• 대한전기학회논문지:전기물성ㆍ응용부문C
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• 제50권10호
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• pp.479-488
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• 2001

The effects of accelerated Ultraviolet (UV) radiation on High temperature vulcanized (HTV), Room temperature vulcanized (RTV) silicone rubber and two types of ethylene propylene diene terpolymer (EPDM) used for composite insulator were inverstigated by hydrophobicity class (HC), surface voltage decay after corona charging, SEM-EDS, FTIR and XPS. The contact angle in two kinds of silicone rubber was scarcely change, but EPDM occurred to the loss of hydrophobicity followed by surface cracking and chalking. The surface voltage decay on UV-treated silicone rubber and EPDM showed a different decay trend with UV treatment. EDS and XPS analysis indicated that the oxygen content increased with UV treatment time in all samples. For silicone rubber, the oxidized groups of inorganic silica-like structure increased with UV treatment time. The oxidized carbon of C=0, O=C-O in EPDM increased. These oxidized surface for each material had different electrostatic characteristics, so deposited charges were expected to have different impacts on their surface hydrophobicity. The degradation mechanism based on our results was discussed.

• 대한전기학회논문지:전기물성ㆍ응용부문C
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• 제50권10호
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• pp.494-499
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• 2001

The effects of accelerated Ultraviolet (UV) radiation on High temperature vulcanized (HTV), Room temperature vulcanized (RTV) silicone rubber and two types of ethylene propylene diene terpolymer (EPDM) used for composite insulator were investigated by hydrophobicity class (HC), surface voltage decay after corona charging, SEM-ES, FTIR and XPS. The contact angle in two kinds of silicone rubber was scarcely change, but EPDM occurred to the loss of hydrophobicity followed by surface cracking and chalking. The surface voltage decay on UV-treated silicone rubber and EPDM showed a different decay trend with UV treatment. EDS and XPS analysis indicated that the oxygen content increased with UV treatment time in all samples. For silicone rubber, the oxidized groups of inorganic silica-like structure increased with UV treatment time. The oxidized carbon of C=O, O=C-O in EPDM increased. These oxidized surface for each material had different electrostatic characteristics, so deposited charges were expected to have different impacts on their surface hydrophobicity. The degradation mechanism based on our results was discussed.

• 한국재료학회지
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• 제5권2호
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• pp.157-168
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• 1995

The effects of Mo additions on the microstructure and the room temperature deformation behavior of polysynthetically twinned (PST) crystals of TiAl were studied in order to get a basic conception for alloying additions on the two-phase TiAl compounds with the lamellar structure. It was found that the Mo additions in TiAl PST crystals increase both the yield stress and tensile elongation to fracture but the increase in yield stress deppend on the angle $\Phi$at which the lamellar boundaries lie from the loading axis. The large difference in yield stress between specimens deformed parallel($\Phi = 0^\circ$)or perpendicular($\Phi = 90^\circ$) to the loading axis and those deformed in intermediate orientations could be plained by the difference in Mo content between the TiAl and the $$Ti_{3}Al$ phases. It was also found that the Mo-doped specimens with intermediate orientation fail by cracking zigzag across to the lamellar boundaris, which is the same fracture mode as that of binary specimens with intermediate orientations tested in vacuum This suggests that Mo atoms are thought to play a role to reduce the environmental embrittlement of binary PST crystals, resulting in increasing the tensile ductility.

• 대한기계학회논문집A
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• 제21권5호
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• pp.842-849
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• 1997

The aim of this study is an investigation of the interaction of two micro hole defects affecting fatigue crack initation life and propagation behavior. The locatio of two micro hole defects was considered as an angle of alignment and the distance between the centers of two micro hole defects. The fatigue cracking behavior is experimented under bending. When micro defects are located close to each other, the fatigue crack initiation lives are varied with their relative locations. In the experiments, the area of local plastic strain strongly played a role in the fatigue crack initiation lives. Therefore we introduce a parameter which contains the plastic deformation area at stress concentrations and propose a fatigue crack initiation life prediction curve. In addition, the directions and propagation rates of fatigue cracks initiated at two micro hole defects are studied experimentally.

• Computers and Concrete
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• 제7권5호
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• pp.455-468
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• 2010

This paper presents a prediction and simulation method of tensile stress-strain curves of Engineered Cementitious Composites (ECC). For this purpose, the bridging stress and crack opening relations were obtained by the fiber bridging constitutive law which is quantitatively able to consider the fiber distribution characteristics. And then, a multi-linear model is employed for a simplification of the bridging stress and crack opening relation. In addition, to account the variability of material properties, randomly distributed properties drawn from a normal distribution with 95% confidence are assigned to each element which is determined on the basis of crack spacing. To consider the variation of crack spacing, randomly distributed crack spacing is drawn from the probability density function of fiber inclined angle calculated based on sectional image analysis. An equation for calculation of the crack spacing that takes into quantitative consideration the dimensions and fiber distribution was also derived. Subsequently, a series of simulations of ECC tensile stress-strain curves was performed. The simulation results exhibit obvious strain hardening behavior associated with multiple cracking, which correspond well with test results.

