• 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 Welding and Joining
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• v.17 no.1
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• pp.124-132
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• 1999

Thermal fatigue strength of the solder joints is the most critical issue for TSOP(Thin Small Outline Package) because the leads of this package are extremely short and thermal deformation cannot be absorbed by the deflection of the lead. And the TSOP body can be subject to early fatigue failures in thermal cycle environments. This paper was discussed distribution of thermal stresses at near the joint between silicon chip and die pad and investigated their reliability of solder joints of TSOP with 42 alloy clad lead frame on printed circuit board through FEM and 3 different thermal cycling tests. It has been found that the stress concentration around the encapsulated edge structure for internal crack between the silicon chip and Cu alloy die pad. And using 42 alloy clad, The reliability of TSOP body was improved. In case of using 42 alloy clad die pad(t=0.03mm). $$\sigma$_{VMmax}$ is 69Mpa. It is showed that 15% improvement of the strength in the TSOP body in comparison with using Cu alloy die pad $($\sigma$_{VMmax}$=81MPa). In solder joint of TSOP, the maximum equivalent plastic strain and Von Mises stress concentrate on the heel of solder fillet and crack was initiated in it's region and propagated through the interface between lead and solder. Finally, the modified Manson-Coffin equation and relationship of the ratio of $N_{f}$ to nest(η) and cumulative fracture probability(f) with respect to the deviations of the 50% fracture probability life $(N_{f 50%})$ were achieved.

• Computational Structural Engineering
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• v.2 no.4
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• pp.59-67
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• 1989

Effect of joints which pre-exist in the rock mass on the behavior of underground structures is studied. A finite element program is developed using a constitutive mode for rock masses exhibiting nonlinear anisotropic behavior. The initial loading scheme combined with reduced region of analysis is employed to minimize the problem size. A circular tunnel within rock mass is analyzed and the results are compared with those of elasto-plastic analysis to verify that the program is reasonable. The effect of joint direction is also analyzed in regard to stress relaxation, displacement, and deformation shape. It is concluded that the joint direction has significant influence on the nonlinear behavior of rock masses such that the vicinity of tunnel perpendicular to the direction of the joints is stressed to slide. It is also observed that the circular shape deforms to an elliptical shape with a major axis in the joint direction.

• Korean Journal of Metals and Materials
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• v.50 no.12
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• pp.931-937
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• 2012

The microstructure of Cu-15 wt%Ag composites fabricated by equal channel angular pressing (ECAP) with intermediate heat treatment at $320^{\circ}C$ was investigated by transmission electron microscopy (TEM) observations. Ag precipitates with a thickness of 20-40 nm were observed in the eutectic region of the Cu-15 wt%Ag composite solution treated at $700^{\circ}C$ before ECAP. The Cu matrix and Ag precipitates had a cube on cube orientation relationship. ECAPed composites exhibited ultrafine-grained microstructures with the shape and distribution dependent on the processing routes. For route A in which the sample was pressed without rotation between each pass, the Cu and Ag grains were elongated along the shear direction and many micro-twins were observed in elongated Cu grains as well as in Ag filaments. The steps were observed on coherent twin boundaries in Cu grains. For route Bc in which the sample was rotated by 90 degrees after each pass, a subgrain structure with misorientation of 2-4 degree by fragmentation of the large Cu grains were observed. For route C in which the sample was rotated by 180 degrees after each pass, the microstructure was similar to that of the route A sample. However, the thickness of the elongated grains along the shear direction was wider than that of the route A sample and the twin density was lower than the route A sample. It was found that more microtwins were formed in ECAPed Cu-15 wt%Ag than in the drawn sample. Grain boundaries were observed in relatively thick and long Ag filaments in Cu-15 wt%Ag ECAPed by route C, indicating the multi-crystalline nature of Ag filaments.

