• International Journal of Korean Welding Society
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• v.3 no.2
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• pp.40-45
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• 2003

A series of experiments has been carried out to investigate the effect of titanium, boron and nitrogen on the microstructure and toughness of simulated heat affected zone (HAZ) in quenched and tempered (QT) type 490MPa yield strength steels. For acquiring the same strength level, the carbon content and carbon equivalent could be lowered remarkably with a small titanium and boron addition due to the hardenability effect of boron during quenching process. Following the thermal cycle of large heat input, the coarsened grain HAZ (CGHAZ) of conventional quenched and tempered (QT) type 490MPa yield strength steels exhibited a coarse bainitic or ferrite side plate structure with large prior austenite grains. While, titanium and boron bearing QT type 490MPa yield strength steels were characterized by the microstructure in the CGHAZ, consisting mainly of the fine intragranular ferrite microstructure. Toughness of the simulated HAZ was mainly controlled by the proper Ceq level, and the ratio of Ti/N rather than titanium and nitrogen contents themselves. In the titanium­boron added QT steels, the optimum Ti/N ratio for excellent HAZ toughness was around 2.0, which was much lower than the known Ti/N stoichiometric ratio, 3.4. With reducing Ti/N ratio from the stoichiometric ratio, austenite grain size in the coarse grained HAZ became finer, indicating that the effective fine precipitates could be sufficiently obtained even with lower Ti/N level by adding boron simultaneously. Along with typical titanium carbo­nitrides, various forms of complex titanium­ and boron­based precipitates, like TiN­MnS­BN, were often observed in the simulated CGHAZ, which may act as stable nuclei for ferrite during cooling of weld thermal cycles

• Restorative Dentistry and Endodontics
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• v.35 no.6
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• pp.461-472
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• 2010

Objectives: This study evaluated microtensile bond strength (${\mu}TBS$) and short-rod fracture toughness to explain fractural behavior of repaired composite restorations according to different surface treatments. Materials and Methods: Thirty composite blocks for ${\mu}TBS$ test and sixty short-rod specimens for fracture toughness test were fabricated and were allocated to 3 groups according to the combination of surface treatment (none-treated, sand blasting, bur roughening). Each group was repaired immediately and 2 weeks later. Twenty-four hours later from repair, ${\mu}TBS$ and fracture toughness test were conducted. Mean values analyzed with two-way ANOVA / Tukey's B test ($\alpha$= 0.05) and correlation analysis was done between ${\mu}TBS$ and fracture toughness. FE-SEM was employed on fractured surface to examine the crack propagation. Results: The fresh composite resin showed higher ${\mu}TBS$ than the aged composite resin (p < 0.001). Mechanically treated groups showed higher bond strength than non-mechanically treated groups except none-treated fresh group in ${\mu}TBS$ (p < 0.05). The fracture toughness value of mechanically treated surface was higher than that of non-mechanically treated surface (p < 0.05). There was no correlation between fracture toughness and microtensile bond strength values. Specimens having high KIC showed toughening mechanism including crack deviation, microcracks and crack bridging in FE-SEM. Conclusions: Surface treatment by mechanical interlock is more important for effective composite repair, and the fracture toughness test could be used as an appropriate tool to examine the fractural behavior of the repaired composite with microtensile bond strength.

• Journal of Powder Materials
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• v.15 no.1
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• pp.1-5
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• 2008

ZnS-$SiO_2$ composite is normally used for sputtering target. In recent years, high sputtering power for higher deposition rate often causes crack formation of the target. Therefore the target material is required that the sintered target material should have high crack resistance, excellent strength and a homogeneous microstructure with high sintered density. In this study, raw ZnS and ZnS-$SiO_2$ powders prepared by a 3-D mixer or high energy ball-milling were successfully densified by spark plasma sintering, the effective densification method of hard-to-sinter materials in a short time. After sintering, the fracture toughness was measured by the indentation fracture (IF) method. Due to the effect of crack deflection by the residual stress occurred by the second phase of fine $SiO_2$, the hardness and fracture toughness reached to 3.031 GPa and $1.014MPa{\cdot}m^{1/2}$, respectively.

• Computers and Concrete
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• v.18 no.3
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• pp.337-354
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• 2016

To study the influence on fracture properties of reinforced concrete wedge splitting test specimens by the addition of reinforcement, and the restriction of steel bars on crack propagation, 7 groups reinforced concrete specimens of different reinforcement position and 1 group plain concrete specimens with the same size factors were designed and constructed for the tests. Based on the double-K fracture criterion and tests, fracture toughness calculation model which was suitable for reinforced concrete wedge splitting tensile specimens has been obtained. The results show that: the value of initial craking load Pini and unstable fracture load Pun decreases gradually with the distance of reinforcement away from specimens's top. Compared with plain concrete specimens, addition of steel bar can reduce the value of initial fracture toughness KIini, but significantly increase the value of the critical effective crack length ac and unstable fracture toughness KIun. For tensional concrete member, the effect of anti-cracking by reinforcement was mainly acted after cracking, the best function of preventing fracture initiation was when the steel bar was placed in the middle of the crack, and when the reinforcement was across the crack and located away from crack tip, it plays the best role in inhibiting the extension of crack.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.04a
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• pp.75-78
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• 2000

There are several effective factors to influence die life in the warm forging process. For instance process design die design and die materials etc This study presented heat treatment method which could improve toughness and wear resistance simultaneously in high temperature to apply high speed tool steels like SKH51 to die material for warm forging process. To verify the feasibility of application of heat treatment method mentioned above wear test was performed under the condition of constant time in 40$0^{\circ}C$ Wear coefficient was examined to search a relation between wear amount and time for each material and heat treatment method in 30, 60, and 130 minutes. To quantify the toughness-behavior between room and high temperature impact test was performed and heat fatigue test also fulfilled to compare with the resistance of heat check in room, 200, 400, and $600^{\circ}C$ temperature. On the basis of experimental results mentioned above high speed tool steel was applied to verify appropriateness of newly proposed heat treatment method for die of rotor pole used in automobile alternator. As a result die life of high speed tool steel applied newly proposed heat treatment is longer than that of STD61.

