• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10a
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• pp.301-301
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• 2003

In current study, Nanocomposites are reinforced with carbon nanofiber, carbon nanotube and SiC, etc. Since the nano reinforcements have the excellent mechanical, thermal and electrical properties compared with that of existing composites, it has lately attracted considerable attention in the various areas. Cu have been widely used as signal transmission materials for electrical electronic components owing to its high electrical conductivity. However, it's size have been limited to small ones due to its poor mechanical properties. Until now, strengthening of the copper alloy was obtained either by the solid solution and precipitation hardening by adding alloy elements or the work hardening by deformation process. Adding the alloy elements lead to reduction of electrical conductivity. In this aspect, if carbon nanofiber is used as reinforcement which have outstanding mechanical strength and electric conductivity, it is possible to develope Cu matrix nanocomposite having almost no loss of electric conductivity. It is expected to be innovative in electric conducting material market. The unidirectional alignment of carbon nanofiber is the most challenging task developing the cooer matrix composites of high strength and electric conductivity. In this study, the unidirectional alignment of carbon nanofibers which is used reinforced material are controlled by drawing process and align mechanism as well as optimized drawing process parameter are verified via numerical analysis. The materials used in this study were pure copper and the nanofibers of 150nm in diameter and of 10∼20$\mu\textrm{m}$ in length. The materials have been tested and the tensile strength was 75MPa with the elongation of 44% for the copper. it is assumed that carbon nanofiber behave like porous elasto-plastic materials. Compaction test was conducted to obtain constitutive properties of carbon nanofiber Optimal parameter for drawing process was obtained by analytical and numerical analysis considering the various drawing angles, reduction areas, friction coefficient, etc. The lower drawing angles and lower reduction areas provides the less rupture of co tube is noticed during the drawing process and the better alignment of carbon nanofiber is obtained.

• Korean Journal of Materials Research
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• v.19 no.9
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• pp.457-461
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• 2009

Recently, metal molding has become essential not only for automobile parts, but also mass production, and has greatly influenced production costs as well as the quality of products. Its surface has been treated by carburizing, nitriding and induction hardening, but these existing treatments cause considerable deformation and increase the expense of postprocessing after treatment; furthermore, these treatments cannot be easily applied to parts that requiring the hardening of only a certain section. This is because the treatment cannot heat the material homogeneously, nor can it heat all of it. Laser surface treatment was developed to overcome these disadvantages, and, when the laser beam is irradiated on the surface and laser speed is appropriate, the laser focal position is rapidly heated and the thermal energy of surface penetrates the material after irradiation, finally imbuing it with a new mechanical characteristic by the process of self-quenching. This research estimates the material characteristic after efficient and functional surface treatment using HPDL, which is more efficient than the existing CW Nd:YAG laser heat source. To estimate this, microstructural changes and hardness characteristics of three parts (the surface treatment part, heat affect zone, and parental material) are observed with the change of laser beam speed and surface temperature. Moreover, the depth of the hardened area is observed with the change of the laser beam speed and temperature.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.3
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• pp.576-585
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• 2002

A three-dimensional Lagrangian explicit time-integration finite element code for analyzing the dynamic impact phenomena was developed. It uses four node tetrahedral elements. In order to consider the effects of strain rate hardening, strain hardening and thermal softening, which are frequently observed in high-velocity deformation phenomena, Johnson-Cook model is used as constitutive model. For more accurate and robust contact force computation, the defense node contact algorithm was adopted and implemented. In order to evaluate the performance of the newly developed three-dimensional hydrocode NET3D, numerical simulations of the oblique impact of mild steel plate by mild steel sphere were carried out. Ballistic limit about various oblique angle between 0 degree and 80 degree was estimated through a series of simulations with different initial velocities of sphere. Element eroding by equivalent plastic strain was applied to mild steel spheres and targets. Ballistic limits and fracture characteristics obtained from simulation were compared with experimental results conducted by Finnegan et al. From numerical studies, the following conclusions were reached. (1) Simulations could successfully reproduce the key features observed in experiment such as tensile failure termed "disking"at normal impacts and outwards bending of partially formed plus segments termed "hinge-mode"at oblique impacts. (2) Simulation results fur 60 degrees oblique impact at 0.70 km/s and 0.91 km/s were compared with experimental results and Eulerian hydrocode CTH simulation results. The Lagrangian code NET3D is superior to Eulerian code CTH in the computational accuracy. Agreement with the experimentally obtained final deformed cross-sections of the projectile is excellent. (3) Agreement with the experimental ballistic limit data, particularly at the high-obliquity impacts, is reasonably good. (4) The simulation result is not very sensitive to eroding condition but slightly influenced by friction coefficient.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.3 s.174
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• pp.777-785
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• 2000

