• Journal of Korea Foundry Society
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• v.21 no.1
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• pp.24-32
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• 2001

Recently, iron aluminides based on Fe3Al and FeAl are ordered intermetallic alloys that offer good oxidation resistance, excellent sulfidation resistance, and potentially lower cost than many high-temperature structural materials. They have better strength, elasticity to weight ratio and high temperature strength, therefore, they can be cosidered as candidate heat resistance structural materials for automobiles, ships, airplanes and spaceships applications. The changes in the mechanical properties and fracture behavior were investigated for Fe-30at.%Al-5at.%Cr alloys when Ti, Hf and Zr were added respectively. For mechanical properties such as Rockwell hardness and yield strength at room temperature, those were decreased in the case of Fe-30at.%Al-5at.%Cr alloy then increased in the case of 5at.% and 10at.% addition of Ti alone. However, Rockwell hardness and yield strength decreased again at 15%Ti then increased dramatically due to the precipitation hardening of the second phase on the specimen at 20%Ti. For fracture modes, cleavage fracture showed in the case of Fe-30at.%Al and Fe-30at.%Al-5at.%Cr alloys. As the amount of Ti addition changed cleavage to transgranular fracture and to quasi-cleavage fracture at 20%Ti. When Hf, Zr and Hf+Zr were added respectively, transgranular, cleavage and quasi-cleavage fracture were coexisted.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.1115-1120
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• 2003

The recent tendency in the automobile industries is toward light weighting vehicle body to improve the problems by environmental pollution as well as improving fuel cost. The effective way to reduce the weight of vehicle body seems to be application of new materials for body structure and such trend is remarkable. Among the various materials for vehicle body, stainless steel sheet (for example, 301L and 304L), TRIP steel and cold rolled steel sheets are under the interests. However, in order to guarantee reliability of new material and to establish the long life design criteria of body structure, it is important and require condition to assess spot weldability of them and fatigue strength of spot welded lap joints which were fabricated under optimized spot welding condition. And, recently, a new issue in the design of the spot welded structure is to predict economically fatigue design criterion without additional fatigue tests. In general, for fatigue design of the spot-welded thin sheet structure, additional fatigue tests according to the welding condition, material, joint type, and fatigue loading condition are generally required. This indicates that much cost and time for it should be consumed. Therefore, in this paper, the maximum stresses at nugget edge of spot weld were calculated through nonlinear finite element analysis first. And next, obtained the ${\Delta}P-N_{f}$ relation through the actual fatigue tests on spot welded lap joints of similar and dissimilar high strength steel sheets. And then, the ${\Delta}P-N_{f}$ relation was rearranged in the ${\Delta}{\sigma}-N_{f}$ relation. From this ${\Delta}{\sigma}-N_{f}$ relation, developed the fatigue design technology for spot welded lap joints of them welded using the optimized welding conditions.

• Journal of the Korean Recycled Construction Resources Institute
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• v.1 no.1
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• pp.58-66
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• 2013

This study investigated quality properties of alkali activated cement free mortar using industrial by-product such as cement kiln dust(CKD), silica fume(SF) and quartz sand powder(SP) to compare with previous research about blast furnace slag(BS) and fly ash(FA). The results were as following. All materials were effective to increase compressive strength, however they showed different tendency on flowability. CKD and SP increased flowability, but on the other hand SF did not because it's blain was great difference with other materials. Flowability and compressive strength were related with grading distributions of binders because CKD, SP and SF which had small particle size filled up BS and FA. Application of industrial by-products with various grading distributions could be effective for the high early strength and flowability of alkali activated cement free mortar using BS.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.3
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• pp.167-173
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• 2018

In general, calcium is required for the reaction of blast furnace slag fine powder and fly ash. The by-products generated during the process of producing recycled aggregates have different calcium contents depending on the crushing stage and the possibility of using the process by-product as a concrete mixture is also different. In this study, the effect of the calcium content of the by-products on the compressive strength was investigated and the block was fabricated by using this. To utilize the by-products as an admixture, the calcium content was analyzed and the bending strength and surface temperature were measured according to the shape of the water storage block. As a result of this study, the possibility of making a block using recycled aggregate by-products was verified and arch type block was constructed to secure storage capacity and bending strength. Also, the surface temperature of the water storage block was reduced by $9^{\circ}C$ or more than that of the general permeable block.

• Transactions of Materials Processing
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• v.27 no.6
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• pp.370-378
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• 2018

Cold forging, carried out at room temperature, leads to high dimensional accuracy and excellent surface integrity as compared to other forging methods such as warm and hot forgings. In the cold forging process, WC-Co (Tungsten Carbide-Cobalt) alloy is the mainly used material as a core dies because of its superior hardness and strength as compared to other structural materials. For cold forging, die life is the most significant factor because it is directly related to the manufacturing cost due to periodic die replacement in mass production. To investigate die life of WC-Co alloy for cold forging, mechanical properties such as strength and fatigue are essentially necessary. Generally, uniaxial tensile test and fatigue test are the most efficient and simplest testing method. However, uniaxial tension is not efficiently application to WC-Co alloy because of its sensitivity to alignment of the specimen due to its brittleness and difficulty in thread machining. In this study, shape of specimen, tools, and testing methods, which are appropriate for uniaxial tensile test for WC-Co alloy, are proposed. The test results such as Young's modulus, tensile strength and stress-strain curves are compared to those in previous literature to validate the proposed testing methods. Based on the validation of test results it was concluded that the newly developed testing method is applicable to other cemented carbides like Titanium carbides with high strength and brittleness, and also can be utilized to carry out fatigue tests for further investigation on die life of cold forging.

