• Computers and Concrete
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• 제7권4호
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• pp.317-329
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• 2010

The paper describes localization of deformation in a bar under tensile loading. The material of the bar is considered as non-linear viscous elastic and the bar consists of two symmetric halves. It is assumed that the model represents behavior of the quasi-brittle viscous material under uniaxial tension with different loading rates. Besides that, the bar could represent uniaxial stress-strain law on a single plane of a microplane material model. Non-linear material property is taken from the microplane material model and it is coupled with the viscous damper producing non-linear Maxwell material model. Mathematically, the problem is described with a system of two partial differential equations with a non-linear algebraic constraint. In order to obtain solution, the system of differential algebraic equations is transformed into a system of three partial differential equations. System is subjected to loadings of different rate and it is shown that localization occurs only for high loading rates. Mathematically, in such a case two solutions are possible: one without the localization (unstable) and one with the localization (stable one). Furthermore, mass is added to the bar and in that case the problem is described with a system of four differential equations. It is demonstrated that for high enough loading rates, it is the added mass that dominates the response, in contrast to the viscous and elastic material parameters that dominated in the case without mass. This is demonstrated by several numerical examples.

• 소성∙가공
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• 제11권5호
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• pp.430-438
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• 2002

This paper suggests the new technology to control metal flow in orther to change of the cold forging from conventional deep drawing forming. This technology can be summarized the complex forming, which consists of bulk forming and sheet forming, and multi-action forging, which be performed double action press. The proposed technology is applied to hub clutch model which is part of auto-transmission for automobile. The purpose of this study is to investigate the material flow behavior of hub clutch through control the relative velocity ratio and the stroke of mandrel and punch using the flow forming technique. First of all, the finite element simulations are applied to analyse optimal process conditions to prevent flow defect(necking defect etc.) from non-uniform metal flow, then the results are compared with the plasticine model material experiments. The punch load for real material is predict from similarity law. Finally, the model material experiment results are in good agreement with the FE simulation ones.

• 소성∙가공
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• 제19권7호
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• pp.393-398
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• 2010

This paper investigates the effect of material properties on the formability of sheet metals based on the Marciniak-Kuczynski model (M-K model). The hardening behavior of the material is modeled as the Hollomon model with the strain rate effect. The yield surfaces are constructed with Hosford79 yield function. The material properties considered in this study include the R-value, the strain hardening exponent, the strain rate hardening exponent, and the crystal structure of the material. The effect of the crystal structure on formability is roughly expressed as the change of the yield surface by varying the value of the exponent in Hosford79 yield function. Results show that the R-value affects neither the magnitude nor the shape of right hand side of forming limit diagrams (FLDs). Higher strain hardening exponent and higher strain rate hardening exponent improve the formability of sheet metals because they stabilize the forming processes.

• Nuclear Engineering and Technology
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• 제54권3호
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• pp.917-925
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• 2022

Steam generator (SG) tubes in a nuclear power plant can undergo rapid changes in pressure and temperature during an accident; thus, an accurate model to predict short-term creep damage is essential. The theta (𝜃) projection method has been widely used for modeling creep-strain behavior under constant stress. However, many creep test data are obtained under constant load, so creep rupture behavior under a constant load cannot be accurately simulated due to the different stress conditions. This paper proposes a novel methodology to obtain the creep curve under constant stress using a modified 𝜃 projection method that considers the increase in true stress during creep deformation in a constant-load creep test. The methodology is validated using finite element analysis, and the limitations of the methodology are also discussed. The paper also proposes a creep-strain model for alloy 690 as an SG material and a novel creep hardening rule we call the damage-fraction hardening rule. The creep hardening rule is applied to evaluate the creep rupture behavior of SG tubes. The results of this study show its great potential to evaluate the rupture behavior of an SG tube governed by creep deformation.

• 산업기술연구
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• 제24권A호
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• pp.227-238
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• 2004

This paper is results of experimental and nunerical works on the behavior of the cut-and-cover tunnel. Centrifuge model tests were performed to simulate the behavior of the cut-and-cover tunnels having cross sections of national road and subway tunnels. Model experiments were carried out with changing the cut slope and the slope of filling ground surface. Displacements of tunnel lining resulted from artificially accelerated gravitational force up to 40g of covered material used in model tests, were measured during centrifuge model tests. In model tests, Jumunjin Standard Sand with the relative density of 80 % and the zinc plates were used for the covered material and the flexible tunnel lining, respectively. Basic soil property tests were performed to obtain it's the property of Jumumjin Standard Sand. Shear strength parameters of Jumunjin Standard Sand were obtained by performing the triaxial compression tests. Direct shear tests were also carried out to find the mechanical properties of the interface between the lining and the covered material. Numerical analysis with the commercially available program of FLAC were performed to compare with results of centrifuge model experiment In numerical modelling. Mohr-Coulomb elasto-plastic constitutive model was used to simulaye the behavoor of Jumunjin Standard Sand and the interface element between the lining and the covered material was implemented to simulate the interaction between them. Compared results between model tests and numerical estimation with respect to displacement of the lining showed in good agreements.

