• Journal of the Korean Society for Precision Engineering
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• v.27 no.11
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• pp.17-23
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• 2010

Titanium alloy is one of the hard processing materials made by the traditional manufacturing method because of the excellent mechanical strength. Ablation of titanium alloy is investigated by using a femtosecond laser which is a regenerative amplified Ti:sapphire laser with 1kHz repetition rate, 184fs pulse duration time and 785nm wavelength. Experiments are carried out under various ablation conditions with different pulse overlap ratios for the rectangular shape and micro hole. Test results show that the ablation characteristic according to pulse overlap ratio of titanium alloy seems to be as non-linear type at the different zone of energy fluence. The optimal condition of rectangular shape processing is obtained at the laser peak power 1.3mW, pulse overlap ratio of 90%, beam gap of $1\;{\mu}m$. The micro hole has a good quality from the pulse overlap ratio of 99% at the same laser peak power. With the optimal processing condition, the fine rectangular shape and micro hole without burr and thermal damage are achieved.

• Journal of Industrial Technology
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• v.26 no.B
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• pp.111-118
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• 2006

The reinforced concrete structure is one of the most popular structures in real construction. Concrete has been strengtened rapidly due to the development of new material and construction technology. But as the concrete has been getting stronger, the brittleness of material has increased and the better ductility has been required. So, the study for strengthening stiffener has been urgently needed. As we said above, it is expected that the use of high strength steel and concrete will be increased. However, The experimental data is not enough for solving problems of the use of high strengthened steel and concrete. In this research, we analyzed 45 combinations of the strength levels of concrete, the thickness of material and the steel strength with regard to simple Reinforced Concrete SLAB Beam bridge. The program MIDAS CIVIL was used to find the optimal combination. As a result, it was found that strength ratio per unit section is in inverse proportion to the strength of material and that the strengths of steel are respectively 400 MPa for low strengthened concrete and 300 MPa for high strengthened concrete. For economic aspect and usability, the effect of high strength steel is not as high as we expected it would be.

• Proceedings of the Korea Concrete Institute Conference
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• 2009.05a
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• pp.277-278
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• 2009

The building that supply tidal and splash zone was constructed near Seamangeum Gate Bridge. The specimens that will be tested for maintenance of gate bridge were exposed on the tidal and splash zone, totally about 650(Fig. 1). The characteristics of strength, salt penetration profile, field application of surface repair material and section recover material will be acquired by periodical test. The program was developed to obtain optimal maintenance strategy of gate bridge as a marine concrete structure and to deposit experimental data, lab. test result, field test result, on its D/B. On this paper, we hope to introduce two years exposure data as compressive strength, the modulus of elasticity, the modulus of dynamic elasticity, field adoption of repair and recover materials. As briefly speaking the results, possion's ratio, elasticity, strength was general, but the recover materials have some problems. There was crack between concrete and recover material and delamination figures.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2018.11a
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• pp.108-109
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• 2018

Cement is the most widely used but generates a lot of CO₂, so we need a material to replace it. Using industrial by-products such as Silica Fume(SF), Blast furnace Slag(BS) and Fly Ash(FA) bring some advantages including CO₂ reduction and resource recycling. However, there is a limit to improve performance when using only one material. Therefore, the synergy effects of No cement binary mortar and ternary mortar were analyzed and compared. As a result, No cement ternary mortar had the strength higher than binary mortar. among ternary mortars, the specimen mixed 50% of BS had the highest strength. However, when SF was mixed by 20%, the flowability reduces. so 10% of SF, 40% of FA and 50% of BS is considered as the optimal mixing ratio.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.11a
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• pp.179-180
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• 2023

The adhesion performance of PCCs for crack repair of RC structures was greater in the case of using ultra high-early strength cement than in the case of using ordinary Portland cement, and the effect of mixing silica fume was higher in the case of ordinary Portland cement than that of ultra high-early strength cement. On the other hand, 130% of W/C was more fluid than 80% of W/C in the same P/C 80%, which increased the fillability and improved the strength, but the strength improvement effect was the greatest in adhesion in flexure. Through this study, the basic characteristics of the adhesion performance of PCCs were identified, and based on this, it is necessary to induce an optimal mixing design that can increase adhesion performance through various mixing designs.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.8 no.2
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• pp.89-98
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• 1988

This research evaluated factors on cement-based solidification process designed for hazardous sludge produced from electroplating industry. Four factors of sand/cement ratio, water/cement ratio, amount of wet sludge and amount of a precipitator, were investigated in terms of leachability and compressive strength of the solidified materials. Results of triplicate tests and statistical analysis indicated that sand/cement ratio(S/C) had the greatest effect on leaching of Cr(VI) from the solidified materials while water/cement ratio(W/C) on Zn and compressive strength. Cr(VI) was fixed better than Zn by portland cement. An experimental modeing was developed to estimate leached metal concentration and compressive strength at a given condition. Proper mixing criteria were also suggested for the use of the solidified mixture as construction materials. In solidification of 30g dry sludge, optimal condition was studied for S/C ratio, W/C ratio and the weight of precipitator which were 1, 1.5 and 1.075g respectively.

