• 한국지반공학회논문집
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• 제18권4호
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• pp.55-64
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• 2002

본 논문에서는 점탄-점소성 구성모델을 다층지반에서의 대형지진 발생시의 조건에 적용하기 위해 일본 고베 포트아일랜드에서 발생한 1995 Hyogoken Nanbu 지진에 대한 지진응답 해석을 수행하였다. 지진응답해석 결과 점성토의탄-점소성 모델과 점탄-점소성 모델로 계산된 가속도 기록은 포트아일랜드에서 계측된 가속도 기록과 거의 일치함을 알 수 있었으며, 점성토 지반 부근에서 점탄-점소성 모델과 탄-점소성 모델은 미세하게 다른 거동 특성을 나타내어 점소성 모델의 타당성을 확인하였다. 따라서 동적 점탄-점소성 구성모델은 대변형률 영역에서 점성토의 소성변형을 유발하는 대형 지진 등의 발생시 점성토의 증폭 및 감쇠특성의 파악을 위해 적용가능한 모델임이 입증되었다.

• Steel and Composite Structures
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• 제5권5호
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• pp.359-374
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• 2005

This paper presents the experimental and theoretical results of the viscoplastic response and collapse of 316L stainless steel tubes subjected to cyclic bending. The tube bending machine and curvature-ovalization measurement apparatus, which was designed by Pan et al. (1998), were used for conducting the cyclic curvature-controlled experiment. Three different curvature-rates were controlled to highlight the characteristic of viscoplastic response and collapse. Next, the endochronic theory and the principle of virtual work were used to simulate the viscoplastic response of 316L stainless steel tubes under cyclic bending. In addition, a proposed theoretical formulation (Lee and Pan 2001) was used to simulate the relationship between the controlled cyclic curvature and the number of cycles to produce buckling under cyclic bending at different curvature-rates (viscoplastic collapse). It has been shown that the theoretical simulations of the response and collapse correlate well with the experimental data.

• Computers and Concrete
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• 제8권4호
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• pp.371-388
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• 2011

In this paper, time-continuous constitutive equations for strain rate-dependent materials are presented first, among which those for the overstress and the consistency viscoplastic models are considered. By allowing the stress states to be outside the yield surface, the overstress viscoplastic model directly defines the flow rule for viscoplastic strain rate. In comparison, a rate-dependent yield surface is defined in the consistency viscoplastic model, so that the standard Kuhn-Tucker loading/unloading condition still remains true for rate-dependent plasticity. Based on the formulation of the consistency viscoplasticity, a computational elasto-viscoplastic constitutive model is proposed for the short fiber-reinforced fresh cementitious paste for extrusion purpose. The proposed constitutive model adopts the von-Mises yield criterion, the associated flow rule and nonlinear strain rate-hardening law. It is found that the predicted flow stresses of the extrudable fresh cementitious paste agree well with experimental results. The rate-form constitutive equations are then integrated into an incremental formulation, which is implemented into a numerical framework based on ANSYS/LS-DYNA finite element code. Then, a series of upsetting and ram extrusion processes are simulated. It is found that the predicted forming load-time data are in good agreement with experimental results, suggesting that the proposed constitutive model could describe the elasto-viscoplastic behavior of the short fiber-reinforced extrudable fresh cementitious paste.

• Geomechanics and Engineering
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• 제17권2호
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• pp.119-131
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• 2019

Time dependent creep settlements are one of the most important causes of material deteriorations for the huge water structures such as concrete faced rockfill dams (CFRDs). For this reason, performing creep analyses of CFRDs is vital important for monitoring and evaluating of the future and safety of such dams. In this study, it is observed how changes viscoplastic behaviour of a CFR dam depending the time. Ilısu dam that is the longest concrete faced rockfill dam (1775 m) in the world is selected for the three dimensional (3D) analyses. 3D finite difference model of Ilısu dam is modelled using FLAC3D software based on the finite difference method. Two different special creep material models are considered in the numerical analyses. Wipp-creep viscoplastic material model and burger-creep viscoplastic material model were rarely used for the creep analyses of CFRDs in the last are taken into account for the concrete slab and rockfill materials-foundation, respectively. Moreover, interface elements are defined between the concrete slab-rockfill materials and rockfill materials-foundation to provide interaction condition for 3D model. Firstly, dam and foundation are collapsed under its self-weight and static behaviour of the dam is evaluated for the empty reservoir conditions. Then, reservoir water is modelled considering maximum water level of the dam and time-dependent creep analyses are performed for maximum reservoir condition. In this paper, maximum principal stresses, vertical-horizontal displacements and pore pressures that may occur on the dam body surface during 30 years (from 2017 to 2047) are evaluated in detail. According to numerical analyses, empty and maximum reservoir conditions of Ilısu dam are compared with each other in detail. 4 various nodal points are selected under the concrete slab to better seen viscoplastic behaviour changes of the dam and viscoplastic behaviour differences of these points during 30 years are graphically presented. It is clearly seen that horizontal-vertical displacements and principal stresses for maximum reservoir condition are more than the empty reservoir condition of the dam and significant pore pressures are observed during 30 years for maximum reservoir condition. In addition, horizontal-vertical displacements, principal stresses and pore pressures for 4 nodal points obviously increased until a certain time and changes decreased after this time.