• 한국자기학회지
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• 제5권5호
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• pp.790-794
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• 1995

Differential equations governing dynamic behavior of toroid-shaped ferro-magnetic material having a small gap of uniform width were derived incorporating Maxwell equations of electromagnetic induction relevent to the system and Newtonian equation of motion. Once the external uniform magnetic field was applied within the material through dc-circuit around the toroid, gap begin to change which lead to the abrupt variation of field in the material and gap according to the differential equations already derived. Characteristics of current and electromotive force with respect to time in the circuit consisting of inductance and resistance in series could be predicted from numerical solutions of these equations. As current in the circuit increasesl, magnetic field in the material increases, thus, the gap starts to shrink due to increased attractive force between gap and elastic restoring force in the material. With an appropriate selection of elastic constant of toroidal ferromagnetic material and design of gap structure it is possible to obtain the specified in both linear and nonlinear magnetic characteristics, such as current dependent and independent inductance.

• Computers and Concrete
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• 제1권1호
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• pp.77-98
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• 2004

This paper presents a numerical model for simulating the nonlinear response of reinforced concrete (RC) shear walls subject to cyclic loadings. The material behavior of cracked concrete is described by an orthotropic constitutive relation with tension-stiffening and compression softening effects defining equivalent uniaxial stress-strain relation in the axes of orthotropy. Especially in making analytical predictions for inelastic behaviors of RC walls under reversed cyclic loading, some influencing factors inducing the material nonlinearities have been considered. A simple hysteretic stress-strain relation of concrete, which crosses the tension-compression region, is defined. Modification of the hysteretic stress-strain relation of steel is also introduced to reflect a pinching effect depending on the shear span ratio and to represent an average stress distribution in a cracked RC element, respectively. To assess the applicability of the constitutive model for RC element, analytical results are compared with idealized shear panel and shear wall test results under monotonic and cyclic shear loadings.

• Earthquakes and Structures
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• 제13권6호
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• pp.519-529
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• 2017

The surrounding rocks of underground openings are natural materials and their mechanical behavior under seismic load is different from traditional man-made materials. This paper proposes a framework to comprehensively model the mechanical behavior of surrounding rocks. Firstly, the effects of seismic load on the surrounding rocks are summarized. Three mechanical effects and the mechanism, including the strengthening effect, the degradation effect, and the relaxation effect, are detailed, respectively. Then, the framework for modelling the mechanical behavior of surrounding rocks are outlined. The strain-dependent characteristics of rocks under seismic load is considered to model the strengthening effect. The damage concept under cyclic load is introduced to model the degradation effect. The quantitative relationship between the damage coefficient and the relaxation zone is established to model the relaxation effect. The major effects caused by seismic load, in this way, are all considered in the proposed framework. Afterwards, an independently developed 3D dynamic FEM analysis code is adopted to include the algorithms and models of the framework. Finally, the proposed framework is illustrated with its application to an underground opening subjected to earthquake impact. The calculation results and post-earthquake survey conclusions are seen to agree well, indicating the effectiveness of the proposed framework. Based on the numerical calculation results, post-earthquake reinforcement measures are suggested.

• 한국항공우주학회지
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• 제37권11호
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• pp.1096-1103
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• 2009

연소기 헤드부는 극저온 유체인 액체산소가 고압으로 작동하고, 동시에 연소기의 추력으로 인한 하중을 받기 때문에, 극저온에서의 헤드의 구조 안정성 해석을 위한 재료의 변형 거동 예측은 매우 중요하다. 헤드부의 변형 거동을 예측하기 위해 재료의 저온에서의 인장 변형 거동을 묘사할 수 있는 구성 방정식을 Kocks의 전위 에너지 장벽 모델을 바탕으로 열적 요소와 비열적 요소의 결합으로 구성하였으며, 극저온에서 장애물들의 증가로 인한 응력의 열적 요소의 증가를 묘사하기 위해서, 장애물로 인해 발생하는 응력 요소를 Ramberg-Osgood 형태의 식으로 구성하였다. 본 모델은 극저온과 상온의 넓은 온도 영역에서 재료의 변형 거동을 잘 예측하였다.

• Geomechanics and Engineering
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• 제13권4호
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• pp.601-619
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• 2017

Cantilever retaining wall movements generally depend on the intensity and duration of ground motion, the response of the soil underlying the wall, the response of the backfill, the structural rigidity, and soil-structure interaction (SSI). This paper investigates the effect of material properties on seismic response of backfill-cantilever retaining wall-soil/foundation interaction system considering SSI. The material properties varied include the modulus of elasticity, Poisson's ratio, and mass density of the wall material. A series of nonlinear time history analyses with variation of material properties of the cantilever retaining wall are carried out by using the suggested finite element model (FEM). The backfill and foundation soil are modelled as an elastoplastic medium obeying the Drucker-Prager yield criterion, and the backfill-wall interface behavior is taken into consideration by using interface elements between the wall and soil to allow for de-bonding. The viscous boundary model is used in three dimensions to consider radiational effect of the seismic waves through the soil medium. In the seismic analyses, North-South component of the ground motion recorded during August 17, 1999 Kocaeli Earthquake in Yarimca station is used. Dynamic equations of motions are solved by using Newmark's direct step-by-step integration method. The response quantities incorporate the lateral displacements of the wall relative to the moving base and the stresses in the wall in all directions. The results show that while the modulus of elasticity has a considerable effect on seismic behavior of cantilever retaining wall, the Poisson's ratio and mass density of the wall material have negligible effects on seismic response.