as well as relative density and vertical confining stress. However, the relative density and vertical confining stress has more significant influence on $G_{max}$ and $q_c$ of uncemented sand than those of cemented sand. It is observed that the $G_{max}/q_c$ ratio of cemented sand decreases with increasing relative density. This result means that state variables have more affect on $q_c$ than $G_{max}$ of cemented sand. Test results also show that the effect of vertical stress on $G_{max}-q_c$ relation is reduced by cementation effect.