• Steel and Composite Structures
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• 제25권4호
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• pp.403-413
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• 2017

The fabric braided braking hose that delivers the driver's braking force to brake cylinder undergoes the large deformation cyclic motion according to the steering and bump/rebound motions of vehicle. The cyclic large deformation of braking hose may give rise to two critical problems: the interference with other adjacent vehicle parts and the micro cracking stemming from the fatigue damage accumulation. Hence, both the hose deformation and the fatigue damage become the critical issue in the design of braking hose. In this context, this paper introduces a multi-objective optimization method for minimizing the both quantities. The total length of hose and the helix angles of fabric braided composite layers are chosen for the design variables, and the maximum hose deformation and the critical fatigue life cycle are defined by the individual single objective functions. The trade-off between two single objective functions is made by introducing the weighting factors. The proposed optimization method is validated and the improvement of initial hose design is examined through the benchmark simulation. Furthermore, the dependence of optimum solutions on the weighting factors is also investigated.

• Earthquakes and Structures
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• 제18권5호
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• pp.581-598
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• 2020

This paper aims to investigate the seismic behavior and influence parameters of the large-scaled thin-walled reinforced concrete (RC) tubular columns in air-cooled supporting structures of thermal power plants (TPPs). Cyclic loading tests and finite element analysis were performed on 1/8-scaled specimens considering the influence of wall diameter ratio, axial compression ratio, longitudinal reinforcement ratio, stirrup reinforcement ratio and adding steel diagonal braces (SDBs). The research results showed that the cracks mainly occurred on the lower half part of RC tubular columns during the cyclic loading test; the specimen with the minimum wall diameter ratio presented the earlier cracking and had the most cracks; the failure mode of RC tubular columns was large bias compression failure; increasing the axial compression ratio could increase the lateral bearing capacity and energy dissipation capacity, but also weaken the ductility and aggravate the lateral stiffness deterioration; increasing the longitudinal reinforcement ratio could efficiently enhance the seismic behavior; increasing the stirrup reinforcement ratio was favorable to the ductility; RC tubular columns with SDBs had a much higher bearing capacity and lateral stiffness than those without SDBs, and with the decrease of the angle between columns and SDBs, both bearing capacity and lateral stiffness increased significantly.

• 대한기계학회논문집A
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• 제30권12호
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• pp.1618-1625
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• 2006

During the last two decades, several guidelines have been developed and used for assessing the integrity of a defective steam generator (SG) tube that is generally caused by stress corrosion cracking or wall-thinning phenomenon. However, as some of SG tubes are also failed due to fretting and so on, alternative failure estimation schemes are required for relevant defects. In this paper, parametric three-dimensional finite element (FE) analyses are carried out under internal pressure condition to simulate the failure behavior of SG tubes with different defect configurations; elliptical wear, tapered and flat wear type defects. Maximum pressures based on material strengths are obtained from more than a hundred FE results to predict the failure of SG tube. After investigating the effect of key parameters such as defect depth, defect length and wrap angle, simplified failure estimation equations are proposed in relation to the equivalent stress at the deepest point in wear region. Comparison of failure pressures predicted by the proposed estimation scheme with corresponding burst test data showed a good agreement.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.407-412
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• 2001

In this research, damage behavior of singly oriented ply (SOP) fiber metal laminate (FML) subject to concentrated load was studied. The static indentation tests were conducted to study fiber orientation effect on damage behavior of FML. During the static indentation tests, Acoustic Emission technique (AE) was adopted to study damage characteristics of FML. AE signals were obtained by using AE sensor with 150kHz resonance frequency and the signals were compared with indentation curves of FML. As fiber orientation angle increases, the crack initiation load of SOP FML increases because the stiffness induced by fiber orientation is increased. The penetration load of SOP FML is influenced by the deformation tendency and boundary conditions. Cumulative AE counts were well predicted crack initiation and crack propagation and AE amplitude were useful for prediction of damage failure mode. During the matrix cracking, fiber debonding and fiber breakage, AE amplitude has $45{\sim}60dB,\;60{\sim}80dB\;and\;90{\sim}100dB$, respectively.

• 한국표면공학회지
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• 제32권6호
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• pp.647-657
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• 1999

Due to the inherently poor adhesion strength of Cu-based leadframe/EMC (Epoxy Molding Compound) interface, popcorn cracking of thin plastic packages frequently occurs during the solder reflow process. In the present work, in order to enhance the adhesion strength of Cu-based leadframe/EMC interface, black-oxide layer was formed on the leadframe surface by chemical oxidation of leadframe, and then oxidized leadframe sheets were molded with EMC and machined to form SDCB (Sandwiched Double-Cantilever Beam) and SBN (Sandwiched Brazil-Nut) specimens. SDCB and SBN specimens were designed to measure the adhesion strength between leadframe and EMC in terms of critical energy-release rate under quasi-Mode I ($G_{IC}$ ) and mixed Mode loading ($G_{C}$ /) conditions, respectively. Results showed that black-oxide treatment of Cu-based leadframe initially introduced pebble-like X$C_2$O crystals with smooth facets on its surface, and after the full growth of $Cu_2$O layer, acicular CuO crystals were formed atop of the $Cu_2$O layer. According to the result of SDCB test, $Cu_2$O crystals on the leadframe surface did not increase ($G_{IC}$), however, acicular CuO crystals on the $Cu_2$O layer enhanced $G_{IC}$ considerably. The main reason for the adhesion improvement seems to be associated with the adhesion of CuO to EMC by mechanical interlocking mechanism. On the other hand, as the Mode II component increased, $G_{C}$ was increased, and when the phase angle was -34$^{\circ}$, crack Kinking into EMC was occured.d.