• Journal of Power System Engineering
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• v.18 no.2
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• pp.62-69
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• 2014

In this paper, in order to investigate the effects of marco and microscopic observations of fatigue crack growth in friction stir welded (FSWed) 7075-T651 aluminum alloy plates, fatigue crack growth tests were performed under constant amplitude loading condition at room temperature with three different pre-cack locations, namely base metal (BM-CL) and two kinds of pre-crack locations in welded joints, weld metal (WM-CL) and heat affected zone (HAZ-CL) specimens. The fatigue crack growth behavior of FSWed 7075-T651 aluminum alloy plates were discussed based on the marco and microscopic fractographic observations. The marcoscopic aspects of surface crack growth path for BM-CL and HAZ-CL specimens indicate relatively straight lines, however, the crack growth paths of WM-CL specimens grow first straight and by followed toward the TMAZ and HAZ. The microscopic aspects of fatigue fracture for BM-CL and HAZ-CL specimens indicate typical fatigue striation, but WM-CL showed intergranular fracture pattern by micro structural changes of FSW process.

• Journal of the Korean Ceramic Society
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• v.20 no.1
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• pp.63-69
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• 1983

It is know that $Si_3-N_4$ bonded SiC has almost all the valuable properties needed for the high temperature material and thus has bery wide range of applicability. Si powder and two different sized SiC powder were used as the raw mateials. Specimens were prepared by heating the green compact mode of the raw materials with polyvinyl alcohol binder in the nitrogen atmosphere. The bond-ing of SiC particles is brought about with the formation of reaction bonded silicon nitride phase between the particles he influences of the variation of the relative amounts of the raw materials and the amount of the organic binder on the density and the bend strength of the specimens were investigated. It was shown that the calculation of the amount of the nitridation of Si is somewhat complicated matter since some portion of the organic binder reacts with the Si during the firing process. Fixing the Si amount to 20w/o the distributions of the size of the SiC particles that gives the maximum density and the maximum strnegth were obtained through experiments. It was observed that the two distributions were not equal to each other. As the amount of Si increased the amount of Si reacted with nitrogen and the strength increased. The fracture mode was intergranular for the most part and the transgranular fracture was scarcely observed.

• Journal of the Korean Ceramic Society
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• v.52 no.5
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• pp.374-379
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• 2015

Mullite-matrix and $Al_2O_3$-matrix composites were fabricated with 30 wt% hydroxyapatite (HA) and tricalcium phosphate (TCP), respectively, as additives to give bioactivity. A diphasic gel process was employed to lower the densification temperature of the mullite matrix to $1320^{\circ}C$. A polymer complexation process was used to synthesize a TCP powder that was fully densified at $1250^{\circ}C$, for application to the matrix. For the HA/mullite composite, HA decomposed during sintering by reactions with the matrix components of $Al_2O_3$ and $SiO_2$, resulting in a mixture of $Al_2O_3$, TCP, and other minor phases with a low densification of less than 88% of the theoretical density (TD). In contrast, the TCP/$Al_2O_3$ composite was highly densified by sintering at $1350^{\circ}C$ to 96%TD with no reaction between the components. Different from the TCP monolith, the TCP/$Al_2O_3$ composite also showed a fine microstructure and intergranular fracture, both of which characteristics are advantageous for strength and fracture toughness.

• The Korean Journal of Ceramics
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• v.1 no.1
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• pp.45-54
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• 1995

Four kinds of silicon nitride based ceramic materials have been hot pressed. Effect of the sintering additives on the phase transformation, microstructural development and mechanical properties was investigated. While sintering under the same condition a big difference among the microstructures of the specimens, they appeared alike if sintered to have a similiar $\alpha$-$\beta$ phase ratio. The specimen of the stoichiometric $\alpha$-$\beta$ sialon composition showed very limited amount of the intergranular glassy phase and a significant degree of the residual stress. It exhibited almost no strength degradation up to $1300^{\circ}C$, and the strength of the specimen degraded more as its composition deviated from the stoichiometry.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.2
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• pp.79-88
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• 1993

