• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집A
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• pp.238-243
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• 2000

The effect of the temperature, the fatigue and the test speed on DEN(double edged notch) specimen which was made by the pp-rubber composites during fracture was stuied. DEN specimen was made on PP-rubber composites through the injection molding. With increasing temperature the fracture strength is linearly decrease and the fracture energy is first increase by $0^{\circ}C$ and after that decrease. In the same temperature the fracture strength during increasing the notch radius is hardly increase. The fracture behaviour at low and high test speed is different entirely. At high test speed plastic region is small and fracture behaviour was seen to brittle fracture tendency. The deformation mechanism of polypropylene-rubber composites during fracture was studied by SEM fractography. A strong plastic deformation of the matrix material ahead of the notch/crack occured. The deformation seem to be enhanced by a thermal blunting of the notch/crack.

• Journal of Mechanical Science and Technology
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• 제16권9호
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• pp.1078-1083
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• 2002

Ceramics are significantly used in many industrial applications due to their excellent mechanical and thermal properties such as high temperature strength, low density, high hardness, low thermal expansion, and good corrosion resistive properties, while their disadvantages are brittleness, poor formability and high manufacturing cost. To combine advantages of ceramics with those of metals, they are often used together as one composite component, which necessiates reliable joining methods between metal and ceramic. Direct brazing using an active filler metal has been found to be a reliable and simple technique, producing strong and reliable joints. In this study, the fracture characteristics of Si$_3$N$_4$ ceramic joined to ANSI 304L stainless steel with a Ti-Ag-Cu filler and a Cu (0.25-0.3 mm) interlayer are investigated as a function of strain rate and temperature. In order to evaluate a local strain a couple of strain gages are pasted at the ceramic and metal sides near joint interface. As a result the 4-point bending strength and the deflection of interlayer increased at room temperature with increasing strain rate. However bending strength decreased with temperature while deflection of interlayer was almost same. The fracture shapes were classified into three groups ; cracks grow into the metal-brazing filler line, the ceramic-brazing filler line or the ceramic inside.

• 한국세라믹학회지
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• 제26권6호
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• pp.789-794
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• 1989

The high-temperature strength of Ce-TZP was measured at 200, 650 and 100$0^{\circ}C$ by 4-point bending test. And its fracture behavior was observed by SEM. Below $650^{\circ}C$ of the temperature, where monoclinic fraction was almost zero, the decreasing rate of bending strength was relatively slow, but above this temperature, high temperature strength was largely decreased as a result of the decrease of stress-induced transformation of zirconia. The observation of fracture surface bended at 100$0^{\circ}C$ indicated that the fracture mode changed from intergranular-into transgranular-form with regardless of ceria contents.

• 대한기계학회논문집A
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• 제27권9호
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• pp.1444-1450
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• 2003

The fatigue life and tensile strength of JLF-1 steel (Fe-9Cr-2W-V-Ta) and its TIG weldment were investigated at the room temperature and $400^{\circ}C$. Four kinds of test specimens, which associated with the rolling direction and the TIG welding direction were machined. The base metal of JLF-1 steel represented almost anisotropy in the tensile properties for the rolling direction. And the base metal of JLF-1 steel showed lower strength than that of TIG weldment. Also, the strength of all materials entirely decreased in accordance with elevating test temperature. Moreover, the fatigue limit of weld metal was largely increase than that of base metal at both temperatures. The fatigue limit of JLF-1 steel decreased in accordance with elevating test temperature. The fatigue limit of JLF-1 steel decreased in accordance with elevating test temperature. The SEM fractography of tensile test specimen showed conspicuous cleavage fracture of a radial shape. In case of fatigue life test specimen, there were so many striations at crack initiation region, and dimple was observed at final fracture region as a ductile fracture mode.

• Journal of Mechanical Science and Technology
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• 제18권9호
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• pp.1604-1613
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• 2004

This paper presents an effective and reliable evaluation method for fracture strength and material degradation of the micro-structure of high temperature service steel weldment using advanced small punch (ASP) test developed from conventional small punch (CSP) test. For the purpose of the ASP test, a lower die with a minimized ${\Phi}$1.5 mm diameter loading ball and an optimized deformation guide hole of ${\Phi}$3 mm diameter were designed. The behaviors of fracture energy (E$\_$sp/), ductile-brittle transition temperature (DBTT) and material degradation from the ASP test showed a definite dependency on the micro-structure of weldment. Results obtained from ASP test were compared and reviewed with results from CSP test, Charpy impact test, and hardness test. The utility and reliability of the proposed ASP test were verified by investigating fracture strength, behavior of DBTT, and fracture location of each micro-structure of steel weldment for test specimen in ASP test. It was observed that the fracture toughness in the micro-structure of FL＋CGHAZ and ICHAZ decreased remarkably with increasing aging time. From studies of all micro-structures, it was observed that FGHAZ microstructure has the most excellent fracture toughness, and it showed absence of material degradation.