• Advances in concrete construction
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• v.16 no.3
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• pp.127-139
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• 2023

In this study, the steel fiber and the polypropylene fiber were used to enhance the mechanical properties of fully recycled coarse aggregate concrete. Natural crushed stone was replaced with recycled coarse aggregate at 100% by volume. The steel fiber and polypropylene fiber were used as additive material by incorporating into the mixture. In this test two parameters were considered: (a) steel fiber volume ratio (i.e., 0%, 1%, 1.5%, 2%), (b) polypropylene fiber volume ratio (i.e., 0%, 0.1%, 0.15%, 0.2%). The results showed that compared with no fiber, the integrity of cubes or cylinders mixed with fibers after failure was better. When the volume ratio of steel fiber was 1~2%, the width of mid-span crack after flexural failure was 5~8 mm. In addition, when the volume ratio of polypropylene fiber was 0.15%, with the increase of steel fiber content, the static elastic modulus and toughness of axial compression first increased and then decreased, and the flexural strength increased, with a range of 6.5%~20.3%. Besides, when the volume ratio of steel fiber was 1.5%, with the increase of polypropylene fiber content, the static elastic modulus decreased, with a range of 7.0%~10.5%. The ratio of axial compression toughness first increased and then decreased, with a range of 2.2%~8.7%. The flexural strength decreased, with a range of 2.7%~12.6%. On the other hand, the calculation formula of static elastic modulus and cube compressive strength of fully recycled coarse aggregate with steel-polypropylene fiber was fitted, and the optimal fiber content within the scope of the test were put forward.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.05a
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• pp.131-132
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• 2023

Recently, the construction industry has seen the emergence of interior and exterior finishes using ultra-high performance concrete (UHPC) and colored concrete products using precast concrete (PC). However, the excessive amount of pigment used for coloring reduces the strength of the concrete. There is a need to improve the durability and chromaticity of colored concrete, and further analytical studies on the properties of colored concrete are also required. Therefore, in this paper, colored ultra-high strength cement composites (C-UHSCC) containing red and green inorganic pigments were prepared, and the compressive strength and color of the specimens were measured according to the age, and the correlation between strength and color was analyzed by simple linear regression analysis using R² value. The results showed that the red color was highly correlated with L^* and a^*, and the green color was highly correlated with a^*. These results can be considered for various concrete formulations, but research is needed to suggest the optimal pigment mixing ratio for proper strength and color development.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.3
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• pp.591-599
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• 1994

A design system for power transmission bevel gears(straight, zerol, and spiral) is developed, in which the strength and durability of bevel gears can be estimated and the size of bevel gears can be minimized by introducing optimal techniques. The size of bevel gear pair as the object function to be minimized is the volume of equivalent spur gear pair at mean normal section, and the design variables to be determined are considered as the number of teeth, face width, diametral pitch, and spiral angle in spiral bevel gear. The strength(bending strength, pitting resistance) according to the AGMA standards, geometrical quantities, and operating characteristics(interference of pinion, contact ratio, etc.) are considered as the constraints in design optimization. The optimization with these constraints becomes nonlinear problem and that is solved with ALM(Augmented Lagrange Multiplier) method. The developed design method is applied to the example designs of straight, zerol, and spiral bevel gears. The design results are acceptable from the viewpoint of strength and durability within the design ranges of all other constraint, and the bevel gears are designed toward minimizing the size of gear pair. This design method is easily applicable to the design of bevel gears used as power transmitting devices in machineries, and is expected to be used for weight minimization of bevel gear unit.

• Advanced Composite Materials
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• v.17 no.3
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• pp.259-275
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• 2008

This study aims to optimize the mechanical and electrical properties of electrically conductive polymer composites (CPCs) for use as a material of bipolar plates for PEM fuel cells. The thin CPCs consisting of conductive fillers and polymer resin were fabricated by a preform molding technique. Expanded graphite (EG), flake-type graphite (FG) and carbon fiber (CF) were used as conductive fillers. This study tested two types of CPCs, EG/FG filled CPCs and EG/CF filled CPCs, to optimize the material properties. First, the characteristics of EG/FG filled CPCs were investigated according to the FG ratio for 7 and $100{\mu}m$ sized FG. CPCs using $100{\mu}m$ FG showed optimal material properties at 60 wt% FG ratio, which were an electrical conductivity of 390 S/cm and flexural strength of 51 MPa. The particle size was an important parameter to change the mechanical and electrical behaviors. The flexural strength was sensitive to the particle size due to the different levels of densification. The electrical conductivity also showed size-dependent behavior because of the different contributions to the conductive network. Meanwhile, the material properties of EG/CF filled CPCs was also optimized according to the CF ratio, and the optimized electrical conductivity and flexural strength were 290 S/cm and 58 MPa, respectively. The electrical conductivity of this case decreased similarly to the EG/FG filled case. On the other hand, the behavior of the flexural strength was more complicated than the EG/FG filled case, and the reason was attributed to the interaction between the strengthening effect of CF and the deterioration of voids.