F.C.C 단결정재에서 기공의 성장과 합체에 관한 연구

하상렬(Sangyul Ha),김기태(KiTae Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10

In this study, we investigate the deformation behavior of f.c.c. single crystals containing micro- or submicron-sized voids by using three dimensional finite element methods. The constitutive model for the ratedependent single crystal is integrated based on the backward Euler method and implemented into a finite element program (ABAQUS) by means of user-defined subroutine (UMAT). The unit cell analysis has been conducted to study the effect of stress triaxiality and crystallographic orientations on the growth and coalescence of voids in f.c.c. single crystals. The results showed that the stress triaxiality has a significant effect on the evolution of voids which includes the growth rate, the shape change and the critical effective strain at which the void coalescence starts. It is also revealed that there is significant influence of crystallographic orientation on the evolution of voids which is mainly represented by the specific deformation modes determined by the orientations.

속도 독립성 결정소성모델의 유한요소해석

하상렬(Sangyul Ha),김기태(KiTae Kim) 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.5

Rate-independent crystal plasticity model suffers from the non-uniqueness of activated slip systems and the determination of the shear slip rates on the active slip systems. In this paper, a time-integration algorithm which circumvents the problem of the multiplicity of the slip systems was developed and implemented into the user subroutine VUMAT of a commercial finite element program ABAQUS. The magnitude of the slip shears on the active slip systems in f.c.c Cu single crystal aligned with the specific crystallographic orientation was investigated to validate our solution procedure. Also, texture developments under various deformation modes such as simple compression, simple tension and plane strain compression were compared with the results of the rate-dependent model by using the rate-independent crystal plasticity model. The computation time employing the rate-independent model is much more reduced than the those of the ratedependent model.

인장형 홉킨슨 바 장치를 이용한 알루미늄 단결정 및 멀티결정재의 동적 실험

하상렬(Sangyul Ha),장진희(Jin Hee Jang),윤효준(Hyo Jun Yoon),김기태(KiTae Kim) 대한기계학회 2016 大韓機械學會論文集A Vol.40 No.1

본 연구에서는 연성 금속재료의 판상형 인장 시편에 대한 동적 물성을 측정하기 위한 인장형 홉킨슨 바(TSHB, Tensile split Hopkinson bar)의 수정 방법에 대해 논의하고, 이를 이용하여 고순도 알루미늄 단결정 및 멀티결정재의 동적 물성을 측정하였다. 시편의 초기 미세조직 및 결정학적 방위는 전자후방 산란회절(EBSD, Electron backscattered diffraction) 분석을 통하여 측정하였으며, 동적 변형 후 파단 형상을 광학 현미경을 통하여 확인하였다. 고속인장 변형 중 시편 내부에 발생하는 변형 분포는 디지털 이미지상관(DIC, Digital image correlation) 기법을 이용하여 측정하였다. 이를 통해 동적 변형 중 나타나는 알루미늄의 거시적인 소성 변형과 결정학적 방위 및 미세 조직과의 상관관계에 대해 논의하였다. In this study, we modified the conventional tensile split Hopkinson bar(TSHB) apparatus typically used for the high strength steel to evaluate the tensile deformation behavior of soft metallic sheet materials under high strain rates. Stress-strain curves of high purity single and multi-crystalline materials were obtained using this experimental procedure. Grain morphology and initial crystallographic orientation were characterized by EBSD(Electron Backscattered Diffraction) method measured in a FE-SEM(Field emission-scanning electron microscopy). The fractured surfaces were observed by using optical microscopy. The relationship between plastic deformation of aluminum crystalline materials under high-strain rates and the initial microstructure and the crystallographic orientations has been addressed.

미세 조직을 고려한 무산소동 다결정재의 변형 거동에 대한 미소역학적 연구

하상렬(Sangyul Ha),김기태(KiTae Kim) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.4

본 연구에서는 다결정재에서 발생하는 결정립 간의 불균일한 변형거동을 조사하기 위하여 조대한 주상 결정립을 가진 무산소동 구리 다결정재의 평면변형압축 실험을 수행하였다. 각 결정립의 결정학적 방위의 변화는 EBSD를 이용하여 측정하였으며, 압축 변형 후의 결정립 형상은 FE-SEM으로 관찰하였다. 또한, 샘플의 전체 형상 및 결정립의 형상 변형과 집합조직의 발전을 속도의존성 결정소성모델을 적용한 유한요소 해석결과와 비교하였다. In this study, plane strain compression experiments of an OFHC copper multicrystal which consists of large columnar grains were conducted to examine heterogeneous distribution of deformation patterns in each grain of the sample. The evolution of the crystallographic orientation in each grain was measured by the electron back scattered diffraction (EBSD) system. Also, the morphologies of the grains normal to TD were observed after compression by using a field emission scanning electron microscope (FE-SEM). Finally, finite element calculations based on the rate-dependent crystal plasticity model were compared with experimental results: the overall shape change of the compression sample, the deformed shapes of the individual grains, and the rotation of the crystallographic orientation.