The low-cycle fatigue behaviors of cast AI-Si alloy and composite with reinforcement of SIC particles were compared with those of extruded unreinforced matrix alloy and composite in order to investigate the influence of cast and extrusion processes on the cyclic deformation and fatigue life. Generally, both cast and extruded composites including the unreinforced alloy exhibited cyclic hardening behaviour, with more pronounced strain-hardening for the composites with a higher volume fraction of the SiC particles. However, cast composite under a low applied cyclic strain showing no observable plastic strain exhibited cyclic softening behavior due to the cast porosities. The elastic modulus and yield strength of the cast composite were found to be quite comparable to those of the extruded composite, however, the extrusion process considerably improved the ductility and fracture strength of the composite by effectively eliminating the cast porosities. Low-cycle fatigue lives of the cast alloy and composite were shorter than those of the extruded counterparts. Large difference in life between cast and extruded composites was attributed to the higher influence of the cast porosities on the fatigue life of the composite than that of the unreinforced alloy material. A fatigue damage parameter using strain energy density effectively represented the inferior life in the low-cycle regime and superior life in the high-cycle regime for the composite, compared to the unreinforced alloy.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.10
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• pp.1793-1799
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• 2003

An optimal design of a multi-layered plate structure to endure high-velocity impact has been suggested by using size optimization after numerical simulations. The NET2D, a Lagrangian explicit time-integration finite element code for analyzing high-velocity impact, was used to find the parameters for the optimization. Three different materials such as mild steel, aluminum for a multi-layered plate structure and die steel for the pellet, were assumed. In order to consider the effects of strain rate hardening, strain hardening and thermal softening, Johnson-Cook model and Phenomenological Material Model were used as constitutive models for the simulation. It was carried out with several different gaps and thickness of layers to figure out the trend in terms of those parameters' changes under the constraint, which is against complete penetration. Also, the measuring domain has been shrunk with several elements to reduce the analyzing time. The response surface method based on the design of experiments was used as optimization algorithms. The optimized thickness of each layer in which perforation does not occur has been obtained at a constant velocity and a designated total thickness. The result is quite acceptable satisfying both the minimized deformation energy and the weight criteria. Furthermore, a conceptual idea for topology optimization was suggested for the future work.

• Journal of Korea Foundry Society
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• v.21 no.1
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• pp.7-14
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• 2001

The research on new DRA(discontinuous reinforced alloy) and CRA(continous reinforced alloy) composites has been carried out to improve the properties of ceramic fiber and particle reinforced metal matrix composites(MMCs). Effects of alloying elements and aging conditions on the microstructures and aging behavior of Al-Si-Cu-Mg-(Ni)-SiCp composite have been examined. The specimens used in this study were manufactured by duplex process. The first squeeze casting is the process to make precomposite and the second squeeze casting is the process to make final composite. The hardening behavior was accelerated with decreasing the size of SiCp particle in the composites. It is considered that the dislocation density increased with increasing SiCp size, due to the different thermal deformation between Al matrix and SiCp during quenching after the solution treatment. Peak aging time to obtain the maximum hardness in 3 ${\mu}m$ SiCp reinforced Al composite was reduced than that in large size(5, 10 ${\mu}m$) of SiCp because of difference in dislocation density. Aging hardening responce(${\Delta}H$ = $H_{Max}.-H_{S.T}$) of composites was greater than that of unreinforced Al alloy because of higher density of second phases in matrix.

