• 소성∙가공
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• 제14권1호
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• pp.65-70
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• 2005

A phase mixture model was employed to simulate the deformation behaviour of metallic materials covering a wide grain size range from micrometer to nanometer scale. In this model a polycrystalline material is treated as a mixture of two phases: grain interior phase whose plastic deformation is governed by dislocation and diffusion mechanisms and grain boundary 'phase' whose plastic flow is controlled by a boundary diffusion mechanism. The main target of this study was the effect of grain size on stress and its strain rate sensitivity as well as on the strain hardening. Conventional Hall-Petch behaviour in coarse grained materials at high strain rates governed by the dislocation glide mechanism was shown to be replaced with inverse Hall-Petch behaviour in ultrafine grained materials at low strain rates, when both phases deform predominantly by diffusion controlled mechanisms. The model predictions are illustrated by examples from literature.

• Geomechanics and Engineering
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• 제8권1호
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• pp.33-52
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• 2015

Based on the Mohr-Coulomb failure criterion, a gradient-dependent plastic model that considers the strain-softening behavior is presented in this study. Both triaxial shear tests on conventional specimen and precut-specimen, which were obtained from an ancient landslide, are performed to plot the post-peak stress-strain entire-process curves. According to the test results of the soil strength, which reduces from peak to residual strength, the Mohr-Coulomb criterion that considers strain-softening under gradient plastic theory is deduced, where strength reduction depends on the hardening parameter and the Laplacian thereof. The validity of the model is evaluated by the simulation of the results of triaxial shear test, and the computed and measured curves are consistent and independent of the adopted mesh. Finally, a progressive failure of the ancient landslide, which was triggered by slide of the toe, is simulated using this model, and the effects of the strain-softening process on the landslide stability are discussed.

• Interaction and multiscale mechanics
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• 제1권3호
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• pp.317-328
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• 2008

When carbon-filled rubber specimens are subjected to cyclic loading, they do not return to their initial state after loading and subsequent unloading, but exhibit a residual strain or permanent deformation. We propose a specific form of the pseudo-elastic energy function to represent cyclic loading for incompressible, isotropic materials with stress softening and residual strain. The essence of the pseudo-elasticity theory is that material behaviour in the primary loading path is described by a common elastic strain energy function, and in unloading, reloading or secondary unloading paths by a different strain energy function. The switch between strain energy functions is controlled by the incorporation of a damage variable into the strain energy function. An extra term is added to describe the permanent deformation. The finite element implementation of the proposed model is presented in this paper. All parameters in the proposed model and elastic law can be easily estimated based on experimental data. The numerical analyses show that the results are in good agreement with experimental data.

• Advances in concrete construction
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• 제9권3호
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• pp.327-335
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• 2020

Concrete pipes are considered important structures playing integral role in spread of cities besides transportation of gas as well as oil for far distances. Further, concrete structures under seismic load, show behaviors which require to be investigated and improved. Therefore, present research concerns dynamic stress and strain alongside deflection assessment of a concrete pipe carrying water-based nanofluid subjected to seismic loads. This pipe placed in soil is modeled through spring as well as damper. Navier-Stokes equation is utilized in order to gain force created via fluid and, moreover, mixture rule is applied to regard the influences related to nanoparticles. So as to model the structure mathematically, higher order refined shear deformation theory is exercised and with respect to energy method, the motion equations are obtained eventually. The obtained motion equations will be solved with Galerkin and Newmark procedures and consequently, the concrete pipe's dynamic stress, strain as well as deflection can be evaluated. Further, various parameters containing volume percent of nanoparticles, internal fluid, soil foundation, damping and length to diameter proportion of the pipe and their influences upon dynamic stress and strain besides displacement will be analyzed. According to conclusions, increase in volume percent of nanoparticles leads to decrease in dynamic stress, strain as well as displacement of structure.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2008년도 춘계 학술발표회 초청강연 및 논문집
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• pp.754-761
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• 2008

In general, stress measurement of existent shotcrete lining be used by pressure cells. but, measuring instrument is lost by high pressure at shotcrete lining construction and pressure cell's measurement value have to low believability by natural conditions like curing temperature. In this study, proposed techniques to measure without utilizing sensitive stress sensor in natural condition at point that want stress of shotcrete lining after shotcrete lining construction. Executed numerical analysis to forecast stress level that interact in tunnel shotcrete lining, measured strain of hole by load action through hole in shotcrete lining. 3D FEM(finite element method) is enforced through various parameters curing time of shotcrete lining, thickness, load condition. Different model cases applied by parametic study. As analysis result, it could grasp development possibility of method that propose this time because it could examine corelation with strain by near hole of shotcrete lining and stress about load condition.

