• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.1
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• pp.26-34
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• 2019

In this paper, a coarse lunar soil model is developed using discrete element method and its computed physical properties are compared with those of the actual lunar soil for its validation. The surface of the actual moon consists of numerous craters and rocks of various sizes, and it is covered with fine dry soil which seriously affects the landing stability of the lunar lander. Therefore, in consideration of the environment of the lunar regolith, the lunar soil is realized using discrete element method. To validate the coarse model of lunar soil, the simulations of the indentation test and the direct shear test are performed to check the physical properties(indentation depth, cohesion stress, internal friction angle). To examine the performance of the proposed model, the drop simulation of finite element model of single-leg landing gear is performed on proposed soil models with different particle diameters. The impact load delivered to the strut of the lander is compared to test results.

• Composites Research
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• v.24 no.2
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• pp.9-13
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• 2011

,In this paper, the symmetric axis parameter method, which was proposed to identify defects, dislocations and stacking fault, with perfect structures in the zinc-blende materials, was introduced as a way to distinguish between elastic and plastic deformation. LAMMPS, a molecular dynamics programme of Sandia National Laboratories, was used to perform nanoindentation simulation on silicon, a zinc-blende material. Defects in silicon (111) under spherical indentation showed the threefold pattern and the slip system in the form of ring crack. Also simulation results show good agreement with experimental results and existing theoretical analyses.

• Journal of the Korean Ceramic Society
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• v.44 no.1 s.296
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• pp.58-64
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• 2007

In this study, we investigated $Li_2ZrO_3$ membrane as a candidate material for high-temperature $CO_2$ separation and evaluated mechanical property. $Li_2ZrO_3$ powder was synthesized by solid state reaction of $Li_2CO_3\;and\;ZrO_2$. Then we fabricated $Li_2ZrO_3$ tape using tape casting method. Dense $Li_2ZrO_3$ membrane prepared by sintering at $1600^{\circ}C$ for 2 h after pressing $Li_2ZrO_3$ tape using lamination machine. Mechanical properties before and after $CO_2$ absorption of fabricated $Li_2ZrO_3$ membrane such as Hertzian indentation, Victors hardness and 3-point bending testing were evaluated.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.5 s.248
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• pp.481-487
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• 2006

Spot-welding is a widely used manufacturing method for thin-sheet components, especially in mass-production industries such as the car industry. Automobiles are often constructed by multi-lap spot welding to secure the passenger from the accident, where optimisation of the welding conditions is a major economic consideration. This research is conducted to investigate weldability characteristics with various welding conditions on the 4-lap spot welded joint of structural steel sheets in automobile. The relationship between the tensile-shear strength and the indentation depth has been investigated to propose the optimum welding conditions. The welding current and the welding time have a greater effect on the welding characteristics than the electrode force. It was found that the electrode force has a relatively close relationship with the expulsion occurrence. The design curves for optimum welding are proposed for the 4-lap spot welded joint.

• Design & Manufacturing
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• v.14 no.1
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• pp.36-41
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• 2020

Recently, due to the development of MEMS technology, researches for the production of effective micro structures and shapes have been actively conducted. However, the process technology based on chemical etching has a number of problems such as environmental pollution and time problems due to multi-process. Various processes to cope with this process are being studied, and one of the mechanical etching processes is the powder blasting process. This process is a method of spraying fine particles, which has the advantage of being an effective process in manufacturing hard brittle materials. However, it is also a process that adversely affects the material surface roughness and material properties due to the impact of the injection of fine particles. In this study, after fabricating micro-channels in fused silica glass with excellent optical properties among the hard brittle materials, we used the nano indentation system to analyze the micro parts using nano-particles as well as machinability and surface roughness analysis of the processed surface. The analysis was performed for the effective processing of powder blasting.

