• 한국군사과학기술학회지
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• 제20권4호
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• pp.520-527
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• 2017

In this study, the experimental methods are compared for obtaining the parameters of the Johnson-Cook constitutive model. The parameters used for numerical simulation are very important in making an accurate estimation of numerical simulation. So, the testing method of obtaining the parameters is also very important. We compared the difference of conventional method, compression method and tensile method of AISI-4340 steel at various strain rate by using MTS, SHPB and SHTB. Taylor impact test and M&S were carried out to compare differences among these three types of JC constitutive parameter.

• Composites Research
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• 제27권4호
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• pp.130-134
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• 2014

본 논문에서는 Kim 등[7]이 제시한 4가지의 다른 밀도를 갖는 원통형 시험편으로한 EPP(Expanded polypropylene)의 준정적(Quasi-static test) 및 충격 시험(Impact test) DB를 바탕으로 Jeong 등[12]이 제시한 폴리우레탄(Polyurethane)에 대한 구성방정식을 EPP 폼에 대하여 다시 정의 하여 충격량-운동량 이론을 접목시켜 새로운 구성방정식을 제안하기위한 DB 구축을 목표로 하였다.

• 소성∙가공
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• 제27권6호
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• pp.331-338
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• 2018

Electrohydraulic forming (EHF) process is a high speed forming process that utilizes the electric energy discharge in fluid-filled chamber to deform a sheet material. This process is completed in a very short time of less than 1ms. Therefore, finite element analysis is essential to observe the deformation mechanism of the material in detail. In addition, to perform the numerical simulation of EHF, the material properties obtained from the high-speed status, not quasi static conditions, should be applied. In this study, to obtain the parameters in the constitutive equation of Al 6061-T6 at high strain rate condition, a surrogate model using an artificial neural network (ANN) technique was employed. Using the results of the numerical simulation with free-bulging die in LS-DYNA, the surrogate model was constructed by ANN technique. By comparing the z-displacement with respect to the x-axis position in the experiment with the z-displacement in the ANN model, the parameters for the smallest error are obtained. Finally, the acquired parameters were validated by comparing the results of the finite element analysis, the ANN model and the experiment.

• 소성∙가공
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• 제26권5호
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• pp.286-292
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• 2017

Magnesium alloy shows strong anisotropy and asymmetric behavior in tension and compression curve, especially at room temperature. These characteristics limit the application of finite element method (FEM) which is based on conventional continuum mechanics. To accurately predict the material behavior of magnesium alloy at microstructural level, a methodology of fully coupled multiscale simulation is presented and a crystal plasticity model as a constitutive equation in the simulation of metal forming process is introduced in this study. The existing constitutive equation for rigid plastic FEM is modified to accommodate deviatoric stress component and its derivatives with respect to strain rate components. Viscoplastic self-consistent (VPSC) polycrystal model was selected as a constitutive model because it was regarded as the most robust model compared to Taylor model or Sachs model. Stiffness matrix and load vector were derived based on the new approach and implemented into $DEFORM^{TM}-3D$ via a user subroutine handling stiffness matrix at an elemental level. The application to extrusion and rolling process of pure magnesium is presented in this study to assess the validity of the proposed multiscale process.

• Computers and Concrete
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• 제7권2호
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• pp.87-102
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• 2010

This paper discusses new capabilities developed for the Virtual Penetration Laboratory (VPL) software package to address the challenges of determining Penetration Resistance (PR) equations for concrete materials. Specifically, the paper introduces a three-invariant concrete constitutive model recently developed by the authors. The Advanced Fundamental Concrete (AFC) model was developed to provide a fast-running predictive model to simulate the behavior of concrete and other high-strength geologic materials. The Continuous Evolutionary Algorithms (CEA) automatic fitting algorithms used to fit the new model are discussed, and then examples are presented to demonstrate the effectiveness of the new AFC model. Finally, the AFC model in conjunction with the VPL software package is used to develop a PR equation for a concrete material.

• 한국재료학회지
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• 제27권1호
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• pp.19-24
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• 2017

This paper is concerned with a test method that can be used to investigate the parameters of the Johnson-Cook constitutive model. These parameters are essential for accurately analyzing material behavior under impact loading conditions in numerical simulation. Ti-6Al-4V alloy (HCP crytal structure) was used as a specimen for the experiments. In the $10^{-3}-10^3/s$ strain rate range, three types of experimental methods (convention, compression and tension) were employed to compare the differences using MTS-810, SHPB and SHTB. Finite element analysis results when applying these parameters were displayed along with the experiment results.