• Transactions of Materials Processing
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• v.31 no.3
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• pp.129-135
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• 2022

The purpose of this study was to quantitatively evaluate the influence of hardness of tool materials on wear resistance in the sheet metal forming process. Punches used in the wear test were made of STD-11 and K340 tool material, and the tempering temperature was set to 530℃ and 500℃, respectively, to control the hardness of the tool materials. The punches mimic the shape of stamping tool of automotive body component to reflect its plastic deformation, and are designed to concentrate wear on the curvature region of punches. Progressive die and coil sheet were used to save time, cost, and raw sheet materials. By quantitatively measuring the wear depth of the punches, the wear behavior and mechanism of the punches were investigated, and characteristics of hardness and wear resistance according to tool materials and tempering temperatures were evaluated. Testing results indicate that the punch made of K340 tool steel with higher hardness had better wear resistance than that of STD-11 tool steel, and the hardness and wear resistance of tool steel were significantly impacted by the tempering temperature.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.10
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• pp.6977-6984
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• 2015

It is faced with the necessity of multi roll forming process of the ball slide rail which is made by adding the separate manufacturing processes, piercing, bending, trimming, to the roll forming process of a continuous plastic deformation, to improve the quality. However, the vibration and noise of the press machine in this process leads to the quality degradation of slide rail manufactured in this process. In this study, the roll was designed considering the optimal strain rates by the roll forming program with finite element method. And to estimate the static stability of the multi process the Von-Mises stress and deformation on the press was calculated with a structural analysis program. Also, to avoid driving systems in the resonance region their natural frequencies in the 1st and 2nd mode were calculated through the modal analysis. To verify its dynamic stability improvement the magnitudes of noise and vibration in the existing and studied system were compared using a microphone and accelerometers. And the widths and surface roughnesses of the rails which had been produced in the existing and studied process were measured. Therefore, it is known that multi roll forming process is stable in the analytical and experimental study.

• Journal of Powder Materials
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• v.9 no.6
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• pp.394-408
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• 2002

Nanostructured high strength metastable Al-, Mg- and Ti-based alloys containing different amorphous, quasicrystalline and nanocrystalline phases are synthesized by non-equilibrium processing techniques. Such alloys can be prepared by quenching from the melt or by powder metallurgy techniques. This paper focuses on one hand on mechanically alloyed and ball milled powders containing different volume fractions of amorphous or nano-(quasi)crystalline phases, consolidated bulk specimens and, on the other hand. on cast specimens containing different constituent phases with different length-scale. As one example. $Mg_{55}Y_{15}Cu_{30}$- based metallic glass matrix composites are produced by mechanical alloying of elemental powder mixtures containing up to 30 vol.% $Y_2O_3$ particles. The comparison with the particle-free metallic glass reveals that the nanosized second phase oxide particles do not significantly affect the glass-forming ability upon mechanical alloying despite some limited particle dissolution. A supercooled liquid region with an extension of about 50 K can be maintained in the presence of the oxides. The distinct viscosity decrease in the supercooled liquid regime allows to consolidate the powders into bulk samples by uniaxial hot pressing. The $Y_2O_3$ additions increase the mechanical strength of the composites compared to the $Mg_{55}Y_{15}Cu_{30}$ metallic glass. The second example deals with Al-Mn-Ce and Al-Cu-Fe composites with quasicrystalline particles as reinforcements, which are prepared by quenching from the melt and by powder metallurgy. $Al_{98-x}Mn_xCe_2$ (x =5,6,7) melt-spun ribbons containing a major quasicrystalline phase coexisting with an Al-matrix on a nanometer scale are pulverized by ball milling. The powders are consolidated by hot extrusion. Grain growth during consolidation causes the formation of a micrometer-scale microstructure. Mechanical alloying of $Al_{63}Cu_{25}Fe_{12}$ leads to single-phase quasicrystalline powders. which are blended with different volume fractions of pure Al-powder and hot extruded forming $Al_{100-x}$$(Al_{0.63}Cu_{0.25}Fe_{0.12})_x$ (x = 40,50,60,80) micrometer-scale composites. Compression test data reveal a high yield strength of ${\sigma}_y{\geq}$700 MPa and a ductility of ${\varepsilon}_{pl}{\geq}$5% for than the Al-Mn-Ce bulk samples. The strength level of the Al-Cu-Fe alloys is ${\sigma}_y{\leq}$550 MPa significantly lower. By the addition of different amounts of aluminum, the mechanical properties can be tuned to a wide range. Finally, a bulk metallic glass-forming Ti-Cu-Ni-Sn alloy with in situ formed composite microstructure prepared by both centrifugal and injection casting presents more than 6% plastic strain under compressive stress at room temperature. The in situ formed composite contains dendritic hcp Ti solid solution precipitates and a few $Ti_3Sn,\;{\beta}$-(Cu, Sn) grains dispersed in a glassy matrix. The composite micro- structure can avoid the development of the highly localized shear bands typical for the room temperature defor-mation of monolithic glasses. Instead, widely developed shear bands with evident protuberance are observed. resulting in significant yielding and homogeneous plastic deformation over the entire sample.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.19 no.1 s.71
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• pp.93-103
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• 2006