• 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.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2008년도 추계학술대회 논문집
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• pp.805-814
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• 2008

This study focused on the resilient modulus prediction model, which is the functions of mean effective principal stress and axial strain, for three types of railroad trackbed materials such as crushed stone, weathered soil, and crushed-rock soil mixture. The model is composed with the maximum Young's modulus and nonlinear values for higher strain in parallel with dynamic shear modulus. The maximum values is modeled by model parameters, $A_E$ and the power of mean effective principal stress, $n_E$. The nonlinear portion is represented by modified hyperbolic model, with the model parameters of reference strain, ${\varepsilon}_r$ and curvature coefficient, a. To assess the performance of the prediction models proposed herein, the elastic response of a test trackbed near PyeongTaek, Korea was evaluated using a 3-D nonlinear elastic computer program (GEOTRACK) and compared with measured elastic vertical displacement during the passages of freight and passenger trains. The material types of sub-ballasts are crushed stone and weathered granite soil, respectively. The calculated vertical displacements within the sub-ballasts are within the order of 0.6mm, and agree well with measured values with the reasonable margin. The prediction models are thus concluded to work properly in the preliminary investigation.

• 소성∙가공
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• 제24권6호
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• pp.395-399
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• 2015

A microstructure-based finite element simulation was conducted to predict the plastic strain ratio (R-value) of a dual-phase (DP) steel. The representative volume elements (RVEs) concept was adopted for the image-based FE modeling and a 3D model was constructed using sequential 2D images. Each phase was considered with the von-Mises yield criterion and the Swift model. The Swift parameters were defined by the empirical equations based on the chemical composition. The developed model was applied to analyze the effect of residual stress on the R-value and stress distribution. In order to consider the residual stress development after cold rolling, 10 % compression was applied in the thickness direction and unloaded before the tensile stress was applied in the rolling direction. The results showed a reasonable prediction for the R-value evolution: a sharp increase at small strains was well described and a transition followed in the downward direction. The R-value evolution was analyzed using the stress distribution change on the π-plane

• 소성∙가공
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• 제12권4호
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• pp.358-363
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• 2003

Adiabatic shear bands have been observed in the serrated chip during high strain rate metal cutting process of medium carbon steel and titanium alloy The recent microscopic observations have shown that dynamic recrystallization occurs in the narrow adiabatic shear bands. However the conventional flow stress models such as the Zerilli-Armstrong model and the Johnson-Cook model, in general, do not predict the occurrence of dynamic recrystallization (DRX) in the shear bands and the thermal softening effects accompanied by DRX. In the present study, a strain hardening and thermal softening model is proposed to predict the adiabatic shear localized chip formation. The finite element analysis (FEA) with this proposed flow stress model shows that the temperature of the shear band during cutting process rises above 0.5Τ$_{m}$. The simulation shows that temperature rises to initiate dynamic recrystallization, dynamic recrystallization lowers the flow stress, and that adiabatic shear localized band and the serrated chip are formed. FEA is also used to predict and compare chip formations of two flow stress models in orthogonal metal cutting with AISI 1045. The predictions of the FEA agreed well with the experimental measurements.s.

• 콘크리트학회논문집
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• 제23권6호
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• pp.765-772
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• 2011

이 연구에서는 알칼리활성 슬래그 콘크리트의 응력-변형률 관계를 평가하기 위한 일련의 콘크리트 실린더의 압축 실험을 요약하였다. 실험된 콘크리트의 압축강도는 8.6 MPa에서 42.2 MPa의 범위이며, 단위용적질량은 $2,168kg/m^3$ 에서 $2,343kg/m^3$의 범위이다. 34개의 콘크리트 시험체에서 얻은 결과들에 근거하여 알칼리활성 슬래그 콘크리트의 응력-변형률 모델을 수학적으로 제시하였다. 콘크리트의 탄성계수, 최대응력 시 변형률 및 곡선의 상승부와 하강부의 기울기는 압축강도와 단위용적질량의 함수로 일반화하였다. 각 시험체에서 측정한 값과 제시된 모델의 예측값 사이에서 산정된 변동계수들의 평균과 표준편차는 각각 6.9%와 2.6%이었다. 따라서 제시된 모델은 보통포틀랜드 시멘트 콘크리트에서 제시된 다른 모델들에 비해 AA 슬래그 콘크리트의 응력-변형률 특성을 보다 더 정확하고 합리적으로 나타내었다.