• Journal of Welding and Joining
• /
• v.33 no.6
• /
• pp.6-12
• /
• 2015

Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of $200^{\circ}C$ to $900^{\circ}C$ due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, $M_6C$ with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.

• Carbon letters
• /
• v.15 no.4
• /
• pp.247-254
• /
• 2014

In this work, thermal treatment accompanied with different acid treatments was applied to a commercial coal tar pitch (CTP) to obtain a spinnable precursor pitch for carbon fiber. In the case of thermal treatment only, a relatively high reaction temperature of between $380^{\circ}C$ and $400^{\circ}C$ was required to obtain a softening point (SP) range of $220^{\circ}C-260^{\circ}C$ and many meso-phase particles were created during the application of high reaction temperature. When nitric acid or sulfuric acid treatment was conducted before the thermal treatment, the precursor pitch with a proper SP range could be obtained at reaction temperatures of $280^{\circ}C-300^{\circ}C$, which were about $100^{\circ}C$ lower than those for the case of thermal treatment only. With the acid treatments, the yield and SP of the precursor pitch increased dramatically and the formation of meso-phase was suppressed due to the lower reaction temperatures. Since the precursor pitches with acid and thermal treatment were not spinnable due to the inhomogeneity of properties such as molecular weight distribution and viscosity, the CTP was mixed with ethanol before the consecutive nitric acid and thermal treatments. The precursor pitches with ethanol, nitric acid, and thermal treatments were easily spinnable, and their spinning and carbon fiber properties were compared to those of air blowing and thermal treated CTP.

• Structural Engineering and Mechanics
• /
• v.65 no.6
• /
• pp.645-656
• /
• 2018

Importance of procuring adequate knowledge about the mechanical behavior of double-layered graphene sheets (DLGSs) incensed the authors to investigate wave propagation responses of mentioned element while rested on a visco-Pasternak medium under hygro-thermal loading. A nonlocal strain gradient theory (NSGT) is exploited to present a more reliable size-dependent mechanical analysis by capturing both softening and hardening effects of small scale. Furthermore, in the framework of a classical plate theory the kinematic relations are developed. Incorporating kinematic relations with the definition of Hamilton's principle, the Euler-Lagrange equations of each of the layers are derived separately. Afterwards, combining Euler-Lagrange equations with those of the NSGT the nonlocal governing equations are written in terms of displacement fields. Interaction of the each of the graphene sheets with another one is regarded by the means of vdW model. Then, a widespread analytical solution is employed to solve the derived equations and obtain wave frequency values. Subsequently, influence of each participant variable containing nonlocal parameter, length scale parameter, foundation parameters, temperature gradient and moisture concentration is studied by plotting various figures.

• Journal of the Korean Ceramic Society
• /
• v.26 no.6
• /
• pp.811-819
• /
• 1989

There have been many studies on the biological phenomena of Bioglasses, which nay be used as implant materials in human body. However, not many works on the Bioglass compositions have been reported. In the present study, the effect of Al2O3 substitution for SiO2 in Bioglass of Na2O-CaO-P2O5-SiO2 system on its structure and properties was examined. Infrared and Raman spectroscopic studies for the glass structural analysis, differential thermal analysis and X-ray diffraction analysis for crystallization of the glass were perfomed. Several physical properties, such as thermal expansion coefficient, softening point, microhardness and reaction phenomena, were also measured. The major crystalline phase, after heat treatment of the glasses, was Na2Ca2(SiO2)3 and the crystal was transformed into other phase with increased substitution of Al2O3. The added Al2O3 reduced non-bridging oxygen in glass structure and thermal expansion coefficient, but increased glass density, sofening point and microhardness. When the glasses are reacted in Tris-buffer solution, the substituted Al2O3 inhibited the formation of hydroxyapatite on the Bioglas surface, and no hydroxyapatite was formed for the sample which contained more than 6wt.% of Al2O3 even if they were reacted for 600 hours.

• Journal of the Korean Ceramic Society
• /
• v.43 no.12 s.295
• /
• pp.865-871
• /
• 2006

In order to examine and compare the characteristics of two vitrified forms (AG8W1 and DG2) simulated for the operation of a commercial vitrification facility being constructed in Ulchin nuclear power plant, the vitrified forms were cooled by the natural cooling and annealing methods respectively. And the Product Consistency Test (PCT), compressive strength, thermal conductivity, specific heat, phase stability, softening point and Coefficient of Thermal Expansion (CTE) of the vitrified farms were experimented. Consequently, it was shown that there were no significant differences on the physiochemical properties of the vitrified forms performed the natural cooling and annealing.

• Journal of the Korean Society for Precision Engineering
• /
• v.28 no.12
• /
• pp.1353-1358
• /
• 2011

LAT(Laser Assisted Turning) is a method that applies a machining process after softening a workpiece in which a preheating process is locally applied to its machining section using laser heat source. LAT shows several advantages, such as high productivity, reduction of manufacturing cost, high quality. Analysis of temperature distribution after preheating for LAT is very difficult due to its very small heat input area and large energy and its movement. Also, the LAT for a square bar is more difficult because the shape of a laser heat source can be changed according to the rotation of the workpiece. In this study, thermal analysis for LAT of square bar was performed using laser heat source projection method. And, the analysis results were compared with the results of the prior study of numerical calculation method. It is thus shown that the proposed method is efficient for the thermal analysis of a shaped bar.