• Journal of Welding and Joining
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• v.3 no.2
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• pp.42-51
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• 1985

The dynamic fracture toughness, fracture characteristics, impact tension and tensile properties of Al-Mg-Si T5 alloy and Al-Zn-Mg T6 alloy respectively welded with filler metal of Alcan 4043 were investigated. The dynamic fracture toughness values were obtained rapidly and simply for the specimen of small size by using instrumented Chirpy impact testing machine. the testing temperatures of the specimen were a range of room temperature and-196.deg. C. The results obtained in this experiment are summarized as follows. With decreasing the testing temperatures, dynamic tensile stress and fracture load were increased, on the other hand the deflection and impact value showed decreasing tendency in order of base metal>HAZ>weld. Changes of total absorbed energy were more influenced by the crack propagation energy than the crack initiation energy. At the low temperatures, the unstable rapid fracture representing the crack propagation appeared for the specimens of Charpy press side notched in Al-Zn-Mg alloy, but it was difficult to obtain the unstable rapid fracture in Al-Mg-Si alloy. Because of the development of plastic zone at the notch root, it was difficult to obtain thevalid $K_{1d}$ value in Al-Mg-Si alloy. Therefore the fatigue cracked specimens were effective in both Al-Mg-Si and Al-Zn-Mg alloys. With decreasing the impact testing temperatures, specimens underwent a transition from dimple-type transgranular fracture to lamella surface-type intergranular fracture because of the precipitate at the grain boundaries, impurities and crystal structure of the precipitates.s.

• Journal of the Korea institute for structural maintenance and inspection
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• v.23 no.2
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• pp.92-98
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• 2019

In general, high strength and high performance fiber reinforced cement composites exclude coarse aggregates basically in order to have homogeneous distributions of material properties. However, these fiber-reinforced cement mortar without coarse aggregate have a tenancy that the modulus of elasticity is low and the unit weight of cement is high, resulting in low economic efficiency. Therefore, in this study, the development of high ductile fiber - reinforced concrete was conducted, which has the adequate level of coarse aggregate but still retains the high flexural toughness and strength and also has the crack - distributing performance. Experimental study was carried out by using the amount of coarse aggregate as an experimental parameter. The results showed that the best flexural toughness and crack dispersion characteristics was obtained when the coarse aggregate was added at 25% by weight of the fine aggregate to the typical mixtures of high ductile cement mortar. PVA fiber was effective in crack distribution and ductility enhancement, and steel fiber was effective in strengthening flexural strength rather than crack distribution.

• Journal of the Korean Ceramic Society
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• v.35 no.3
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• pp.231-238
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• 1998

Cr3C2 -dispersed TiC composites were prepared via HPCS(high pressure-self combustion sintering) pro-cess using mixtures of Ti, Cr and a carbon source for the purpose of increasing the facture toughness and sinterability of TiC. In this study the microstructure and properties of the composites were investigated in terms of relation to the carbon source the particle size of Ti and the amount of Cr. It was found that car-bon black was the most effective carbon source among the various carbon sources tested and the reaction was more effective as the particle size of Ti decreased. Among the sintered composites of Ti-C-Cr system the one with 30wt% Cr showed the best physical properties with 0.5% in apparent porosity 98.8% in re-lative density 18.2 GPa in hardness and 4.46 MPa.m1/2 in fracture toughness. In addition it was observed that the lattice constant of TiC decreased gradually with increasing the amount of Cr.

• Journal of Welding and Joining
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• v.7 no.2
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• pp.35-48
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• 1989

This study is concerned with a correlation of microstructure and local brittle zone (LBZ) in offshore structural steel welds. The influence of the LBZ on fracture toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in void and cleavage microcrack formation were also identified using notched round tensile tests and subsequent SEM observations. The LBZ in the HAZ of a multiphase welded joint is the interstitially reheated coarse grained HAZ, which is influenced by metallurgical factors such as effective grain size, the major matrix structure and the amount of high-carbon martensite-austenite (M-A) constituents. The experimental results indicate that Chirpy energy was found to scale monotonically with the amount of M-A constituents, confirming that the M-A constituent is the major microstructural factor controlling the HAZ toughness. In addition, voids and microcracks are observed to initiate at M-A constituents by the shear cracking process. Thus, the M-A constituent played an important role in initiating the voids and microcracks, and consequently caused brittle fracture.

• Journal of the Korea Concrete Institute
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• v.26 no.4
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• pp.483-489
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• 2014

In this study, the flexural test of amorphous steel fiber-reinforced concrete was performed according to ASTM C 1609 to investigate its flexural performances. The amorphous steel fibers have different configurations from conventional steel fibers : thinner sections and coarser surfaces. Primary test parameters are fiber type (amorphous and conventional steel fibers), concrete compressive strength (27 and 50 MPa), and fiber volume fraction (0.25, 0.50, and 0.75%). Based on the test results, flexural strength and flexural toughness of the amorphous and conventional steel fiber-reinforced concrete were investigated. The results showed that the addition of the amorphous steel fibers into concrete could enhance both flexural strength and toughness while the addition of the conventional steel fibers into concrete was mainly effective to increase the flexural toughness.