• Journal of Advanced Marine Engineering and Technology
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• 제27권4호
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• pp.494-502
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• 2003

The near-tip field of mode-I dynamic cracks steadily propagating in a strain softening material is investigated under plane strain conditions. The material is assumed to be incompressible and its deformation obeys the $J_2$ flow theory of plasticity. A power-law stress-strain relation with strain softening is adopted to account for the damage behavior of materials near the dynamic crack tip. By assuming that the stresses and strain have the same singularity at the crack tip. this paper obtains a fully continuous dynamic crack-tip field in the damage region. Results show that the stress and strain components the same logarithmic singularity of (In(R/r))$\delta$, and the angular variations of filed quantities are identical to those corresponding to the dynamic cracks in the elastic-perfectly plastic material.

• Computers and Concrete
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• 제1권4호
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• pp.433-455
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• 2004

The paper presents results of FE-calculations on shear localizations in quasi-brittle materials during both an uniaxial plane strain compression and uniaxial plane strain extension. An elasto-plastic model with a linear Drucker-Prager type criterion using isotropic hardening and softening and non-associated flow rule was used. A non-local extension was applied in a softening regime to capture realistically shear localization and to obtain a well-posed boundary value problem. A characteristic length was incorporated via a weighting function. Attention was focused on the effect of mesh size, mesh alignment, non-local parameter and imperfections on the thickness and inclination of shear localization. Different methods to calculate plastic strain rates were carefully discussed.

• 한국산학기술학회논문지
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• 제18권6호
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• pp.263-268
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• 2017

상용 마그네슘 합금의 경우, 상온에서 낮은 성형성을 갖기 때문에, 온간 성형 조건 하에서 성형 공정을 수행하는 것이 일반적이다. 마그네슘 합금은 온간 성형 과정 중에 동적 재결정(dynamic recrystallization, DRX)이 발생하여, 초기 결정립 사이즈가 급격하게 작아지며, 내부 전위 밀도가 낮아지게 된다. 이에 따라, 유동 응력 곡선은 세 단계의 복잡한 변형 경화 및 연화 현상을 보이게 된다. 첫 번째 구간에서는 변형률이 증가함에 따라, 가공 경화에 의해 응력이 증가하는 경향을 보이며, 두 번째 구간에서는 동적 재결정 현상에 의한 가공 연화로 응력이 갑작스럽게 감소한다. 세 번째 구간에서는 가공 경화와 가공 연화 사이의 평형에 의해, 응력이 일정하게 나타난다. 본 연구에서는, 성형 온도 $300^{\circ}C$, 변형률 속도는 0.001, 0.1, 1, 10/sec에서 AZ80 합금의 구성 방정식의 18개 변수들을 체계적으로 최적화하며, 유동 곡선의 정확도를 높일 수 있는 방식에 대해 제안하려고 한다. 또한 AZ80외에 AZ61도 추가적으로 최적화여 본 논문에서 제안한 최적화 방식의 성능을 증명하였다.

• 소성∙가공
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• 제17권7호
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• pp.466-472
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• 2008

In this paper, tension tests and formability tests are performed to construct a database related to mechanical properties and the formability of the AZ31B Mg alloy sheet. A forming test with a hemi-spherical punch is conducted at varying temperatures to establish a forming limit diagram. In order to verify the applicability of the analysis using the conventional flow hardening model, a finite element analysis is performed on the hemi-spherical punch forming process and the results are compared with experimental ones. The study investigates problems involving a computational analysis that does not consider flow softening of the magnesium alloy at elevated temperatures.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1997년도 춘계학술대회논문집
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• pp.77-80
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• 1997

The torsion tests in the range of 900~110$0^{\circ}C$, 5.0$\times$10-2~5.0$\times$100/sec were performed to study the recry stallization behavior of 304 stainless steel in the high temperature multistage deformation. The no-recrystallization temperature(Tnr) and fractional softening(FS) were determined by the change of flow curves. The inflection points of stress slope were moved to lower temperature area as the strain rate and the interrupt time were increased. From the multipass flow curve, the intersection between pass stress and FS curve was corresponding to the pass which the FS dropped abruptly and it was shown that the recrystallization area could be determined by the FS measurement in multipass deformation.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 춘계학술대회 논문집
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• pp.366-369
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• 2008

In this paper, tension tests and formability tests are performed to construct a database related to mechanical properties and the formability of the AZ31B Mg alloy sheet. A forming test with a hemi-spherical punch is conducted at varying temperatures to establish a forming limit diagram. In order to verify the applicability of the analysis using the conventional flow hardening model, a finite element analysis is performed on the hemi-spherical punch forming process and the results are compared with experimental ones. The study investigates problems involving a computational analysis that does not consider flow softening of the magnesium alloy at elevated temperatures.

