• Steel and Composite Structures
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• v.15 no.1
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• pp.1-14
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• 2013

Predicting the fire resistance of structures has been significantly advanced by full scale fire tests in conjunction with improved understanding of compartmental fire. Despite the progress, application of insulation is still required to parts of structural steelwork to achieve over 60 minutes of fire rating. It is now recognised that uncertainties on insulation properties hinder adaptation of performance based designs for different types of structures. Intumescent coating has recently appeared to be one of most popular insulation types for steel structures, but its design method remains to be confirmed by empirical data, as technical difficulties on the determination of the material properties at elevated temperatures exist. These need to take into account of further physiochemical transitions such as moving boundary and endothermic reaction. The impetus for this research is to investigate the applicability of the conventional differential equation solution which examines the temperature rise on coated steel members by an inorganic intumescent coating, provided that the temperature-dependent thermal/mechanical insulation properties are experimentally defined in lab scale tests.

• Proceedings of the KWS Conference
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• 2006.05a
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• pp.137-139
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• 2006

In order to establish the optimum hot forming temperature and solution heat treatment temperature for 25% chromium super duplex stainless steel weld, a commercial 25%Cr-10%Ni-4%Mo weld metal for super duplex stainless steel(UNS S32750) with different solution heat treatment conditions at $1100^{\circ}C,\;1050^{\circ}C,\;1025^{\circ}C\;and\;1000^{\circ}C$ for 1.5 hours has been investigated by means of optical metallography, and estimated mechanical properties. It is found that exposure to elevated temperatures at $1050^{\circ}C,\;1025^{\circ}C\;and\;1000^{\circ}C$ except $1100^{\circ}C$ brings partial decomposition of ferrite to austenite and sigma phase, which deteriorates their properties and heat treatment at $1100^{\circ}C$ shows acceptable mechanical properties.

• Steel and Composite Structures
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• v.15 no.6
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• pp.705-724
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• 2013

The behaviour of steel column at elevated temperature is significantly different than that at ambient temperature due to its changes in the mechanical properties with temperature. Reported literature suggests that steel column may become vulnerable when exposed to fire condition, since its strength and capacity decrease rapidly with temperature. The present study aims at investigating the lateral load resistance of non-insulated steel columns under fire exposure through finite element analysis. The studied parameters include moment-rotation behaviour, lateral load-deflection behaviour, stiffness and ductility of columns at different axial load levels. It was observed that when the temperature of the column was increased, there was a significant reduction in the lateral load and moment capacity of the non-insulated steel columns. Moreover, it was noted that the stiffness and ductility of steel columns decreased sharply with the increase in temperature, especially for temperatures above $400^{\circ}C$. In addition, the lateral load capacity and the moment capacity of columns were plotted against fire exposure time, which revealed that in fire conditions, the non-insulated steel columns experience substantial reduction in lateral load resistance within 15 minutes of fire exposure.

• Transactions of the Korean Society of Mechanical Engineers
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• v.7 no.4
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• pp.425-433
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• 1983

Fatigue tests by axial loading (R=0.1) were carried out to investigate fatigue crack growth properties of small surface cracks in mild steel at room temperature, 250.deg. C and 400.deg. C, by using flat specimens with a small artificial pit. All the data of the fatigue crack growth rate obtained in the present tests are determined as a function of the stress intensity factor range, so that the applicability of liner fracture mechanics to the fatigue crack growth of surface cracks at elevated temperatures is investigated and discussed in comparison with the data of type 304 stainless steel at room temperature and elevated temperature. The obtained results are as follows: 1) Relations of both surface fatigue crack length and its depth to cycle ratio fall within a narrow scatter band in spite of different stress levels. 2) The .DELTA. .sigma. .root. .pi. a-da/dN relation of surface fatigue crack growth at room temperature is independent of the stress level and can be plotted as a straight line at log-log diagram, but the relation at 400.deg. C depends partly on the stress level. 3) Relations of the fatigue crack growth into depth d(2b)/dN and is stress intensity factor range .DELTA. $K_{I}$, accounted for the aspect ratio variation, fall within a narrow scatter band for wide range of the applied stress levels. And .DELTA. $K_{I}$E-d(2b)/dN relations of mild steel at different stress level coincide relatively well with the data of type 304 stainless steel. 4) The value of aspect ratio obtained by a beach mark method and a temper coloring method approaches about 0.9 in common with crack growth and it is independent of stress level and temperatures. 5) The equi-crack length curve is parallel to S-N$_{f}$ curve at elevated temperatures.s.s.s.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.4
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• pp.432-436
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• 1999

Fiber growth of $Al_{2}O_{3}/R-Al-O$ (R = Y, Gd, Dy, Ho, Er) eutectic by the micro-pulling down method is described. The thermal stability and strength at elevated temperature of each materials is evaluated in relation to the microstructure.

• 연구논문집
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• s.22
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• pp.115-125
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• 1992

Mechanical properties and microstructure were investigated on vacuum induction melted $Fe_3A1$base alloys of $DO_3$ structure. Specal emphasis were put on the effect of alloy chemistry, grain size and process(rolling, directional solidification) on mechanical properties of Fe-22.5-39at.%Al at elevated temperature between room temperature and $800^{\circ}C$. grain size of as-cast alloys is refined by rolling from 1mm to $80\mum$. Tensile strength of Fe-24.lat.%AI was about 404MPa at the critical ordering temperature, and the fracture strain of the alloy was 1-2% at room temperature. An inverse temperature dependence of the strength is noticed as-cast $Fe_3A1$. The presence of Cr and Zr do not affect the room temperature ductility and high temperature strength. Fracture strain of directionally solidified(DS) $Fe_3A1$ is about 1%at room temperature, but is about 60%at. $T_C$(550^{\circ}C)$. Tensile strength of DS alloy is lower than that of as-cast alloy at $530^{\circ}C$ and $430^{\circ}C$. Failure mode at room temperature varies from transgranular fracture to intergranular fracture with the addition of Al. the failure mode also varies from mixed(transgranular+ intergranular) mode between room temperature and $500^{\circ}C$ to intergranular mode above $550^{\circ}C$

