• Polymer(Korea)
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• v.27 no.3
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• pp.265-270
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• 2003

In spite of high ionic conductivity, the polymer gels have poor mechanical properties and high reactivity with lithium metal anode. To solve these problems, the dry solid systems and polymer composites have been intensively studied, due to their good mechanical, thermal, chemical, and electrochemical stability. The objectives of this experiment were to improve ionic conductivity and mechanical properties of the solid polymer electrolytes based on PEO-LiClO$_4$. To obtain higher ionic conductivity and better mechanical properties, ceramic or rubber phase was added in the PEO-LiClO$_4$(8:1) matrix. The results showed that ionic conductivity and mechanical properties were improved. The ionic conductivity of the samples was as high as 10$\^$-5/ S cm$\^$-1/. This value is similar to the best ionic conductivity ever reported in the solid drying system. To obtain better results, we used PEO with various molecular weights (600∼8000) and changed the salt contents. By using DSC, we found that the addition of salt reduced the crystallinity of PEO. The mobility of polymer dependence on salt contents was examined by FT-IR.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.8
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• pp.913-920
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• 2011

We describe the evaluation of the fatigue life and strength of a lightweight railway bogie frame made of glass fiber/epoxy 4-harness satin-woven composites. To obtain the S-N curve for the evaluation of the fatigue characteristics of the composite bogie frame, we performed a tension-compression fatigue test for composite specimens with different stacking sequences of the warp direction, fill direction, and $0^{\circ}/90^^{\circ}$ direction. We used a stress ratio (R) of -1, a frequency of 5 Hz, and an endurance limit of $10^7$. The fatigue strength of the composite bogie frame was evaluated by a Goodman diagram according to JIS E 4207. The results show that the fatigue life and strength of the lightweight composite bogie satisfy the requirements of JIS E 4207. Given its weight, its performance was better than that of a conventional metal bogie frame based on an SM490A steel material.

• Composites Research
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• v.34 no.6
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• pp.357-372
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• 2021

The energy harvesting device is known to be promising as an alternative to solve the resource shortage caused by the depletion of petroleum resources. In order to overcome the limitations (environmental pollution and low mechanical properties) of piezoelectric elements capable of converting mechanical motion into electrical energy, many studies have been conducted on a polymer matrix-based composite piezoelectric energy harvesting device. In this paper, the output performance and related applications of the reported piezoelectric composites are reviewed based on the applied materials and processes. As for the piezoelectric fillers, zinc oxide, which is advantageous in terms of eco-friendliness, biocompatibility, and flexibility, as well as ceramic fillers based on lead zirconate titanate and barium titanate, were reviewed. The polymer matrix was classified into piezoelectric polymers composed of polyvinylidene fluoride and copolymers, and flexible polymers based on epoxy and polydimethylsiloxane, to discuss piezoelectric synergy of composite materials and improvement of piezoelectric output by high external force application, respectively. In addition, the effect of improving the conductivity or the mechanical properties of composite material by the application of a metal or carbon-based secondary filler on the output performance of the piezoelectric harvesting device was explained in terms of the structure of the composite material. Composite material-based piezoelectric harvesting devices, which can be applied to small electronic devices, smart sensors, and medicine with improved performance, can provide potential insights as a power source for wireless electronic devices expected to be encountered in future daily life.

• Structural Engineering and Mechanics
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• v.63 no.3
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• pp.361-370
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• 2017

Through the use of finite element analysis and acoustic emission techniques we have evaluated the interfacial failure of a carbon fiber reinforced polymer (CFRP) repair patch on a notched aluminum substrate. The repair of cracks is a very common and widely used practice in the aeronautics field to extend the life of cracked sheet metal panels. The process consists of adhesively bonding a patch that encompasses the notched site to provide additional strength, thereby increasing life and avoiding costly replacements. The mechanical strength of the bonded joint relies mainly on the bonding of the adhesive to the plate and patch stiffness. Stress concentrations at crack tips promote disbonding of the composite patch from the substrate, consequently reducing the bonded area, which makes this a critical aspect of repair effectiveness. In this paper we examine patch disbonding by calculating the influence of notch tip stress on disbond area and verify computational results with acoustic emission (AE) measurements obtained from specimens subjected to uniaxial tension. The FE results showed that disbonding first occurs between the patch and the substrate close to free edge of the patch followed by failure around the tip of the notch, both highest stress regions. Experimental results revealed that cement adhesion at the aluminum interface was the limiting factor in patch performance. The patch did not appear to strengthen the aluminum substrate when measured by stress-strain due to early stage disbonding. Analysis of the AE signals provided insight to the disbond locations and progression at the metal-adhesive interface. Crack growth from the notch in the aluminum was not observed until the stress reached a critical level, an instant before final fracture, which was unaffected by the patch due to early stage disbonding. The FE model was further utilized to study the effects of patch fiber orientation and increased adhesive strength. The model revealed that the effectiveness of patch repairs is strongly dependent upon the combined interactions of adhesive bond strength and fiber orientation.