• Journal of Mechanical Science and Technology
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• 제20권5호
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• pp.621-633
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• 2006

To reflect the size effect of material $(1\sim15{\mu}m)$ during plastic deformation of polycrystalline copper, a constitutive equation which includes the strain gradient plasticity theory and intrinsic material length model is coupled with the finite element analysis and applied to plane strain deformation problem. The method of least square has been used to calculate the strain gradient at each element during deformation and the effect of distributed force on the strain gradient is investigated as well. It shows when material size is less than the intrinsic material length $(1.54{\mu}m)$, its deformation behavior is quite different compared with that computed from the conventional plasticity. The generation of strain gradient is greatly suppressed, but it appears again as the material size increases. Results also reveal that the strain gradient leads to deformation hardening. The distributed force plays a role to amplify the strain gradient distribution.

• 열처리공학회지
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• 제31권4호
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• pp.180-186
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• 2018

High temperature flow behaviors of Fe-Cr-Mo-V-W-C tool steel were investigated using isothermal compression tests on a Gleeble simulator. The compressive test temperature was varied from 850 to $1,150^{\circ}C$ with the strain rate ranges of 0.05 and $10s^{-1}$. The maximum height reduction was 45%. The dynamic softening related to the dynamic recrystallization was observed during hot deformation. The constitutive model based on Arrhenius-typed equation with the Zener-Hollomon parameter was proposed to simulate the hot deformation behavior of Fe-Cr-Mo-V-W-C steel. An artificial neural network (ANN) model was also developed to compare with the constitutive model. It was concluded that the ANN model showed more accurate prediction compared with the constitutive model for describing the hot compressive behavior of Fe-Cr-Mo-V-W-C steel.

• Advances in materials Research
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• 제3권4호
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• pp.217-225
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• 2014

In this study, a forging steel alloyed with both Nb and V was used as experimental material and the hot deformation behavior has been studied for this steel by conducting the compressive deformation test at temperature of $900-1150^{\circ}C$ and strain rate of $0.01-0.01s^{-1}$ in a MMS-300 thermo-mechanical simulator. The microstructure evolution, particularly the dynamically recrystallized microstructure, of the experimental steel at elevated temperatures, strain rates and strain levels, was characterized by optical microstructural observation and the constitutive equation in association with the activation energy and Zener-Hollomon parameter. The curves of strain hardening rate versus stress were used to determine the critical strain and peak strain, and their relation was connected with Zener-Hollomon parameter. Under the conditions of processing temperature $900^{\circ}C$ and strain rate $0.01s^{-1}$, the dynamic recrystallization took place and the austenite grain size was refined from $164.5{\mu}m$ to $28.9{\mu}m$.

• 한국지반공학회논문집
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• 제27권11호
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• pp.55-69
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• 2011

다양한 지반공학적 문제들에서 불포화 상태의 중요성이 강조되면서, 불포화 지반의 열-수리-역학적 현상들에 대한 거동특성을 모사하기 위한 역학적 구성모델 개발이 진행되고 있다. 본 연구에서는 Bishop의 유효응력 정의에 근거한 불포화 지반의 역학적 탄소성 구성모델을 제시하였다. 유효응력에 근거한 구성관계는 유효응력과 온도를 주 변수로 증분 형식으로 표현되었으며, 이를 이용하여 응력 갱신과 강성 텐서를 산정하였다. 개발된 구성모델을 이용하여 THM 현상을 포함하는 불포화토의 1차원 거동, 불포화토의 삼축 압축시험, 그리고 고준위 방사성폐기물 시설의 완충재의 거동 특성에 관한 예제 해석을 수행하여 해의 안정성과 구성모델의 적용성에 대하여 논의하였다. 수치해석결과는 개발된 역학적 구성모델이 THM 현상의 매우 복잡한 거동을 효과적으로 모사할 수 있었으며, 일반적인 불포화토의 거동 해석뿐만 아니라 다양한 환경 조건하에서의 THM 거동 해석에 적용이 가능할 것으로 판단된다.