• Proceedings of the KWS Conference
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• 2005.11a
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• pp.189-191
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• 2005

Dissimilar joining of AI 6013-T4 alloys and austenite stainless steel was carried out using friction stir welding technique. Microstructures near the weld zone and mechanical properties of the joint have been investigated. Microstructures in the stainless steel side and AI alloy were depended on the thermo-mechanical condition which they received. TEM micrographs revealed that the interface region was composed of the mixed layers of elongated stainless steel and ultra-fine grained AI alloy and intermetallic compound layer which was identified as the $Al_{4}Fe$ with hexagonal close packed structure. Mechanical properties were lower than those of 6013 AI alloy base metal, because tool inserting location was deviated to AI alloy from the butt line, which resulted in the lack of the stirring.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.259-260
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• 2008

In this paper, we fabricated elastomer epoxy specimens by added liquid elastomer to improve the mechanical and electrical properties instead of previous high-voltage epoxy materials. As increased additive contents, glass transient temperature (Tg) was continually decreased in DSC (differential scanning calorimetry). Among specimens, 15 phr sample showed the mechanical and electrical properties similar of high-voltage epoxy in modulus, break-down and arc test. From the optimized condition of elastic epoxy, we confirmed a chance of application for high-voltage materials and power electrical instruments.

• Advanced Composite Materials
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• v.16 no.4
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• pp.335-347
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• 2007

Green composites composed of long maize fibers and poly $\varepsilon$-caprolactone (PCL) biodegradable polyester matrix were manufactured by the thermo-mechanical processing termed as 'Sequential Molding and Forming Process' that was developed previously by the authors' research group. A variety of processing parameters such as fiber area fraction, molding temperature and forming pressure were systematically controlled and their influence on the tensile properties was investigated. It was revealed that both tensile strength and elastic modulus of the composites increase steadily depending on the increase in fiber area fraction, suggesting a general conformity to the rule of mixtures (ROM), particularly up to 55% fiber area fraction. The improvement in tensile properties was found to be closely related to the good interfacial adhesion between the fiber and polymer matrix, and was observed to be more pronounced under the optimum processing condition of $130^{\circ}C$ molding temperature and 10 MPa forming pressure. However, processing out of the optimum condition results in a deterioration in properties, mostly fiber and/or matrix degradation together with their interfacial defect as a consequence of the thermal or mechanical damages. On the basis of microstructural observation, the cause of strength degradation and its countermeasure to provide a feasible composite design are discussed in relation to the optimized process conditions.

• Steel and Composite Structures
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• v.29 no.3
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• pp.319-332
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• 2018

This paper investigates the free vibration of geometrically imperfect functionally graded car-bon nanotube-reinforced composite (FG-CNTRC) beams that are integrated with two sur-face-bonded piezoelectric layers and subjected to a combined action of a uniform temperature rise, a constant actuator voltage and an in-plane force. The material properties of FG-CNTRCs are assumed to be temperature-dependent and vary continuously across the thick-ness. A generic imperfection function is employed to simulate various possible imperfections with different shapes and locations in the beam. The governing equations that account for the influence of initial geometric imperfection are derived based on the first-order shear deformation theory. The postbuckling configurations of FG-CNTRC hybrid beams are determined by the differential quadrature method combined with the modified Newton-Raphson technique, after which the fundamental frequencies of hybrid beams in the postbuckled state are obtained by a standard eigenvalue algorithm. The effects of CNT distribution pattern and volume fraction, geometric imperfection, thermo-electro-mechanical load, as well as boundary condition are examined in detail through parametric studies. The results show that the fundamental frequency of an imperfect beam is higher than that of its perfect counterpart. The influence of geometric imperfection tends to be much more pronounced around the critical buckling temperature.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.267-270
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• 2008

Thermo-mechanical simulation of the friction stir butt welding process was performed for AA5083-H18 sheets, utilizing commercial FVM code, STAR-CCM+, which is based on Eulerian formulation. Temperature and strain rate histories along the material flow were calculated under the steady state condition and simulated temperature distributions (profiles and peak values) were compared with experiments for verification. It was found that by including proper thermal properties of the backing plate (anvil) the accuracy of the simulation results increased significantly.