It is absolutely essential that safety assessment of the containment buildings during service life because containment buildings are last barrier to protect radioactive substance due to the accidents. Therefore, this study describes an enhanced degenerated shell finite element(FE) which has been developed for nonlinear FE analysis of reinforced concrete(RC) containment buildings with elasto-plastic material model. For the purpose of the material nonlinear analysis, Drucker-Prager failure criteria is adapted in compression region and material parameters which determine the shape of the failure envelop are derived from biaxial stress tests. Reissner-Mindlin(RM) assumptions are adopted to develop the degenerated shell FE so that transverse shear deformation effects is considered. However, it is found that there are serious defects such as locking phenomena in RM degenerated shell FE since the stiffness matrix has been overestimated in some situations. Therefore, shell formulation is provided in this paper with emphasis on the terms related to the stiffness matrix based on assumed strain method. Finally, the performance of the present shell element to analysis RC containment buildings is tested and demonstrated with several numerical examples. From the numerical tests, the present results show a good agreement with experimental data or other numerical results.

• Journal of the Korea Concrete Institute
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• v.19 no.3
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• pp.265-273
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• 2007

The most common structural system for apartment buildings in Korea is adopted to combine structural systems: for example, a moment-resisting frame will be used for lower stories and bearing wall system for the upper stories. This type of buildings have soft and/or weak stories in lower stories, and it may lead to collapse of those buildings during the large earthquake. Reversed cyclic load tests were conducted to estimate the performance and behavioral characteristics of deep beam and exterior column Joints. Experimental parameter is the amount of transverse reinforcement (designed by ACI code and Sheikh's procedure). The results of this study are as follows: (1) The required transverse reinforcement of column designed by Sheikh's procedure requires 2.9 times larger than that designed by ACI procedure. Large amount of transverse reinforcement increase the ductility of the column. (2) Most of the lateral drift in the column is due to the flexural deformation in the joint and plastic hinge region and up-lift rotation. (3) Transverse reinforcement in the exterior column shall be required not only in the hinge region but also in the joint.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.5
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• pp.363-372
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• 2020

Dynamic compressive material properties at high strain rates is essential for improving the reliability of finite element analysis in dynamic environments, such as high-speed collisions and high-speed forming. In general, the dynamic compressive material properties for high strain rates can be obtained through SHPB equipment. In this study, SHPB equipment was used to acquire the dynamic compressive material properties to cope with the collision analysis of Woven tpye CFRP material, which is being recently applied to unmanned aerial vehicles. It is also used as a pulse shaper to secure a constant strain rate for materials with elastic-brittle properties and to improve the reliability of experimental data. In the case of CFRP material, since the anisotropic material has different mechanical properties for each direction, experiments were carried out by fabricating thickness and in-plane specimens. As a result of the SHPB test, in-plane specimens had difficulty in securing data reproducibility and reliability due to fracture of the specimens before reaching a constant strain rate region, whereas in the thickness specimens, the stress consistency of the specimens was excellent. The data reliability is high and a constant strain rate range can be obtained. Through finite element analysis using LS-dyna, it was confirmed that the data measured from the pressure rod were excessively predicted by the deformation of the specimen and the pressure rod.