The static creep mechanism and behaviour of Al-Zn-Mg alloy have been investigated under condition of constant stress tension creep test in the temperature and stress range of $170-260^{\circ}C$ and 5-12.5 $kg/mm^2$ respectively. The experimental result are follows : The stress exponent value for creep was observed to about 7.3-6.43 and the activation energy for creep deformation was 44-41 kcal/mol. Larson-Miller parameter P for the crept specimens under the creep condition was obtained as P = (T + 460) (log $t_r$ + 8.6). Emperical equation for the creep rate was obtained by the computer simulation as follows. $${\varepsilon}\;=\;\exp[(-5.519{\times}10^{-4}{\sigma}+2.33{\times}10^{-2})T-6.98{\sigma}+18.295]{\times}{\sigma}^{-0.0142+10.18}\exp[\frac{(-6{\sigma}+47.8)1000}{RT}]$$ Fracture was dominated by intergranular mechanism over the experimental range.

• Transactions of Materials Processing
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• v.6 no.3
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• pp.213-220
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• 1997

The microstructure of the inconel 690 alloy was varied by the solution treatment and the thermal treatment. The specimens having different microstructures were examined in order to understand the strengthening mechanism of the inconel 690. The level of supersaturation of carbon in the solid solution was increased by applying a longer solution treatment at 115$0^{\circ}C$. As increased carbon content in the solid solution, more carbides precipitated during the thermal treatment at $700^{\circ}C$. Since the carbides played a role of obstacle on the movement of dislocations, a higher tensile strength was obtained in the sample having a large number of carbider. The accumulation of dislocations at the grain boundary carbides caused the development of intergranular fracture which led to a lower elongation.

• Journal of the Korean Ceramic Society
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• v.32 no.10
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• pp.1131-1138
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• 1995

Variations of piezoelectric properties and compressive strength of Sr-doped PZT ceramics were investigated with poling direction. The electro-mechanical coupling constants (k31 and k33) were increased linearly with increasing poling strength. The volume fraction of intergranular fracture also increased with incresing poling strength due to weakening of grain boundaries by domain rearrangement during poling process. The internal stresses induced from the poling at 2.5 kV/mm parallel and perpendicular to the poling direction poled were 405 MPa and 89 MPa, respectively. The compressive strength of the specimen poled parallel to the poling direction was higher than that perpendicular to the poling direction.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.244-249
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• 2008

It happened a failure on special bolt which supported main landing gear actuator up-lock rod of 00 aircraft. Fracture was occurred at end of center drilled hole and thread machined on bolt. Metallographic, fractographic, and other characteristics of embrittlement analysis and experiments carried out on the failed bolt to find out the reason. Bolt surface was cadmium electroplated(EP) to give lubrication and provide excellent corrosion resistance. Resultly, Bolt was failed due to cadmium embrittlement occurred during baking treatment as well as center drilled hole. for the failure that are relevant to failure analysis and prevention. For their successful functional application, cadmium EP bolts require proper and adequate baking treatment after electroplating, and is complete with no center drilled hole

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.2
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• pp.16-22
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• 2006

Stainless steel has widely been used in various industrial fields because it has high corrosion resistance. But, we have little design data about the creep life prediction of SUS316L stainless steel. Therefore, in this study, a series of creep tests and study on them under 16 constant stress and temperature combined conditions have been performed to get the creep design data and life prediction of SUS316L stainless steels and we have gotten the following results. First, the stress exponents decrease as the test temperatures increase. Secondly, the creep activation energy gradually decreases as the stresses become bigger. Thirdly, the constant of Larson-Miller parameters on this alloy is estimated about 10. And last, the creep rupture fractographs show the intergranular ductile fracture with many dimples.

• Journal of Welding and Joining
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• v.20 no.5
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• pp.63-71
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• 2002

One of the shortcoming of ferritic stainless steels is their limited toughness. The most important factor governing the toughness of ferritic stainless steels is hewn to be their interstitial contents. Due to the limited solubility of carbon and nitrogen in the ferrite matrix, it is difficult to avoid carbide and nitride precipitates. In the study, the role of nitrogen on the toughness of 260r-2Mo superferritic stainless steel welds has been investigated using alloys containing various nitrogen levels between 100 and 1640 ppm. Mechanical properties of weld metals have been evaluated by microhardness, Charpy impact test and notch tensile test. The alloys are mainly embrittled by the grain boundary and intragranular nitride precipitation. Grain boundary precipitates are considered to be more deleterious than intrauanular nitrides. Fracture mechanism have been elucidated through microscopic evaluation of notch tensile test