• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2007년도 춘계학술대회 논문집
• /
• pp.191-194
• /
• 2007

In hot embossing lithography which has shown to be a good method to fabricate polymeric patterns for IT and bio components, it is very important to determine the proper process conditions of pressure, temperature, and time. It is also a key factor for predicting the optical properties of final product to calculate residual stress distribution after the embossing process. Therefore, to design the optimum process with right conditions, the ability to predict viscoelastic behavior of polymer during and after the hot embossing process is required. The objective of the present investigation is to establish simulation technique based on constitutive modeling of polymer with experiments. To analyze deformation behavior of viscoelastic polymer, the large strain material properties were obtained from quasi-static compression tests at different strain rates and temperatures and also stress relaxation tests were executed. With this viscoelastic material model, finite element simulation of hot embossing was executed and stress distribution is obtained. Proper process pressure is very important to predict the defect and incomplete filling.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2001년도 춘계학술대회 논문집
• /
• pp.20-23
• /
• 2001

Hypereutectic Al-25Si alloy, which is expected to be applied to the cylinder-liner-part of the engine-block of an automobile due to its excellent wear resistance, low density and low thermal expansion coefficient, has been fabricated through a spray forming process. The obtained microstructure of the hypereutectic Al-25Si alloy appeared to consist of Al matrix and equiaxed Si particles of average diameter of 5-7 mm. To characterize the deformation behavior of this alloy, a series of load relaxation and compression tests have been conducted at temperatures ranging from RT to $500^{\circ}C$. The strain rate sensitivity parameter (m) of this alloy has been found to be very low (0.1) below $400^{\circ}C$ and reached maximum value of about 0.2 at $500^{\circ}C$. During the deformation above $300^{\circ}C$ in compression, strain softening has been observed. The diagram of extrusion pressure vs. ram-speed has been constructed. The extrusion has been successfully conducted at the temperatures of $300^{\circ}C$ and above with the ratio of area reduction of 28 and 40 in this study.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2001년도 춘계학술대회 논문집
• /
• pp.167-171
• /
• 2001

High temperature deformation behavior of Fe-28Al-5Cr alloy has been investigated known to show anomalous temperature dependence of yield strength. Specifically, the effect of Cr addition has been examined. A series of tensile and load relaxation tests have been carried out to obtain the flow behavior of Fe-28Al-5Cr alloy at the elevated temperatures. The flow curves have then been analyzed using the inelastic deformation theory recently proposed. Firstly, high temperature flow stress of iron aluminides can be resolved into internal stress and frictional stress. Secondly, the temperature corresponding to peak strength gets higher level at faster strain rate, which presumably due to the increased contribution of internal stress in observed flow stress. And thirdly, the alloying of Cr seems to cause solid-solution strengthening of frictional stress level and the elevation of 2nd order transition temperature. In this analogy, Fe-28Al-5Cr exhibits better strength especially at relatively higher temperature and lower strain rate than Fe-28Al.

• 소성∙가공
• /
• 제18권4호
• /
• pp.317-322
• /
• 2009

Superplastic deformation behavior of a Zn-0.3 wt.% Al was investigated. Grain sizes of $1{\mu}m$ and $10{\mu}m$ were obtained by a thermomechanical treatment. A series of load relaxation and tensile tests were conducted at various temperatures ranging from RT ($24^{\circ}C$) to $200^{\circ}C$. A large elongation of 1400% was obtained at room temperature in the specimens with the grain size of $1{\mu}m$. In the case of specimens with the grain size of $10{\mu}m$, relatively lower elongation at room temperature was obtained and, as the temperature increases above $100^{\circ}C$, a high elongation of about 400 % has been obtained at $200^{\circ}C$ under the strain rate of $2{\times}10^{-4}/s$. Dynamic materials model (DMM) has been employed to explain the contribution from GBS of Zn-Al alloy. Power dissipation efficiency for GBS was evaluated as above 0.4 and found to be very close to the unity as strain rate decreased and temperature increased, suggesting that GBS could be regarded as Newtonian viscous flow.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2008년도 춘계학술대회 논문집
• /
• pp.539-542
• /
• 2008

Recently, GMP(Glass Molding Press) process is mainly used to produce aspheric glass lenses. Because glass lens is heated at high temperature above Ty (yielding point) for forming glass, the quality of aspheric glass lens is deteriorated by residual stresses which are generated in a aspheric glass lens after forming. Before this study, as a fundamental study to develop forming conditions for progressive GMP process, compression, strain relaxation and thermal conductivity tests were carried out to obtain the visco-rigid plastic, the visco-elastic and thermal properties of K-PBK40 which is newly developed and applied for precision molding glass material, In this study, using the experimental results we obtained, a glass lens forming simulation in progressive GMP process was carried out and we could forecast the shape of deformed glass lenses and residual stresses contribution in the structure of deformed glass lenses after forming.

• 한국자동차공학회논문집
• /
• 제22권6호
• /
• pp.104-112
• /
• 2014

Generally, numerical approaches of evaluation for vehicle seat comfort have been studied without considering time-dependent characteristics and the only seating moment have been considered in seat design. However, the comfort not only at the seating moment but also in the long-term should be evaluated because the passengers are sitting repeatedly on the seat to drive the vehicle for hours. So, the aim of this paper is to carry out a quantitative evaluation of the time-dependent mechanical characteristics of seat foams and to suggest a process for predicting the viscoelastic deformation of seat foam in response to long-term driving. To characterize the seat materials, uniaxial compression and tension tests were carried out for the seat foam and stress relaxation tests were performed for evaluating the viscoelastic behavior of the seat foam. A unit solid element model was used to verify the reliability of the material model with respect to the compression behavior of the seat foam. It is not straightforward to evaluate the time-dependent compression of foams using the explicit solver because the viscoelastic material model is limited. To use the explicit solver, the material model must be modified using stress-degradation data. Normalized stress relaxation moduli were added to the stress-strain curves obtained under static conditions to achieve a time-dependent set of stress-strain relations that were compatible with the implicit solver. There was good agreement between the analysis results and experimental data.

