• Advances in materials Research
• /
• v.7 no.1
• /
• pp.1-16
• /
• 2018

Thermal bifurcation buckling behavior of fully clamped Euler-Bernoulli nanobeam built of a through thickness functionally graded material is explored for the first time in the present paper. The variation of material properties of the FG nanobeam are graded along the thickness by a power-law form. Temperature dependency of the material constituents is also taken into consideration. Eringen's nonlocal elasticity model is employed to define the small-scale effects and long-range connections between the particles. The stability equations of the thermally induced FG nanobeam are derived via the principal of the minimum total potential energy and solved analytically for clamped boundary conditions, which lead for more accurate results. Moreover, the obtained buckling loads of FG nanobeam are validated with those existing works. Parametric studies are performed to examine the influences of various parameters such as power-law exponent, small scale effects and beam thickness on the critical thermal buckling load of the temperature-dependent FG nanobeams.

• Journal of the Korean Ceramic Society
• /
• v.53 no.2
• /
• pp.178-183
• /
• 2016

Ferroelectric relaxor ceramics with $BaTiO_3-NaNbO_3-Bi(Mg_{1/2}Ti_{1/2})O_3$ ternary compositions (BT-NN-BMT) have been prepared by sol-gel powder synthesis and consequent bulk ceramic processing. Through the modified chemical approach, fine and single-phase complex perovskite compositions were successfully obtained. Temperature and frequency dependent dielectric properties indicated typical relaxor characteristics of the BT-NN-BMT compositions. The ferroelectric-paraelectric phase transition became diffusive when NN and BMT were added to form BT based solid solutions. BMT additions to the BT-NN solid solutions affected the high temperature dielectric properties, which might be attributable to the compositional inhomogeneity of the complex perovskite and resulting weak dielectric coupling of the Bi-containing polar nanoregions (PNRs). The temperature stability of the dielectric properties was good enough to satisfy the X9R specification. The quasi-linear P-E response and the temperature- stable dielectric properties imply the high potential of this ceramic compound for use in high temperature capacitors.

• Journal of Electrical Engineering and Technology
• /
• v.9 no.1
• /
• pp.114-120
• /
• 2014

In this paper, a method for multi-physics analysis of the temperature-dependent properties of an oil-immersed transformer is discussed. To couple thermal fields with electromagnetic and fluid fields, an algorithm employing a user defined function (UDF) is proposed. Using electromagnetic analysis, electric power loss dependent on temperature rise is calculated; these are used as input data for multi-physics analysis in order to predict the temperature rise. A heat transfer coefficient is applied only at the outermost boundary between transformer and the atmosphere in order to reduce the analysis region. To verify the validity of the proposed method, the predicted temperature rises in high-voltage (HV) and low-voltage (LV) windings and radiators were compared with the experimental values.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.22 no.5
• /
• pp.596-606
• /
• 1998

Thermophysical properties and the compressive strength of concrete used in nuclear power plants in Korea were measured. The chemical composition of the concrete was also analyzed. The measured thermophysical properties include the density, the thermal conductivity, the thermal diffusivity and the specific heat for a wide temperature range of 20.deg. C to 1100.deg. C. The chemical composition of Korean concrete is similar to that of US basaltic concrete and the thermophysical properties are strongly temperature dependent. The density, the conductivity and the diffusivity decrease with an increase in temperature, and particularly the conductivity and the diffusivity are a 50-perdent decrease at 900.deg. C as compared with these values at room temperature. The specific heat increases until 500.deg. C, decreases from 700.deg. C to 900 .deg. C, and then increases again when temperature is above 900.deg. C. The measurement beyond 1100.deg. C is not acceptably accurate because the concrete decomposes to a liquid phase from a solid phase at that temperature. The results of this study can be applied, for example, to an analysis of the molten core-concrete interaction (MCCI) phenomenon of concrete structures at high temperature will also require those property data, especially for high temperature ranges.

• Korean Journal of Materials Research
• /
• v.27 no.2
• /
• pp.69-75
• /
• 2017

To establish low-temperature process conditions, process-property correlation has been investigated for Ga-doped ZnO (GZO) thin films deposited by pulsed DC magnetron sputtering. Thickness of GZO films and deposition temperature were varied from 50 to 500 nm and from room temperature to $250^{\circ}C$, respectively. Electrical properties of the GZO films initially improved with increase of temperature to $150^{\circ}C$, but deteriorated subsequently with further increase of the temperature. At lower temperatures, the electrical properties improved with increasing thickness; however, at higher temperatures, increasing thickness resulted in deteriorated electrical properties. Such changes in electrical properties were correlated to the microstructural evolution, which is dependent on the deposition temperature and the film thickness. While the GZO films had c-axis preferred orientation due to preferred nucleation, structural disordering with increasing deposition temperature and film thickness promoted grain growth with a-axis orientation. Consequently, it was possible to obtain a good electrical property at relatively low deposition temperature with small thickness.