• Journal of the Korean Ceramic Society
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• v.35 no.12
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• pp.1316-1322
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• 1998

The amount of VC and C/N ratio in Ti(CN) was varied to investigate the effect of VC addition on the mi-crostructural change in Ti(CN)-Ni system. As the amount of VC addition increases in Ti(C0.7N0.3)-20Ni sys-tem a complete solid solution was observed in Ti(C0.7N0.3)-20Ni-25VC system. It implies that the ratio of the dissolution rates of Ti(C0.7N0.3)to that of VC is nearly 2:1 at the sintering conditions used in this study. It was found from the experiments that the system composed of the Ti(C0.7N0.3) phase exhibits a rimless structure and relatively small amount of solid solution. That is among Ti(C0.7N0.3) phase exhibits a rimless structure and relatively small amount of solid solution. This is among Ti(C1-xNx) phases the dissolution rate of Ti(C0.3N0.7) is the lowest. Also fracture toughness(KIC) of the cermet was measured by indentation method. Attentions were paid to crack propagation path to look for a dominant fracture mode and to cor-relate it with fracture toughness values. The fracture toughness was relatively high with the addition of VC content. But the addition of a large VC content reduced the overall toughness of the cermet. This result is explained with the difference in fracture mode.

• Corrosion Science and Technology
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• v.7 no.3
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• pp.145-151
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• 2008

Material degradation such as high temperature oxidation of metallic material is a severe problem in energy generation systems or manufacturing industries. The metallic materials are oxidized to form oxide films in high temperature environments. The oxide films act as diffusion barriers of oxygen and metal ions and thereafter decrease oxidation rates of metals. The metal oxidation is, however, accelerated by mechanical fracture and spalling of the oxide films caused by thermal stresses by repetition of temperature change, vibration and by the impact of solid particles. It is therefore very important to investigate mechanical properties and adhesion of oxide films in high temperature environments, as well as the properties in a room temperature environment. The oxidation tests were conducted for Ni and Ni-Co alloy under high temperature corrosive environments. The hardness distributions against the indentation depth from the top surface were examined at room temperature. Dynamic indentation tests were performed on Ni oxide films formed on Ni surfaces at room and high temperature to observe fractures or cracks generated around impact craters. As a result, it was found that the mechanical property as hardness of the oxide films were different between Ni and Ni-Co alloy, and between room and high temperatures, and that the adhesion of Ni oxide films was relatively stronger than that of Co oxide films.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.9 no.6
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• pp.170-177
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• 2000

Dense $Al_2$O$_3$-30%TiC and Si$_3$N$_4$ ceramic tool materials with various grain size were produced by sintering-HIP treatment and by gas-pressure sintering. The fracture toughness was measured by indentation fracture and indentation strength method for both ceramics with various grain size. The effect of the grain size on the fracture toughness was evaluated, and the correlation between fracture toughness and mechanical properties such as hardness, Young\`s modulus and flexural strength of these ceramic were also investigated. The highest fracture toughness of around 6.7 MPa.m(sup)1/2 was obtained in Si$_3$N$_4$ ceramics with grain size of 1.58${\mu}{\textrm}{m}$. With a larger grain size of $Al_2$O$_3$-30%TiC and Si$_3$N$_4$ ceramics, the fracture toughness was generally increased. The increased fracture toughness of these ceramic also improved the flexural strength although the hardness decreased considerably. Similar results were obtained in grain size and mechanical properties on both $Al_2$O$_3$-30%TiC and Si$_3$N$_4$ ceramic tool materials.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.10a
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• pp.63-68
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• 2005

The nanoindentation technique is widely used to investigate the mechanical properties of nano-microscale materials. The nanoindentation method for assessing mechanical properties at low loads and shallow depths is already well established fur the characterization of thin films as well as bulk materials. In this study, we evaluated residual stress in DLC and Au thin films usign nanoindentation technique with a new stress-relaxation model. Moreover, We suggest a composite hardness equation and quantify the magnitude of hardness increase by using an equation based on the interface hardness and the interface thickness, derived by comparing results derived from this equation and those determined in nanoindentation tests. Finally, We present an indentation size effect (ISE) model that extends the available contact depth for ISE application down to several tens of nanometers by considering the tip bluntness effect.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.17 no.6
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• pp.1-6
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• 2018

Recently nano meter size pattern (sub-micro scale) can be machined mechanically using a diamond tool. Many studies have found a 'size effect' which referred to a specific cutting energy increase with the decrease in the uncut chip thickness at micro scale machining. A new analysis method was suggested in order to observe 'size effect' in nano scale machining and to verify the cause of the 'size effect' in this study. The diamond tool was indented to a vertical depth of 1,000nm depth in order to simplify the stress state and the normal force was measured continuously. The tip rounding was measured quantitatively by AFM. Based on the measurements and theoretical analysis, it was verified that the main cause of the 'size effect' in nano scale machining is geometrically necessary dislocations, one of the intrinsic material characteristics. st before tool failure.