• 한국식품과학회지
• /
• 제26권2호
• /
• pp.103-109
• /
• 1994

응력완화 현상을 측정하여 생선단백질 겔의 선형적 점탄성과 열처리 공정에 따른 물성의 변화를 수식적 모델로 분석하였다. 생선단백질 겔은 정변형도 $0.105{\sim}0.693$, 압축속도 $50{\sim}250\;mm/min$의 범위에서 선형적 점탄성을 나타내었으며, generalized Maxwell 모형에 의해 분석한 결과, 압축변형도과 압축속도가 증가함에 따라 가열에 의해 형성된 내부조직 중 탄성 성분의 증가와 점성성분의 감소현상을 보였다. $4^{\circ}C$와 $40^{\circ}C$에서 전처리하여 $90^{\circ}C$에서 제조한 겔은 전처리 없이 $90^{\circ}C$에서 직접 열처리한 겔보다 탄성율(E) 및 평형탄성율$(E_e)$이 높았으나, 점성성분$({\eta})$은 적용된 모델에 따라서 그 값의 차이가 나타났다. 따라서 식품의 물성을 측정하는데 있어서 두 수학적 모델의 접근방법 및 정확도에 대하여 설명하였다.

• 복합신소재구조학회 논문집
• /
• 제3권1호
• /
• pp.8-15
• /
• 2012

고력볼트의 초기 체결력은 미끄럼표면조건에 따라, 일정시간이 경과될 때까지, 축력저하가 발생한다. 이 연구는 미끄럼접합부 표면에 도장이 되어있는 경우, 도장의 크리프현상에 따른 축력저하에 관한 예측 모델을 찾는 것이다. 이 실험연구 범위는 무기질 아연 프라이머로 도포된 볼트접합부의 장기축력저하에 한정한다. 실험에 적용된 볼트종류는 다크로 도포된 토크쉬어 볼트이다. 대상 표면의 도막 두께는 각각 96, 168, $226{\mu}m$ 이었다. 도막두께가 증가될수록, 초기 체결이후 축력이완율은 도막의 크리프 때문에 10%에서 18%로 증가되었다. 장기축력예측을 위한 정량적인 모델은 도막두께에 따른 크리프 스트레인과 경과된 시간사이에 회귀분석 결과로 얻어진다. 이 실험연구를 통해 미끄럼표면 도막의 크리프 거동특성을 알 수 있다면, 일정시간 경과후 고력볼트 체결력은 초기 체결력으로부터 구할 수 있다. 본 실험결과를 근거로 각 도막두께에 대한 장기축력이완이후의 고력볼트 체결력에 관한 정량적인 수식을 제안하였다.

• 한국농공학회논문집
• /
• 제55권2호
• /
• pp.1-8
• /
• 2013

This study was performed for the purpose of analyzing the effect of tide on the stress-strain behavior in the boundary surface of sea dike embankment. Tide is a dynamic condition, but there are not suitable numerical models to solve the dynamic embankment condition caused by tide. So the analysis was simplified to quasi dynamic as follow. First, seepage by tide was analyzed according to elapsed time, and the results of the analysis at every hour during one periodic cycle time of 12 hours were applied to the pore water pressure conditions of stress-strain analysis with hyperbolic model by Duncan and Chang. The place at which maximum shear strain took place in the analysis result moved up and down repeatedly along the boundary of the dredged sand fill section and the crashed stone filter section. The value of maximum shear strain was large at high water level of tide. This result means that contraction and relaxation occur in turn repeatedly at every specific position along the boundary, and the repeated action compact loose position with sand moved down from the upper position by gravity. The experiment with the small sea dike model showed the result consistent with the numerical analysis. The surface of sea side on the dike collapsed at high water level after a couple of repetition of the rising and falling of water.

• 대한토목학회논문집
• /
• 제29권4A호
• /
• pp.291-297
• /
• 2009

실험을 통해 고장력 볼트의 릴렉세이션를 측정하였다. 종류와 표면 처리가 다른 볼트를 사용하여 체결길이가 서로 다른 블록형태와 겹침이음형태의 시험체를 조립하였다. 항온항습실에서 시간 경과에 따라 볼트에 도입된 축력을 변형률 게이지를 통하여 측정하였다. 볼트축력의 감소율은 모든 시험체에서 10% 미만으로 나타났다. 결합 형태, 볼트종류, 체결길이 그리고 볼트의 표면 처리가 서로 다른 여러 경우에 대해 실험결과를 이 연구에서 제안한 릴렉세이션 추정 모델을 이용하여 비교, 검토하였다. 제안한 모델은 여러 경우에 대한 실험 결과와 모두 합리적으로 잘 합치하고 있다. F13T와 F10T 볼트사이에 차이점을 발견할 수 없으나 다크로 볼트의 최종 체결력 감소율은 일반 볼트보다 30% 정도 높으며, 또한 전체적으로 볼트 체결길이가 짧아짐에 따라, 전단면의 수가 증가함에 따라 높아지는 경향을 보여주고 있다.