• 소성∙가공
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• 제9권4호
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• pp.372-378
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• 2000

A process model including the effects of both the texture development and ductile damage evolution In plane strain rolling is presented. In this process model, anisotropy from deformation texture and deterioration of mechanical properties due to growth of micro voids are directly coupled Into the virtual work expressions for the momentum and mass balances. Special treatments in obtaining the initial values of field variables in the nonlinear simultaneous equations for the anisotropic, dilatant viscoplastic deformation are also given. Mutual effects of the texture development and damage evolution during plate rolling are carefully examined in terms of the distribution of strain components, accumulated damage, R-value as well as yield surfaces.

• 소성∙가공
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• 제13권7호
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• pp.600-607
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• 2004

This paper addresses a viscoplastic constitutive model that allows a consistent way of modeling positive and negative rate sensitivities of flow stress concerned with dynamic strain aging. Based on the concept of continuum mechanics, a phenomenological constitutive model includes the use of a yield surface within the framework of unified viscoplasticity theory. To model negative rate sensitivity, rate-dependent back stress is introduced and flow stress in fully developed inelastic deformation regime is thus decomposed into the plastic contribution of rate independency and the viscous one of rate dependency.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 1991년도 추계학술발표회 논문집 지반공학에서의 컴퓨터 활용 COMPUTER UTILIZATION IN GEOTECHNICAL ENGINEERING
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• pp.362-375
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• 1991

Background and Necessity of the study : For more than 20 years, the soil engineering reserach group of Chonnam National University has been performing the deformation analysis of soft clayey foundation, since the University is located near the south-western coast of Korean Peninsulla, along which tide reclamation works have been under proaressing. Associsted with the fact mentioned above, the researchers have been developing a computer program in order to carry out deformation analysis of soft foundation since early 1980. Case-studies : In this research, the Biot's equation was selected as the governing equation coupled with several constitutive models including original and modified Cam-clay models, elasto-viscoplastic model, Lade's model etc. The anisotropy of soi1 can be considered in this program. To validate the accuracy of the computer program developed a couple of case-studies were performed. These include the pilot banking, sand drain considering smear effect and compound foundation reinforced with sheet pile into soft foundation.i) The pilot banking Good results could be acquired by assuming banking load as the body force composed of finite element mesh rather than equivalent concentrated load.ii) The sand drain Due to smear, the delay of consolidation was remarkable at the early stsge. so safety for the failure of foundation should be checked for the initial step of consolidation. iii) The compound foundation Accurate results were obtained by introducing the joint element method for the soft foundation reinforced with sheet pile into soiㅣ.

• 소성∙가공
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• 제16권8호
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• pp.614-620
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• 2007

Deformation characteristics and damage evolution during warm backward extrusion of bulk AZ31 Mg alloy were investigated using finite element analyses. AZ31 Mg alloy was assumed as a hardening viscoplastic material. The tensile tests of AZ31 Mg alloy in previous experimental works showed the ductile fracture even at the warm temperature of $175^{\circ}C$. In this study, damage evolution model proposed by Lee and Dawson, which was developed based on the growth of micro voids in hardening viscoplastic materials, was combined into DEFORM 2D. Effects of forming temperature, punch speed, extrusion ratio and size of work piece on formability in warm backward extrusion as well as on mechanical properties of extruded products were examined. In general, finite element predictions matched the experimental observations and supported the analyses based on experiments. Distributions of accumulated damage predicted by the finite element simulations were effective to identify the locations of possible fracture. Finally, it was concluded that the process model, DEFORM2D combined with Lee & Dawson#s damage evolution model, was effective for the analysis of warm backward extrusion of AZ31 Mg alloys.

• 한국정밀공학회지
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• 제15권5호
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• pp.167-175
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• 1998

A mathematical formulation is presented to model anisotropy from the deformation textures developed in a forming process. In this work, a micro-mechanical-based polycrystalline analysis is implemented into a consistent finite element method for the anisotropic, viscoplastic deformation of polycrystalline metals. As suggested by Taylor, the deformation of each grain in an aggregate is assumed to be same as the macroscopic deformation of an aggregate or a macro-continuum point. Algorithms are developed to represent the plastic anisotropy, such as the anisotropic yield surface and R-value, from the predicted deformation texture. As applications, the evolution of texture in rolling, upsetting and drawing/extrusion processes are simulated and the corresponding changes of mechanical properties such as yield surface and R-value are predicted.