• Composites Research
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• v.37 no.3
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• pp.209-214
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• 2024

Graphene oxide (GO), known for its high stiffness, thermal conductivity, and electrical conductivity, is being utilized as a reinforcement in nanocomposite materials. This study evaluates the mechanical properties of epoxy nanocomposites incorporating GO and edge modified GO (E-GO), which has hydroxyl groups substituted only on its edges. GO/E-GO was uniformly dispersed in epoxy resin using ultrasonic dispersion, and mechanical properties were assessed through tensile testing. The results showed that the addition of nanoparticles increased both tensile strength and toughness. The tensile strength of the epoxy without nanoparticles was 74.4 MPa, while the highest tensile strength of 90.7 MPa was observed with 0.3 wt% E-GO. Additionally, the modulus increased from 2.55 GPa to 3.53 GPa with the addition of nanoparticles. Field emission scanning electron microscopy of the fracture surface revealed that the growth of cracks was impeded by the nanoparticles, preventing complete fracture and causing the cracks to split in multiple directions. E-GO, with surface treatment only on the edges, exhibited higher mechanical properties than GO due to its superior dispersion and surface treatment effects. These results highlight the importance of nanoparticle surface treatment in developing high-performance nanocomposite materials.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.49 no.8
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• pp.445-450
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• 2000

Many successful developments and microscopic studies have been made on the high quality insulating materials. However, a little attention have given to the macroscopic interface in HV(High Voltage) insulating systems. In this study, AC interfacial breakdown strength and V-t characteristic of the interface between Epoxy/EPDM(ethylene propylene diene terpolymer) are investigated. Electrode system is designed to reduce the charges from electrodes and to have the tangential potentials along the interface between Epoxy/EPDM by FEM(finite elements method). The AC breakdown strength is observed when HV is given to the interface. It is shown that AC interfacial breakdown strength is improved by increasing interfacial pressure and oiling. In particular, it was saturated at certain interfacial pressure level. V-t characteristic is able to extend to the life time of the interface between Epoxy/EPDM. Oiling also plays a good roll in prolongation of the life time.

• International Journal of Concrete Structures and Materials
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• v.2 no.2
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• pp.71-81
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• 2008

A macroscopic finite element model is applied to investigate the effect of fire induced spalling on the response of reinforced concrete (RC) beams. Spalling is accounted for in the model through pore pressure calculations in concrete. The principles of mechanics and thermodynamics are applied to compute the temperature induced pore pressure in the concrete structures as a function of fire exposure time. The computed pore pressure is checked against the temperature dependent tensile strength of concrete to determine the extent of spalling. Using the model, case studies are conducted to investigate the influence of concrete permeability, fire scenario and axial restraint on the fire induced spalling and also on the response of RC beams. Results from the analysis indicate that the fire induced spalling, fire scenario, and axial restraint have significant influence on the fire response of RC beams. It is also shown that concrete permeability has substantial effect on the fire induced spalling and thus on the fire response of concrete beams. The fire resistance of high strength concrete beams can be lower that that of normal strength concrete beams due to fire induced spalling resulting from low permeability in high strength concrete.

• Transactions of Materials Processing
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• v.16 no.4 s.94
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• pp.254-259
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• 2007

Dent resistance is an important characteristic to avoid damage on automotive outer panels. From a practical point of view, dents can be caused in a number of ways. Considering doors as an example, denting can occur from stone impacts or from the careless opening of an adjacently parked vehicle door. Denting can occur where the door surface is smooth and may not have sufficient curvature to resist dent. These exterior body parts are designed to improve dent resistance using a combination of work hardening and bake hardening. In brief, dent is affected by the shape of the parts and the material properties such as yield strength, strain and thickness. In this work, forming of door outer panel is investigated by Taguchi method. Main parameters are yield strength, thickness, blank size, blank holding force and so on. For the given value of design parameters, forming analysis of the eighteen cases are carried out according to L18 orthogonal array. After comparing the performance by simple conversion of simulation results into dent resistance, the final suggestion of the forming parameters is verified for the optimal improvement of dent resistance.

• Advances in nano research
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• v.16 no.1
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• pp.81-90
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• 2024

In the oil and gas industry, composite materials should exhibit high flexibility and strength for offshore structures. Therefore, weak points in the composites should be improved, such as brittleness, moisture penetration, and diffusion of detrimental ions into nanometric pores. This study aimed to increase the strength, flexibility, and plugging of nanopores using single-layer graphene oxide (SGO) nanosheets. Therefore, SGO is added to unsaturated polyester resin at concentrations of 0.015 and 0.15 % with Normal Methyl Pyrrolidone (NMP) as a solvent for the formation of Nanographene Oxide Reinforced Polymer (NGORP). The mechanical properties of the prepared samples were tested using tensile testing (ASTM-D 638). It has been shown that incorporating SGO, approximately 0.015%, into the base resin resulted in enhanced properties such as rupture resistance forces increased by 745.61 N, applied stress tolerances increased by 4.1 MPa, longitude increased to 1.58 mm, elongation increased by about 2.38%, and rupture energy increased by about 204.51 J. Despite the decrease in tensile force strength properties in the manufactured nanocomposite with 0.15% SGO, it has exclusive flexibility properties such as a high required energy level for rupture of 5,576 times and a formability of 40% more than the base sample. It would be best to use NGORP manufactured from 0.015% nanosheets with exclusive properties rather than base samples for constructing parts and equipment, such as rebars, composite sheets, and transmission pipes, on offshore platforms.