• Journal of the Korean Society for Nondestructive Testing
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• v.17 no.2
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• pp.89-99
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• 1997

Although discontinuously reinforced metal matrix composite(MMC) is one of the most promising materials for applications of aerospace, automotive industries, the thermal residual stresses developed in the MMC due to the mismatch in coefficients of thermal expansion between the matrix and the fiber under a temperature change has been pointed out as one of the serious problem in practical applications. There are very limited nondestructive techniques to measure the residual stress of composite materials. However, many difficulties have been reported in their applications. Therefore it is important to establish analytical model to evaluate the thermal residual stress of MMC for practical engineering application. In this study, an elastic model is developed to predict the average thermal residual stresses in the matrix and fiber of a misoriented short fiber composite. The thermal residual stresses are induced by the mismatch in the coefficient of the thermal expansion of the matrix and fiber when the composite is subjected to a uniform temperature change. The model considers two-dimensional in-plane fiber misorientation. The analytical formulation of the model is based on Eshelby's equivalent inclusion method and is unique in that it is able to account for interactions among fibers. This model is more general than past models to investigate the effect of parameters which might influence thermal residual stress in composites. The present model is to investigate the effects of fiber volume fraction, distribution type, distribution cut-off angle, and aspect ratio on thermal residual stress for in-plane fiber misorientation. Fiber volume fraction, aspect ratio, and distribution cut-off angle are shown to have more significant effects on the magnitude of the thermal residual stresses than fiber distribution type for in-plane misorientation.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.11a
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• pp.7-7
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• 2001

The increasing interest in light weight materials coupled to the need for cost -effective processing have combined to create a significant opportunity for aluminum P/M. particularly in the automotive industry in order to reduce fuel emissions and improve fuel economy at affordable prices. Additional potential markets for Al PIM parts include hand tools. Where moving parts against gravity represents a challenge; and office machinery, where reciprocating forces are important. Aluminum PIM adds light weight, high compressibility. low sintering temperatures. easy machinability and good corrosion resistance to all advantages of conventional iron bm;ed P/rv1. Current commercial alloys are pre-mixed of either the AI-Si-Mg or AL-Cu-Mg-Si type and contain 1.5% ethylene bis-stearamide as an internal lubricant. The powder is compacted in closed dies at pressure of 200-500Mpa and sintered in nitrogen at temperatures between $580~630^{\circ}C$ in continuous muffle furnace. For some applications no further processing is required. although most applications require one or more secondary operations such as sizing and finishing. These sccondary operations improve the dimension. properties or appearance of the finished part. Aluminum is often considered difficult to sinter because of the presence of a stable surface oxide film. Removal of the oxide in iron and copper based is usually achieved through the use of reducing atmospheres. such as hydrogen or dissociated ammonia. In aluminum. this occurs in the solid st,lte through the partial reduction of the aluminum by magncsium to form spinel. This exposcs the underlying metal and facilitates sintering. It has recently been shown that < 0.2% Mg is all that is required. It is noteworthy that most aluminum pre-mixes contain at least 0.5% Mg. The sintering of aluminum alloys can be further enhanced by selective microalloying. Just 100ppm pf tin chnnges the liquid phase sintering kinetics of the 2xxx alloys to produce a tensile strength of 375Mpa. an increilse of nearly 20% over the unmodified alloy. The ductility is unnffected. A similar but different effect occurs by the addition of 100 ppm of Pb to 7xxx alloys. The lend changes the wetting characteristics of the sintering liquid which serves to increase the tensile strength to 440 Mpa. a 40% increase over unmodified aIloys. Current research is predominantly aimed at the development of metal matrix composites. which have a high specific modulus. good wear resistance and a tailorable coefficient of thermal expnnsion. By controlling particle clustering and by engineering the ceramic/matrix interface in order to enhance sintering. very attractive properties can be achicved in the ns-sintered state. I\t an ils-sintered density ilpproaching 99%. these new experimental alloys hnve a modulus of 130 Gpa and an ultimate tensile strength of 212 Mpa in the T4 temper. In contest. unreinforcecl aluminum has a modulus of just 70 Gpa.