• Korean Journal of Materials Research
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• v.11 no.10
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• pp.879-888
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• 2001

The effects of final annealing temperature on the microstructure and creep characteristics were investigated for the Zr-lNb-0.2X (X=0, Mo, Cu) and Zr-lNb- 1Sn-0.3Fe-0.1X (X=0, Mo, Cu) alloys. The microstructures were observed by using TEM/EDS, and grain size and distributions of precipitates were analyzed using a image analyzer. The creep test was performed at $400^{\circ}C$ under applied stress of 150 MPa for 10 days. The $\beta$-Zr was observed at annealing temperature above $600^{\circ}C$. In the temperature above$ 600^{\circ}C$, the grain sizes of both alloy systems appeared to be increased with increasing the final annealing temperature. The creep strengths of Zr-1Nb-1Sn-0.3Fe-0.1X alloys were higher than those of Zr-1Nb-0.2X ones due to the effect of solid solution hardening by Sn in Zr-lNb-lSn-0.3Fe-0.1X alloy system. Also, Mo addition showed the strong effect of precipitate hardening in both alloy systems. The creep strength rapidly decreased with increasing the annealing temperature up to $600^{\circ}C$. However, a superior creep resistance was obtained in the sample that annealed to have a second phase of $\beta$-Zr. It was considered that the appearance of $\beta$-Zr would play an important role in the strengthening mechanism of creep deformation.

• Composites Research
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• v.17 no.5
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• pp.54-60
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• 2004

A new three-dimensional model for predicting the relationship between the prestrain of the composite and the amount of phase transformation of shape memory alloy inducing shape memory effect has been proposed by using Eshelby's equivalent inclusion method with Mori-Tanaka's mean field theory. The model composite is aluminum matrix reinforced with short TiNi fiber shape memory alloy, where the matrix is work-hardening material of power-law type. The analytical results predicted by the current model show that most of the prestrain is induced by the plastic deformation of the matrix, except the small prestrain region. The strengthening mechanism of the composite by the shape memory effect should be explained by excluding its increase of yield stress due to the work-hardening effect of the matrix.

• Journal of the Korean Society for Heat Treatment
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• v.25 no.1
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• pp.22-26
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• 2012

An influence of the addition of copper (0.5, 1.0 and 1.5 mass% Cu) on the microstructure and mechanical properties of high purity iron (99.998 mass%) was characterized. The microstructure and microhardness of high-purity iron based samples, which were rolled at room temperature and subsequently annealed, were investigated in this work. The microstructure of the samples has been observed by electron back scattering diffraction (EBSD) and the mechanical properties have been studied by using micro-Vickers hardness test. The results of microstructural observation showed that deformation band was formed in high purity iron by rolling at room temperature, and it was recovered by annealing up to about 900 K. The microhardness results showed that the softening of high-purity iron occurred by annealing up to about 900 K, while the hardness of iron added with about 0.5-1.5 mass% copper was kept over 100 Hv and at the early time of annealing reached a maximum. The hardness of iron added with a small amount of copper may be attributed to precipitation hardening as well as solution hardening. The orientation of crystal in recrystallized grain was almost same as that of deformed grain.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.6
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• pp.876-888
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• 1986

The Study is concerned with the analysis of unconstrained axisymmetric piercing as a nonsteady forging process by the rigid-plastic finite element method. In the numerical analysis of axisymmetric piercing, the initial velocity field is generated by assuming the material as a linear viscous material to begin with in order to facilitate the input handling and to ensure better convergencey. The strain-hardening effect for nonsteady deformation and the friction of the die-material interial interface are considered in the formulation. Rigid body treatment is also incorporated in the developed program. The experiments are carried out for aluminum alloy specimens (A1204) with different specimen heights. It is shown that the experimental results are in excellent agreement with the finite element simulations is deformed configuration. For load prediction the theoretical prediction shows excellent agreement with th eexperimental laod in the initial stage of loading before fracture of the specimen is not initiated. Distribution of stresses, strains and strain rates has been found for the given cases in computation. On this basis several fracture criteria are introduced in order to check the fracture initiation. It is found that maximum shear criterion is capable of good fracture prediciton.