• 대한기계학회논문집A
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• 제24권3호
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• pp.710-717
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• 2000

Tensile and fracture toughness tests of the cold-rolled STS 304 steel plate for membrane material of LNG storage tank were performed at wide range of temperatures, 11 IK(boiling point of LNG), 153K , 193K and 293K(room temperature). Tensile strength significantly increases with a decrease in temperature, but the yield strength is relatively insensitive to temperature. Elongation at 193K abruptly decreases by 50% of that at 293K, and then decreases slightly in the temperature range of 193K to 111K. Strain hardening exponents at low temperatures are about four times as high as that at 293K. Elastic-plastic fracture toughness($J_c$) and tearing modulus($T_{mat}$) tend to decrease with a decrease in temperature. The $J_c$ values are inversely related to effective yield strength in the temperature range of 111K to 293K. These phenomena result from a significant increase in the amount of transformed martensite in low temperature regions.

• 대한기계학회논문집A
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• 제25권7호
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• pp.1082-1088
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• 2001

Nowadays study on recycling disused plastics for automobiles was lively progressed. Rubber and talcum powder was added to retrieve degradation of physical properties caused by recycling disused polypropylene. The effect of the temperature, the fatigue load and the loading speed on DEN(double edged notch) specimen which was made by the pp-rubber composites during fracture was studied by. DEN specimen was made on PP-rubber composites through the injection molding. With increasing temperature the fracture strength was linearly decreased and the fracture energy was increased by $0^{\circ}C$ and after that decreased. In the same temperature the fracture strength during increasing the notch radius was hardly increased. The fracture behaviour at low and high loading speed was different entirely. At high loading speed plastic region was small and fracture behaviour was seen to brittle fracture tendency. With increasing fatigue load fracture energy was first rapidly decreased and subsequently steady when radius of notch tip was 2mm, but Maximum load during fracture scarcely varied. The deformation mechanism of polypropylene-rubber composites during fracture was studied by SEM fractography. A strong plastic deformation of the matrix ahead of the notch/crack occurred. The deformation seem to be enhanced by a thermal blunting of the notch/crack.

• 대한기계학회논문집A
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• 제33권9호
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• pp.957-962
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• 2009

Sialon was produced by hot-pressing the mixtures of $Si_3N_4$, AlN and $Y_2O_3$ powders. All fracture tests were performed on a three-point loading system with a 30 mm bending span. Fracture toughness and Vickers hardness of smooth specimen were average 7.05 $MPa{\cdot}m^{0.5}$ and Hv = 1580, respectively. Density of three kinds of specimens, smooth specimen, smooth and healed specimen, smooth with $SiO_2$ colloidal coating and healed specimen, had beyond 99 % of theoretical density. Bending strength of smooth healed specimens had high strength more than 1 GPa. Crack healed specimens recovered as strength as smooth specimen. That is, cracked specimen with $SiO_2$ colloidal coating on cracked part recovered strength by heat treatment, completely. Crack healing of $Si_3N_4$ composite ceramics had contributed glassy $SiO_2$ to strength recovery. Limiting high temperature for bending strength of heat treated smooth specimen for bending strength was about 1273 K.

• 연구논문집
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• 통권22호
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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 $550^{\circ}C$

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2004년도 학술대회지
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• pp.326-330
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• 2004

Thermosetting matrix composites have disadvantages in terms of moulding time, repairability and manufacturing cost. Thus the high-performance thermoplastic composites to eliminate such disadvantages have been developed so far. As a result of environmental and economical concerns, there is a growing interest in the use of thermoplastic composites. However, since their mechanical properties are very sensitive to the environment such as moisture, temperature etc., those behaviors need to be studied. Particularly the temperature is a very important factor influencing the mechanical behavior of thermoplastic composites. The effect of temperature have not yet been fully quantified. Since engineering applications of reinforced composites necessitate their fracture mechanics characterization, work is in progress to investigate the fracture and related failure behavior. An approach which predicts the tensile strength was perpormed in the tensile test. The main goal of this work is to study the effect of temperature on the result of tensile test with respect to GF/PE composite. The tensile strength and failure mechanisms of GF/PE composites were investigated in the temperature range $60^{\circ}C\;to\;-50^{\circ}C$. The tensile strength increased as the fiber volume fraction ratio increased. The tensile strength showed the maximum at $-50^{\circ}C$, and it tended to decrease as the temperature increased from $-50^{\circ}C$. The major failure mechanisms was classified into the fiber matrix debonding, the fiber pull-out, the delamination and the matrix deformation.