• Advances in nano research
• /
• v.16 no.2
• /
• pp.187-200
• /
• 2024

This study focuses on wave propagation analysis in the curved nanobeam exposed to different thermal loadings based on the Nonlocal Strain Gradient Theory (NSGT). Mechanical properties of the constitutive materials are assumed to be temperature-dependent and functionally graded. For modeling, the governing equations are derived using Hamilton's principle. Using the proposed model, the effects of small-scale, geometrical, and thermo-mechanical parameters on the dynamic behavior of the curved nanobeam are studied. A small-scale parameter, Z, is taken into account that collectively represents the strain gradient and the nonlocal parameters. When Z<1 or Z>1, the phase velocity decreases/increases, and the stiffness-softening/hardening phenomenon occurs in the curved nanobeam. Accordingly, the phase velocity depends more on the strain gradient parameter rather than the nonlocal parameter. As the arc angle increases, more variations in the phase velocity emerge in small wavenumbers. Furthermore, an increase of ∆T causes a decrease in the phase velocity, mostly in the case of uniform temperature rise rather than heat conduction. For verification, the results are compared with those available for the straight nanobeam in the previous studies. It is believed that the findings will be helpful for different applications of curved nanostructures used in nano-devices.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.11a
• /
• pp.321-321
• /
• 2008

To obtaingood resolution in PDP, one of the important factors is to achieve the accuracy of barrier ribs. The photolithographic process can be used to form patterns of barrier rib with high accuracy and a high aspect ratio. The composition for photolithography is based on the $B_2O_3-SiO_2-Al_2O_3$ glass system including additives such as alkali oxides and alkali earth oxides. The refractive index and thermal properties in glass system are changed by amount of alkali oxides and alkali earth oxides. Therefore, it is important that additives are controlled to have proper refractive index and thermal properties. The additives are contributed to non-bridging oxygen within the glass network, causing a change of density. In addition to a change of the structural cross-link density, the refractive index, dielectric and thermal properties glass are correlated with ionic radius and polarizability of cations. In this study, we investigated the refractive index and the thermal properties such as glass transition temperature, glass softening temperature and coefficient of thermal expansion by changing composition in the $B_2O_3-SiO_2-Al_2O_3$ glass system.

• Journal of the Korean institute of surface engineering
• /
• v.32 no.2
• /
• pp.144-156
• /
• 1999

Thermal Barrier Coating with Functional Gradient Materials (FGM-TBC) can play an important role to protect the parts from harmful environments in high temperatures such as oxidation, corrosion, and wear and to improve the efficiency of aircraft engine by lowering the surface temperature on turbine blade. FGM-TBC can increase the life spans of product and improve the operating properties. Therfore, in this study the evaluations of mechanical and thermal properties of FGM-TBC such as fatigue, oxidation and wear-resistance at high temperatures have been conducted. The samples of both the TBC with 2, 3, 5 layers (YSZ/NiCrAlY) to be produced by Air Plasma Spray method (APS) and the bulk TBC with 6 layers to be produced by Plasma Assisted Sintering method (PAS) were used. Furthermore, residual stress, bond strength, and thermal conductivity were evaluated. The average thickness of the APS was 500$\mu\textrm{m}$ to 600$\mu\textrm{m}$ and the average thickness of the PAS was 3mm. The hardness number of the top layer of APS was 750 Hv to 810Hv and that of PAS was 950 Hv to 1440Hv. The $ZrO_2$ coating layer of APS was composed of tetragonal structure after spraying as the result of XRD analysis. As shown in the results of the high temperature wear test, the 3 layer coating of APS had the best wear resistance at $800^{\circ}C$ and the 5 layer coating of APS had the best wear resistance at $600^{\circ}C$. But, these coatings had the tendency of the low-temperature softening at $300^{\circ}C$. The main mechanism of wear was the adhesive wear and the friction coefficient of coatings was increased as increasing the test temperatures. A s results of thermal conductivity test, the ${\Delta}T$ of the APS coating was increased as number of layer and the range of thermal conductivity of the PAS was $800^{\circ}C$ to $1000^{\circ}C$.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.23 no.1
• /
• pp.44-50
• /
• 2013

E-glass fiber is the most widely used glass fiber for reinforced composite materials of aircrafts, automobiles and leisure equipments. But recently researches are being progressed to reduce boric oxide from 8 % to 0 (zero), as is called 'Boron free E-glass', because of increasing material cost, environmental problem, and improving chemical resistance and mechanical properties of E-glass fiber. In this study, we fabricated the bulk glass and fiber glass of 'Boron free E-glass (BF) compositions', and characterized thermal properties and optical properties. 'Boron free E-glass (BF)' was obtained by the melting of mixed batch materials at $1550^{\circ}C$ for 2 hrs with different $Al_2O_3$ compositions 5~10 %. We obtained transparent clear glass with high visible light transmittance value of 81~86 %, and low thermal expansion coefficient of $4.2{\sim}4.9{\times}10^{-6}/^{\circ}C$ and softening point of $907{\sim}928^{\circ}C$. For the chemical resistance test of 'BF' fiber samples, we identified that the higher alumina contents gives the better corrosion resistance of glass fiber.