• 소성∙가공
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• 제30권5호
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• pp.226-235
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• 2021

The strain aging behavior of a low carbon dual phase steel was examined in two conditions: representing room temperature strain aging (100 ℃ × 1 hr after 7.5 % prestrain) and bake hardening process (170 ℃ × 20 min after 2 % prestrain), basing on carbon effective diffusion and dislocation distribution. The first principle calculations revealed that (Mn or Cr)-vacancy-C complexes exhibit the strongest attractive interaction compared to other complexes, therefore, act as strong trapping sites for carbon. For room temperature strain aging condition, the carbon effective diffusion distance is smaller than the dislocation distance in the high dislocation density region near ferrite/martensite interfaces as well as ferrite interior considering the carbon trapping effect of the (Mn or Cr)-vacancy-C complexes, implying ineffective Cottrell atmosphere formation. Under bake hardening condition, the carbon effective diffusion distance is larger compared to the dislocation distance in both regions. Therefore, formation of the Cottrell atmosphere is relatively easy resulting in to a relatively large increase in yield strength under bake hardening condition.

• 콘크리트학회지
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• 제2권1호
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• pp.109-119
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• 1990

이 연구는 충진재의 유무에 따른 철근 콘리크트의 경화수축과 초기 크리이프 특성을 실험적으로 구명한 것이다. 그 결과, 경화수축은 보관온도가 높아질수록 커졌고, 충진재를 사용한 경우가 온도의 영향을 더 크게 받는 것으로 나타났다. 또한, 초기 크리이프는 재하시간과 응력일강도화가 증가함에 따라 증가했고, 강성변형이 클수록 컸는데, 이는 충진재를 첨가한 경우가 더 크게 나타났다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1999년도 춘계학술대회논문집
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• pp.225-228
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• 1999

In a plastically deformed body the formation of a shear band is widely observed in the engineering materials during rapidly forming process for a thermally rate-sensitive material. The localized shear bond stems from evolution of a narrow region in which intensive plastic flow occurs. The shear band often plays as a precursor of the ductile fracture during a forming process. The objectives of this study are to investigate the localization behaivor by using numerical method thus predict the failure. In this work the implicit finite difference scheme is preformed due to the ease of covergence and the numerical stability. This study is based on an analysised material with hardening as well as thermally softening behavior which includes isotropy strain hardening. Furthermore this paper suggests that an anticipated and suggested a kinematic hardening constitutive equation be requried to predicte a more accurate strain level wherein a shear band occurs.

• 소성∙가공
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• 제21권6호
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• pp.372-377
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• 2012

In this study, the plastic flow behaviors of extra deep drawing quality (EDDQ) steel subjected to non-proportional strain paths were investigated. Two-step uniaxial tension tests, in which the first step was performed in the rolling direction (RD) and the subsequent test in different directions in $15^{\circ}$ increments from the RD, were conducted. The experiments clearly showed that stress overshooting and strain hardening stagnation were the dominant features, which were captured reasonably well using a recently proposed distortional hardening model.

• 소성∙가공
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• 제17권3호
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• pp.161-169
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• 2008

In the present work, the two-back stress model is proposed and continuum damage mechanics (CDM) is incorporated into the plastic constitutive relation in order to describe the plastic deformation localization and the damage evolution in a deforming continuum body. Coupling between damage mechanics and isothermal rate independent plasticity is performed using the kinematic hardening rule, which in turn is formulated by combining the nonlinear Armstrong-Frederick rule and the Phillips rule. The numerical analyses are carried out within h deformation theory. It is noted that the damage evolution within a work piece accelerates the plastic deformation localization such that the material with lower hardening exponent results in a rapid shear band formation. Moreover, the results from the numerical analysis reflected closely with the micro-structures around the fractured regime. The effects of the various hardening parameters on deformation localization are also investigated. As the nonlinear strain rate description in the back stress evolution becomes dominant, the strain localization becomes intensified as well as the damage evolution.