• Structural Engineering and Mechanics
• /
• v.59 no.2
• /
• pp.343-371
• /
• 2016

In this paper thermo-mechanical vibration analysis of a porous functionally graded (FG) Timoshenko beam in thermal environment with various boundary conditions are performed by employing a semi analytical differential transform method (DTM) and presenting a Navier type solution method for the first time. The temperature-dependent material properties of FG beam are supposed to vary through thickness direction of the constituents according to the power-law distribution which is modified to approximate the material properties with the porosity phases. Also the porous material properties vary through the thickness of the beam with even and uneven distribution. Two types of thermal loadings, namely, uniform and linear temperature rises through thickness direction are considered. Derivation of equations is based on the Timoshenko beam theory in order to consider the effect of both shear deformation and rotary inertia. Hamilton's principle is applied to obtain the governing differential equation of motion and boundary conditions. The detailed mathematical derivations are presented and numerical investigations are performed while the emphasis is placed on investigating the effect of several parameters such as porosity distributions, porosity volume fraction, thermal effect, boundary conditions and power-low exponent on the natural frequencies of the FG beams in detail. It is explicitly shown that the vibration behavior of porous FG beams is significantly influenced by these effects. Numerical results are presented to serve benchmarks for future analyses of FG beams with porosity phases.

• Steel and Composite Structures
• /
• v.46 no.4
• /
• pp.553-563
• /
• 2023

In the current research, the nonlinear free vibrations of curved pipes made of functionally graded (FG) carbon nanotube reinforced composite (CNTRC) materials are investigated. It is assumed that the FG-CNTRC curved pipe is supported on a three-parameter nonlinear elastic foundation and is subjected to a uniform temperature rise. Properties of the curved nanocomposite pipe are distributed across the radius of the pipe and are given by means of a refined rule of mixtures approach. It is also assumed that all thermomechanical properties of the nanocomposite pipe are temperature-dependent. The governing equations of the curved pipe are obtained using a higher order shear deformation theory, where the traction free boundary conditions are satisfied on the top and bottom surfaces of the pipe. The von Kármán type of geometrical non-linearity is included into the formulation to consider the large deflection in the curved nanocomposite pipe. For the case of nanocomposite curved pipes which are simply supported in flexure and axially immovable, the motion equations are solved using the two-step perturbation technique. The closed-form expressions are provided to obtain the small- and large-amplitude frequencies of FG-CNTRC curved pipes rested on a nonlinear elastic foundation in thermal environment. Numerical results are given to explore the effects of CNT distribution pattern, the CNT volume fraction, thermal environment, nonlinear foundation stiffness, and geometrical parameters on the fundamental linear and nonlinear frequencies of the curved nanocomposite pipe.

• Steel and Composite Structures
• /
• v.26 no.4
• /
• pp.513-531
• /
• 2018

In this article, static, buckling and free vibration analyses of a sinusoidal micro composite beam reinforced by single-walled carbon nanotubes (SWCNTs) with considering temperature-dependent material properties embedded in an elastic medium in the presence of magnetic field under transverse uniform load are presented. This system is used at micro or sub micro scales to enhance the stiffness of micro composite structures such as bar, beam, plate and shell. In the present work, the size dependent effects based on surface stress effect and modified strain gradient theory (MSGT) are considered. The generalized rule of mixture is employed to predict temperature-dependent mechanical and thermal properties of micro composite beam. Then, the governing equations of motions are derived using Hamilton's principle and energy method. Numerical results are presented to investigate the influences of material length scale parameters, elastic foundation, composite fiber angle, magnetic intensity, temperature changes and carbon nanotubes volume fraction on the bending, buckling and free vibration behaviors of micro composite beam. There is a good agreement between the obtained results by this research and the literature results. The obtained results of this study demonstrate that the magnetic intensity, temperature changes, and two parameters elastic foundations have important effects on micro composite stiffness, while the magnetic field has greater effects on the bending, buckling and free vibration responses of micro composite beams. Moreover, it is shown that the effects of surface layers are important, and observed that the changes of carbon nanotubes volume fraction, beam length-to-thickness ratio and material length scale parameter have noticeable effects on the maximum deflection, critical buckling load and natural frequencies of micro composite beams.

• Textile Coloration and Finishing
• /
• v.25 no.3
• /
• pp.206-214
• /
• 2013

There has been an increasing demand for high performance and energy-saving of blind. In order to develop the eco-friendly blind textiles, heat treatment process has been utilized for LM(Low Melting) polyester fabrics and the changes of the physical properties of the treated fabrics were examined according to temperature of heat treatment. Morphology, surface reflectance, contact angle, luster, thermal property and mechanical property of heat treated LM polyester fabrics were investigated. As results, morphology analysis of thermal treated fabrics confirmed that degree of fusion of LM polyester yarns improved with increase of temperature. Surface reflectance of thermal treated fabrics decreased with increase of temperature. Luster and contact angle of a water droplet on thermal treated fabrics increased slightly with increase of temperature. The mechanical properties of the fabrics by KES-FB system were found to be temperature-dependent and especially, bending and shear properties among them were highly related to temperature.

• Structural Engineering and Mechanics
• /
• v.30 no.5
• /
• pp.577-592
• /
• 2008

Laplace-Fourier transform techniques are used to investigate the interaction caused by mechanical, thermal and microstress sources in a generalized thermomicrostretch elastic medium with temperature-dependent mechanical properties. The modulus of elasticity is taken as a linear function of reference temperature. The integral transforms are inverted using a numerical technique to obtain the normal stress, tangential stress, tangential couple stress, microstress and temperature distribution. Effect of temperature dependent modulus of elasticity and thermal relaxation times have been depicted graphically on the resulting quantities. Comparisons are made with the results predicted by the theories of generalized thermoelasticity. Some particular cases are also deduced from the present investigation.