• 지질공학
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• 제3권3호
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• pp.215-230
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• 1993

암반은 지질학적 속성에 따라 다양한 형태의 구조적 특징을 나타내며, 특히 암반 내부에 발달되어 있는 절리계의 공간적, 역학적 특성은 외부응력 변화에 대한 전체 암반의 거동 및 파괴 강도 등에 중대한 영향을 끼치고 있다. 본 연구에서는 절리가 발달되어 있는 암반의 점소성 거동(viscoplastic behavior)을 연속체적 개념으로 분석할 수 있는 능력을 또한 암반구조물 보강에 일반적으로 사용되는 록볼트의 점소성 거동을 분석할 수 있는 능력을 추가시켜, 록볼트 지보효과가 고려된 절리암반의 점소성 거동에 대한 수치해석 모델을 완성하였다. 절리암반 및 록볼트의 점소성 거동에 대한 연속체 모델의 정확도 및 신뢰도는 예제적 모델분석을 수행하여 입증하였다. 실제적 암반구조물 거동분석에 대한 활용성을 고찰하기 위하여 현재 굴착이 진행되고 있는 지하 원유비축공동의 시간적 거동을 분석하여 현장 계측결과와 비교하였으며, 록볼트 설치비율에 따른 지보효과를 산출하였다.

• 한국추진공학회지
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• 제10권4호
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• pp.69-76
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• 2006

Bodner-Partom 점소성 모델을 이용하여 액체로켓 연소기 재생냉각 챔버의 구조해석을 수행하였다. 구조해석에 사용한 점소성 모델의 재료상수를 구하기 위하여 구리합금에 대하여 변형률 속도를 변화시켜 인장시험을 상온 및 고온에서 수행하였다. 점소성 모델은 상용유한요소 해석 프로그램인 Marc의 사용자 서브루틴을 이용하여 구현하였다. 구조해석 결과 냉각 채널은 압력에 의한 하중보다 열하중에 의하여 대부분의 변형이 발생하며 연소기의 작동조건에서 냉각 채널의 구조적인 안정성 여부를 확인할 수 있었다.

• 열처리공학회지
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• 제9권2호
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• pp.147-158
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• 1996

Generally, analytical consideration on the behaviour of metallic structures during quenching process, and analysis on the thermal stress and deformation after heat treatment are very important in presumption of crack and distorsion of quenched material. In this study a set of constitute equations relevant to the analysis of thermo elasto-viscoplastic materials with strain hysteresis during quenching process way presented on the basis of contimuum thermo-dynamics mechanics. The thermal stresses were numerically calculated by finite element technique of weighted residual method and the principle of virtual work. In the calculation process, the temperature depandency of physical and mechaniclal properties of the material in consideration. On the distribution of elasto-viscoplastic thermal stresses according to radial direction, axial and tangential stress are tensile stress(50MPa, 1.5GPa and 300MPa) in surface and compressive stress(-1.2GPa, -1.14GPa and -750MPa) in the inner part on the other hand, radial stress is tensile stress(900MPa) in area of analysis. According to axial direction, tangential stress gradients are average 60MPa/mm on the whole. The reversion of stress takes place at 11.5 to 16.8mm from the center in area of analysing.

• 대한기계학회논문집A
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• 제27권9호
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• pp.1555-1561
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• 2003

Finite element analysis using modern constitutive equation is one of the most general tools to simulate the deformation behavior and to predict the life of the structure. Constitutive equation becomes complicated so as to predict the material behavior more accurately than the classical models. Because of the complexity of constitutive model, numerical treatment becomes so difficult that the calculation should be verified carefully. One-element tests, simple tension or simple shear, are usually used to verify the accuracy of finite element analysis using complicated constitutive model. Since this test is mainly focused on the time integration scheme, it is also necessary to verify the equilibrium iteration using material stiffness matrix and to compare FE results with solution of structures. In this investigation, viscoplastic solution of thick walled cylinder was derived considering axial constraints and was compared with the finite element analysis. All the numerical solutions showed a good coincidence with FE results. This numerical solution can be used as a verification tool for newly developed FE code with complicated constitutive model.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 추계학술대회 논문집
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• pp.89-92
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• 2001

A rigid-viscoplastic finite element code for superplastic forming processes has been developed The material is assumed to be isotropic and a modified Coulomb friction law is adopted to explain contact between tool and sheet. This code uses the triangular element based on the membrane approximation and a hierarchical contact searching method is implemented The optimum pressure-time relationships for target strain rate are calculated by several pressure control algorithms. By the analysis, optimum pressure-time curves and deformation behavior are predicted.