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.95.1-95.1
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• 2012

Very High Temperature gas cooler Reactor (VHTR) has been considered as one of the most promising nuclear reactor because of many advantages including high inherent safety to avoid environmental pollution, high thermal efficiency and the role of secondary energy source. The TRISO coated fuel particles used in VHTR are composed of 4 layers as OPyC, SiC, IPyC and buffer PyC. The significance of CVD-SiC coatings used in tri-isotropic(TRISO) nuclear coated fuel particles is to maintain the strength of the whole particle. Various methods have been proposed to evaluate the mechanical properties of CVD-SiC film at room temperature. However, few works have been attempted to characterize properties of CVD-SiC film at high temperature. In this study, micro tensile system was newly developed for mechanical characterization of SiC thin film at elevated temperature. Two kinds of CVD-SiC films were prepared for micro tensile test. SiC-A had [111]-preferred orientation, while SiC-B had [220]-preferred orientation. The free silicon was co-deposited in SiC-B coating layer. The fracture strength of two different CVD-SiC films was characterized up to $1000^{\circ}C$.The strength of SiC-B film decreased with temperature. This result can be explained by free silicon, observed in SiC-B along the columnar boundaries by TEM. The presence of free silicon causes strength degradation. Also, larger Weibull-modulus was measured. The new method can be used for thin film material at high temperature.

• Tribology and Lubricants
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• v.11 no.4
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• pp.45-52
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• 1995

The wear behavior of alumina short fiber reinforced tin-bronze matrix composites was studied at the room temperature and an elevated temperature. The effect of the composition of specimens and the variation of wear conditions on the wear properties was examined by a pin-on-disc type wear testing machine. The wear mechanism according to the compositon of specimens at various wear conditions was discussed by the observation of the microstructure and the analysis of the composition on the worn surfaces. A thicker oxide layer on worn surfaces led to a lower wear loss because of the lubricating effect of oxide layers between pin and disc. As the testing temperature was raised to 350$^{\circ}$C, the fiber reinforced composites exibited markedly increased wear resistance even at a higher applied load since the reinforcement of composites with alumina fibers was not affected to a large extent by raising temperature. The results obtained by AES and EDS analysis indicated that the oxide layer of the worn surfaces formed at 350$^{\circ}$C was proved as Fe-oxide. This was explained by the faster formation of Fe-oxide than Cu-oxide at 350$^{\circ}$C.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.3
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• pp.355-361
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• 1987

Creep and fatigue tests were carried out on crack growth properties of small surface cracks in 304 stainless steel at 538.deg.C, 593.deg. C and 650.deg. C in air, by using small plate specimens with a small artificial pit. All the data of the crack growth rate per hour obtained in the present tests were correlated with the maximum stress intensity factor, so that the applicability of linear fracture mechanics to the crack growth of surface cracks at elevated temperature was investigated. In the creep test, relatiion of .sigma.$\^$n/.t$\_$f/=C is obtained between failure time and nominal stress at each temperature level, where n has the value of 11-14 depending on the temperature level. In the creep and fatigue crack growth properties of surface cracks at the elevated temperatures, the maximum stress intensity factor, $_{4}$$\_$max/, is some extent applicable parameter to describe the surface crack growth rate under the present experimental conditions. The crack growth rate per hour increases when the holding time decreases, and creep crack growth rate per hour becomes the lowest limit of crack growth rate per hour in this tests.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.144.2-144.2
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• 2016

Diamond-like carbon (DLC) coatings have been widely applied to the mechanical components, cutting tools due to properties of high hardness and wear resistance. Among them, hydrogenated amorphous carbon (a-C:H) coatings are well-known for their low friction properties, stable production of thin and thick film, they were reported to be easily worn away under high temperature. Non-hydrogenated tetrahedral amorphous carbon (ta-C) is an ideal for industrial applicability due to good thermal stability from high $sp^3$-bonding fraction ranging from 70 to 80 %. However, the large compressive stress of ta-C coating limits to apply thick ta-C coating. In this study, the thick ta-C coating was deposited onto Inconel alloy disk by the FCVA technique. The thickness of the ta-C coating was about $3.5{\mu}m$. The tribological behaviors of ta-C coated disks sliding against $Si_3N_4$ balls were examined under elevated temperature divided into 23, 100, 200 and $300^{\circ}C$. The range of temperature was setting up until peel off observed. The experimental results showed that the friction coefficient was decreased from 0.14 to 0.05 with increasing temperature up to $200^{\circ}C$. At $300^{\circ}C$, the friction coefficient was dramatically increased over 5,000 cycles and then delaminated. These phenomenon was summarized two kinds of reasons: (1) Thermal degradation and (2) graphitization of ta-C coating. At first, the reason of thermal degradation was demonstrated by wear rate calculation. The wear rate of ta-C coatings showed an increasing trend with elevated temperature. For investigation of relationship between hardness and graphitization, thick ta-C coatings(2, 3 and $5{\mu}m$) were additionally deposited. As the thickness of ta-C coating was increased, hardness decreased from 58 to 49 GPa, which means that graphitization was accelerated. Therefore, now we are trying to increase $sp^3$ fraction of ta-C coating and control the coating parameters for thermal stability of thick ta-C at high temperatures.