• Structural Engineering and Mechanics
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• 제14권2호
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• pp.209-221
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• 2002

In this paper a fracture criterion is proposed for cracked cylindrical samples of high-strength prestressing steels of different yield strength. The surface crack is assumed to be semi-elliptical, a geometry very adequate to model sharp defects produced by any subcritical mechanism of cracking: mechanical fatigue, stress-corrosion cracking, hydrogen embrittlement or corrosion fatigue. Two fracture criteria with different meanings are considered: a global (energetic) criterion based on the energy release rate G, and a local (stress) criterion based on the stress intensity factor $K_I$. The advantages and disadvantages of both criteria for engineering design are discussed in this paper on the basis of many experimental results of fracture tests on cracked wires of high-strength prestressing steels of different yield strength and with different degrees of strength anisotropy.

• Journal of Advanced Marine Engineering and Technology
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• 제32권4호
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• pp.570-577
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• 2008

Magnesium alloys are potentially useful as structural materials due to higher strength/weight ratio, heat conductivity and recyclability compared to other alloys. These alloys have attracted the interest of modern manufacturing such as the automobile, computer, communication and consumer electronic appliances industries. Hence welding techniques are required to be developed for these applications. Laser are known to be an excellent tool for them. This paper presents the laser weldability of AZ31 magnesium alloy with CW Nd:YAG laser. The low viscosity and surface tension of the melt pool make magnesium more difficult to weld than steel. As a result of this study, optimal process parameters could be obtained without weld defects. Also it was certain that cutting methods had influence on butt weldability.

• Journal of Magnetics
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• 제15권4호
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• pp.185-189
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• 2010

Spin polarized tunneling through a hybrid tunnel barrier of a Spin filter (SF) based on a EuO ferro-magnetic semiconductor and an organic semiconductor (OSC) (rubrene in this case) was investigated. For quasi-magnetic tunnel junction (MTJ) structures, such as Co/rubrene/EuO/Al, we observed a strong spin filtering effect of the EuO layer exhibiting I-V curves with high spin polarization (P) of up to 99% measured at 4 K. However, a magnetoresistance (MR) value of 9% was obtained at 4.2 K. The low MR compared to the high P could be attributed to spin scattering caused by structural defects at the interface between the EuO and rubrene, due to nonstoichiometry in the EuO.

• 반도체디스플레이기술학회지
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• 제3권3호
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• pp.23-25
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• 2004

The defect formation energy of boron nitride (BN) nanotubes is investigated using molecular-dynamics simulation. Although the defect with tetragon-octagon pairs (4-88-4) is favored in the flat cap of BN nanotubes, BN clusters, and the growth of BN nanotubes, the formation energy of the 4-88-4 defect is significantly higher than that of the pentagon-heptagon pairs (5-77-5) defect in BN nanotubes. The 5-77-5 defect reduces the effect of the structural distortion caused by the 4-88-4 defect, in spite of homoelemental bonds.

• Structural Engineering and Mechanics
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• 제8권3호
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• pp.273-283
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• 1999

Two physical experiments are performed to verify the effectiveness of beam-particle model for simulating the progressive failure of particulate composites such as sandstone and concrete. In the numerical model, the material is schematized at the meso-level as an assembly of discrete, interacting particles which are linked through a network of brittle breaking beams. The uniaxial compressive tests of cubic and parallelepipedal specimens made of carbon steel rod assembly which are glued together by a mixture are represented. The crack patterns and load-displacement response observed in the experiments are in good agreement with the numerical results. In the application respect of beam-particle model to the particulate composites, the influence of defects, particle arrangement and boundary conditions on crack propagation is approached, and the correlation existing between the cracking evolution and the level of loads imposed on the specimen is characterized by fractal dimensions.