F.C.C. 단결정재에서 기공의 성장과 합체에 관한 연구

하상렬(Sangyul Ha),김기태(Kitae Kim) 대한기계학회 2008 大韓機械學會論文集A Vol.32 No.4

In this study, we investigate the deformation behavior of F.C.C. single crystals containing micro- or submicron-sized voids by using three dimensional finite element methods. The locally homogeneous constitutive model for the rate-dependent crystal plasticity is integrated based on the backward Euler method and implemented into a finite element program (ABAQUS) by means of user-defined subroutine (UMAT). The unit cell analysis has been investigated to study the effect of stress triaxiality and crystallographic orientations on the growth and coalescence of voids in F.C.C. single crystals.

알루미늄 결정재의 전위밀도 기반 결정소성모델과 실험

하상렬(Sangyul Ha) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.열공학 No.-

세라믹 분말의 변형거동 해석을 위한 미소역학모델

하상렬(Sangyul Ha),박태욱(TaeUk Park),김기태(KiTae Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.11

In this paper, we developed a physically-based micromechanical model for inelastic deformation of ceramic powders. The aggregate response of ceramic particles was modeled using the two-surface yield function which considered the shear-induced dilatancy caused by friction, rolling resistance and cohesion between powder particles and consolidation caused by plastic deformation of powder themselves under high compression. The constitutive equations were implemented into the user-subroutine VUMAT of finite element program ABAQUS/Explicit. The material parameters in the constitutive model were identified by calibrating the model to reproduce data from triaxial compression tests and simple compression tests. The density distribution obtained by using the proposed model was in good quantitative agreement with the experimental results of the triaxial compression and cold isostaic compression as well.

알루미늄 멀티결정재 변형거동 예측을 위한 전위밀도 기반 변형률 구배 결정소성 모델

하상렬(Sangyul Ha),장진희(Jin-Hee Jang) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11

In this study, a strain gradient crystal plasticity model based on dislocation densities is presented aiming at predicting the scale-dependent deformation response of crystalline materials. Two types of dislocation densities, edge and screw, have been incorporated into the framework of the crystal plasticity and the geometrically necessary dislocation(GND) are introduced to take into the strain gradient effects. A corresponding time-integration algorithm for the constitutive model has been developed and implemented into the user-subroutine UEL of a finite element program ABAQUS. Representative comparisons between the numerical predictions and experiments are given to verify the robustness and stability of the current implementation of the dislocation-density-based crystal plasticity model: uniaxial tension and simple shear of aluminum multicrystals.

응집영역모델을 이용한 다공질 재료의 파괴 거동 연구

최승현(Seung Hyun Choi),하상렬(Sangyul Ha),김기태(KiTae Kim) 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.6

The effect of porosity on the crack propagation is studied by using the cohesive zone model. Standard mode Ⅰ fracture test were done by using compact tension specimens with various porosities. Load-load line displacement curves and δ5-crack resistance curves for various porosities were obtained from experiments. The cohesive zone model proposed by Xu and Needleman was employed to describe the crack propagation in porous media, and the Gurson model is used for constitutive relation of porous materials. These models were implemented into user subroutines of a finite element program ABAQUS. The fracture mode changes from ductile fracture to brittle fracture as the porosity increases. Numerical calculations agree well with experimental results.

온도와 변형 속도의 영향에 따른 방향성 단결정 전기강 박판의 소성 거동

윤효준(Hyo Jun Yoon),하상렬(Sangyul Ha),이용석(Yong Seok Lee),성우석(Woo Seok Sung),박창수(Changsoo Park),김기태(Ki Tae Kim) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11

Oriented silicon steel sheets having (110)[001] goss texture, are usually produced by cold rolling processes at a rolling reduction above 80%, which can develop surface crack and failure and eventually cause yield drop. Thus, it is essential to characterize a deformation behavior of silicon steels under various forming conditions. In this study, tensile tests were carried out at a strain rate 0.00038/s and in temperature range from room temperature to 300℃. A digital image correlation(DIC) method combined with the miniaturized tensile stage was used to characterize deformation field of the single crystal specimen. The initial and final crystallographic orientations of the tensile specimen were measured in a field-emission microscopy(FE-SEM) by using electron backscatter diffraction(EBSD). A strong dependency of the strain-rate and temperature on the stress-strain curves was observed as a typical characteristic of the body-centeredcubic(BCC) metals. Also, it is shown that even small misalignment of the crystallographic orientation can cause significant change of the stress level by changing the activation of slip system during plastic deformation.