• Polymer(Korea)
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• v.27 no.4
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• pp.275-284
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• 2003

The positive temperature coefficient (PTC) effects of high density polyethylene (HDPE)/carbon black composite materials were investigated by enhancing adhesive characteristics of electrodes and controlling HDPE chemical crosslinking. When the silver paste was used as an electrode for the same 45 wt% HDPE/carbon composites, the resistance was over 1 $\Omega$, which should be compared with the resistance of 0.2 $\Omega$ for the dendritic copper electrode. In general, the silver-paste electrode exhibited higher electrical resistance than cupper electrode due to the interfacial resistance between the electrode and PTC composites. The HDPE/carbon composite exhibited typical PTC characteristics maintaining a constant resistance up to vicat point and showing a maximum at the melting point of HDPE. The crosslinked HDPE significantly decreased the negative temperature coefficient (NTC) phenomena, and desirably showed a constant or slightly increasing feature of electrical resistance in the high temperature region.

• Journal of the Korean Society for Nondestructive Testing
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• v.29 no.4
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• pp.368-373
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• 2009

A human tissue mimicking phantom is constructed to assess the performance of a medical ultrasound elasticity imaging system. In a human body, the tumor or cancer is stiffer than its surrounding normal tissue. A technique fur imaging the elasticity of such a tissue is referred to as elastography. Homogeneous elasticity phantoms with differing Young's moduli are constructed using a plastic hardener and softener to simulate the mechanical characteristics of a diseased human tissue. The Young's modulus of the fabricated homogeneous phantom materials were measured from 11.1 to 79.6 kPa depending on the mixing ratio of the amount of the hardener to that of the softener. An ultrasound lesion mimicking phantom was made of these materials, and ultrasound elasticity imaging was performed on it. It is confirmed in this paper that the fabricated plastic-based elasticity phantom is useful in representing the elastic characteristics of a human tissue.

• Journal of the Korea Concrete Institute
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• v.24 no.2
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• pp.185-193
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• 2012

In this study, an estimation equation was proposed to predict the shear strength of RC interior beam-column connections. The proposed equation considered the effect of both truss and arch mechanisms, while the existing equations in the ACI and AIJ design codes consider only arch mechanism. In addition, the proposed equation estimates the shear strength of RC joints by considering the contribution of the vertical and horizontal steel bars on the effective compressive strength of concrete. The shear strength of RC joints calculated by the proposed equation was compared with the test results of 54 RC joints, which failed in shear before plastic hinges developed at the end of the adjacent beams. The comparison study showed that the proposed equation estimated the strength of the 54 specimens with a mean value of 1.14 and the coefficient of variation of 20%. The proposed equation provides improved prediction compared to those obtained from the equations in the ACI and AIJ design codes.

• Magazine of the Korea Concrete Institute
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• v.9 no.5
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• pp.115-125
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• 1997

콘크리트의 변형률국소화는 콘크리트의연화거동에 수반되어 변형이 국부적으로 집중되는 현상이다. 본 연구의 목적은 인장과 압축하중상태에서 콘크리트 부재에 발생하는 콘크리트 변형률 국소화 거동을 해석적으로 재현할 수 있는 통일된 모형을 제안하는 것이다. 본 논문에서는 인장과 압축에 대하여 변형률국소화가 일어나는 콘크리트 부재를 변형률 연화가 일어나는 국소화영역과 탄성제하가 발생하는 비국소화영역으로 구분하여 모델링하는 통일된 모형을 제안하였다. 또한 제안된 모형에서 미시역학적 평균화기법을 이용해 평균등가탄성계수와 수정된 평균등가탄성계수를 구하여 시편의 크기와 국소화영역의 크기에 따는 해석을 수행하였으며 기존의 실험값과 비교하였다. 연구결과, 본 연구에서의 변형률국소화모형이 크기효과를 포함한 콘크리트의 변형률국소화거동 해석에 타당하게 적용될 수 있음을 보여주었다.