• Composites Research
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• v.32 no.6
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• pp.355-359
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• 2019

The high efficient water splitting system should involve the reduction of high overpotential value, which was enhanced by the electrocatalytic reaction efficiency of catalysts, during the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) reaction, respectively. Among them, transition metal-based compounds (oxides, sulfides, phosphides, and nitrides) are attracting attention as catalyst materials to replace noble metals that are currently commercially available. Herein, we synthesized optimal monodisperse Co₃[Co(CN)₆]₂ PBAs by FESEM, and confirmed crystallinity by XRD and FT-IR, and thermal behavior of PBAs via TG-DTA. Also, we synthesized monodispersed Co₃O₄ nanocubes by calcination of Co₃[Co(CN)₆]₂ PBAs, confirmed the crystallinity by XRD, and proceeded OER measurement. Finally, the synthesized Co₃O₄ nanocubes showed a low overpotential of 312 mV at a current density of 10 mA·cm^-2 with a low Tafel plot (96.6 mV·dec^-1).

• Composites Research
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• v.29 no.3
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• pp.111-116
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• 2016

The tilting pad journal bearing for the turbo compressor application has a role to support high speed and heavy loading rotor. White metal has been widely used for the bearing material but the conventional bearing is immediately suspended and induces serious serious damage to the rotor under the unexpected oil cut situation or the insufficient oil film formation. The carbon fiber reinforced composite having high specific stiffness, specific strength and excellent tribological characteristics can solve these seizure problems. In this work, the study on the durability of high thermal resistance carbon fiber/epoxy composite tilting pad journal bearing under oil cut situation was conducted. The material properties of the composite materials including tensile, compressive and interlaminar properties were measured at room and high temperature of oil cut situation. To investigate the possibility of failure of composite tilting pad journal bearing under oil cut situation, the stress distribution of the composite bearing was analyzed via finite element analysis and the Tsai-Wu Failure index was calculated. To verify the failure analysis results, the oil cut tests for the composite tilting pad journal bearing were conducted using industrial test bench.

• Journal of the Korean Magnetics Society
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• v.15 no.2
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• pp.67-75
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• 2005

For th aim of lightweight microwave absorbers, conductive and magnetic microspheres are fabricated by plating of Co films on hollow ceramic microspheres of low density. Metal plating was carried out in a two-step electroless plating process (pre-treatment of activation and plating). Uniform coating of the film with about $2{\~}3{\cal}um$ thickness was identified by SEM. High-frequency magnetic and microwave absorbing properties were determined in the rubber composites containing the Co-coated microspheres. Due to conductive and ferromagnetic behavior of the Co thin films, high dielectric constant and magnetic loss can be obtained in the microwave frequencies. Due to those electromagnetic properties, high absorption rate (25 dB) and thin matching thickness ($2.0{\~}2.5{\cal}mm$) are predicted in the composite layers containing the metal-coated microspheres of low density (about 0.84 g/cc) for the electromagnetic radiation in microwave frequencies.

• Elastomers and Composites
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• v.42 no.4
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• pp.249-258
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• 2007

In this study, the EPDM powder which was surface activated by high temperature and shear pulverization process was prepared and the mechanical properties and odor material analysis were investigated. Analysis for particle size and size distribution of waste of the EPDM powder has been performed. The waste EPDMs used in this study were 4 types of solid, sponge, solid+sponge, and solid+metal. According to the results, the solid type showed the smallest particle size among the 4 types of EPDM powder. Effective surface devulcanization of EPDM powder could be obtained by the addition of the reclaiming agent. The dicumyl peroxide was considered as the best crosslink agent for dynamic vulcanization when the surface activated EPDM powder was blended with polyolefin in order to make TPE. Also, the optimum amounts of DCP was 6 phr in terms of surface crosslink reaction and mechanical properties of EPDM powder. The processes of water adsorption and rose oil addition were employed to remove the odor of EPDM powder caused by reclaiming agent. The GC/MS was used to analyze the odor compounds.