• 대한화학회지
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• 제40권1호
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• pp.81-86
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• 1996

반도체를 보호하기 위하여 사용하는 반도체 성형 재료로, 현재 주로 사용되고 있는 Epoxy Molding Compound(EMI)의 주성분인 올소 크레졸 노볼락 에폭시 수지의 특성과 성형 재료의 관계를 조사하기 위하여 올소 크레졸 노볼락 에폭시 수지의 분자량과 깊은 관련이 있는 수지의 연화점 변화에 따른 EMC의 물성변화를 살펴보았다. 사용된 epoxy 수지의 연화점은 각각 65.1$^{\circ}C$, 72.2$^{\circ}C$, 83.0$^{\circ}C$ 인 3종을 사용하였으며 연화점 변화에 따른 EMC의 물성변화를 살펴보기 위하여 기계적 물성으로 굴곡 강도와 굴곡탄성율을, 열적 특성 변화를 관찰하기 위하여 열팽창 계수와 열전도도 그리고 유리 전이온도를 각각 측정하였다. 그리고 성형 특성과의 관계를 살펴보기 위하여 스피랄 플로우(Spiral Flow)를 측정하였다, 연화점이 증가함에 따라 굴곡 탄성율과 유리상에서의 열팽창 계수(${\alpha}_1$), 그리고 열전도도는 거의 변화가 없었으나 유리전이온도는 연화점 증가에 따라 증가함을, 스피랄 플로우는 연화점 증가에 따라 감소함을 보여주었다. 이는 에폭시 수지의 분자량이 증가함에 따라 가교밀도가 증가하는 현상에 기인한다고 판단된다. 고무상에서의 여팽창 계수(${\alpha}_2$)와 굴곡강도의 경우는 전이점을 보여주고 있는데, 이는 수지점도 증가에 따른 충전제의 분산성에 기인한 것으로 판단된다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.369-369
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• 2009

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.778-779
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• 2006

Manufacturing technologies of micro parts were studied in nano grained Al-1.5mass%Mg alloy. During compressive test at $300^{\circ}C$, the Al alloy showed stain softening phenomenon by grain boundary sliding regardless of strain rate. Micro spur gear with ten teeth (height of $200{\mu}m$ and pitch of $250{\mu}m$) was fabricated with sound shape by micro forging. During micro forging, increase of applied stress induced by friction between material and die surface was effectively compensated by decrease of stress by strain softening behavior and as a result, flow stress increased only about 50 MPa more than that in compressive test

• 상하수도학회지
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• 제17권4호
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• pp.534-541
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• 2003

Membrane processes are now frequently considered for application in drinking water treatment. The biggest impediment for applying membrane processes is fouling that comes from mass flux (such as particle and organic matter) to the membrane surface and its pores due to convection flow through the membrane. Natural organic matter (NOM) has been reported as the most detrimental foulant. Some research also indicated that particles were often the dominant cause of fouling. Therefore, both NOM and particle fouling need to be examined to better understand fouling in ultrafiltration. Two waters from natural sources, Lake Austin water and Missouri River water, were selected. Both waters are relatively hard waters but has significantly different particle concentrations, which will elucidate effects of particles on membrane fouling. Precipitative softening is traditionally designed to remove hardness ions in hard waters but it can also remove particles and organic matter. Therefore, the integrated water treatment with softening and ultrafiltration is proposed as a promising option for hard waters. The three levels of softening were used to represent different degrees of pretreatment to ultrafiltration in terms of organic matter (i.e., NOM fouling) and precipitates (i.e., particle fouling by further precipitation). Results showed that natural particles in Missouri River water was detrimental foulants of ultrafiltration. As the levels of softening were increased, NOM and particle removal was increased, and thus fouling was decreased. Direct images of the surface of the membranes by scanning electron microscopy allowed observation of the different properties of particles caught in fibril networks of natural organic matter.