• Journal of the Korean Ceramic Society
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• v.38 no.7
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• pp.628-633
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• 2001

전자 패키징에 적용 가능한 저융점 유리 중에서 납성분이 포함되어 있지 않는 Bi계 유리의 연구를 행하였다. 기판과의 일체화를 위한 유리의 응용으로, 소자와의 반응 억제와 열응력 완화를 위해 유리의 저융점화 및 열팽창계수를 제어하였다. 본 연구에서 제조된 Bi계 유리는 DTA와 TMA의 열분석을 통해 5$50^{\circ}C$ 부근에서 연화점이 있고, 열팽창계수는 7.52~12.09$\times$$10^{-6}$/$^{\circ}C$의 범위였으며, 비유전율은 9~13이었다. 본 연구의 납성분을 포함되어 있지 않는 유리조성은 우수한 내산성을 나타내었다.

• Korean Journal of Food Science and Technology
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• v.47 no.1
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• pp.56-62
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• 2015

Various pretreatment methods were evaluated to prevent tissue softening of heated onion. Changes in onion tissue firmness during heating were explained by 3-mechanism model consisting of texture hardening at low temperature ($60-80^{\circ}C$) and substrate softening at high temperature. Preheating of onion in a $Ca^{2+}$-containing solution significantly improved its texture after high-temperature heating. The improvement of firmness by preheating at low temperature was related to the formation of strong cross-linking between carboxyl groups and $Ca^{2+}$ by the action of pectin methylesterase in onion. The highest firmness was obtained by pre-heating at $70^{\circ}C$ for 120 min in 0.5% calcium solution. This result was supported by chemical analysis showing that the amount of bound calcium was the highest at $70^{\circ}C$. Further investigation should be carried out to establish the optimal conditions to prevent the softening of various vegetables.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.11
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• pp.2421-2428
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• 2002

In this study, the failure probability of the degraded LP turbine blade steel was performed using the Monte Carlo simulation to apply variation of applied stress and strength. For this purpose, applied stress under the service condition of steady state was obtained by theoretical stress analysis and the maximum Von-Mises stress was 219MPa. The fatigue strength under rotating-bending load was evaluated by the staircase method. Furthermore, 3-parameter Weibull distribution was found to be most appropriate among assumed distributions when the probabilistic distributions of tensile and fatigue strength were determined by the proposed analysis. The failure probability with various loading conditions was derived from the strength-stress interference model and the characteristic factor of safety was also estimated.

• Journal of the Korea institute for structural maintenance and inspection
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• v.10 no.5
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• pp.115-121
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• 2006

This paper deals with the influence of behavior of a variety of cable elements on the limit strength of steel cable-stayed bridges. The softening plastic-hinge model, which is represented in this study for the limit strength evaluation of the example bridge, considers both geometric and material nonlinearites. Geometric nonlinearity of beam-column members are accounted by using stability function, and material nonlinearity - by using CRC tangent modulus and parabolic function. Cable sag effect is considered for cable members. The result of this study shows that the limit strength of the example bridge using the equivalent of elasticity for truss straight elements is smaller than those using the cable or the catenary elements.

• Tunnel and Underground Space
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• v.19 no.2
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• pp.86-94
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• 2009

A three dimensional rock joint element was developed considering fracture mechanics and subcritical crack growth to simulate non-linear behavior and the progressive failure of rock joints. Using this 3-D joint element, joint shear tests of rock discontinuities were simulated by a numerical method. The asperities on the joint surface began to fail at stress levels lower than the rock fracture toughness and continued progressively due to subcritical crack growth. As a result of progressive failing in each and every asperity, the joint showed non-linear stress-time behavior including stress hardening/softening and the reaching of a residual stress.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.10a
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• pp.627-631
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• 2005

In this study, low fatigue behavior of laser welded sheet metal were investigated. Before welding, the cross section of butt joint was prepared only by fine shearing without milling process. Specimens were same sheet metal and welding condition that using automobile manufacturing company at present. Butt joint of cold rolled sheet metal was welded by $CO_2$ laser. It is used that welding condition such as laser welding speed was 5.5m/sec and laser output power was 5kW for 0.8mm and 1.2mm sheet metal. The laser weldments were machined same or different thickness and same or different material. In order to mechanical properties of around welding zone, hardness test was performed. Hardness of welding bead is about 2 times greater than base material. We performed the low cycle fatigue tests for obtaining fatigue properties about thickness and the weld line direction of specimen. The results of strain controlled low cycle fatigue test indicate that all specimens occur cyclic softening, as indicated by the decrease in stress to reach a prescribed strain.