• 한국진공학회지
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• 제5권1호
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• pp.73-76
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• 1996

We have prepared hydrogenated amophous silicon (a-si : H) films with superlattice structure by hydrogen radical anneling(HRA) technique. We have studied the preparation of a-Si :H films by HRA and the optical & electronic characteristics. Optical band gap and the hydrogen contents in the a Si : H film is decreased as HRA time increased. We first report a -Si : H film prepared by periodicdeposition of a-Si : H layer and HRA have the superlattice structure using TEM . After 1 hour light soaking on the a-Si :H film prepared by HRA, there are no difference in the temperatre dependence of dark conductivity and the conductivity activation energy. An excellent stability for light in a-Si :H films by HRA can be explained using the long-range structural relaxation of the amorphous network and the propertiesof light -induced defects(LID) proposed by Fritzsche.

• 한국해양공학회지
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• 제13권1호통권31호
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• pp.88-93
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• 1999

This paper presents the experimental examinations and statistical quantitative analysis of the correlation between the cumulative counts of acoustic emission(AE) during plastic deformation periods of the welding and the tensile strength and other properties of the bar-to-bar welded joints of O.D. 16mm shaft(SM40C) as well as the various welding variables. And this is a new approach which attempts finally to develop real-time quality monitoring system for friction welding. The results lead to a practical possibility of real-time quality control more than 100% joint efficiency showing good weld with no micro structural defects.

• 한국조명전기설비학회:학술대회논문집
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• 한국조명전기설비학회 2004년도 춘계학술대회 논문집
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• pp.546-549
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• 2004

This paper describes the electrical characteristics of the polymer lightning arresters(L.As) used to protect distribution lines and power facilities from surge. For the relative comparison, the characteristics assessment was done for 2 states of L.As before and after water immersion treatment. The L.As for the comparative test were immersed with water of $95[^{\circ}C]$ considered a severe condition. After the immersion treatment, the assessment was carried out for material analyses in chemical structure(FTIR : fourier transform infrared spectrophotometer). Electrical performance measured insulation resistance measurement, leakage current measurement, critical operation voltage measurement etc.. In addition to, the results were compared with those for the L.As deteriorated in the field. Thus, the accelerative deterioration degree of polymer materials by immersion treatment was evaluated, by which the structural defects of the polymer L.As could be inferred.

• EDISON SW 활용 경진대회 논문집
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• 제5회(2016년)
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• pp.308-312
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• 2016

2차원 화합물 반도체인 GaS single tetralayer에 존재하는 vacancy defect의 원자구조 및 전자구조 특성을 제일원리계산을 이용하여 연구하였다. 고립된 Ga과 S vacancy를 모델링하기 위해, GaS $4{\times}4$ supercell을 이용하였고 각 vacancy에 대해 symmetry-preserving 구조와 broken symmetry 구조들의 에너지를 계산하여 가장 안정한 결함 원자 구조를 결정하였다. Ga-rich, S-rich condition에서의 formation energy 계산을 통해 vacancy 구조의 생성 가능성을 예측하였다. 안정한 vacancy 구조들에 대해 projected density of states (PDOS)를 clean GaS의 PDOS와 비교 분석함으로써 vacancy에 의한 defect states들을 찾고, 결과적으로 나타나는 전자구조 특성의 변화를 규명하였다.

• 한국정밀공학회지
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• 제20권3호
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• pp.157-164
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• 2003

The pulse-echo method is one of the most widely used ultrasonic techniques for application of nondestructive evaluation. Particularly, quantitative nondestructive evaluation of defects has been considered more important to assure the reliability and the safety of structure. Frequency energy in adhesive joints is based on the ultrasonic wave analysis. The attenuation coefficient upon wave amplitude and the frequency energy that is expressed in the term of wave pressure amplitude were utilized for the primary wave experiment. By means of a control experiment, it was confirmed that the variation of the frequency energy in adhesive joints depends on transition by stress variation. In this paper, the ultrasonic characteristics were measured for single lap joint and Double Cantilever Beam specimen with different fracture modes that was subjected to stress. Consequently, the data that was obtained from the adhesive specimen was analytically compared to the fracture mechanics parameter