• 대한조선학회지
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• 제26권2호
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• pp.41-48
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• 1989

본 연구에서는 기계적 성질이 상이한 4가지 박용재료에 대한 피로실험을 통하여, 일정 진폭하중 피로실험에서의 피로균열 전파거동과 단일 과부하하중 피로실험에서의 피로균열 전파거동을 비교함으로써, 재료의 변형경화지수(n)와 단일과부하하중에 의한 피로균열 지연효과와의 관계를 규명하고자 하였다. 본 실험의 결과를 요약하면 다음과 같다. (1) 단일 과부하하중을 작용시키면 피로 균열 전파속도는 지연되며, 이 지연효과의 정도는 재료의 기계적 성질 특히 변형경화지수와 밀접한 관계가 있다. (2) 단일 과부하하중에 의한 피로 균열 전파속도는 변형경화지수의 값이 작은 재료에서는 피로균열 지연효과가 작고, 변형경화지수의 값이 큰 재료일수록 지연효과가 커진다. (3) 단일 과부하하중에 의한 균열 closure의 현상은 변형경화지수의 값이 큰 재료에서 뚜렷이 나타나며, n이 0.2보다 큰 재료에서 단일 과부하하중에 의한 피로수명 연장이 효과적임을 알 수 있다.

• Food Science and Biotechnology
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• 제18권4호
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• pp.1013-1018
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• 2009

Several rheological terms were introduced to estimate the properties of cooked noodles with different starch content using tensile tests. Ring-shaped specimens were prepared by connecting both ends of the noodle strip before cooking. Hencky strain and rate, as well as true stress were applied in constant deformation tests. The elastic region on the curves of strain vs. stress was not clearly identified. Strain hardening in the subsequent plastic region was more prominent in low-starch noodles. Elongational viscosities at lower strain rates were used to differentiate noodles with different starch content, representing the dominant effect of protein content in the range of lower strain rates. In stress relaxation tests, the reciprocal of Peleg's constant $K_1$ (initial decay rate) and $K_2$ (asymptotic level) increased and decreased respectively, with an increase in starch content. This indicated that addition of starch contributed to the noodles becoming viscous liquid rather than elastic solid.

• 열처리공학회지
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• 제35권5호
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• pp.255-261
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• 2022

In this work, the strain-amplitude independent and strain-amplitude dependent damping capacities of Mg-5%Sn alloy have been investigated as a function of its age-hardening response. The hardness increased with an increase in aging time, reached a peak value after 48 h, and then it gradually decreased. The damping capacities of the Mg-5%Sn alloy exhibited a decreasing tendency in the order of solution-treated, under-aged, peakaged, and over-aged states in the strain-amplitude dependent region, whereas they increased continuously with aging time in the strain-amplitude independent region. The microstructural examination during aging revealed that the lower concentration of Sn solutes in the α-(Mg) matrix and the lower density of the Mg₂Sn precipitate particles may well be the crucial factors for better damping values in the strain-amplitude independent and strain-amplitude dependent regions, respectively.

• 한국재료학회지
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• 제23권3호
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• pp.199-205
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• 2013

The mechanical properties and microstructures of aluminum-matrix composites fabricated by the dispersion of fine alumina particles less than $20{\mu}m$ in size into 6061 aluminum alloys are investigated in this study. In the as-quenched state, the yield stress of the composite is 40~85 MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix introduced due to the difference in the thermal expansion coefficients between the matrix and the reinforcement. The difference in the yield stress between the composite and the 6061 alloy decreases with the aging time and the age-hardening curves of both materials show a similar trend. At room temperature, the strain-hardening rate of the composite is higher than that of the 6061 alloy, most likely because the distribution of reinforcements enhances the dislocation density during deformation. Both the yield stress and the strain-hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.

• 소성∙가공
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• 제15권8호
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• pp.544-549
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• 2006

In this work, we have employed the strain gradient plasticity theory to investigate the effect of material size on the deformation behavior in metal forming process. Flow stress is expressed in terms of strain, strain gradient (spatial derivative of strain) and intrinsic material length. The least square method coupled with strain gradient plasticity was used to calculate the components of strain gradient at each element of material. For demonstrating the size effect, the proposed approach has been applied to plane compression process and micro rolling process. Results show when the characteristic length of the material comes to the intrinsic material length, the effect of strain gradient is noteworthy. For the microcompression, the additional work hardening at higher strain gradient regions results in uniform distribution of strain. In the case of micro-rolling, the strain gradient is remarkable at the exit section where the actual reduction of the rolling finishes and subsequently strong work hardening take places at the section. This results in a considerable increase in rolling force. Rolling force with the strain gradient plasticity considered in analysis increases by 20% compared to that with conventional plasticity theory.