• Proceedings of the Polymer Society of Korea Conference
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• 2006.10a
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• pp.289-289
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• 2006

The carboxylated MWNTs were successfully prepared by conventional acid treatment, and their structures were confirmed by FT-IR, Raman and TEM analysis. The water-dispersibility of the surface modified WNTs were good. The COOH-MWNT will show better stability during the emulsion polymerization as compared with Pristine MWNT. In-situ emulsion polymerizations of methyl methacrylate N(MMA) and n-butyl acrylate (BA) were carried out. Aggregate size and dispersion stability of the CNTs in water phase were measured using dynamic light scattering, turbidity, UV-visible spectrophotometer, and electron microscope. In addition, thermo-mechanical properties of MWNT/polymer nanocomposites were investigated.

• Transactions of Materials Processing
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• v.23 no.8
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• pp.475-481
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• 2014

Water quenching is one method of cooling after hot forming, which is presently being used for the manufacturing of automobile parts. The formed parts at room temperature are heated and then cooled rapidly in a water bath to produce high strength. The formed parts may undergo excessive thermal distortion during the water quench. In order to predict the distortion during water quenching, a coupled thermo-mechanical simulation is needed. In the current study, the simulation of heating and cooling of boron steel cylinders was performed. The material properties for the simulation were calculated from JMatPro, and the convective heat transfer coefficient was obtained from experimental tests. The results show that the thermal distortion and the residual stresses are well predicted by the coupled simulation.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.399-404
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• 2000

Functionally graded materials(FGMs) involve dual-phase graded layers in which two different constituents are mixed continuously and functionally according to a given volume fraction. For the analysis of their thermo-mechanical response, conventional homogenized methods have been widely employed in order to estimate equivalent material properties of the graded layer. However, such overall estimations are insufficient to accurately predict the local behavior. In this paper, we compare the thermo-elastic behaviors predicted by several overall material-property estimation techniques with those obtained by discrete analysis models utilizing the finite element method, for various volume fractions and loading conditions.

• Structural Engineering and Mechanics
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• v.20 no.4
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• pp.435-450
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• 2005

Here, the dynamic instability characteristics of aero-thermo-mechanically stressed functionally graded plates are investigated using finite element procedure. Temperature field is assumed to be a uniform distribution over the plate surface and varied in thickness direction only. Material properties are assumed to be temperature dependent and graded in the thickness direction according to simple power law distribution. For the numerical illustrations, silicon nitride/stainless steel is considered as functionally graded material. The aerodynamic pressure is evaluated based on first-order high Mach number approximation to the linear potential flow theory. The boundaries of the instability region are obtained using the principle of Bolotin's method and are conveniently represented in the non-dimensional excitation frequency-load amplitude plane. The variation dynamic instability width is highlighted considering various parameters such as gradient index, temperature, aerodynamic and mechanical loads, thickness and aspect ratios, and boundary condition.

• Structural Engineering and Mechanics
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• v.68 no.6
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• pp.701-711
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• 2018

Herein, the thermo-magneto-elastic wave dispersion answers of functionally graded (FG) double-nanobeam systems (DNBSs) are surveyed implementing a nonlocal strain gradient theory (NSGT). The kinematic relations are derived employing the classical beam theory. Also, scale influences are covered precisely in the framework of NSGT. Moreover, Mori-Tanaka homogenization model is introduced in order to obtain the effective material properties of FG nanobeams. Meanwhile, effects of external forces such as thermal and Lorentz forces are included in this research. Also, based upon the Hamilton's principle, the Euler-Lagrange equations are developed; afterwards, these equations are incorporated with those of NSGT to reach the nonlocal governing equations of FG-DNBSs. Furthermore, according to an analytical approach, the governing equations are solved to obtain the wave frequencies and phase velocities of FG-DNBSs. At the end, some illustrations are rendered to clarify the influences of a wide range of involved parameters.