• 한국농공학회논문집
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• 제53권3호
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• pp.19-26
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• 2011

In order to estimate the viscous effects of clay over a wide range of strain levels, we confirmed the performance of a viscoelastic-viscoplastic constitutive model by simulating cyclic undrained triaxial tests to determine the cyclic strength and deformation characteristics of natural marine clay. The viscoelastic-viscoplastic constitutive model was then incorporated into an effective stress-based seismic response analysis to estimate the effects of an intermediate clay layer on the behavior of sand layers. Seismic response was simulated by the cyclic viscoelastic-viscoplastic constitutive model created with data recorded at Rokko Island, Kobe, Japan. The results show that a cyclic viscoelastic-viscoplastic constitutive model can provide a good description of dynamic behavior including viscoelastic effects, within a small strain range.

• 대한기계학회논문집A
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• 제29권2호
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• pp.253-261
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• 2005

In decades, a substantial body of work on a unified viscoplastic model which considers the mechanism of plastic deformation and creep deformation has developed. The systematic scheme for numerical analysis of unified model is necessary because the dominant failure mechanism is the defect growth and coalescence in materials. In the present study, the unified viscoplastic model for materials with defects suggested by Suquet and Michel was employed for numerical analysis. The constitutive equations are integrated based on the generalized mid-point rule and implemented into a finite element program (ABAQUS) by means of user-defined subroutine (UMAT). To evaluate the validity of the developed UMAT code and the assessment of the adopted viscoplastic model, the results obtained from the UMAT code was compared with the numerical reference solution and experimental data. The unit cell analysis also has been investigated to study the effect of strain rate, temperature, stress triaxiality and initial defect volume fraction on the growth and coalescence of the defect.

• Structural Engineering and Mechanics
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• 제7권5호
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• pp.457-471
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• 1999

In this paper, different curvature-rates are controlled to highlight the characteristic of viscoplastic response in cyclic bending tests. The curvature-ovalization apparatus, which was designed by Pan et al. (1998), is used for conducting the curvature-controlled experiments on thin-walled tubular specimens for AISI 304 stainless steel under cyclic bending. The results reveals that the faster the curvature-rate implies, the fast degree of hardening of the metal tube. However, the ovalization of the tube cross-section increases when the curvature-rate increases.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1998년도 가을 학술발표논문집(II)
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• pp.468-473
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• 1998

The objective of this paper is to develop a consistent algorithm for the finite element analysis for behavior of concrete under cyclic loading using viscoplastic-damage model. For modeling the behavior of concrete under cyclic loading, consistent algorithms of rate-dependent viscoplastic-damage are employed with a Willam-Warnke 5-parameter failure criterion which can consider the softening behavior of concrete and consistent tangent moduli are derived. Using finite element program implemented with the developed algorithms, the algorithms are verified and the behaviors of concrete under cylic loading are simulated and compared with experimental data.

• 한국지반공학회논문집
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• 제15권6호
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• pp.17-28
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• 1999

본 연구에서는 자연점토의 압밀에 대한 변형률속도의 영향을 해석하기 위하여 비선형 점탄소성 모델을 제안하였다. 연약지반의 점소성 거동은 유일한 유효응력-변형률-변형률속도 (equation omitted)의 관계식으로부터 구할 수 있다. Berthierville 점토에 대한 크리프실험, 다단계 하중실험, Relaxation실험과 같은 실내실험결과와 수치해석결과를 비교하였다. 제안된 점탄소성 모델을 사용하여 자연점토의 압밀거동을 비교적 정확히 예측할 수 있다.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2011년도 춘계학술대회 논문집
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• pp.191-192
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• 2011